One of the underlying tenets of ecology is the principle of competitive exclusion. This principle states that when two species compete for the same vital resource, the better adapted species will ultimately displace its competitor. Simply put, it’s survival of the fittest.
More recently, some ecologists have suggested that nature’s not quite so brutal – that the species composition in an ecosystem is determined more by random fluctuations in population numbers than by direct competition.
But last month, this "neutral theory" was directly challenged by evidence on three continents which compared the abundance of particular tree species, both in the fossil record and in existing forest ecosystems. The similarities were so close among all the comparisons that it’s most likely due to direct competition rather than random fluctuations.
While this information might seem pretty esoteric, it does have direct application to gardens and landscapes. Among your plants, you will have some that compete better for water, nutrients, and other resources. The concept of "companion plantings" as plants actively helping each other survive is a wishful projection on our part.
And this all ties into the discussions we’ve been having about mulch. While living mulches – turf, ground covers, etc. – help protect soil structure and reduce erosion, they also compete with other plants in the landscape. Maintaining landscapes with living mulches will require more water than the same landscape with organic mulches. It doesn’t matter if the plants are native or not – it’s just a question of limiting resources and who’s going to be the most competitive in extracting them.
(Forgot to include the reference the first time I posted this – here it is: Ludwig-Maximilians-Universitaet Muenchen (LMU). “Jostling for position: Competition at the root of diversity in rainforests.” ScienceDaily, 26 Jan. 2012.)