We are in the middle of a global pandemic (#coronavirus). I am working from home, going out just once a day for a walk with our dog, keeping in touch with my family and friends in Spain with the worry that one day they will have a cough… Challenging times.
But that is not why I decided to add another blog post today (still, I could not not mention it, Could I?). No, the reason is the publication of a paper to which I contributed. The work was lead by Marcos Moleon and involved 27 scientists (many more than my usual papers). It “started” with a workshop in Sevilla back in October 2016 (started has quote marks because Marcos and several co-authors had already put considerable work into it by then). It finally saw the (publication) light this year in Proceedings of the Royal Society B, where it made the cover of the March issue.
The project goal was to evaluate how researchers understand and use the term megafauna. Etymologically the term is formed by mega: an ancient Greek word which meant large, and the late latin term fauna, the name of a Roman fertility goddess which we now use to describe the total of the animal life of a certain region or time following its use by Linnaeus in his Fauna Svecica. So megafauna = large animals. But, What do we mean by large animals? The biggest animal in the planet is the blue whale, but surely we also think of an elephant as large. But how about a tiger, sea turtle or albatross? Let’s see what we found.
How do researchers use the term megafauna?
To answer this question we did a literature review and identified 276 published studies that used the term. There are three main findings from that review:
First, studies that used and defined the term megafauna fitted largely into two definition types: megafauna means large body vs. megafauna means relatively large size for a given system, taxonomic group or ecological role. Lesson 1: megafauna is an ambiguous term, we may not all mean the same when talking about megafauna.
Second, most studies that use the term megafauna fail to provide a definition, so it may be unclear what they mean (because the term is ambiguous). Lesson 2: always provide a working definition when using terms for which different readers may have different interpretations, like megafauna.
Third, the term was used a lot in papers talking about current marine environments and those about prehistorical terrestrial environments. Very few studies of freshwater systems used the term megafauna. Lesson 3: know your audience: marine biologists and paleontologist are likely to think about megafauna (although in different ways), but river ecologists will rarely do.
How do researchers identify megafauna?
So the literature is a bit of a mess, but surely if we showed ecologists and palaeontologists images of animals they will be able to tell which are megafauna and which aren’t, and if we asked they can tell us how to identify megafauna, right?
Well, there was some agreement, everyone thought megafauna implies large body size and agreed elephants are megafauna, but then large body was more often considered in relative terms, so for some people there were megafauna insects (which are smaller than the average mammal, which they did not classified as megafauna).
So How do we identify megafauna? of course size is important, but respondents thought other qualifiers including taxonomy or ecological role should be also considered. Lesson 4: size matter but needs to be put into context.
Rethinking (the term) megafauna
So, Can we propose a unifying definition of megafauna? We identify three possible ways to go (spoiler alert: we like the last one better).
We could have a definition based on a size threshold alone. But saying bigger than 45 kg seems too limiting. To start it will only apply to some animal groups (no insects here). Maybe we are fine with that idea. However, we don’t always use weight to describe size. So this threshold based definition will need to have values in body length, volume…So nope, we think larger than a given threshold is not a good way to define megafauna.
We could have relative thresholds, so the largest of a given group of animals in a given environment. But we rarely know all the sizes of the animals in a given environment or community. And many organisms change size during their life, so the largest organism at a juvenile stage may not be the largest adult.
Finally, we could think of megafauna as more than just large animals but animals that due to their size play distinct ecological and functional roles in their systems or have distinct traits or characteristics. Something like the large and important or unique.
We think the last definition is more interesting and has potential to contribute to our understanding of ecological systems by exploring roles. It should also facilitate more interdisciplinary work across environments and animal groups for which threshold based definitions are poorly suited.
Still we realize this is far from a strict definition, and it may be not fully or rapidly embraced by all. Some people like the simple thresholds or have different ideas of what a functional role is (I am actually working with another team to evaluate how researchers define functional traits, not a simple history either). So the final lesson: if you use the term megafauna, define what you mean (yes, that is lesson 2 again, glad you were paying attention).
Paper citation Moleón, M; Sánchez-Zapata, JA; Donázar, JA; Revilla, E; Martín-López,B; Gutiérrez-Cánovas, C; Getz,WM; Morales-Reyes,Z; Campos-Arceiz, A; Crowder, LB; Galetti, M; González-Suárez, M; He, F; Jordano, P; Lewison, R; Naidoo, R; Owen-Smith,N; Selva, N; Svenning, J-C; Tella, JL; Zarfl, C; Jähnig, SC; Hayward, M; Faurby, S; García, N; Barnosky, AD; Tockner, K. (in press) Rethinking megafauna. Proceedings of the Royal Society B 287: 20192643 DOI: 10.1098/rspb.2019.2643
Ramblings of an ecóloga 