Monday, 30 September 2013

Theropoda Sympatrico Pt.2



In the previous post we had started to look at the importance of theropods within the ecosystem and I suggested that the possible extinction of large carnivorous dinosaurs may have been the catalyst for selective extinction for some animals and also the consequential acceleration and proliferation of others. In other words, large theropod extinction may have had direct implications for faunal turnover within ecosystems.
Although I have essentially been looking at large theropods it is worth remembering that the diversity of theropods in most ecosystems was generally very complex. Regardless of size and/or niche even the disappearance of a small theropod may have caused limited environmental problems (more of this later). In simple terms, and if we use a large predator as an example, if a particular carnivore was in decline or on the road to extinction, for whatever reason, then it is likely that the herbivorous animals that they preyed upon would increase in numbers. This, in turn, would have added pressure to the plants, trees and other flora since, as the herbivores increased in number, and then there would be less chance of a successful germination by those plants affected.
When plants are affected then, not only non-avian dinosaurs, but  everything becomes affected including, in Mesozoic times, insects, pterosaurs, mammals and birds and these effects would have changed their distribution, relative abundance  and, most importantly of all, their intraspecific behaviour. The loss of a single large theropod therefore would have had enormous effects on an ecosystem.
Another interesting thought is whether theropods displayed variable feeding tendencies like we see in carnivores today. It is certain that the majority of carnivorous theropod dinosaurs could at least be termed as hypercarnivorous – that is their diet would have consisted of at least 70% meat and it is fair to say that the majority of stereotypical theropod teeth are blade like and serrated for exactly this kind of diet.
However, it is difficult to comprehend that theropods, by their very nature, would have been able to utilise the other 30% worth of plant matter and fungi to be genuinely referred to as hypercarnivores and the vast majority of theropods should, indeed, be known as obligate carnivores since it is certain that they had neither the dentition or, even more importantly, the digestive capability to assimilate such matter.
But theropods were still exceptionally diverse and we have discovered many weird and wonderful animals now that were indeed cosmopolitan in their tastes. Ornithomimids appear to be clearly omnivorous, therizinosaurs were certainly herbivorous and the peculiar oviraptorids appear to be capable of consuming a great variety of foodstuffs. The fascinating Alvarezsaurids seem ideally suited to be insectivores whilst, at the other end of the scale, we have the great baryonychines and spinosaurs that appear to be both great fish and flesh eaters.
Regardless, the largest carnivorous theropod in any given ecosystem would still have been the most important ecological factor in determining the health and prosperity of the system. They were at the very top of the trophic cascade and would exert a level of influence on those species at the next level down – their prey. These animals too exert influence on those species below them and so on until we reach the very bottom of the trophic ladder. This is known as top-down regulation.
Large theropod dinosaurs, although relatively few in number, would have exerted considerable pressure on species below them. Fear would have been a strong driving force and their presence alone would have indirect effects on prey animals and, of course, predation would have very direct effects indeed. These effects would have had a ripple action throughout an entire community of animals so that the entire ecosystem was affected.   
And yet, as we have already observed, palaeocommunities were also inhabited by smaller theropods and these were much more diverse and common than their larger cousins. Although not at the apex of the trophic ladder they would have filled many varied predatory niches and it was this adaptability that enabled them to diversify and, as a consequence, their numbers would have been far greater than their giant cousins.
However it does seem likely that a small theropod’s disappearance would have only had a moderate effect on the ecosystem. Many of the smaller carnivores’ predatory niches would have almost certainly overlapped thus the effect on the environment would have been reduced as opposed to those effects caused by the loss of an apex carnivore.
Large apex theropods would have needed large territories to fulfil their nutritional requirements especially as it is now generally accepted that they were active fast growing animals - if not fully endothermic. In an extant mammalian ecosystem, the rule of ecological efficiency suggests that there needs to be ten times the amount of prey animals to support a modest carnivorous population. Even if dinosaurs were not fully endothermic we do understand their physiology enough to realise that this general rule of thumb would probably apply to large theropods but perhaps on a slightly lesser scale.
Allowing for sampling bias and other taphonomic distortions the fossil record for large theropods in various ecosystems tend to support this equation and that they are generally found in low population densities and would therefore have required large territories. So what were the implications for ecosystems if a top carnivore declined in numbers on the road to extinction?

Because of their limited numbers, any decline in large sized theropod populations would have been keenly felt – sooner rather than later. Their numbers would have become increasingly scattered thereby decreasing the likelihood of intraspecific encounters with others of their own species which, in turn, greatly decreased their chances of a successful mating. This leads to an accelerated rate of decline as the animals that remain may inbreed leading to further genetic complications and disease. This whole process would likely have been extremely rapid leading only to the one conclusion – extinction.  
As the large theropod population declined it is certain that the population of their main prey animals would boom and the effects on the flora and fauna would be dramatic. Plants and trees would be seriously compromised and degraded due to increased and unregulated grazing. The unchecked herbivorous dinosaurs, without fear of predation, would have also probably increased their own ranges which, in turn, would have enabled them to perhaps exploit other food sources leading to yet more environmental consequences for other species. So many things would have changed including those that would even seem to be relatively mundane such as longer feeding spells at different feeding times and, of course, the sizes of the groups and herds as they got bigger.
Would the disappearance of the giant apex theropods affect their smaller brethren?   The answer to this is an uncompromising yes. Not only did the giants keep the herbivorous population in check but it is very likely that their presence kept the numbers of their smaller carnivorous cousins under control. Released of these shackles then small carnivorous theropods may have also gone through something of a population boom which, naturally enough, would have increased predation of other taxa thus increasing further the pressure on the ecosystem. These smaller prey animals are often essential parts of ecological stability that filter right through to the bottom of the trophic ladder.
And just as the herbivorous dinosaurs behaviour would have changed so the behaviour of the smaller theropods would have changed too. It is fairly certain that the small theropod population would have been fearful in the presence of their larger cousins and very likely kept their distance. Freed of this fear factor small theropods would also probably have changed their behavioural patterns and this would also have added yet more pressure to an already distressed ecosystem.
So we can see how the sudden loss of a group of large apex theropods could possibly be the spark that leads to the degradation of an ecosystem which affects every kind of plant and animal on the trophic ladder. Understanding the trophic ladder is key to understanding ancient ecosystems and it is this realisation that everything is linked together from top to bottom that helps us see life for what it was – and is.
So perhaps you may be thinking that there may be something to all this - and I believe there is. Certainly there is every reason to believe that localised extinction events and regular faunal turnover were almost certainly driven when large theropod carnivores came under pressure and started to disappear. There are a number of factors that may have led to large theropod extinction and these include, for example, disease and environmental stress.    
And yet, as I have alluded to before, dinosaurian ecosystems represent something of a conundrum in as much that a quite a few of them now seem almost impossible to have functioned efficiently – and yet they obviously did. In the final part of this mini-series we will try and work out some of the permutations that led to the success of these ecosystems, try to understand the apparent high faunal turnover levels and why sympatry in large theropods is still kind of weird.

10 comments:

Alessio said...

Very interesting; by the way, what do you think about the possibility that some theropods could have implemented plant material in their diet, sort of like modern felids, wolves and bears do?

Andrea Cau said...

Hi Mark,
an important ecological role of large-bodied theropods is also due to their multi-niche life-histories. Contrary to mammals, where the juveniles are ecologically linked to the adults due to lactation and intensive parental cares, theropods show evidence that each life phase played a different and distinct ecological role. Accordingly, large-bodied theropods played the largest spectrum of ecologies among predatory vertebrates known. Thus, juveniles of large-bodied species played part of the roles usually seen in small-bodied species, and probably were direct competitors of them. This means that the exctintion of a large-bodied theropod was very relevant since every large-bodied species directly acted along all the predatory ecological roles of their environment, from turkey-sized insectivorous (played by early juveniles) to small-vertebrate predators (played by older juveniles) to mid-sized predators (played by subadults) to giant-sized superpredators (played by adults).
A single large-bodied theropod species played all the roles we usually consider played by several distinct species of different body sizes: every discussion on large theropod ecological roles must include this very relevant factor.

As noted in the post, hypercarnivory does not exclude inclusion of vegetable in the diet. Although in popular literature the carnivorous vertebrates are depicted as exclusively eating animals, it's just a myth. Not only mammals, also predatory reptiles are known to eat vegetables as part of the diet.
For example, crocodiles eat fruits and this is not occasional but a possibly relevant ecological role in the dispersal of the seeds:
S. G. Platt, R. M. Elsey, H. Liu, T. R. Rainwater, J. C. Nifong, A. E. Rosenblatt, M. R. Heithaus, F. J. Mazzotti. Frugivory and seed dispersal by crocodilians: an overlooked form of saurochory? Journal of Zoology, 2013; DOI: 10.1111/jzo.12052

Mark Wildman said...

Hi Andrea,

That's a teriffic point about large theropods filling multiple niches throughout their ontogeny. This actually makes the issue of sympatric large theropods even more of a fascinating yet perplexing issue.

@ Alessio
As you saw in the post, and from Andrea's comments above, it is entirely possible, perhaps likely, that theropods may have supplemented their diets with fruit or similar matter. The point I was making is that they appear completely unsuited to be able to feed effectively or process such matter in their digestive tracts. But since crocodiles eat fruit then it appears it may not seem to be so unreasonable after all.

Alessio said...

Thanks for the answer!
As for the niche partitioning subjest, well, it could have worked for some theropods, but i'd play it safe and would not apply to ALL theropod clades; there's evidence many theropods were already well suited for catching their own meal since their earliest years, granted, but this doesn't automatically means they went on their own completely abandoning the possible shelter their parents could provide... Chickens are already "full functional" as hatchlings yet they stay close to their mother for some time, the same can be said with ostriches and cassowaries; not saying that Andrea's view is wrong, far from it, but i think the niche partition card could not be the only answer to resolve the issue of theropod symphatry.

Andrea Cau said...

Did I wrote that niche partitioning is the "only answer"? Please, re-read carefully my words...
Also, remember that ecology is linked to biology (that is, morphology, life-history, and physiology) not just phylogenetic proximity: you cannot mention a modern bird as good analogy of a non-avian taxon if the former lacked a good series of comparable features known in the theropod. The mention of modern birds is not a valid argument here, since it needs and assumes a series of features that we know Mesozoic theropods lack.
1- Neornithines show the largest egg size compared to adult body size and the lowest number of eggs for clutch among theropods. This means they show the most extreme K-selection among theropods, thus we may expect that all known non-avian theropods were more r-selected (thus, in a rough way, more 'reptilian' in reproductive strategy) than all modern birds (including galliforms and ratites, that are the least K-selected birds).
2- Compared to birds, all juvenile theropods we know usually are much much smaller than their adults (in particular, since we are talking about large taxa, a few kg against some tons), show different snouts, and had a different dentition than the adults (these differences are the reason "Nanotyrannus" was considered a different species than Tyrannosaurus due to skull proportions and dentition morphology): these marked differences in size, snouts and dentitions mean that juveniles and adults ate different prey and lived in different environmental contests. This is further supported by their r-selected regime, which means precociality, thus indipendence from adults. Since they were precocial and "fully armed", and had cranio-dental distinctions, it is nonsense to believe that a juvenile allosaur or tyrannosaur needed the adult help to kill small reptiles, mammals or insects. Given that a fully grown theropod was much larger, with different dentition and much slower than juveniles, it seems poorly plausible that it was adapted to kill the usual prey of juveniles. Do you believe adult T. rex killed small insects or mammals for its juveniles? Do you a chicken-sized young allosaur with its tiny teeth and gracile jaws was able to eat the flesh, skin and bones of a dead sauropod?
3- Modern birds like galliforms and ratites do not show the enormous size difference and skull difference than those present among large theropods. In fact, both adult and juveniles in modern birds eat the same kind of food (this is particularly true for predatory birds, since adults are the source of juvenile food: this parental strategy is absent - and very unlikely - for non-avian theropods).
Again, what we see in modern reptiles (varanids are a good example) is the best analogy for Mesozoic theropod ecology: size and ecological partition among different age classes. Large theropods in fact are just the extreme trend of what we see in modern reptiles in term of size and ecology disparity among ontogenetic stages. Mentioning the highly derived modern birds is just an error of extrapolating the avian autapomorphies into their Mesozoic relatives.
The best approach is to assume that non-avian theropods showed the ecological features shared by the diapsids (including birds), not to assume the unusual and peculiar specialisation of a highly derived branch (avians).

raptor_044 said...

@Everyone

The stuff about fruit-eating crocs reminded me of Komodo dragons & how they seemingly go out of their way to avoid the vegetable contents of their prey ("No matter how hungry a dragon might get, it will never resort to eating anything other than meat. In fact the dragons have such an aversion to all vegetable matter that that they will violently shake out the stomach and intestine contents of their vegetarian prey before devouring it": http://www.seatrekbali.com/blog/item/253-fun-frightening-facts-about-the-world-s-largest-lizard-the-komodo-dragon ). Makes me wonder whether other large terrestrial hypercarnivores do anything similar & whether large theropods did anything similar.

@Mark Wildman

"It is certain that the majority of carnivorous theropod dinosaurs could at least be termed as hypercarnivorous – that is their diet would have consisted of at least 70% meat and it is fair to say that the majority of stereotypical theropod teeth are blade like and serrated for exactly this kind of diet."

Correct me if I'm wrong, but I thought it was >95% ( http://dml.cmnh.org/1994Nov/msg00148.html ). Just making sure.

@Alessio

"As for the niche partitioning subjest, well, it could have worked for some theropods, but i'd play it safe and would not apply to ALL theropod clades; there's evidence many theropods were already well suited for catching their own meal since their earliest years, granted, but this doesn't automatically means they went on their own completely abandoning the possible shelter their parents could provide..."

I concur to a certain extent. I wouldn't say "many" b/c, based on what I've read (E.g. Currie et al 1990; Sweetman, 2004), most young theropods have the same teeth as the adults & thus probably ate the same things. In other words, most young theropods were probably shown/brought food by the adults (How else could they have eaten prey so much larger than themselves?). Bakker & Bir 2004 in particular shows that there's good evidence for parental feeding in at least some large theropods (Allosaurids more so than ceratosaurids). If you don't have access to the paper, then Levin's "Dino Family Values" ( http://discovermagazine.com/2003/jun/cover#.Ul-CWDnhA4Y ) is a good summary. Young tyrannosaurids are the exceptions b/c they have different teeth from the adults & thus probably ate different things. That combined with the tyrannosaurid bonebeds reminds me of young crocs (which hunt for themselves with parental supervision). I'm not sure about coelophysoids or megalosauroids (excluding Sciurumimus b/c it's probably a coelurosaur), given the lack of info. It's also worth mentioning that, based on what I've read (E.g. Horner 2002; Breithaupt et al. 2004), there's good evidence for semi-precociality (See "Semi-precocial": http://www.stanford.edu/group/stanfordbirds/text/essays/Precocial_and_Altricial.html ) in at least some small to medium-sized tetanurans.

-Hadiaz

Mark Wildman said...

Hypercarnivory, by definition, is indicative of an animal whose diet comprises of more than 70% meat (apparently). Most references appear to support this line of thought - even the almighty (*cough*) Wikipedia ( http://en.wikipedia.org/wiki/Hypercarnivore ).

My thoughts on niche partitioning are in part three of this series just posted.

Andrea Cau said...

@Hadiaz: "most young theropods have the same teeth as the adults & thus probably ate the same things. In other words, most young theropods were probably shown/brought food by the adults".

Sorry, but this is not a logical argument. With the "same teeth" term do you mean that the teeth were the same size? It is obviously not, I hope. Note that most theropod teeth are ziphodont: generalistic carnivorous teeth, that provide very few information on the diet, aside carnivory. The fact the teeth share the same shape means that both sliced flesh the same way, BUT it does not mean that they ate the same things. Differently-sized teeth of same shape performed differently on flesh tissue, due to allometric and mechanical effects. In short, small bodied ziphodont animals killed small-bodied prey, large bodied ziphodont animals killed big prey.
The rest of your argument, in particular the etological implication, is purely arbitrary and conjectural.

Also, Bakker and Bir (2004) interpretation of the Como Bluff association is a very speculative and poorly supported hypothesis, which is very bad from a taphonomic point of view. In facts, that interpretation has been almost completely ignored by the paleontologists.
I strongly suggest you to read this as an example of how a taphonomic investigation of a theropod bonebed has to be performed:
Gates T.A., 2005. The Late Jurassic Cleveland-Lloyd Dinosaur Quarry as a Drought-Induced Assemblage Palaios 20(4):363-375.

raptor_044 said...

@Andrea Cau

I just want to clear some things up.

"Sorry, but this is not a logical argument...The rest of your argument, in particular the etological implication, is purely arbitrary and conjectural."

I respectfully disagree b/c, based on what I've read both independent (E.g. The "Our Wild World" series) & dependent of dinos (E.g. See the Bakker quote in my next comment), that's how it works with living predators.

"With the "same teeth" term do you mean that the teeth were the same size?"

I'm referring to the fact that young theropod teeth "are miniature replicas of the adult teeth."

"Also, Bakker and Bir (2004) interpretation of the Como Bluff association is a very speculative and poorly supported hypothesis, which is very bad from a taphonomic point of view. In facts, that interpretation has been almost completely ignored by the paleontologists."

I get that anything about dino behavior is controversial, but that last sentence in particular is very misleading (if not just plain wrong). In fact, there are several references to Bakker's allosaur-related conclusions in both the popular (E.g. "Walking With Dinosaurs: The Evidence"; Chapter 5 of "The Scientific American Book of Dinosaurs") & technical literature (E.g. Chapters 4, 25, & 26 of "The Dinosauria"; Chapter 10 of "The Carnivorous Dinosaurs"; "Jurassic West"), most of which are either positive (E.g. Both popular examples & chapter 25) or neutral. The only negative references I know of are a paper by a BANDit supporter & a paper in which the authors either didn't do their research or ignored their research completely. This is for good reason, given the evidence (E.g. The discovery of bones marked by/associated with teeth from both young & adult allosaurs, but no other predators; Said bones being "from giant herbivores' meaty parts"; Said bones being found in fine-grained sediments).

-Hadiaz

raptor_044 said...

Quoting Bakker ( http://www.amazon.com/Dinofest-International-Proceedings-Symposium-University/dp/0935868941/ref=sr_1_1?s=books&ie=UTF8&qid=1358800950&sr=1-1 ): "A striking difference exists in modern communities between cold-blooded predators and hot-blooded predators. Most bird and mammal species feed their young until the youngsters are almost full size; then and only then do the young set out to hunt on their own. Consequently, the very young mammals and birds do not chose food items independently of the parents. Young lions and eagles feed on parts of carcasses from relatively large prey killed by the parents. Most snakes, lizards, and turtles do not feed the young after birth, and the new-born reptiles must find prey suitably diminutive to fit the size of the baby reptilian jaws and teeth. A single individual lizard during its lifetime usually feeds over a much wider size range of prey than a single individual weasel or hawk, because the lizard begins its life hunting independently.
Therefore, a predatory guild of three lizard species with adult weights 10g, 100g and 1000g would require a much wider range of prey size than a guild of three mammal predator species with the same adult weights. If allosaurs had a lizard-like parental behavior, then each individual allosaur would require a wide size range in prey as it grew up. The evidence of the Como lair sites strongly suggests that the dinosaur predatory guild was constructed more like that of hot-blooded carnivores than that of lizards or snakes.
This theory receives support from the shape of the baby allosaur teeth. In many cold-blooded reptilian predators today, the crown shape in the very young is quite different from the adult crown shape. For example, hatchling alligators have the same number of tooth sockets in each jaw as do the adults, but the hatchling crowns are very much sharper and more delicate. In the hatchling all the teeth are nearly the same shape, and the young gators have less differentiation of crown size and shape along the tooth row; the hatchlings lack the massive, projecting canine teeth and the very broad, acorn-shaped posterior crowns of the adults. Young gators feed extensively on water insects, and the sharp crowns are designed for such insectivorous habits. Adult gator species use their canine teeth for killing large prey, such as deer, and employ the acorn crowns to crush large water snails and turtles (Chabreck, 1971; Delaney and Abercrombie, 1986; McNease and Joanen, 1977; Web et al, 1987).
If allosaur hatchlings fed independent of adults, I would not expect the hatchling tooth crowns to be the same over-all shape as that of the adult. However, the over-all tooth crown shape in the tiniest allosaur IS identical to that of the adult (figs. 3,4). Thus it appears that hatchlings were feeding on prey tissue of the same general texture and consistency as that fed upon by adults."

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