I’ve just handed in the first draft of my dissertation (the final version is due in mid-January). As much as is possible with a large and stressful piece of coursework, I’d say I enjoyed writing it. I was lucky enough to be able to do it on a topic that really interests me – the link between biological diversity and ecosystem functioning.
This is an area that has received growing attention in the last couple of decades, since the global decline in biodiversity was recognised. Earlier ecologists had looked at biodiversity to try and answer why we have it: why are there so many species? How do they coexist? What environmental and ecological factors maintain diverse communities? And over the longer term, how does diversity evolve? Why do species split and give rise new species?
But the recognition that biodiversity was under threat suggested a new question. What would happen to ecosystems if species were lost? Instead of asking what causes diversity, ecologists started to wonder what would happen if we lost that diversity.
Answering this question requires an understanding of what species do. What function do they perform? Living things can be categorised into functional groups. For example, the objects of my practical project are the detritivores. These are organisms which facilitate the breakdown of organic matter, helping to recycle nutrients back into the soil. Their function is decomposition. Another functional group is the pollinating insects, such as the bees, which have received a much attention in recent years due to worrying losses of honeybees which are used to pollinate many commercial crops. The much quoted statement (allegedly from Einstein) sums up the potential importance of such a functional group:
“If the bee disappears from the surface of the earth, man would have no more than four years to live.”
Bees pollinate flowers. Pollinated flowers bear fruit. Without bees, crops might fail.
We can assume that the “pollinators” are an important functional group. Having no pollinators at all would surely cause problems. But how important is diversity within that group? Could honeybees do everything, or do we need lots of species of bees and other insects?
At the heart of the diversity-ecosystem function debate is the question of redundancy. The redundant hypothesis suggests that in any functional group there are probably several species that do the same thing, perform the same role. If true, then perhaps we could safely lose some of these species without any loss of function. However there is likely to be a certain minimum diversity level beyond which function will decline. How many species is this? Five? One? We do not know, and it is likely to vary widely for different ecosystems, different functions and different species.
Contrasting with the redundant hypotheses is the “rivet” hypothesis. The rivet hypothesis gets its name from an analogy with the rivets holding together the body of an aeroplane: the builders of the aeroplane would have planned for a degree of redundancy. It might be safe to lose a few rivets, but there will be a point where the integrity of the aeroplane breaks down and fails. The problem is, that when it comes to ecosystems and species, we don’t know the safety limit. We’re not sure how many rivets it is safe to lose, or if it is safe to lose any at all. An extreme form of the rivet hypothesis suggests there is no redundancy: every species lost is important and will have a detrimental effect on ecosystem function.
At heart, the rivet and redundant hypotheses are similar. Both suggest caution – we simply do not know how many species it is “safe” to lose, if any.
The authors all write as good scientists too, concentrating on the facts and what is practical, and leaving aside the wider ethical implications of species loss. The functional debate can be a way of highlighting the importance of species loss to groups who might not otherwise acknowledge its importance: it brings the debate about species loss into the sphere of human concern. How does the loss of a species affect me? Will I have less food to eat? Will I have to use more fertiliser to improve soils that lack detritivores?
It’s an approach that needs to be taken with caution and always placed into the wider context of man’s relationship to the natural world. From an ecologist’s point of view it is a fascinating approach, combining approaches from previously separate fields of ecology such as population biology and community ecology. Exploring the questions requires a deep understanding of individual species’ roles, which can only be gained with thorough and attentive field work of the sort which is threatened with becoming unfashionable; but the complexity of the question also requires complex theoretical models that can approximate the tangled systems and feedbacks within real-world ecosystems comprised of tens or hundreds of species.
Although much progress has been made in the last twenty years, much more needs to be done. One thing seems certain: we should proceed with caution, lest we end up playing Russian roulette with species loss.
Some further reading:
Ehrlich, P. R. and Ehrlich, A. H. (1981) Extinction. The causes and consequences of the disappearance of species., New York: Random House.
Ehrlich, P. and Walker, B. (1998) ‘Rivets and redundancy’, Bioscience, 48(5), 387-387.
Lawton, J. H. (1994) ‘What do species do in ecosystems?’, Oikos, 71(3), 367-374.
Walker, B. H. (1992) ‘Biodiversity and ecological redundancy’, Conservation Biology, 6(1), 18-23.
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