Feed me, Seymour

October 25, 2010 § 3 Comments

This post was chosen as an Editor's Selection for ResearchBlogging.orgThe diversity of life is a puzzle for ecologists and evolutionary biologists.  The principle of competitive exclusion suggests that if two species are competing for the same resource, one of them should eventually win and the other should become extinct.  So if you have n different food sources, you should end up with (at most) n different species.  But real biological communities are far more diverse than this analysis would suggest.

Pink sundew. From http://www.biophilia.net/

In this context, I found a recent paper about competition between carnivorous plants and spiders interesting.  (Jennings et al. 2010. Evidence for competition between carnivorous plants and spiders. Proc. R. Soc. B 277 3001-3008 PMID: 20462904).  This paper looked at the dietary habits of the pink sundew, Drosera capillaris, and the wolf spider Sosippus floridanus.  Both eat insects — do they compete?

They do: they eat the same prey (as measured by what gets caught in the spider’s web or on the sticky leaves of the sundew); the presence of a sundew affects the behavior of the spider (the distance between a spider web and a sundew gets larger as the trapping area of the sundew gets larger, and the web gets larger too); and when a spider and a sundew are put in the same terrarium with a limited food supply, the presence of the spider reduces the number of seeds the sundew can produce.  Poor starving plant.

Why do I connect this to the principle of competitive exclusion?  The idea that competing species cannot

Wolf spider. From http://bugguide.net/

co-exist is based on “complete” competitors in constant environments: there is no distinction between the species in terms of access to different areas, or responses to environmental changes.  And therein, of course, lies the problem.  Two species cannot be perfect competitors without actually being the same species.  Ecologists recognize that spatial heterogeneity, changing environments, the presence of multiple food sources, and other such factors (including species-species differences) make competition more complex; in fact they make it so complex as to be, often, analytically intractable.

Reading about this discovery of competition between a plant and a spider made me realize the level of difficulty we face in understanding species diversity.  Here is a pair of species that consume the same resource, but have nothing else in common. [Well.  They’re both carbon-based life forms.  That might be it.]  The spider is mobile, the plant sessile; but the spider is entirely dependent on catching enough insects, while the plant can live and produce seeds even at very low levels of food.  The plant might fall victim to grazing goats, but the spider is probably OK unless there are sharp-eyed birds around. How do you analyze competition between two species of such different lifestyles?

Roy Kishony, who recently published a paper arguing that adding mutation/selection processes to competitive exclusion makes the whole problem worse — it’s even harder to understand why there are so many species if a species can evolve to be a winner, in addition to winning by starving its competitors — points out that ecologists are aware of this problem and have attempted to solve it by dividing resources into different subparts, for example nuts or grains eaten at night versus those eaten in the day.  This approach doesn’t seem entirely satisfactory in this case, even if it is in other situations; the resource subsets for this competition would be “insects close enough to a sundew to be eaten by one” and “insects not close enough to a sundew to be eaten by one”, which seems to me to lack explanatory power.  If anyone’s searching for an unsolved problem to get their teeth into, may I suggest the real-world application of competitive exclusion as a possibility?

Jennings DE, Krupa JJ, Raffel TR, & Rohr JR (2010). Evidence for competition between carnivorous plants and spiders. Proceedings. Biological Sciences / The Royal Society, 277 (1696), 3001-8 PMID: 20462904

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§ 3 Responses to Feed me, Seymour

  • Bodo Stern says:

    I just visited the Harvard Forest, where one of their researchers, Clarisse Hart, studies a rather similar competitive species interaction: between web-building spiders and carnivorous pitcher plants. If I understood correctly the spiders can actually construct a web right across the opening of the pitcher plant. Bad news for the pitcher plant since the presence of the spider does indeed reduce the prey they capture. But I wonder whether the spiders occasionally lose their footing and fall into the plant thus becoming a delicious meal for the pitcher plant. You can read more at

    http://eco.confex.com/eco/2009/techprogram/P17561.HTM

  • The competitive exclusion principle needs to take the complete niche into account (i.e., all resources), and not just one. As you hint at, a plant and a spider use many resources that are not overlapping. The plant will need water from the ground, and many other nutrients, and a place with enough sunlight. Not sure what a Wolf spider needs, but they have predators that will keep their growth in check. So, the fact that they are competing over one resource makes them competitors, but the competitive exclusion principle would in this case not predict them to compete so much that one is expected to go extinct because of it.

  • […] When we typically think of competitive exclusion, we think of two similar plants or animals overlapping in niche-space. But sometimes, the two species that compete for the same resource are a plant AND an animal. […]

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