Settlement intensification and coastline topography: understanding the role of habitat availability in the pelagic−benthic transition

Among the factors that influence intertidal settlement of rocky shore invertebrates, habitat availability has a direct effect on potential settlement per unit area. Consequently, the amount of suitable habitat available is relevant to explanations of intertidal distributions and abundance. Results concerning the direction of the relationship between the availability of habitat and settlement have, however, been contradictory. Several studies have concluded that settlement rates are positively correlated with free space, whereas others have identified inverse relationships, typified by the settlement intensification hypothesis based on the finding that barnacle settlement has a negative non-linear relationship with habitat availability. Combining a simple method of measuring alongshore rocky habitat availability with intertidal mussel settlement data from 20 sites and 4 sampling occasions, we examined the relationship between these 2 factors at 3 spatial scales (10, 100 and 1000 m) to test for an intensification effect and to identify the scale(s) at which it may take place. Sites were split between bay and open coast areas, allowing additional tests of habitat availability patterns between bays and open coast areas. Significant negative settlement−habitat correlations were identified on 3 of the sampling occasions, and regression analyses confirmed that these relationships were mostly exponential in nature, with as much as 45% of the variation in settlement explained by rocky habitat availability. This finding of settlement intensification highlights the need to understand fully settlement data, particularly in relation to the interpretation of bentho-pelagic links and large-scale patterns of mussel settlement.