Boosting the natural population of New Zealand sea urchins may help to eliminate an invasive Asian sea kelp, but at what cost?
Invasive species are regarded as one of the greatest threats to biodiversity worldwide and frequently cause serious damage to local ecosystems. They are particularly difficult to deal with in marine habitats, where efforts to control their spread tend to be extremely labour-intensive.
One promising method of reducing harmful invasive species is through the use of biological control programmes. Augmentative biocontrol works by massively boosting the pest’s natural enemies which then act to supress the pest population. But this approach has rarely been used in marine habitats, largely because of the fear that the biocontrol agent may have unforeseen negative effects elsewhere in the marine environment. However, an opportunity recently arose to test this method when the invasive seaweed Undaria pinnatifida was found in Breaksea Sound in Fiordland, New Zealand.
A huge response was initiated to assess the extent of the problem. Due to the remote location and logistical challenges, the agencies involved were open to new methods to treat Undaria, including looking at the use of the New Zealand sea urchin Evechinus chloroticus as a biocontrol agent.
Researchers set up an experiment to see if the sea urchin (locally known as kina) would have any negative effects on the rocky reef habitats where the invasive kelp was found. Divers collected sea urchins from the outer reaches of Breaksea Sound and placed huge numbers (over 50 individuals per m2) on the rocky reef study sites.
Over time, the number of sea urchins declined to about 5 individuals per m2 as they moved into more suitable flat and rocky areas. Despite this movement, by the end of the study, most of the sites had shifted from kelp forests to ‘urchin barrens’. The initial extremely high densities of sea urchins had rapidly grazed the natural seaweeds (Ecklonia and Carpophyllum), eventually leaving only encrusting algal forms to dominate the ‘urchin barrens’. This was a dramatic change to the natural rocky reef community, and certainly shows the potential of biocontrol agents to have far-reaching effects on organisms other than the target pest species.
Nonetheless, these effects are still nothing compared to the negative impacts that are associated with the establishment of an invasive species in a unique marine area such as Breaksea Sound. The study also showed that the changes that occurred were extremely localised and could easily be revered by simply removing sea urchins from the treated areas. As the number of sea urchins continues to decline, it is likely that the native brown seaweeds will eventually re-establish.
So there is some potential for augmentative biocontrol to be a useful tool in the management of marine pests. However, this method has its limitations and is probably only of real use in small-scale programmes where the aim is to control rather than eradicate the pest population.
Reference: Javier Atalah, Grant A. Hopkins, & Barrie M. Forrest (2013). Augmentative Biocontrol in Natural Marine Habitats: Persistence, Spread and Non-Target Effects of the Sea Urchin Evechinus chloroticus PLoS ONE, 8 (11) DOI: 10.1371/journal.pone.0080365
Image: Paddy Ryan