Excluding the Competitive Exclusion Principle: Why the World is Not a Zoo

As an undergraduate, I worked several summers in a microbiology lab, as a technician. I made and poured culture media. I learned to streak agar Petri dishes in a way which would allow isolation of multiple organisms. One problem, when culturing gram negative organisms, is the way that a motile species, such as the Proteus mirabilis species shown above, may rapidly overrun other colonies, making isolation for identification of other organisms more difficult.

In biology, the competitive exclusion principle is a theory of ecology which states that two species with identical resource requirements cannot coexist. This theory, as developed using micro-organisms in cultures by Georgy Gause around 1930, suggests that, given any particular niche in an environment, only one species will come to occupy that niche.

The competitive exclusion principle can be seen when we culture a mix of micro-organisms in a Petri dish. If, at first, we streak the agar properly to form multiple isolated colonies, the colonies usually coexist until they overlap, after which usually one colony type will overgrow and replace the other. Many mathematical attempts to model competition for resources in an ecosystem also suggest that one species per niche should be the end result at equilibrium.

Yet in nature, diversity prevails within most such niches. Diversity, not monoculture, is the long term norm, outside of human systematic interventions.

Why? Likely, it is because the Petri dish's zoo is artificially simplistic, as are the mathematical models. The physical environment is varied in seasons and weather, and most species have many challenges, in the physical environment and with parasite and predator, which keep that species' numbers below what is required to fully take over a given niche. This allows other species in that niche room to reproduce.

The competitive exclusion principle is thus one which applies best in artificially constrained situations, such as those created by experimental designs. A zoo is not a complete ecosystem.

Diversity may then thrive in nature in large part because of the variety of types and levels of both opportunity and adversity. When people homogenize the environment for their own purposes, we interfere with those mechanisms. The competitive exclusion principle is an anthropogenic principle.

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ABSTRACT

Influence of Interspecific Competition and Landscape Structure on Spatial Homogenization of Avian Assemblages

Authors: Oliver J. Robertson, Clive McAlpine, Alan House, Martine Maron.

Published: May 28, 2013

DOI: 10.1371/journal.pone.0065299

Human-induced biotic homogenization resulting from landscape change and increased competition from widespread generalists or ‘winners’, is widely recognized as a global threat to biodiversity. However, it remains unclear what aspects of landscape structure influence homogenization. This paper tests the importance of interspecific competition and landscape structure, for the spatial homogeneity of avian assemblages within a fragmented agricultural landscape of eastern Australia. We used field observations of the density of 128 diurnal bird species to calculate taxonomic and functional similarity among assemblages. We then examined whether taxonomic and functional similarity varied with patch type, the extent of woodland habitat, land-use intensity, habitat subdivision, and the presence of Manorina colonies (a competitive genus of honeyeaters). We found the presence of a Manorina colony was the most significant factor positively influencing both taxonomic and functional similarity of bird assemblages. Competition from members of this widespread genus of native honeyeater, rather than landscape structure, was the main cause of both taxonomic and functional homogenization. These species have not recently expanded their range, but rather have increased in density in response to agricultural landscape change. The negative impacts of Manorina honeyeaters on assemblage similarity were most pronounced in landscapes of moderate land-use intensity. We conclude that in these human-modified landscapes, increased competition from dominant native species, or ‘winners’, can result in homogeneous avian assemblages and the loss of specialist species. These interacting processes make biotic homogenization resulting from land-use change a global threat to biodiversity in modified agro-ecosystems.

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