Friday, September 14, 2012

This Island Earth Part IIIc: Theropod Wars

So you may have noticed it has got a little toothy around here as of late... what with all the talk about sharks, top down control, intraguild predation, theropods of Kem Kem- I thought this was a blog about plants man. What all these posts are building up to is this critical installment of the This Island Earth series. Basically what I am trying to build a case for in the This Island Earth series is that the dinosaurian megaherbivores, despite their larger size, were actually less stressful on their respective vegetative communities than modern mammalian herbivores. If you want to review my argument points so far- go ahead and do it at my rundown here. But basically what I have argued so far is that dinosaurian herbivores, despite their absolute larger size, were not as adapted to the extent of large mammals as "whole plant predators" and to take the gardening analogy further- more like "pruners than chainsaws". But now back to meat eating...

Recent years have seen a revival of the importance of diverse predators, and especially top predators, in the health, vitality, and diversity of the earth's ecosystems. Not only do top predators keep down the numbers of other animals and prevent excessive overbrowsing and loss of microhabitats, but they also shape the ecosystem just by being around. While I went through some of the main points in top predator benefits in shark bay describing how tiger sharks maintain the seagrass banks, other studies are pointing to even more intricate and profound effects of top predators on ecosystem health.

Grasshoppers frightened by spiders affect whole ecosystem. Very interesting study. Basically researchers introduced "defanged" (harmless) spiders into enclosures with grasshoppers. Although the spiders could not predate on the grasshoppers directly the grasshoppers nevertheless went into a "fear mode" and concentrated on more carbohydrate rich parts of the plants. The "scared" grasshoppers were in a sense carb-loading (a little like stressed people?). Control studies of grasshoppers without spiders were more nitrogen enriched- they were taking in more amines/amides- basically concentrating on parts of the plant with more proteins.  Soil microbes, responsible for the transfer of nutrients from dead organics to plants, work at a higher pace with abundant nitrogen. Both the feces and the dead bodies of the "scared" grasshoppers showed a slower turnover rate of nutrients through soil microbes than the control group which was N-enriched. That is not to suggest the speedy turnover rate of the control group is more desirable- in fact quite the opposite- slow release of nutrients to plants is much more ideal in the longrun. What the study explicitly demonstrates is that predators have unexpected, and often dramatic, roles in their ecosystems.

All right, so once we realize the importance of predators in modern ecosystems it is not a stretch to imagine that these same ecological principals of "top down control" were also at play in the earth's past ecosystems. But all intact ecosystems (modern and ancient) maintain a dynamic equilibrium of top down (predators, parasites, virus, pathogens) and bottom up ( abiotic factors, rain, temp, plant availability, nutrient in soil) controls. What is a challenge to decode is the relative importance of each. Teasing apart these intricacies is hard enough in modern ecosystems- where the stuff is still alive- but trying to argue the relative importance of these controls in deep time is even more of a challenge, many would argue beyond the limit of our dataset...but let's go there anyways.

I am going to argue that theropods, as well as other archosaurian predators, exerted strong top down control on their respective Mesozoic faunas. This top down control was pervasive and systemic to dinosaurian faunas and on a level beyond that of modern mammalian dominated communities. The net effect of this top down control was to maintain biodiversity and, especially, floral diversity and health. Archosaurian predators were the "guardians of the Mesozoic garden" and kept plant predators at bay by not only active predation, but by maintaining a "climate of fear" that kept plant predators slightly stressed and always on the move.

Herrerasaurus and Eoraptor. wiki

Let us first look at how dinosaur reproductive physiology "set the table" (pun intended) so to speak for not only dinosaur success- but theropod success. It is now widely regarded that dinosaurs were R-strategist reproducers, as opposed to most large mammals which are classic K-strategist. R-strategist species are characterized by high reproductive output, small offspring, minimal to nonexistent parental investment in the offspring, independent offspring, and relatively early maturation and are not especially long lived. Generally speaking these species are "weedy" in the sense that they can quickly proliferate in disturbed ecosystems, especially when not at carrying capacity. K-strategists in contrast have few offspring which they invest tremendous resources into, are generally long lived, and maintain population at or near carrying capacity. Elephants, whales and humans are on the extreme scale of R-strategists all being long lived, large, and invest heavily in the young.

Now following from these differences between dino and mammal reproductive style are several corollaries which are important for future arguments.

1) Immature dinosaurs acted as "separate ecological species" from the adults (Codron, 2012). This creates a dinosaur fauna skewed towards either very large species or relatively small species- but few species in the middle range. In any given dinosaur population one may find a wide spectrum of different age/size groups represented with the population's modal size possibly quite smaller than the full size adult.

2) Unlike mammalian K-strategists, young dinosaurs- due to minimal ecological overlap with their parents- did not compete directly for food. This is opposite from the case with mammals or most modern aves as well- where neonates are directly stealing resources from the parents (i.e. milk and direct provisioning of food stuffs). These are resources the parents could have utilized for growing larger themselves- like dinosaurs did. And when the K-strategist finally weans its offspring they are full or nearly full grown and now direct competitors with the parents in their respective ecosystem.

"Feed Me!"

Alright then, dinosaurs, both predators and herbivores are reproducing at rates relatively faster than most modern birds and mammals. Obviously the vast majority of dinos are not going to make it- where do they go- down the gullet of theropods. A large, constantly replenishing prey base of immature dinos would have provided an easily accessible standing crop of food for theropods.

Like theropods, carnivorous mammals love to go after the young- they are easier to catch and not as dangerous as the adults. But contrast the situation in a mammal dominated ecosystem- for example the Serengeti plains- with what was going on in say a dino dominated ecosystem such as the Morrison. Each year every wildebeest cow has but one offspring- which are heavily sought after by any number of predators. Each year in the Morrison every Camarasaurus may lay several dozens of eggs- which turn into several dozens of baby sauropods- which feed several dozens of various theropod species. It can thus be argued that dinosaur dominated habitats would have provided a lot more easily caught, abundant protein packets in the form of baby dinos than mammal dominated ecosystems with much less abundant, more heavily guarded offspring. This important distinction is why dino habits would have been slightly more enriched in predators than mammal ones. And if you follow my line of thought you should also be reminded of my first Planet Predator where I discussed the hyperabundance of large predators on pristine reefs- central to that seeming ecological anomaly was the rapid reproduction of the reef fish prey base which constantly replenished itself even with 85% predator biomass. Again- I am not suggesting theropods were 85% of the biomass in their ecosystems. But what I would expect is predator prey ratios slightly enriched compared to modern mammalian predator prey ratios.

 And this is actually what we see in many dino habits with sufficient sampling.

A study by Horner found T-Rex to represent about 25% of ALL dino bones in the famous Hell Creek formation. In some locations it was equal or even greater than Edmontosaurus, supposedly one of its main prey species. Likewise in the Morrison, we have super abundant Allosaurus alongside a bevy of smaller and larger theropods. In the book Jurassic West predator biomass is estimated at 8.6%, on the extreme high end for mammalian communities. But the adjusted ratio for large predators is still 7.3%. Keep in mind though that sauropods (73% biomass), being substantially larger, would therefore not have been as numerous as one might expect compared to theropods. Trackway fossils in the Morrison bare this out, with theropod footprints being nearly as numerous as sauropod footprints by their data. Quite simply theropods were possibly only slightly less visible on the Morrison landscape than sauropods (barring grievous preservational bias).

And this is an important point for consideration- it would have been the visual threat of predation as much as the actual predation event itself that may have mattered in terms of how prey moved across the landscape.

We may never know to what extent theropods tackled prey larger than itself. But chances are any dinosaur that makes it to full grown status (predator or herbivore) has felt the pangs of being bit by a theropod several times while growing up. And there is no reason to doubt that the fear invoked while growing up remains in the adult. So if we apply sound ecological observations on the effects predators have on their prey- by predation or simply by presence- ( tiger sharks and dolphins, wolves and elk, spiders and grasshoppers) it is not unreasonable to assume theropods had profound impacts on herbivore movements/browsing intensity. The behavior of the herbivores under predator stress would consequently affect their prey- namely plants. And given theropods were relatively more abundant than mammalian predators (due to the abundance of baby dino apertifs), this "climate of fear" may have been ultimately greater than that in mammal communities.

And so what about corollary #2- that many dino species occupied distinct niches through their growth allomatry and this minimized competition between the adults and immature stages? Well if something wiped out the adult stage- some type of disease, lack of prey- there was always a younger generation ready to fill in the gap. Well this predicts two things in my mind- theropods ( and all dinos really) were very hard to get rid of- and herbivores got not break.

Well I will continue this post later- wrote more than I thought I would for this . Coming up next- theropod feeding envelope buoys up population and more on fishing theropods.



Naish, Darren. Cristina Eisenberg's The Wolf's Tooth: Keystone Predators, Trophic Cascades and Biodiversity. Tet Zoo.

Foster, John. Jurassic West. Indiana University Text, 2002.

Hone, D.W.E. & Rauhut, O.W.M. 2010: Feeding behaviour and bone utilization by theropod dinosaurs.
Lethaia, Vol. 43, pp. 232–244

Daryl Codron, Chris Carbone, Dennis W. H. Müller, and Marcus Clauss. Ontogenetic niche shifts in dinosaurs influenced size, diversity and extinction in terrestrial vertebratesBiology Letters, April 18, 2012 DOI:10.1098/rsbl.2012.0240

Horner JR, Goodwin MB, Myhrvoid N (2011) Dinosaur Census Reveals Abundant Tyrannosaurus and Rare Ontogenetic Stages in the Upper Cretaceous Hell Creek Formation (Maastrichtian), Montana, USA PLoS One 6(2) doi:10.1371/journal.pone.0016574

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