Ecological Archives E088-041-A10

Donna Drury McCall and Chet F. Rakocinski. 2007. Grass shrimp (Palaemonetes spp.) play a pivotal trophic role in enhancing Ruppia maritima. Ecology 88:618–624.

Appendix J. Possible artifacts, biases, and scaling issues associated with the field experiments.

Various artifacts, biases, and scaling issues may influence the outcome of in situ field experiments. By incorporating ambient controls in a blocked split-plot design, we attempted to address some of these challenges. Although field experiments ensued within small areas, concurrent samples of ambient conditions subject to ecological effects at larger spatial scales served as controls. Scaling issues were also addressed in this study through the spatial arrangement of sites within the blocked split-plot design. Three sites were configured as blocks separated on a spatial scale coincident with the spatial distribution of Ruppia maritima beds in this system.

Marginally significant Site effects on shoot density occurred in Experiments 1 and 3. Pairwise post hoc comparisons of sites via LSD tests in SPSS 11.0 confirmed that shoot density was significantly different between two of the sites in Experiment 1 (P = 0.019). Shoot density was also significantly lower at the middle site than at the two outer sites in Experiment 3 (P = 0.003 and 0.001). Although Site effects on Shoot Density and Biomass showed that SAV condition varied on a landscape scale, main effects were not confounded by interactions with site effects.

Inherent biases in using microcosms to contain shrimp and SAV could not be avoided. For example, movements of grass shrimp were restricted by the microcosms. Consequently, diet choices of grass shrimp may not have matched those of shrimp under ambient conditions, due to differences in the availability of alternative food types. Effects of predation threats on grass shrimp behavior were also certainly diminished (see Appendix K) while competitive interactions were probably enhanced within microcosms (see below).

A microcosm carrying-capacity-effect was implied by shrimp recovery pattern. An overall recovery success across all three experiments of 83% (i.e., 2.5) of the original grass shrimp from the Medium Shrimp treatment was considerably higher than that of 65% (i.e., 6.5) from the High Shrimp treatment. Limiting resources (e.g., food, space, DO) inside microcosms may have facilitated greater density-dependent mortality within the High Shrimp treatment. Consequently, recovery success from the High Shrimp treatment corresponded to an effective density of 400 shrimp m2. These results are analogous to a mesocosm study conducted by Duffy et al. (2001) in which variation in amphipod grazer biomass became comparable across experimental treatments, leading the authors to conclude that limiting resources imposed a common carrying capacity within the mesocosms.

LITERATURE CITED

Duffy, J. E., K. S. Macdonald, J. M. Rhode, and J. D. Parker. 2001. Grazer diversity, functional redundancy, and productivity in seagrass beds: an experimental test. Ecology 82:2417–2434.



[Back to E088-041]