Case 8: Can competition drive species diversification?

CASE 8 BIODIVERSITY HOTSPOTS: RAIN FORESTS AND CORAL REEFS

In Chapter 46, we discussed the Anolis lizards on the island of Hispaniola and other islands of the Caribbean. Within their fundamental niche of tropical forests, different Anolis species eat insects found in distinct parts of forest trees (see Fig. 46.19). That is, over time, they have evolved different adaptations for feeding (Fig. 47.4). How did these different species and different feeding strategies evolve?

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FIG. 47.4 Adaptations of Anolis lizards for feeding. (a) Anolis species that feed on twigs have short limbs and a prehensile tail that they use to support themselves when they sleep; (b) those that feed on trunks have long limbs.

It is likely that competition for similar resources promoted species diversification in this group. That is, competitive exclusion, played out over time, resulted in initially similar species diverging in habitat use and feeding strategies. This process is known as resource partitioning, in which species whose niches overlap diverge and become different subspecies or species. The result is that the overlap between closely related groups is minimized.

Interactions with predators such as lizard cuckoos and curly-tailed lizards further influence, and limit, the distributions of different Anolis species. Moreover, there is reason to believe that the ability (or inability) to migrate contributes to the distributions of the individual species: When Anolis species are introduced by accident or deliberately to places beyond their dispersal capabilities, they commonly thrive. Therefore, competition, predation, and dispersal are all factors that influence the distribution of species.

Quick Check 2 What is the difference between competitive exclusion and resource partitioning?

Quick Check 2 Answer

In competitive exclusion, there is ongoing competition between two species for a particular resource, leading one to change its niche in the presence of the other. In resource partitioning, species evolve to use different resources, so there is no longer competition for that particular resource. Over time, competitive exclusion can lead to resource partitioning.