Quantifying biological success

28 April 2009

Tuesday

Is homo sapiens sapiens the most successful species? By what measure?

It is easy to suppose that human beings — homo sapiens — constitute the most successful species in the natural history of the planet, but it is somewhat more difficult to quantify this claim. How ought we to measure the biological success of a species?

When I was thinking about this a couple of days ago, without too much effort I could think of six ways in which the biological success of a species might be quantified, and these methods of quantification would yield different results for different species.

1. The biological success of a species could be measured by the absolute number of individual organisms belonging to the species in question.

By this measure, homo sapiens is not the most biologically successful species. For example, at any given time there are approximately 16 billion chickens living on Earth. The title of most numerous organism would probably go to some insect species, or perhaps some marine invertebrate, like plankton. But because each individual of the species homo sapiens is so large, our absolute numbers can be less significant than the total biomass that we represent (see 3 below).

When we think of vast swarms of insects (or even vast swarms of vertebrate mammals) it is obvious that homo sapiens has no monopoly on absolute numbers, and we aren't even talking microbe species yet.

2. The biological success of a species could be measured by the number of distinct biomes in which the species in question has been able to make a home.

By this measure, homo sapiens has a good shot at the title of most biologically successful species, since human beings have inhabited every biome on the planet from equatorial desert to arctic tundra to tropical forest to temperate grassland, but there are probably other species — for example, species of microbes — that have been similarly successful in colonizing diverse habitats.

A map of terrestrial biomes from Wikipedia.

3. The biological success of a species could be measured by the absolute quantity of biomass (in weight) represented by the collected members of the species.

In other words, if we could gather up all human beings in a big net and weigh them, if together they all weighed more than another other species (say, for example, more than the weight of all the killer whales in all the oceans of the world, or all the chickens in the world) then we would be the most biologically successful species. By this measure, human beings have a good shot at being named the most biologically successful species in the earth, since human bodies are large, and taken together they constitute a substantial biomass, but this is far from certain. However, being at the top of the food chain virtually guarantees that a more plentiful biomass of primary producers is supporting the later consumers at or near the top of an ecological pyramid.

A biomass pyramid shows the amount of biomass at each trophic level. When energy is transferred from one trophic level to the next, typically only ten percent is used to build new biomass. The remaining ninety percent goes to metabolic processes or is dissipated as heat. This energy loss means that productivity pyramids are never inverted, and generally limits food chains to about six levels. However, in oceans, biomass pyramids can be wholly or partially inverted, with more biomass at higher levels. (Wikipedia)

4. The biological success of a species could be measured by the ability of a given species to alter its habitat for its own use, i.e., niche construction.

This seems like a category contrived strictly for the purpose of making humankind the most biologically successful species, but that is not necessarily the case. Whereas our changes to our environment — like the building of cities — are dramatic, the coevolution of many microbial species with their non-living environment would constitute another, and perhaps more pervasive, example — and an example that has persisted for a far longer period of time. There are also more conventional examples like beavers, who alter their habitat, but I doubt beaver numbers approach human numbers, so that human beings modify their environment far more than beavers, speaking quantitatively.

Homo sapiens has profoundly altered its environment for its own purposes, but there are many ways for an organism to modify its environment.

5. The biological success of a species could be measured by the ability of a given species to inhabit every available ecological niche.

This may not be too different from 2 above, except that a biome and a niche are two very different things, differing in terms of order of magnitude (though, for present purposes, qualitatively similar), so a careful definition would allow us to distinguish this as a category of biological success. Biological success defined in terms of niches is a far more fine-grained account than biological success defined in terms of biomes. Within the biome of, say, tropical rainforests, there will be many niches. Few biological niches are sufficiently robust to support a species as large as a human being, but of those that are, we can quantify whether or not these niches are so exploited as a relative measure of the biological success of the species in question.

A graphic representation of ecological niches from http://www.metafysica.nl/nature/insect/nomos_26.html

6. The biological success of a species could be measured by the ability of a species to supplant and replace other species.

This again sounds like a contrived category provided merely for the purpose of finding human beings to be the most biologically successful species, since we certainly have supplanted a great many species. But this is true of “weedy” species generally, and a careful quantification, once again, would be necessary to determine, so far as it is possible, the exact number of other species supplanted by a given weedy species. This could be defined in more than one way, whether in terms of the total number of individuals of any one species displaced, the total number of species displaced, or the total number of individuals of any species whatever displaced. Each of these formulations is likely to yield a distinct result.

There is, however, a yet more radical way in which we might define the biological success for a species. The biological success of an individual is measured by the success of the individual organism in passing on its genes to the next generation. When this happens the species survives (we could say that it has historical viability, or even existential viability). Obviously, this definition of biological success cannot be used to define the biological success of a species, but it could be reformulated, mutatis mutandis, to apply to species on the whole, and not just to individuals of a species.

Successful species pass along their genetic material to successor species and in this way continue to be represented in living populations even after extinction.

The biological success of a species, then, could be measured by the genetic information that it passes along to other, distinct species after the species in question itself has become extinct. (When a species goes extinct but leaves direct descendants of a distinct species, this is sometimes called “pseudoextinction,” and the larger taxon to which both species belong is called a “chronospecies”; cf. Hobson’s Choice, Evolution, and Civilization) Death is the extinction of the individual. Extinction is the death of a species. An individual is survived by the offspring that carries its genetic information. Similarly, species that undergo adaptive radiation bequeath their genetic information to successor species. After a given species has become extinct, its relative biological “success” could be measured by the amount of genetic information that it passed along to successor species. In other words, the biological success of a species could be measured by its total contribution to the genetic legacy to the planet.

In this last and most radical sense, homo sapiens cannot be called the most biologically successful species on the planet, and we would not want to earn that title soon, as it can only be conferred upon extinction. Moreover, the institutions of civilization have militated against human adaptive radiation, at least in terms of biology — in terms of social technology, human beings have an impressive legacy of adaptive radiation, and it is just this that has made it possible for us to inhabit as many biomes and niches that we do inhabit. But it is worthwhile to think of our legacy, and our potential legacy, in this context.

In any case, what I hope to have accomplished in this post is to have convinced the reader that we cannot simply assume that human beings are the most “successful” terrestrial species. In order to determine the relative success of a species we would need to embark upon a systematic scientific research program specifically formulated with the intention to analyze the question of what constitutes biological success. To the best of my knowledge, no such research program currently exists. If any reader is in fact so convinced, and decides as a consequence to formulate a scientific research program, that would be the first step toward answering the question of what constitutes biological success.

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