Rethinking megafauna

Abstract

Concern for megafauna is increasing among scientists and non-scientists. Many studies have emphasized that megafauna play prominent ecological roles and provide important ecosystem services to humanity. But, what precisely are 'megafauna'? Here, we critically assess the concept of megafauna and propose a goal-oriented framework for megafaunal research. First, we review definitions of megafauna and analyse associated terminology in the scientific literature. Second, we conduct a survey among ecologists and palaeontologists to assess the species traits used to identify and define megafauna. Our review indicates that definitions are highly dependent on the study ecosystem and research question, and primarily rely on ad hoc size-related criteria. Our survey suggests that body size is crucial, but not necessarily sufficient, for addressing the different applications of the term megafauna. Thus, after discussing the pros and cons of existing definitions, we propose an additional approach by defining two function-oriented megafaunal concepts: 'keystone megafauna' and 'functional megafauna', with its variant 'apex megafauna'. Assessing megafauna from a functional perspective could challenge the perception that there may not be a unifying definition of megafauna that can be applied to all eco-evolutionary narratives. In addition, using functional definitions of megafauna could be especially conducive to cross-disciplinary understanding and cooperation, improvement of conservation policy and practice, and strengthening of public perception. As megafaunal research advances, we encourage scientists to unambiguously define how they use the term 'megafauna' and to present the logic underpinning their definition.

Keywords: apex predators; body size; functional traits; keystone species; large animals; megaherbivores.

Figure 1.

A representation of several examples of megafauna according to explicit-size-based-threshold definitions that are commonly found in the scientific literature (see electronic supplementary material, table S1). Mass-based definitions are typically used in vertebrate studies in terrestrial, pelagic marine and freshwater ecosystems, while length-based definitions are typically used in invertebrate studies in benthic marine and soil ecosystems. A list of the species represented and photograph credits is provided in the electronic supplementary material, appendix S2. (Online version in colour.)

Figure 2.

Number of megafauna publications according to ecosystem (terrestrial, marine and freshwater) and period (historical and prehistorical). For each pathway, we indicate in parentheses the number and percentage of the total reviewed articles (n = 276) that provide a definition of megafauna and those that do not provide any definition; in the former case, we indicate if the definition is supported by citations, arguments, both or none. Line width is proportional to the number of studies. When an article referred to more than one ecosystem and/or period—6% of cases—we depicted as many lines as needed. Note that some ‘terrestrial’ studies do not explain in detail the species considered and may include also freshwater-dwelling species. Only articles with the term ‘megafauna’ in the title were considered for this purpose. (Online version in colour.)

Figure 3.

Relationship between species body mass and the proportion of respondents to the questionnaire that classified the showed species as megafauna, either for the whole set of species (a) or broken down by taxonomic group (b). Solid lines represent the fitted values of the model including only body mass as predictor (for (a): F_1,118 = 510.3, p < 0.001; R² = 0.81). According to a regression tree analysis (see electronic supplementary material, appendix S4), the species included in the questionnaires with body mass greater than or equal to 61 kg (vertical dotted line) had the highest probability of being classified as megafauna (probability greater than or equal to 0.69; horizontal dotted line). (Online version in colour.)

Figure 4.

A general, conceptual definition of megafauna based on body size and its coupling to the effect of the species population on ecosystems. (a) The largest animals exert strong, consistently high impacts on local ecosystems. By contrast, the effect of small animals on local ecosystems is highly variable, with different species having low or high effects. The empirical challenge is to identify the shape of the size–effect relationship. (b) Qualitative distribution of animal species in the two-dimensional space defined by body size and ecosystem effects. Animals exerting high effects are defined as keystone species [,–68], but only the largest keystone species are considered as megafauna. Note that large animals exerting low/medium effects are rare. (Online version in colour.)