Situating defaunation in an operational framework to advance biodiversity conservation

Abstract

Anthropogenic pressures are causing the widespread loss of wildlife species and populations, with adverse consequences for ecosystem functioning. This phenomenon has been widely but inconsistently referred to as defaunation. A cohesive, quantitative framework for defining and evaluating defaunation is necessary for advancing biodiversity conservation. Likening defaunation to deforestation, we propose an operational framework for defaunation that defines it and related terms, situates defaunation relative to intact communities and faunal degradation, and encourages quantitative, ecologically reasonable, and equitable measurements. We distinguish between defaunation, the conversion of an ecosystem from having wild animals to not having wild animals, and faunal degradation, the process of losing animals or species from an animal community. The quantification of context-relevant defaunation boundaries or baselines is necessary to compare faunal communities over space and time. Situating a faunal community on the degradation curve can promote Global Biodiversity Framework targets, advancing the 2050 Vision for Biodiversity.

Keywords: defaunation; ecological erosion; extinction; extirpation; faunal degradation.

Figure 1.

Defaunation conceptual framework. (a) Defaunation is the state where the animal community no longer functionally exists. There are an infinite number of paths an intact community can take toward defaunation. The lines represent different scenarios of faunal degradation. The dashed line depicts a community that is rapidly converted to defaunation, perhaps by habitat conversion. The black line depicts a community that is slowly degraded over time, perhaps by hunting. The dotted line depicts either faunal conversion, where species from the original community are replaced by invasives or species with expanded ranges or successful conservation in which the original community rebounds. (b) The level of faunal degradation can be quantified as multiple attributes of species (dissimilarity, biomass, abundance, density, occurrence, occupancy, functional distinctness). In the present figure, we represent it simply as the difference in proportion of species. Such defaunation indices could be compared across space and time. In this example, the degradation index has decreased by roughly 45% from x to y and now lies close to the defaunation boundary. (c) Defaunation and faunal degradation affect ecological processes such as seed predation, herbivory, and seed dispersal. The magnitude or strength of these processes changes with different levels of animal depletion.

Figure 2.

Schematic diagram of intact, degraded, and defaunated communities from Afrotropical rainforest, marine pelagic, and African savanna ecosystems along a defaunation gradient. These are example communities that represent the defaunation process. Faunal degradation and defaunation occur in communities worldwide, with heterogenous paths and consequences for ecosystem function and structure. In most scenarios, this process disproportionately targets large-body animals and results in an increase in abundance of small-body animals released from resource competition and predation. Some large-body species can persist in human-modified landscapes because of their large home range (Poulsen et al. 2011); therefore, it is possible for a community to be defaunated despite the occurrence of large-body species. In addition to the loss of species biodiversity and changes in vegetation biomass, as can be seen above, defaunation can also lead to more nuanced environmental changes such as decreases in nutrient translocation and ecosystem productivity. As was described in the text, defaunation is not necessarily the complete absence of animals. Crossing boundaries between intact communities, faunal degradation, and defaunation is possible and bidirectional. Source: Figure created in BioRender with additional clipart from pinclipart.com, imgbin.com, clipartmax.com, dimensions.com, creazilla.com.