Niche separation in the fossil record

There is a slow but steady publication papers that describe new fossil taxa that state or imply that the presence of some new species is evidence for niche partitioning in the animal’s ecosystem. This is basically redundant and is akin to the classic ‘this new species adds to the known diversity’ as if it could do anything else. One of the fundamental ideas of niche theory is that two species will not occupy the exact same niche. If they do, one will go extinct (or be forced out) or will adapt. In other words, for two species to live alongside each other there will be, by definition, niche separation, so pointing it out as some revelation or important insight or gained knowledge from two species being present is really not the case.

Worse, it might be wrong. Most of the time with new finds we don’t have very much to go on, so we don’t actually know that it truly did occupy a unique niche. Perhaps it was a transient species and so could survive briefly in full competition with another species as it passed through or had only just invaded and in the fullness of time would outcompete (or be outcompeted) by another species. So a statement about two (or more) species with similar biology (perhaps they are close relatives) being evidence for niche partitioning is most likely either redundant or wrong. Either way, as currently reported, it’s really not needed as commentary on a lot of papers about palaeoecology.

As a related point, there is an idea floating around (online and in discussions rather than I think in the scientific literature) that large predators can’t coexist normally, especially if they are closely related. There’s an expectation that ecosystems with say three large theropods in (or even three large tyrannosaurs) would be really weird and somehow not normal or possible. I’m not sure why this idea is out there but I think it’s a misunderstanding of the above point about niche separation and is somehow a conflation of the idea that species somehow doing the same thing or eating the same prey means one will inevitably come out on top, but again this isn’t really correct.

First off, species can be catching prey in very different ways, but still competing with each other. You only need to see videos of a baitball and any combination of sharks, dolphins, whales, sealions, large fish, diving birds and others all going after the same small fish. Each has its own technique and feeds and processes food very differently, but they are in direct competition for that same resource. So just because birds are flying and have no teeth, doesn’t mean they don’t compete with the dolphins and there is at least some niche overlap between them. Even very similar and near identical species can still partition successfully depending on quite what they are eating and when. They may be separated by seasonal or daylight cycles, or be targeting different prey species even if they are hunting it in the same manner, they will be likely shifting their niches as they grow, and there could be all kinds of local differences in habitat that are hard enough to spot in living species let alone in the fossil record.

Back in my paper describing Zhuchengtyrannus I made this point and the idea that multiple similar species in ecosystems is not actually that strange. However, some ongoing work on a related issue with theropods has made me look again at this and pull out a couple of relevant examples from modern / recent ecosystems. A quick look at the distribution maps on Wikipedia (hardly the last word in science I know, but sufficient to make the point) shows that the estuarine crocodile C. prorosus overlaps with C. johnstoni in part of Australia, C. mindorensis in the Philippines and C. novaeguinea in New Guinea. C. siamensis and Tomistoma both overlap with it and each other in parts of Indonesia, and there is a similar 3-way overlap with both C. palustris and Gavialis in India. Similarly, in South America, Caiman crocodilus, Melanosuchus and two species of Paleosuchus all overlap with each other. Now, at a very local level in a given pond or a short stretch of river there might be only one or perhaps two species present, and in some cases there are some dramatic differences in skull shape and gross feeding ecology, but these overlaps and the inevitable occasional migrations or transport of individuals means they must be truly sympatric at times and probably under some competition.

For a more terrestrial example, the 2019 paper by Schnitzler and Hermann looking at fairly recent (historical) overlaps of large mammals in Asia (especially lions and tigers) has this wonderful quote [that I have modified a little for clarity] about carnivores in part of Western Asia. “The Western Asian area of the Palearctic Biogeographic Realm includes part of the continental interior of the Near East (the northeastern part of Anatolia in Turkey, and Transcaucasia – Georgia, Armenia, Azerbaijan), part of the Caspian lowlands and western part of Pakistan. [It] had an impressive assemblage of large mammalian carnivores (Asiatic lion, Caspian tiger, Asiatic cheetah, Anatolian leopard, lynx, brown bear, grey wolf, jackal, and striped hyena).”

That’s really quite a set of animals and while jackals were probably not much competing with lions for food, there’s a huge amount of overlap here. Even modern India has striped hyena, jackal, dhole, wolves, leopard, lion, tiger and sloth bears in various parts and until recently cheetah too (plus, of course, three large crocodylians) and while their ranges are now much restricted there would have been much greater overlap in the past. In short, while obviously dinosaurs are very different to mammals and crocs, the idea that a Mesozoic ecosystem couldn’t support two or three large theropod genera looks like a poor hypothesis against the kind of overlaps we see even in modern depauperate and stressed ecosystems. Multiple large carnivores, even including closely very related species from the game genus, that are known to hunt similar prey in similar ways, are commonly sympatric and there’s no clear reason to assume ancient systems were that different.

Schnitzler, A. and Hermann, L., 2019. Chronological distribution of the tiger Panthera tigris and the Asiatic lion Panthera leo persica in their common range in Asia. Mammal Review49(4), pp.340-353.

4 Responses to “Niche separation in the fossil record”

  1. 1 Stephen Gunnell 14/04/2021 at 5:18 am

    I think your example needs a bit of tightening up. C. porosus and C. johnstoni are very different beasts. Their common names of Saltwater Crocodile and Freshwater Crocodile are a dead giveaway. C. porosus is a true top predator well capable of snacking on C. johnstoni but the reverse is not true. I can’t speak for the other species but I would suspect the same is true that C. porosus dominates it’s chosen biome and the others subsist where they can. But having got this far I’m not sure whether this supports or repudiates your argument. I think it supports niche separation and restricted numbers of top predator species and I initially thought you were arguing against the latter but now I’m not sure.

    • 2 David Hone 14/04/2021 at 8:44 am

      There were two points I was trying to go with here. The first was the idea that almost by definition if there are multiple species that could conceivably compete in an environment then there must be niche separation so stating this as some kind of exciting fact about say two theropods being found in a single geological formation is redundant (and quite possibly wrong if one of them isn’t actually a resident). The second one is that there is also an idea that you tend to get single large predators in ecosystems and there being two or even more of vaguely similar sizes with some niche overlap is rare when instead it’s actually very common.

      With that in mind, yes, I agree that some of those croc species are doing rather different things. But they are still big predators that are pretty close relatives of each other and at some levels will be competing (not just for food but also things like nesting sites, sunbathing spots etc.). Any overlap is competition so while big adults might be feeding on very different things and even living in different local environments, that doesn’t rule out competition. Still, they are both present and if we found the two as fossils together I think some people would be surprised that two big crocs could coexist.

  2. 3 kris michael 14/04/2021 at 1:01 pm

    You make a good point here, it’s relevant where I’m at in the American west. The grey wolf has re-colonized a small part of it’s former range it was eradicated from a century ago. There’s an idea that along with the mountain lions (and grizzly bears in some areas) there’s more apex predators then the range can support. Which is silly, maybe a few hunters will lose their tags for a couple years in some places. The real culprit is livestock grazing and industrial production on national and state lands along with urban sprawl. Cutting off seasonal migration routes and access to local resources. The animals are carefully ‘managed’ to keep their populations barely stable so a few wolves can throw the numbers off.

    Crocodiles are neat because as they grow they occupy different niches. Young salties might share an area and resources with the smaller local crocs but when they get big they take over like Stephen describes. They seem to be both territorial and migrate quite a bit. If there’s plenty of snacks to go around they lose quite a bit of their aggression towards each other. They probably recognize the pecking order automatically, sensing each others sizes and sexes through water displacement and other signals.

  3. 4 Jordan M 20/04/2021 at 2:49 pm

    The other point to make is that competition only happens when resources are limiting, which is rather difficult (though I would argue not impossible) to demonstrate in the fossil record.

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