Box 3. Mechanisms promoting species coexistence
When taxa with similar resource utilization occur in both allopatry and sympatry, competition in sympatry resulting from niche overlap can promote niche divergence, facilitating species coexistence [83,84]. However, two different processes can be invoked to explain greater divergence in sympatry than in allopatry: either ecological character displacement [85], in which functionally similar species evolve in different directions, or species assortment, in which only taxa with preexisting differences in niche can successfully colonize the same habitat and coexist. Character displacement is consistent with trait lability and competition-driven niche segregation promot- ing coexistence, whereas species assortment suggests trait con- servatism and competitive exclusion. A molecular phylogenetic approach has the potential to assess the relative roles of both ecological character displacement and species assortment. Although only few in number, these studies provide evidence for the importance of both processes (see Ref. [86] for a recent review).
Besides ecological character displacement and species assort- ment, other conditions can also facilitate coexistence. In the absence of limiting resources, perhaps the existence of particular demo- graphic features within one or more species, such as a metapopula- tion structure, might permit coexistence with niche overlap [87]. The application of molecular phylogenetic methodology, both among and within species, can help to reveal the relative importance of trait divergence and demographic explanations for species coexistence. Within the beetle genus Aphanarthrum (Coleoptera: Scolytidae) on the Canary Islands, a four-gene phylogeographic analysis has uncovered the evolutionary origin of two sympatric and ecologically indistinguishable species on the island of La Palma, A. subglabrum and A. glabrum nudum [88]. Analyses point to reproductive character displacement more consistent with reinforcement theory [89] than ecological character displacement, resulting in the coexistence of two ecologically indistinguishable but reproductively isolated taxa. A metapopulation structure for these species, due to patchy host plant distribution or lack of resource limitation, remains a testable hypothesis for their coexistence. It is interesting to note that the few studies of morphological patterns of character divergence among other beetles [90–93] have also been shown to reflect possibly reproductive rather than ecological character displacement [86], supporting a neutral ecological model of coexistence. Neutral models of community assembly [33,41] predict a minor role for niche segregation for coexistence, and insect communities deserve further investigation to evaluate such models.
Box 3. Mechanisms promoting species coexistence
When taxa with similar resource utilization occur in both allopatry and sympatry, competition in sympatry resulting from niche overlap can promote niche divergence, facilitating species coexistence [83,84]. However, two different processes can be invoked to explain greater divergence in sympatry than in allopatry: either ecological character displacement [85], in which functionally similar species evolve in different directions, or species assortment, in which only taxa with preexisting differences in niche can successfully colonize the same habitat and coexist. Character displacement is consistent with trait lability and competition-driven niche segregation promot- ing coexistence, whereas species assortment suggests trait con- servatism and competitive exclusion. A molecular phylogenetic approach has the potential to assess the relative roles of both ecological character displacement and species assortment. Although only few in number, these studies provide evidence for the importance of both processes (see Ref. [86] for a recent review).
Besides ecological character displacement and species assort- ment, other conditions can also facilitate coexistence. In the absence of limiting resources, perhaps the existence of particular demo- graphic features within one or more species, such as a metapopula- tion structure, might permit coexistence with niche overlap [87]. The application of molecular phylogenetic methodology, both among and within species, can help to reveal the relative importance of trait divergence and demographic explanations for species coexistence. Within the beetle genus Aphanarthrum (Coleoptera: Scolytidae) on the Canary Islands, a four-gene phylogeographic analysis has uncovered the evolutionary origin of two sympatric and ecologically indistinguishable species on the island of La Palma, A. subglabrum and A. glabrum nudum [88]. Analyses point to reproductive character displacement more consistent with reinforcement theory [89] than ecological character displacement, resulting in the coexistence of two ecologically indistinguishable but reproductively isolated taxa. A metapopulation structure for these species, due to patchy host plant distribution or lack of resource limitation, remains a testable hypothesis for their coexistence. It is interesting to note that the few studies of morphological patterns of character divergence among other beetles [90–93] have also been shown to reflect possibly reproductive rather than ecological character displacement [86], supporting a neutral ecological model of coexistence. Neutral models of community assembly [33,41] predict a minor role for niche segregation for coexistence, and insect communities deserve further investigation to evaluate such models.
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