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 analyze associated terminology in the scientific literature. Second, we conduct a survey among ecologists and paleontologists 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.

We surveyed researchers working on megafauna to get a better understanding of how they understand the concept when using it.

For this purpose, we designed a questionnaire to investigate the megafauna concept among researchers. The questionnaire was divided in three parts: Part 1 was devoted to researchers’ personal and expertise data; Part 2 showed pictures of 120 species (15 mammals, 15 birds, 15 reptiles, 15 amphibians, 15 fishes, 15 terrestrial invertebrates, 15 marine invertebrates and 15 freshwater invertebrates, spanning the whole body size range of each group; habitat refers to the main habitat during the whole life, being also the main habitat for adults); and Part 3 formulated questions regarding different definitions of megafauna. We used foreground color pictures of animals without human-related scale references. All photographs avoided backlighting, represented the entire body of the animal, had less than 10% visible sky and pictured adult individuals. Among the list of pictures, we asked respondents to indicate the species that fell into their concept of megafauna.

We selected the respondents in several steps following Sutherland et al. (2013) and Hays et al. (2016). First, we identified leading experts in the ecology and conservation of megafauna (particularly, human-wildlife conflicts and ecosystem services associated with megafauna), as well as in paleontology (mainly, vertebrate paleontology and paleoecology), based on their publication record and extent of work in these fields. These experts were invited to participate in a workshop organized in the Doñana Biological Station-CSIC (EBD; Seville, Spain, November 9-11 2016; http://www.ebd.csic.es/web/megafauna-workshop/home) to discuss on megafauna and their benefits and detriments to humans. Before the meeting, experts were asked to fill in the questionnaire on-line. Second, the same questionnaires were distributed to researchers who attended the first workshop session, which was open to all EBD researchers. These EBD researchers were asked to fill in the questionnaires at the beginning of the session, i.e. before hearing invited researchers and participating in discussions. Third, experts attending the workshop (n=20) were asked to contact and distribute the questionnaires to other researchers of their fields with the aim of obtaining a minimum of ten researchers belonging to the 12 broad fields resulted from all combinations of ecosystem of expertise (terrestrial, marine and freshwater), period of expertise (prehistorical and historical) and taxonomic expertise (vertebrates and invertebrates). We obtained a total of 93 questionnaires (note that one researcher can be expert in more than one combination of ecosystem, period and taxonomic expertise).

To represent species traits and other relevant characteristics that may determine the probability of a species to be classified as megafauna, we compiled information on the taxonomy, biology, ecology, behavior, conservation status and popularity among the general public of the species included in the questionnaires. Also, we recorded respondents’ characteristics that might have an influence on the questionnaires’ results (personal details, and research experience and expertise).