Abstract
Ecological theory predicts that two species with similar requirements will fail to show long-term co-existence in situations where shared resources are limiting, especially at spatial scales that are small relative to the size of the organisms. Two species of intertidal mussels, the indigenous Perna perna and the invasive Mytilus galloprovincialis, form mixed beds on the south coast of South Africa in a situation that has been stable for several generations of these species, even though these populations are often limited by the availability of space. We examined the spatial structure of these species where they co-exist at small spatial scales in the absence of apparent environmental heterogeneity at two sites, testing: whether conspecific aggregation of mussels can occur (using spatial Monte-Carlo tests); the degree of patchiness (using Korcak B patchiness exponent), and whether there was a relationship between percent cover and patchiness. We found that under certain circumstances there is non-random conspecific aggregation, but that in other circumstances there may be random distribution (i.e. the two species are mixed), so that spatial patterns are context-dependent. The relative cover of the species differed between sites, and within each site, the species with higher cover showed low Korcak B values (indicating low patchiness, i.e. the existence of fewer, larger patches), while the less abundant species showed the reverse, i.e. high patchiness. This relationship did not hold for either species within sites. We conclude that co-existence between these mussels is possible, even at small spatial scales because each species is an ecological engineer and, while they have been shown to compete for space, this is preceded by initial facilitation. We suggest that a patchy pattern of co-existence is possible because of a balance between direct (competitive) and indirect (facilitative) interactions.
Highlights
General ecological theory predicts that stable coexistence of two or more similar species will be unusual and dependent on resource availability or predation [1,2,3]
At spatial scales that are small relative to body size, it is considered unlikely that two competing species can coexist, unless the inferior competitor is a superior colonizer of empty patches or there is spatial variability in their competitive abilities so that the superior competitor is at a disadvantage in at least some part of its distribution [2,6,8]
Study area and sampling design We studied two sites where M. galloprovincialis occurs at high densities on the south coast of South Africa [22]
Summary
General ecological theory predicts that stable coexistence of two or more similar species will be unusual and dependent on resource availability or predation [1,2,3]. Coexistence of competing species at larger local scales, that is scales beyond those at which species directly interact, is often explained by environmental heterogeneity, or by species interactions generating spatially segregated distributions even in the absence of environmental heterogeneity [1,3,4,5,6,7]. Interference between interacting species can be both costly and beneficial for an inferior resource exploiter, e.g. the production of chemicals as defence involves a cost, while predation on the eggs and larvae of the competitor is beneficial in terms of growth etc. Interference between interacting species can be both costly and beneficial for an inferior resource exploiter, e.g. the production of chemicals as defence involves a cost, while predation on the eggs and larvae of the competitor is beneficial in terms of growth etc. [2]
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