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30 March 2021 Kangaroos at maximum capacity: health assessment of free-ranging eastern grey kangaroos on a coastal headland
Maquel E. Brandimarti, Rachael Gray, Fabiola R. O. Silva, Catherine A. Herbert
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Abstract

Sprawling urban development is fragmenting the landscape and native wildlife habitats on the Australian east coast. The impact of this rapid urbanization on wildlife health is largely unknown. This study surveyed the health of a high-density (5.4 individuals per ha) population of eastern grey kangaroos (Macropus giganteus) affected by urban encroachment and prolonged drought. Blood parameters (hematological and serum protein), trace element and heavy metal concentrations, and parasite counts (fecal worm egg counts, ticks, and mites) are reported for a sample of ≤ 54 kangaroos at Look at Me Now Headland, New South Wales, Australia. These parameters were compared to lower density kangaroo populations from other sites in New South Wales. We found the health and welfare of this population to be severely compromised, with nonregenerative anemia and nutritional deficiencies evident. Our results indicate that high-density kangaroo populations isolated by urban encroachment are at significant health risk. To prevent further decline in this population's health, we discuss management strategies that could be employed, concurrent with ongoing health and disease monitoring, to mitigate the poor health outcomes in this population. We conclude that it is essential to retain habitat connectivity when altering land use in areas with resident kangaroo populations if managers are to maintain healthy populations.

Humans have changed the natural environment and altered the dynamics of wildlife populations (Foster et al. 2002). This can lead to an increase in density of some species (Ditchkoff et al. 2006), up to the point at which they are considered “overabundant.” These populations reduce natural diversity, affect human life or livelihood, and affect the fitness of individuals within the overabundant population itself (Adderton Herbert 2004). While most studies have focused on the negative impacts of overabundant species on the ecosystem they inhabit, less is known about the impacts on the overabundant species themselves.

Overabundant populations often are visually perceived as flourishing, and the immediate impacts that increasing density can have on animal health, welfare, and long-term population viability often are overlooked (Garrott et al. 1993). High-density populations are exposed to an array of physiological stressors that can affect an individual's diet, reproductive success, disease status, welfare, and survival (Scott 1988; Gortázar et al. 2006). Animals at high density can overexploit food resources (McCarthy 1996) causing malnutrition, starvation, and mortality. This can increase their susceptibility to disease and parasitism, resulting from greater intraspecies contact and opportunities for disease transmission (Gortázar et al. 2006). For urbanized native species, these stressors are exacerbated by habitat fragmentation, infrastructure expansion, and motor vehicle collisions (Brunton et al. 2019). As the urban environment expands and continues to distort and disrupt native populations and their habitats, there is a growing need to explicate the indirect costs of these changes to wildlife health, welfare, and population resilience.

Wildlife health surveillance is increasingly important due to ongoing biodiversity loss and increasing threats of zoonotic disease (Ryser-Degiorgis 2013; Han et al. 2016). Wildlife health investigations may require species-specific baseline health parameters for long-term health monitoring of populations. If species-specific baseline hematology and biochemical reference intervals (RIs) are known, the potential to gather physiological data and identify disease can be improved (Maceda-Veiga et al. 2015). The number of individuals in a population that fall outside of the RI provides a quantifiable measure of the impact of parasites, disease, and physiological stressors such as malnutrition (Huber et al. 2017). Blood samples also can enable measurement of stress-related parameters, which are intimately linked to animal health and welfare (Huber et al. 2017). Classical measures of stress include glucocorticoid concentration (Sheriff et al. 2011); however, there is a mounting body of evidence advocating the superiority of using the immune system as an indicator of stress (Davis et al. 2008; Huber et al. 2017). Specifically, the neutrophil to lymphocyte ratio (N:L) can be used as a proxy measure for stress as a result of characteristic changes in the white blood cell (WBC) profile within 4–8 h of exposure to a stressor (Davis et al. 2008; Schultze 2010; Huber et al. 2017). The occurrence of declining health status and/or increased incidence of disease and bio-markers of stress are sensitive indicators of environmental and ecological change for a species (Scott 1988) and can be used to direct management efforts. This is particularly important for overabundant native species in which management often involves controversial techniques such as culling (Descovich et al. 2016).

Eastern grey kangaroos (Macropus giganteus, hereafter kangaroos) are a common macropod species with a wide geographic distribution along eastern and southeastern Australia (Coulson 2008). Overabundant kangaroo populations commonly occur on the urban fringes (peri-urban areas) because large-scale movement of animals is uncommon and largely constrained by infrastructure development (Coulson et al. 2014). Kangaroos have been able to persist in these often-isolated pockets of habitat because they actively use urban green spaces, remnant forests, and land cleared for livestock grazing (Coulson et al. 2014). Despite the apparent success of