Nessa E. O'Connor

Threats and knowledge gaps for ecosystem services provided by kelp forests: a northeast Atlantic perspective

Kelp forests along temperate and polar coastlines represent some of most diverse and productive habitats on the Earth. Here, we synthesize information from >60 years of research on the structure and functioning of kelp forest habitats in European waters, with particular emphasis on the coasts of UK and Ireland, which represents an important biogeographic transition zone that is subjected to multiple threats and stressors. We collated existing data on kelp distribution and abundance and reanalyzed these data to describe the structure of kelp forests along a spatial gradient spanning more than 10° of latitude. We then examined ecological goods and services provided by kelp forests, including elevated secondary production, nutrient cycling, energy capture and flow, coastal defense, direct applications, and biodiversity repositories, before discussing current and future threats posed to kelp forests and identifying key knowledge gaps. Recent evidence unequivocally demonstrates that the structure of kelp forests in the NE Atlantic is changing in response to climate- and non-climate-related stressors, which will have major implications for the structure and functioning of coastal ecosystems. However, kelp-dominated habitats along much of the NE Atlantic coastline have been chronically understudied over recent decades in comparison with other regions such as Australasia and North America. The paucity of field-based research currently impedes our ability to conserve and manage these important ecosystems. Targeted observational and experimental research conducted over large spatial and temporal scales is urgently needed to address these knowledge gaps.

On the dimensionality of ecological stability

Ecological stability is touted as a complex and multifaceted concept, including components such as variability, resistance, resilience, persistence and robustness. Even though a complete appreciation of the effects of perturbations on ecosystems requires the simultaneous measurement of these multiple components of stability, most ecological research has focused on one or a few of those components analysed in isolation. Here, we present a new view of ecological stability that recognises explicitly the non-independence of components of stability. This provides an approach for simplifying the concept of stability. We illustrate the concept and approach using results from a field experiment, and show that the effective dimensionality of ecological stability is considerably lower than if the various components of stability were unrelated. However, strong perturbations can modify, and even decouple, relationships among individual components of stability. Thus, perturbations not only increase the dimensionality of stability but they can also alter the relationships among components of stability in different ways. Studies that focus on single forms of stability in isolation therefore risk underestimating significantly the potential of perturbations to destabilise ecosystems. In contrast, application of the multidimensional stability framework that we propose gives a far richer understanding of how communities respond to perturbations.

Simulated predator extinctions: Predator identity affects survival and recruitment of oysters

The rate of species loss is increasing at a global scale, and human-induced extinctions are biased toward predator species. We examined the effects of predator extinctions on a foundation species, the eastern oyster (Crassostrea virginica). We performed a factorial experiment manipulating the presence and abundance of three of the most common predatory crabs, the blue crab (Callinectes sapidus), stone crab (Menippe mercenaria), and mud crab (Panopeus herbstii) in estuaries in the eastern United States. We tested the effects of species richness and identity of predators on juvenile oyster survival, oyster recruitment, and organic matter content of sediment. We also manipulated the density of each of the predators and controlled for the loss of biomass of species by maintaining a constant mass of predators in one set of treatments and simultaneously using an additive design. This design allowed us to test the density dependence of our results and test for functional compensation by other species. The identity of predator species, but not richness, affected oyster populations. The loss of blue crabs, alone or in combination with either of the other species, affected the survival rate of juvenile oysters. Blue crabs and stone crabs both affected oyster recruitment and sediment organic matter negatively. Mud crabs at higher than ambient densities, however, could fulfill some of the functions of blue and stone crabs, suggesting a level of ecological redundancy. Importantly, the strong effects of blue crabs in all processes measured no longer occurred when individuals were present at higher-than-ambient densities. Their role as dominant predator is, therefore, dependent on their density within the system and the density of other species within their guild (e.g., mud crabs). Our findings support the hypothesis that the effects of species loss at higher trophic levels are determined by predator identity and are subject to complex intraguild interactions that are largely density dependent. Understanding the role of biodiversity in ecosystem functioning or addressing practical concerns, such as loss of predators owing to overharvesting, remains complicated because accurate predictions require detailed knowledge of the system and should be drawn from sound experimental evidence, not based on observations or generalized models.

Environmental context determines multi-trophic effects of consumer species loss

Loss of biodiversity and nutrient enrichment are two of the main human impacts on ecosystems globally, yet we understand very little about the interactive effects of multiple stressors on natural communities and how this relates to biodiversity and ecosystem functioning. Advancing our understanding requires the following: (1) incorporation of processes occurring within and among trophic levels in natural ecosystems and (2) tests of context-dependency of species loss effects. We examined the effects of loss of a key predator and two groups of its prey on algal assemblages at both ambient and enriched nutrient conditions in a marine benthic system and tested for interactions between the loss of functional diversity and nutrient enrichment on ecosystem functioning. We found that enrichment interacted with food web structure to alter the effects of species loss in natural communities. At ambient conditions, the loss of primary consumers led to an increase in biomass of algae, whereas predator loss caused a reduction in algal biomass (i.e. a trophic cascade). However, contrary to expectations, we found that nutrient enrichment negated the cascading effect of predators on algae. Moreover, algal assemblage structure varied in distinct ways in response to mussel loss, grazer loss, predator loss and with nutrient enrichment, with compensatory shifts in algal abundance driven by variation in responses of different algal species to different environmental conditions and the presence of different consumers. We identified and characterized several context-dependent mechanisms driving direct and indirect effects of consumers. Our findings highlight the need to consider environmental context when examining potential species redundancies in particular with regard to changing environmental conditions. Furthermore, non-trophic interactions based on empirical evidence must be incorporated into food web-based ecological models to improve understanding of community responses to global change.

Biodiversity among mussels: separating the influence of sizes of mussels from the ages of patches

The role of habitat structure in controlling the composition of assemblages has often been studied, but is rarely manipulated so that it is distinguishable from other factors. Differences in habitat structure as determined by differences in mussel size structure may affect the diversity of assemblages associated with mussel beds. Previous studies examining the effect of the size of individual mussels in a patch on the diversity of associated macro-faunal assemblages confounded the age of the patch with the size of the mussels. We manipulated the age of mussel patches and the size of the mussels within them to test experimentally whether the size of mussels influenced the structure of associated assemblages. At one of the two locations considered, the structure of macro-faunal assemblages in patches of larger mussels differed significantly from those in patches of the same age composed of smaller mussels. At this location, the size of mussels did not affect species richness but the abundance and proportion of organisms present differed depending on the size of the mussels. Here patches of larger mussels contained greater numbers ofNematodes and Oligochaetes and a lower abundance oftaxa such as Jaeraforsmani and Lepidonotus clava. We also found that invertebrate assemblages in general differed between the two locations. The effect of the size structure of mussels, however, varied spatially demonstrating that the effect of habitat structure on the diversity of associated assemblages is context dependent.

Wave action modifies the effects of consumer diversity and warming on algal assemblages

To understand the consequences of biodiversity loss, it is necessary to test how biodiversity-ecosystem functioning relationships may vary with predicted environmental change. In particular, our understanding will be advanced by studies addressing the interactive effects of multiple stressors on the role of biodiversity across trophic levels. Predicted increases in wave disturbance and ocean warming, together with climate-driven range shifts of key consumer species, are likely to have profound impacts on the dynamics of coastal marine communities. We tested whether wave action and temperature modified the effects of gastropod grazer diversity (Patella vulgata, Littorina littorea, and Gibbula umbilicalis) on algal assemblages in experimental rock pools. The presence or absence of L. littorea appeared to drive changes in microalgal and macroalgal biomass and macroalgal assemblage structure. Macroalgal biomass also decreased with increasing grazer species richness, but only when wave action was enhanced. Further, independently of grazer diversity, wave action and temperature had interactive effects on macroalgal assemblage structure. Warming also led to a reversal of grazer-macroalgal interaction strengths from negative to positive, but only when there was no wave action. Our results show that hydrodynamic disturbance can exacerbate the effects of changing consumer diversity, and may also disrupt the influence of other environmental stressors on key consumer-resource interactions. These findings suggest that the combined effects of anticipated abiotic and biotic change on the functioning of coastal marine ecosystems, although difficult to predict, may be substantial.

Nutrient enrichment alters the consequences of species loss

Summary 1. Loss of biodiversity and nutrient enrichment are two of the most pervasive drivers of change in ecosystems globally. However, little is known about how these disturbances interact to affect ecosystem functioning. 2. We established a field experiment to test for effects of loss of consumer species on algal assemblages (richness and assemblage structure) and ecosystem functioning (ecosystem productivity and algal biomass accumulation) on rocky shores. We manipulated the presence of three common molluscan grazer species in newly created rock pools over 13 months. Our grazer manipulation was based on a subtractive experimental design, and we also manipulated nutrient concentrations to evaluate explicitly how resource availability affected the consequences of species loss. 3. We found that the effects of loss of grazer species on algal assemblages and their functioning depended strongly on both the identity of the grazer species lost and of those remaining in the community. Moreover, the effects of loss of species depended on nutrient availability, making it extremely difficult to predict the effects of species loss generally without detailed knowledge of a system. 4. Our results demonstrate clearly that the effects of grazer species loss on primary producers and ecosystem functioning may not be generalisable based on grazer functional roles or traits. 5. Synthesis. Our findings highlight the need to include key physical drivers, such as nutrient availability, explicitly into biodiversity–ecosystem functioning models in order to move towards a predictive framework that incorporates the effects of both environmental heterogeneity and anthropogenic stressors.

Benthic assemblages associated with native and non-native oysters are similar

Invasive species can impact native species and alter assemblage structure, which affects associated ecosystem functioning. The pervasive Pacific oyster, Crassostrea gigas, has been shown to affect the diversity and composition of many host ecosystems. We tested for effects of the presence of the invasive C. gigas on native assemblages by comparing them directly to assemblages associated with the declining native European oyster, Ostrea edulis. The presence of both oyster species was manipulated in intertidal and subtidal habitats and reefs were constructed at horizontal and vertical orientation to the substratum. After 12months, species diversity and benthic assemblage structure between assemblages with C. gigas and O. edulis were similar, but differed between habitats and orientation, suggesting that both oyster species were functionally similar in terms of biodiversity facilitation. These findings support evidence, that non-native species could play an important role in maintaining biodiversity in systems with declining populations of native species.

Microplastics Affect the Ecological Functioning of an Important Biogenic Habitat

Biological effects of microplastics on the health of bivalves have been demonstrated elsewhere, but ecological impacts on the biodiversity and ecosystem functioning of bivalve-dominated habitats are unknown. Thus, we exposed intact sediment cores containing European flat oysters (Ostrea edulis) or blue mussels (Mytilus edulis) in seawater to two different densities (2.5 or 25 μg L-1) of biodegradable or conventional microplastics in outdoor mesocosms. We hypothesized that filtration rates of the bivalves, inorganic nitrogen cycling, primary productivity of sediment dwelling microphytobenthos, and the structure of invertebrate benthic assemblages would be influenced by microplastics. After 50 days, filtration by M. edulis was significantly less when exposed to 25 μg L-1 of either type of microplastics, but there were no effects on ecosystem functioning or the associated invertebrate assemblages. Contrastingly, filtration by O. edulis significantly increased when exposed to 2.5 or 25 μg L-1 of microplastics, and porewater ammonium and biomass of benthic cyanobacteria decreased. Additionally the associated infaunal invertebrate assemblages differed, with significantly less polychaetes and more oligochaetes in treatments exposed to microplastics. These findings highlight the potential of microplastics to impact the functioning and structure of sedimentary habitats and show that such effects may depend on the dominant bivalve present.

Loss of predator species, not intermediate consumers, triggers rapid and dramatic extinction cascades

Abstract Ecological networks are tightly interconnected, such that loss of a single species can trigger additional species extinctions. Theory predicts that such secondary extinctions are driven primarily by loss of species from intermediate or basal trophic levels. In contrast, most cases of secondary extinctions from natural systems have been attributed to loss of entire top trophic levels. Here, we show that loss of single predator species in isolation can, irrespective of their identity or the presence of other predators, trigger rapid secondary extinction cascades in natural communities far exceeding those generally predicted by theory. In contrast, we did not find any secondary extinctions caused by intermediate consumer loss. A food web model of our experimental system—a marine rocky shore community—could reproduce these results only when biologically likely and plausible nontrophic interactions, based on competition for space and predator‐avoidance behaviour, were included. These findings call for a reassessment of the scale and nature of extinction cascades, particularly the inclusion of nontrophic interactions, in forecasts of the future of biodiversity. &NA; We show that loss of single predator species in isolation can, irrespective of their identity, trigger rapid secondary extinction cascades in natural communities far exceeding those generally predicted by theory. A food web model of our experimental system—a marine rocky shore community—could reproduce these results only when biologically likely and plausible nontrophic interactions, based on competition for space and predator‐avoidance behaviour, were included. Our findings call for a reassessment of the scale and nature of extinction cascades, particularly the inclusion of nontrophic interactions, in forecasts of the future of biodiversity. Figure. No caption available.