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.