Determining the extent and spatial scale of population connectivity: decapods and coral reef fishes compared
Introduction
Patchily distributed, demersal marine species possessing pelagic larvae have the potential for considerable levels of connectivity among local populations. Mediated by larval dispersal, this connectivity is potentially of considerable importance for determining demography of local populations, because it determines the magnitude of immigration and emigration (Sale, 1991b, Caley et al., 1996). It also can be important in determining the dynamics of the overall population comprised of some number of inter-connected local populations, particularly if connectivity falls within the range appropriate for the formation of a metapopulation (Sale, 1998, Mora and Sale, 2002). Although both local and global demographics may be interesting in their own right, knowledge of the extent of connectivity, and its demographic consequences, is particularly important for population management for fisheries or conservation.
Knowledge of patterns of connectivity will be important for any form of spatially explicit management of marine populations. This is particularly the case if management tools include use of marine protected areas (MPAs) (Sale, 2002b). Whether intended for conservation or for fisheries management, MPAs function by segregating some local population(s) within their borders, where they experience lessened direct human impacts, particularly fishing pressure. The immediate result is improved survivorship with resulting larger size distributions and concomitant greater fecundity of the protected individuals. The primary rationale for use of MPAs, particularly as a fisheries management tool, is that this improved survivorship and fecundity will have positive secondary demographic effects on populations lying outside the boundaries, either by “spillover” of individuals across the boundaries, or by “subsidy” of recruitment in these outside populations due to dispersal of larvae spawned within the MPA. While spillover will be limited to the immediate proximity of the MPA, the expectation is that subsidy will be important to enhancing production in a number of populations in the broad region surrounding the MPA. This expectation is not yet supported by empirical data for any MPA (Carr and Reed, 1993, Allison et al., 1998, Russ, 2002) despite some claims to the contrary (e.g. Roberts et al., 2001), and it is worth reflecting that, to enhance fishery yields sustainably, an MPA must enhance fishery yields outside its borders, by spillover and/or subsidy, by an amount that is greater than the yield lost by its establishment. Otherwise, the MPA is simply a way of surreptitiously reducing effort, while pretending to do something more. Although current levels of effort in many fisheries may need to be reduced, using tools that do this while claiming that they do something more is not an effective way to manage relationships between managers and the fishing community. We believe there is an urgent need to obtain reliable estimates of connectivity in order to validate this primary argument for use of MPAs as fishery management tools. There is also a real need to use estimates of connectivity when designing MPAs, so that they are sited and sized appropriately, in order to maximize their effectiveness in subsidizing neighbouring populations’ growth. At present, this is not being attempted.
Unfortunately, estimates of the extent of connectivity are difficult to make, because larval dispersal is a complex process mediated by a number of different factors, and pelagic larvae are minute creatures difficult to tag or to track (Stobutzki, 1998, Cowen et al., 2000, Montgomery et al., 2001, Mora and Sale, 2002). The fishes of coral reefs and crabs and lobsters of reefs and other spatially heterogeneous environments share a number of biological characteristics, particularly the possession of life histories featuring a relatively short pelagic larval stage followed by a longer, relatively sedentary adult stage. Ecologists studying both groups have begun to explore the question of connectivity, so a comparison of the progress made and the approaches taken is timely. This paper, therefore, provides an overview of the types of empirical data that can be used to examine the process of larval dispersal and the resultant connectivity of spatially segregated sub-populations of adult coral reef fishes and benthic decapods. We do not aim to summarize the emergent biological patterns (this has been done effectively by McConaugha, 1992, Bradbury and Snelgrove, 2001, among others), but rather to assess the state of the research process. There is an extensive literature on larval ecology for a variety of other taxa, but we confine our review to studies on lobsters, crabs and coral reef fishes due to the similarities in their post-settlement ecology. Even having restricted our focus to these taxa, there is undoubtedly useful work that we have overlooked, especially within the grey literature. Nevertheless, we hope that we have succeeded in surveying what can be done and what has been done to investigate dispersal of these two broad groups of organisms, and therefore what will be fruitful directions for researchers working on each group in the future.
Section snippets
Types and application of data
There are two broad classes of data with which to investigate larval dispersal. The first is composed of information on the processes that determine dispersal. Process data consist of: (i) movement of water masses, (ii) timing and location of egg or larval release, (iii) pelagic larval duration, (iv) behaviour and sensory abilities of larvae, and (v) demographic rates of larvae. These causal agents collectively determine how many larvae move between any two locations at any given time.
Hydrodynamics
Hydrodynamics have relevance for biologists studying dispersal, but are not directly reliant upon the biology of focal organisms and therefore can be examined separately. However, physical oceanographers, like all scientists, need to make choices about the spatio-temporal scales and degree of resolution of their work, and the biological aspects of dispersal help define the relevant scales. Although dispersal has been considered in the context of very large-scale current patterns (hundreds to
Conclusions
Of the various topics relevant to understanding larval dispersal we have reviewed, there do not seem to be any that have been completely neglected by researchers working on either coral reef fish or benthic decapods. Rather, the relative emphasis placed upon different topics varies between the two groups and each can follow the lead of the other in filling those gaps. For example, coral reef fish ecologists have been more active in estimating larval durations in the field, due largely to the
Acknowledgements
PFS thanks the EDFAM conference organizers for the opportunity to participate. We acknowledge the generous support of the Canadian National Science and Engineering Research Council (particularly Collaborative Research Opportunity grant #227965-00), which supports half of JPK’s post-doctoral appointment. The University of Windsor supports the other half of JPK’s appointment.
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