Top-down effects on rocky reef ecosystems in north-eastern New Zealand: a historic and qualitative modelling approach

Since New Zealand was first settled, humans have had a profound impact on the abundance of many of the larger predatory marine species including white sharks (Carcharodon carcharias), killer whales (Orcinus orca), hapuku or groper (Polyprion oxygenios), sea lions (hocarctos hookeri), and small and medium sized sharks such as school sharks (Galeorhinus galeus) and bronze whaler (Carcharhinus brachyurus). In this study, we explored the potential impact of the removal of these top predators on rocky reef ecosystems in the Hauraki Gulf in order to determine the critical interactions amongst species and other ecosystem components, and identify those that should be a priority for future research. We used an existing qualitative model of a present day rocky reef ecosystem and included each top predator sequentially in the model by applying a positive or negative perturbation to the abundance of one or more of the existing groups in the model to simulate the effects of predation impact or release. In essence, using this approach, we worked from the present day situation back in time in four steps, at each step adding another group of top predators that were previously abundant in the Hauraki Gulf. For each step or iteration of the model, system stability was examined, and where necessary negative feedback loops were added to ensure that the system was stable.

The inclusion of small and medium sized sharks in the qualitative model had the most predictable impacts with highly certain negative impacts on the abundance of small and large lobsters, and highly certain increases in the abundance of large predatory invertebrates, such as snake stars and sea stars, and macro-invertebrate predatory fish, such as snapper. In the model none of these groups are directly preyed upon by small and medium sized reef sharks, indicating that the effect of shark predation propagates through the food web indirectly via linkages of prey groups to the affected groups in a complex manner.

Subsequent and sequential addition of white sharks and orca, hapuku, and sea lions to the model system had a variety of positive and negative impacts, but all interactions had predictability values of 0.32 or less indicating that the probability of obtaining the correct direction of impact on groups in the modelled ecosystem was moderate to low. This uncertainty indicates that more information than the direction of the first order interactions (i.e. a positive or negative impact on the abundance of a modelled group) is required to successfully explore the impact of these predators on reef ecosystems in northern New Zealand. Such additional information may be about the intensity of interactions which are likely to be density or encounter rate dependent as these larger predators range over very large distances revisiting the same area only intermittently or seasonally. The uncertainty in the outcome of modelling the impact of white sharks and orca, hapuku, and sea lions underscores the limitation of qualitative models. While they require less information to setup and run than mass-balance models, for example, they may also leave out potentially critical information.

The qualitative modelling undertaken in this study suggests that historically higher school shark and bronze whaler populations in the Hauraki Gulf were very likely to increase the abundance of reef fish, such as snapper, that prey upon macro-invertebrates, but depress rock lobster abundance. That more sharks may mean more snapper is an unexpected finding. It suggests that it may be fruitful to undertake a field sampling programme to examine changes in reef community structure across a gradient of small and medium shark population abundance in northern New Zealand, that the diet of these sharks should be investigated in greater detail, and the relationships between these predators and other components of the reef ecosystem should be remodelled once further data are available.