Eelgrass structural complexity mediates mesograzer herbivory on epiphytic algae
Structural complexity mediates ecological processes such as predation, competition, and recruitment in marine systems, but relatively little is known about its effects on herbivory. In temperate seagrasses, such as eelgrass (Zostera marina), the primary herbivores are small crustacean and gastropod mesograzers that promote seagrass persistence by preferentially consuming competing epiphytic algae. We used a laboratory grazing experiment, a field colonization experiment, and stable isotope analysis to determine whether one component of eelgrass structural complexity, shoot density, dictates the strength of mesograzer top-down effects on epiphytic algae, and whether this is influenced by mesograzer community composition. Our results suggest that increasing structural complexity shifted eelgrass communities from a bottom-up to a top-down controlled system. In the lab, mesograzers reduced epiphyte standing stock only in high-shoot density experimental communities, though grazing impact varied among different combinations of dominant mesograzer taxa. In our field experiment, epiphyte biomass was inversely correlated with mesograzer density in high but not in low-shoot density eelgrass plots. High-shoot density plots contained lower epiphyte biomass despite housing lower densities of mesograzers, when compared to low-density plots, suggesting potential effects of mesograzer behavior, community composition, or self-shading on epiphyte growth. Our results suggest that structural complexity can strongly influence rates of top-down and bottom-up processes in eelgrass habitat, and should be incorporated into future experiments on the role of herbivores in seagrass ecosystems.
KeywordsGrazing Habitat structure Seagrass Shoot density Stable isotope
We thank C. Bayne, J. Boucree, W. Dailey, R. Dunn, A. Harrington, S. Hengen, J. Jaeger, J. Joseph, J. Ledbetter, P. Shukla, and M. Yeager for their advice and assistance both in the laboratory and the field. We would also like to thank J. Long, A. Palacios, J.E. Duffy, J.J Stachowicz, P.L. Reynolds, and members of the Zostera Experimental Network (ZEN) for advice and guidance throughout the study. Our research was supported by a Segal Americorps Education Award, a San Diego State University Harold and June Grant Memorial Scholarship, a CSU COAST Graduate Student Award for Marine Science Research, a Lerner-Grey Grant for Marine Research, and a National Science Foundation grant (OCE- 1336905) to K. Hovel. This is contribution number 62 from the San Diego State University Coastal and Marine Institute.
Author contribution statement
EPV and KAH conceived of and designed the experiments. EPV performed the experiments and analyzed the data. EPV and KAH wrote the manuscript.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
- Clarke KR, Warwick RM (2001) Change in marine communities: an approach to statistical analysis and interpretation. PRIMER E Ltd, PlymouthGoogle Scholar
- Hunter MD, Price PW (1992) Playing chutes and ladders: heterogeneity and the relative roles of bottom-up and top-down forces in natural communities. Ecology 73:724–732Google Scholar
- Larkum AWD, Orth RJ, Duarte CM (2006) Seagrasses: biology, ecology, and conservation. Springer, DordrechtGoogle Scholar
- Loreau M, Naeem S, Inchausti P (2002) Biodiversity and ecosystem functioning: synthesis and perspectives. Oxford University Press, OxfordGoogle Scholar
- Michel L, Dauby P, Dupont A, Gobert S (2015) Selective top-down control of epiphytic biomass by amphipods from Posidonia oceanica meadows: implications for ecosystem functioning. Belgian J Zool 145:83–93Google Scholar
- Valentine JF, Duffy JE (2006) The central role of grazing in seagrass ecology. In: Larkum AWD et al (eds) Seagrasses: biology, ecology, and conservation. Springer, Dordrecht, pp 463–501Google Scholar
- Williams SL, Heck KL Jr (2001) Seagrass community ecology. Marine Community Ecology. Sinauer Associates Inc, Sunderland, pp 317–337Google Scholar