Detecting regional variation using meta-analysis and large-scale sampling: Latitudinal patterns in recruitment

Regional-scale variation of recruitment by marine organisms may reflect geographic patterns in adult stock sizes or fecundities, large-scale hydrodynamic features that influence the transport of larvae (e.g., currents, upwelling), and patterns of early mortality. In turn, recruitment may play a vital role in determining patterns of adult abundance and community structure, from local to biogeographic scales. We examined spatial variation in recruitment by corals at a regional scale, along 3300 km of the tropical and subtropical coast of eastern Australia (10degrees-31degrees S). We used two complementary approaches: (1) a meta-analysis of 21 different studies undertaken over a 16-yr period, each of which was generally conducted at a single reef, and (2) a large-scale sampling effort in which recruitment was measured in two years on 33 reefs arrayed along the length of the Great Barrier Reef (GBR). Our goal is to compare the emergent large-scale picture derived from many small-scale studies with patterns revealed by shorter-term regional sampling. The two approaches show very similar large-scale patterns. Recruitment by spawning corals (mainly acroporids) was highest in the central GBR and declined steadily with increasing latitude by up to more than 20-fold. A smaller decline occurred on the northern GBR between Australian and Papua New Guinea. Recruitment by brooding corals (mostly pocilloporids) was greatest in the northern GBR and also declined to the south. The latitudinal decline in brooders was three- to fivefold, i.e., not as great as for spawners. Consequently, the proportion of brooded recruits increased to the south, and they generally exceeded spawners on the southern GBR and on isolated subtropical reefs at higher latitudes. Our meta-analysis shows that fully half of the variation in the ratio of spawners to brooders is attributable to one of 11 variables that we extracted from the published studies: the month when the recruitment panels were deployed. This result suggests that the intensity and timing of spawning have a crucial impact on large-scale patterns of recruitment. Elsewhere, we tested this hypothesis in the field, and confirmed that regional variation in recruitment by spawning acroporid corals was driven by spatial and temporal variation in the extent of mass spawning. Together, large-scale sampling and meta-analyses provide a powerful, combined approach for investigating large-scale patterns and the mechanisms underlying them.