Environmental stress drives herbivory rates and species selection in subtidal seagrass communities
Grazing is a fundamental ecological process structuring seagrass ecosystems, yet the environmental drivers influencing grazing are poorly understood. Whilst the stress gradient hypothesis (SGH) is commonly used to predict competitive species interactions under environmental stress, it is possible to...
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Figshare
2016
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| Online Access: | https://dx.doi.org/10.6084/m9.figshare.3506276.v2 https://figshare.com/articles/Bell_2016/3506276/2 |
| Summary: | Grazing is a fundamental ecological process structuring seagrass ecosystems, yet the environmental drivers influencing grazing are poorly understood. Whilst the stress gradient hypothesis (SGH) is commonly used to predict competitive species interactions under environmental stress, it is possible to adapt the mechanistic model behind the concept to include predictions for herbivory. In subtidal marine systems, however, this modified SGH has seldom been tested. We deployed forage-choice assays using the five most common seagrass species of Shark Bay ( Amphibolis antarctica, Posidonia australis, Halodule uninervis, Cymodocea angustata and Halophila ovalis ), to determine whether herbivory pressure and feeding choice changed across a salinity-stress gradient from normal oceanic salinities (~38‰) to hyper-saline conditions (>50‰). Biomass of seagrass tissue removed from forage-choice assays decreased as environmental stress increased, showing salinity is a key environmental driver of grazing. The salinity stress threshold was identified as the marine environment approached hyper-saline conditions at around 41‰. Here, herbivory pressure decreased to negligible levels, meaning seagrass community structure was influenced by other ecosystem processes. With salinity-stress such a predominant feature within this area it is therefore likely to be a major contributor. Herbivores preferred the smaller tropical/sub-tropical seagrass species compared to the larger temperate species that dominate seagrass cover in Shark Bay. This preference was upheld across the entire salinity-stress gradient and was correlated with enriched seagrass nitrogen and phosphorous concentrations. Our work supports the modified SGH and presents the first example of this novel hypothesis predicting herbivory interactions along a salinity-stress gradient within the marine environment. By demonstrating the fundamental relationship between trophic interactions and environmental conditions we underscore the importance of including a suite of abiotic and biotic processes when studying seagrass dynamics. Synthesis: The complexity of interactions within and between biotic and abiotic components of marine systems plays an uncommonly recognised role in determining the structure and dynamics of subtidal seagrass environments. We show that the relative importance of trophic interactions can be a function of environmental conditions, and advance our understanding of the effectiveness of field approaches in developing predictive frameworks to determine potential mechanisms of impact on marine communities. |
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