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Planktonic Settlement in the Rocky Intertidal |
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Communities that are high in diversity are considered to be healthier than species-poor communities because the chances that a community will adapt to a changing environment are greater when multiple strategies are present. This concept is becoming increasingly important as species are continually lost and conservation issues are more and more dire. The factors that influence biodiversity levels have thus been accordingly studied, environmental stability and productivity are among the two receiving the most attention (Worm and Duffy, 2003). Productivity is the rate of conversion in an area from nutrients to plant biomass per unit time (Smith and Smith, Waide et.al. 1999). Studies that relate productivity to biodiversity mostly focus on communities of primary producers, and the general school of thought suggests that the relationship is unimodal; that is, as productivity increases, biodiversity increases less and less until it begins to decline at an increasing rate. The logic behind this model relates increases in nutrient availability to higher niche overlap potential, in which a greater number of species might theoretically fit into. Negative relationships are reportedly related to evolutionary immaturity and system infeasibilities (Wade et.al. 1999). However, because biodiversity can also change the productivity potential, and productivity shifts can cause fluctuations in community abundance and composition, predicting the outcome of a change in productivity may be complicated. One publication on this subject matter reported that a survey on 200 relationships between biodiversity and productivity deemed 30% of relationships unimodal, 26% were positive linear, 12% negative linear, and 32% were not significant (ibid). The outcome seems to be related closely to the scale at which the relationship is examined, with larger scales tending to be more positive and more significant (Chase and Leibold, 2002). The effect of productivity on community regulation in the marine environment has been studied to a much lesser extent, and usually focuses on top-down and bottom-up effects. A general trend coming out of this work is that community regulation mechanisms (especially for sessile organisms) can vary with wave exposure, tidal height, and nutrient availability. In nutrient-limited marine environments, areas of high wave action, or high intertidal regions, community assemblages are likely to be determined by bottom-up processes. Alternatively, top-down control mechanisms regulate community structure in nutrient-rich, wave-protected, or low tidal regions (Menge, 2000, Raghukumar and Anil, 2003). These patterns are apparently community-dependent, as in one study, the abundance of an algal species that was resistant to predation by grazers increased with nutrient upwelling. This caused a decrease in abundance of algae prone to grazing, thereby changing the community structure through bottom-up processes in nutrient-rich areas (Nielsen and Navarrete, 2003). The effects of productivity on the settlement marine zooplankton species may be dependent on the life history strategies of each particular species, as well as the availability of nutrients already present in the experimental location. If nutrients are not already limiting, then adding nutrients should have no further effect on productivity at small scales, unless the relationship is negative, in which case it might be expected that additional nutrients would cause productivity to decrease. Whether or not a species actively settles due to food/nutrient availability will depend on the life history and trophic strategy for that organism. Some species have larval stages that do not require food because they carry their own yolk sacks. For other species, the extent of their nutrient needs might be dependent on how much time the larvae have spent in the pelagic, and how readily available their food sources are (see figure 1). Nutrient availability will have an effect on settlement substrate and location because natural selection is so intense for most larvae that permanently settle in the rocky intertidal. If an individual happens to settle in an area without the proper food, it could mean a fitness of zero. Some species have adapted to actively select suitable habitat based on chemical cues and substrate texture (see figure 2).
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