Density dependence, stochasticity, compensation and predator regulation. -Oikos 75: 164-173.This paper presents conceptual models of how density dependence and regulation operates in populations. Limitation is a process which sets the potential equilibrium level, and is caused by all forms of mortality or reproductive loss. Random variations in these mortalities will cause the population to fluctuate about the equilibrium. Regulation is the tendency of the population to return, through density dependent factors, to the equilibrium level when disturbed from it. The strength of density dependence determines whether a stable equilibrium, limit cycle or chaotic behaviour occurs. A population rarely, if ever, remains at its equilibrium because of disturbances. Because the amplitude of population fluctuations is determined by both the strength of density dependence and the size of the density independent mortalities, knowledge of both are required to understand population dynamics. Compensation is the ability of a mortality factor to counteract the effects of another one and requires a change in strength of the density dependence. Exact compensation rarely, if ever, occurs. Predators can act in a density dependent way to hold prey numbers at low levels. They can also act in an inverse density dependent (depensatory) way, which at high prey densities limits but not regulates the prey, but at low densities may cause the extinction of prey. Predation can produce special cases of multiple stable states and stable limit cycles. We discuss some of the criticisms of density dependence.
In parts of the world such as the Pacific Islands, Australia, and New Zealand, introduced vertebrate predators have caused the demise of indigenous mammal and bird species. A number of releases for reestablishment of these mammal species in mainland Australia have failed because predators extirpated the new populations. The nature of the decline of both extant populations and reintroduced colonies provides information on the dynamics of predation.Predator-prey theory suggests that the effects of predation are usually inversely dependent on density (depensatory) when the prey are not the primary food supply of exotic predators. Thus, such predators can cause extinction of endemic prey species. Three types of evidence can be deduced from the predator-prey interactions that allow predictions for conservation: (1) whether per capita rates of change for prey increase or decrease with declining prey densities, (2) whether predation is depensatory or density-dependent, and (3) the overall magnitude of predation. If this magnitude is too high for coexistence, then the degree of predator removal required can be predicted. If the magnitude of predation is sufficiently low, then the threshold density of prey that management must achieve to allow predator and prey to coexist can also be predicted. We analyzed published reports of both declining populations and reintroduced colonies of endangered marsupial populations in Australia. The observed predation curves conformed to the predictions of predator-prey theory. Some, such as the black-footed rock-wallaby ( Petrogale lateralis ), were classic alternate prey and were vulnerable below a threshold population size. Others, such as the brush-tailed bettong ( Bettongia penicillata ), have a refuge at low numbers and thus offer the best chance for reintroduction. Our predictions suggest a protocol for an experimental management program for the conservation of sensitive prey species: (1) determination of net rates of change of prey with declining population, (2) improvement of survivorship through habitat manipulation, (3) improvement of survivorship through predator removal, (4) determination of the threshold density above which reintroductions can succeed, and (5) manipulations to change interactions from Type II to Type III. The task in the future is to determine how to change the vulnerability of the prey so that they can have a refuge at low numbers. Predicción de los Efectos de la Depredación en la Conservación de una Presa en Peligro de ExtinciónResumen: En algunas partes del mundo como son las Islas del Pacífico, Australia y Nueva Zelandia, los veretebrados depredadores introducidos han ocasionado la desaparición de especies indígenas de mamíferos y aves. Un gran número de liberaciones para el restablecimiento de estas especies de mamíferos en tierras continentales de Australia han fracasado debido a que los depredadores han extirpado las poblaciones nuevas. La naturaleza de la declinación tanto de las poblaciones existentes como de las colonias reintroducidas provee inform...
Invasive species are frequently the target of eradication or control programmes to mitigate their impacts. However, manipulating single species in isolation can lead to unexpected consequences for other species, with outcomes such as mesopredator release demonstrated both theoretically and empirically in vertebrate assemblages with at least two trophic levels. Less is known about the consequences of species removal in more complex assemblages where a greater number of interacting invaders increases the potential for selective species removal to result in unexpected changes in community structure. Using a replicated Before-After Control-Impact field experiment with a four-species assemblage of invasive mammals we show that species interactions in the community are dominated by competition rather than predation. There was no measurable response of two mesopredators (rats and mice) following control of the top predator (stoats), but there was competitive release of rats following removal of a herbivore (possums), and competitive release of mice following removal of rats.
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