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1.
The coexistence of multiple species on a smaller number of limiting resources is an enduring ecological paradox. The mechanisms that maintain such biodiversity are of great interest to ecology and of central importance to conservation. We describe and prove a unique and robust mechanism for coexistence: Species that differ only in their dispersal abilities can coexist, if habitat patches are distributed at irregular distances. This mechanism is straightforward and ecologically intuitive, but can nevertheless create complex coexistence patterns that are robust to substantial environmental stochasticity. The Great Barrier Reef (GBR) is noted for its diversity of reef fish species and its complex arrangement of reef habitat. We demonstrate that this mechanism can allow fish species with different pelagic larval durations to stably coexist in the GBR. Further, coexisting species on the GBR often dominate different subregions, defined primarily by cross-shelf position. Interspecific differences in dispersal ability generate similar coexistence patterns when dispersal is influenced by larval behavior and variable oceanographic conditions. Many marine and terrestrial ecosystems are characterized by patchy habitat distributions and contain coexisting species that have different dispersal abilities. This coexistence mechanism is therefore likely to have ecological relevance beyond reef fish.  相似文献   

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A phylogeny and timescale derived from analyses of multilocus nuclear DNA sequences for Holarctic genera of plethodontid salamanders reveal them to be an old radiation whose common ancestor diverged from sister taxa in the late Jurassic and underwent rapid diversification during the late Cretaceous. A North American origin of plethodontids was followed by a continental-wide diversification, not necessarily centered only in the Appalachian region. The colonization of Eurasia by plethodontids most likely occurred once, by dispersal during the late Cretaceous. Subsequent diversification in Asia led to the origin of Hydromantes and Karsenia, with the former then dispersing both to Europe and back to North America. Salamanders underwent rapid episodes of diversification and dispersal that coincided with major global warming events during the late Cretaceous and again during the Paleocene-Eocene thermal optimum. The major clades of plethodontids were established during these episodes, contemporaneously with similar phenomena in angiosperms, arthropods, birds, and mammals. Periods of global warming may have promoted diversification and both inter- and transcontinental dispersal in northern hemisphere salamanders by making available terrain that shortened dispersal routes and offered new opportunities for adaptive and vicariant evolution.  相似文献   

4.
Microorganisms and zooplankton are both important components of aquatic food webs. Although both inhabit the same environment, they are often regarded as separate functional units that are indirectly connected through nutrient cycling and trophic cascade. However, research on pathogenic and nonpathogenic bacteria has shown that direct association with zooplankton has significant influences on the bacteria''s physiology and ecology. We used stratified migration columns to study vertical dispersal of hitchhiking bacteria through migrating zooplankton across a density gradient that was otherwise impenetrable for bacteria in both upward and downward directions (conveyor-belt hypothesis). The strength of our experiments is to permit quantitative estimation of transport and release of associated bacteria: vertical migration of Daphnia magna yielded an average dispersal rate of 1.3 × 105·cells·Daphnia−1·migration cycle−1 for the lake bacterium Brevundimonas sp. Bidirectional vertical dispersal by migrating D. magna was also shown for two other bacterial species, albeit at lower rates. The prediction that diurnally migrating zooplankton acquire different attached bacterial communities from hypolimnion and epilimnion between day and night was subsequently confirmed in our field study. In mesotrophic Lake Nehmitz, D. hyalina showed pronounced diel vertical migration along with significant diurnal changes in attached bacterial community composition. These results confirm that hitchhiking on migrating animals can be an important mechanism for rapidly relocating microorganisms, including pathogens, allowing them to access otherwise inaccessible resources.  相似文献   

5.
In the heterogeneous environment surrounding plant roots (the rhizosphere), microorganisms both compete and cooperate. Here, we show that two very different inhabitants of the rhizosphere, the nonmotile fungus Aspergillus fumigatus and the swarming bacterium Paenibacillus vortex, can facilitate each other's dispersal. A. fumigatus conidia (nonmotile asexual fungal spores) can be transported by P. vortex swarms over distances of at least 30 cm and at rates of up to 10.8 mm h(-1). Moreover, conidia can be rescued and transported by P. vortex from niches of adverse growth conditions. Potential benefit to the bacteria may be in crossing otherwise impenetrable barriers in the soil: fungal mycelia seem to act as bridges to allow P. vortex to cross air gaps in agar plates. Transport of conidia was inhibited by proteolytic treatment of conidia or the addition of purified P. vortex flagella, suggesting specific contacts between flagella and proteins on the conidial surface. Conidia were transported by P. vortex into locations where antibiotics inhibited bacteria growth, and therefore, growth and sporulation of A. fumigatus were not limited by bacterial competition. Conidia from other fungi, similar in size to those fungi from A. fumigatus, were not transported as efficiently by P. vortex. Conidia from a range of fungi were not transported by another closely related rhizosphere bacterium, Paenibacillus polymyxa, or the more distantly related Proteus mirabilis, despite both being efficient swarmers.  相似文献   

6.
Chemotaxis, the guided motion of cells by chemical gradients, plays a crucial role in many biological processes. In the social amoeba Dictyostelium discoideum, chemotaxis is critical for the formation of cell aggregates during starvation. The cells in these aggregates generate a pulse of the chemoattractant, cyclic adenosine 3’,5’-monophosphate (cAMP), every 6 min to 10 min, resulting in surrounding cells moving toward the aggregate. In addition to periodic pulses of cAMP, the cells also secrete phosphodiesterase (PDE), which degrades cAMP and prevents the accumulation of the chemoattractant. Here we show that small aggregates of Dictyostelium can disperse, with cells moving away from instead of toward the aggregate. This surprising behavior often exhibited oscillatory cycles of motion toward and away from the aggregate. Furthermore, the onset of outward cell motion was associated with a doubling of the cAMP signaling period. Computational modeling suggests that this dispersal arises from a competition between secreted cAMP and PDE, creating a cAMP gradient that is directed away from the aggregate, resulting in outward cell motion. The model was able to predict the effect of PDE inhibition as well as global addition of exogenous PDE, and these predictions were subsequently verified in experiments. These results suggest that localized degradation of a chemoattractant is a mechanism for morphogenesis.

Eukaryotic cell motion guided by gradients of diffusible chemoattractants plays a crucial role in wound healing, embryology, the movement of immune cells, and cancer metastasis (14). In some cell types, including neutrophils and the social amoeba Dictyostelium discoideum, gradient generation occurs through a relay mechanism where cells stimulated by the chemoattractant secrete additional chemoattractant (57). To prevent continuous build-up of chemoattractants, many systems use enzymes to degrade the chemoattractant. These enzymes are either membrane-bound, with their active sites facing the extracellular environment, or can diffuse in the surrounding environment (8, 9). In addition, chemoattractants may be removed by other methods, including receptor uptake or decoy receptors (10, 11).Compared to chemoattraction, relatively little is known about chemorepulsion, exemplified by the dispersal of immune cells out of a tissue during the resolution of inflammation. Under some conditions in chamber-based assays, chemoattractant-degrading enzymes can create a local minimum in chemoattractant concentration, resulting in chemoattractant gradients that point away from areas with high cell density (1214).In a nutrient-rich environment, Dictyostelium cells grow as separate, independent cells. When deprived of food, these amoebae start to secrete the chemoattractant cyclic adenosine 3′,5′-monophosphate (cAMP) in an oscillatory manner (15, 16). The secreted cAMP rapidly diffuses to neighboring cells, which, in turn, start to secrete cAMP as well. The resulting relay mechanism generates periodic waves that sweep through the cell population with a period of 6 min to 10 min (17). For wave periods smaller than 10 min, cells only respond to incoming waves, ignoring the “back of the wave,” and directionally move toward higher concentrations of cAMP (18, 19), form streams, and eventually aggregate into mounds of up to 100,000 cells. Cells within the aggregate subsequently differentiate and form a fruiting body that contains the majority of the original population of cells as a mass of spores held up off of the substrate by a stalk to maximize dispersal of spores (15, 16). In addition to cAMP, Dictyostelium cells also secrete phosphodiesterases (PDEs), which hydrolyze cAMP and which prevent continuous build-up of cAMP (8, 20). The presence of PDE is essential for aggregation, and mutants that cannot secrete PDE fail to form viable aggregation centers (21).We reasoned that the competition between a time-varying chemoattractant signal and an inhibitor can result in guidance that changes direction as a function of time. To test this hypothesis, we examined the aggregation of Dictyostelium cells at low cell densities. We present evidence that aggregates of developing Dictyostelium cells display dispersal behavior in which cells are “repelled” from, rather than attracted to, aggregates. This behavior was only present for small aggregates. Furthermore, we show that, during this dispersal behavior, oscillatory cAMP signaling is still active, but that its period is abruptly increased at the onset of dispersal. We develop a model for cell aggregation and show that periodic signaling of cAMP, together with a spatial profile of PDE, can explain the observed dispersal. Furthermore, the model predicts that the disruption of the PDE profile, either by removal of PDE or by globally adding PDE, will result in the abolishment of dispersal. These predictions were subsequently verified in experiments. Our results suggest that, by modulating the frequency of cAMP signaling, small aggregates can shed their cells, potentially avoiding mounds that would form small and thus relatively ineffective fruiting bodies.  相似文献   

7.
Though chickens are the most numerous and ubiquitous domestic bird, their origins, the circumstances of their initial association with people, and the routes along which they dispersed across the world remain controversial. In order to establish a robust spatial and temporal framework for their origins and dispersal, we assessed archaeological occurrences and the domestic status of chickens from ∼600 sites in 89 countries by combining zoogeographic, morphological, osteometric, stratigraphic, contextual, iconographic, and textual data. Our results suggest that the first unambiguous domestic chicken bones are found at Neolithic Ban Non Wat in central Thailand dated to ∼1650 to 1250 BCE, and that chickens were not domesticated in the Indian Subcontinent. Chickens did not arrive in Central China, South Asia, or Mesopotamia until the late second millennium BCE, and in Ethiopia and Mediterranean Europe by ∼800 BCE. To investigate the circumstances of their initial domestication, we correlated the temporal spread of rice and millet cultivation with the first appearance of chickens within the range of red junglefowl species. Our results suggest that agricultural practices focused on the production and storage of cereal staples served to draw arboreal red junglefowl into the human niche. Thus, the arrival of rice agriculture may have first facilitated the initiation of the chicken domestication process, and then, following their integration within human communities, allowed for their dispersal across the globe.

Despite the global ubiquity and cultural importance of chickens (Gallus gallus domesticus L., 1758), the timing and circumstances of their domestication and subsequent dispersal remain both obscure and controversial (1). Although the spatiotemporal patterns of chicken dispersal following their domestication have been addressed by several studies (24), two key publications (5, 6) pertaining to the early history of chicken domestication are almost always cited but rarely challenged (SI Appendix, Table S1). In brief, the first argues for a Southeast Asian and possible South Indian origin for chickens (5), and the second claims that domestic chickens first appeared in northern China before following a northern trajectory toward Europe (6). Both of these oft-cited syntheses summarized, but did not critically assess either the original osteological and stratigraphic data or the cultural implications for the presence of chickens across the Old World.Inferring chicken origins and dispersal have primarily been hampered by a paucity of archaeological remains, and more specifically, issues related to excavation and recovery biases, faunal identification, and dating (7). Excavations that do not consistently employ fine sieving, for example, are unlikely to systematically retrieve chicken bones. When bird remains are recovered, confident identification can be problematic in the absence of reference collections, since chicken bones are difficult to distinguish from other related galliform species. Although early Holocene bird remains from northern China were initially claimed to be chickens (8), a reanalysis of those bones based upon discrete osteomorphological criteria demonstrated that the specimens were derived from pheasants (1, 9). More generally, ongoing hybridization between wild red junglefowl (G. gallus) populations and those birds living in human settlements (10) complicates the task of identifying early poultry husbandry in the archaeological record.The low density of excavated archaeological sites, specifically within the distribution of red junglefowl, means that the earliest shifts in the relationship between people and these birds remain opaque. Additionally, chicken bones are prone to taphonomic loss through scavenger activity (11) and stratigraphic displacement (12). The latter can lead to invalid claims regarding their dating and cultural context, and a recent program of radiocarbon dating demonstrated that numerous early claims for the first appearance of chickens in Europe were spurious (13).Regarding their geographic origin, genetic studies have demonstrated that of the four extant junglefowl species, chickens were primarily derived from the red junglefowl (14, 15). A more recent study analyzed 863 genomes from modern Gallus specimens, including all five subspecies of red junglefowl, and identified the subspecies Gallus gallus spadiceus as the most likely progenitor of domestic chickens (16). This result suggests that the chicken domestication process began within the distribution of this subspecies in Southeast Asia (Fig. 1 and SI Appendix, Fig S1). Genomic analyses further suggested that the divergence between the ancestral population of modern domestic chickens and G. gallus spadiceus occurred between 12,800 and 6,200 y ago. Importantly, this range cannot be equated with the initiation of a domestication process. Instead, this timeframe represents the divergence between G. gallus spadiceus and the lineage from which domestic chickens were derived (17), and thus represents an upper bound on the chicken domestication timeframe.Open in a separate windowFig. 1.A map depicting the distribution of both the gray and Ceylon junglefowl species and three subspecies of red junglefowl: G. gallus murghi, G. gallus spadiceus, and G. gallus jabouillei. The distribution of G. gallus gallus is depicted as the remainder of mainland southeast Asia and Sumatra following the general distribution in ref. 16. The G. gallus murghi distribution follows that of SI Appendix, Fig. S1, which draws on published maps in ornithological sources and the Global Biodiversity Information Facility (GBIF) records (119121). For G. gallus spadiceus and G. gallus jabouillei, the GBIF records were augmented by specimens with genetic data reported by refs. 16 and 122.Here, in order to establish a robust spatial and temporal framework for the early occurrence of chickens both within and beyond the range of red junglefowl, including Africa and Oceania, we reevaluated reports of chicken remains from >600 archaeological sites in 89 countries. We did so by assessing the claims for chickens in their chrono-cultural and geographic contexts, and wherever possible, by reassessing the taxonomic determination of existing specimens by measuring either published photographs or the actual bones. We then combined zoogeographic, contextual, and osteometric data to confirm or question the domestic status of the birds. We also critically reviewed the stratigraphic position of each of the remains and assessed their intrusive potential. We complemented these efforts by investigating iconographic, written, and linguistic records pertaining to chickens. In addition, we interpreted these records within the context of the ecological characteristics and distributions of all the jungle fowl subspecies.These analyses allowed us to generate two datasets: a comprehensive table of archaeological remains consisting of chickens that were confidently assigned as domestic using conservative measures (SI Appendix, Table S2), and a list of remains whose identification or stratigraphic position was ambiguous (SI Appendix, Table S3). We then correlated the resulting spatiotemporal pattern of archaeological chickens with human societies and their subsistence strategies. This correlation allowed us to address the process, circumstances, and cultural context in which the initial shift in the human–chicken relationship that led to domestication took place, and the contexts of their subsequent translocations.  相似文献   

8.
Many planktonic suspension feeders are attached to particles or tethered by gravity when feeding. It is commonly accepted that the feeding flows of tethered suspension feeders are stronger than those of their freely swimming counterparts. However, recent flow simulations indicate the opposite, and the cause of the opposing conclusions is not clear. To explore the effect of tethering on suspension feeding, we use a low-Reynolds-number flow model. We find that it is favorable to be freely swimming instead of tethered since the resulting feeding flow past the cell body is stronger, leading to a higher clearance rate. Our result underscores the significance of the near-field flow in shaping planktonic feeding modes, and it suggests that organisms tether for reasons that are not directly fluid dynamical (e.g., to stay near surfaces where the concentration of bacterial prey is high).  相似文献   

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Active dispersal by wild Triatoma infestans in the Bolivian Andes   总被引:1,自引:0,他引:1  
Triatoma infestans is the main vector of Chagas disease and target of control programmes in the Southern Cone countries. So far Bolivia is the only country where true T. infestans wild foci are documented. The dispersal ability for wild T. infestans was studied at microgeographical scale in Bolivian Andes, to assess the possibility for wild populations to actively recolonize insecticide-treated villages. Nine microsatellite loci were used to detect the extent of gene flow between neighbouring collecting sites. The detection of restricted gene flow between close but distinct sylvatic sites supports the hypothesis that wild T. infestans does not disperse by flying at high altitude (2,750 m asl). It gradually disperses over small distances by walking within a 'patch' of continuous land cover. The genetic differentiation detected between sylvatic and domestic populations suggests a limited short-term role of wild insects in the process of recolonization of insecticide-treated houses in the Andes.  相似文献   

12.
It has been known for centuries that microorganisms are ubiquitous in the atmosphere, where they are capable of long-distance dispersal. Likewise, it is well-established that these airborne bacteria and fungi can have myriad effects on human health, as well as the health of plants and livestock. However, we have a limited understanding of how these airborne communities vary across different geographic regions or the factors that structure the geographic patterns of near-surface microbes across large spatial scales. We collected dust samples from the external surfaces of ∼1,200 households located across the United States to understand the continental-scale distributions of bacteria and fungi in the near-surface atmosphere. The microbial communities were highly variable in composition across the United States, but the geographic patterns could be explained by climatic and soil variables, with coastal regions of the United States sharing similar airborne microbial communities. Although people living in more urbanized areas were not found to be exposed to distinct outdoor air microbial communities compared with those living in more rural areas, our results do suggest that urbanization leads to homogenization of the airborne microbiota, with more urban communities exhibiting less continental-scale geographic variability than more rural areas. These results provide our first insight into the continental-scale distributions of airborne microbes, which is information that could be used to identify likely associations between microbial exposures in outdoor air and incidences of disease in crops, livestock, and humans.For nearly 2 centuries, we have known that microbes are ubiquitous in dust and outdoor air (13). In the near-surface atmosphere, microbial cells likely account for a significant fraction of aerosolized organic carbon (4), with microbial cell numbers typically ranging from 104 to 106 cells·m−3 over land (5). This means we inhale thousands of microbial cells every hour spent outdoors, and the potential effects of these airborne microbes on human health and the health of plants and animals are well recognized. As just one example, there are currently ∼16 million people living in the United States suffering from allergic asthma (6), and there has been considerable attention focused on understanding the airborne and dust-associated microbes influencing asthma and how those microbial triggers vary across space and time (69). In addition, the number of virulent fungal infections affecting human populations, wildlife, and plant crops is increasing (10, 11). The effects of those microbes capable of atmospheric transport can even extend to entire ecosystems: Microbes in Saharan dust clouds, for example, have been shown to affect the ecology of alpine lakes in Spain (12) and coral reefs in the Caribbean Sea (13).Despite the well-recognized importance of airborne microbes and the long history of research on microbial transport through the atmosphere (3), we have only recently been able to describe the full extent of microbial diversity found in the atmosphere by using molecular approaches to characterize the microbial taxa that are difficult to identify via cultivation-dependent or microscopy-based surveys. Such molecular approaches not only have yielded new insight into the enormous diversity of airborne microbes (14, 15) but also have been instrumental in helping us understand how the composition of the airborne microbial communities varies across time and space (16, 17). However, nearly all of this work has focused on local-scale variability, examining the bacterial or fungal communities found in outdoor air at selected sites. What is missing is a continental-scale understanding of microbial diversity in the near-surface atmosphere and its deposition patterns over land. We do not know whether there are distinct microbial taxa found in the outdoor air from different geographic regions, nor do we know what biotic and abiotic factors may be driving the geographic patterns of dust-associated microbial communities across larger spatial scales. There is some evidence to suggest there are regional differences in exposures to specific bacterial and fungal taxa (18) that may be associated with geographic patterns in allergenic disease (19), but the continental-scale biogeographic patterns exhibited by the broad range of microbes that can be found in outdoor air remain undetermined.Here we used dust samples collected from the external surfaces of ∼1,200 homes located across the continental United States to gain our first insight into the continental-scale patterns of bacterial and fungal diversity in the near-surface atmosphere and the factors driving the distributions of these airborne microbial taxa. Specifically, we investigated two questions: First, to what extent are the diversity and types of bacteria and fungi found in settled dust collected from outside homes a function of geographic location, climatic variables, soil characteristics, crop production, and land use type? Second, we sought to determine whether urbanization has a significant effect on bacterial and fungal exposures, given that it has long been assumed that airborne microbial exposures differ across urban and rural areas (3), with recent work suggesting that these differential microbial exposures may be one explanation for the higher rates of allergic disorders often observed in more urbanized areas (7, 8).  相似文献   

13.
Hydrothermal vent fields in the western Pacific Ocean are mostly distributed along spreading centers in submarine basins behind convergent plate boundaries. Larval dispersal resulting from deep-ocean circulations is one of the major factors influencing gene flow, diversity, and distributions of vent animals. By combining a biophysical model and deep-profiling float experiments, we quantify potential larval dispersal of vent species via ocean circulation in the western Pacific Ocean. We demonstrate that vent fields within back-arc basins could be well connected without particular directionality, whereas basin-to-basin dispersal is expected to occur infrequently, once in tens to hundreds of thousands of years, with clear dispersal barriers and directionality associated with ocean currents. The southwest Pacific vent complex, spanning more than 4,000 km, may be connected by the South Equatorial Current for species with a longer-than-average larval development time. Depending on larval dispersal depth, a strong western boundary current, the Kuroshio Current, could bridge vent fields from the Okinawa Trough to the Izu-Bonin Arc, which are 1,200 km apart. Outcomes of this study should help marine ecologists estimate gene flow among vent populations and design optimal marine conservation plans to protect one of the most unusual ecosystems on Earth.Hydrothermal vent fields in the western Pacific have received substantially less attention than have eastern Pacific vents. Western Pacific vents are mostly distributed along spreading centers in submarine basins behind convergent plate boundaries, whereas those of the eastern Pacific occur mainly at midocean ridges. It is estimated that vent-endemic species in back-arc basins were introduced along now-extinct midocean ridges that bridged the eastern and western Pacific Oceans ∼55 million years ago, with a potential origin at the East Pacific Rise (1, 2). More recent studies suggest the possibility that Indian Ocean ridge systems once connected Atlantic and Pacific vent fields (3). Spreading centers in back-arc basins are active for typically 5–10 million years (4, 5). Thus, life spans of back-arc spreading centers are significantly longer than population lifetimes of vent animals observed in the eastern Pacific (∼1 million years) (6).Recent genetic studies have addressed the matter of genetic differentiation among vent populations (711). Genetic data imply that back-arc basin populations are well-mixed genetic pools (12, 13). In contrast, vent populations in distant basins (∼3,000 km apart) are genetically distinct, suggesting that occasional migrations may have occurred over the course of several hundred thousand generations (14). There is one example of a widespread species (Bathymodiolus septemdierum complex) occurring in all western Pacific back-arc basins (15). To interpret gene flows of vent species, it is necessary to understand larval dispersal by ocean circulation, as well as tectonic history (1618). However, quantitative data regarding dispersal processes in the western Pacific are still woefully inadequate, leaving many unanswered questions. Dispersal patterns among vent populations in the western Pacific basins have not been previously addressed.Detailed observations and models for eastern Pacific vents have revealed mechanisms of near-bottom circulation strongly influenced by distinct topographic features of midocean ridges (1923). Conduit-like structures of midocean ridges may shield larvae from cross-axial dispersal and also may enable long-distance dispersal that connects distant vent fields (20). Similar long-dispersal mechanisms, however, do not apply to species in the western Pacific, where midocean ridges do not exist. If dispersal were limited to near-bottom depths, vent species of the western Pacific would largely be contained within a given back-arc basin.Although most species likely remain near the bottom, some strong-swimming larvae (e.g., shrimp and crabs) may disperse higher in the water column, possibly ∼1,000 m above the bottom, where they can be transported by faster currents (24, 25). Lagrangian measurement methods, using deep-ocean profiling floats programmed to drift at a specified depth or constant density surface, can be used to measure dispersal in the water column. This approach has been used for hydrothermal vent surveys as well (26, 27). One example was the Lau Basin Float Experiment (27), which captured boundary currents within the back-arc basin and westward outflow from the basin resulting from the South Equatorial Current. For various reasons, it is challenging to quantify vent-to-vent transport using only in situ experiments; therefore, one promising approach is to combine dispersal experiments with ocean circulation models.Properly analyzed, such observation and modeling data should yield reasonable estimates of dispersal processes by ocean circulation and should help marine ecologists understand biogeography and gene flow among vent populations in the western Pacific Ocean. We assessed potential larval dispersal from hydrothermal vent fields in the western Pacific on varying spatial scales, from intra- to interbasin vent communications, by integrating information from a deep-ocean profiling float experiment and predictions derived from an ocean circulation model.  相似文献   

14.
In the cat or primate primary visual cortex (V1), normal vision corresponds to a state where neural excitation patterns are driven by external visual stimuli. A spectacular failure mode of V1 occurs when such patterns are overwhelmed by spontaneously generated spatially self-organized patterns of neural excitation. These are experienced as geometric visual hallucinations. The problem of identifying the mechanisms by which V1 avoids this failure is made acute by recent advances in the statistical mechanics of pattern formation, which suggest that the hallucinatory state should be very robust. Here, we report how incorporating physiologically realistic long-range connections between inhibitory neurons changes the behavior of a model of V1. We find that the sparsity of long-range inhibition in V1 plays a previously unrecognized but key functional role in preserving the normal vision state. Surprisingly, it also contributes to the observed regularity of geometric visual hallucinations. Our results provide an explanation for the observed sparsity of long-range inhibition in V1--this generic architectural feature is an evolutionary adaptation that tunes V1 to the normal vision state. In addition, it has been shown that exactly the same long-range connections play a key role in the development of orientation preference maps. Thus V1's most striking long-range features--patchy excitatory connections and sparse inhibitory connections--are strongly constrained by two requirements: the need for the visual state to be robust and the developmental requirements of the orientational preference map.  相似文献   

15.
Despite the ecological importance of long-distance dispersal in insects, its mechanistic basis is poorly understood in genetic model species, in which advanced molecular tools are readily available. One critical question is how insects interact with the wind to detect attractive odor plumes and increase their travel distance as they disperse. To gain insight into dispersal, we conducted release-and-recapture experiments in the Mojave Desert using the fruit fly, Drosophila melanogaster. We deployed chemically baited traps in a 1 km radius ring around the release site, equipped with cameras that captured the arrival times of flies as they landed. In each experiment, we released between 30,000 and 200,000 flies. By repeating the experiments under a variety of conditions, we were able to quantify the influence of wind on flies’ dispersal behavior. Our results confirm that even tiny fruit flies could disperse ∼12 km in a single flight in still air and might travel many times that distance in a moderate wind. The dispersal behavior of the flies is well explained by an agent-based model in which animals maintain a fixed body orientation relative to celestial cues, actively regulate groundspeed along their body axis, and allow the wind to advect them sideways. The model accounts for the observation that flies actively fan out in all directions in still air but are increasingly advected downwind as winds intensify. Our results suggest that dispersing insects may strike a balance between the need to cover large distances while still maintaining the chance of intercepting odor plumes from upwind sources.

If asked to picture a migrating insect, the first image that comes to mind might be a large charismatic species such as the monarch butterfly, whose seasonal movements across North America have inspired naturalists for centuries. However, as pointed out by David and Elizabeth Lack (1), our impression of insect migration is strongly biased toward large animals; many species are so small that their geographic relocations escape our attention, especially if their population densities are not strongly concentrated by geological features such as narrow mountain passes. As research using high-altitude traps (2) and upward-looking radar (3, 4) indicates, long-distance migration may be more ubiquitous and ecologically important among both large and small insects than previously appreciated (5, 6). Long-distance dispersal (i.e., the noncyclic movement from one area to another) is even harder to observe and study in small insects, because the events are not generally predictable, and the animals are far too small to be captured on radar or outfitted with tracking devices. The dispersal of small insects across a landscape has often been modeled as stochastic processes governed by diffusion and advection (7), processes that may underestimate the ability of the animals to actively maintain constant trajectories over large spatial scales. Understanding long-distance migration and dispersal is quite important, because these phenomena are responsible for biomass relocation on both local and global scales (8, 9). Furthermore, as insect population densities decline due to environmental degradation and climate change (1012), understanding the dispersal capacity of insects and the behavioral algorithms that underlie them will be crucial in predicting the ecological impact of population decline.Although not generally renowned for its capability to disperse over long distances, a series of release-and-recapture experiments over 40 y ago suggest that the fruit fly, Drosophila melanogaster, may be capable of movements on the order of 15 km in a single night, a distance equivalent to 6 million body lengths (13, 14). These experiments were conducted by releasing tens of thousands of fluorescently labeled flies in the evening and then censusing the contents of traps baited with yeast and banana placed at distant oases the next morning. Although these pioneering studies suggested that the dispersal capacity of Drosophila was much greater than previously estimated, they left open several critical questions. First, it was not clear whether individual flies dispersed in random directions or whether the population movement was biased by external conditions, such as the wind, geographical features, or celestial cues. Second, because the precise transit times of the flies were not known, it was impossible to estimate the actual groundspeeds used by the animals as they dispersed. To provide more clarity to these and other questions related to long-distance dispersal, we conducted a series of release-and-recapture experiments in the Mojave Desert. We equipped circular arrays of chemically baited traps with simple machine vision systems that captured the arrival times of flies as they landed and repeated the experiments under a variety of ambient wind conditions. The results provide key insight into the behavioral algorithms used by Drosophila while dispersing in the wild and serve as the basis for a general agent-based model of wind-assisted dispersal in insects.  相似文献   

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17.
Plant pathogens are responsible for the annual yield loss of crops worldwide and pose a significant threat to global food security. A necessary prelude to many plant disease epidemics is the short-range dispersal of spores, which may generate several disease foci within a field. New information is needed on the mechanisms of plant pathogen spread within and among susceptible plants. Here, we show that self-propelled jumping dew droplets, working synergistically with low wind flow, can propel spores of a fungal plant pathogen (wheat leaf rust) beyond the quiescent boundary layer and disperse them onto neighboring leaves downwind. An array of horizontal water-sensitive papers was used to mimic healthy wheat leaves and showed that up to 25 spores/h may be deposited on a single leaf downwind of the infected leaf during a single dew cycle. These findings reveal that a single dew cycle can disperse copious numbers of fungal spores to other wheat plants, even in the absence of rain splash or strong gusts of wind.

Spores of plant pathogenic fungi are spread through the atmosphere in three stages: liberation from the host by some active or passive method(s), drift by biotic or abiotic factors, and deposition onto a new host (1). Examples of active liberation mechanisms include osmotic pressure–driven ejection of ascospores of Fusarium graminearum (the causal agent of Fusarium head blight of wheat) and ballistospore ejection from the tip of a sterigma due to the chemical secretion of a Buller’s drop (2, 3). In the absence of wind, the resulting dispersal distance is a function of both the weight of the spore(s) and the initial discharge velocity, with the range of discharge varying from 40 μm for basidiospores (4) to 6 m for the artillery fungus (5). Passive liberation and dispersal mechanisms, such as wind and rain splash, can spread fungal diseases in plants (6). For wind to successfully liberate dry spores, an unusually strong and/or sudden gust of wind is often required (1, 69). In contrast, rain splash can liberate spores from a plant either through transferring momentum to the leaf to launch spores off (10, 11) or by adhering spores to splashed satellite droplets (12, 13). Spores ejected by active methods or rain splash can only disperse over a very short distance in the absence of wind (14) but when carried in moderate winds, can travel for many kilometers (15, 16).One recent study reported an entirely new mode of pathogen liberation, where coalescing dew droplets on superhydrophobic wheat leaves jump with considerable velocity (0.1 to 1.0 m/s) and carry adhered spores of a fungal plant pathogen (17). Mechanistically, the out-of-plane motion is a result of symmetry breaking as the expanding liquid bridge during coalescence impinges upon the bottom substrate (1820) (Fig. 1A). While this initial report characterized the jumping-droplet liberation of spores in the absence of wind (17), it did not consider the subsequent dispersal or deposition, which ultimately governs the rate of disease spread. Here, we characterize the dispersal of spores of leaf rust (Puccinia triticina) after they are liberated from a diseased wheat leaf via jumping-droplet condensation. Two different scenarios are explored: short-range and long-range drift and deposition in the absence and presence of wind flow, respectively (Fig. 1 B and C). We found that even a low wind speed (0.5 m/s) is capable of dispersing as many as 100 jumping droplets and 25 spores to a single leaf downwind of a diseased leaf saturated in dew. Our ability to quantify both the liberation and dispersal of fungal spores from a diseased leaf during a dew cycle improves our understanding of disease spread within and among plants (1, 2123).Open in a separate windowFig. 1.Spore dispersal via jumping-droplet condensation. (A) Jumping-droplet condensation on a healthy wheat leaf. Two condensed droplets coalesce (second frame) and jump off from the superhydrophobic wheat leaf (third frame). (B) Without any wind, the jumped droplets with spores can land on an adjacent healthy leaf, spreading the disease within the plant. (C) In low (0.5 m/s) to moderate (1.5 m/s) wind speed, the spore-laden jumped droplets can travel long-range to land on different healthy plants within the field.  相似文献   

18.
It has previously been noted that there are drops in global sea level (GSL) after some major volcanic eruptions. However, observational evidence has not been convincing because there is substantial variability in the global sea level record over periods similar to those at which we expect volcanoes to have an impact. To quantify the impact of volcanic eruptions we average monthly GSL data from 830 tide gauge records around five major volcanic eruptions. Surprisingly, we find that the initial response to a volcanic eruption is a significant rise in sea level of 9 ± 3 mm in the first year after the eruption. This rise is followed by a drop of 7 ± 3 mm in the period 2–3 years after the eruption relative to preeruption sea level. These results are statistically robust and no particular volcanic eruption or ocean region dominates the signature we find. Neither the drop nor especially the rise in GSL can be explained by models of lower oceanic heat content. We suggest that the mechanism is a transient disturbance of the water cycle with a delayed response of land river runoff relative to ocean evaporation and global precipitation that affects global sea level. The volcanic impact on the water cycle and sea levels is comparable in magnitude to that of a large El Niño–La Niña cycle, amounting to ≈5% of global land precipitation.  相似文献   

19.
20.
Determining how widespread human-induced changes such as habitat loss, landscape fragmentation, and climate instability affect populations, communities, and ecosystems is one of the most pressing environmental challenges. Critical to this challenge is understanding how these changes are affecting the movement abilities and dispersal trajectories of organisms and what role conservation planning can play in promoting movement among remaining fragments of suitable habitat. Whereas evidence is mounting for how conservation strategies such as corridors impact animal movement, virtually nothing is known for species dispersed by wind, which are often mistakenly assumed to not be limited by dispersal. Here, we combine mechanistic dispersal models, wind measurements, and seed releases in a large-scale experimental landscape to show that habitat corridors affect wind dynamics and seed dispersal by redirecting and bellowing airflow and by increasing the likelihood of seed uplift. Wind direction interacts with landscape orientation to determine when corridors provide connectivity. Our results predict positive impacts of connectivity and patch shape on species richness of wind-dispersed plants, which we empirically illustrate using 12 y of data from our experimental landscapes. We conclude that habitat fragmentation and corridors strongly impact the movement of wind-dispersed species, which has community-level consequences.Habitat loss and fragmentation sever movement pathways, posing major risks to population persistence and community diversity (1). As a result, landscape connectivity––the degree to which landscapes facilitate movement––is receiving growing attention as a means to increase long-distance dispersal (LDD) of individuals and persistence of species under global change (2, 3). In particular, habitat corridors, or linear strips of habitat connecting otherwise isolated habitat patches, have become one of the most commonly applied conservation tools (4, 5). However, corridors (and connectivity more generally) are almost exclusively considered a conservation strategy for animals or animal-dispersed organisms, not for the great diversity of species that are passively transported by wind.Wind is a frequent means of movement for many organisms, including plant seeds, pollen, spores, insects, and pathogens (68). The changes in habitat structure (e.g., edge creation) that accompany habitat fragmentation and connectivity may strongly influence the flow of air, particularly the amount of vertical uplifting––a critical factor known to drive LDD of seeds by wind (911). Over the past decade, our mechanistic understanding of wind-driven seed dispersal has substantially increased (12, 13) and models have begun to incorporate landscape heterogeneity (11, 1418). Combining mechanistic insight from dispersal models with dispersal patterns from real landscapes is the next frontier in understanding dispersal in fragmented habitats (12, 1921).Gaining a mechanistic understanding of how habitat fragmentation and connectivity affect wind dispersal (and dispersal in general), however, has proven challenging: whereas short-distance dispersal events can be empirically quantified, LDD events––the unusually long movements accomplished by only a small fraction of individuals in a population (Materials and Methods and SI Materials and Methods)––are difficult to detect empirically. As a consequence, LDD events are typically predicted using mechanistic models, but these predictions are rarely empirically tested (19).Here, we surmount these challenges by combining predictions for wind dynamics and seed dispersal patterns from an advanced fluid-dynamics model with empirical wind and seed dispersal data from a unique large-scale landscape fragmentation experiment that controls for patch area, shape, and connectivity. This comprehensive approach allows for evaluation of habitat fragmentation and connectivity impacts on both short- and long-distance dispersal of seeds by wind. Our work reveals several revealing mechanisms by which landscape structure impacts the movement of wind-dispersed plants, and demonstrates how landscape effects on wind can have consequences for plant community diversity.We developed and tested our model predictions within a large-scale experimental landscape (∼50 ha) at the Savannah River Site, SC (Fig. 1) comprising 1.375 ha connected and unconnected open-habitat longleaf pine savanna patches surrounded by mature pine plantation. “Connected” patches test for connectivity effects whereas unconnected “rectangular” and “winged” patches test for patch area and shape effects, respectively (Materials and Methods). Longleaf pine savanna supports some of the most diverse plant communities in the world (22, 23) and is typified in part by a large proportion of wind-dispersed plant species (22). At our study site, for example, wind-dispersed species constitute the most common plant dispersal mode.Open in a separate windowFig. 1.Experimental landscape at Savannah River Site, SC. Patch types are connected (with a corridor), unconnected winged, or unconnected rectangular. The long axis of the corridor is aligned along 90° and 270° to correspond with Figs. 2 and and33.To understand how connectivity and habitat fragmentation affect wind and seed dispersal dynamics, we applied and tested a mechanistic model of wind-driven dispersal in our experimental landscape. We used the Regional Atmospheric Modeling System-based Forest Large Eddy Simulation model (RAFLES; see Materials and Methods, ref. 24, and Fig. S1), a mechanistic model that explicitly incorporates 3D heterogeneous habitat structure at meter-scale resolution. We tested the model’s predictions by empirically measuring wind dynamics and LDD patterns of experimentally released artificial seeds in our highly controlled landscape. We also tested the predicted implications of these model results for plant community dynamics by evaluating changes in species richness of wind-dispersed plants among our experimental patch types across 12 y of community development.  相似文献   

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