Security in being different
(appeared in Oct 2015)

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Alternate routes that lead to the same place may be nature’s way of being sure to get there, , says S.Ananthanarayanan.

A problem in evolutionary biology has been to explain how there could be such a large number different species. While there are forces that limit the length of a food chain, many communities would affect each other by competing for the same resources. It is hence difficult to understand how one level in the food chain could maintain a large number of different communities.

Marten Scheffer, RemiVergnon, Egbert H. van Nes, Jan G.M. Cuppen, Edwin T.H.M. Peeters, RemkoLeijs and Anders N. Nilsson, of Wageningen University in The Nederlands, the University of Umeå, in Sweden and institutes in Adelaide. Australia, report in the journal, PLoS One, their study of the 4,168 species of the diving beetle, which belong the group of animals which is the most species-rich in nature, and show the way of nature to separate groups of species to occupy a habitat and still stay apart in their use of resources.

The foundation of the idea that there cannot be large numbers of species was described in a classic, 1959 paper by the American ecologist G Evelyn Hutchinson, where he suggests that there is a built-in limit,known as ‘limiting similarity’, to how close, biologically, that is, that species can get. This follows from the competitive exclusion principle, that either one of two species which need to use the same resource must overpower the other. When one species has even the slightest advantage or edge over another then the one with the advantage will dominate in the long term, and the other will either go extinct or, unless there are environmental changes, migrate to another ecological niche. But this did not seem to work in practice, as there were clearly not as many separate ecological ‘niches’ as there were species. Furthermore, the principle of competitive exclusion alone, did not prove useful to explain evolution dynamics, as species depend on a brace of resources and it was found difficult to create conditions where the principle could be tested or demonstrated. In contrast is the neutral theory, which suggests that many species can, in fact, co-exist, because they their dependence on the same resources renders them equivalent and hence incapable of overcoming each other. But neutrality also has deficiencies and what is proving more fertile, as a theory, is the possibility of near neutrality, where the pressure of competitive exclusion is weaker and other mechanisms, like population dynamics or environmental conditions,are able to allow species to survive.

Even this, however, is difficult to prove or demonstrate, again as there are often differences in the specific resource complex that species depend on. But the problem becomes easier to address if the dimensions of the pattern in the ecological niche have a gradation, such as the increasing size of food items, where the different, near neutral, resident species differ in size and hence the items of food. Theoretical considerations predict that in such conditions, species would distribute themselves in specific size compartments, with a separation between one compartment and the next.

The principle behind such self-organisation, the PLoS One paper says, was first described by the legendary mathematician Alan Turing, in his paper,’The Chemical Basis of Morphogenesis’. Turing, in this paper, considers a mathematically tractable case of a ring of living cells and shows that with the onset of any instability, the ring may develop a wavy structure which is more persistent than the symmetric or homogenous, initial form. This might explain, for instance, the ‘tentacle pattern on Hydra, or for whorled leaves’, Turing’s paper says. Similar ‘self organising’ is in play, the PLoS One paper says, in the appearance of striped or dappled patterns in animal skin or in the distribution of desert vegetation, in the physical space, or, when there is competition, in clubbing of species in the niche space. Robust ‘self organisation’ of this kind has been shown to exist, analytically and mathematically and is also supported by some field data, the paper says. Nevertheless, the idea of ‘near neutrality’ remains controversial, as it seems to contradict the idea of a species occupying an exclusive ‘niche’. “Indeed, if self-organized similarity shaped much of nature’s diversity we should fundamentally rethink the way we look at the ecological identity of species,” the paper says.

Verification

Prof Marten Scheffer and others made use of massive global data of Dytiscidae, the diving beetle, beetles being the group of animals that have the greatest number of species found in nature. As the species share the environment, their distribution, both in local settings and in settings far removed, offers a view of how species evolve and an opportunity to test the theories that numerous competing species may organise themselves into regularly separated groups. The data was analysed in term of sizes of all known species of the diving beetle in different parts of the world,the pattern in size distribution of co-occurring species of beetles in 1,507 samples collected from ditches in Holland and also body size patterns in groups of species that have evolved independently while they were isolated for five million years in subterraneanaquifers in Australia. Body size was the item of data considered because size would indicate the preferred dimensions of prey, and hence tendency to compete. The data analyses then looked for similarities in the distributions in different groups, with the help of data statistical data processing software.

The result of the data analysis show with statistical rigour that body measurements are clumped around specific sizes, so that there is reduced competition for resources between individuals in different groups. At the same time, there are large numbers of species that co-exist within each group. It is also found that species tend to be the same size or to be larger or smaller by a factor 1.3, a value that corresponds to the ‘limiting proximity’ suggested by Hutchinson in his 1959 paper. The relationship is also found to be maintained in communities in different locations or with different evolution history.

The key finding, the PLoS One paper says, is that while similarity or dissimilarity, i.e., the difference of 1.3 times, are common, intermediate size rations are rare. As the beetles are ‘generalistcarnivores’, the size ranges indicate, approximately, the ‘niche’ that the groups of beetles occupy. The regularity of the size distribution, locally and long range, and their separation, hence shows that evolution drives species to either be sufficiently similar, or sufficiently different, so that they do not compete. Size distribution was equally seen in the isolated communities that had evolved in cramped and resource-starved conditions. It was seen that fewer species survived, and their size-spacing was also more spaced out, which conforms to how it should be if there are fewer species to fit into the ‘niche-space’.

In conclusion, the paper says, evolution seems to create ‘not only functional complementarity but also functional redundancy.’ While complementarity distributes the agents that carry forward the large interests of the community, the redundancy of several species occupying the same function makes the system resilient. Although the different species in a niche may be near neutral when it comes to competing for resources, they do have different resistance to stress, like from parasites or disease. An instance of diversity at the micro level!

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