Diversity and stability of herbivorous fishes on coral reefs

Abstract

Biodiversity may provide insurance against ecosystem collapse by stabilizing assemblages that perform particular ecological functions (the “portfolio effect”). However, the extent to which this occurs in nature and the importance of different mechanisms that generate portfolio effects remain controversial. On coral reefs, herbivory helps maintain coral dominated states, so volatility in levels of herbivory has important implications for reef ecosystems. Here, we used an extensive time series of abundances on 35 reefs of the Great Barrier Reef of Australia to quantify the strength of the portfolio effect for herbivorous fishes. Then, we disentangled the contributions of two mechanisms that underlie it (compensatory interactions and differential responses to environmental fluctuations [“response diversity”]) by fitting a community‐dynamic model that explicitly includes terms for both mechanisms. We found that portfolio effects operate strongly in herbivorous fishes, as shown by nearly independent fluctuations in abundances over time. Moreover, we found strong evidence for high response diversity, with nearly independent responses to environmental fluctuations. In contrast, we found little evidence that the portfolio effect in this system was enhanced by compensatory ecological interactions. Our results show that portfolio effects are driven principally by response diversity for herbivorous fishes on coral reefs. We conclude that portfolio effects can be very strong in nature and that, for coral reefs in particular, response diversity may help maintain herbivory above the threshold levels that trigger regime shifts.

Biodiversity enhances reef fish biomass and resistance to climate change

“…However, the explanation suggested by theory and prior empirical results is that the naturally low species richness at cooler sites results in low functional redundancy. Thus, at cooler, less diverse sites species fill more unique roles on average (40) and loss of a single species reduces biomass production more than it does at richer sites, where the remaining ecologically similar species can compensate for that loss (41). This finding implies that high-latitude marine ecosystems are especially ecologically vulnerable to climate change, due not only to higher projected invasion rates (19), but also to stronger impacts of species loss on production (Fig.…”

Strong but opposingβ-diversity–stability relationships in coral reef fish communities

“…Under such conditions, structurally diverse communities are likely to display a range of responses to environmental fluctuations, thus generating asynchronous abundance dynamics among the component communities and thereby stabilizing the metacommunity as a whole. This dynamic underpins the portfolio effect in ecological communities [7,13] and metapopulations [14] and is, in theory, also applicable to metacommunity dynamics. It follows that high b-diversity should increase the odds that at least some communities will resist disturbances, such as invasions, diseases or extreme climate events, better than others, and thereby act as refuges from which neighbouring patches can be replenished [11].…”

“…Although initially challenged by evidence for reduced stability in species-rich communities [2], the diversity-stability hypothesis has become a widely used rule of thumb and assumption in ecology [3,4] and conservation planning [5]. A range of ecological mechanisms are likely to contribute to the manifestation of this relationship, including: (i) weaker biotic interactions, such as competition and predation in species-rich communities, promoting species persistence [4,6]; (ii) different responses of species to environmental fluctuations, promoting coexistence between species as a result of greater niche partitioning [4,7]; and (iii) greater functional redundancy in species-rich communities, whereby multiple species in each functional group help to maintain that ecological function and thereby contribute to overall community stability [8]. Alternatively, diversity-stability relationships might arise as a sampling artefact, where species present at a site are detected in some but not all instances [9], spuriously inflating estimates of temporal turnover in community structure.…”

Quantifying effects of abiotic and biotic drivers on community dynamics with multivariate autoregressive (MAR) models

“…To the best of our knowledge, the first uses of MAR modeling to analyze marine fisheries data were applications involving the forecast of anchovy and sardine populations in the Mediterranean Sea (Stergiou 1991, Stergiou andChristou 1996). Since then, MAR modeling has been used to analyze the effects of climate and catch prices on fishing effort for albacore tuna, Chinook salmon, sablefish, and squid in California's Monterey Bay (Dalton 2001); to investigate the ecosystem linkages that affect the carite, croaker, and honey shrimp fisheries in Trinidad's Gulf of Paria (Dhoray and Teelucksingh 2007); to examine fish declines in the San Francisco Bay estuary (Mac Nally et al 2010); to assess the portfolio effect in coral fishes (Thibaut et al 2012); and to evaluate the effects of fishing pressure and environmental fluctuations on dynamics of commercially important cod (Clupea harengus) in the Black Sea with additional explorations of alternative management scenarios (Lindegren et al 2009). …”