An increase in exploited species within marine reserves is often expected to benefit adjacent fished areas through the movement or ‘spill‐over’ of exploited species across reserve boundaries. ...However, harvest at and near reserve boundaries can also decrease abundances inside reserves, forming ‘edge‐effects’. Spatial analysis of gradients in abundance across reserve boundaries have been used to interpret the extents of edge‐effects and spill‐over. While the shape of these gradients will vary depending on species mobility and fishing intensity on reserve boundaries, they have largely been assumed to be gradual and quantified using continuous modelling strategies.
This study shows that continuous modelling strategies can lead to overestimations of spill‐over and edge‐effects by underfitting the rapid transition in abundance that can occur from reserve to fished areas. The abundances of spiny lobster (Jasus edwardsii) across the boundaries of two north‐eastern New Zealand marine reserves were quantified using catch survey data collected over 2 years. Several modelling approaches were used, and the predictive performance of each approach was compared.
Abrupt changes in lobster abundances were found across reserve boundaries and were best represented with models that incorporated a stepwise function across boundaries or where data from reserve and fished areas were modelled independently. Despite the mobile nature of this species, we found little evidence of edge‐effects and spill‐over within and adjacent to the surveyed marine reserves, which may reflect the low population status within these reserves. In contrast, standard continuous modelling approaches overestimated the extent of edge‐effects and spill‐over in both marine reserves.
Policy implications. This study shows how accurate assessments of edge‐effects and spill‐over can be used to evaluate the effectiveness of existing marine reserves in protecting exploited species. These findings emphasise the importance comparing several different modelling strategies when evaluating edge‐effects and spill‐over to ensure appropriate model fits. Although modelling approaches such as generalised additive models present a simple solution to exploring spatial variation in ecological data, we emphasise the importance of questioning the applicability of such strategies when assessing variation across areas of rapid change such as protected area boundaries.
This study shows how accurate assessments of edge‐effects and spill‐over can be used to evaluate the effectiveness of existing marine reserves in protecting exploited species. These findings emphasise the importance comparing several different modelling strategies when evaluating edge‐effects and spill‐over to ensure appropriate model fits. Although modelling approaches such as generalised additive models present a simple solution to exploring spatial variation in ecological data, we emphasise the importance of questioning the applicability of such strategies when assessing variation across areas of rapid change such as protected area boundaries.
Blooms of benthic dinoflagellates belonging to the tropical genus
Ostreopsis are an increasingly common phenomenon in temperate regions worldwide. This is reflected in the rapid upsurge of ...publications on
Ostreopsis from temperate regions since 2000. Relatively little is known about these blooms or their effects on other organisms. An unprecedented bloom of
Ostreopsis siamensis occurred on shallow reefs in northern New Zealand in 2004 providing an opportunity to examine the dynamics of an
O. siamensis bloom and its effect on community structuring sea urchins (
Evechinus chloroticus). The bloom occurred following a period of calm sea conditions with warmer than average water temperatures. The cover of
O. siamensis was highly ephemeral and strongly related to temporal and spatial variation in wave action. Blooms were most prevalent at sites protected from prevailing swells where
O. siamensis covered 30–60% of the reef with the concentrations on macroalgae reaching 1.4
×
10
6
cells
g
−1 wet weight, some of the highest recorded worldwide. Surveys of the health of sea urchins in relation to the cover of
O. siamensis suggested strong negative effects on this ecologically important herbivore and urchin densities declined by 56–60% at bloom sites over the study period. Further research is needed to examine the factors controlling the distribution and intensity of this new phenomenon, and into the ecological effects of such blooms on marine communities and the potential mechanisms responsible.
Summary
Diversity measurement techniques can present logistical and financial obstacles to conservation efforts. Ecoacoustics has recently emerged as a promising solution to these issues, providing a ...mechanism for measuring diversity using acoustic indices, which have proven to be beneficial in terrestrial habitats. This study investigates the application of acoustic measures as a tool for quick and effective marine diversity monitoring via direct, in situ comparison of ecoacoustics indices with species assemblage diversity measures from temperate rocky reefs.
Acoustic recordings and visual surveys of reef fish abundance were collected at nine sites in north‐eastern New Zealand. Three ecoacoustic indices originally developed for terrestrial use were then compared to three species assemblage diversity measures and compared using Pearson correlations. Additionally, four criteria for successful ecoacoustic indices were developed and tested as a means of standardizing future evaluation and use of acoustic indices: (i) positive correlations between species diversity and ecoacoustic indices in relevant frequency ranges, (ii) robustness to changes in spectral resolution, (iii) robustness to the presence of natural noise interference (i.e. wind) and (iv) robustness to the presence of anthropogenic noise.
Acoustic Complexity Index (ACI) was significantly correlated with Pielou's Evenness (J′) and Shannon's index (H′). Neither Acoustic Richness (AR) nor ACI was impacted by changes in spectral resolution, but values of the Acoustic Entropy Index (H) increased significantly between fast Fourier transformation (FFT) sizes 512 and 1024. H was consistently positively correlated with both H′ and estimated number of species (S) above a spectral resolution of c. 140·6 Hz (FFT size 1024). Wind did not affect any of the acoustic indices. As anthropogenic noise was included in these investigations, both ACI and H were considered robust to its presence.
While AR failed to meet all four criteria for a successful ecoacoustic indices, both ACI and H appeared to be appropriate for use on temperate reefs. In a time of accelerated global diversity loss, these two ecoacoustic indices show strong potential for use as efficient, non‐invasive marine diversity measures.
Decadal-scale observations of marine reserves suggest that indirect effects on taxa that occur through cascading trophic interactions take longer to develop than direct effects on target species. ...Combining and analyzing a unique set of long-term time series of ecologic data in and out of fisheries closures from disparate regions, we found that the time to initial detection of direct effects on target species (±SE) was 5.13 ± 1.9 years, whereas initial detection of indirect effects on other taxa, which were often trait mediated, took significantly longer (13.1 ± 2.0 years). Most target species showed initial direct effects, but their trajectories over time were highly variable. Many target species continued to increase, some leveled off, and others decreased. Decreases were due to natural fluctuations, fishing impacts from outside reserves, or indirect effects from target species at higher trophic levels. The average duration of stable periods for direct effects was 6.2 ± 1.2 years, even in studies of more than 15 years. For indirect effects, stable periods averaged 9.1 ± 1.6 years, although this was not significantly different from direct effects. Populations of directly targeted species were more stable in reserves than in fished areas, suggesting increased ecologic resilience. This is an important benefit of marine reserves with respect to their function as a tool for conservation and restoration.
Predator mediation of sea urchin grazing pressure may occur via lethal removal of individual sea urchins or non-lethal modification of sea urchin behaviour. Several studies have shown that ...predation-related cues can affect sea urchin movement and grazing rates, but generalisations about the types of cues that prompt responses and the magnitude of those responses will require further research on a wider variety of species. We examined the effects of potential alarm cues on behaviour of the habitat-forming sea urchin Evechinus chloroticus (Echinometridae) on fished rocky reefs in northeastern New Zealand, where predators are uncommon and the sea urchins form barrens. Exposed E. chloroticus (i.e. those not in crevices) rapidly fled from injured conspecifics within a 1 m radius of the cue, but showed no apparent reaction to injured sea urchins belonging to another family (Centrostephanus rodgersii, Diadematidae), diced pilchards or the disturbance caused by fish attracted to the cues. Densities of exposed sea urchins in an area containing injured conspecifics did not return to control values for at least 20 h, while cryptic individuals remained crevice-bound when injured conspecifics were nearby. Injured conspecifics thus provide a strong, albeit localised, cue for E. chloroticus. By restricting sea urchins to crevices where they have a reduced impact on living kelp, this non-consumptive effect may complement the lethal effects of predation in marine reserves where populations of predators such as rock lobsters and large fish are allowed to recover from overharvesting by humans, thereby reinforcing the trophic cascade initiated by those predators.
Underwater light is essential for fuelling coastal productivity. However, elevated turbidity, resulting from land-based activities and climate change, is often overlooked as a threat to coastal ...ecosystems. Understanding how low light, and specifically the temporal delivery of light, affects the productivity of forest-forming species is necessary to predict how ecosystems and species will respond to future increases in turbidity. Outdoor mesocosm experiments were used to compare the low-light tolerance of 2 forest-forming macrophytes that vary in their distribution in relation to turbidity, and investigate how the temporal delivery of light, i.e. press vs. pulse low-light disturbance, affects net primary productivity (NPP). We showed that the kelp Ecklonia radiata, which dominates reefs with low turbidity, ismore productive per unit biomass under high-light conditions than the fucoid Carpophyllum flexuosum, which typifies more turbid waters. Under low light, E. radiata suffered greater tissue loss and had lower NPP than C. flexuosum. Under both press and pulse treatments, E. radiata showed significant losses of lamina biomass and reduced NPP, while C. flexuosum showed net growth under press disturbance, and only lost tissue and had reduced NPP under pulse disturbance. The greater tolerance of C. flexuosum to de creased light, and differential responses of E. radiata and C. flexuosum to press and pulse low-light conditions, provide mechanistic support for C. flexuosum being better suited to turbid low-light environments than E. radiata. These results suggest future increases in turbidity may facilitate a shift from kelp-dominated forests to alternate states, resulting in reduced primary productivity.
Replicated ecological studies in marine reserves and associated unprotected areas are valuable in examining top-down impacts on communities and the ecosystem-level effects of fishing. We carried out ...experimental studies in two temperate marine reserves to examine these top-down influences on shallow subtidal reef communities in northeastern New Zealand. Both reserves examined are known to support high densities of predators and tethering experiments showed that the chance of predation on the dominant sea urchin, Evechinus chloroticus, within both reserves was approximately 7 times higher relative to outside. Predation was most intense on the smallest size class (30-40 mm) of tethered urchins, the size at which urchins cease to exhibit cryptic behaviour. A high proportion of predation on large urchins could be attributed to the spiny lobster, Jasus edwardsii. Predation on the smaller classes was probably by both lobsters and predatory fish, predominantly the sparid Pagrus auratus. The density of adult Evechinus actively grazing the substratum in the urchin barrens habitat was found to be significantly lower at marine reserve sites (2.2±0.3 m-2) relative to non-reserve sites (5.5±0.4 m-2). There was no difference in the density of cryptic juveniles between reserve and non-reserve sites. Reserve populations were more bimodal, with urchins between 40 and 55 mm occurring at very low numbers. Experimental removal of Evechinus from the urchin barrens habitat over 12 months lead to a change from a crustose coralline algal habitat to a macroalgal dominated habitat. Such macroalgal habitats were found to be more extensive in both reserves, where urchin densities were lower, relative to the adjacent unprotected areas that were dominated by urchin barrens. The patterns observed provide evidence for a top-down role of predators in structuring shallow reef communities in northeastern New Zealand and demonstrate how marine reserves can reverse the indirect effects of fishing and re-establish community-level trophic cascades.
Between 1978 and 1996 benthic communities in the Leigh Marine Reserve shifted from being dominated by sea urchins to being dominated by macroalgae. This was a result of a trophic cascade thought to ...be an indirect effect of increased predator abundance. We assessed further changes in communities from 1996 to 2000, differences in benthic communities between reserve and adjacent unprotected sites, and the stability of these patterns from 1999 to 2001. Since 1996, densities of sea urchinsEvechinus chloroticushave continued to decline in shallow areas of the reserve (<8 m), and all sites classified as urchin barrens in 1978 are now dominated by large brown algae. Comparisons between reserve and non-reserve sites revealed differences consistent with a trophic cascade at reserve sites. The greatest differences in algal communities between reserve and non-reserve sites occurred at depths whereE. chloroticuswas most abundant (4 to 6 m). Reserve sites had lower urchin densities and reduced extent of urchin barrens habitat with higher biomass of the 2 dominant algal species (Ecklonia radiataandCarpophyllum maschalocarpum). At reserve sites densities of exposedE. chloroticus(openly grazing the substratum) declined so that urchin barrens were completely absent by 2001. Lower density of the limpetCellana stelliferaand higher densities of the turbinid gastropodCookia sulcataat reserve sites are thought to be responses to changes in habitat structure, representing additional indirect effects of increased predators. The overall difference in community types between reserve and non-reserve sites remained stable between 1999 and 2001. Localised urchin mortality events due to an unknown agent were recorded at some sites adjacent to the marine reserve. Only at 1 of these sites did exposed urchins decline below the critical density of 1 m–2, which resulted in the total replacement of urchin barrens with macroalgae-dominated habitats. At other sites urchin barrens have remained stable. Declines in the limpetC. stelliferaoccurred across all sites between 1999 and 2001 and may be indirectly associated with urchin declines. Long-term changes in benthic communities in the Leigh reserve and the stability of differences between reserve and non-reserve sites over time are consistent with gradual declines in urchin densities due to increased predation on urchins, thus providing further evidence for a trophic cascade in this system. The rapid declines in urchin numbers at some unprotected sites, however, demonstrate how short-term disturbances, such as disease, may result in shifts in community types over much shorter time frames.
Kelp forests are among the most productive ecosystems in the world. Accurately quantifying net primary productivity (NPP) of kelps is challenging, as the carbon being produced is continually being ...lost through erosion of tissue and as dissolved organic carbon. Here, we use a physiological model that incorporates in situ estimates of photosynthesis vs. irradiance (P-E) parameters, biomass and underwater irradiance to estimate NPP of a subsurface Ecklonia radiata kelp forest in northern New Zealand over the seasonal cycle and depth range. Model testing found that predicted gross primary production (GPP) based on seasonal and depth-specific P-E parameters, biomass and underwater irradiance levels was strongly related to rates of GPP measured on individual kelp in the field (r² = 0.83). Sensitivity analysis indicated that NPP estimates were most sensitive to variation in biomass and photosynthetic parameters. Using this approach, estimated annual production (±SE) at 6 m (615 ± 183 g C m−2 yr−1) was found to be 1.6× greater than at 14 m depth (374 ± 102 g C m−2 yr−1), which was proportional to the difference in biomass between depths. This indicates that the greater photosynthetic abilities (higher maximum photosynthetic rates and photosynthetic efficiency) of deeper kelp offset the effects of reduced irradiance on overall NPP at depth. This study demonstrates how in situ P-E curves of adult kelp can be incorporated with biomass and irradiance data to estimate NPP of kelp forests, and highlights the importance of understanding depth-related variation in photosynthetic performance when estimating primary production of subsurface kelp forests.
Marine reserves provide a large-scale experimental framework to investigate the effects of fishing on food web dynamics and how they vary with environmental context. Because marine reserves promote ...the recovery of previously fished predators, spatial comparisons between reserve and fished sites are often made to infer such effects; however, alternative explanations for differences between reserve and fished sites are seldom tested (e.g., environmental variation among sites). We investigated the context dependency of the predator—urchin—kelp trophic cascade reported in northeastern New Zealand by comparing the abundance of herbivorous sea urchins (Evechinus chloroticus), the extent of urchin barrens habitat, and macroalgal biomass between reserve and fished sites within six locations that span an environmental gradient in wave exposure, sedimentation, and water clarity. At depths where differences in urchin abundance or macroalgal biomass were found between reserve and fished sites we used a model selection approach to identify which variables (fishing or environmental factors) best explained the variation among sites. Differences between reserve and fished sites were not ubiquitous across the locations examined and were highly depth specific. At sheltered locations, urchins were rare and barrens absent at both reserve and fished sites. At moderately exposed coastal locations, actively grazing urchins were most abundant at 4—6 m depth, and significant differences in macroalgal biomass between reserve and fished sites were observed. In contrast, at offshore island locations, urchins extended into deeper water, and differences between reserve and fished sites were found at 4—9 m depth. These differences could only be attributed to trophic cascades associated with protection from fishing in two of the six locations examined. In other cases, variation between reserve and fished sites was equally well explained by differences in sediment or wave exposure among sites. These results suggest that trophic cascades are not ubiquitous to northeastern New Zealand's subtidal reefs and the importance of sea urchins, and indirectly predators, in controlling macroalgal biomass will vary at local and regional scales in relation to abiotic factors. A better mechanistic understanding of how environmental variation affects the strength of species interactions across multiple spatial scales is needed to predict the ecosystem-level effects of fishing.
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