High-latitude marginal reefs support fewer but bigger corals than their
tropical counterparts
Abstract
Anthropogenic impacts are typically detrimental to tropical coral reefs,
but the effect of increasing environmental stress and variability on the
size structure of coral communities remains poorly understood. This
limits our ability to effectively conserve coral reef ecosystems because
size specific dynamics are rarely incorporated. Our aim is to quantify
variation in the size structure of coral populations across 20 sites
along a tropical-to-subtropical environmental gradient on the east coast
of Australia (~23°S to 30°S), to determine how size
structure changes with a gradient of sea surface temperature, turbidity,
productivity and light levels. We use two approaches: 1) linear
regression with summary statistics (such as median size) as response
variables, a method frequently favoured by ecologists; and 2)
compositional functional regression, a novel method using entire
size-frequency distributions as response variables. We then predict
coral population size structure with increasing environmental stress and
variability. Together, we find fewer but larger coral colonies in
marginal reefs than in tropical reefs, where environmental conditions
are more variable and stressful for tropical corals. Our model predicts
that coral populations may become gradually dominated by larger colonies
(> 148 cm2) with increasing environmental stress. Fewer but
bigger corals suggest low survival of smaller corals, slow growth, and /
or poor recruitment. This finding is concerning for the future of coral
reefs as it implies populations may have low recovery potential from
disturbances. We highlight the importance of continuously monitoring
changes to population structure over biogeographic scales.