Wednesday, 14 October 2015

Frozen in time: The illusion of stable ecological communities and the geological perspective

Evolution as a process is, at its most basic level, relatively easy to understand: Slow adaptation over many generations favoring forms more well suited to their environment. This is of course so overly simplified as to border on flat out inaccuracy, there are so many complicated processes at play in evolution that one could not possibly do them all justice in a brief summary. The point however stands that the general perception of what evolution broadly speaking is can be defined as an endless process of refinement. We tend to perceive the species that exist today as perfectly adapted to their environments, fully capable of persisting and surviving on their own. After all, if this was not the case, how could they even be here in the first place? This mindset is not necessarily wrong at face value, species must be competitive in order in order to persist. The issue here is the lack of perspective. Species and populations that may seem stable and self-sustaining in the short term may actually be in a state of chronic decline over a longer course of time. The lifespan of an individual person is so fleeting compared to the vastness of deep time, that perceiving such declines is essentially impossible without the benefit of hindsight.

When we look at a pristine forest, an un-managed oldgrowth, we see a vast and complex community of plants and animals, each fulfilling its own role in the ecosystem. Trees of varies species tower above us, often forming dense stands, seemingly impenetrable and ageless. Yet this is an illusion. Pollen cores and fossil evidence clearly shows that forests, and all other ecosystems for that matter, are in a state of constant flux, everchanging, but on a timescale too slow for us to perceive. The communities of trees that a person visiting a Himalayan forest today might see would be almost wholly different from that experienced by a person walking through the same area just a few thousand years prior. But this state of constant change is not just an endless reshuffling, many species once common are now gone, and many that we today would consider "staples" of the forests will no longer be here millennia in the future.

A Himalayan forest, not as unchanging as it may seem

This is not just a matter of local diversity, entire species that are presently highly abundant may very well disappear entirely in the geological near future. Looking back into the fossil record, we see all sorts of examples of this. Temporarily, for but a few million years, a species appears, becomes ubiquitous and abundant, and then suddenly dies out. There was no great calamity, few or no other species were affected, yet one moment they were there, and a million years later they have gone, leaving no descendants. It must be said that we are still talking about what we would consider long spans of time, several million years, but when compared to the history of life on earth, or even the 66 million years since the non-avian dinosaurs died out, it is not very long at all. Allow me to give an example: Barylambda. Living from the Paleocene to Eocene, it was a member of the order pantodonta. Pantodonta itself was quite a successful group, lasting for almost 30 million years, but Barylambda was far more short lived. It first appeared during the late Paleocene of North America, and was gone by the end of the early Eocene, seemingly without descendants. Quite a large animal, around the size of a small cow, it would have been an important component of the ecosystems it inhabited, a keystone species. Yet it did not last, and like 99% of all other things that have ever lived, it died out. 

The key here is not that it went extinct, that was of course inevitable. What is interesting is the seeming lack of a cause. A major climatic event, the Paleocene-Eocene Thermal Maximum, or PETM, was ongoing while Barylambda existed. During this period, the global average temperature was raised by several degrees, and the planet was upwards of 5 degrees warmer than it is today. This did not however have a large effect on the terrestrial ecosystems of the world, as there was no noticeable spike in extinctions at the time, and as such was probably not the cause of Barylambda's demise. It seems, quite simply, that it eventually grew increasingly less abundant, and was slowly replaced by other animals such as Coryphodon. The latter appeared simultaneously with Barylambda, and coexisted with it for millions of years, yet lasted longer, before itself suddenly vanishing. That Coryphodon or any animals like it should have been directly responsible for the disappearance of Barylambda is unlikely, so the only real option we are left with is that slow environmental changes caused a gradual population decrease, while not having the same effect on other animals, eventually culminating in its extinction. 

A pair of Barylambda in a Paleogene forest, by Roman Uchytel

Looking at the more recent past, the Great American Biotic Interchange, or GABI, offered several prime examples of this. The GABI itself is an incredibly interesting and complicated subject that deserves an article all of its own, but for now I will give a brief description: The GABI was an event that took place 3 million years ago, when the isolated island continent of South America finally came into contact with North America, after the creation of the Isthmus of Panama. This land bridge allowed for animals from both continents to freely disperse both north and south, the so-called interchange. It was however a very one-sided event, as the North American faunas were by far the most successful. Many native South American animals were wiped out by the newly arrived competition, as the northern invaders spread throughout the continent, but far fewer succeeded in spreading north themselves. Only a few groups of South American animals ever managed to make it into NA, and of those that did, the vast majority died out shortly after. It is these that we will focus on. Remember that these are species which successfully not only compete and persist, but expand their ranges, who actively adapt and evolve, and had you seen only the first million years or so of the interchange, you would point to as clear winners. Yet once again, without any clear cause, they suddenly died out. 

One particular example of this is Titanis, a large flightless bird of the group phorusrhacidae, better known as "terror birds". It was the only phorusrhacid to make it into North America, and it managed to spread as far as Florida. While it lived, it would certainly have been a fearsome animal, a large and lethal predator, capable of both efficiently hunting and competing with other predators. It disappeared a bit less than a million years after the interchange, long after its South American kin had already died out. Why it survived when its relatives did not is in and of itself a great question, but it also highlights a very important point. Just because an animal is adaptable, just because it is capable of expanding its range, competing, dominating, and evolving, does not mean that it is going to last. Of the other South American animals that spread north, three groups persist today, the armadillos, the opossums, and the porcupines. Today they are generally regarded as winners of the interchange, groups which managed to adapt and spread, but as Titanis has shown, this is no guarantee of persistence. It has still not been very long since the GABI, and it is fully possible that at least some of these South American colonists will eventually die out as did their kin. 

Skeleton of T. walleri, mounted at the Florida Museum Of Natural History

The main take-home from all of this is that nature is not as static nor as stable as we tend to think. It is comforting to believe that while the economy of your country or the politics of your tribe may change, the trees will always remain stable, a constant, but it is simply not the case. So let us not take what we have today for granted, because whatever happens in the future, however our influence on this planet may turn out, this will not last, not forever, maybe not even for very long.

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