Claire de Mazancourt

Grazing optimization and nutrient cycling : when do herbivores enhance plant production

In a general theoretical ecosystem model, we investigate the conditions under which herbivores increase primary production and lead to grazing optimization through recycling of a limiting nutrient. Analytical and simulation studies of the model lead to several general results. Grazing optimization requires that (1) the proportion of nutrient lost along the herbivore pathway be sufficiently smaller than the proportion of nutrient lost throughout the rest of the ecosystem; and that (2) inputs of nutrient into the system be greater than a threshold value, which depends on the sensitivity of plant uptake rate to an increase in soil mineral nutrient. An increase in nutrient turnover rate is not sufficient to explain grazing optimization in the long term. When a nutrient is the single limiting factor, plant biomass and productivity at equilibrium are determined only by the balance of ecosystem inputs and outputs of nutrient. Processes that do not have an impact on these inputs or outputs have no effect on primary producers. On the other hand, turnover rates are important for the transient dynamics of the system, and the equilibrium analysis is relevant only if it can be reached in a reasonable time scale. The equilibrium is not reached by a compartment with a very slow turnover rate, such as the resistant soil organic matter, before several centuries. On a small time scale, such a compartment can be considered constant, and the trend of the system is predicted with a simplified system. The results at equilibrium are insensitive to the functional form used to describe herbivore consumption: the results obtained for simple, linear, donor-controlled herbivory also apply to most forms of more realistic, recipient-controlled herbivory. We conclude that grazing optimization is most likely to occur in systems with large losses of the limiting nutrient during recycling of plant detritus, or where herbivores bring nutrient from outside the ecosystem considered (which acts to reduce, or even make negative, the fraction of nutrient lost along the herbivore detritus pathway).

Species Synchrony and Its Drivers: Neutral and Nonneutral Community Dynamics in Fluctuating Environments

Independent species fluctuations are commonly used as a null hypothesis to test the role of competition and niche differences between species in community stability. This hypothesis, however, is unrealistic because it ignores the forces that contribute to synchronization of population dynamics. Here we present a mechanistic neutral model that describes the dynamics of a community of equivalent species under the joint influence of density dependence, environmental forcing, and demographic stochasticity. We also introduce a new standardized measure of species synchrony in multispecies communities. We show that the per capita population growth rates of equivalent species are strongly synchronized, especially when endogenous population dynamics are cyclic or chaotic, while their long‐term fluctuations in population sizes are desynchronized by ecological drift. We then generalize our model to nonneutral dynamics by incorporating temporal and nontemporal forms of niche differentiation. Niche differentiation consistently decreases the synchrony of species per capita population growth rates, while its effects on the synchrony of population sizes are more complex. Comparing the observed synchrony of species per capita population growth rates with that predicted by the neutral model potentially provides a simple test of deterministic asynchrony in a community.

The evolutionary ecology of metacommunities.

Research on the interactions between evolutionary and ecological dynamics has largely focused on local spatial scales and on relatively simple ecological communities. However, recent work demonstrates that dispersal can drastically alter the interplay between ecological and evolutionary dynamics, often in unexpected ways. We argue that a dispersal-centered synthesis of metacommunity ecology and evolution is necessary to make further progress in this important area of research. We demonstrate that such an approach generates several novel outcomes and substantially enhances understanding of both ecological and evolutionary phenomena in three core research areas at the interface of ecology and evolution.

Trade‐Off Geometries and Frequency‐Dependent Selection

Life‐history evolution is determined by the interplay between natural selection and adaptive constraints. The classical approach to studying constrained life‐history evolution—Richard Levins’s geometric comparison of fitness sets and adaptive functions—is applicable when selection pressures are frequency independent. Here we extend this widely used tool to frequency‐dependent selection. Such selection pressures vary with a population’s phenotypic composition and are increasingly recognized as ubiquitous. Under frequency dependence, two independent properties have to be distinguished: evolutionary stability (an evolutionarily stable strategy cannot be invaded once established) and convergence stability (only a convergence stable strategy can be attained through small, selectively advantageous steps). Combination of both properties results in four classes of possible evolutionary outcomes. We introduce a geometric mode of analysis that enables predicting, for any bivariate selection problem, evolutionary outcomes induced by trade‐offs of given shape, shapes of trade‐offs required for given evolutionary outcomes, the set of all evolutionary outcomes trade‐offs can induce, and effects of ecological parameters on evolutionary outcomes independent of trade‐off shape.

To Freeze or Not to Freeze? An Evolutionary Perspective on the Cold-Hardiness Strategies of Overwintering Ectotherms

We address the question of whether freeze‐tolerance, freeze‐avoidance, or mixed strategy represents the best adaptation for overwintering ectotherms to endure severe winter. To this end, we develop an optimization fitness model that takes into account different physiological parameters such as energetic level, the physiological stress associated with each strategy, and climatic variables. The results show that the freeze‐tolerance strategy is strongly dependent on a low sensitivity to the number of freezing days and on a capacity to reduce stress associated with freezing. This strategy is also favored when the initial energetic level is low compared to the freeze‐avoidance strategy, which is favored by a high initial energetic level, a low stress associated with the supercooling, and a low sensitivity of this strategy to climatic conditions. From a theoretical point of view, the mixed strategy permits survival in harsher environments but requires the optimization of all parameters involved in both cold‐hardiness strategies. However, the mixed strategy shows energetic advantages in variable environments allowing animals to resist the harshest periods. From the model results, it appears that the physiological processes developed by ectotherms to reduce these stresses might be a key to understanding the evolution of the cold‐hardiness strategies.

Effect of Herbivory and Plant Species Replacement on Primary Production

Grazing optimization occurs when herbivory increases primary production at low grazing intensities. In the case of simple plant‐herbivore interactions, such an effect can result from recycling of a limiting nutrient. However, in more complex cases, herbivory can also lead to species replacement in plant communities, which in turn alters how primary production is affected by herbivory. Here we explore this issue using a model of a limiting nutrient cycle in an ecosystem with two plant species. We show that two major plant traits determine primary production at equilibrium: plant recycling efficiency (i.e., the fraction of the plant nutrient stock that stays within the ecosystem until it is returned to the nutrient pool in mineral form) and plant ability to deplete the soil mineral nutrient pool through consumption of this resource. In cases where sufficient time has occurred, grazing optimization requires that herbivory improve nutrient conservation in the system sufficiently. This condition sets a minimum threshold for herbivore nutrient recycling efficiency, the fraction of nutrient consumed by herbivores that is recycled within the ecosystem to the mineral nutrient pool. This threshold changes with plant community composition and herbivore preference and is, therefore, strongly affected by plant species replacement. The quantitative effects of these processes on grazing optimization are determined by both the recycling efficiencies and depletion abilities of the plant species. However, grazing optimization remains qualitatively possible even with plant species replacement.

GRAZING OPTIMIZATION AND NUTRIENT CYCLING: POTENTIAL IMPACT OF LARGE HERBIVORES IN A SAVANNA SYSTEM

Using a model, we test the prediction that herbivory can result in grazing optimization of primary production in a nitrogen-limited system where large losses of nitrogen occur in annual fires. The model is based on the nitrogen budget of the humid savanna of Lamto, Ivory Coast, estimated from field data. At present, the ecosystem contains few herbivores, but buffalo and kob populations are increasing. We show that grazing optimization through recycling of nitrogen would occur at Lamto in the short term (i.e., several decades) if the percentage of nitrogen lost from the system out of the amount ingested by herbivores is <24%, and in the long term (i.e., several centuries) if it is <19%. When 25% of nitrogen is lost by herbivores, primary production is maintained at a high level up to very high consumption rates. Because losses due to herbivores are likely to be lower than these values in this particular ecosystem, we conclude that grazing optimization is likely to occur in the Lamto savanna.

Increased plant growth from nitrogen addition should conserve phosphorus in terrestrial ecosystems

Inputs of available nitrogen (N) to ecosystems have grown over the recent past. There is limited general understanding of how increased N inputs affect the cycling and retention of other potentially limiting nutrients. Using a plant–soil nutrient model, and by explicitly coupling N and phosphorus (P) in plant biomass, we examine the impact of increasing N supply on the ecosystem cycling and retention of P, assuming that the main impact of N is to increase plant growth. We find divergent responses in the P cycle depending on the specific pathway by which nutrients are lost from the ecosystem. Retention of P is promoted if the relative propensity for loss of plant available P is greater than that for the loss of less readily available organic P. This is the first theoretical demonstration that the coupled response of ecosystem-scale nutrient cycles critically depends on the form of nutrient loss. P retention might be lessened, or reversed, depending on the kinetics and size of a buffering reactive P pool. These properties determine the reactive pools ability to supply available P. Parameterization of the model across a range of forest ecosystems spanning various environmental and climatic conditions indicates that enhanced plant growth due to increased N should trigger increased P conservation within ecosystems while leading to more dissolved organic P loss. We discuss how the magnitude and direction of the effect of N may also depend on other processes.

A resource ratio theory of cooperation

Resource ratio theory predicts that two species may coexist in the presence of two limiting nutrients provided that each species is limited by the resource it is least able to deplete. We modify this classical competition model to allow interspecific cooperation through trading. We show that resource trade expands the realm of stable coexistence, and that optimal trading partners competitively invade and exclude any other trading or non-trading strategy. We show that natural selection favours evolution towards establishment of a trading relationship so long as partners can establish long-term associations even though cooperation may result in a decrease in abundance of one species. This theory substantively expands traditional applications of resource competition models and suggests additional empirical experimentation.

Can the evolution of plant defense lead to plant-herbivore mutualism?

Moderate rates of herbivory can enhance primary production. This hypothesis has led to a controversy as to whether such positive effects can result in mutualistic interactions between plants and herbivores. We present a model for the ecology and evolution of plant‐herbivore systems to address this question. In this model, herbivores have a positive indirect effect on plants through recycling of a limiting nutrient. Plants can evolve but are constrained by a trade‐off between growth and antiherbivore defense. Although evolution generally does not lead to optimal plant performance, our evolutionary analysis shows that, under certain conditions, the plant‐herbivore interaction can be considered mutualistic. This requires in particular that herbivores efficiently recycle nutrients and that plant reproduction be positively correlated with primary production. We emphasize that two different definitions of mutualism need to be distinguished. A first ecological definition of mutualism is based on the short‐term response of plants to herbivore removal, whereas a second evolutionary definition rests on the long‐term response of plants to herbivore removal, allowing plants to adapt to the absence of herbivores. The conditions for an evolutionary mutualism are more stringent than those for an ecological mutualism. A particularly counterintuitive result is that higher herbivore recycling efficiency results both in increased plant benefits and in the evolution of increased plant defense. Thus, antagonistic evolution occurs within a mutualistic interaction.