The Ecological Theatre and the Evolutionary Play

The Ecological Theatre and the Evolutionary Play

The above title is from Hutchinson’s influential book (Hutchinson, 1965), and encapsulates nicely my topic for this essay – that is, the nature of the ‘ecosystem’ in an evolutionary context. Here I have used the term ‘ecosystem’ as an approximation for Hutchinson’s ‘Ecological Theatre’. The level of understanding of the astonishingly popular term ‘ecosystem’, both by the public and among many professional biologists, is imperfect. Indeed, it has been suggested that the ecosystem concept should be abandoned altogether (McLachlan & Ladle, 2011). To define what we are dealing with - an ecosystem is the complex of living organisms and their physical environment and all interactions among them, in a particular place (Tansley, 1934). Here I wish to address the following questions. First, is an ecosystem, such as a warm little pond or tangled bank, an adaptation, as many people appear to think, or is it something quite different i.e. an economy of cohabiting species (Dawkins, 2004), p 266. This question leads, in turn, to the nature of adaptation itself. I give special attention to the testing context of invasions i.e. the arrival of species in a novel habitat (Elton, 1958). These are largely unresolved and indeed largely unrecognised questions. Let’s take Charles Darwin’s Tangled Bank to set the scene (Darwin, 1859).

“It is interesting to contemplate a tangled bank. Clothed with many plants of many kinds, with birds singing on the bushes, with various insects flitting about and with worms crawling through the damp earth, and to reflect that these elaborately constructed forms, so different from each other, and dependent upon each other in so complex a manner, have all been produced by laws acting around us…… “

Thus organisms and their physical environment interact in complex ways. I start with the vexing matter of ecosystem, adaptation or economy. One line of reasoning is that, applying natural selection to an ecosystem as a self sustaining entity leads to the conclusion that the best balanced ecosystem is most likely to persist. This idea is neatly expressed in what has been called the BBC Theorem (Dawkins, 1982), pp 236-238 with natural selection operating at the level of the ecosystem. But, and it is a very big ‘but’, there is no evidence in support of higher level selection of this kind (Williams, 1966). Thus seeing an ecosystem as a quasi-organism is not permissible and may, among other things, have encouraged Lovelock Ellis’ flawed Gaia hypothesis, with earth itself as the adaptive ecosystem to reason about (Lovelock, 1995).True, some of the organisms living in a particular place are closely interlocked and evolve adaptations to local conditions but with natural selection operating in the conventional neo-Darwinian fashion – i.e. on the reproductive success of individuals (Dawkins, 2004), pp 265-266. For these reasons an ecosystem as an adaptation can be dismissed. We are therefore left with the ecosystem as an economy of organisms living together, essentially temporarily, and exploiting the place (habitat), to their best advantage. An organism living within an economy is not locked into a particular ecosystem by rigid co-adaptations but is free to go wherever it can find an amenable habitat.

We now come to invasion of novel habitats. Invasion seems to carry with it conceptual difficulties which, I believe, have not attracted sufficient attention. I refer to the fact that organisms would seem maladapted to invasion. Any novel habitat must necessarily carry with it a suite of novel selective pressures and invaders are unlikely to have encountered all these in their home habitat. To understand how invasion is achieved in the face of this difficulty, r-K selection theory comes to our aid (McNaughton, 1975). r – K selection theory makes an important distinction between two kinds of organism. One of these, the r-selected species, is composed of individuals that are specifically adapted to the invasion of new habitats. They have good dispersal ability, get to novel habitats first and breed rapidly. To take an example from my own interest, the chironomid midges of the genus Chironomus. Ovipositing Chironomus females are typically the first to arrive at any new body of water. They immediately lay eggs from which larvae emerge (McLachlan, 1970, 1974). But Chironomus is soon replaced by other midge species that are slow to invade but push Chironomus out through superior competitive ability. These are the K selected species.

So we can easily understand how r-selected species are able to act as invaders but we are left with the difficulty of understanding how K-selected species manage the trick. And K– selected species are indeed common invaders (Elton 1958). This is the nub of my second question. The evidence supports the idea that invasion by K-selected species is permitted by two effects. Initially they undergo rapid adaptive phenotypic change through essentially non-heritable phenotypic plasticity (West-Eberhard, 2003). The well known phenomenon of character displacement (Schulter, 1994), would seem to be a component of phenotypic plasticity. Phenotypic plasticity is followed by slower, evolutionary adaptation involving heritable changes in gene frequency in the conventional Darwinian fashion. Population numbers are necessarily low for invaders and E. B. Ford shows that selection, and therefore adaptation to new conditions are strongest when at low population densities (Ford, 1964).

To summarise: I have attempted to raise questions about the value of the ecosystem concept. Perhaps it could be replaced by the better understood terms ‘habitat’ and ‘environment’. At the same time, taking an evolutionary stance, I have attempted to show how invasion can be understood in an evolutionary context.  

Thanks to Clive Howard-Williams for interesting discussion.

References

Darwin, C. D. (1859). The origin of species by means of natural selection, or the preservation of favoured races in the struggle of life. (Fascimile 1901 ed.). London: John Murray.

Dawkins, R. (1982). The Extended Phenotype. (1999 edition ed.). Oxford: Oxford University Press

Dawkins, R. (2004). A devil's chaplain. London: Phoenix.

Elton, C. S. (1958). The Ecology of Invasions by Animals and Plants. New York: John Wiley.

Ford, E. B. (1964). Ecological Genetics. London: Methuen.

Hutchinson, G. E. (1965). The Ecological Theatre and the Evolutionary Play. London: Yale University Press.

Lovelock, J. E. (1995). The Ages of Gaia. Oxford: Oxford University Press.

McLachlan, A. J. (1970). Some Effects of Annual Fluctuations in Water Level on the Larval Chironomid communities of Lake Kariba.  . Journal of Animal Ecology, 39, 79-90.

McLachlan, A. J. (1974). Recovery of the Mud Substrate and its Associated Fauna Following a Dry Phase in a Tropical Lake. . Limnology and Oceanography, 10, 74 - 83.

McLachlan, A. J., & Ladle, R. (2011). Barriers to Adaptive Reasoning in Community Ecology. Biological Reviews, 86, 543-548.

McNaughton, S. J. (1975). r- and k-selection in Typha. . American Naturalist, 109, 251-261.

Schulter, D. (1994). Experimental Evidence that Competition Promotes Divergence in Adaptive Radiation. Science, 266, 998-801.

Tansley, A. G. (1934). The Use and Abuse of Vegetation Terms and Concepts. Ecology, 16, 284-307.

West-Eberhard, M. J. (2003). Developmental Plasticity and Evolution. Oxford: Oxford University  Press.

Williams, G. C. (1966). Adaptation and Natural Selection. Princeton: Princeton University Press.