Patrick Lavelle: Competition versus Cooperation Underground
Among the first topics a biologist studies is Darwin’s trip to the Galapagos in 1859, and his subsequent (very famous) Theory of Evolution. This theory, which outlines the process by which organisms speciate over time, is based on the idea that competition drives species interaction. In other words, the competition for food, water and nutrients causes species to evolve certain characteristics that allow them to outcompete surrounding organisms, and achieve a higher probability of survival. In our seminar with Patrick Lavelle, however, we were introduced to a very different view of species interaction: that the relationship that predominantly governs species interaction is cooperation.
Cooperation versus competition is a controversial viewpoint for Darwinian biologists: the idea that competition drives speciation is widely accepted. Take, for example, the rainforest: the relentless competition for sunlight and a spot in the canopy has favored the evolution of (tens of) thousands of different kinds of plant species, from opportunist pioneer species that grow rapidly when canopy space opens, to epiphytes (plants that grow on other plants) that grow on tree branches high in the canopy. Bizarre species such as the strangler fig achieve their spot in the canopy by subterfuge: they start their life as epiphytes, and then send roots down to the ground that strangle their host tree and steal its place in the canopy. Meanwhile certain tree species produce flaky or toxic bark that prevents plants from growing on them and strangling them or weighing down their branches. It is easy to describe adaptations such as these in terms of competition.
During our time with Professor Lavelle, we were encouraged to look at systems like this from a different viewpoint: rather than being defined by competition, these systems could be defined by collaborative symbioses between species. Take the example of the rainforest, again: the pioneer species that grow rapidly in clearings usually grow after a large tree has fallen, which provides them with sufficient nutrients to grow. By germinating and filling the gap immediately, the pioneer species capture and store important and limited nutrients from the fallen tree, such as phosphorous, before these nutrients are washed away by rain and permanently lost from the forest. As the pioneer trees grow, slower-growing secondary species have time to establish: when the pioneer species fall after 10 or 15 years, the secondary species are sufficiently mature to grow and achieve the canopy. They, in turn, can capture phosphorous and other minerals from the fallen primary species. The rapid drive to grow and fill space in the rainforest, therefore, can be considered a system to maintain nutrients: preventing the loss of valuable phosphorous from the rainforest.
To test whether we supported the idea of competition or cooperation, Professor Lavelle presented us with a scenario involving earthworms. In a field in Fiji there are two species of earthworm: one large, and one small. The small worms leave small pellets/aggregates; and the large ones leave large pellets. Soil samples were taken from the field, and the distribution of pellets was mapped in the attached white and green grid. The white squares indicate soil containing small pellets, and green squares indicate soil with large pellets. Our exercise was to predict the relationship between the two species of earthworm, based on the distribution of their pellets.
An interpretation of the relationship based on competition might assume that one of these species has an advantage: maybe the small worms are unable to break down large pellets, and therefore cannot move into areas that have been occupied by the larger worms. If the larger worms are able to break down small pellets and leave large pellets behind, then they are gradually excluding the smaller worms: only one earthworm species can exist in this space.
An interpretation based on cooperation would look like this: the small worms can break down the large pellets, and the large worms can break down the small pellets. So each mutually supports the other species: the two earthworm species can coexist in this space.
What do you think?
By Holly, Lina & Laura.