Elsevier

Biological Conservation

Volume 164, August 2013, Pages 98-106
Biological Conservation

Incorporating mortality into habitat selection to identify secure and risky habitats for savannah elephants

https://doi.org/10.1016/j.biocon.2013.04.006Get rights and content

Highlights

  • Mortality location data was used to predict attractive but risky habitats for elephants.

  • Risky habitats were closely associated with people.

  • Protected area designation did little to reduce mortality incidents.

  • Mortality information can be used to better inform habitat selection models.

Abstract

Empirical models of habitat selection are increasingly used to guide and inform habitat-based management plans for wildlife species. However, habitat selection does not necessarily equate to habitat quality particularly if selection is maladaptive, so incorporating measures of fitness into estimations of occurrence is necessary to increase model robustness. Here, we incorporated spatially explicit mortality events with the habitat selection of elephants to predict secure and risky habitats in northern Botswana. Following a two-step approach, we first predict the relative probability of use and the relative probability of mortality based on landscape features using logistic regression models. Combining these two indices, we then identified low mortality and high use (primary habitat) and areas of high mortality and high use (primary risk). We found that mortalities of adult elephants were closely associated with anthropogenic features, with 80% of mortalities occurring within 25 km of people. Conversely, elephant habitat selection was highest at distances of 30–50 km from people. Primary habitat for elephants occurred in the central portion of the study area and within the Okavango Delta; whereas risky areas occurred along the periphery near humans. The protected designation of an area had less influence on the proportion of prime habitat therein than did the locations of the area in relation to human development. Elephant management in southern Africa is moving towards a more self-sustaining, habitat-based approach, and information on selection and mortality could serve as a baseline to help identify demographic sources and sinks to stabilize elephant demography.

Introduction

Using telemetry data to predict habitat utilization is a common practice in ecology, and the results are often used as a guide to protect and preserve habitats important to wildlife species (e.g. Johnson et al., 2004, Meyer et al., 1998). However, habitat utilization is only one aspect within a complex set of factors that ultimately relate to individual fitness. Selection, for example, could be maladaptive, whereby individuals select areas that ultimately increase their mortality risk or decrease their reproductive success (Battin, 2004, Delibes et al., 2001). To avoid the misclassification of highly used habitats as high-quality habitats, it is necessary to temper estimations of habitat selection with some indication of fitness or risk, particularly when habitat selection appraisals are to be used as an impetus for conservation action.

The main limitation of incorporating indices of fitness into animal occurrence models is that spatially-explicit fitness data is often difficult to obtain (Nielsen et al., 2006). Studies that incorporate offspring survival are most common for avian species, where fledging success at the nest site can readily be established (Aldridge and Boyce, 2007, Donovan and Thompson, 2001). However, with mammalian species the estimation of recruitment and survival are generally less straightforward. Changes in the demographic parameters of large mammals in particular occur over relatively long timescales because of their multi-year or multi-decade life spans and their low reproductive and mortality rates. Most large mammals are also highly mobile with large ranging patterns, making it difficult to relocate individuals to monitor survival and fecundity. Studies linking reproductive success to habitats have been most successful in ungulates on islands and other closed systems (McLoughlin et al., 2007, McLoughlin et al., 2008). Given limitations in time, resources, study species, and study area, researchers resort to data that are more readily available, such as mortality location data (Dzialak et al., 2011, Falcucci et al., 2009, Nielsen et al., 2006).

Just as live animals inform habitat selection estimations, carcass locations provide spatially explicit information on where animals die and can be used to inform the riskiness of habitats (Nielsen et al., 2006). However, mortality location data is often underutilized in the literature. African elephants, for example, have carcasses that are easily visible from the air for several years after death (Douglas-Hamilton and Hillman, 1981), and while carcasses have been used to inform local mortality rates (e.g. Dudley et al., 2001, Dunham, 2008) and CITES status (e.g. Wittemyer et al., 2013), we found no peer-reviewed study exploiting the spatial location of carcasses. Knowing where animals die can provide valuable insight into risky landscapes, which is helpful information to guide conservation and management plans. Grizzly bears mortalities in Alberta, for example, were concentrated around roads or hiking trails (Benn and Herrero, 2002), prompting calls to regulate human access in grizzly bear habitats (Alberta Sustainable Resource Development and Alberta Conservation Association, 2010). Relating mortality locations to habitat selection models has also been used to better inform habitat-based management plans (i.e. Nielsen et al., 2006).

Habitat heterogeneity contributes to the spatial pattern of use and mortalities of elephants, and describing those patterns is particularly important in Africa, where the management of savannah elephants is a continuing concern (van Aarde and Jackson, 2007). Hunting and poaching in the late 19th and early to mid 20th century reduced some populations to near extinction (Roth and Douglas-Hamilton, 1991, Whyte et al., 2003), but actions taken to decrease poaching in the mid to late-20th century were largely successful in southern Africa (Whyte et al., 2003). The decline and subsequent recovery of elephant populations also may explain changes in woodland habitat (Guldemond and van Aarde, 2008, Nasseri et al., 2011), prompting concern for elephant-related tree damage (van Aarde et al., 2008). Their role as ecosystem engineers and the susceptibility to population decline from legal and illegal hunting makes it particularly important to quantify how habitats contribute to elephant habitat selection and mortality risk.

Using the information theoretic approach, we modelled habitat selection from elephant occurrence data obtained during aerial surveys. We then created an index of habitat use by elephants. Following a similar procedure, we next modelled the relative probability of elephant mortality using the locations of elephant carcasses. Combining the relative probability of use and mortality indexes, we then defined areas of high use and low mortality as secure habitats, and areas of high use and high mortality as risky areas. By interrelating conditions where elephants live with where they die, we can begin to establish a habitat-based approach to elephant management and work towards understanding and maintaining natural regulatory processes where needed, as proposed by van Aarde and Jackson (2007) and supported by others (Chamaillé-Jammes et al., 2008). These indices, while not a direct measure of demographic sources and sinks, do provide insights for the prioritization of conservation actions and can serve as a baseline to direct future studies into elephant demography.

Section snippets

Study area

The study area in northern Botswana incorporated an area of 74,355 km2. Study area boundaries to the north and east coincide with national borders for Namibia, Zambia, and Zimbabwe, respectively (Fig. 1). Twenty percent of the study area was protected within the confines of Chobe National Park (NP), Makgadikgadi NP, Moremi Game Reserve (GR), and Nxai Pan NP, while an additional 65% occurred within Wildlife Management Areas (WMAs; Chase, 2011). WMAs were designated with a two letter code and

Results

The top-ranked model for all three habitat selection analyses (elephant vs. random, mortality vs. random, and mortality vs. elephant) was model 7 (the global model with water; see Appendix C in Supplementary material for full model results). This model had strong support in the elephant vs. random (weight = 1.00) and mortality vs. random (weight = 1.00) analyses. Conversely, in the analysis comparing carcass locations to live elephant locations, the global model with water (Model 7) had a weight of

Discussion

Our results indicate that elephant use and mortality locations were spatially separated. Elephant mortality locations were concentrated in areas close to human settlements, with 80% of all elephant mortalities occurring within 25 km of high human use areas, an area that accounts for 52% of the study area. Conversely, 50% of live elephants were observed in that same area, and live elephants selected areas of intermediate distance from people (Fig. 4). Elephant mortalities were, therefore, not

Acknowledgements

We would like to thank Elephants Without Borders, the International Fund for Animal Welfare, and the University of Pretoria for research funding. The aerial survey was sanctioned and supported by the Botswana Department of Wildlife and National Parks, through a grant administered by the Conservation Trust Fund (CTF/2010/56). Additional funding was received from the Zoological Society of San Diego, Madeleine and Jerry Delman Cohen, Mr. Brett Warren, Nathan Jamieson Memorial Fund, Abu Camp, and

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