Sunday, March 30, 2014

Megaherbivore Multiplier Effect: Maybe We Should Not Get So Excited About Giant Herbivorous Dinosaurs Coexisting

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
I should clarify Tobias' work by pointing out that he maintains character displacement does occur- it has just been overstated by researchers. Allopatric speciation and then later cohabitation with niche partitioning is the more common scenario Tobias suggests. Controversial for sure, but very interesting and pertinent to dinosaur speciation/character displacement.

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.

Dzanga Bai
Ok so now back to the dinosaur dilemma where multiple mega ton species are stacked on top of each other in a seemingly untenable manner.

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.


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Anonymous said...

Hmmm. First off, I'd strongly suggest reading Jan Sapp's Evolution by Association if you think that the emphasis on competition is about some sort of masculine bias.

Sapp makes a very good case that the emphasis on competition is straight-up capitalist ideology: competition became associated with capitalism, symbiosis (and especially mutualism) with communism, and the bias rolled out from there on both sides of the former iron curtain. Even though anyone with a brain knows that companies only exist due to their contracts with each other and/or their special relationships with governments, still our ideology is all about how competition makes America (or the West) strong, yada yada yada.

The other simple, dumb question I have is this: did big dinosaurs really coexist with each other? A few months ago I saw an article about the most precise date achieved for the K-Pg boundary. This is a distinct layer, easy to find, and by consensus laid down in a very short time. The precision for this date was 10,000 years.

Similarly, most of our data on what forests looked like during the 100,000 year-long PETM comes from two fossil leaf sites and a bunch of pollen samples, and our vision of Paleocene and Eocene forests comes from a similar scattering of fossils from over 10,000,000 years or so. We know climate changed radically over that time (see the PETM), but simultaneously, everyone who "knows" anything about the Paleocene "knows" that it was an unchanging jungle to the poles during that time. Even the smattering of fossils we have refutes this, but it doesn't stop it from becoming popular knowledge.

While I'm sure this pains paleontologists, I have to ask what paleontologists mean when they say two organisms lived together. If the fossils were found in the same layer, what's the range of accuracy: a year? A decade? The same millennium? 100,000 years?

The reason I'm asking is that animals move around, habitats change over time, and if animals are only associated with each other plus or minus 10,000 years, well, faunas can change in that time, not through evolution, but through migration, succession, and so forth.

That's the argument I'd suggest refuting, before we start discussions about how so many big herbivores could coexist in one place at one time. After all, the people doing the dating and stratigraphy can give us some idea of how close the fossils actually were in time.

Duane Nash said...

Thanks for comments Heteromeles, good points.
I will check out that book Evolution by Association sounds interesting. Personally my take on evolution and social systems, is I am a bit of a blender. Competition, commensalism, isolation and just plain luck are all working to greater or lesser degrees I feel. Socially I think capitalism, socialism, and communism all have their strengths and weaknesses- but I feel a blending of these systems might offer some advantages. But that is a rabbit hole I don't really want to go down at least in this forum.

Regarding the accuracy with which we can assert the cohabitation of dinosaurian megafauna very pertinent points and questions. Doubtless species came and went and there was waxing and waning of species, range shifts etc etc. But I think trace fossils, especially trackways on the same bedding plane indicating a short period of deposition are very strong indicators of cohabiting megafauna. In my last post reviewing Anthony Martins book I talked about dinoturbation and the paper which I linked to documents at least two types of big sauropods laying down tracks on a lagoonal substrate. Additional places like Dino nat'l park seem to indicate drought induced die offs with a variety of sauropods/theropods dying together.

I don't know enough about the PETM to comment. When you say it changed radically did it go from humid to drier or hot to hotter? AFAIK there was no type of ice age oscillations going on?

But yeah paleontologists often find themselves in a bit of a pickle when asserting that species lived together. The resolution with which you can gage these relationships also gets fuzzier and many would argue that you should not even try. That is why I attempted to paint with as wide a brush as possible. But this was really more of a concept piece and I was arguing against the prevailing, in my view at least, sentiment espoused: "Ehrmagard so many large animals living together... how did they do it?" Large animals living together is the rule rather than the exception. In fact I can't think of a single situation with just one megaherbivore living without a diverse assemblage of other small, medium, and large herbivores.

Paleo King said...

It's pretty likely that most of these coexisting mega-herbivores didn't directly compete for resources. I doubt that Diplodocus and Brachiosaurus directly competed for food, low-grazers don't usually go for the same plants as high-browsers.

In fact Brachiosaurus remains tend to be found far from the lowland plains that most diplodocids dominated. They appear to have avoided each other, as brachiosaurs were tree-feeders, but diplodocids would have been mostly tree-destroyers, crushing conifers to get at pockets of ferns or create new clearings instinctively, which would mean more room for ferns to grow. Brachiosaurus was more of a highland animal, and may not have looked too kindly on diplodocus and its destructive behavior wrecking the food supply.

In the ecosystems where brachiosaurs and diplodocoids coexisted close together in the same quarries (like at Tendaguru), the diplodocoids tend to be the round-mouthed high browsing types like Tornieria. Even little Dicraeosaurus has a rounded mouth which is better for browsing than grazing. Thus everyone was sharing the tall trees, nobody was knocking them down. In those situations, judging how so many browsing species can coexist (Giraffatitan, the Archbishop, possibly another brachiosaur, Tornieria, Australodocus, Janenschia, 2 species of Dicraeosaurus, Tendaguria, and perhaps others) is a real challenge.

The other tricky part is stratigraphy, radically different faunas can be separated by less than a million years of sedimentation (or a few tens of feet). Giraffatitan is mostly found in the upper layers of Tendaguru (Tithonian epoch), the so-called Upper Saurian Marl. The middle and lower Saurian Marls tend to contain more diplodocoids, which may be an indicator that the ecosystem was changing and even among browsers, these changes tended to favor different families and designs at different times.

Duane Nash said...

Thanks for comments Nima.
We might just fundamentally disagree on diplodocid feeding ecology in the Morrison. I actually like the jack of all trade niche that GSP inferred for them- ferns, groundcover, short, medium and high browse, aquatic charophytes- I am not convinced by the 'vacuum cleaner' fern grazer interpretation. Why wouldn't a more conservative neck design, such as one like Nigersaurus work for diplodocus if it was a hoover? As for habitual tree felling I am not convinced that such a behavior was common for sauropods or diplodocids although maybe some of the robust necked apatosaurus species possibly did knock down trees commonly.

I think the most important landscape modification with regards to vegetation that sauropods performed was not performed with their mouths or through dramatically knocking over trees. Simply the sheer weight of sauropods, combined with repeated travels- could have compressed and damaged tree roots until the trees die. Similar to what multi-tonne tractors, bobcats, forklifts do today when working around trees.

Several papers in the book Mesozoic Plants Through Time question the view of 'fern prairies' being a dominant feature in the Morrison: Palynological evidence for conifer dominance within a heterogenous landscape in the Late Jurassic Morrison Formation USA Hotton, Baghai-Riding. A mosaic of characters in a new whole plant Araucara from the L. Jurassic Morrison formation. Gee, Tidwell

For me open gallery forests- with enough space between trees allowing for movements of sauropods and diverse herbaceous groundcover -makes a lot of sense. The notion that conifers would be restricted to riparian sections does not make sense in light of the way conifers, especially auracarians, grow today over the whole landscape where they live. If diplodocids were modifying their habit towards fern meadows i think we should have seen a strong signal that they were doing so. And this signal would have been the ubiquitous dominance of a few fern species in the spore record. But we do not see this- instead we see a highly diverse record of ferns, conifers, ginkgoes, sporophytes etc etc

In the Tendaguru I have read less about the vegetation there. But, from my viewpoint, that it may have housed multiple large taxa cohabitating is not something that surprises me. As I argued throughout this post multiple large herbivores cohabitating is the rule not the exception. Even with substantial competition.

Evolutionary biology has a certain hangup with competition. We love to talk about competition as being a primary impetus towards evolution. But we tend to overlook commensal, symbiotic, coevolutionary pathways. Just look at the cell- a symbiotic mashup if there ever was one.

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