Urchin Barrens: The Spiny Threat to our Native Shores

Marine Biology | Jarod McTaggart

Kelp forests are one of New Zealand’s most valuable and biodiverse marine ecosystems. Unfortunately, they are under threat of becoming urchin barrens. These barrens form due to the overgrazing of sea urchins, transforming this biodiverse ecosystem into a barren undiverse stable state. Anthropocentric fishing of urchin predators causes a trophic cascade that initiates a regime shift of the alternative stable state from kelp forest to urchin barren. Conservation of existing kelp forests and restoration of degraded ecosystems is necessary and should be complemented by fisheries management and marine protected areas, driven by legislation and cultural awareness of the issue.

Introduction

New Zealand is home to many unique, beautiful, and valuable marine ecosystems. The rocky reef kelp forest is one of the most economically, ecologically, and culturally valuable on our shores. However, due to sea urchin overgrazing, these ecosystems are being transformed into barren wastes that are far less valuable. This alternative stable state is known as an urchin barren. These barrens are beginning to dominate New Zealand shores, where once green forests of kelp stood, is now bare rock dominated by urchins (Evechinus chloroticus).

Kelp Forest Benefits

Ecosystem benefits are defined as ecological characteristics, functions, or processes that directly or indirectly contribute to human well-being. These services are further divided into categories that relate to how they benefit humans. Provisioning services are services that are utilised or exploited by humans to produce resources humans use directly. Regulating services are passive benefits that ecosystems have, whereby existing in a healthy state they protect human populations from environmental and biological threats. Cultural services are related to the recreational, aesthetic, and spiritual benefits that an ecosystem provides. Supporting services are the basic ecological services that an ecosystem provides, where through functioning it provides a passive benefit [1].

The major provisioning service these kelp forests provide is to fisheries, where the ecosystem acts as a source of food and habitat for economically important species and the food webs that support these stocks. In many cases, they are a nursery habitat for juveniles of important fish species [2]. Filbee-Dexter and Wernberg estimated the value kelp forests provide directly from provisioning services globally is ~$1.6 million NZD per km per year [3]. This is not even counting the other services kelp forest provides, just the economic value from direct use. There is also potential for direct harvest and use of kelp in developing new markets from agriculture to the food and supplement industry [4].

Kelp forests provide a number of regulating services that include lowering wave action by forming physical barriers, which reduces erosion of coastlines as well as protecting coastal settlements from storms [5]. Kelp forests have also been researched as a carbon sink that captures atmospheric CO2 supporting blue carbon schemes [6].

Healthy rocky reef kelp forests also provide a number of cultural services. These include recreational fishing and diving, as well as providing biodiversity people can observe and appreciate. Additionally, many of the species that occupy the rocky reef, including urchins/kina and their predators are taonga species. Taonga is the Te Reo Māori term for something that is treasured and/or culturally significant to Māori - many native species are classified as taonga due to their importance to Māori culture and tradition. These taonga are protected under Te Tiriti o Waitangi (The Treaty of Waitangi), with taonga species being specifically addressed in Wai 262, a claim lodged with the Waitangi Tribunal in 1991. In 2011, the Tribunal released their report detailing reform and recommendations that protect Māori rights over taonga, thus there is a legal obligation to protect and preserve these species. Furthermore, having native ecosystems that are biodiverse makes them more resilient to exotic invasion, further protecting native species [5].

The supporting services are perhaps the most significant services this ecosystem provides. Kelp is important in a number of cycles, from nutrient cycling, to carbon cycles, and larval distribution [7-8]. Primary productivity is critical to the function of any ecosystem where organisms, including kelp, use light to produce energy. These photosynthetic organisms are at the base of every food chain and thus support all higher trophic levels in the ecosystem. The kelp in these forests provides food for every species that inhabit the ecosystem, thus this primary productivity is integral to life. Kelp provides complex habitat and refuge for organisms within the ecosystem, many of which are native and protected species. Paired with the fact that it is a rich source of food, there is a high level of biodiversity in kelp forests. Biodiversity is incredibly important as it provides communities with resilience as well as a greater number of services the assemblage may offer.

Urchin Barren Causes

Rocky reef kelp forest ecosystem is under threat, with increasing numbers of urchin barrens forming. These barrens form where urchins overgraze kelp and prevent the kelp population from recovering. This is an ecosystem regime shift, where the ecosystem shifts to a different stable state, often due to disturbance in ecosystem function.

Urchin barrens are a discontinuous phase shift, where the threshold to reverse the regime shift is higher than what was required to initiate the shift. The principle of hysteresis describes this lag, where to return to the alternative stable state of a kelp forest, the urchin barren needs to fall well below the urchin density required to initiate the shift [9]. This creates an issue as the more valuable ecosystem state (kelp forest) is more sensitive than the alternative state, which is compounded by hysteresis, making it more difficult to maintain and conserve. In a study looking at urchin barrens globally, Ling et al. found that the hysteresis of this particular regime shift is approximately one order of magnitude between stable states [9]. Thus to revert back to a kelp forest, urchin numbers must be significantly lower than is required to initiate the original change in stable state.

The commonly understood reason for the regime shift of rocky reefs is the overfishing of urchin predators. These predators are higher in the trophic level and thus control urchin numbers in normal circumstances, however, due to anthropogenic fishing of these species, urchin numbers become uncontrolled [10]. The removal of these predators triggers a trophic cascade, where due to lower pressure on the urchin population from predators, urchin numbers increase drastically. This as aforementioned applies pressure to kelp, as with larger urchin populations kelp is overgrazed, resulting in the regime shift. This negatively affects all species that rely on kelp in this ecosystem, including urchins and their predators, as well as various other organisms.

The most common predator species that are fished are snapper (Pagrus auratus) and lobster (Jasus edwardsii & Sagmariasus verreauxi), both of which are recreationally and economically valuable species. When populations of these and other urchin predators drop due to either anthropogenic or environmental disturbance, urchin populations explode. This leads to the overgrazing and destruction of kelp forests. Not only does this impact every other organism in the environment, but the urchins themselves are negatively affected when kelp populations cease to recover. Studies show that urchins occupying barrens tend to have lower fitness than those in a healthy kelp forest ecosystem. In a number of different aspects, urchins are less healthy after a barren is established, from having lower growth rates to poorer body conditions and smaller gonad sizes [11]. It is clear that urchin barrens have a negative impact on every species that would normally occupy a healthy kelp forest, not to mention the anthropocentric benefits of having healthy kelp forests.

Urchin Barren Prevention

While it is clear that having healthy kelp forests is universally more valuable, urchin barrens are persistent, especially due to them having a high level of hysteresis. So what can be done in order to combat the formation of these barrens as well as facilitate the restoration of existing barrens to kelp forests? There are two facets to the problem of urchin barrens; conservation of healthy kelp forests and restoration of degraded barren ecosystems. Both approaches must be considered and utilised in tandem in order to reverse urchin barrens and maintain kelp forests.

Like any multifaceted problem, there is a litany of solutions and actions that must be considered. Further research is important. The more data there is to understand how kelp forests and barrens function, the more tools can be developed and introduced. For instance, little is known about the exact density of urchins per square metre required to trigger a regime shift from kelp forest to urchin barren. Monitoring techniques and technology are a necessary piece of the puzzle that is restoration and conservation. For instance, in an interview with Arie Spyksma, he explained that he is involved in developing artificial intelligence to classify urchin barrens from still and video imagery. Numerous other researchers are working to find solutions to aid in the understanding and restoration of kelp forests.

Another dimension that we must consider is the political discourse around Aotearoa’s oceans. Fishing, both commercially and recreationally, is the human pressure that has the largest impact on urchin predators. If any long-term success is to be had then there must be management and legislation put in place to protect predator species from overexploitation. The key is effective fisheries management that considers ecosystem health and longterm species population success over purely economic gains. Establishing more marine protected areas (MPA) in order to provide refuge for predator species, also results in spill-over, so surrounding areas will be more resilient to the formation of barrens. This spill-over effect has been shown as beneficial to fisheries [12], with this economic advantage of MPA’s potentially offering additional support for their creation.

Contemporary science is coming to accept that multifaceted approaches and transdisciplinary action is required for large-scale environmental issues. Urchin barrens are no exception, where researchers may be able to understand and provide tools to help with the restoration and conservation of kelp forests. Policymakers and community action must also work to contribute, be it by regulating commercial and recreational fisheries or by creating more marine protected zones. Science and technology are not the only tools that must be used to combat the urchin barren problem - science communication, education, policy, and community action are required to make a positive long-term change. Without a cultural understanding of the value these ecosystems have, legislation will not have the support it requires to pass. Without legislation, cultural support science and technology can only do so much, fighting an uphill battle. Like trophic levels we observe in nature, all of these dimensions are linked, they rely on each other to function. Thus, a multifaceted effort must be undertaken to make a long-term and sustainable impact in protecting one of New Zealand's most valuable and diverse marine ecosystems.

Conclusion

Rocky reef kelp forests are an incredibly valuable native ecosystem. Anthropocentric fishing has reduced the number of urchin predators to the point that urchin populations have exploded. Due to this, urchin overgrazing leads to a regime shift turning the once productive and biodiverse ecosystem into a barren that is comparatively undiverse and unproductive. To combat these changes, a mixed approach is required, where passive restoration of degraded ecosystems must be paired with active management (eg. removal or harvesting of urchins). This is further complemented by the formation of more marine protected areas and more effective fisheries management. For the long-term health of New Zealand shores, fisheries, biodiversity, taonga, and people, kelp forests must be restored.

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[6] Bradly, N., Syddall, V., Ingram, C., Clarkson, R., Elliot, A., Major, R., & Adams, S. (2021). Stocktake and characterisation of Aotearoa New Zealand’s seaweed sector: market and regulatory focus.

[7] Bayley, D., Brickle, P., Brewin, P., Golding, N., & Pelembe, T. (2021). Valuation of kelp forest ecosystem services in the Falkland Islands: A case study integrating blue carbon sequestration potential. One Ecosystem, 6.

[8] Cuba, D., Guardia-Luzon, K., Cevallos, B., Ramos-Larico, S., Neira, E., Pons, A., & AvilaPeltroche, J. (2022). Ecosystem Services Provided by Kelp Forests of the Humboldt Current System: A Comprehensive Review. Coasts, 2(4), 259-277.

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[11] Ling, S. D., Kriegisch, N., Woolley, B., & Reeves, S. E. (2019). Density‐dependent feedbacks, hysteresis, and demography of overgrazing sea urchins. Ecology, 100(2), e02577.

[12] McClanahan, T. R., & Mangi, S. (2000). Spillover of exploitable fishes from a marine park and its effect on the adjacent fishery. Ecological applications, 10(6), 1792-1805.

Jarod McTaggart - BA/BSc, Communication, Ecology

Jarod is studying a BA (Communications / Media, Film and Television) / BSc (Biological Science - Ecology) conjoint degree at the University of Auckland. He has a passion for filmmaking as well as the environment. His goal is to make a substantial contribution to conservation in New Zealand, be it through education, research, policy, or environmental management.