WHAT IS WILDLIFE HEALTH?
Wildlife health is a complex, dynamic topic that can be difficult to define and measure. Wildlife health encompasses more than simply the presence or absence of disease caused by pathogens, parasites, and toxicants (e.g., contaminants or other poisonous substances produced by animals, plants, or people); rather, wildlife health is defined by a population’s ability to withstand stressors and challenges, such as habitat loss, the emergence of disease-causing agents, and other environmental, climatic and anthropogenic threats. The term “resilience” can be used instead of “health” and, as such, health is the result of interacting biological, social, and environmental determinants (Stephen 2014).
Why is wildlife health important?
The health of wildlife populations determines their ability to persist in the wild at sustainable levels for the long term. Wildlife health is linked to environmental and ecosystem health, as well as the health of humans and domestic animals. Monitoring wildlife health provides important information about population, community, and ecosystem dynamics and can inform measures needed to protect human and domestic animal health, as well as other wildlife populations. Further, wildlife health is linked with ecosystem health, and healthy ecosystems provide a plethora of services, from supporting productive agriculture to facilitating enjoyment of natural areas through recreation to mitigating the effects of severe weather events, among many others. Healthy, resilient wildlife populations contribute to biodiversity, which also supports healthy ecosystems.
How does disease relate to wildlife health?
Disease occurs when a host responds to the presence of a pathogen, parasite, or toxicant and it can be both a determinant of health, as well as a stressor. Disease-causing agents can have significant impacts on wildlife populations. Diseases can have direct (e.g., causing overt morbidity and mortality) and indirect (e.g., decreased fitness) impacts on populations, both of which can result in or exacerbate population declines.
The development of disease depends on factors related to the host organism, the environment, and the disease-causing agent. In healthy wildlife populations, pathogens (e.g., bacteria, fungi, viruses, parasites) and toxicants are present, and disease associated with these pathogens and toxicants typically occurs at low levels that do not have population-level impacts; however, when the host (i.e., wildlife) population experiences additional stressors or threats, the prevalence of disease may increase.
How do emerging diseases and other health threats impact wildlife populations?
An emerging disease is defined as “one that has recently been discovered; has recently increased in incidence, geography, or host range; or is newly evolved” (Rachowicz et al. 2005). Two hypotheses can apply to an emerging disease: the novel pathogen hypothesis states that the disease has recently spread into new geographic areas, whereas the endemic pathogen hypothesis suggests that it has been present in the environment but recently has affected new hosts or increased in its ability to cause disease (Rachowicz et al. 2005).
Diseases pose unique challenges to the conservation of wildlife populations, particularly in many species that are already experiencing other stressors. Disease may be the primary cause of population declines, or it may exacerbate population declines. Although in many instances we lack full comprehension of the impacts of diseases on wildlife populations, the impact of disease is well-documented in certain populations. For example, in North America, chytridiomycosis, caused by the fungus Batrachochytrium dendrobatidis, and white-nose syndrome, caused by the fungus Pseudogymnoascus destructans, have emerged following pathogen introduction and have had devastating impacts on amphibian and bat populations, respectively.
The role of wildlife health in One Health
There are numerous definitions of One Health, but generally, One Health is considered a collaborative approach that recognizes that the health of humans, domestic and wild animals, and the ecosystems/landscapes they inhabit are interconnected (AFWA 2023). For example, sometimes disease-causing agents/toxicants that impact wildlife health may also threaten human (e.g., zoonotic diseases) or domestic animal health and vice versa. Further, anthropogenic changes to the landscape can also affect wildlife populations. Discussions of wildlife health should consider linkages with humans, domestic animals, and ecosystems through a variety of measures and collaborations.
How do we measure wildlife health?
Many different approaches can be used to measure health and evaluate health outcomes in wildlife. In a population of interest, potential health threats should be evaluated, followed by estimating the possible subsequent health outcomes. The health of wildlife populations (i.e., a population’s resilience to stressors) is difficult to measure without the presence of an obvious challenge or threat that causes overt mortality. Evaluating potential threats to the health of wildlife populations will inform and guide the best way(s) to measure, evaluate, and subsequently manage health outcomes, which may vary by individual species, population, threat, and scenario.
Factors that may influence the health of populations include: genetic diversity; population demographics; weather events; resource availability; habitat continuity/quality; exposure to toxicants, pathogens, or parasites; and other stressors. These factors may directly affect population health or may work in conjunction with other factors to affect populations. For example, hurricanes may alter the geographic distribution of certain pathogens, leading to exposure of naive wildlife populations to novel pathogens. Additionally, decreased resource availability or poorer quality and/or quantity of habitat may weaken a population’s ability to respond to pathogens, thus resulting in more significant impacts of pathogens in certain populations.
Factors that may be reflective of the health of individuals and populations, and thus the resilience of a population, include: genetic diversity; immune function; reproductive fitness; population parameters; body condition; organ function; active infections; parasite loads; and causes of morbidity and mortality. These lists are not meant to be exhaustive but do provide good examples of various factors to consider when evaluating both potential health threats and measuring/evaluating health outcomes and the health of wildlife populations.