Ecologists contrast r- and ^-strategy life histories (see r-Strategist/K-Strategists) to identify the broad pattern of life-history tradeoffs between short-lived organisms with fast maturation, good dispersal ability, and many, small propagules and long-lived organisms with slow maturation, strong competitive ability, and few, well-supplied propagules, respectively. Because resources are finite, selection has forced a tradeoff between colonization ability and competitive ability. This tradeoff also produces a mechanism of species coexistence, called competition-colonization tradeoff.
In systems where disturbance creates colonization opportunities, competition-colonization tradeoffs can lead to coexistence between multiple competitors that share a single, limiting resource. This tradeoff is most easily imagined when the limiting resource is space; the dominant species is a better competitor and will overgrow the fugitive species, which is a better colonizer. Disturbance results in mortality of both species and opens space on the landscape. If the fugitive species can colonize and exploit this new space fast enough, then the dominant and fugitive species can coexist.
We can see competition-colonization tradeoff as one case of colonization-extinction balance. Species coexistence is determined by the relative colonization rates of each species, the mortality rate of both species due to disturbance, and the mortality rate of the fugitive species by competitive exclusion. For coexistence to occur, the fugitive species and dominant species must each solve the problem of colonization-extinction balance; the dominant species must have a high-enough colonization rate to avoid extinction by disturbance alone, while the fugitive species must have a higher colonization rate to withstand extinction from both disturbance and competitive exclusion. It acts at the scale of an individual or small group of individuals and determines whether one or both species will persist.
This pattern of coexistence has been best documented between organisms that compete for space. One classic example of competition-colonization tradeoff is the interaction between the annual sea palm, Postelsia palmaeformis, and the long-lived mussel, Mytilus californianus, in the rocky intertidal of the northeast Pacific. M. californianus is the competitive dominant, which will exclude P. palmaeformis over time. However, in areas of high wave energy, patches of M. californianus are ripped from the rocky substrate and P. palmaeformis colonizes these open patches more quickly than M. californianus. Transplant experiments demonstrate that P. palmaeformis can survive and reproduce in areas of low wave energy; however, in the absence of wave disturbance, it is overgrown by M. californianus. Therefore, when wave energy is high enough, M. californianus mortality due to disturbance and the faster colonization rate of P. palmaeformis allow these two species to coexist. For more on this examples, see section entitled Further Reading.
In Laikipia district of Kenya, fire-prone bushland savannah is dominated by a single species of swollen-thorn acacia tree, Acacia drepanolobium, which is host to four acacia-ant species that are obligate mutualists with A. drepanolobium. The ants nest within the swollen thorns and feed on nectaries of A. drepanolobium; the presence of ants deters herbivores from browsing on A. drepanolobium. Any one acacia tree hosts a single colony of ants. The four species of ants coexist on a single limiting resource of host trees; the species form a strict competitive hierarchy, in which more dominant species displace subordinate species from neighboring host trees. Unoccupied trees arise due to fire and elephant disturbance, which destroys colonies but not trees, and by small trees growing into a habitable size. A tree may be colonized by a colony from a neighboring tree or by a foundress queen. The two subordinate ant species both have higher colonization rates; the most subordinate species produces many more foundress queens to colonize newly available mature trees while the second most subordinate species has a higher than expected rate of colonizing empty neighboring trees by colony expansion. This tradeoff between competitive dominance and colonization ability is one mechanism of coexistence for these four species sharing a single limiting resource. For more on this examples, see the section titled 'Further reading'.
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