The legacies of land-clearance and trophic downgrading accumulate to affect structure and function of kelp forests

View paper

Aotearoa New Zealand is the last major land mass settled by people, and therefore provides a recent record of ecological legacy effects in the coastal zone. Large-scale land clearances of forests accelerated over the last century, affecting the concentration of suspended sediments, light environment and nutrient composition on rocky reefs, and consequently the distribution, abundance and composition of algal forests. Environmental effects were compounded in many places by overfishing and long-term declines of large predatory species, often leading to proliferation and extensive grazing by sea urchins.

Here we examine these processes in three biogeographic regions (North, Central and South) that have been differentially affected by ecological legacy effects. Effective management of kelp forest habitats relies on our understanding of how environmental conditions have changed on rocky reefs, particularly the effects of fine sediments on kelp growth and survival, how regional scale changes have occurred in populations of sea urchins particularly, and how changes in abundance of large predatory species such as rock lobsters, snapper and blue cod due to overfishing influence the local extent of sea urchin grazing in kelp forests. Our results highlight important differences in how centuries-old legacies of land clearance and overfishing affect regional-scale dynamics of sea urchins and kelp, and define the appropriate spatial scales, land-sea and ecosystem connections to consider for effective ecosystem-based management of critical kelp forest habitats.