Is biomechanical warfare important in ecology?

Ecological theory emphasizes the importance of competition for resources in
structuring ecological communities. However, the interaction of a species
with the environment might also be important in causing structures. Some
species have a considerable influence on their environment by modifying it.
Such species are called ecosystem engineers. We report on a new class of
interspecific competitive interactions: competition by modification of
environmental conditions. A general, mathematical model is proposed, based
on the classical Lotka-Volterra competition model. In this model a function
for the environmental factor, dependent on the density of both species, is
added to each of the competition equations. The outcome of the model
indicates that species interactions by means of modification of the
environment may lead to competitive exclusion.
This type of competitive interaction was found on Dutch tidal flat/marsh
transitions. Some salt-marsh pioneer zones are characterized by alternating
patches of Spartina anglica and Arenicola spec. Both species are known as
ecosystem-engineers. Field observations suggest that both species are
mutually exclusive. We tested the hypothesis that each of the species, by
actively changing environmental factors, prevents the other species from
invading. Experiments in mesocosmos and in the field revealed that Arenicola
negatively affects the establishment of young Spartina plants. Futhermore,
field experiments showed that Arenicola does not invade Spartina tussock
pieces that are transplanted into Arenicola patches. Arenicola might have
problems penetrating the soil in a Spartina tussock because of soil
stability and dense root mass. Both species might influence the amount of
pore water present in the soil.