Patchiness and Co-Existence of Indigenous and Invasive Mussels at Small Spatial Scales: The Interaction of Facilitation and Competition

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

Characterising epibenthic diversity and physical drivers in unconsolidated marine habitats of Algoa Bay, South Africa

Unconsolidated marine sediment habitats spatially make up the majority of global ocean seabed, yet benthic faunal patterns and their abiotic drivers remain poorly understood. Benthic research in Algoa Bay, on the south coast of South Africa, has largely focused on rocky reefs, while the dominant unconsolidated sediment habitats have been poorly studied. This study describes epibenthic assemblages associated with unconsolidated sediment in Algoa Bay, at between 30 and 100 m depth, and investigates the relationship between biotic patterns and physical drivers. Epibenthic abundance data were quantified from benthic imagery and tested against the long-term means and coefficients of variation of 12 abiotic factors, including depth, mean grain size and bottom temperature. Multivariate analyses revealed two statistically distinct epibenthic communities. This pattern was largely explained by depth, mean grain size, mean bottom temperature and mean current speed (cumulative variation of 52.49%). To a lesser extent, the long-term variability of bottom temperature, current speed and dissolved oxygen also influenced the community (cumulative variation of 34.44%). Visual classification of the substrates indicated that a mixed substrate type (i.e. sand and a low percentage of rock) significantly influences the benthic community. The findings suggest that a combination of depth and substrate type are largely responsible for the epibenthic assemblages observed