Regional differences in kelp forest interaction chains are influenced by both diffuse and localized stressors

Abstract

On temperate rocky reefs, overexploitation of high‐trophic‐level omnivores can result in the decimation of kelp forest habitats by releasing sea urchin population networks from top‐down control. However, the local dynamics of the resulting trophic cascades are context‐dependent. Here, we investigate the community‐wide patterns associated with both diffuse stressors and localized protection of high‐trophic‐level omnivores in kelp forest ecosystems by comparing communities among marine reserves and fished areas in two contrasting regions in terms of fishing intensity and land‐based stressors, Fiordland and the Marlborough Sounds, New Zealand. We find higher densities of the potential sea urchin predators, red rock lobsters (Jasus edwardsii) and banded wrasse (Notolabrus fucicola), in the Fiordland region, and larger effect sizes of fishing on the exploited fish communities in the Marlborough Sounds. Patterns in sea urchin density were consistent with the idea that high‐trophic‐level species, such as large fish and rock lobsters, regulate sea urchin population density, with lower densities of Evechinus chloroticus observed inside marine reserves, in both regions. Nevertheless, densities of E. chloroticus were generally high (>3 m2) in the Marlborough Sounds, likely above a grazing threshold in both fished and reserve sites. The proportion of habitat where sea urchins were absent was 29% in Marlborough Sounds and 90% in Fiordland. Consequently, we observe 49% barren habitat in Fiordland vs. 70%, and a larger effect of fishing on kelp community structure, in the Marlborough Sounds, where fishing effect sizes and land‐based stressors were more severe. We propose that a combination of diffuse stressors including regional overexploitation of important sea urchin predators, sedimentation, and warming of coastal waters likely contributed to regional differences in the responses of trophic interaction chains to localized reductions in fishing within marine reserves. The present study highlights how physiologically stressed and modified kelp forest ecosystems are more susceptible to detrimental phase shifts at a regional spatial scale.

The legacies of land clearance and trophic downgrading accumulate to affect structure and function of kelp forests

“…Rock lobster stocks have been depleted but are recovering after the implementation of the Fiordland Marine Management Act in 2005, and the density of potential predators of Evechinus is high compared with T A B L E 2 Results of corrected Akaike information criterion (AIC c ) analysis by depth zone (a) 0-5 m, (b) 6-10 m, and (c) 11-15 m for the statistical effects of (1) total suspended solids (TSS, in grams per cubic meter), (2) average regional density of Evechinus chloroticus (EVER, number per square meter), and (3) marine reserve effects on density of Evechinus chloroticus (MREve, number per square meter) on density of Ecklonia radiata at marine reserve and control sites distributed across the three regions. the other regions considered in this study (Udy, Wing, Jowett, et al, 2019). Ecklonia was found at relatively high densities in the deepest depth strata of subtidal surveys (Figure 4b), which is consistent with deep penetration of PAR in a clear water column at the fjord entrances and low levels of smothering by fine sediments (Wing et al, 2007).…”

“…The Fiordland region is covered in mature stands of native podocarp and Nothofagus beech forest, and except for the influence of a large hydroelectric power station in one Sound (Jack et al, 2009; Tallis et al, 2004), has unmodified catchments (Wing & Jack, 2014). Rock lobster stocks have been depleted but are recovering after the implementation of the Fiordland Marine Management Act in 2005, and the density of potential predators of Evechinus is high compared with the other regions considered in this study (Udy, Wing, Jowett, et al, 2019). Ecklonia was found at relatively high densities in the deepest depth strata of subtidal surveys (Figure 4b), which is consistent with deep penetration of PAR in a clear water column at the fjord entrances and low levels of smothering by fine sediments (Wing et al, 2007).…”

“…The observed patterns in the central region are likely influenced by the increased susceptibility of physiologically stressed kelp to overgrazing (Udy, Wing, Jowett, et al, 2019). Theory predicts that grazing distributed along gradients in primary production can result in sharp thresholds for persistence as grazing pressure exceeds vegetative regeneration (Oksanen et al, 1981).…”

“…A complementary solution is to contrast different present‐day ecological systems with marked differences in their histories of disturbance, land conversion, exploitation, or other anthropogenic influences (e.g., Schlieman et al, 2022; Udy, Wing, O'Connell‐Milne, et al, 2019). The approach relies on processes within contrasted regions to be relatively independent and isolated from broad‐scale degradation, as well as being comparable in terms of key ecological processes at play (Udy, Wing, Jowett, et al, 2019). In marine systems, the study of “intact” ecosystems has been vital to achieving this goal by providing key insights into the importance of land–sea connections, predator‐mediated ecological processes, animal‐mediated nutrient fluxes and, importantly, the interactions among cumulative long‐term and broad‐scale stressors.…”

Shifting trophic architecture of marine fisheries in New Zealand: Implications for guiding effective ecosystem‐based management

“…Fisheries‐induced shifts in community structure can have dramatic impacts on ecosystem function, affecting species trophic levels (Popp et al, 2007), food web dynamics and reproduction (Jack & Wing, 2010; Persson & Hansson, 1999), organic matter fluxes, energetics and distribution of biomass (Udy, Wing, Jowett, et al, 2019; Udy, Wing, O’Connell‐Milne, et al, 2019), and ultimately catch rates (Hinz et al, 2017). In the present study, average trophic levels of individual species were retrieved from a large online database (http://Fishbase.org).…”