The notion of large scale patterns in biotic interaction strength has a long history in ecology, with the expectation that interactions will be stronger in the tropics than temperate regions. However, the empirical evidence has been mixed. In light of ongoing climate change and redistribution of species it is important to study interactions, such as those between predators and prey that shape community structure and ecosystem function. Prey defensive traits dictate susceptibility to predators, and behavioural, physical and chemical traits will influence the outcomes of predator-prey interactions in existing and novel systems. Here, a meta-analysis was used to test the prediction that predation pressure on sea urchins, a group of consumers that influence community structure, is strongest in the tropics (Chapter 2).
Predator impacts on sea urchins were highest in tropical coral reefs and decreased towards the poles in rocky reefs. However, latitude was weakly correlated with effect sizes, and the strongest predictor of predator impacts was sea urchin species. This suggests an important role of prey identity (i.e. behavioural, physical, and chemical traits) rather than large-scale abiotic factors in determining variation in interaction strengths. In light of this, the sea urchin Heliocidaris erythrogramma was sampled along a latitudinal gradient from Tasmania to Queensland in Australia to test the hypothesis that traits associated with anti-predator defence - including size, attachment force, and test strength - are most strongly expressed in warmer waters (Chapter 3). Test size increased, and spine length and test strength decreased with increasing latitude and decreasing temperature. The results highlight temperature as predicting phenotype, but also corroborate the findings of the meta-analysis in that anti- predator defences were stronger at lower latitudes.
Climate change is facilitating the redistribution of species, and species traits will shape their interactions in novel communities. Two sea urchins that are shifting their ranges are tropical Tripneustes gratilla and temperate Centrostephanus rodgersii, both of which have the ability to denude macroalgae forests. I quantified the non-consumptive effects of predators on these species to test the hypothesis that T. gratilla and C. rodgersii have fundamentally different behavioural traits that will lead to differences in algal consumption (Chapter 4).
Centrostephanus rodgersii preferentially fed at night, but decreased activity in the presence of predators resulting in reduced algal consumption. In contrast, T. gratilla exhibited no difference in diel activity patterns and no change in behaviour in the presence of predators. A mechanism that could facilitate continual foraging by T. gratilla is a unique chemical defence: the release of small venomous appendages with jaws and teeth, the globiferous pedicellariae (Chapter 5). In experiments, pedicellariae were unpalatable to fish consumers and when subjected to simulated predator attack, T. gratilla released a cloud of pedicellaria heads into the water column. Flume experiments established the presence of a water-borne pursuit-deterrent signal associated with this release of pedicellariae. These novel results add to our understanding of how T. gratilla is able to reach high densities in many reef habitats with subsequent impacts on algal cover. This thesis highlights the importance of understanding the function of individual traits of sea urchin species in determining the outcomes of predator-prey interactions in a changing ocean.