Understanding the long history of biodiversity helps us to appreciate the immense value of the biosphere we now dominate, writes Victoria University’s James Crampton
There has been much publicity about the search for extra-terrestrial life; most recently, it has been suggested the sub-surface ocean of Saturn’s moon Enceladus could, perhaps, harbour life. Indeed, the question “is there life out there?” captures the imagination like no other, and has been the subject of countless movies, songs, books and nightmares — what would Doctor Who be without a few Daleks trundling in the background?
Despite all this, however, Earth remains the only place to host life as far as we know. Life of spectacular, wonderful, colourful, noisy, smelly, tasty, tactile richness and variety.
The history of this life is documented in a sort of ‘diary’. Admittedly, some of it is written in code and many pages are damaged or missing, but it is a remarkable diary. I refer, of course, to the fossil record of life studied by palaeontologists.
We know from this record that simple, single-celled life forms have been around for most of the 4.6-billion-year history of planet Earth, but complex life has ‘only’ been around for about 600 million years, give or take. Even today, in terms of numbers of individuals, life is dominated by single-celled organisms — for every human cell in your body there are about 10 microbes living in and on you. So, given most life on Earth for most of its history has been microbial, we might expect extra-terrestrial life, should we ever find it, will be microscopic. This makes our rain forests and coral reefs seem even more special.
One key task of palaeontology has been to try to figure out exactly when and why complex life and ecosystems evolved, and to understand what controls the numbers of species that can occupy our small planet. Is the number of species living at any time — biodiversity — controlled mainly by chance, by interactions between species such as predation, parasitism and competition for resources, or by environmental factors such as climate or continental configuration? Is there a limit to the number of species Earth can house? Given enough time, will species’ ecological niches become more and more specialised, so ever more forms can be packed into the same space? Or is there a fixed carrying capacity of species?
Biologists are interested in such problems and can design short-term experiments with living organisms to tackle some of these questions. Palaeontologists, however, can also look at patterns of changing fossil biodiversity over time, patterns that record a whole suite of natural biodiversity ‘experiments’. Such paleontological studies are becoming increasingly powerful as sophisticated numerical analyses are applied to large databases of fossil occurrences. These databases include the New Zealand Fossil Record File — a compilation unrivalled in any other country — and the international Paleobiology Database.
Using these tools, we can now tease out and correct for the effects of imperfections in the fossil record — the damaged and missing pages in Earth’s diary mentioned above — to reveal true signals of biodiversity change. Three patterns are proving particularly interesting.
First, palaeontologists are gaining an ever more nuanced view of the mass extinctions that punctuate geological history. These short intervals of catastrophic biodiversity loss are highly relevant today because many scientists have suggested we are currently entering a human-caused global mass extinction. It seems there is no single, simple, cause for the major past mass extinctions, which instead may record unfortunate coincidences of ’bad‘ things that cause runaway chain reactions of ecosystem collapse. Prior to the present day, the most recent large mass extinction was 66 million years ago, when the impact of a 10km-diameter asteroid occurred in concert with vast volcanic eruptions to cause the extinction of 75 percent of species, including all the non-bird dinosaurs. This illustrates the role of rare chance events in biodiversity history, events that may in fact change the entire course of evolution on the planet — without that asteroid, perhaps this article would have been written by a reptile.
Second, we know biodiversity is controlled by interactions between species, and this suggests there is an upper limit on the number of species that can coexist, a fixed carrying capacity. Above this number, intense competition between species will drive some to extinction, bringing the total diversity down. Below this carrying capacity, competition will be relaxed so more species can survive, allowing evolution to bring the total diversity back up. For some intervals of geological time both globally and within New Zealand, it seems biodiversity did indeed remain approximately constant, suggesting it had hit a ceiling controlled by interactions between species. At other times, notably approaching the recent, global biodiversity seems to have been increasing seemingly without limit. This may indicate species carrying capacity is now higher than it used to be and the planet has not yet reached this new limit.
Finally, although interactions between species control biodiversity, environmental factors can also have a large influence. After all, we only need to contrast the enormous diversities found today in warm tropical jungles and on coral reefs with the more modest species numbers seen in cold polar regions. So, all else being equal, we might think variations in climate and associated environmental variables will have controlled biodiversity over geological time. Very recent work confirms this. In fact, research by myself and colleagues has shown that regular fluctuations in global climate on time scales of 2 million years and shorter were significantly controlling biodiversity between 400 and 500 million years ago. These fluctuations relate to wobbles in Earth’s orbit driven by the physics of the solar system — biodiversity on Earth is affected by the motion of Saturn!
To summarise, the history of biodiversity on Earth reflects the interplay of limits set by biological interactions, changing environmental conditions, and chance, catastrophic events. A task of palaeontology is to quantify the relative importance of these different factors, and to clarify how they shape the biodiversity we see today and how they might shape life in the future. Understanding the long history of biodiversity helps us to appreciate the staggering value of the biosphere we now dominate, which is, to the best of our knowledge, unique in the universe.
James Crampton gives his inaugural public lecture as a professor at Victoria University of Wellington on Tuesday 30 May.