Predicting Effects of Predation on Conservation of Endangered Prey

Abstract

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...

You can hide but you can’t run: apparent competition, predator responses and the decline of Arctic ground squirrels in boreal forests of the southwest Yukon

“…In cases where most prey mortality is caused by predation, the predator response can be determined by inspection of the instantaneous rates of change in the prey species over a realistic range of prey densities. Such analyses across a wide range of taxa (Messier, 1994; Sinclair et al, 1998) confirm two general categories of predator response curves predicted by predator–prey theory (Holling, 1959; Holling, 1973; Sinclair & Pech, 1996). Where predators suppress but do not eliminate prey at low density (Type III predator response; Fig.…”

“…To distinguish between possible Type II and Type III predator relationships, the instantaneous rates of increase were calculated for each active-season (May–August). The shape of the predator prey relationship was determined by plotting the instantaneous per capita rate of change for AGS between N t and N t +1 ( dN ∕ dT ∕ Nt ) over population density at N t (Sinclair et al, 1998). Data from 1990 and 1999 were excluded from this plot because these years coincide with intense prey-switch events that result in total population collapse (such perturbation events are treated separately when analysing cyclic dynamics; Sinclair & Krebs, 2002).…”

“…Strong correlation between hare and ground squirrel numbers across a wide range of hare densities is the expected outcome of asymmetric apparent competition (Holt, 1977). Furthermore, the population trajectory of the ground squirrels at various population densities provides useful information concerning the dynamics of the predation at low prey numbers (Sinclair et al, 1998). We evaluate the following hypotheses to account for observed changes in ground squirrel abundance over a 25-year period.…”

Continued Decline of an Atlantic Cod Population: How Important Is Gray Seal Predation?

“…Proposals to reduce predator populations come from the notion that they are the principal source of natural mortality. While this may be true in some cases (Sinclair et al 1998, Courchamp et al 2003, Wittmer et al 2005, in others it is not (Punt andButterworth 1995, Valkama 2005). Our model indicates that during the 11-yr period of the fishing moratorium (i.e., through 2003) there is little evidence that gray seals (Halichoerus grypus) were the principal source of natural mortality on the ESS Atlantic cod (Gadus morhua) stock.…”

“…Understanding the factors, including predation, that limit the recovery of depressed populations is clearly important to design effective recovery strategies (Sinclair et al 1998). Even if the limiting factors are beyond our control (e.g., ocean temperature), a better understanding of the limiting factors provides a firmer basis for establishing expectations about both the timeframe and extent of recovery.…”