Species coexistence is often ensured by niche separation. The 'niche shift' can follow from the competitive exclusion of one species from the part of ecological space where the niches overlap, or from coevolution of competing species, favoring in each species phenotypes differing from the phenotype of the competitor. The latter case is often referred to as the 'ghost of competition past', emphasizing that current niche segregation can be due to the processes that took place in distant evolutionary past. If morphological differences arose due to divergent evolution of sympatric competitors, we speak of 'character displacement'. Typically, sympatric populations of competing species evolve toward more different sizes of characters associated with food consumption (beaks, teeth) than allopatric populations - if there is only one species of Galapagos finches on an island, it has an intermediate beak size enabling to utilize wide spectrum of seed sizes, whereas if there are two species, one has bigger and the other has smaller beak than the species occurring without competitors. If there are more than two locally coexisting species, we often observe regularly spaced sizes of morphological characters, again indicating past competition leading to maximum niche separation.
Simple separation of niche optima is not, however, the only way that stable local coexistence of species is attained. Many species pairs, for instance, consist of one species which is competitively dominant, and the other species which is less specialized and can thrive in a broader range of ecological conditions. An example is the pair of two closely related species of redstarts, where the black redstart Phoenicurus ochruros is bigger and more aggressive, but the common redstart Phoenicurus phoeni-curus can utilize a wider spectrum of habitats, such that it has always an option to thrive out of the range of conditions preferred by black redstart. Such niche division between dominant aggressive specialist and subordinate generalist has also been observed in many mammal species, and is apparently stable. In plants, competitively inferior species are often those with higher rate of spreading and growth, which enables them to quickly occupy empty places before arrival and eventual overgrowth of competitively superior species. In this case we speak about 'regenerative niche', representing a time window for competitively inferior, but fast-spreading and fast-growing species, thus ensuring long-term coexistence of competitors in the same habitat.
If species are very similar to each other, such that they do not differ substantially in their utilization of resources, the competitive exclusion can take a very long time. If the replacement of old individuals by young ones is basically a random process, that is, all individuals regardless of species identity have equal chances to give birth to their descendants within an environment, populations of all involved species will fluctuate randomly and the prevalence of a particular species is just a matter of chance. However, due to these stochastic fluctuations and due to the fact that the species which incidentally prevails in a time step will have higher probability to further increase its abundance, this process will finally lead to apparent competitive exclusion. This process, called 'community drift', can be relatively slow and may be further slowed down by dispersal limitations (leading to random prevalence of different species in different local communities isolated by migration barriers) and balanced by the emergence of new species (i.e., speciation or migration from elsewhere).
Communities where dispersal limitation and community drift play a major role are called 'dispersal-assembled communities', in contrast to 'niche-assembled communities'
where niche differences play a major role in determining species distributions and abundances. Trees in tropical forests represent a very good candidate for dispersal-assembled communities. Most tropical tree species are very similar in terms of their ecology and growth characteristics, and it has been documented that for their recruitment the proportion of parent individuals in a given locality (i.e., dispersal limitation of more distant individuals) is much more important than any habitat characteristics. Still, an incredible number of species can coexist locally. It is hardly believable that there are several hundreds of different narrow ecological niches (i.e., combinations of environmental characteristics) on a hectare of tropical forest to enable coexistence of several hundreds of tree species on the basis of their niche differences - the dispersal assembly and coexistence without significant niche differentiation seem more likely. However, an unusual aspect of niche differences can still be involved in this classical case of species coexistence. It has been demonstrated that coexistence of tropical trees is facilitated by frequency dependence, where relatively rarer species have an advantage of not being so severely attacked by natural enemies which strongly limit recruitment of more common species on which they specialize. In a sense, all species compete for 'enemy-free space', and this 'niche' for a given species is open only if the species is not too abundant to allow population growth of specialized natural enemies. Separation of 'niches' of tropical trees seems thus to be determined by the community of species-specific pathogens.
In conclusion, coexistence among species can be certainly maintained both by niche differences and - at least in a nonequilibrium world - by niche similarity. Coexistence of species with similar niches maintained by dispersal-assembly processes could be a reason why we often observe that species are not regularly distributed in a niche space, but form clumps of species whose niches are closer to each other than to other species.
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