Showing papers on "Allee effect published in 2000"

Inbreeding depression in conservation biology

Abstract: ▪ Abstract Inbreeding depression is of major concern in the management and conservation of endangered species. Inbreeding appears universally to reduce fitness, but its magnitude and specific effects are highly variable because they depend on the genetic constitution of the species or populations and on how these genotypes interact with the environment. Recent natural experiments are consistent with greater inbreeding depression in more stressful environments. In small populations of randomly mating individuals, such as are characteristic of many endangered species, all individuals may suffer from inbreeding depression because of the cumulative effects of genetic drift that decrease the fitness of all individuals in the population. In three recent cases, introductions into populations with low fitness appeared to restore fitness to levels similar to those before the effects of genetic drift. Inbreeding depression may potentially be reduced, or purged, by breeding related individuals. However, the Speke's ...

Allee effects and compensatory population dynamics within a stock complex

Abstract: Sparked by the observation that (i) many collapsed stocks have failed to recover despite the apparent prevalence of compensatory population dynamics and (ii) many stocks have a complex structure, w...

The Effects of a Bottleneck on Inbreeding Depression and the Genetic Load.

Abstract: We study the effects of a population bottleneck on the inbreeding depression and genetic load caused by deleterious mutations in an outcrossing population. The calculations assume that loci have multiplicative fitness effects and that linkage disequilibrium is negligible. Inbreeding depression decreases immediately after a sudden reduction of population size, but the drop is at most only several percentage points, even for severe bottlenecks. Highly recessive mutations experience a purging process that causes inbreeding depression to decline for a number of additional generations. On the basis of available parameter estimates, the absolute fall in inbreeding depression may often be only a few percentage points for bottlenecks of 10 or more individuals. With a very high lethal mutation rate and a very slow population growth, however, the decline may be on the order of 25%. We examine when purging might favor a switch from outbreeding to selfing and find it occurs only under very limited condition...

Evidence for Allee effects in an over-harvested marine gastropod: density-dependent mating and egg production

Abstract: Conservation programs often focus on studying extinction risks encountered by small populations and de- termining minimum population sizes below which they can- not recover. In certain cases, per capita rates of population growth become negative at low population density. This 'Allee effect' (or 'depensation') is rarely considered in marine systems. We conducted surveys of adult density, reproductive behavior, and spawning in natural populations of Caribbean queen conch Strombus gigas at 2 locations in the Exuma Cays, Bahamas, to test for Allee effects. Mating never occurred when density was < 56 conch ha -1 , and spawning never occurred at <48 conch ha -1 , clearly demonstrating the operation of depensatory mechanisms. Reproductive behav- ior then increased rapidly to asymptotes at densities near 200 conch ha -1 . Heavily exploited populations of queen conch in the Caribbean have been slow to recover despite fishery closures. Failure to recover could result from spawning stock densities that are reduced to the point at which Allee effects begin to operate on reproductive behavior.

Inbreeding depression due to mildly deleterious mutations in finite populations: size does matter.

Abstract: We studied the effects of population size on the inbreeding depression and genetic load caused by deleterious mutations at a single locus. Analysis shows how the inbreeding depression decreases as population size becomes smaller and/or the rate of inbreeding increases. This pattern contrasts with that for the load, which increases as population size becomes smaller but decreases as inbreeding rate goes up. The depression and load both approach asymptotic limits when the population size becomes very large or very small. Numerical results show that the transition between the small and the large population regimes is quite rapid, and occurs largely over a range of population sizes that vary by a factor of 10. The effects of drift on inbreeding depression may bias some estimates of the genomic rate of deleterious mutation. These effects could also be important in the evolution of breeding systems in hermaphroditic organisms and in the conservation of endangered populations.

Multipack dynamics and the Allee effect in the African wild dog, Lycaon pictus

Abstract: The current decline of the highly endangered African wild dog (Lycaon pictus) may be partly due to the population dynamics induced by their social system. African wild dogs are obligate co-operators, and their need for helpers could generate inverse density dependence at the pack level. We show, through a mathematical model, that this can create an Allee effect, leading to a lower population size and a higher risk of population extinction, compared to populations with direct density dependence. This is due to three different processes. All three processes can increase population extinction individually and probably occur simultaneously in African wild dogs. First, inverse density dependence causes significantly higher rates of pack extinction. Second, it also increases the probability of failure to colonize territories by founders. Third, the Allee effect at the pack level (with a critical number of individuals), generates an Allee effect at the population level (with a critical number of packs). These three processes are likely to apply to other obligate co-operative breeders. Furthermore, our results suggest that habitat fragmentation and destruction, as well as increased human pressure, increase the effects of inverse density dependence. Direct and indirect anthropogenic effects may thus be more detrimental to obligate co-operative breeders than to other species.

Genetic Allee effects on performance, plasticity and developmental stability in a clonal plant

Abstract: 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.

Impact of natural enemies on obligately cooperative breeders

Abstract: Obligately cooperative breeders (cooperators) display a negative growth rate once they fall below a minimum density. Constraints imposed by natural enemies, such as predators or competitors, may push cooperator groups closer to this threshold, thus increasing the risk that stochastic fluctuations will drive them below it. This may indirectly drive these groups to extinction, thereby increasing the risk of population extinction. In this paper, we construct mathematical models of the dynamics of groups of cooperators and non-cooperators in the presence of two types of enemies: enemies whose dynamics do not depend on the dynamics of their victim (e.g., amensal competitor, generalist predator) and those whose dynamics do. In the latter case, we distinguish positive (e.g., specialist predator) and negative (e.g., bilateral competitor) reciprocal effects. These models correspond to the classical amensal, predation and competition models, in the presence of an Allee effect. We then develop the models to study consequences at the population level. By comparing models with or without an Allee effect, we show that enemies decrease the group size of cooperators more than that of non-cooperators, and this increases their group extinction risk. We also demonstrate how an Allee effect at a lower dynamical level can have consequences at a higher level: inverse density dependence at the group level generated lower population sizes and higher risks of population extinction. Our results also suggest that demographic compensation can be achieved by cooperators through an increased intrinsic growth rate, or by decreasing the enemy constraint. Both of these types of compensation have been observed in empirical studies of cooperators.

Vertebrate mating systems, allee effects and conservation

Abstract: Recent interest has focused increasingly on the role of behavioural research in conservation. Within this field, the study of mating systems can make a highly important contribution. Mating systems both affect, and are affected by, interactions between conspecifics. Of particular importance are Allee effects, which arise as a result of the benefits of conspecific presence. Recognition of the consequences of Allee effects for mating systems, conservation, and behaviour, has also increased recently. The inter-relationship between these areas is highly complex, and both mating systems and Allee effects have consequences for each other, as well as for conservation. In this way, both have direct and indirect consequences for conservation. We discuss these in the light of current, and potential, contributions of the study of mating systems to conservation.

Effect of aggregating behavior on population recovery on a set of habitat islands.

Abstract: We consider a single-species model which is composed of several patches connected by linear migration rates and having logistic growth with a threshold. We show the existence of an aggregating mechanism that allows the survival of a species which is in danger of extinction due to its low population density. Numerical experiments illustrate these results.

Effects of Density Dependence and Environment on Recruitment of Coastal Invertebrates

Abstract: Author(s): Lundquist, Carolyn J. | Abstract: The processes influencing recruitment in sea urchins are poorly understood. To examine the depensatory density-dependent reproduction of broadcast spawning invertebrates, I created a simulation model of fertilization dynamics. The model described decreased fertilization success as density decreased, and increased fertilization as the aggregation of adult spawners increased. Highest fertilization success resulted from simulations with wide gamete dispersal distributions with an extended tail, representing low flow conditions. Surprisingly, the model did not predict a deterministic Allee effect or minimum threshold density of spawners for successful fertilization. but instead produced a gradual non-linear decline in larval production at low densities. A stochastic Allee effect was observed, as the variability in larval production increased substantially at low density.To estimate current larval production of the red sea urchin Strongylocentrotus franciscanus, I applied this fertilization model to observed spatial distributions. Adult sea urchins were censused at eleven sites to determine density and local spatial distribution. Reserve sites generally had higher densities than fished sites. All sites were significantly aggregated, and aggregation increased as density decreased. Fertilization success was strongly influenced by both density and aggregation. Estimated current larval production was approximately one-third of estimated pre-fishery larval production, with 75% of the decline attributable to declines in mean density, and the fertilization Allee effect responsible for the remainder of the decline.To evaluate density-independent influences on recruitment, I examined settlement patterns of juvenile sea urchins and crabs in northern California Prior research had determined an intra-annual pattern of settlement during local wind relaxation events. This pattern was present during the 1998 El Nino, resulting in high settlement of cancrid crabs and sea urchins. The 1999 La Nina lacked the typical upwelling/relaxation pattern, and low settlement was observed.These results illustrate several aspects of population dynamics that should be considered in fisheries management and reserve design. Decreased adult density can have long-lasting effects if reproductive potential is affected. Local oceanographic transport of planktonic larvae can result in spatial and temporal variability in juvenile settlement. Researchers will need to consider both reproductive potential and settlement variability when designing reserve networks.

Population processes during establishment and spread of invading species: implications for survey and detection programs

Abstract: Three important population processes occur during any biological invasion: arrival, establishment, and spread. Arrival is the process by which individual(s) of the invading organism are transported to their new habitat. Establishment can be considered the opposite of extinction and represents the growth of a newly arrived population sufficient such that extinction is impossible. Spread is the process by which the species expands its range into the new habitat. Because most biological invasions are caused by human activities, these activities are important to understanding the arrival process and the design of detection programs. Establishment is a highly stochastic process and Allee dynamics may be of considerable importance depending upon the life history of the invading species. These characteristics should be incorporated into the timing of responses to positive detection. Population spread is an area of considerable research currently. Early attempts to understand population spread were based upon a simple theory of reaction-diffusion. However, more recent studies indicate that spread often involves two or more forms of stratified dispersal whereby isolated colonies are founded ahead of the expanding population front; these colonies expand and coalesce with the expanding population front. Efforts to monitor spread should incorporate these characteristics.