Abstract
Despite edges being common features of many natural habitats, there is little general understanding of the ways assemblages respond to them. Every edge between two contrasting habitats has characteristics governed by the composition of adjoining habitats and/or by the nature of any transitions between them. To develop better explanatory theory, we examined the extent to which edges act independently of the composition of the surrounding landscape and to which transitions between different types of habitats affect assemblages. Using experimental landscapes, we measured the responses of assemblages of marine molluscs colonising different experimental landscapes constructed with different compositions (i.e. different types of habitats within the landscape) and different types of transitions between habitats (i.e. sharp vs gradual). Edge effects (i.e. proximity to the edge of the landscape) were independent of the internal composition of experimental landscape; fewer species were found near the edges of landscapes. These reductions may be explained by differences in differential larval settlement between edges and interiors of experimental landscapes. We also found that the sharpness of transitions influenced the magnitude of interactions in the different types of habitats in experimental landscapes, most probably due to the increased number of species in areas of transition between two habitats. Our experiments allowed the effects of composition and transitions between habitats to be disentangled from those of proximity to edges of landscapes. Understanding and making predictions about the responses by species to edges depends on understanding not only the nature of transitions across boundaries, but also the landscape in which the edges are embedded.
Highlights
Ideas from landscape ecology have advanced our conceptual understanding of how boundaries between and within habitats may mediate biotic interactions with surrounding habitats [see reviews in 1,2]
Fewer species were close to edges (0 – 5 cm) in any type of experimental landscape (SNKLxD at P,0.05, Fig. 2; Table 1a), which supports hypothesis 1 that the effect of distance from edge and type would be independent of the type of landscape
Our main findings were that: (1) fewer species occurred close to edges (0–5 cm) of experimental landscape; (2) differences in composition of habitat had major effects on assemblages in different types of habitats, (3) reductions in numbers of species closer to edges were independent of the composition of landscape; (4) sharp as opposed to gradual transitions influenced the magnitude of interactions among different types of habitats and (5) intermediate turfs had more species than did sparse or denser turfs
Summary
Ideas from landscape ecology have advanced our conceptual understanding of how boundaries between and within habitats may mediate biotic interactions with surrounding habitats [see reviews in 1,2]. There can be no generality in understanding of biotic responses to edges until the effects of edges themselves are disentangled from influences due to composition of a landscape (i.e. varieties or amounts of different types of habitats in the landscape). Edges are usually defined as boundaries between distinct types of habitat [8, or ‘‘systems’’, 9], or as areas of a habitat near a perimeter adjoining an unsuitable matrix [10]. In these cases, the nature of the surrounding habitats (i.e. matrix) may influence the response of organisms to edges [8]. We will use ‘‘edges’’ to refer to areas near the perimeters of a habitat in contact with an unsuitable matrix. ‘‘Transitions’’ are boundaries between different habitats (some of which might be unsuitable for some species) within a landscape
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