Eighty-three lineages that took over the world: a first review of terrestrial cosmopolitan tetrapods

Şerban Procheş and Syd Ramdhani. Journal of Biogeography (early access): DOI:10.1111/jbi.12125. Eighty-three lineages that took over the world: a first review of terrestrial cosmopolitan tetrapods.

Cosmopolitan species... geddit? From Esquire magazine

Cosmopolitan species… geddit? From Esquire magazine

Will Pearse

Will Pearse

I really don’t know how I feel about this paper; I found the introduction the most interesting part because it introduced me to many things I never think about. The authors are asking why some species are found everywhere throughout the world, and they think up some pretty cool ways of looking at it. This seems very much like a first-pass at these ideas, and I’d be quite interested to see what more analyses these authors will do.

While many species are not cosmopolitan, their clade is – the authors use shrews as an example of a clade that is widespread, but the individual species within the clade are not. This paper examines how wide you have to make your definition of a clade to make it cosmopolitan; for instance, how many close relatives of the Arctic fox do we have to add before we have a widespread clade? I think looking at the number of species is much less interesting than looking at how old a clade has to be for it to be widespread. Moreover, if we know how widespread species are, can we reconstruct that as if it were (maybe it is) an ancestral state, and look at relationships between the evolution widespread-ed-ness and traits?

Their trait analysis says that flying things and big things are more cosmopolitan, which is essentially the same as saying that things that need more space (to hunt, to feed, etc.) cover a wider space. This in of itself is kind of interesting, because it makes me think about scaling. Large birds are at the top of the food chain, and as such they’re not so affected by individual aspects of ecology – they just need other birds or larger prey they can kill. They can spread across a number of ecoregions because the ecologically limiting factors for them are more abstract (some kind of bird, not just one kind of bird) and different to those for other species. Maybe there’s a relationship between trophic position and how widespread a species is.

Lynsey McInnes

Lynsey McInnes

I have mixed feelings about this paper, just like Will. It definitely felt like a first pass exploration, with lots of potential for extensions and I am totally happy to overlook some misgivings on the approach the authors took in exchange for their presenting an interesting and most definitely overlooked side to macroecological patterns.

I’ve thought a little about cosmopolitan/widespread species mostly with respect to dispersal ability. While the authors suggest good dispersal ability helps generate cosmopolitan taxa, I’d be inclined to take a more careful look at dispersal ability and what we mean by it. A good ‘disperser’ (let’s loosely define that as a species that can move ‘far’) can get to far-flung areas, but will this action produce one cosmopolitan taxon or lots of restricted range taxa? This will of course depend on other intrinsic traits, the nature of the environments the dispersed lineages find themselves in, the frequency of dispersal (continuing gene flow preventing divergence). While it feels logical that species that can reach a large area are the most likely to have large ranges, I think we also have to think long-term (evolutionary time?) to really understand how dispersal relates to distributions.

The above paragraph also relates to the authors’ attempt to define cosmopolitan with respect to single species but also w.r.t. multiple species (i.e., how many lineages do we need to make a cosmopolitan taxon?). This was an interesting element of the paper and I felt the authors could have pursued this line of reasoning more thoroughly. ‘What is a species’ is of course a controversial subject and one I won’t go into here, but, when it comes down to it, most of us are happy to recognise a ‘species’ as a special unit such that a single species cosmopolitan taxon is much more striking than a multi-species one (requiring different traits perhaps, and strong population connectivity). One route to untangling this conundrum might be to look at species’ ages. Are all single-species cosmopolitan taxa really young, and just en route to splitting up into multiple species? I imagine yes in many cases, but probably not all. What about this subset? How do they manage it? Of course, we need robust phylogenies for this. Damn.

The authors were also keen to relate their analyses back to invasion biology and there are clear parallels here. It would be fun to do analyses on cosmopolitan/non-cosmopolitan species and on invasive/non-invasive species concomitantly and see if the same set of traits (or indeed species) come up. Are cosmopolitan species already pests? Will they become so? Do we need to separate out old and young species? Do we need to pay particular attention to restricted-range species in the paths of invading cosmopolitans.

In passing, some of the discussion in this paper reminded me of this work by Purvis et al. looking at mammals on the edge of trait space. Without re-reading the paper, I think I recall the authors finding ‘weird’ species are able to make it when they occupy ‘weird’ niches. They might not diversify, but they can persist and persist because there’s nothing else like them. Are old cosmopolitan species also weird?

In short, an imperfect, but thought-provoking paper. I’d like to see these questions being pursued further in other groups and from different angles.


About will.pearse
Ecology / evolutionary biologist

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