DESCRIPCIÓN DE IMPACTOS

Summary

Given the increasing human pressures on global biodiversity, there is an urgent need to conserve species and species populations more effectively. In conservation biology, information on species and population viability is typically obtained via population via- bility analysis (PVA). The applicability of PVA is, however, limited by the availability of species-specific input data. The main aim of this thesis is therefore to quantify popula- tion viability 1) for a large number of species and 2) as an explicit function of anthro- pogenic pressures by making better use of available data. Ultimately, this type of infor- mation will help to better underpin biodiversity conservation efforts. The focus in this thesis is on birds and mammals, and chemical pollution and water scarcity.

To obtain population viability information on as many species as possible, the extinction vulnerability of birds and/or mammals was quantified based on body size (or body mass) and feeding guild (chapters 2 and 3). In chapter 2, a new framework was developed that systematically quantifies extinction risk based on allometric relationships between vari- ous wildlife demographic parameters and body size. Extinction risk metrics included the probability of extinction (PE), the mean time to extinction (MTE), and the critical patch size (CPS). The framework was applied to assess the extinction vulnerability of terres- trial carnivorous and non-carnivorous birds and mammals globally. Irrespective of the metric used, large-bodied mammals and non-carnivorous bird species were found to be more vulnerable to extinction than their smaller counterparts. For carnivorous birds, a multi-modal relationship was found, with larger extinction vulnerabilities for both large and small species. Overall, birds were more prone to extinction than mammals, while carnivorous mammals were found to have higher extinction risks than non-carnivorous mammals. The patterns were confirmed by a comparison with the proportions of extant threatened species as retrieved from the International Union for Conservation of Nature. In chapter 3, minimum viable population targets were estimated for a large number of species by using models of population dynamics across a range of life-history traits related to species’ body size. The method was applied from the smallest to the largest mammal species (from 2 g [Suncus etruscus] to 3825 kg [Loxodonta africana]). The mini- mum viable population targets decreased asymptotically with increasing body size and were in the same order of magnitude as minimum viable population estimates from ear- lier studies. The approach thus enables a first estimation of minimum viable population targets based on an easily retrievable species trait.

To more explicitly account for the influence of anthropogenic pressures on population viability, quantitative pressure-response relationships for demographic rates or habitat suitability were connected to population models (chapter 4 - 6). In chapter 4, a method was developed to quantify the effects of chemical pollution on wildlife population per- sistence based on field monitoring data. Field-based vital rate response functions for toxicants were established with quantile regression, in order to correct for the influ- ence of confounding factors on the vital rates observed in the field, and the response curves were combined with population viability models. The method was then applied to quantify the impact of the toxicant DDE (a breakdown product of the insecticide DDT) on three bird species: the white-tailed eagle, bald eagle and osprey. Population viability

was expressed via five population extinction vulnerability metrics: population growth rate (r1), critical patch size (CPS), minimum viable population size (MVP), probability of population extirpation (PE) and median time to population extirpation (MTE). Past DDE exposure concentrations were found to have increased population extirpation vulnera- bilities of all three bird species.

Next, an approach was developed to predict the costs of releasing captive-bred individ- uals in order to mitigate the impacts of particular environmental pressures on wildlife populations (chapter 5). To that end, quantitative stressor-response relationships for vital rates were combined with wildlife demographic models to compute the impacts of the pressure on the size of the target population. Subsequently, cost estimates were obtained by quantifying the number of captive-reared individuals needed per year in order to maintain a user-defined population size at a given degree of impact of the stressor of concern. To illustrate the approach, it was applied to calculate the total costs required to mitigate the impacts of DDE on the population of peregrine falcons in California over the period 1970-1994. To increase the breeding adult population as soon as possible to a minimum viable size of 116 individuals, 670 captive-reared young were required. The corresponding restoration costs were in total ~$1,520,000, with the highest yearly costs of ~$1,343,000 in 1970. Gradually increasing reintroduction efforts reduced the number of young required and the reintroduction costs to 581 individuals and ~$1,029,000, respectively. However, the number of breeding adults then exceeded the minimum population size only after 1983 (instead of 1972), thus reflecting a trade- off between costs and extinction risk.

In chapter 6, the influence of water scarcity on the spatial distribution of several mam- mal species in Kruger National Park (South Africa) was assessed. The long-term effects of different scenarios of water point closure on the spatial distribution of elephants and the consequential effects on the vegetation and other herbivores were investigated. Using a dynamic ecosystem model, scenarios were evaluated that varied in water availability (both artificial and natural) and elephant densities. The modelling results showed that the closure of artificial water points hardly had any effect during natural wet conditions. Under dry conditions, the spatial distribution of both elephant bulls and cows changed when the availability of artificial water was severely reduced. This, in turn, triggered changes in the spatial distribution of woody biomass availability over the simulation period of 80 years, which led to increased local population densities of all herbivores except for giraffe and steenbok in areas close to rivers.

Based on the findings of this thesis, it was concluded that feeding guild and body size can be profitably used to obtain first estimates of the extinction vulnerability of a large num- ber of species and populations (chapter 7). Further, it is possible to explicitly account for the influence of anthropogenic pressures on population viability in multiple, com- plementary ways, making maximum use of both laboratory and field data. Finally, the various metrics of population viability as applied in this thesis provide complementary information relevant to underpin conservation measures. With increasing data availa- bility, population viability information may be quantified for more taxonomic groups, in relation to more anthropogenic pressures, and become more detailed and precise, which may further help to conserve biodiversity more effectively.