A taxonomy of multiple stable states in complex ecological communities.

Natural systems are built from multiple interconnected units, making their dynamics, functioning and fragility notoriously hard to predict. A fragility scenario of particular relevance concerns so-called regime shifts: abrupt transitions from healthy to degraded ecosystem states. An explanation for these shifts is that they arise as transitions between alternative stable states, a process that is well-understood in few-species models. However, how multistability upscales with system complexity remains a debated question. Here, we identify that four different multistability regimes generically emerge in models of species-rich communities and other archetypical complex biological systems assuming random interactions. Across the studied models, each regime consistently emerges under a specific interaction scheme and leaves a distinct set of fingerprints in terms of the number of observed states, their species richness and their response to perturbations. Our results help clarify the conditions and types of multistability that can be expected to occur in complex ecological communities.

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.

Increasing atmospheric CO2 overrides the historical legacy of multiple stable biome states in Africa.

The dominant vegetation over much of the global land surface is not predetermined by contemporary climate, but also influenced by past environmental conditions. This confounds attempts to predict current and future biome distributions, because even a perfect model would project multiple possible biomes without knowledge of the historical vegetation state. Here we compare the distribution of tree- and grass-dominated biomes across Africa simulated using a dynamic global vegetation model (DGVM). We explicitly evaluate where and under what conditions multiple stable biome states are possible for current and projected future climates. Our simulation results show that multiple stable biomes states are possible for vast areas of tropical and subtropical Africa under current conditions. Widespread loss of the potential for multiple stable biomes states is projected in the 21st Century, driven by increasing atmospheric CO2 . Many sites where currently both tree-dominated and grass-dominated biomes are possible become deterministically tree-dominated. Regions with multiple stable biome states are widespread and require consideration when attempting to predict future vegetation changes. Testing for behaviour characteristic of systems with multiple stable equilibria, such as hysteresis and dependence on historical conditions, and the resulting uncertainty in simulated vegetation, will lead to improved projections of global change impacts.

Effects of Enrichment on Three-Level Food Chains with Omnivory.

Although omnivory (the consumption of resources from more than one trophic level) is widespread, this fundamental limitation to the applicability of food chain theory to real communities has received only limited treatment. We investigated effects of enrichment (increasing carrying capacity, K, of the resource) on a system consisting of a resource (R), an intermediate consumer (N), and an omnivore (P) using a general mathematical model and tested the relevance of some of its predictions to a laboratory system of mixed bacteria (=R) and the ciliates Tetrahymena (=N) and Blepharisma (=P). The model produced six major predictions. First, N may facilitate or inhibit P. Enrichment may revert the net effect of N on P from facilitation to inhibition. Second, along a gradient of K, up to four regions of invasibility and stable coexistence of N and P may exist. At the lowest K, only R is present. At somewhat higher K, N can coexist with R. At intermediate K, either N and P coexist, or either consumer excludes the other depending on initial conditions. At the highest K, N may be excluded through apparent competition and only R and P can coexist. The pattern of persistence of Tetrahymena and Blepharisma along an enrichment gradient conformed fairly well to the scenario allowing coexistence at intermediate K. Third, for stable equilibria of the omnivory system, R always increases and N always decreases with K. The abundances of bacteria and Tetrahymena were suggestive of such a pattern but did not allow a strict test because coexistence occurred at only one level of enrichment. Fourth, an omnivore can invade an R-N system at a lower K than an otherwise identical specialist predator of N. Fifth, an omnivore can always invade a food chain with such a specialist predator. Sixth, over ranges of K where both omnivory systems and otherwise identical three-level food chains are feasible, N is always less abundant in the omnivory system, whereas the relative abundances of R and P in omnivory systems compared to food chains may change with K. It is thus possible that total community biomass at a given K is lower in an omnivory system than in a food chain. Both the model and the experimental results caution that patterns of trophic-level abundances in response to enrichment predicted by food chain theory are not to be expected in systems with significant omnivory.