Natural population die-offs : lessons for conservation

Abstract

Population die-offs generally refer to the loss of a large number of individuals in a population over a short time interval. When die-offs are attributable to one or more extreme natural events, they are referred to as “natural” population die-offs (also abbreviated as “NPDOs”). Despite the increased number of studies reporting natural population die-offs we still have only a limited understanding of the processes driving such phenomena. The first section of this thesis, a review chapter, highlights the variety of pathways by which extreme natural events (including extreme climatic events) can drive episodes of high mortality. It also introduces a framework for assessing vulnerability to population declines applied to terrestrial mammals. The vulnerability to these declines can be heightened if species have high susceptibility and/or a poor adaptive capacity. Using a database of 72 natural population die-offs (NPDOs) of terrestrial mammal species we tested a set of hypothesis related to species’ biological traits (body mass, home range, foraging strategy, territoriality) the main aim of chapter two investigate whether these traits are significant predictors of the observed degree of population losses caused by extreme natural events (“NPDO severity”). In parallel with intrinsic biology, the degree of exposure to threat processes determines the fate of species and their constituent populations. In this context, chapter three maps the degree of overlap between the extant geographical distribution of terrestrial mammals and areas of recent past exposure to cyclones and droughts. Species whose distributions have a high overlap with extreme climatic events are expected to be at greater threat than currently recognized by their biology alone. Chapter four then introduces a vulnerability assessment to cyclone-driven population declines based on the interactions between species biology, the species’ extant geographical range and recent past exposure to tropical cyclones. This is with the aim of determining species that are both, intrinsically vulnerable and highly exposed. This objective is addressed for the 19 terrestrial mammals listed as critically endangered by Mexican Law: Among Mexican mammalian fauna they stand already at the first line to disappear due to human-mediated impacts and extreme climate-related phenomena can increase such risk. Therefore there is a critical need to identify those further jeopardized by climatic impacts and implement comprehensive management practices to protect them. Great changes in the environment have the potential to generate large mortality events. However, the processes by which extreme climatic events shape population growth and trajectory are overlooked. In the final chapter of this thesis a population viability analysis is performed for a predator-free, well-monitored and undisturbed primate population harboured in Agaltepec Island, Los Tuxtlas, Mexico. This translocated population share ecological and demographic features within the range of variation reported for the subspecies in Los Tuxtlas region. In light of the expected increment in the frequency of extreme climatic events (e.g., cyclones for Los Tuxtlas region), systems as the one presented here can help to understand demographic consequences that extreme agents of disturbance could trigger on isolated populations embed in hostile landscapes where data is still not available.