The legacies of land-clearance and trophic downgrading accumulate to affect structure and function of kelp forests
Wing, Stephen; Shears, Nicholas; Tait, Leigh; Schiel, David (2022), The legacies of land-clearance and trophic downgrading accumulate to affect structure and function of kelp forests, Dryad, Dataset, https://doi.org/10.5061/dryad.05qfttf5z
Aotearoa New Zealand is the last major landmass 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 that have been differentially affected by ecological legacy effects. The study was based on the depth-specific associations between sea urchins (Evechinus chloroticus) and the common kelp (Ecklonia radiata) from multiple sites within each region, some of which were sampled over two decades within no-take marine reserves and in actively fished areas. Satellite-derived estimates of water column properties were used as proxies for the relative effects of coastal sedimentation on kelp forest habitats. We then used an information-theoretic framework to assess the relative factor weightings of marine reserve effects on urchin density, total suspended solids and regional urchin density on the depth-specific density of E. radiata. Fishing effects were significant within and outside of reserves in the Northern and Central regions, but the effect-sizes were by far strongest in the Northern region. In the Central region characterised by extensive land clearance and forestry combined with high coastal retention of water, the concentration of total suspended solids had a major influence on the depth distribution of kelp, confining it to shallow water (<10m depth) in small patches where urchins did not overgraze. These patterns are in sharp contrast to the small marine reserve effects and deep distribution of E. radiata in the Southern region, which has intact native forested catchments, comparatively low fishing pressure and a large regional network of marine reserves. The results highlight important differences in how centuries-old legacies of land clearance and exploitation affect regional-scale dynamics of sea urchins and kelp, and define the appropriate spatial scale of ecosystem-based management of kelp forest habitats.
New Zealand National Science Challenge: Sustainable Seas, Award: 4.1.1
New Zealand National Science Challenge: Sustainable Seas, Award: 2.1