Using Management Policies as Ecosystem Experiments

It has become increasingly common to use management policies as experiments and test their effects on ecosystem dynamics. An excellent example of this approach is the use of marine reserves to investigate the ecosystem-level consequences of fishing. Essentially, well-enforced marine reserves constitute large-scale human-exclusion experiments and provide controls by which to test the ecosystem effects of reducing consumer biomass via fishing at an ecologically relevant scale. Dramatic shifts in nearshore community structure have been documented in well-established and well-protected marine reserves in both Chile and New Zealand. In northeastern New Zealand's two oldest marine reserves, the Leigh Marine Reserve and Tawharanui Marine Park, previously fished predators, snapper (Pagrus auratus) and rock lobster (Jasus edwardsii), have increased in abundance by 14- and 3.8-fold, respectively, compared to adjacent fished waters. Increased predation leading to reduced survivorship and cryptic behavior of their herbivorous prey, the sea urchin (Evechinus chloroticus), has allowed the macroalga (Ecklonia radiata) to increase significantly within the reserves, a trend that has been developing in the Leigh reserve for the past 25 years (Figure 7). Although this provides evidence that fishing can indirectly reduce ecosystem productivity, the trophic dynamics described above are context dependent and vary as a function of depth, wave exposure, and oceano-graphic circulation (Figure 8). For example, both in the presence and absence of fishing, urchin densities decline

Figure 7 (a) In nearshore fished ecosystems in northeastern New Zealand, snapper and lobster densities have been reduced due to fishing pressure resulting in high sea urchin densities, urchin barrens, and reduced kelp production. (b) In marine reserves, where previously fished snapper and lobster have recovered, sea urchins that have not been consumed by these predators behave cryptically, hiding in crevices. Consequently, kelp forests of Ecklonia radiata dominate. Photos by Nick Shears, Hernando Acosta, and Timothy Langlois.

Figure 7 (a) In nearshore fished ecosystems in northeastern New Zealand, snapper and lobster densities have been reduced due to fishing pressure resulting in high sea urchin densities, urchin barrens, and reduced kelp production. (b) In marine reserves, where previously fished snapper and lobster have recovered, sea urchins that have not been consumed by these predators behave cryptically, hiding in crevices. Consequently, kelp forests of Ecklonia radiata dominate. Photos by Nick Shears, Hernando Acosta, and Timothy Langlois.

Figure 8 The effects of fishing on nearshore ecosystems are influenced locally by wave exposure and regionally by oceanographic circulation. (a) In northeastern New Zealand, ocean circulation patterns influence nutrient delivery and thus (b) spring and (c) summer pelagic primary production. Satellite images: SeaWiFs Project, Ocean Color Web.

Figure 8 The effects of fishing on nearshore ecosystems are influenced locally by wave exposure and regionally by oceanographic circulation. (a) In northeastern New Zealand, ocean circulation patterns influence nutrient delivery and thus (b) spring and (c) summer pelagic primary production. Satellite images: SeaWiFs Project, Ocean Color Web.

to nearly 0 individuals per m2 below depths greater than 10 m due to unfavorable conditions for recruitment, despite the presence or absence of snapper and lobster, while at depths above 3 m, wave surge can preclude urchin grazing both inside and outside the reserves. Furthermore, where oceanic conditions hinder urchin recruitment, the effects of fishing on macroalgae become less clear-cut. These physical constraints highlight the importance of abiotic context on biotic interactions. Ultimately, one can gain a lot of information by using management policies as experiments.

Although policy experiments have played an important role in elucidating ecosystem dynamics, in many cases, it is politically intractable or logistically impossible to experiment with whole ecosystems. Under such circumstances, researchers have used alternative techniques to explore ecosystem dynamics. Models in ecology have a venerable tradition for both teaching and understanding complex processes. Ecosystem models are now being used to gain insight into the ecosystem-level consequences of management policies, from fisheries to carbon emissions. For more information on ecosystem models and using management policies as experiments, see the section entitled 'Social-ecological systems, Humans as key ecosystem components'.

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