Home About us Contact
|
Home About us Contact | ||
|
|
||
Allee Effects (allee + effects)
Selected AbstractsAllee effects in biological invasionsECOLOGY LETTERS, Issue 8 2005Caz M. Taylor Abstract Understanding the dynamics of small populations is obviously important for declining or rare species but is also particularly important for invading species. The Allee effect, where fitness is reduced when conspecific density is low, can dramatically affect the dynamics of biological invasions. Here, we summarize the literature of Allee effects in biological invasions, revealing an extensive theory of the consequences of the Allee effect in invading species and some empirical support for the theory. Allee effects cause longer lag times, slower spread and decreased establishment likelihood of invasive species. Expected spatial ranges, distributions and patterns of species may be altered when an Allee effect is present. We examine how the theory can and has been used to detect Allee effects in invasive species and we discuss how the presence of an Allee effect and its successful or unsuccessful detection may affect management of invasives. The Allee effect has been shown to change optimal control decisions, costs of control and the estimation of the risk posed by potentially invasive species. Numerous ways in which the Allee effect can influence the efficacy of biological control are discussed. [source] Genetic Allee effects on performance, plasticity and developmental stability in a clonal plantECOLOGY LETTERS, Issue 6 2000M. Fischer Negative effects of small population size on fitness, so-called Allee effects, may threaten population persistence even in intact habitat remnants. We studied genotypes of 14 isolated populations of the clonal plant Ranunculus reptans, for which molecular genetic (RAPD-) variability is higher for large than for small populations. In a competition-free greenhouse environment vegetative offspring of genotypes from large populations produced more rosettes and flowers, indicating higher fitness. Within-genotype coefficients of variation in performance traits, indicating developmental instability, were lower for genotypes from populations with higher RAPD-variability. In competition with a taller grass, we found relative reduction in leaf length less pronounced for plants from large populations, suggesting higher adaptive plasticity. Our experimental study of a plant with predominantly vegetative reproduction suggests, that negative genetic effects of recent habitat fragmentation, which so far rather were expected in plants with frequent sexual reproduction, are more severe and more common than previously acknowledged. [source] Safety in numbers: extinction arising from predator-driven Allee effectsJOURNAL OF ANIMAL ECOLOGY, Issue 3 2010Stephen D. Gregory A.M. Kramer & J.M. Drake (2010) Experimental demonstration of population extinction due to a predator-driven Allee effect. Journal of Animal Ecology, 79, 633,639. Experimental evidence of extinction via an Allee effect (AE) is a priority as more species become threatened by human activity. Kramer & Drake (2010) begin the International Year of Biodiversity with the important , but double-edged , demonstration that predators can induce an AE in their prey. The good news is that their experiments help bridge the knowledge gap between theoretical and empirical AEs. The bad news is that this predator-driven AE precipitates the prey extinction via a demographic AE. Although their findings will be sensitive to departures from their experimental protocol, this link between predation and population extinction could have important consequences for many prey species. [source] Experimental demonstration of population extinction due to a predator-driven Allee effectJOURNAL OF ANIMAL ECOLOGY, Issue 3 2010Andrew M. Kramer Summary 1. Allee effects may result in negative growth rates at low population density, with important implications for conservation and management of exploited populations. Theory predicts prey populations will exhibit Allee effects when their predator exhibits a Type II functional response, but empirical evidence linking this positively density-dependent variation in predator-induced individual mortality to population growth rate and probability of extinction is lacking. 2. Here, we report a demonstration of extinction due to predator-driven Allee effects in an experimental Daphnia-Chaoborus system. A component Allee effect caused by higher predation rates at low Daphnia density led to positive density dependence in per capita growth rate and accelerated extinction rate at low density. 3. A stochastic model of the process revealed how the critical density below which population growth is negative depends on the mechanistic details of the predator,prey interaction. 4. The ubiquity of predator,prey interactions and saturating functional responses suggests predator-driven Allee effects are potentially important in determining extinction risk of a large number of species. [source] Reproductive assurance through self-fertilization does not vary with population size in the alien invasive plant Datura stramoniumOIKOS, Issue 8 2007Mark van Kleunen Autonomous self-fertilization is suggested to be associated with invasiveness in plants because it offers reproductive assurance when there is a shortage of suitable mates or pollinators. Given that shortages of mates and pollinators are a common cause of Allee effects in small plant populations, we predict that the benefits of self-fertilization in terms of reproductive assurance should be greatest in small populations. We tested this idea for the invasive herb Datura stramonium, a self-fertilizing species which is also cross-pollinated to some extent by insects (mainly hawkmoths and honeybees). During two consecutive years, we studied 20 and 55 populations, respectively, of different sizes. Untreated flowers of D. stramonium showed high levels of fruit and seed set in all populations studied. Although, fruit and seed set were generally reduced by about 90% in flowers in which self-fertilization was prevented through emasculation, this effect did not vary according to population size. By using a natural color (anthocyanin) dimorphism in 12 populations, we showed that the average outcrossing rate was low (1.3%) and that there was no relationship between outcrossing rate and population size. Pollen removal from flowers also did not vary according to population size, suggesting that the pollinator visitation rate is not lower in small populations. However, decreasing deviations of observed from expected fruit set with population size imply that small populations may have an increased chance of extinction due to demographic stochasticity. Overall, our results suggest that reproductive assurance through self-fertilization in invasive plants may be important for all stages of population establishment, and not just in the founder population. [source] Effects of food supplementation on home-range size, reproductive success, productivity and recruitment in a small population of Iberian lynxANIMAL CONSERVATION, Issue 1 2010J. V. López-Bao Abstract In a conservation context, food supplementation is a management tool used to reverse the decline of food-limited populations by means of positive changes in behaviour and fitness that may be reflected in population parameters. The critically endangered Iberian lynx Lynx pardinus has suffered a dramatic decline primarily because of the severe drop of its main prey, the European wild rabbit Oryctolagus cuniculus. To reverse this situation, a food supplementation programme has been implemented in Doñana, south-west Spain, since 2002. In this study, we assess the utility of providing artificial food to reduce home-range (HR) size, and to increase productivity, survival and recruitment in a scenario of low lynx density, as compared with reference data from the same population in the absence of extra food. Food supplementation produced a significant contraction of core areas, but not of complete lynx HRs. We did not detect any significant change in productivity or dispersal rates, but supplementation could have helped transient adult lynx to settle down. The positive effects of food supplementation may have been partly countered by factors such as inbreeding, Allee effects and disease outbreaks, whose effects may have been exacerbated in this small lynx population. Food supplementation, however, proved useful to retain individuals, to keep range sizes within their normal range of values, thus maintaining spatial organization, and to allow lynx reproduction and kitten survival in areas with very low prey density. Therefore, we recommend keeping an extensive and intensive supplementary feeding programme until the density of wild rabbits will enable the viability of this endangered lynx population. [source] A test for Allee effects in the self-incompatible wasp-pollinated milkweed Gomphocarpus physocarpusAUSTRAL ECOLOGY, Issue 6 2009GARETH COOMBS Abstract It has been suggested that plants that are good colonizers will generally have either an ability to self-fertilize or a generalist pollination system. This prediction is based on the idea that these reproductive traits should confer resistance to Allee effects in founder populations and was tested using Gomphocarpus physocarpus (Asclepiadoideae: Apocynaceae), a species native to South Africa that is invasive in other parts of the world. We found no significant relationships between the size of G. physocarpus populations and various measures of pollination success (pollen deposition, pollen removal and pollen transfer efficiency) and fruit set. A breeding system experiment showed that plants in a South African population are genetically self-incompatible and thus obligate outcrossers. Outcrossing is further enhanced by mechanical reconfiguration of removed pollinaria before the pollinia can be deposited. Self-pollination is reduced when such reconfiguration exceeds the average duration of pollinator visits to a plant. Observations suggest that a wide variety of wasp species in the genera Belonogaster and Polistes (Vespidae) are the primary pollinators. We conclude that efficient pollination of plants in small founding populations, resulting from their generalist wasp-pollination system, contributes in part to the colonizing success of G. physocarpus. The presence of similar wasps in other parts of the world has evidently facilitated the expansion of the range of this milkweed. [source] Critical thresholds associated with habitat loss: a review of the concepts, evidence, and applicationsBIOLOGICAL REVIEWS, Issue 1 2010Trisha L. Swift A major conservation concern is whether population size and other ecological variables change linearly with habitat loss, or whether they suddenly decline more rapidly below a "critical threshold" level of habitat. The most commonly discussed explanation for critical threshold responses to habitat loss focus on habitat configuration. As habitat loss progresses, the remaining habitat is increasingly fragmented or the fragments are increasingly isolated, which may compound the effects of habitat loss. In this review we also explore other possible explanations for apparently nonlinear relationships between habitat loss and ecological responses, including Allee effects and time lags, and point out that some ecological variables will inherently respond nonlinearly to habitat loss even in the absence of compounding factors. In the literature, both linear and nonlinear ecological responses to habitat loss are evident among simulation and empirical studies, although the presence and value of critical thresholds is influenced by characteristics of the species (e.g. dispersal, reproduction, area/edge sensitivity) and landscape (e.g. fragmentation, matrix quality, rate of change). With enough empirical support, such trends could be useful for making important predictions about species' responses to habitat loss, to guide future research on the underlying causes of critical thresholds, and to make better informed management decisions. Some have seen critical thresholds as a means of identifying conservation targets for habitat retention. We argue that in many cases this may be misguided, and that the meaning (and utility) of a critical threshold must be interpreted carefully and in relation to the response variable and management goal. Despite recent interest in critical threshold responses to habitat loss, most studies have not used any formal statistical methods to identify their presence or value. Methods that have been used include model comparisons using Akaike information criterion (AIC) or t -tests, and significance testing for changes in slope or for polynomial effects. The judicious use of statistics to help determine the shape of ecological relationships would permit greater objectivity and more comparability among studies. [source] | ||||||||||