Why study larval transport, settlement, and recruitment? Why follow settlers all the way to reproduction?

The number of young larvae transported to the adult habitats could determine adult abundance of many benthic species. The figure below shows a representation of how different processes influence population size of adults. The larger the cone the more critical the process because, for example, if there are no larvae transported from offshore larval habitats to adult coastal habitats then no reproductive individuals would be subject to mortality due to competition, disturbance or predation.

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Hierarchy of settlement processes

Settlement processes are hierarchical in more than one axis. They are both serial, and ordered in space, starting with processes in the larval pool, far from the settlement site, and ending in processes occurring around the settlement site. There is also a scale correspondence, with larger spatial scale processes occurring in the larval pool, followed by physical transport processes, and smaller scale processes occurring at the settlement site. Larger scale hydrodynamic ("oceanographic") factors tend to be more pervasive in the larval pool and transport stages, with smaller-scale processes more pervasive during transport (e.g. accumulation processes) or settlement (substrate availability; micro-hydrodynamics). Finally, there is also an abundance relationship, where processes in the larger scales operate over a larger number of individuals than processes in the smaller scales. Large-scale offshore oceanographic processes operate first over higher abundances than the small-scale nearshore processes that operate last over fewer individuals (Pineda, 1994, and 2000).

Studying larval transport, dispersal and settlement may not be sufficient for understanding determination of population growth

Knowledge of larval transport and dispersal is important for understanding the temporal and spatial dynamics of benthic populations. Nonetheless, if the motivation to study larval transport, dispersal and settlement lies on understanding determination of population growth, spread of invasive species, and other phenomena that assume survival to reproduction, understanding which individuals survive to settlement is only part of the story because (a) in most species, the majority of the settlers die before reproducing and (b) individuals settlers that survive to reproduction are not a random sample of the settlers (see Pineda et al, 2006). Thus, ultimately, studies should follow individuals that disperse and settle all the way to reproduction (see discussion in Pineda, Hare and Sponaugle, 2007).

The importance of recruitment in benthic populations

David et. al (1997) describe well why it is important to study larval transport, settlement, and recruitment:

"Studies of marine invertebrates have led to the emergence of new paradigms in population dynamics, which differ sharply from classical models of population growth that apply, for example, in terrestrial taxa. Indeed, recruitment, rather than reproductive output, is the key concept for the demography of marine species, and has been recognized as one of the main determinants of spatial and temporal variations in population size. Most marine taxa have complex life cycles with planktonic larvae that settle and metamorphose into sessile benthic adults (Barnes 1986). In this case, populations must be considered as open systems (Roughgarden et al. 1985; Bence and Nisbet 1989), in that settling larvae may come from remote places. For this reason, recruitment is a critical factor, and is not necessarily correlated with the local reproductive output. Understanding the determinants of recruitment is therefore one of the major goals in marine population dynamics."

Quoted from David, P., P. Berthou, P. Noel and P. Jarne 1997. Patchy recruitment patterns in marine invertebrates: a spatial test of the density-dependent hypothesis in the bivalve Spisula ovalis. Oecologia (Berl.). 111, 331-340

Click for a 300 Kb animation of a setting Chthamalus larva