Don’t stand so close to me: a new view on how species coexist

Plants and animals living together in communities don’t rub shoulders too closely because evolution has caused them to compromise on key life measures, say ecologists at Imperial College London and Royal Holloway, University of London, writing in the journal Science today (1 October).

The researchers suggest a new basis for explaining how communities of species assemble: they have to give up being good at everything and ’trade off’ their life histories. ’Life histories’ is ecological jargon for the important measures, shaped by evolution, such as how often you can reproduce; how many children you will have; how long you can live for; and crucially, how good you are at getting food on which to survive. “You can’t be good at doing everything,” says Dr Mike Bonsall, a Royal Society University Research Fellow working at Imperial College London, and first author of the paper. “Most people do one thing really well, another thing fairly well and then aren’t very good at anything else. So it is with any other species. Now we know that they coexist precisely because they each have different life histories.”

The London researchers assembled a simple artificial community of parasitoid wasps within a computer model, and then watched what happened over very long periods of time – up to 100,000 generations. Parasitoid wasps, insects that kill other insects by laying eggs in them, account for a fifth of all known multi-celled species. Their 200,000 species places them approximately next to land plants in terms of diversity.

To their surprise they found that over long periods of time, ’gaps’, or differences in their life histories, opened up between the evolving parasitoid wasp species, which are not filled by others. They suggest this may explain the great diversity of wasps seen in nature. “There is a fixed amount of difference necessary,” says Dr Bonsall. “This allows evolution to affect patterns of diversity such as how many similar species we see.”

Dr Vincent Jansen, author of the paper from Royal Holloway, University of London, added: “The bottom line of this work is that patterns of diversity are shaped both by ecology and by evolution.”

One way of capturing the essence of the new work, he added, is expressed in the concept of the ’Darwinian demon’ – a hypothetical species that develops rapidly, reproduces continuously and does not age. Trade offs in life histories are thought to prevent Darwinian demons from evolving. Rather, similar species are allowed to coexist.