Intertidal estimates of sea urchin abundance reveal congruence in spatial structure for a guild of consumers

Abstract

We hypothesized congruence in the spatial structure of abundance data sampled across multiple scales for an ecological guild of consumers that exploit similar nutritional and habitat resources. We tested this hypothesis on the spatial organization of abundance of an herbivorous guild of sea urchins. We also examined whether the amount of local along-shore rocky habitat can explain the observed spatial patterns of abundance. Standardized estimates of abundance of four intertidal sea urchins-Diadema cf. savignyi, Echinometra mathaei, Parechinus angulosus, and Stomopneustes variolaris-were determined by six observers at 105 sites across 2,850 km of coast of South Africa. For each species and observer, wavelet analysis was used on abundance estimates, after controlling for potential biases, to examine their spatial structure. The relationship between local sea urchin abundance and the amount of upstream and downstream rocky habitat, as defined by the prevailing ocean current, was also investigated. All species exhibited robust structure at scales of 75-220 km, despite variability among observers. Less robust structure in the abundances of three species was detected at larger scales of 430-898 km. Abundance estimates of sympatric populations of two species (D. cf. savignyi and E. mathaei) were positively correlated with the amount of rocky habitat upstream of the site, suggesting that upstream populations act as larval sources across a wide range of scales. No relationship between abundance and habitat size was found for P. angulosus or S. variolaris. Within the range of scales examined, we found robust congruence in spatial structure in abundance at the lower, but not the larger, range of scales for all four species. The relationship between abundance and upstream habitat availability in two species suggests that larval supply from upstream populations was probably the mechanism linking habitat size and abundance.

Keywords: South Africa; co‐occurrence; echinoderm; echinoids; rocky shores; spatial heterogeneity; spatial scales; spectral analysis.