Long-term trends and ecological impacts of the subtropical sea urchin Centrostephanus rodgersii in northeastern Aotearoa New Zealand

Abstract

Changing climatic conditions along with other anthropogenic stressors can lead to irruptions of native species, with potentially similar widespread effects to that of range extending or invasive species. These are especially profound when they cause the loss of habitat forming organisms (e.g. kelp), resulting in the subsequent degradation of associated biodiversity and ecosystem function. In northeastern Aotearoa New Zealand, there are anecdotal reports that populations of the native subtropical sea urchin Centrostephanus rodgersii are increasing; however, little is known about the long-term trends or associated impacts on reef ecosystems. This thesis aims to examine population trends, feeding preferences and ecological impacts of C. rodgersii in northeastern New Zealand, relative to the common sea urchin Evechinus chloroticus. This study provides the first documentation of long-term increases in C. rodgersii in kelp forests and on invertebrate-dominated rock walls in this region. This has resulted in the emergence of C. rodgersii barrens, both within a long-standing marine reserve and fished archipelago, with a greater depth distribution than historic E. chloroticus barrens. Conversely, densities of E. chloroticus declined and associated barrens disappeared within the marine reserve while remaining stable at the fished location, following the expected response to marine protection. The increase in C. rodgersii coincided with substantial oceanic warming and low numbers of key predatory lobster in both locations. Long-term increases in C. rodgersii on rock walls led to declines in habitat-forming sessile invertebrates and increases in encrusting algae and bare rock. These effects were supported by feeding preference experiments with C. rodgersii exhibiting a more omnivorous diet and consistently feeding at higher trophic levels than E. chloroticus. This reveals that C. rodgersii has potential to impact a greater range of habitats and depths than E. chloroticus and could drive the widespread homogenisation of multiple rocky reef habitats. Overall, these results highlight how a changing climate combined with other stressors can cause changes to the abundance of native species with far-reaching impacts on marine ecosystems. This research demonstrates the novel and unprecedented threat C. rodgersii poses and stresses that steps are urgently needed to help prevent catastrophic shifts in New Zealand’s rocky reef ecosystems.