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41 results for “Allee effects”

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Data from: Timing between successive introduction events determines establishment success in bacteria with an Allee effect.

Propagule pressure is a leading determinant of population establishment. Yet, an experimental understanding of how propagule size and number (two principle parts of propagule pressure) determine establishment success remains incomplete. Theoretical studies suggest that the timing between introduction events, a component of propagule number, can influence establishment success. However, this dynamic has rarely been explored experimentally. Using Escherichia coli engineered with an Allee effect, we investigated how the timing of two introduction events influences establishment. For populations introduced below the Allee threshold, establishment occurred if the time between two introduction events was sufficiently short, with the length of time between events further reduced by reducing growth rate. Interestingly, we observed that as the density of bacteria introduced in one introduction event increased, the time between introduction events that allowed for establishment increased. Using a mathematical model, we provide support that the mechanism behind these trends is the ability of the first population to modify the environment, which can pave the way for establishment of the second population. Our results provide experimental evidence that the temporal distribution of introduction events regulates establishment, furthering our understanding of propagule pressure and may have implications in invasion biology and infectious disease.

opencc-zeroDec 2018View details →

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Data from: Plant sex alters Allee effects in aggregating plant parasites

Species interactions are central to our understanding of population dynamics. While density typically strengthens competition, reducing absolute fitness, Allee effects can reverse this pattern, increasing fitness with density. Allee effects emerge in host-parasite systems when higher parasite densities dilute immune responses or increase resource-mobilization. The optimal density of individuals in these systems should be influenced by how host quality alters the rates at which facilitative and competitive effects change across densities. We tested these ideas using sumac (Rhus typhina) and a gall-forming parasite (Melaphis rhois) that attacks sumac leaves. Fitness peaked at intermediate densities , indicating an Allee effect, but the fitness peak depended on host sex. Patterns of abundance mirrored fitness patterns, with galls clustered on leaves and female hosts supporting greater numbers of galls. Within leaves, galls near the stem were more fit, and gall-makers preferentially oviposited near to the stem. The patterns of fitness and abundance are consistent with Allee effects caused by increased resource mobilization at higher gall-maker densities rather than diluted immune responses. Our results suggest that Allee effects in parasites can be described as the summative effects of competitive and facilitative processes and, because both are common, Allee effects are likely common in host-parasite systems.

opencc-zeroDec 2016View details →

dryad28/100

Data from: Experimental demonstration of an Allee effect in microbial populations

Microbial populations can be dispersal limited. However, microorganisms that successfully disperse into physiologically ideal environments are not guaranteed to establish. This observation contradicts the Baas-Becking tenet: 'Everything is everywhere, but the environment selects'. Allee effects, which manifest in the relationship between initial population density and probability of establishment, could explain this observation. Here, we experimentally demonstrate that small populations of Vibrio fischeri are subject to an intrinsic demographic Allee effect. Populations subjected to predation by the bacterivore Cafeteria roenbergensis display both intrinsic and extrinsic demographic Allee effects. The estimated critical threshold required to escape positive density-dependence is around 5, 20 or 90 cells ml−1 under conditions of high carbon resources, low carbon resources or low carbon resources with predation, respectively. This work builds on the foundations of modern microbial ecology, demonstrating that mechanisms controlling macroorganisms apply to microorganisms, and provides a statistical method to detect Allee effects in data.

opencc-zeroDec 2015View details →

dryad28/100

Data from: Mate finding, Allee effects, and selection for sex-biased dispersal

Although dispersal requires context-dependent decision-making in three distinct stages (emigration, transit, immigration), these decisions are commonly ignored in simple models of dispersal. For sexually reproducing organisms, mate availability is an important factor in dispersal decisions. Difficulty finding mates can lead to an Allee effect where population growth decreases at low densities. Surprisingly, theoretical studies on mate finding and on sex-biased dispersal produce opposing predictions: in the former one sex is predicted to move less if the other sex evolves to search more, whereas in the latter mate-finding difficulties can select for less sex bias in dispersal when mate finding occurs after dispersal. Here we develop a pair of models to examine the joint evolution of dispersal and settlement behaviour. Our first model resolves the apparent contradiction from the mate-search and dispersal literatures. Our second model demonstrates that the relationship between mating system and sex-biased dispersal is more complex than a simple contrast between resource-defence monogamy and female-defence polygyny. Our results highlight that a key factor is the timing of mating relative to dispersal (before, during, or after). We also show that although movement has the potential to alleviate a mate-finding Allee effect, in some cases it can actually exacerbate the effect.

opencc-zeroDec 2013View details →

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Atlantic cod recovery from the Allee effect zone: contrasting ecological and evolutionary rescue

<p>The ability of a population to recover from disturbances is fundamental for its persistence. Impaired population recovery might be associated with a demographic Allee effect. Immigration from adjacent populations could accelerate the recovery not only by promoting population growth beyond the Allee effect threshold but also by bringing in advantageous genotypes. We explore the nature and role of ecological and evolutionary rescue in an Atlantic cod (Gadus morhua) population fished below its Allee effect threshold. We utilize an eco-evolutionary model and simulate scenarios, where the target population evolves in response to selective fishing and sample immigrants from i) a source population similarly adapted to fishing (post-fishing genotypes) or ii) an unexploited source population with natural genetic and phenotypic diversity (pre-fishing genotypes). Immigration of pre-fishing genotypes enhances the recovery due to the larger body sizes and the flow of associated genes. Post-fishing immigrants can promote the population abundance recovery, but they slow down evolutionary recovery from fishing-induced selection and increase uncertainty about recovery times. Our results stress the importance of maintaining genetic and phenotypic diversity and suggest that marine protected areas can serve as an important source of immigrants to promote both the demographic and evolutionary recovery of exploited populations.</p>

opencc-zeroMay 2020View details →

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Data from: Spatial variation in Allee effects influences patterns of range expansion

Allee effects are thought to slow range expansion and contribute to stable range boundaries. Recent studies have shown Allee effects to vary spatiotemporally due to influences of environmental heterogeneity on population processes. Gradients in Allee effects might occur as a species' range approaches suboptimal conditions while expanding into new territory. Allee effects could exhibit patchiness if drivers of positive density dependence (e.g., mate finding rates) are influenced by habitat patchiness. However, theoretical studies have largely assumed Allee effects to be spatially constant. The goal of this study was to evaluate how spatiotemporal patterns of range expansion respond to spatial variations in Allee effects. We simulated spread in landscapes that differed in the spatial configuration and range of Allee thresholds. We compared spread with a constant Allee effect to spread in landscapes where the Allee threshold varied along a gradient or in a patchy fashion. Landscape configuration affected patterns of range expansion when Allee thresholds were near or exceeded the number of colonizing immigrants. In gradient landscapes, spread decelerated as the range edge approached higher Allee thresholds. In patchy landscapes, spread advanced quickly through areas with lower Allee thresholds and stalled in areas with higher Allee thresholds. Both focal and neighboring locations influencing spread. Spatial variation in Allee effects may be an underappreciated source of heterogeneity in patterns of range expansion. When Allee effects vary, spread estimates based on a spatially averaged Allee threshold may not accurately predict realized rates of spread. Our findings suggest that spread can occur despite generally high Allee thresholds if Allee thresholds are low in a subset of patches. This result has negative implications for controlling the spread of invasive species, but it also suggests range shifts by native species in response to climate change may be possible with even sparsely distributed refugia from Allee effects.

opencc-zeroDec 2015View details →

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Data from: A cryptic Allee effect: spatial contexts mask an existing fitness–density relationship

Current theories predict that Allee effects should be widespread in nature, but there is little consistency in empirical findings. We hypothesized that this gap can arise from ignoring spatial contexts (i.e. spatial scale and heterogeneity) that potentially mask an existing fitness–density relationship: a 'cryptic' Allee effect. To test this hypothesis, we analysed how spatial contexts interacted with conspecific density to influence the fertilization rate of the freshwater mussel Margaritifera laevis. This sessile organism has a simple fertilization process whereby females filter sperm from the water column; this system enabled us to readily assess the interaction between conspecific density and spatial heterogeneity (e.g. flow conditions) at multiple spatial levels. Our findings were twofold. First, positive density-dependence in fertilization was undetectable at a population scale (approx. less than 50.5 m2), probably reflecting the exponential decay of sperm density with distance from the sperm source. Second, the Allee effect was confirmed at a local level (0.25 m2), but only when certain flow conditions were met (slow current velocity and shallow water depth). These results suggest that spatial contexts can mask existing Allee effects.

opencc-zeroDec 2014View details →

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