Ok for this post I want to get back to the terrestrial realm, although I do have a load of posts on aquatic stuff planned, and talk about some ideas in ecology/evolution that may have gone unnoticed in paleo circles with some pertinent thoughts to dinosaur niche partitioning. Why would ecology papers go unnoticed in paleontology? Well because Nanuqasaurus, and umm Cambrian filter feeding invertebrates, and Cosmos was on TV and creationists got all butt-hurt about it... No really it is important for paleontology to stay abreast with ecology/evolutionary theory because if you accept that the present is the key to the past then maybe you should keep your mind open to all avenues of thought and guard against a myopic view of the history of life.
Now dinosaurian niche partitioning has been a rather en vogue avenue of study with regards to dinosaur paleoecology. It is an elegant and, with morphometrics, quantitative way to potentially explain how such large animals, orders of magnitude larger than current land mammals, coexisted in chunks of land often times significantly smaller than contemporary continents (higher eustatic sea levels in Mesozoic). The skull ecomorphology paper on late Cretaceous Albertan herbivores by Mallon and Anderson (2013) which I talked about here (although I have changed my views a bit as noted in post) might be good review and also the various papers on sauropod skull differences/niche partitioning you can find on the web/blogs etc etc may be useful. What these papers suggest is that through niche partitioning, resources can be in a way divvied up allowing situations like we see in Cretaceous Laramidia or the late Jurassic Morisson formation with multiple multi-tonne taxa coexisting. Through character displacement, species- especially closely related and/or morphologically similar ones -that might compete for food, space, resources will tend to diverge more and more from each other where their ranges over lap. Sauropods might have differing neck lengths to feed at different heights or perhaps have stronger jaws/teeth to handle rougher forage (i.e. Camarasaurus) versus weaker slender jawed taxa (diplodocids). Ornithischians coexisting on Laramidia are construed to be high to mid browsers (hadrosaurs), mid level browsers (ceratopsids), or low level grazers (ankylosaurs). But all in all this partitioning of resources is construed as a necessary corollary of the crucible of evolution- competition - as the guiding light that allowed such a diverse Mesozoic bestiary to coexist without overwhelming the resources.
But how secure are we in positing competition as the engine that drove these adaptations towards niche partitioning in dinosaur communities?
I want to direct your attention to an article Competition May Not Be the Driving Force of Species Diversity After All, which suggests an alternative take on the evolution of ecological diversity. The article, more or less a summary of several papers by Joseph Tobias of Oxford University, calls into question the seeming ubiquity of character displacement driven by competition as the primary impetus towards diversity. Looking at the natural and evolutionary histories of species often interpreted as textbook examples of character displacement driven by competition Tobias found the evidence lacking in a majority of these cases. Instead, Tobias offers, species tend to diverge- with or without competition -given enough time to evolve. Looking at ovenbirds, a new world tropical bird family, Tobias performed a rather exhaustive study of bill shapes. Although he found that the species that lived together had the most divergent bills, as predicted with classic Darwinian competition, when the evolutionary history of these cohabiting birds was taken into account the signal for an evolutionary bump due to competition was lacking. Instead the ovenbirds that lived in the same environment also had the longest evolutionary histories. It was evolution over time and in isolation, allopatric speciation, that best explained the signal of species diversity in ovenbirds. That species which cohabited had the longest evolutionary histories made sense- they had enough time to evolve on their own unique evolutionary trajectories- that ultimately allowed them to effectively partition resources when their ranges once again overlapped after isolation.
|To study the role of competition in evolution, Joseph Tobias and collaborators mapped out the evolutionary relationships and variation in beak size among 350 lineages of ovenbirds. Image: Joseph A. Tobias and D. Seddon|
On more of a cultural note ( I do have a background in anthro after all) it does make sense for biologists to perhaps foster a bit of bias towards competition as being a chief catalyst towards diversity. Competition is very Darwinian, competition is a very masculine, and competition is very western and often times intimately associated with capitalism (social Darwinism). And let's face it, biology and science is still very masculine and western dominated. Of course scientists imagine that they are above social biases and cultural leanings, but I beg to differ. We are all cultural animals and bring some amount of baggage with us into any endeavor, no matter how rigorous, we pursue.
Ok now with those thoughts in mind I want to direct you to a recent paper on Plos One: Reconstructing Grazer Assemblages for Protected Area Restoration (March 2013). You should go read it but what these researchers were looking at is the most optimal way to recover successful and complete herbivore guilds on a managed, but depauperate, African range. What is critical is that they looked at the question from the lens of size classes, competition, and facilitatory effects. And basically what they found was that competition inhibited a high diversity of species that occupied the same general size cluster- which plays into what is fairly well established in terms of the link between herbivore and digestive efficiency, optimal grazing sites etc etc. But what was most critical in terms of stabilizing the whole system and allowing for maximum diversity, sustainability, and health of all size classes of herbivores was what they referred to as the facilitatory effects of large megaherbivores such as rhinos, hippos, and elephants. Basically the movements and feeding envelopes of these megaherbivores enhanced pasture, broke up woody debris, altered fire regimes, and created pathways that benefitted all the players in a highly disproportionate manner.
|Venter et al. 2014. Plos One|
"... the lack of large grazers creates an ecosystem devoid of facilitatory effects which in turn leads to an ecosystem which is unable to maintain its herbivore assemblage structure."
Hopefully you can see that the two examples to the far right with abundant megaherbivores, show the most diversity and balance between all the size guilds. It might at first seem counterintuitive but a healthier, more stable system is the one with abundant and diverse megaherbivores.
I like to call this phenomena the megaherbivore multiplier effect and if you start to look at where you see megaherbivores today guess what you see generally- more megaherbivores - as well as more small and medium sized herbivores. African savanna- large herbivores and all sorts of small, medium, and large guys. In India places like Kaziranga park host Indian elephants, Indian one-horned rhinos, wild water buffalo, gaur, sambar, swamp deer, Indian muntjac, wild boar, hog deer. You do not need to go very far back into the Pleistocene to see that this pattern of abundant and diverse megaherbivores with complete guilds of various sized herbivores was the rule rather than the exception. Now you might be thinking well that is just because those systems are productive enough to support a diverse assemblage... But even in semiarid habitats such as in the Namib and Kalahari deserts there are more abundant and diverse large herbivores- including elephants, rhinos, and giraffes as well as smaller guys -found there than in say a place like the Mojave desert. At the other extreme let's compare rain forests. West African rain forests are heavily modified by forest elephants which are pivotal engineers of open bai habitats which provide food/habitat/mineral resources for forest buffalo, lowland gorilla, giant forest hog, red-river hog, and bongo among others. Contrast that situation with the depauperate large herbivore fauna of south/central American rain forests- what do you have... a couple of tapirs, some peccaries, some rodents and a couple of deer? But chances are you will not see much in the way of large herbivore activity in south American rain forests if you go there. Just loads of leaf-cutter ants.
But the dinosaurs were sooooo much bigger- does this comparison really hold true for berbivores several orders of magnitude larger?
True but bear in mind that dinosaur reproductive strategy was a lot different than large mammalian reproductive strategy. At any one time in a dinosaur population there would be gazillions of newborn/hatchlings running around, one year olds, two years olds, three year olds from previous breeding cycles..... and then a bunch of promiscuous teenage dinos making babies even before their skeletons were ossified and paying no heed at all to responsible family planning. And then maybe just a couple of old stodgy, weather-beaten adults talking about the good ol' days. It was a live fast die young strategy and for the most part seems to have worked for dinos. More importantly, and diverging strongly from mammals, is that the average size of a dinosaur would be a lot smaller than the maximum adult size that could be reached. The mean was brought down quite a bit. Which is why whenever I see a herd of fully mature ceratopsids/sauropods in movies/pictures, with no variety in size classes, I throw up in my mouth just a little bit... Look at the range of sizes in a population of say nile crocodiles, this gives a better view, in the broadest strokes, of how these populations may have varied in size. Again contrast with large mammals, where small broods and intensive natal care via mothers milk brings up baby quickly to adult size.
Ok but where are all the small and medium dinosaur herbivore species? Many locations show really small species and then really big species- but few in betweeners?
Two points here:
1) The notion of dinosaurs acting as various ecological species- ontogenetic species -throughout their growth trajectories has gained considerable traction. Additionally if we look at this idea through the lens of what I discussed earlier where megaherbivores enhance systems for small and medium species- then it could be possible that the activities of adults megaherbivorous dinos augmented habitat for their own young!!
2) We may in fact be missing quite a bit of the diversity of small and medium sized herbivorous dinosaurs. Fossil bias exists and if they tended to stay in more upland/drier habits they may have stood less chance of fossilization.
And one final word on niche partitioning/competition in dinosaurs. I think it is a bit overstated. Did it exist? Sure. Various sized mouths, tooth structure, jaw muscle leverage, neck length all point to various herbivores being better equipped to handle such and such resource better than others. But if we are talking about the big guys here, the megaherbivores, I think these suggestions of partitioning are more or less moot. They just ate everything. I know, I know what about tooth wear studies? Well I will offer how much do tooth studies really tell us? If dinos were shedding their teeth constantly maybe the wear patterns reflect what they were eating during that particular season? There are lots of examples of modern herbivores partitioning resources but I see a lot of examples of herbivores overlapping considerably and there just being enough of that green stuff that it doesn't really matter. Maybe there was room for 'em all with some partitioning but often times high degrees of dietary overlap. And in the Mesozoic with high eustatic seas, year round balmy temperatures, dino-dung fertilizer, monsoonal climate regimes, high CO2, and often times rich volcanic soils maybe these were just systems that could foster multiple taxa of berbivores with highly overlapping diets and that was just ok.