Alternative cliques of coexisting species in complex ecosystems

We congratulate Drake and colleagues (Drake et al. 1993) for their creative and elegant experimental study of some critical issues in community ecology; we count ourselves among a minority of ecologists who believe that such 'bottle experiments' (Kareiva 1989) can tell us much about the real world. However, we wish to raise some caveats, involving both technical aspects of Drake et al.'s experimental design, and the interpretations of the results proffered by these authors. It is useful to define some terms. 'Community convergence' occurs when one or more communities reach the same 'state', in terms of identities, absolute and relative abundances of constituent species. Historically, this has usually meant convergence to the same point equilibrium, but more recent thinking on higher order attractors (e.g. limit cycles) and stochastic boundedness (Chesson & Ellner 1989) allows a possibility that species abundances might only converge in terms of some dynamic pattern, e.g. to oscillations with the same frequency spectra, or to random variables with the same probability distributions. Communities could fail to converge if external, abiotic conditions differed. More interestingly, communities with the same external conditions would fail to converge if the state space of the governing dynamical system had 'alternative stable states'-more than one locally stable, point equilibrium, or higher-order attractor. It is useful to distinguish cases where one or more of the alternative stable states is on the boundary of the state space, because such boundary attractors correspond to community states in which one or more species are extinct. Such cases we call 'priority effects': the classical example is provided by the Lotka-Volterra competition equations, when each species suffers more from interspecific than intraspecific competition, and the species with a sufficiently large initial relative abundance excludes the other. If alternative stable states occur only in the interior of the state space, communities have the same species composition, but differ in absolute and relative abundances of species. When priority effects occur, communities fail to converge in the identity of constituent species. These are important issues. In the real world, communities are frequently disturbed in ways that alter species abundances. If a community has alternative stable states, such a disturbance can potentially move the community from one state to another. In such cases, good knowledge of local processes and interactions would at best enable one to catalogue a set of possible community states. More exact prediction of community state would require historical knowledge of disturbances. Despite a number of plausible theoretical models in which alternative'stable states occur (Polis, Myers & Holt 1989; Sinclair 1989; Case 1990; Drake 1990; Jones, Hassell & Pacala 1993), and their reported occurrence, including priority effects, in natural communities (Cole 1983; Kneidel 1983; Barkai & McQuaid 1988; Moss 1989; Sinclair 1989; Dublin, Sinclair & McGlade 1990; Knowlton, Lang & Keller 1990; Pech et al. 1992), the existence of, and mechanisms generating these phenomena in the field remain poorly researched. Occam's razor would assume a single equilibrium until proved otherwise. Parsimony has no logical primacy, however. Invasion experiments in laboratory microcosms are ideal tools to look for alternative stable states in communities, because replicated communities under the same external conditions can be created, and invasions provide radically differing initial relative abundances, from which divergence to alternative states might be revealed. Priority effects would be detected if such experimental communities diverged in species identities, and more generally, alternative stable states would be detected if absolute and relative abundances (or their dynamic patterns) diverged. Of course, a sufficiently long period of undisturbed observation of each community is needed to allow convergence to occur, if it is going to. Several generations of the longest-lived resident organisms, or the time required for achievement of the apparent carrying capacity for total biomass, or of a few turnovers of total biomass, are probably minimum estimates of the time required. Where several invasions are imposed on local communities (as in Drake et al. 1993), the time allowed for potential convergence must be counted from the date of the last invasion, since invasions are a form of disturbance, even if unsuccessful, and may slow convergence. Moreover, if convergence to higherorder attractors, rather than point equilibria, is possible, very long runs of data may be needed to assess whether dynamic patterns of species abundances have converged or not. 484

Nonlinear ecological responses to anthropogenic forcing are common, and in some cases, the ecosystem responds by assuming a new stable state. This article is an overview and serves as the introduction to several articles in this BRIDGES cluster that are directed toward managers interested in dealing with nonlinear responses in freshwaters, particularly streams. A threshold or breakpoint occurs where the system responds rapidly to a relatively small change in a driver. The existence of a threshold can signal a change in system configuration to an alternative stable state, although such a change does not occur with all thresholds. In general, a mechanistic understanding of ecological dynamics is required to predict thresholds, where they will occur, and if they are associated with the occurrence of alternative stable states. Thresholds are difficult to predict, although a variety of univariate methods has been used to indicate thresholds in ecological data. When we applied several methods to one type of response variable, the resulting threshold values varied 3-fold, indicating that more research on detection methods is necessary. Numerous case studies suggest that the threshold concept is important in all ecosystems. Managers should be aware that human actions might result in undesirable rapid changes and potentially an unwanted alternative stable state, and that recovery from that state might require far more resources and time than avoiding entering the state in the first place would have required. Given the difficulties in predicting thresholds and alternative states, the precautionary approach to ecosystem management is probably the most prudent.

T. Fukami and W. G. Lee argue that the logical expectation from ecological theory is that competitively‐structured assemblages will be more likely to exhibit alternative stable states than abiotically‐structured assemblages. We suggest that there are several important misinterpretations in their arguments, and that the substance of their hypothesis has both a weak basis in ecological theory and is not supported by empirical evidence which shows that alternative stable states occur more frequently in natural systems subject to moderate‐ to harsh abiotic extremes. While this debate is founded in ecological theory, it has important applied implications for restoration management. Sound theoretical predictions about when to expect alternative stable states can only aid more effective restoration if theoretical expectations can be shown to translate into predictable empirical outcomes. If strongly abiotically‐ or disturbance‐structured systems are more likely to exhibit catastrophic phase shifts in community structure that can be resilient to management efforts, then restoration ecologists will need to treat these systems differently in terms of the types of management inputs that are required.

The models investigated in this lab demonstrate how an ecological system can exist in alternative stable states and how the dynamics of alternative stable states change when moving from a site to an entire landscape. In this exam-ple, a site could persist as either a young, frequently burned site (dominated by longleaf pine) or an old, infrequently burned site (dominated by oaks). In-terestingly, however, the dynamics of a particular site changed when the site was placed within the context of its landscape, and the dynamics of a site greatly depended on the specific landscape context in which it was placed. Also, the dynamics of any single site were quite different from those of the entire landscape. When sites were grouped together to create a landscape, a site tended to push its neighboring sites toward its own particular state, re-ducing the range of system parameters for which alternative stable states could persist. In the forests of northern Florida, the ability of fire to spread, and conse-quently the rate at which patches of hardwood or pine either grow or shrink, is influenced by the pattern of hardwoods and pine across the landscape. The combination of spatial heterogeneity and positive feedbacks makes the dy-namics of forests difficult to predict from the study of a local site, since the processes that control a site can be affected by the properties of its neighbors. However, the use of models allows one to examine how the configuration of habitats across a landscape can affect the resilience of landscapes in the face of disturbance. Furthermore, while the models used in this lab are based on the forest dynamics of northern Florida, many ecosystems exhibit alternative stable states (Dublin et al., 1990; Knowlton, 1992; Scheffer et al., 1993). The dynamics of these ecosystems can be analyzed using similar models and techniques.

Increasingly frequent and intense wildfires threaten to push ecosystems beyond their ecological thresholds. Fire-adapted forests can be vulnerable to ecosystem conversion under shortening fire-return intervals. To evaluate the impacts of fire regime change, we contrasted Eucalyptus forests that retained canopy cover under tolerable regimes (reference forests) with alternative states of markedly reduced canopy. These alternative states established following short-interval wildfires and have persisted through subsequent fires. Alternative states were dominated by more uniform distributions of small trees (< 20 cm diameter) than reference states, with 97% fewer large trees (> 20 cm diameter). These changes in structure translated into declines in several key ecosystem functions. The loss of large, hollow-forming trees and limited Eucalyptus regeneration indicate long-term constraints on habitat availability for hollow-dependent fauna. Coarse woody debris mass was 38% lower in alternative states, indicating decreased resources for detritivores, fungi, and ground-dwelling mammals. Aboveground carbon stocks were reduced by 59%, with a much higher proportion (98%) stored in small trees than reference states (20%). Lower fire tolerance of small trees suggests a greater vulnerability of carbon stocks to subsequent fires in alternative states compared to reference states. These findings highlight the long-term ecological consequences of altered fire regimes. In addition, emphasising the management and restoration of forest structural integrity is essential to maintain ecosystem functions in fire-prone landscapes that are increasingly vulnerable under changing climates.

How nutrient loading leads to alternative stable states in microbially mediated N-cycle pathways: A new insight into bioavailable nitrogen removal in urban rivers

No abstract available.

The interpretation of biological communities as complex adaptive systems aims to demonstrate their ecological patterns at different scales and their environmental relationships. In this survey, we analyzed species diversity and alternative stable states patterns of the bird community in an Andean agroecosystem located on the central-western slope of the Peruvian Andes (Cajatambo District, Lima - Peru), on a temporal and spatio-temporal scale. We evaluated their bird community through 24 line transects, located in seven plant formations along two ecoregions, between 2 700 and 4 600 masl, during the dry and rainy seasons of 2015 and 2016. The temporal patterns did not show differences in species diversity between seasons. We determined the existence of a single temporary alternative stable states; however, we evidenced changes in their species composition between dry and rainy seasons, suggesting a greater functional redundancy in dry season than rainy season due to temporal variability of small granivore-insectivore species. Regarding the spatio-temporal patterns, the plant formations of Polylepis forests, some scrubs and agricultural lands presented high values of species diversity. In addition, we determined eight alternative stable states in relation to one or more plant formations and seasonality. Finally, a higher proportion of regime changes was found in the community alternative stable states located in the Meso-Andean ecoregion as opposed to the Puna-Humid ecoregion, which would suggest that this bird community has a greater self- organization and adaptability in the face of environmental stochasticity.

Alternative stable states of multi-trophic microbiota in long-distance water diversion systems.

Regime shifts between clear and turbid water states are commonly found in shallow lakes. These shifts are attributed to a positive feedback between water clarity and submerged macrophytes (underwater plants). Altering the retention time of the water may influence these interactions and thus potentially reduce the probability of alternative stable states. Here we assessed the effect of water retention time on the occurrence of alternative states in water quality of flushed lakes, chains of lakes and rivers using a spatially explicit simple model. Our results indicate that increased flushing of lakes rapidly decreases the range of parameters with alternative stable states up to their total disappearance at a flushing rate of about 50% the algal growth rate. Similarly, in a chain of lakes or in rivers with low flowing velocity, our model predicts that alternative stable states can only occur for systems with a high retention time. Despite the lack of hysteresis at lower water retention times, we predict that abrupt changes between clear and turbid states are still possible both in time and in space. Over a wide range of parameters, the equilibrium state of the chain of lakes shows a steep gradient of vegetation cover. Further, the transient dynamics of the model often include rapid shifts in time. For example, a local regime shift that occurs upstream may propagate through the whole lake chain or river due to a domino effect. All results of the simple model could qualitatively be reproduced with a more mechanistic model. The abrupt rather than gradual response of submerged macrophytes to reduced turbidity levels still makes river systems rather resilient to management measure. The importance of the initial turbidity and the observed domino effect suggest that restoration measures should start upstream and that these measures should eventually trigger regime shifts downstream.

A self-reinforcing positive feedback is regarded as a critical process for maintaining alternative stable states (ASS); however, identification of ASS and quantification of positive feedbacks remain elusive in natural ecosystems. Here, we used large-scale field surveys to search for ASS and a positive feedback mechanism under a wide range of habitats on the Tibetan Plateau. Using multiple methods, we proved that three stable states exist that accompany alpine marsh degradation. Positive feedbacks between changing soil moisture and plant community composition forced the ecosystem into another stable state, and the alteration of water use efficiency (WUE) of the component species contributed to this shift. This study provides the first empirical evidence that positive feedback loops maintain ASS in the alpine marsh ecosystem on the Tibetan Plateau. Our research revealed the powerful driving role of plants in transitions between states, which may support the conservation and restoration of global alpine marsh ecosystems.

Ecological systems can exist under multiple stable states for a specific set of environmental conditions. Perturbations can shift ecosystems to alternative stable states, and these changes are difficult to reverse. This is a concept known as hysteresis. Current management methods for system recovery require considerable effort. A model for system recovery that requires small changes in a target community would be of high value for ecosystem restoration. In this study, we focused on rocky reefs that exhibit two alternative stable states: one dominated by macroalgae and another dominated by sea urchins and crustose coralline algae. Community recovery requires a state shift. To better understand the state shift, we simulated the ecosystem using a lattice model incorporating local interactions, rather than equations representing homogenized spaces. Using spatial information and simulating conditions in the target ecosystem, we concluded that a shift to an alternative stable state is possible by adjusting the spatial arrangement of the organisms. These results indicate that spatial arrangements are significant in systems exhibiting hysteresis and that the spatial manipulation of organisms may be useful for ecosystem management and restoration.

The vegetation of the Wet Tropics bioregion of Far North Queensland is a complex system whose components (mainly tropical rainforests and fire-prone forests and woodlands) have mostly been studied independently from each other. We suggest that many characteristics of the vegetation are consistent with those of a complex, dynamic, spatially heterogeneous system which exhibits alternative stable states. We propose these states are driven and maintained by the interaction of vegetation-specific positive feedback loops with the regions’ environmental parameters (such as topography, steep humidity gradients and seasonality) and result in the rainforest/fire-prone vegetation mosaic that characterises the area. Given the regions’ magnitude, biodiversity and complexity, we propose the Wet Tropics as an important new example and a good testing ground for alternative stable state and resilience theories in large heterogeneous natural systems. At the same time, thinking in terms of alternative stable states and resilience creates a new context for understanding the regions’ biological dynamics.

Alternative stable state and its evaluation in wetland reconstruction based on landscape design

Summary Temporal changes in semi‐arid ecosystems can include transitions between alternative stable states, involving thresholds and multiple domains of attraction, but can also include relatively continuous, symmetric and reversible shifts within a single stable state. Conceptual state‐and‐transition models (STMs) describe both types of ecosystem dynamics by including state transitions (plant community changes difficult‐to‐reverse without substantial input or effort) and phase shifts (easily reversible community changes) as consequences of management practices and environmental variability. Grazing management is purported to be the primary driver of state transitions in current STMs for North American grasslands, but there is limited empirical evidence from these grasslands showing that grazing can cause difficult‐to‐reverse transitions between alternate stable states. In a northern mixed‐grass prairie in Wyoming, USA, we examined plant community responses to (i) long‐term (33‐year) grazing intensity treatments (none, light, moderate and heavy stocking rates) and (ii) 8 years of light or no grazing in pastures that were grazed heavily for the previous 25 years. Long‐term grazing treatments were associated with distinct, but not stable, plant communities. From year 22 to 33, heavier stocking rates decreased cover of dominant C3 grasses and increased cover of the dominant C4 grass Bouteloua gracilis. Reversing stocking rates from heavy to light or no grazing resulted in reversal of changes induced by prior heavy stocking for dominant C3 grasses, but not for B. gracilis. For both groups, rates of change following grazing treatment reversals were consistent with rates of change during the initial years of the experiment (1982–1990). Synthesis and applications. In a semi‐arid rangeland with a long evolutionary history of grazing, different long‐term grazing intensity treatments caused slow, continuous and directional changes with important management implications, but did not appear to induce alternative stable states. For this and similar ecosystems, quantifying the time‐scales and compositional gradients associated with key phase shifts may be more important than identifying thresholds between alternative stable states.