Rapid upslope shifts in New Guinean birds illustrate strong distributional responses of tropical montane species to global warming

Temperate-zone species have responded to warming temperatures by shifting their distributions poleward and upslope. Thermal tolerance data suggests that tropical species may respond to warming temperatures even more strongly than temperate-zone species, but this prediction has yet to be tested. We addressed this data gap by conducting resurveys to measure distributional responses to temperature increases in the elevational limits of the avifaunas of two geographically and faunally independent New Guinean mountains, Mt. Karimui and Karkar Island, 47 and 44 y after they were originally surveyed. Although species richness is roughly five times greater on mainland Mt. Karimui than oceanic Karkar Island, distributional shifts at both sites were similar: upslope shifts averaged 113 m (Mt. Karimui) and 152 m (Karkar Island) for upper limits and 95 m (Mt. Karimui) and 123 m (Karkar Island) for lower limits. We incorporated these results into a metaanalysis to compare distributional responses of tropical species with those of temperate-zone species, finding that average upslope shifts in tropical montane species match local temperature increases significantly more closely than in temperate-zone montane species. That tropical species appear to be strong responders has global conservation implications and provides empirical support to hitherto untested models that predict widespread extinctions in upper-elevation tropical endemics with small ranges.Temperate species are responding to anthropogenic temperature increases by rapidly shifting geographic distributions to track their climatic niche (1–3). These shifts appear to be increasing in pace—a recent metaanalysis concluded that species are shifting their distributions poleward and upslope much faster than previously estimated (1, 2). Range shifts are less studied in tropical regions however (1, 4, 5), despite being home to the vast majority of biodiversity (6). Notwithstanding strong latitudinal bias in empirical studies, climate change-driven range shifts are predicted to cause widespread extinctions in both temperate and tropical species within the next century (7–10).With scarce empirical data, models of tropical species’ response to temperature increases predict a wide range of responses (11). At one extreme, tropical species may be relatively unaffected, as the magnitude of temperature increases is relatively low in the tropics (12). Alternately, vulnerability to warming temperatures could be highest in the tropics if tropical species are physiologically specialized to narrow thermal niches (13–18). Such thermal specialization has been documented in tropical ectotherms (16, 17), but it is unclear whether similar patterns may apply to tropical endotherms, whose distributional shifts in response to warming may result from indirect rather than direct impacts of temperature increases (5).We resurveyed geographically and faunally independent elevational gradients in New Guinea nearly a half-century after they were first surveyed. The original transect surveys were conducted by J. Diamond to determine bird species’ elevational limits on Mt. Karimui (July–August 1965) (19) and Karkar Island (May 1969) (20). These environments differ significantly: Mt. Karimui is located in New Guinea’s biodiverse Central Ranges and harbors a diverse resident avifauna of ca. 250 resident landbirds (19), whereas Karkar Island is a small oceanic island off New Guinea’s north coast with a depauperate flora and fauna (ca. 50 resident landbirds) dominated by highly dispersive taxa (20) (Fig. 1).Open in a separate windowFig. 1.Map of resurvey sites in Papua New Guinea. The elevational transects recently revisited by the authors are marked by dashed lines (Mt. Karimui: 1,130–2,520 m; Karkar Island: 800–1,600 m). Mt. Karimui is an extinct volcano in the southern Central Ranges of New Guinea, whereas Karkar Island is an oceanic island located 10 miles from the New Guinean mainland. These elevational gradients were originally surveyed by Diamond in the 1960s [Mt. Karimui: 1965 (19); Karkar Island: 1969 (20)], and remain covered in primary forest.We used elevational limits measured during historical transects and modern resurveys to investigate New Guinean montane birds’ response to warming temperatures. We predicted that species have moved upslope relative to historical range limits. Given that tropical species are hypothesized to be especially sensitive to temperature increases (either directly or via indirect ecological interactions), we additionally predicted that the magnitude of upslope shifts would closely match predicted shifts based on local temperature increases. We simultaneously tested two additional hypotheses, investigating whether upslope shifts at the leading range margin outpaced upslope shifts at the trailing range edge (21), and whether species’ dietary preferences influenced upslope shifts (22, 23). We then used our data in conjunction with recent tropical resurveys to test the tropical-species-are-strong-responders hypothesis, predicting that upslope shifts measured in tropical resurveys match predicted upslope shifts significantly more closely than for temperate-zone resurveys.     

From the Cover: Strong upslope shifts in Chimborazo's vegetation over two centuries since Humboldt

Global climate change is driving species poleward and upward in high-latitude regions, but the extent to which the biodiverse tropics are similarly affected is poorly known due to a scarcity of historical records. In 1802, Alexander von Humboldt ascended the Chimborazo volcano in Ecuador. He recorded the distribution of plant species and vegetation zones along its slopes and in surrounding parts of the Andes. We revisited Chimborazo in 2012, precisely 210 y after Humboldt’s expedition. We documented upward shifts in the distribution of vegetation zones as well as increases in maximum elevation limits of individual plant taxa of >500 m on average. These range shifts are consistent with increased temperatures and glacier retreat on Chimborazo since Humboldt’s study. Our findings provide evidence that global warming is strongly reshaping tropical plant distributions, consistent with Humboldt’s proposal that climate is the primary control on the altitudinal distribution of vegetation.The biological impacts of ongoing climate change (1) are already apparent in species’ poleward and upslope range shifts and earlier spring events (2–9). However, most studies stem from high-latitude areas and are generally restricted to dynamics across the past few decades (10). To our knowledge, only three previous resurveys have studied range shifts of tropical plant taxa, all at <4,000 m in elevation (7, 8, 11). Modeling (12) and paleoecological studies (13) suggest that tropical montane vegetation should be highly sensitive to climate change. However, researchers strongly debate whether tropical plants are tracking warming temperatures along elevation gradients, with most (although scarce) studies indicating they are lagging behind (cf. 14, 15). Such lags could have negative effects on the distributions of species dependent on certain plant taxa, e.g., as a food source (16). The question is particularly urgent given the growing evidence of systematically stronger warming rates in high-mountain environments (17).The legacy and works of Alexander von Humboldt (1769–1859) not only constitute the foundation of biogeography, but also what is likely the oldest dataset on altitudinal ranges of plant species. The observations recorded by Humboldt and Aimé Bonpland (1773–1858) during their travels in Central and South America, and synthesized in a Tableau of Mt. Chimborazo (summit 6,268 m above sea level) and accompanying essay (18), provide a unique opportunity to study tropical vegetation changes over a period of 210 y. To our knowledge, this period is more than twice as long as any previous resurvey study based on historical biodiversity records (11, 19). We revisited the upper slopes of the Chimborazo volcano in June 2012. Our aim was to record the current elevational distribution of plants and test for upward shifts since Humboldt’s expedition, as a response to anthropogenic global warming. We sampled plant species presence and abundance along transects every 100 m of elevation between 3,800 and 5,200 m. Three main findings, comparing our surveys to Humboldt’s data, support strong upward shifts of plant distributions: a higher upper limit for plant growth, increased elevation of vegetation zones, and upward shifts in the upper range limits of most individual taxa.     

Cenozoic imprints on the phylogenetic structure of palm species assemblages worldwide

Despite long-standing interest in the origin and maintenance of species diversity, little is known about historical drivers of species assemblage structure at large spatiotemporal scales. Here, we use global species distribution data, a dated genus-level phylogeny, and paleo-reconstructions of biomes and climate to examine Cenozoic imprints on the phylogenetic structure of regional species assemblages of palms (Arecaceae), a species-rich plant family characteristic of tropical ecosystems. We find a strong imprint on phylogenetic clustering due to geographic isolation and in situ diversification, especially in the Neotropics and on islands with spectacular palm radiations (e.g., Madagascar, Hawaii, and Cuba). Phylogenetic overdispersion on mainlands and islands corresponds to biotic interchange areas. Differences in the degree of phylogenetic clustering among biogeographic realms are related to differential losses of tropical rainforests during the Cenozoic, but not to the cumulative area of tropical rainforest over geological time. A largely random phylogenetic assemblage structure in Africa coincides with severe losses of rainforest area, especially after the Miocene. More recent events also appear to be influential: phylogenetic clustering increases with increasing intensity of Quaternary glacial-interglacial climatic oscillations in South America and, to a lesser extent, Africa, indicating that specific clades perform better in climatically unstable regions. Our results suggest that continental isolation (in combination with limited long-distance dispersal) and changing climate and habitat loss throughout the Cenozoic have had strong impacts on the phylogenetic structure of regional species assemblages in the tropics.     

Recent increases in tropical cyclone precipitation extremes over the US east coast

The impacts of inland flooding caused by tropical cyclones (TCs), including loss of life, infrastructure disruption, and alteration of natural landscapes, have increased over recent decades. While these impacts are well documented, changes in TC precipitation extremes—the proximate cause of such inland flooding—have been more difficult to detect. Here, we present a latewood tree-ring–based record of seasonal (June 1 through October 15) TC precipitation sums (ΣTCP) from the region in North America that receives the most ΣTCP: coastal North and South Carolina. Our 319-y-long ΣTCP reconstruction reveals that ΣTCP extremes (≥0.95 quantile) have increased by 2 to 4 mm/decade since 1700 CE, with most of the increase occurring in the last 60 y. Consistent with the hypothesis that TCs are moving slower under anthropogenic climate change, we show that seasonal ΣTCP along the US East Coast are positively related to seasonal average TC duration and TC translation speed.

Landfalling tropical cyclones (TCs) produce high winds, storm surges, and inland flooding that can have devastating impacts on human and natural landscapes (1). TC-related flooding can cause billions of dollars in structural damage (2) and is one of the deadliest aspects of TCs (3, 4). Moreover, climate model simulations suggest that TCs produce more precipitation under anthropogenic forcing, particularly within the center of the TCs (5–9). However, our understanding of the impacts and potential trends in the flood hazard caused by excess precipitation from TCs in the United States is limited by the length of the instrumental TC precipitation (TCP) record (1948 to present) (10).Along the US east coast—an area considered to have the most complete record of TCs through time worldwide (11–13)—the translation speed of TCs has decreased in recent decades (14), which could result in higher TCP totals (ΣTCP) (15). This slowdown is in line with global decreases in translation speed by 10% from 1949 to 2016 (16) and has been implicitly related to the weaker global wind circulation from anthropogenic warming (17, 18). However, part of this slowing of TC speed is an artifact of the introduction of satellite data recording a larger number of weaker and smaller TCs, biasing the record of TC translation speed (19–21). To place modern changes in TC characteristics—and this recent slowdown—in a historical perspective and to investigate potential links to anthropogenic climate change, long-term (i.e., multicentury) records are needed. Slower TCs produce more TCP (15), and long-term trends in ΣTCP could therefore provide further evidence that TCs are slowing.Reliable detection of changes in ΣTCP, however, has proven challenging and therefore produced mixed results. Annual ΣTCP in the United States (10, 22) and globally (23) show no significant trends over the observed record. Yet, studies that focus on TCP amounts from individual storms have found increases in 1) TC-induced “drought-busting” events (24, 25), 2) TC-associated heavy rainfall events (26), and 3) Individual storm-related TCP amounts in US coastal cities (15). This inconsistency can be explained by the substantial interannual variability within the short instrumental TCP record, which limits our ability to reliably detect changes in ΣTCP through time.Here, we use 300+ years of tree-ring data to extend the record of seasonal (June 1 to October 15; Materials and Methods) ΣTCP for the coast of North Carolina and South Carolina (Fig. 1). This region experiences the highest annual ΣTCP in North America (10), averaging ∼50 mm/year (1948 to 2018) and the highest contribution of ΣTCP to annual precipitation (up to 8%; Fig. 1 A and B). In many years, ΣTCP accumulates within a few days and can create widespread flooding. For instance, of the 5 y with the largest ΣTCP in this region, three were generated predominantly by a single storm that produced ΣTCP exceeding 200 mm (Fig. 1C). However, the 2 y with the largest ΣTCP were generated by multiple storms: Connie and Diane (190 and 110 mm) in 1955 and Bertha and Fran (110 mm and 175 mm) in 1996 (Fig. 1C).Open in a separate windowFig. 1.Instrumental TCP seasonal and extreme year totals. Average (1948 through 2018) June 1 to October 15 ΣTCP (A) and seasonal average contribution of ΣTCP to overall precipitation (B) (1948 through 2018). Red squares represent the 0.25° grids where data were averaged to calculate the regional ΣTCP. (C) Maps of individual storms that had large ΣTCP (millimeters) over the instrumental record. Orange diamonds are tree-ring sites. Data for all plots were created from TCPDat (10).To reconstruct seasonal ΣTCP and extend the ΣTCP record to 1700 CE, we use the latewood portion of tree rings from Pinus palustris Mill. (longleaf pine). Our reconstruction is based on tree-ring data collected from seven sites in North Carolina and South Carolina (Fig. 1), which allows the development of a regional ΣTCP reconstruction that is calibrated against a gridded dataset of ΣTCP: TCPDat (10). By creating a regional ΣTCP reconstruction, we expand upon earlier ΣTCP reconstruction work (27, 28) spatially as well as temporally. This allows us to determine spatiotemporal ΣTCP variability and in particular, whether extreme ΣTCP have increased over time, as is expected in a warmer world.     

Downscaling CMIP5 climate models shows increased tropical cyclone activity over the 21st century

A recently developed technique for simulating large [O(10⁴)] numbers of tropical cyclones in climate states described by global gridded data is applied to simulations of historical and future climate states simulated by six Coupled Model Intercomparison Project 5 (CMIP5) global climate models. Tropical cyclones downscaled from the climate of the period 1950–2005 are compared with those of the 21st century in simulations that stipulate that the radiative forcing from greenhouse gases increases by over preindustrial values. In contrast to storms that appear explicitly in most global models, the frequency of downscaled tropical cyclones increases during the 21st century in most locations. The intensity of such storms, as measured by their maximum wind speeds, also increases, in agreement with previous results. Increases in tropical cyclone activity are most prominent in the western North Pacific, but are evident in other regions except for the southwestern Pacific. The increased frequency of events is consistent with increases in a genesis potential index based on monthly mean global model output. These results are compared and contrasted with other inferences concerning the effect of global warming on tropical cyclones.     

Deep ocean communities impacted by changing climate over 24 y in the abyssal northeast Pacific Ocean

The deep ocean, covering a vast expanse of the globe, relies almost exclusively on a food supply originating from primary production in surface waters. With well-documented warming of oceanic surface waters and conflicting reports of increasing and decreasing primary production trends, questions persist about how such changes impact deep ocean communities. A 24-y time-series study of sinking particulate organic carbon (food) supply and its utilization by the benthic community was conducted in the abyssal northeast Pacific (∼4,000-m depth). Here we show that previous findings of food deficits are now punctuated by large episodic surpluses of particulate organic carbon reaching the sea floor, which meet utilization. Changing surface ocean conditions are translated to the deep ocean, where decadal peaks in supply, remineralization, and sequestration of organic carbon have broad implications for global carbon budget projections.Contemporary climate change marked by increasing water temperature, density stratification, and acidification is impacting the world ocean. These changes are especially evident in oceanic surface waters and coastal areas (1), where surface water production of organic carbon and trophic exchanges are affected. However, little is known of how these changes influence the food supply to the deep ocean. Can we expect decreased production of organic carbon produced in the upper ocean, and thus less food delivered to the sea floor? Because the deep ocean occupies the vast majority of the world, such answers are critical to evaluating impacts of climate variation on the global carbon cycle, particularly regarding long-term carbon sequestration.A major unknown component of the global carbon cycle is the amount of organic carbon that reaches the deep ocean and its ultimate utilization or long-term sequestration in the sediments. This supply starts with primary production by phytoplankton in surface waters. There is no consensus on whether phytoplankton biomass is decreasing as a result of a reduction in upwelled nutrients, caused by warming surface waters and increasing stratification (2). In open ocean areas, these conditions can lead to a decrease in primary production by phytoplankton (1, 3) and a geographic expansion of oligotrophic (low chlorophyll) waters (3). In contrast, along-shore winds and increased land–sea temperature disparities are leading to increased nutrient supply and primary production in coastal upwelling areas (1, 2, 4–6). A portion of the organic carbon produced in surface waters is exported to the deep ocean by a variety of mechanisms, including mineral ballasting, aggregation, fecal pellet production, and sinking or vertical migration of large zooplankton (7–12).How do such conditions impact the food supply to the deep ocean, which relies on surface water primary production? Previous studies have shown an unexpected shortage of food reaching deep ocean depths to sustain benthic communities over an 18-y period, with carbon utilization consistently exceeding local supply over that time scale (13, 14). New technology added to long time-series studies now allows us to compare high-resolution measurements of food supply and benthic community carbon consumption to trends over the past 24 y to resolve the question of food shortage, and to examine how it might be changing in the context of global warming. We set out to test the hypothesis that food produced in and settling from overlying surface waters cannot sustain benthic community utilization on multiyear to decadal scales at an abyssal station in the northeast Pacific. To test this hypothesis, we used a combination of autonomous instrumentation on the sea floor and water column, along with satellite-derived measurements, to estimate sinking particulate food as organic carbon reaching and being used by deep-sea communities. These studies were conducted as part of an ongoing 24-y time series at Station M, where measurements of deep-sea processes, combined with atmospheric and surface ocean conditions, have been monitored since 1989 (15). Overlying waters at this abyssal site (∼4,000-m water depth) show strong seasonal primary production corresponding to upwelling events within the California Current.     

The Past as a Lens for Biodiversity Conservation on a Dynamically Changing Planet: Improving the relevance of paleontology to climate change policy

Paleontology has provided invaluable basic knowledge on the history of life on Earth. The discipline can also provide substantial knowledge to societal challenges such as climate change. The long-term perspective of climate change impacts on natural systems is both a unique selling point and a major obstacle to becoming more pertinent for policy-relevant bodies like the Intergovernmental Panel on Climate Change (IPCC). Repeated experiments on the impacts of climate change without anthropogenic disturbance facilitate the extraction of climate triggers in biodiversity changes. At the same time, the long timescales over which paleontological changes are usually assessed are beyond the scope of policymakers. Based on first-hand experience with the IPCC and a quantitative analysis of its cited literature, we argue that the differences in temporal scope are less of an issue than inappropriate framing and reporting of most paleontological publications. Accepting that some obstacles will remain, paleontology can quickly improve its relevance by targeting climate change impacts more directly and focusing on effect sizes and relevance for projections, particularly on higher-end climate change scenarios.     

Climate-driven diversity loss in a grassland community

Local ecological communities represent the scale at which species coexist and share resources, and at which diversity has been experimentally shown to underlie stability, productivity, invasion resistance, and other desirable community properties. Globally, community diversity shows a mixture of increases and decreases over recent decades, and these changes have relatively seldom been linked to climatic trends. In a heterogeneous California grassland, we documented declining plant diversity from 2000 to 2014 at both the local community (5 m²) and landscape (27 km²) scales, across multiple functional groups and soil environments. Communities became particularly poorer in native annual forbs, which are present as small seedlings in midwinter; within native annual forbs, community composition changed toward lower representation of species with a trait indicating drought intolerance (high specific leaf area). Time series models linked diversity decline to the significant decrease in midwinter precipitation. Livestock grazing history, fire, succession, N deposition, and increases in exotic species could be ruled out as contributing causes. This finding is among the first demonstrations to our knowledge of climate-driven directional loss of species diversity in ecological communities in a natural (nonexperimental) setting. Such diversity losses, which may also foreshadow larger-scale extinctions, may be especially likely in semiarid regions that are undergoing climatic trends toward higher aridity and lower productivity.Large-scale elevational and latitudinal range shifts, altered seasonal timing, and disrupted interactions among interdependent species are well-known consequences of recent global warming, all of which are predicted to intensify in coming decades and to be accompanied by increasing rates of global extinction (1–4). Consequences of rapid climate change for the diversity of local ecological communities are far less clear; diversity might increase or decrease at any given location, depending on the particular nature of climatic changes and the potential for dispersal (5–8). For two decades, gains in plant species richness have been observed on European mountain summits in boreal-temperate regions, where climatic warming has led to longer growing seasons and higher productivity (5), and where steep topography may have facilitated upward dispersal from lower elevations (6, 7). However, in one of the first documentations of the opposite trend, declines in species richness were reported on 10 European mountain summits, mostly in the Mediterranean region where warming has led to more severe climatic water deficits (8). Globally, the species diversity of ecological communities has shown neither consistent increases nor decreases in recent decades (9, 10). It remains unknown how widely across biomes, and at what spatial scales, climatically driven losses of plant community diversity may be expected in the near future. Diversity at relatively local spatial scales has been linked experimentally to resource use efficiency, productivity, temporal stability, resistance to invasion, and other desirable functional properties (11–13). To the extent that climate is causing declining diversity at local scales, there is increased support for concerns about ecosystem service loss, and reasons to expect larger-scale extinctions in the not too distant future.Climatic drying (aridification), arising from both increases in temperature and declines in precipitation during the growing season, is a major facet of contemporary and anticipated climate change throughout the world’s arid and semiarid climates (14–19). Aridification in the western United States has already been linked to large-scale tree dieoffs and other vegetation changes (20–23). In California, where overall aridity has increased in recent decades (18) and coastal and inland fog have declined dramatically (24, 25), aridification is predicted to dominate the effects of climate change on natural vegetation over the coming century (18, 19, 26). Although directional declines in species richness in western US grasslands in response to long-term drying trends are not yet documented, they may be expected based on evidence that grassland species richness is higher in wetter than drier years, geographical locations, and experimental treatments (27–29). Seed dormancy, especially by annuals, is likely an important facet of short-term fluctuations in grassland species richness in response to water availability (e.g., ref. 28); however, dormancy during dry years is clearly not a strategy by which species can survive longer-term, directional drying of the climate.We monitored grassland species richness for 15 y (2000–2014) at 80 sites in a heterogeneous California landscape. Thirty-eight sites were on infertile serpentine soils, considered an important refuge for native species, and 42 sites were on fertile soils dominated by exotic grasses. Each site consisted of five, 1 m² quadrats at which species presence or absence (2000–2014) and visual estimates of species cover (2006–2014) were recorded in April and June annually (see Dataset S1 for a complete species list). Native annual forbs were the most numerous group (110 of 237 species) although they were individually uncommon (median cover 2%, median occupancy five sites). These species germinate in late fall to early winter and most of them flower in April–May, although a few flower earlier or later. Local richness of native annual forbs was higher on serpentine soils (mean 17 native annual forbs of 29 total species) than on more fertile soils (mean 9 native annual forbs of 24 total species). Livestock grazing on half the sites slightly enhanced local species richness before ceasing in 2001, and a fire affecting some sites in 1999 had a modest positive effect on local species richness in 2000 (ref. 30; Methods). Local species richness was higher in years of higher rainfall (31, 32), as has been seen in other studies in similar grasslands (e.g., ref. 28). The study area lies within a region that has warmed and dried in recent decades (18, 19).We analyzed trends in grassland species richness at the local (5 m²) and landscape scales (27 km², all 80 sites). We compared trends among native and exotic species, sites on fertile (nonserpentine) and infertile (serpentine) soils, and sites with and without histories of livestock grazing. We similarly analyzed time trends in climatic variables recorded at the site, focusing on precipitation because of its well-documented effects on grassland diversity, but also including temperature, solar radiation (an inverse measure of cloudiness), and humidity, because of the potential of these variables to either ameliorate or exacerbate the effects of declining precipitation. We examined trends in these climatic variables for the whole rainy season (Sep–Jun) and its early (Sep–Nov), middle (Dec–Feb), and late (Mar–Jun) thirds; the middle period (Dec–Feb) is especially critical for native annual forbs, which are then present as small seedlings dependent on shallow soil moisture. Upon identifying significant time trends in species richness and in precipitation, we used time-series models (as in ref. 27) to test for a direct link between richness and precipitation.To further explore the community consequences of aridification, we examined plant functional traits (33). Variation among species in mean specific leaf area (SLA; leaf area/dry mass; or its inverse, termed leaf mass per unit area) has been found to correlate well with among-species variation in key physiological attributes such as leaf longevity (LL), water use efficiency (WUE), and relative growth rate (RGR). Species with high SLA and RGR, and low LL and WUE, are more prevalent in wetter climates (34) and in wetter years (35) and tend to increase disproportionately in response to experimental watering (36) and natural precipitation increase (37). Several previous studies have found that high-SLA species are especially vulnerable to decline or loss under aridification (38, 39).We hypothesized that aridification would lead to grassland communities poorer in overall species diversity. We also hypothesized that community functional composition would change, such that the native annual forb component would show a lower community mean value of SLA, consistent with the disproportionate local disappearance of drought-intolerant species. Because high SLA is linked to rapid litter decomposition and nutrient cycling (40), such a shift in community mean trait values could affect community function. We tested for time trends in community mean SLA, and for a direct link between precipitation and community mean SLA, using the same models as for species richness.     

The Past as a Lens for Biodiversity Conservation on a Dynamically Changing Planet: A resilient and connected network of sites to sustain biodiversity under a changing climate

Motivated by declines in biodiversity exacerbated by climate change, we identified a network of conservation sites designed to provide resilient habitat for species, while supporting dynamic shifts in ranges and changes in ecosystem composition. Our 12-y study involved 289 scientists in 14 study regions across the conterminous United States (CONUS), and our intent was to support local-, regional-, and national-scale conservation decisions. To ensure that the network represented all species and ecosystems, we stratified CONUS into 68 ecoregions, and, within each, we comprehensively mapped the geophysical settings associated with current ecosystem and species distributions. To identify sites most resilient to climate change, we identified the portion of each geophysical setting with the most topoclimate variability (high landscape diversity) likely to be accessible to dispersers (high local connectedness). These “resilient sites” were overlaid with conservation priority maps from 104 independent assessments to indicate current value in supporting recognized biodiversity. To identify key connectivity areas for sustaining species movement in response to climate change, we codeveloped a fine-scale representation of human modification and ran a circuit-theory-based analysis that emphasized movement potential along geographic climate gradients. Integrating areas with high values for two or more factors, we identified a representative, resilient, and connected network of biodiverse lands covering 35% of CONUS. Because the network connects climatic gradients across 250,000 biodiversity elements and multiple resilient examples of all geophysical settings in every ecoregion, it could form the spatial foundation for targeted land protection and other conservation strategies to sustain a diverse, dynamic, and adaptive world.

Conservationists in the United States are not winning the battle to sustain biological diversity. Despite broad public support and unprecedented bipartisan agreement on Earth Day 1970, followed by landmark environmental laws, expanded regulatory efforts, and the establishment of hundreds of private conservation organizations, the species and ecosystems that characterize the natural world continue to decline. In North America, the abundance of birds has fallen 29% since 1970 (1); 32% of insect taxa are in decline (2); and 56% of mammalian carnivore and ungulates have shown notable range contractions since 1950 (3). Amphibians have declined an average (avg.) of 33% since 2002 (4). Of the 51,936 species of plants, vertebrates, and macroinvertebrates tracked by NatureServe for the conterminous United States (CONUS), 9% are ranked vulnerable, 12% imperiled, and 1% possibly extinct (5).^*Changes in climate are exacerbating species declines, especially for small, isolated populations. As temperature and moisture regimes change, species ranges are shifting with speed and magnitude unprecedented in recent millennia. In the eastern United States, trees have shifted their centers of distribution 10 km north and 11 km west per decade since 1980 (6). Southern bird ranges have shifted northward by an avg. of 23.5 km per decade (7). These changes are on par with global shifts of 10 km north and 11 m upslope per decade across taxa groups (8).A primary driver of biodiversity decline is habitat loss and degradation resulting from land-use change (9, 10). Land- and water-conservation efforts can reverse these trends when strategically located and enabled by the necessary investments. In North America, billions of dollars spent on wetland restoration and management, combined with more stringent hunting regulations, reversed bird-abundance declines in wetlands (1). Globally, conservation investment from 1996 to 2008 reduced the extinction risk for mammals and birds by a median value of 29% (10). However, the effectiveness of land and water conservation in sustaining biodiversity depends on the representativeness of the conserved area network, the resilience and condition of the sites, and the connectivity between sites to allow for movement and adaptation (11, 12).To sustain biodiversity and facilitate adaptation of species to a changing climate, the Convention on Biological Diversity (CBD) Target 2 (13) calls for the protection of well-connected and effective systems of protected areas covering at least 30% of the planet. However, as climate change drives changes in species distributions and ecosystem composition, conservation plans based on current biodiversity patterns may become less effective at sustaining species (14). In particular, the current configuration of protected areas may fail to adequately provide access to the diverse climatic conditions needed for species populations to persist amid changing regional climates (12, 15, 16). Accordingly, conservation planners are beginning to focus on conserving sites that represent the earth’s eco-physiographic regions (hereafter “ecoregions”) and the spectrum of geophysical variation and a diversity of connected topographic microclimates (hereafter “topoclimates”) to allow species to adapt in situ or move to newly favorable areas, an approach known as Conserving Nature’s Stage (CNS) (15–19).Most studies of climate effects on biota use regional-scale climate-projection models combined with species vulnerability assessments to identify areas of relatively high threat or stability at a coarse scale. Here, we take a different approach. By focusing on geophysical diversity that shapes species distributions and fine-scale climate variation directly relevant to species persistence (20, 21), we aimed to identify enduring climate strongholds relevant under many climate scenarios and to map them at scales appropriate for land-conservation decisions.For species in topographically diverse locations, variability in temperature locally may exceed the degree of warming expected over the next century (22, 23). These areas have the potential to provide species with microclimatic buffering from regional climatic change by allowing local dispersal to more favorable microclimates or providing stepping stones to facilitate longer-distance range shifts (24, 25). Paleoecological records highlight the dynamic nature of species responses to Quaternary climate change, including the role of topography in creating climate refugia (26–28), and suggest that the CNS strategy may be appropriate for many taxa if it is purposefully designed to accommodate species responses to climate change (29).Species persisted under past climatic changes through in situ refugia combined with range shifts to track suitable climates (30–32). Rapid warming projected for the next century will likely require many species to adapt in a similar way (33–35), and many species’ ranges are already shifting (8). However, high levels of habitat loss and fragmentation due to anthropogenic activities are isolating populations and creating barriers to species movement that were not present during past periods of rapid climate change (29, 36, 37). Thus, conservation actions that maintain or increase connectivity are essential for effective conservation under climate change, as connectivity facilitates movement and gene flow, bolstering adaptive capacity by maintaining genetic diversity (38–40).To sustain biodiversity, a conservation network must also include sites that support living biotic assemblages reflecting each ecoregion’s geophysical properties, such as dominant habitats, unique communities, and viable examples of rare and specialist species populations. We refer to these as sites with “recognized biodiversity value.” Including them in a conservation network ensures that it is embedded with species and habitats that provide the capacity for adapting to climate change (41, 42). In the United States, state agencies and nongovernment organizations (NGOs) have identified over a thousand areas with recognized biodiversity value through comprehensive ecoregional or state-based assessments specifically targeting viable rare species populations, exemplary natural communities, and intact ecosystems. Integrating the footprint of these sites with spatial information on connected topoclimates and representative geophysical features helps confirm that the sites are collectively distributed across all abiotic “stages” needed to sustain biodiversity into the future.     

Community ecology in a changing environment: Perspectives from the Quaternary

Community ecology and paleoecology are both concerned with the composition and structure of biotic assemblages but are largely disconnected. Community ecology focuses on existing species assemblages and recently has begun to integrate history (phylogeny and continental or intercontinental dispersal) to constrain community processes. This division has left a “missing middle”: Ecological and environmental processes occurring on timescales from decades to millennia are not yet fully incorporated into community ecology. Quaternary paleoecology has a wealth of data documenting ecological dynamics at these timescales, and both fields can benefit from greater interaction and articulation. We discuss ecological insights revealed by Quaternary terrestrial records, suggest foundations for bridging between the disciplines, and identify topics where the disciplines can engage to mutual benefit.     

Rates of change in natural and anthropogenic radiative forcing over the past 20,000 years

The rate of change of climate codetermines the global warming impacts on natural and socioeconomic systems and their capabilities to adapt. Establishing past rates of climate change from temperature proxy data remains difficult given their limited spatiotemporal resolution. In contrast, past greenhouse gas radiative forcing, causing climate to change, is well known from ice cores. We compare rates of change of anthropogenic forcing with rates of natural greenhouse gas forcing since the Last Glacial Maximum and of solar and volcanic forcing of the last millennium. The smoothing of atmospheric variations by the enclosure process of air into ice is computed with a firn diffusion and enclosure model. The 20th century increase in CO(2) and its radiative forcing occurred more than an order of magnitude faster than any sustained change during the past 22,000 years. The average rate of increase in the radiative forcing not just from CO(2) but from the combination of CO(2), CH(4), and N(2)O is larger during the Industrial Era than during any comparable period of at least the past 16,000 years. In addition, the decadal-to-century scale rate of change in anthropogenic forcing is unusually high in the context of the natural forcing variations (solar and volcanoes) of the past millennium. Our analysis implies that global climate change, which is anthropogenic in origin, is progressing at a speed that is unprecedented at least during the last 22,000 years.     

Warming increases the risk of civil war in Africa

Armed conflict within nations has had disastrous humanitarian consequences throughout much of the world. Here we undertake the first comprehensive examination of the potential impact of global climate change on armed conflict in sub-Saharan Africa. We find strong historical linkages between civil war and temperature in Africa, with warmer years leading to significant increases in the likelihood of war. When combined with climate model projections of future temperature trends, this historical response to temperature suggests a roughly 54% increase in armed conflict incidence by 2030, or an additional 393,000 battle deaths if future wars are as deadly as recent wars. Our results suggest an urgent need to reform African governments'' and foreign aid donors'' policies to deal with rising temperatures.     

Drought stress and hurricane defoliation influence mountain clouds and moisture recycling in a tropical forest

Mountain ranges generate clouds, precipitation, and perennial streamflow for water supplies, but the role of forest cover in mountain hydrometeorology and cloud formation is not well understood. In the Luquillo Experimental Forest of Puerto Rico, mountains are immersed in clouds nightly, providing a steady precipitation source to support the tropical forest ecosystems and human uses. A severe drought in 2015 and the removal of forest canopy (defoliation) by Hurricane Maria in 2017 created natural experiments to examine interactions between the living forest and hydroclimatic processes. These unprecedented land-based observations over 4.5 y revealed that the orographic cloud system was highly responsive to local land-surface moisture and energy balances moderated by the forest. Cloud layer thickness and immersion frequency on the mountain slope correlated with antecedent rainfall, linking recycled terrestrial moisture to the formation of mountain clouds; and cloud-base altitude rose during drought stress and posthurricane defoliation. Changes in diurnal cycles of temperature and vapor-pressure deficit and an increase in sensible versus latent heat flux quantified local meteorological response to forest disturbances. Temperature and water vapor anomalies along the mountain slope persisted for at least 12 mo posthurricane, showing that understory recovery did not replace intact forest canopy function. In many similar settings around the world, prolonged drought, increasing temperatures, and deforestation could affect orographic cloud precipitation and the humans and ecosystems that depend on it.

The interaction between forests, atmosphere, and precipitation in mountainous areas is key to the maintenance of clean, steady water supplies, yet factors contributing to this outcome are still being identified. Forest-covered land impacts water and energy cycles at multiple scales (1). At a global scale, recent (2–4) and historical (5) evidence indicates that deforestation can affect precipitation patterns at great distances and over large regions (6). At a watershed scale, forest evapotranspiration returns large amounts of water to the atmosphere and regulates local meteorological conditions. Despite significant transpiration outflux, forest soils store and purify water for downstream uses, and forest land cover increases infiltration and moderates runoff to control flooding, thus often providing a net advantage for water resources (7–9).Prevailing winds carry moist air from upwind oceans or landmasses; when this air is lifted by topography and cooled, orographic clouds form. Mountain regions are crucial to water supply worldwide (10) because orographic clouds produce precipitation, increase rainfall from regional scale weather systems (11), and, when in contact with mountain slopes (12), both add water and suppress evapotranspiration losses.The lifting condensation level (LCL) is a quasi-planar feature in the atmosphere at the temperature and pressure that determine the transition of water from gas to liquid phase in rising air; its altitude can vary on timescales from diurnal to seasonal (LCL is just below cloud base; it can be “observed” on days with flat-bottomed clouds in the sky). Numerous studies hypothesized that global warming could raise the mean LCL or change frequency of coastal advective fog, thereby eliminating cloud-dependent ecosystems and hydrological inputs (13–17). Modeling studies in Central America (18, 19), the Amazon basin (20, 21), and the Western Ghats (22) suggested regional forest removal would decrease orographic precipitation due to lack of recycled evapotranspiration vapor input along air mass trajectories. In the Luquillo Experimental Forest (LEF), Scatena and Larsen (23) described higher cloud base, orographic rainfall decline, and temperature increase in the 3 mo after defoliation from Hurricane Hugo in 1989; however, insufficient data were available to confirm observations.Few land-based measurements support modeled predictions of forest disturbance effects on mountain clouds (24, 25) because the complex interactions between these systems are difficult to detect from short-term data. Additionally, cloud base height cannot be measured by satellite at the spatial and temporal resolution required for hydrological analysis. This study presents in situ observations quantifying response of a mountain cloud system to forest disturbances (drought stress and canopy removal).     

A rapid upward shift of a forest ecotone during 40 years of warming in the Green Mountains of Vermont

Detecting latitudinal range shifts of forest trees in response to recent climate change is difficult because of slow demographic rates and limited dispersal but may be facilitated by spatially compressed climatic zones along elevation gradients in montane environments. We resurveyed forest plots established in 1964 along elevation transects in the Green Mountains (Vermont) to examine whether a shift had occurred in the location of the northern hardwood-boreal forest ecotone (NBE) from 1964 to 2004. We found a 19% increase in dominance of northern hardwoods from 70% in 1964 to 89% in 2004 in the lower half of the NBE. This shift was driven by a decrease (up to 76%) in boreal and increase (up to 16%) in northern hardwood basal area within the lower portions of the ecotone. We used aerial photographs and satellite imagery to estimate a 91- to 119-m upslope shift in the upper limits of the NBE from 1962 to 2005. The upward shift is consistent with regional climatic change during the same period; interpolating climate data to the NBE showed a 1.1 degrees C increase in annual temperature, which would predict a 208-m upslope movement of the ecotone, along with a 34% increase in precipitation. The rapid upward movement of the NBE indicates little inertia to climatically induced range shifts in montane forests; the upslope shift may have been accelerated by high turnover in canopy trees that provided opportunities for ingrowth of lower elevation species. Our results indicate that high-elevation forests may be jeopardized by climate change sooner than anticipated.     

Natural variability contributes to model–satellite differences in tropical tropospheric warming

A long-standing discrepancy exists between general circulation models (GCMs) and satellite observations: The multimodel mean temperature of the midtroposphere (TMT) in the tropics warms at approximately twice the rate of observations. Using a large ensemble of simulations from a single climate model, we find that tropical TMT trends (1979–2018) vary widely and that a subset of realizations are within the range of satellite observations. Realizations with relatively small tropical TMT trends are accompanied by subdued sea-surface warming in the tropical central and eastern Pacific. Observed changes in sea-surface temperature have a similar pattern, implying that the observed tropical TMT trend has been reduced by multidecadal variability. We also assess the latest generation of GCMs from the Coupled Model Intercomparison Project Phase 6 (CMIP6). CMIP6 simulations with muted warming over the central and eastern Pacific also show reduced tropical tropospheric warming. We find that 13% of the model realizations have tropical TMT trends within the observed trend range. These simulations are from models with both small and large climate sensitivity values, illustrating that the magnitude of tropical tropospheric warming is not solely a function of climate sensitivity. For global averages, one-quarter of model simulations exhibit TMT trends in accord with observations. Our results indicate that even on 40-y timescales, natural climate variability is important to consider when comparing observed and simulated tropospheric warming and is sufficiently large to explain TMT trend differences between models and satellite data.

Pronounced tropical tropospheric warming is a consistent feature of general circulation model (GCM) simulations of historical climate change. Although observations show a significant increase in the temperature of the midtroposphere (TMT) over the last four decades (1), satellite-based estimates of the rate of tropical tropospheric temperature change are substantially smaller than the multimodel average (2). This puzzling and controversial discrepancy has persisted through multiple generations of increasingly sophisticated GCMs and different versions of the satellite datasets.Spaceborne observations of tropospheric temperature have been recorded by microwave sounding unit (MSU) instruments since 1978 (3). By the late 2000s, the multimodel average tropical tropospheric warming from phase 3 of the Coupled Model Intercomparison Project (CMIP3) was two to six times larger than in satellite observations, depending on the time period, atmospheric layer, and dataset considered (4). In the subsequent generation of climate model simulations (CMIP5), despite updates to satellite datasets (5–7) and efforts to better isolate the tropospheric warming signal from stratospheric cooling in model–satellite comparisons (2), the multimodel average tropical TMT trend continued to exhibit two to three times more warming than observational products (2, 8). A recent analysis of CMIP6 simulations indicates that most models significantly overestimate the rate of tropospheric warming within the tropics and globally (9).Several nonmutually exclusive explanations have been proposed to explain these model–observation differences. One explanation is that GCMs are too sensitive to increases in the atmospheric concentration of greenhouse gases (9–11). Natural climate variability and systematic model-forcing errors are also possible explanations. Recent studies note that the divergence between modeled and observed tropical tropospheric warming began in the early 2000s (12, 13). During this period, internal climate variability contributed to a slowdown in observed tropical tropospheric warming (14). The timing of such decadal variations in climate is random and should not be captured by coupled atmosphere–ocean model simulations, except by chance. Deficiencies in external forcing over the 2000s also contributed to exaggerated warming in climate models (15–17). It is likely that some combination of these factors, in addition to substantial observational uncertainty, explains the apparent differences in TMT change in models and observations (16, 18, 19).Our focus here is on natural variability. We use all available historical simulations from CMIP6 (the newest generation of GCMs) and a large initial condition ensemble to determine whether natural internal climate variability can explain longstanding differences between model simulations and satellite observations.     

Tracing the effects of the Little Ice Age in the tropical lowlands of eastern Mesoamerica

The causes of late-Holocene centennial to millennial scale climatic variability and the impact that such variability had on tropical ecosystems are still poorly understood. Here, we present a high-resolution, multiproxy record from lowland eastern Mesoamerica, studied to reconstruct climate and vegetation history during the last 2,000 years, in particular to evaluate the response of tropical vegetation to the cooling event of the Little Ice Age (LIA). Our data provide evidence that the densest tropical forest cover and the deepest lake of the last two millennia were coeval with the LIA, with two deep lake phases that follow the Spörer and Maunder minima in solar activity. The high tropical pollen accumulation rates limit LIA''s winter cooling to a maximum of 2°C. Tropical vegetation expansion during the LIA is best explained by a reduction in the extent of the dry season as a consequence of increased meridional flow leading to higher winter precipitation. These results highlight the importance of seasonal responses to climatic variability, a factor that could be of relevance when evaluating the impact of recent climate change.     

Predicting and understanding forest dynamics using a simple tractable model

The perfect-plasticity approximation (PPA) is an analytically tractable model of forest dynamics, defined in terms of parameters for individual trees, including allometry, growth, and mortality. We estimated these parameters for the eight most common species on each of four soil types in the US Lake states (Michigan, Wisconsin, and Minnesota) by using short-term (≤15-year) inventory data from individual trees. We implemented 100-year PPA simulations given these parameters and compared these predictions to chronosequences of stand development. Predictions for the timing and magnitude of basal area dynamics and ecological succession on each soil were accurate, and predictions for the diameter distribution of 100-year-old stands were correct in form and slope. For a given species, the PPA provides analytical metrics for early-successional performance (H₂₀, height of a 20-year-old open-grown tree) and late-successional performance (*, equilibrium canopy height in monoculture). These metrics predicted which species were early or late successional on each soil type. Decomposing * showed that (i) succession is driven both by superior understory performance and superior canopy performance of late-successional species, and (ii) performance differences primarily reflect differences in mortality rather than growth. The predicted late-successional dominants matched chronosequences on xeromesic (Quercus rubra) and mesic (codominance by Acer rubrum and Acer saccharum) soil. On hydromesic and hydric soils, the literature reports that the current dominant species in old stands (Thuja occidentalis) is now failing to regenerate. Consistent with this, the PPA predicted that, on these soils, stands are now succeeding to dominance by other late-successional species (e.g., Fraxinus nigra, A. rubrum).     

From the Cover: Species distributions in response to individual soil nutrients and seasonal drought across a community of tropical trees

Tropical forest vegetation is shaped by climate and by soil, but understanding how the distributions of individual tree species respond to specific resources has been hindered by high diversity and consequent rarity. To study species over an entire community, we surveyed trees and measured soil chemistry across climatic and geological gradients in central Panama and then used a unique hierarchical model of species occurrence as a function of rainfall and soil chemistry to circumvent analytical difficulties posed by rare species. The results are a quantitative assessment of the responses of 550 tree species to eight environmental factors, providing a measure of the importance of each factor across the entire tree community. Dry-season intensity and soil phosphorus were the strongest predictors, each affecting the distribution of more than half of the species. Although we anticipated clear-cut responses to dry-season intensity, the finding that many species have pronounced associations with either high or low phosphorus reveals a previously unquantified role for this nutrient in limiting tropical tree distributions. The results provide the data necessary for understanding distributional limits of tree species and predicting future changes in forest composition.