Variation in recruitment and the establishment of alternative community states

Variation in Successional Dynamics Shape Biodiversity Patterns over a Tropical-Temperate Latitudinal Gradient

AbstractSuccessional dynamics can vary because of a range of ecological and environmental factors, but our understanding of biogeographic variation in succession, and the processes contributing to community development across ecosystems, is limited. The pattern and rate of recruitment of dispersive propagules likely differs over large spatial scales and can be an important predictor of successional trajectory. Over a 20° tropical-temperate latitudinal gradient, we measured sessile invertebrates over 12 months of community development and successive 3-month recruitment windows to understand succession and how it is influenced by recruitment. Succession and recruitment patterns varied over latitude. In the tropics, fast temporal turnover, fluctuating abundances, and lack of successional progression suggest that the contribution of stochastic processes was high. As latitude increased, successional progression became more apparent, characterized by increasing species richness and community cover and a shift to more competitive taxa over time. At temperate locations, species identities were similar between older communities and recruiting assemblages; however, community composition became more variable across space over time. Such divergence suggests an important role of early colonizers and species interactions on community structure. These findings demonstrate differences in the processes contributing to community development and biodiversity patterns over latitude. Understanding such biogeographic variation in community dynamics and identifying the prevalence of different processes can provide insights into how communities assemble and persist in response to environmental variability.

Resilience assessment in complex natural systems

Ecological resilience is the capability of an ecosystem to maintain the same structure and function and avoid crossing catastrophic tipping points (i.e. undergoing irreversible regime shifts). While fundamental for management, concrete ways to estimate and interpret resilience in real ecosystems are still lacking. Here, we develop an empirical approach to estimate resilience based on the stochastic cusp model derived from catastrophe theory. The cusp model models tipping points derived from a cusp bifurcation. We extend cusp in order to identify the presence of stable and unstable states in complex natural systems. Our Cusp Resilience Assessment (CUSPRA) has three characteristics: (i) it provides estimates on how likely a system is to cross a tipping point (in the form of a cusp bifurcation) characterized by hysteresis, (ii) it assesses resilience in relation to multiple external drivers and (iii) it produces straightforward results for ecosystem-based management. We validate our approach using simulated data and demonstrate its application using empirical time series of an Atlantic cod population and marine ecosystems in the North Sea and the Mediterranean Sea. We show that Cusp Resilience Assessment is a powerful method to empirically estimate resilience in support of a sustainable management of our constantly adapting ecosystems under global climate change.

Understanding change in benthic marine systems

BACKGROUND

The unprecedented influence of human activities on natural ecosystems in the 21st century has resulted in increasingly frequent large-scale changes in ecological communities. This has heightened interest in understanding such changes and effective means to manage them. Accurate interpretation of state changes is challenging because of difficulties translating theory to empirical study, and most theory emphasizes systems near equilibrium, which may not be relevant in rapidly changing environments.

SCOPE

We review concepts of long-transient stages and phase shifts between stable community states, both smooth, continuous and discontinuous shifts, and the relationships among them. Three principal challenges emerge when applying these concepts. The first is how to interpret observed change in communities - distinguishing multiple stable states from long transients, or reversible shifts in the phase portrait of single attractor systems. The second is how to quantify the magnitudes of three sources of variability that cause switches between community states: (1) 'noise' in species' abundances, (2) 'wiggle' in system parameters and (3) trends in parameters that affect the topography of the basin of attraction. The third challenge is how variability of the system shapes evidence used to interpret community changes. We outline a novel approach using critical length scales to potentially address these challenges. These concepts are highlighted by a review of recent examples involving macroalgae as key players in marine benthic ecosystems.

CONCLUSIONS

Real-world examples show three or more stable configurations of ecological communities may exist for a given set of parameters, and transient stages may persist for long periods necessitating their respective consideration. The characteristic length scale (CLS) is a useful metric that uniquely identifies a community 'basin of attraction', enabling phase shifts to be distinguished from long transients. Variabilities of CLSs and time series data may likewise provide proactive management measures to mitigate phase shifts and loss of ecosystem services. Continued challenges remain in distinguishing continuous from discontinuous phase shifts because their respective dynamics lack unique signatures.

Settlement Preferences in Temperate Sea Stars

AbstractMany marine invertebrates possess biphasic life histories, during which larvae develop in the plankton and adults inhabit the benthos. The transition between phases entails the settlement of larvae onto substrata, completion of metamorphosis, and survival as vulnerable early juveniles. The perimetamorphic period, encompassing settlement and the interval immediately following settlement, is a key determinant of adult abundance and distribution. However, because settling larvae and early juveniles are difficult to observe in the field, the ecology of this period remains poorly understood. We performed experiments to elucidate the settlement preferences of Asterias forbesi and Asterias rubens, keystone predators on the east coast of North America, on substrata common to their intertidal habitats. Larval Asterias exhibit clear selectivity in settlement, with shells of the blue mussel, Mytilus edulis, most preferred. The algae Chondrus crispus and crustose coralline algae also induced high rates of settlement, while little settlement was observed on rocks with biofilm and no settlement occurred in controls. When inductive cues were subsequently added to controls, high frequencies of settlement occurred immediately, confirming the competency of larvae to settle and their ability to delay metamorphosis in the absence of appropriate cues. Our results demonstrate that Asterias larvae have specific settlement preferences and that settlement can be postponed in this species if no suitable substrate is available.

Variability effects by consumers exceed their average effects across an environmental gradient of mussel recruitment

The implicit assumption that properties of natural systems deduced from the average statistics from random samples suffice for understanding them focuses the attention of ecologists on the average effects of processes and responses, and often, to view their variability as noise. Yet, both kinds of effects can drive dynamics of ecological systems and their covariation may confound interpretation. Predation by crabs and snails on competitively dominant mussels has long been recognized as an important process structuring communities on rocky shores of the Northwest Atlantic Ocean. We experimentally manipulated the average intensity of predation in plots across a gradient of mussel recruitment to separately estimate the average and variability of responses of mussel recruitment and community composition. Predation did not affect the average number of mussels recruited to plots, nor the average multivariate composition of the community. Plots from which predators were excluded showed a ~ 30% increase in spatial variability of mussel recruitment. After 1 year, the spatial variability in community composition was greater than that observed among plots that predators could access. An important, but less recognized, aspect of predation is its dampening effect on variability of community structure. As accelerating rates of environmental change disrupt species interactions, variability effects of ecological processes and corresponding responses are likely to be increasingly important determinants of community dynamics.

Community disruption in small biogenic habitats: A coastal invader overcomes habitat complexity to alter community structure

Non-indigenous species are often identified as threats to native species and communities. Yet, the mechanisms that enable many of these invaders to thrive and alter their newly invaded habitats are still not fully understood. This applies to habitats such as widespread sedimentary shorelines characterized by the presence of scattered biogenic clumps of blue mussels (Mytilus edulis) structurally more complex than bare sediments. In Atlantic Canada, some of these shorelines are numerically dominated by native mud crabs (Dyspanopeus sayi) but have been gradually invaded by the European green crab (Carcinus maenas). This study describes between-habitat (mussel clump vs. bare sediment) differences in density and diversity of invertebrates. It also tests the impact of juvenile green crabs in comparison to native mud crabs using two approaches: First, measuring habitat-related differences in these crabs' feeding rates on a common prey (soft-shell clams, Mya arenaria). Second, measuring their influence on invertebrate communities associated with mussel clumps. The results show that mussel clumps hold higher invertebrate density and diversity than surrounding sedimentary bottoms. In the laboratory, the feeding rates of native mud crabs were dependent on the type of habitat (sand flat > mussel clump), whereas those of green crabs were significantly higher and unrelated to the habitat in which predation occurred. In field experiments, juvenile green crabs were also the only predators that changed community structure in the mussel clump habitat. These results indicate that green crabs can cause a significant impact on native species and communities. Moreover, they suggest that the ability of this species to overcome the refuge provided by complex biogenic habitats for prey may represent an unexplored mechanism to explain this invader's expansion here and elsewhere.

Declines over the last two decades of five intertidal invertebrate species in the western North Atlantic

Climate change has already altered the environmental conditions of the world's oceans. Here we report declines in gastropod abundances and recruitment of mussels (Mytilus edulis) and barnacles (Semibalanus balanoides) over the last two decades that are correlated with changes in temperature and ocean conditions. Mussel recruitment is declining by 15.7% per year, barnacle recruitment by 5.0% per year, and abundances of three common gastropods are declining by an average of 3.1% per year (Testudinalia testudinalis, Littorina littorea, and Nucella lapillus). The declines in mussels and the common periwinkle (L. littorea) are correlated with warming sea temperatures and the declines in T. testudinalis and N. lapillus are correlated with aragonite saturation state, which affects rates of shell calcification. These species are common on shores throughout the North Atlantic and their loss is likely to lead to simplification of an important food web on rocky shores.

Catastrophic dynamics limit Atlantic cod recovery

Collapses and regime changes are pervasive in complex systems (such as marine ecosystems) governed by multiple stressors. The demise of Atlantic cod (Gadus morhua) stocks constitutes a text book example of the consequences of overexploiting marine living resources, yet the drivers of these nearly synchronous collapses are still debated. Moreover, it is still unclear why rebuilding of collapsed fish stocks such as cod is often slow or absent. Here, we apply the stochastic cusp model, based on catastrophe theory, and show that collapse and recovery of cod stocks are potentially driven by the specific interaction between exploitation pressure and environmental drivers. Our statistical modelling study demonstrates that for most of the cod stocks, ocean warming could induce a nonlinear discontinuous relationship between fishing pressure and stock size, which would explain hysteresis in their response to reduced exploitation pressure. Our study suggests further that a continuing increase in ocean temperatures will probably limit productivity and hence future fishing opportunities for most cod stocks of the Atlantic Ocean. Moreover, our study contributes to the ongoing discussion on the importance of climate and fishing effects on commercially exploited fish stocks, highlighting the importance of considering discontinuous dynamics in holistic ecosystem-based management approaches, particularly under climate change.

After the Fall: Legacy Effects of Biogenic Structure on Wind-Generated Ecosystem Processes Following Mussel Bed Collapse

Blue mussels (Mytilus edulis) are ecosystem engineers with strong effects on species diversity and abundances. Mussel beds appear to be declining in the Gulf of Maine, apparently due to climate change and predation by the invasive green crab, Carcinus maenas. As mussels die, they create a legacy of large expanses of shell biogenic structure. In Maine, USA, we used bottom traps to examine effects of four bottom cover types (i.e., live mussels, whole shells, fragmented shells, bare sediment) and wind condition (i.e., days with high, intermediate, and low values) on flow-related ecosystem processes. Significant differences in transport of sediment, meiofauna, and macrofauna were found among cover types and days, with no significant interaction between the two factors. Wind condition had positive effects on transport. Shell hash, especially fragmented shells, had negative effects, possibly because it acted as bed armor to reduce wind-generated erosion and resuspension. Copepods had the greatest mobility and shortest turnover times (0.15 d), followed by nematodes (1.96 d) and the macrofauna dominant, Tubificoides benedeni (2.35 d). Shell legacy effects may play an important role in soft-bottom system responses to wind-generated ecosystem processes, particularly in collapsed mussel beds, with implications for recolonization, connectivity, and the creation and maintenance of spatial pattern.

Transition probabilities help identify putative drivers of community change in complex systems

Understanding the role of larger-scale processes in modulating the assembly, structure, and dynamics of communities is critical for forecasting the effects of climate-change and managing ecosystems. Developing this comprehensive perspective is difficult though, because species interactions are complex, interdependent, and dynamic through space and time. Typically, experiments focus on tractable subsets of interactions that will be most critical to investigate and explain shifts in communities, but qualitatively base these choices on experience, natural history, and theory. One quantitative approach to identify the putative forces regulating communities, without reducing system complexity, is estimating transition probabilities among species occupying space (i.e., multispecies Markov chain models). Although not mechanistic, these models estimate the relative frequency and importance of ecological pathways in community assembly and dynamics, and can serve as a framework to identify how pathways change across large scales and which are most important to investigate further. Here, we demonstrate this method in the Gulf of Maine (GOM) intertidal zone, where research has largely focused on the local-scale processes that influence communities, while the mechanisms responsible for more regional shifts in communities are less clear. Transition probabilities of faunal elements were quantified bimonthly for ~2.5 yr in local intertidal communities at three replicate sites in the southern, mid-coast, and northern GOM. Transitions related to mortality, colonization, and replacement by mussels, barnacles, red algae, and encrusting corallines differed regionally, suggesting specific pathways related to consumer pressure and recruitment vary across the GOM with shifting intertidal community structure. Combined with species abundance data and insights from previous research, we develop and evaluate the pathways by which communities likely change in the GOM. Species interactions in local communities can be complex, and this complexity should be incorporated into hypothesis building, experiments, theory, interpretations, and forecasts in ecology. Such a comprehensive approach will be critical to understand how regional shifts in local interactions can drive large-scale community change.

Persistence and space preemption explain species-specific founder effects on the organization of marine sessile communities

Community assembly may not follow predictable successional stages, with a large fraction of the species pool constituted by potential pioneering species and successful founders defined through lottery. In such systems, priority effects may be relevant in the determination of trajectories of developing communities and hence diversity and assemblage structure at later advanced states. In order to assess how different founder species may trigger variable community trajectories and structures, we conducted an experimental study using subtidal sessile assemblages as model. We manipulated the identity of functionally different founders and initial colony size (a proxy of the time lag before the arrival of later species), and followed trajectories. We did not observe any effects of colony size on response variables, suggesting that priority effects take place even when the time lag between the establishment of pioneering species and late colonizers is very short. Late community structure at experimental panels that started either with the colonial ascidian Botrylloides nigrum, or the arborescent bryozoan Bugula neritina, was similar to control panels allowed natural assembling. In spite of high potential for fast space domination, and hence negative priority effects, B. nigrum suffered high mortality and did not persist throughout succession. Bugula neritina provided complex physical microhabitats through conspecific clustering that have enhanced larval settlement of late species arrivals, but no apparent facilitation was observed. Differently, panels founded by the encrusting bryozoan Schizoporella errata led to different and less diverse communities compared to naturally assembled panels, evidencing strong negative priority effects through higher persistence and space preemption. Schizoporella errata founder colonies inhibited further conspecific settlement, which may greatly relax intraspecific competition, allowing resource allocation to colony growth and space domination, thus reducing the chances for the establishment of other species.

Long-term declines in an intertidal foundation species parallel shifts in community composition

The earth is in the midst of a biodiversity crisis, and projections indicate continuing and accelerating rates of global changes. Future alterations in communities and ecosystems may be precipitated by changes in the abundance of strongly interacting species, whose disappearance can lead to profound changes in abundance of other species, including an increase in extinction rate for some. Nearshore coastal communities are often dependent on the habitat and food resources provided by foundational plant (e.g., kelp) and animal (e.g., shellfish) species. We quantified changes in the abundance of the blue mussel (Mytilus edulis), a foundation species known to influence diversity and productivity of intertidal habitats, over the past 40 years in the Gulf of Maine, USA, one of the fastest warming regions in the global ocean. Using consistent survey methods, we compared contemporary population sizes to historical data from sites spanning >400 km. The results of these comparisons showed that blue mussels have declined in the Gulf of Maine by >60% (range: 29-100%) at the site level since the earliest benchmarks in the 1970s. At the same time as mussels declined, community composition shifted: at the four sites with historical community data, the sessile community became increasingly algal dominated. Contemporary (2013-2014) surveys across 20 sites showed that sessile species richness was positively correlated to mussel abundance in mid to high intertidal zones. These results suggest that declines in a critical foundation species may have already impacted the intertidal community. To inform future conservation efforts, we provide a database of historical and contemporary baselines of mussel population abundance and dynamics in the Gulf of Maine. Our results underscore the importance of anticipating not only changes in diversity but also changes in the abundance and identity of component species, as strong interactors like foundation species have the potential to drive cascading community shifts.