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41 results for “Allee effects”
An experimental test of the Allee effects range limitation hypothesis
<p>Data and code for a simple N-mixture model in JAGS Merker, S.A and Chandler, R.B. In Press. An experimental test of the Allee effects range limitation hypothesis. Journal of Animal Ecology.</p> <p>JAE-data.gzip contains the data required to run the model including:</p> <ol> <li>scaled and center covariates of detection</li> <li>a 3 dimensional array containing counts of Canada warbler (cardellina canadensis) at 71 point count locations over 4 years</li> <li>A climate PCA derived from PRISM data.</li> </ol> <p>The model itself is described in the .jAG file "Abundance_EN_clim-trt-D.JAG"</p> <p>Merker_Chandler_Appendices is an Rmarkdown file for the appendices of the article</p>
Data from: The magnitude of Allee effects varies across Allee mechanisms, but not taxonomic groups
<p>The Allee effect is a density-dependent phenomenon in which individual fitness increases as population density increases at low population densities. Over the past few decades, a growing number of studies have identified Allee effects in populations using experimental approaches and statistical modelling techniques. These studies have investigated multiple Allee mechanisms (e.g., mate-finding, predation, resource limitation), across a range of systems and taxa (e.g., plants, vertebrates, invertebrates). This meta-analysis aims to synthesize studies that experimentally manipulated population density and measured either per capita population growth or fitness components, with the goal of determining whether the "magnitude" of the Allee effect (defined here as the correlation between population density and population growth or fitness) varies with Allee mechanism across taxonomic groups. A total of 2305 studies were screened, and 62 of these studies met our meta-analysis inclusion criteria. Within these 62 studies, 155 effect sizes encompassing 9 different Allee mechanisms were identified across 5 broad taxa. When grouped by Allee mechanism and taxa, the magnitude of the Allee effect differed across mechanisms, whereas taxonomic group was less useful at explaining variation in the magnitude of Allee effects. Of the nine Allee mechanisms identified, interspecific competition was associated with the strongest Allee effects, followed by fear, pollen limitation and mate limitation. These findings suggest that Allee effects may be more dependent on mechanism than taxa and may function similarly within different taxonomic groups. However, as the majority of experimental Allee effect studies included in this meta-analysis focused on plants and invertebrates, more research is needed on Allee effects in other taxonomic groups to confirm this conclusion. This first quantitative synthesis of Allee effect research in ecology offers novel insight into how Allee mechanisms affect the manifestation of Allee effects in populations, providing important information for ecologists and conservation managers.</p>
Temporary Allee effects among non-stationary recruitment dynamics in depleted gadid and flatfish populations
<p>We investigated whether low-abundance recruitment dynamics can change in time between compensation and depensation, the latter implying the presence of the Allee effect. For this, we studied the stock-recruitment time series of 17 gadid and flatfish populations in the RAM Legacy Stock Assessment Database using a Bayesian change point model. The recruitment dynamics were represented with the sigmoidal Beverton-Holt and the Saila-Lorda stock-recruitment models, and the parameters of the models were allowed to shift at a priori unknown change points.</p>
Allee effects and the allee-effect zone in northwest atlantic cod
<p>According to the theory of compensatory dynamics, depleted populations should recover when the threat responsible for their decline is removed because per capita population growth is assumed to be highest when populations are at their smallest viable sizes. Yet, many seriously depleted fish populations have failed to recover despite threat mitigation. Atlantic cod (<i>Gadus morhua</i>) off Newfoundland despite thirty years of dramatically reduced fishing mortality and numerous fishery closures has not recovered suggesting that drivers other than fishing regulate the growth of collapsed fish populations, inhibiting or preventing their recovery. Here, using Bayesian inference, we show strong evidence of Allee effects in a south Newfoundland cod population, based on data on recruitment and spawning stock biomass. We infer the Allee-effect threshold, below which recovery is impaired. We demonstrate the necessity of data at low population sizes to make inferences about the nature of low-abundance dynamics. Our work indicates that Allee effects are not negligible in commercially exploited fish populations, as commonly projected, and that they represent an inhibitory force that can effectively prevent recovery from overfishing. Our findings contrast with prevailing fisheries management practices that assume compensatory dynamics at low abundances with potential to seriously overestimate the recovery potential of collapsed populations.</p>
Data from: The magnitude of Allee effects varies across Allee mechanisms, but not taxonomic groups
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Temporary Allee effects among non-stationary recruitment dynamics in depleted gadid and flatfish populations
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Allee effects and the allee-effect zone in northwest atlantic cod
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No Allee effect detected during the natural recolonization by a large carnivore despite low growth rate
<p>Eurasian lynx (Lynx lynx) have recently naturally recolonized southern Sweden. The first documented reproduction of lynx in recent times occurred in 2003, and the population increased from two to 48 family groups (the unit of measurement in Swedish monitoring) during its first 18 years (2003/04 – 2020/21). We did not detect any Allee effect, i.e., lower growth rate at low population density, during the recolonization of southern Sweden, although our population simulations revealed a non-negligible (30 %) chance that population observed development could include an Allee effect. The probable absence of an Allee effect was likely because colonizing females did not lack mating partners, as a larger number of wide-ranging males were established in the area before documented reproduction took place. Despite the absence of an Allee effect, the growth rate during recolonization was lower in southern Sweden (lambda = 1.20) than in central Sweden (lambda = 1.29). We have no evidence of higher mortality, including that from poaching, or lower reproduction in southern Sweden could explain the lower growth rate. Instead, we suggest that the lower growth rate during the recolonization of southern Sweden was explained by fewer immigrants arriving from central Sweden due to areas of less suitable habitat between central and southern Sweden, partially preventing immigration southward. From a conservation point of view, it is positive that this small population could recover without being negatively influenced by an Allee effect, as small populations with an Allee effect experience lower viability than those without.</p>
Catastrophes, connectivity and Allee effects in the design of marine reserve networks
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Population dynamics of a communally rearing mammal is driven by population but not group-level Allee effects
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No Allee effect detected during the natural recolonization by a large carnivore despite low growth rate
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Data from: Can a mate-finding Allee effect be elicited to help eradicate invasive mammals?
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Evidence for an Allee effect in a declining fur seal population
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Data from: Sex differences and allee effects shape the dynamics of sex-structured invasions
The rate at which a population grows and spreads can depend on individual behaviour and interactions with others. In many species with two sexes, males and females differ in key life history traits (e.g. growth, survival, dispersal), which can scale up to affect population rates of growth and spread. In sexually reproducing species, the mechanics of locating mates and reproducing successfully introduce further complications for predicting the invasion speed (spread rate), as both can change nonlinearly with density. Most models of population spread are based on one sex, or include limited aspects of sex differences. Here we ask whether and how the dynamics of finding mates interact with sex-specific life history traits to influence the rate of population spread. We present a hybrid approach for modelling invasions of populations with two sexes that links individual-level mating behaviour (in an individual-based model) to population-level dynamics (in an integrodifference equation model). We find that limiting the amount of time during which individuals can search for mates causes a demographic Allee effect which can slow, delay or even prevent an invasion. Furthermore, any sex-based asymmetries in life history or behaviour (skewed sex ratio, sex-biased dispersal, sex-specific mating behaviours) amplify these effects. In contrast, allowing individuals to mate more than once ameliorates these effects, enabling polygynandrous populations to invade under conditions where monogamously mating populations would fail to establish. We show that details of individuals' mating behaviour can impact the rate of population spread. Based on our results, we propose a stricter definition of a mate-finding Allee effect, which is not met by the commonly used minimum mating function. Our modelling approach, which links individual and population-level dynamics in a single model, may be useful for exploring other aspects of individual behaviour that are thought to impact the rate of population spread.
Data from: Aggregation and a strong Allee effect in a cooperative outbreak insect
Most species that are negatively impacted when their densities are low aggregate to minimize this effect. Aggregation has the potential to change how Allee effects are expressed at the population level. We studied the interplay between aggregation and Allee effects in the mountain pine beetle (Dendroctonus ponderosae Hopkins), an irruptive bark beetle that aggregates to overcome tree defenses. By cooperating to surpass a critical number of attacks per tree, the mountain pine beetle is able to breach host defenses,oviposit and reproduce. Mountain pine beetles and Hymenopteran parasitoids share some biological features, the most notable of which is obligatory host death as a consequence of parasitoid attack and development. We developed spatiotemporal models of mountain pine beetle dynamics that were based on the Nicholson-Bailey framework but which featured beetle aggregation and a tree-level attack threshold. By fitting our models to data from a local mountain pine beetle outbreak, we demonstrate that due to aggregation, attack thresholds at the tree level can be overcome by a surprisingly low ratio of beetles per susceptible tree at the stand level. This results confirms the importance of considering aggregation in models of organisms that are subject to strong Allee effects.
Data from: Allee effects may slow the spread of parasites in a coastal marine ecosystem
Allee effects are thought to mediate the dynamics of population colonization, particularly for invasive species. However, Allee effects acting on parasites have rarely been considered in the analogous process of infectious disease establishment and spread. We studied the colonization of uninfected wild juvenile Pacific salmon populations by ectoparasitic salmon lice (Lepeophtheirus salmonis) over four years. From a dataset of 67,896 fish, we observed 88 occurrences of pre-copular pair formation among 1258 pre-adult female and 611 adult male lice. The probability of pair formation was dependent on the local abundance of lice, but this mate limitation is likely offset somewhat by mate-searching dispersal of males among host fish. A mathematical model of macroparasite population dynamics that incorporates the empirical results suggests a high likelihood of a demographic Allee effect, which can cause the colonizing parasite populations to die out. These results may provide the first empirical evidence for Allee effects in a macroparasite. Furthermore, the data give a rare detailed view of Allee effects in colonization dynamics and suggest that Allee effects may dampen the spread of parasites in a coastal marine ecosystem.
Data from: Allee effects may slow the spread of parasites in a coastal marine ecosystem
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Data from: Sex differences and allee effects shape the dynamics of sex-structured invasions
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Data from: Aggregation and a strong Allee effect in a cooperative outbreak insect
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Mate-finding Allee effects can be exacerbated or relieved by sexual cannibalism
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