Abstract
Combined environmental and ecological stressors are causing degradation of kelp forest ecosystems. Where kelp biomass is diminished, communities will be affected in ways that depend on attachments to macroalgal food webs throughout the life history of consumers. Simultaneously, weakened interactions between large predators and herbivores following overfishing or habitat loss may exacerbate kelp deforestation. To advance understanding of these size-structured processes in kelp forests of Aotearoa New Zealand, I first examined trends and variation in lifetime trophic shifts of three fish species, especially patterns of macroalgal reliance during early growth. I then tested whether predator detection affected feeding behaviour of a sea urchin (Evechinus chloroticus), providing novel evidence for a non-consumptive and size-specific interaction through which predators might influence herbivory on local reefs.
In Chapter 2, I used stable isotope analyses to quantify associations of basal resource use, trophic position, and size in two carnivores (Notolabrus fucicola, Parapercis colias) and an herbivorous fish (Odax pullus). I estimated heightened but variable reliance on macroalgae-derived prey during juvenile dispersal of N. fucicola and P. colias, followed by stabilisation of basal resource mixtures and trophic breadth contraction in mature fish. I also provided fine-scale assessments of individual variation in the magnitude and timing of critical ontogenetic diet shifts in these species, such as peaks in omnivory of young O. pullus.
In Chapter 3, I compared trophic (isotopic) chronologies of P. colias subpopulations that were likely experiencing differences in fishing pressure and resource availability. Estimated feeding on macroalgae-fuelled diets correlated with improved physiological condition, especially growth (length-at-age). In some subpopulations, early macroalgal reliance seemed to be a consistent trophic strategy that supported post-settlement growth and long-term survival. In other cases, it appeared that juvenile diet shifts were much more variable, with adult success instead facilitated by limited fishing pressure or an abundance of high-quality prey.
In Chapter 4, I measured changes in urchin feeding rates in the presence of lobster excretions (predator cues) across a range of predator and prey body sizes. Lobster-exposed urchins reduced per-capita feeding rates by 30-40% at larger sizes (>5 cm test diameter). Although smaller urchins responded similarly to the presence of a lobster, their low feeding capacity yielded negligible differences in the mass of kelp consumed across predator treatments. Overall, especially as larger lobsters did not drive stronger feeding reductions, this risk-mediated effect did not appear to scale with size-based vulnerability to predation.
In Chapter 5, I tested effects of P. colias presence on urchin feeding rates and again measured a 30-40% reduction in kelp consumption by larger urchins exposed to predator cues. In this experiment, small urchins did not show a clear response to predator excretions despite being more vulnerable to P. colias than larger individuals. Paired with estimates of urchin activity, I determined that heightened vigilance was a plausible mechanism for observed feeding reductions. Finally, I concluded that responses of larger urchins to predator cues from lobsters or P. colias were proportionally similar, and therefore did not correspond to predator-specific capacity for urchin consumption.
In Chapter 6, I summarised the primary contributions of the present work, including novel assessments of size as a driver of basal resource reliance in kelp forest fishes and non-consumptive interactions among E. chloroticus and their predators. I then discussed outstanding research questions, such as the specific benefits gained from juvenile feeding in macroalgal food webs and the identification of risk-signalling compounds that are detectable by urchins. Collectively, my findings can help generate insights and hypotheses regarding consequences of kelp forest restoration for individual fishes throughout development, as well as shifts in urchin grazing behaviour that might follow recovery of predator populations.