Bell 2016

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.<br><br>We deployed forage-choice assays using the five most common seagrass species
of Shark Bay (<i>Amphibolis antarctica,
Posidonia australis, Halodule uninervis, Cymodocea angustata </i>and<i> Halophila ovalis</i>), to determine whether
herbivory pressure and feeding choice changed across a salinity-stress gradient
from normal oceanic salinities (~38‰) to hyper-saline conditions (&gt;50‰). <br><br>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. <br><br>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.<br><i><br></i>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.