We celebrate Endangered Species Day on May 18th. We have reasons to celebrate because though human activities have many species on the brink of annihilation, there is serious commitment ranging from individuals to nations states to both protect those species, and to bring them back from the brink. Nevertheless, the threats to species are growing in number and severity. The following essay will not be cheery, and I hope to convince you that avoiding extinction is a very difficult problem and its consequences are severe. I will therefore begin with the optimistic message: The fact that we humans are the cause of the current species crisis is reason to be hopeful. We cannot save species from devastating physical events such as asteroids and volcanoes, but we can save them from ourselves.
It is well known, based on the fossil record, that the majority of species that have ever existed are now extinct. That's why you will often hear the phrases, "99% of all species that have ever existed are extinct", and "extinction is the fate of all species". Let's examine these statements for a moment. The first is a somewhat factual measure. No one knows if 99% is the correct figure, but we do know that most species that have ever evolved are now extinct. The reason that the Earth is still teeming with millions of species is, of course, because new ones evolve all the time. The second statement is a bit more problematic though; it's an assumption, not an observation. To scientifically predict the fate of a thing is to presume that we know and understand all the forces controlling it, and that we know what those forces will do in the future. That of course is certainly not the case here. Even more problematic is the fact that even though changing conditions might cause the extinction of a species, changing conditions also drive evolution! In a sense then, extinction is an evolutionary failure. Okay, it's not quite that simple, but here's what I mean. Under what circumstances does evolution fail and lead to extinction?
Such cool headgear (Wikipedia)
Imagine that you are lucky enough to be a spectacular dinosaur living somewhere on the planet 65 million years ago (some of you know what's coming...). You are the culmination of archosaurian evolution which got started some 175 million years before, in the wake of the devastating end Permian mass extinction. Unfortunately for you, today is the day that a giant asteroid from outer space collides with the Earth somewhere in the vicinity of today's Yucatan Peninsula. The energy released by the collision is some 19,000 times greater than the explosive force of the world's current nuclear weapons arsenal. The survival of your species depends on enough of its members surviving the ensuing environmental catastrophe, and that could happen in two ways. First, you could have the individual capacity to acclimatize to the changes happening around you. For example, it's possible that many animals survived by taking advantage of underground dwellings, or their abilities to enter into some sort of resting phase. Second, your species could adapt to the changes. In either case, evolution is at work. The capacity to acclimatize is generally a function of the physiology, behaviors and so on that evolved in your ancestry, while adaptation is the result of current genetic variation and natural selection. Extinction will occur if the magnitude or severity of the environmental changes overwhelm your capacity to acclimatize, or your species's capacity to adapt (e.g. limited genetic variation) or the rate at which it can adapt. Sadly, non-avian dinosaurs neither acclimatized nor adapted, and today exist only as fossils or in the movies. This has happened repeatedly during the history of life. The end Permian mass extinction of 251 million years ago resulted from overwhelming changes of climate, ocean conditions and atmospheric composition. Ultimately, it was driven by massive volcanism in the Siberian region. The mass extinction 65 million years ago was also the result of significant changes in climate, driven by massive volcanism in India's Deccan region, coupled with that rock from outer space. That collision would have heated large regions of the North American continent, darkened the skies for months, and subsequently cooled the planet for years.
Today the Earth is on the brink of another mass extinction, but this time we humans are the volcanoes and asteroids. Driven by an exploding population and rapidly increasing rates of resource consumption, we are sequestering landscapes and habitats or destroying them outright, over-exploiting wild species, and changing the climate at rates that overwhelm the ability of species to acclimatize or adapt. Climate change is by far the most dangerous of the bunch because while we can confer protection from exploitation on landscapes and species, we cannot protect them from changing temperatures, seasons, patterns of rainfall and ocean acidification. Even as we work to curb our climate-changing habits by developing alternatives to fossil fuels, engineering more efficient transportation systems and planning our own socio-economic adaptations, species must also acclimatize and adapt. This will be accomplished either by species movement to track favourable climates, or staying put and acclimatizing and adapting. Unfortunately, there are severe limitations to each.
There is no doubt that many species are now on the move in response to climate change. Most pronounced are expanding ranges of many tropical or warm temperate species as tropical air and water temperatures expand outward from the equator. Examples include the Humboldt squid, normally restricted to waters of the tropical and south eastern Pacific, but is now a frequent and abundant species in waters as far north as Alaska. The increasing incidence of normally tropical diseases such as West Nile virus are also testimony to increasingly favourable conditions in regions that were previously too cool. Habitat expansion for those species occurs at the expense of other habitats of course. Regions of cooler temperatures are shrinking, leaving no room for migration of the species there. In alpine regions many species are moving upward, but of course can go no higher than the highest mountains. Polar species are not only faced with changing habitats, such as the drastic reductions of summer ice coverage in the Arctic Ocean, but must deal with the newcomers from warmer regions. And therein lies the another limitation. Given the rate at which conditions are changing, on a timescale of decades, there is absolutely no guarantee that a species will have the genetic capacity to evolve and adapt rapidly enough to survive. For scientists, the answer will in many cases be a wait and see experiment. The results will no doubt be intriguing and valuable, but that will be small comfort for any species that come up short.
There is a final limitation, and that's based on relationships. No species exists in isolation, humans included. Every species interacts with other species and is dependent on other species for survival. That's how we evolved! Species adapt to changing conditions, and in turn their new adaptations alter the world around them. We are united in a gigantic global network of biological relationships which includes production, predation, competition, parasitism, reproductive services, habitat engineering, and recycling. When species move, when they change, some of those relationships are disrupted. Even now we see the synchrony of spring flowerings, fruitings, nesting etc. becoming de-synchronized. And these relationships are not formed on the fly! They are the evolutionary result of species interactions occurring over time, and indeed the systems or networks that they produce are likewise products of evolution. In studies by myself and colleagues (see Further Resources below) of ecosystems in the wake of the end Permian mass extinction, we found that while the number of species recovered very quickly within a million years of the extinction, the networks of relationships so formed were unstable and weak. It took several million more years before system robustness recovered to pre-extinction levels. And during that time, the dominant vertebrates of the land, the ancient relatives of humans and other mammals, were eclipsed by the rise of the dinosaurs. They would have to wait another 175 million years before another opportunity for dominance would present itself. Those timescales would try the most patient of humans.
So what do we do? I believe that we must become better stewards of the planet. I know that some will argue that we should not be stewards at all, but consider this. Humans already occupy to varying extents some 48% of the planet's dry surface and we directly utilize or otherwise co-opt 24-41% of global photosynthetic production. We are already stewards of the planet! The real problem is that we are rather poor stewards. We either absolve ourselves collectively of this responsibility or we embrace it. The choice is ours, and the fates of an uncounted number of species now depend on us. Therefore, let us indeed celebrate Endangered Species Day, for while it is a reminder of the dire state of our environment, it also recognizes our acceptance of an awesome responsibility.
Further Resources
Some of these are fairly technical (sorry!), but please feel free to submit any and all questions.
- Roopnarine, P. D. 2013. Ecology and the Tragedy of the Commons. Sustainability 5:749-773.
- Roopnarine, P. D. and K. D. Angielczyk. 2012.The evolutionary palaeoecology of species and the tragedy of the commons. Biology Letters 8:147-150. DOI:10.1098/rsbl.2011.0662
- Barnosky, A. et al. 2012. Approaching a state-shift in Earth's biosphere. Nature 486:52-58.
- Mitchell, J. S., P. D. Roopnarine and K. D. Angielczyk. 2012. Late Cretaceous restructuring of terrestrial communities facilitated the End-Cretaceous mass extinction in North America. Proceedings of the National Academy of Sciences 109:18857-18861. DOI:10.1073/pnas.1202196109.
- Call of Life