Scale of dispersal in varying environments and its implications for life histories of marine invertebrates

Abstract

Summary.We present several models concerning the short term consequences of spreading offspring in varying environments. Our goal is to determine what patterns of spatial and temporal variation yield an advantage to increasing scale of dispersal. Of necessity, the models are somewhat artificial but we feel they are a reasonable approximation of and hence generalizable to natural systems. With these models we examine consequences of dispersal arising from environmental variation: increased environmental variance, different degrees of spatial and temporal correlation, some arbitrary spatial patterns of favorability and finally some patterns derived from long-term, large-scale weather data collected along a contiguous stretch of coastline from southern Oregon to northern Washington (USA). We examine the costs and benefits of increasing scale of dispersal in both density dependent and density independent models. Several conclusions may be drawn from the results of these models. In the absence of any spatial or temporal order to favorability (where favorability is directly proportional to either fitness or carrying capacity) increasing scale of spread produces a higher rate of population increase. At larger scales, though, an asymptote of maximum relative advantage is approached, so each added increment of spread has a smaller contribution to fitness. This asymptote is higher and the approach to it relatively slower with increasing environmental variance. For a given environmental variance, increasing spatial correlation results in a slower approach to the same asymptote. In density independent models, increasing temporal correlation of fitness selects against increased dispersal if expected differences between sites are sufficiently great relative to variation within sites; but in this instance, density dependence yields a somewhat different result: dispersers have a refuge at sites of low carrying capacity or sites lacking non-dispersers. Finally, optimum intermediate scales of dispersal can occur where differences in expected fitness increase with increasing distance from the parental site, such as in a gradient, but where the environmental variation at a given site is fairly large relative to differences in expected fitness between adjacent sites.The foregoing results are extended for the following predictions. When greater longevity in a resistant phase of the life cycle reduces temporal variation in survival and fecundity, increased generation time should decrease the benefits of spreading offspring in an environment that would otherwise favor spread * Order of authorship alphabetical and by increasing age and height ** Present Address:

Extensive gene flow within sibling species in the deep-sea fish Beryx splendens

“…isolation by distance) is hard to admit for this reason, and also because this hypothesis is hardly compatible with the worldwide distribution of haplotypes a, h and i. Hypothesis (a.b), according to which the haplotype frequency cline would be transitory, supposes that the sampling was undertaken at the very place of secondary contact, and that the latter is very recent. The rate of colonisation of marine species with long pelagic stage is on the order of 10-100 km per generation [64], which is very fast and renders hypothesis (a.b) as highly unlikely.…”

The Ecology of Rafting in the Marine Environment. Ii. the Rafting Organisms and Community

“…This aspect may be particularly important when dispersal occurs over wide ocean expanses as compared with dispersal in an alongshore direction where larvae may have repeated chances to encounter suitable habitats. Palmer & Strathmann (1981) emphasised that pelagic larvae may pass many suitable settlement sites before reaching the competent stage -this may occur to a much lesser extent in species with direct development that are dispersed on floating objects, where both adults and offspring can be considered to be always competent to settle onto newly encountered habitats. For the genus Lasaea, Ó Foighil (1989) summarised in this context that species with direct development have equal abilities of long-distance rafting as species with planktonic larvae given that both have access to floating substrata.…”

Effect of wind and tidal advection on distribution patterns of rock crab Cancer irroratus megalopae in Block Island Sound, Rhode Island

“…Larval dispersal is thought to increase the probability of persistence for a population by increasing individual fitness. The benefit of dispersal is limited because individuals dispersing too far are expected to experience higher mortality than those that disperse shorter distances (Palmer & Strathmann 1981). Larvae have developed mechanisms to limit dispersal and increase the probability of delivery to appropriate settlement habitat.…”

“…The dispersive larval period can link patches of relatively sedentary adults into a metapopulation (Roughgarden et al 1985(Roughgarden et al , 1988. A tradeoff exists between dispersing too far from natal habitat (and settling in inappropriate areas) versus short distance dispersal which may increase competition between newly settled juveniles and adults (Palmer & Strathmann 1981). D~spersal distances of larvae may be pan-oceanlc as they spend months or years in the plankton (Scheltema 1968(Scheltema , 1986, or dispersal may be only a few tens of meters and last a short time (Young 1989, Stoner 1990.…”