Coevolutionary transitions from antagonism to mutualism explained by the Co-Opted Antagonist Hypothesis

There is now good evidence that many mutualisms evolved from antagonism; why or how, however, remains unclear. We advance the Co-Opted Antagonist (COA) Hypothesis as a general mechanism explaining evolutionary transitions from antagonism to mutualism. COA involves an eco-coevolutionary process whereby natural selection favors co-option of an antagonist to perform a beneficial function and the interacting species coevolve a suite of phenotypic traits that drive the interaction from antagonism to mutualism. To evaluate the COA hypothesis, we present a generalized eco-coevolutionary framework of evolutionary transitions from antagonism to mutualism and develop a data-based, fully ecologically-parameterized model of a small community in which a lepidopteran insect pollinates some of its larval host plant species. More generally, our theory helps to reconcile several major challenges concerning the mechanisms of mutualism evolution, such as how mutualisms evolve without extremely tight host fidelity (vertical transmission) and how ecological context influences evolutionary outcomes, and vice-versa.

While there is strong evidence that many mutualisms evolved from antagonism, how or why remains unclear. A study combining theory and a data-based model sheds light on how mutualisms evolve without extremely tight host fidelity and how ecological context affects evolutionary outcomes and vice-versa.

A unified framework for plant life-history strategies shaped by fire and herbivory

Fire and herbivory both remove aboveground biomass. Environmental factors determine the type and intensity of these consumers globally, but the traits of plants can also alter their propensity to burn and the degree to which they are eaten. To understand plant life-history strategies associated with fire and herbivory we need to describe both response and effect functional traits, and how they sort within communities, along resource gradients, and across evolutionary timescales. Fire and herbivore functional traits are generally considered separately, but there are advances made in understanding fire that relate to herbivory, and vice versa. Moreover, fire and herbivory interact: the presence of one consumer affects the type and intensity of the other. Here, we present a unifying conceptual framework to understand plant strategies that enable tolerance and persistence to fire and herbivory. Using grasses as an example, we discuss how flammability and fire tolerance, palatability, and grazing tolerance traits might organize themselves in ecosystems exposed to these consumers, and how these traits might have evolved with reference to other strong selective processes, like aridity. Our framework can be used to predict both the diversity of life-history strategies and plant species diversity under different consumer regimes.

Overcompensation: a 30-year perspective

Biomass removal by herbivores usually incurs a fitness cost for the attacked plants, with the total cost per unit lost tissue depending on the value of the removed tissue (i.e., how costly it is to be replaced by regrowth). Optimal defense theory, first outlined in the 1960s and 1970s, predicted that these fitness costs result in an arms race between plants and herbivores, in which selection favors resistance strategies that either repel herbivores through morphological and chemical resistance traits in order to reduce their consumption, or result in enemy escape through rapid growth or by timing the growth or flowering to the periods when herbivores are absent. Such resistance against herbivores would most likely evolve when herbivores are abundant, cause extensive damage, and consume valuable plant tissues. The purpose of this Special Feature is to celebrate the 30th anniversary of the phenomenon of overcompensation, specifically, where the finding has brought us and where it is leading us 30 yr later. We first provide a short overview of how the phenomenon of overcompensation has led to broader studies on plant tolerance to herbivory, summarize key findings, and then discuss some promising new directions in light of six featured research papers.

Simulating Eco-evolutionary Processes in an Obligate Pollination Model with a Genetic Algorithm

Pollination interactions are common, and their maintenance is critical for many food crops upon which human populations depend. Pollination is a mutualism interaction; together with predation and competition, mutualism makes up the triumvirate of fundamental interactions that control population dynamics. Here we examine pollination interactions (nectar reward for gamete transport service) using a simple heuristic model similar to the Lotka-Volterra models that have underpinned our understanding of predation and competition so effectively since the 1920s. We use a genetic algorithm to simulate the eco-evolutionary interactions of the plant and pollinator populations and examine the distributions of the parameter values and zero isoclines to infer the relative ubiquity of the various eco-evolutionary outcomes possible in the model. Our results suggest that trade-offs between costs and benefits for the pollinator may be a key component of obligate pollination systems in achieving adaptive success creating and stably occupying mutualist niches.

Evidence of horizontal gene transfer between obligate leaf nodule symbionts

Bacteria of the genus Burkholderia establish an obligate symbiosis with plant species of the Rubiaceae and Primulaceae families. The bacteria, housed within the leaves, are transmitted hereditarily and have not yet been cultured. We have sequenced and compared the genomes of eight bacterial leaf nodule symbionts of the Rubiaceae plant family. All of the genomes exhibit features consistent with genome erosion. Genes potentially involved in the biosynthesis of kirkamide, an insecticidal C7N aminocyclitol, are conserved in most Rubiaceae symbionts. However, some have partially lost the kirkamide pathway due to genome erosion and are unable to synthesize the compound. Kirkamide synthesis is therefore not responsible for the obligate nature of the symbiosis. More importantly, we find evidence of intra-clade horizontal gene transfer (HGT) events affecting genes of the secondary metabolism. This indicates that substantial gene flow can occur at the early stages following host restriction in leaf nodule symbioses. We propose that host-switching events and plasmid conjugative transfers could have promoted these HGTs. This genomic analysis of leaf nodule symbionts gives, for the first time, new insights in the genome evolution of obligate symbionts in their early stages of the association with plants.

Variation in Arabidopsis developmental responses to oomycete infection: resilience vs changes in life history traits

Although plant resistance to aggressors has been well described, there is still little knowledge about the mechanisms underlying their tolerance to pathogens. Tolerance often appears to be mediated by changes in life history traits, shifting host resource investment from growth to reproduction, but whether host phenotype modifications induced after attack are adaptive is not always clear. Here, we investigated the details of the impact of Hyaloperonospora arabidopsidis infection on several biomass, phenology and architectural traits of Arabidopsis thaliana, for three pathogen genotypes and three host plant genotypes that have been shown previously to differ greatly in fecundity and tolerance to infection. We found that, although host genotype explains most of the variance in life history traits, these three lines differ critically in their response to infection, with delays and biomass losses at bolting, together with changes in inflorescence architecture, observed at one extreme host line, and an advantage at bolting for infected plants and no inflorescence alteration for the other. These results suggest that the differences in tolerance observed previously in this pathosystem do not involve plasticity in inflorescence architecture, but may arise from induced changes at the vegetative stage, before plant transition to reproduction.

Coupling effect of grazing pressure and nutrient enrichment on system stability

A three dimensional nutrient-plant-herbivore model was proposed and conditions for boundedness, positive invariance, existence and stability of different equilibrium points, Hopf-bifurcation and global stability were obtained. We performed numerical simulations to observe the simultaneous effect of the top-down and the bottom-up mechanism on the system. It was found that nutrient enrichment destroyed the coexistence steady state of the system. This nutrient enrichment could be due to high nutrient input rate or high nutrient recycling rate. In both cases the system showed instability. Moreover, these results were independent of the grazing pressure and the predation functional form.

Linking biodiversity and ecosystems: towards a unifying ecological theory

Community ecology and ecosystem ecology provide two perspectives on complex ecological systems that have largely complementary strengths and weaknesses. Merging the two perspectives is necessary both to ensure continued scientific progress and to provide society with the scientific means to face growing environmental challenges. Recent research on biodiversity and ecosystem functioning has contributed to this goal in several ways. By addressing a new question of high relevance for both science and society, by challenging existing paradigms, by tightly linking theory and experiments, by building scientific consensus beyond differences in opinion, by integrating fragmented disciplines and research fields, by connecting itself to other disciplines and management issues, it has helped transform ecology not only in content, but also in form. Creating a genuine evolutionary ecosystem ecology that links the evolution of species traits at the individual level, the dynamics of species interactions, and the overall functioning of ecosystems would give new impetus to this much-needed process of unification across ecological disciplines. Recent community evolution models are a promising step in that direction.

Above- and below-ground vertebrate herbivory may each favour a different subordinate species in an aquatic plant community

At least two distinct trade-offs are thought to facilitate higher diversity in productive plant communities under herbivory. Higher investment in defence and enhanced colonization potential may both correlate with decreased competitive ability in plants. Herbivory may thus promote coexistence of plant species exhibiting divergent life history strategies. How different seasonally tied herbivore assemblages simultaneously affect plant community composition and diversity is, however, largely unknown. Two contrasting types of herbivory can be distinguished in the aquatic vegetation of the shallow lake Lauwersmeer. In summer, predominantly above-ground tissues are eaten, whereas in winter, waterfowl forage on below-ground plant propagules. In a 4-year exclosure study we experimentally separated above-ground herbivory by waterfowl and large fish in summer from below-ground herbivory by Bewick’s swans in winter. We measured the individual and combined effects of both herbivory periods on the composition of the three-species aquatic plant community. Herbivory effect sizes varied considerably from year to year. In 2 years herbivore exclusion in summer reinforced dominance of Potamogeton pectinatus with a concomitant decrease in Potamogeton pusillus, whereas no strong, unequivocal effect was observed in the other 2 years. Winter exclusion, on the other hand, had a negative effect on Zannichellia palustris, but the effect size differed considerably between years. We suggest that the colonization ability of Z. palustris may have enabled this species to be more abundant after reduction of P. pectinatus tuber densities by swans. Evenness decreased due to herbivore exclusion in summer. We conclude that seasonally tied above- and below-ground herbivory may each stimulate different components of a macrophyte community as they each favoured a different subordinate plant species.

Theory of grazing optimization in which herbivory improves photosynthetic ability

Herbivory had been generally considered to have a negative effect on plants, but a lot of studies have recently indicated that continuous herbivory pressure has a positive effect on plant performance, known as "grazing optimization." Based on field observations, we analytically examined a hypothesis of grazing optimization in which herbivory improves the photosynthetic ability of individual plants. We examined plant performance under various herbivory pressures and considered the evolution of plant phenology in response to a given herbivory pressure. First, we compared plant performances measured under their native conditions with specific herbivory levels. This was called the long-term response. Second, we examined the performances of plants adapting to a certain level of herbivory pressure under a non-native herbivory intensity. This was called the short-term response. According to numerical analysis, in realistic situations, grazing optimization is unlikely to be observed as a long-term response. However, grazing optimization can occur as short-term response if a plant is adapted to a certain level of herbivory pressure and the photosynthetic ability decreases significantly with the increasing size of vegetative parts. Our results suggest that improved photosynthetic ability by herbivory can result in grazing optimization, although it is constrained by the functional form of photosynthetic ability, native conditions, and experimental design.

Mutualistic stability in the arbuscular mycorrhizal symbiosis: exploring hypotheses of evolutionary cooperation

The 450-million-year-old symbiosis between the majority of land plants and arbuscular mycorrhizal fungi (AMF) is one of the most ancient, abundant, and ecologically important mutualisms on Earth. Yet, the evolutionary stability of mycorrhizal associations is still poorly understood, as it follows none of the constraints thought to stabilize cooperation in other well-known mutualisms. The capacity of both host and symbiont to simultaneously interact with several partners introduces a unique dilemma; detecting and punishing those exploiting the mutualism becomes increasingly difficult if these individuals can continue to access resources from alternative sources. Here, we explore four hypotheses to explain evolutionary cooperation in the arbuscular mycorrhizal symbiosis: (1) pseudo-vertical transmission and spatial structuring of plant and fungal populations leading to local adaptation of partners; (2) luxury resource exchange in which plants trade surplus carbon for excess fungal nutrients; (3) partner choice allowing partners to associate with better cooperators; and (4) host and symbiont sanctions which actively reward good partners and punish less cooperative ones. We propose that mycorrhizal cooperation is promoted by an exchange of surplus resources between partners and enforced through sanctions by one or both partners. These mechanisms may allow plant and fungal genotypes to discriminate against individuals employing exploitative strategies, promoting patterns of partner choice. Together these selection pressures provide a framework for understanding the stabilization of mycorrhizal cooperation over evolutionary time.

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.

Adaptive walks on changing landscapes: Levins' approach extended

The assumption that trade-offs exist is fundamental in evolutionary theory. Levins (Am. Nat. 96 (1962) 361-372) introduced a widely adopted graphical method for analyzing evolution towards an optimal combination of two quantitative traits, which are traded off. His approach explicitly excluded the possibility of density- and frequency-dependent selection. Here we extend Levins method towards models, which include these selection regimes and where therefore fitness landscapes change with population state. We employ the same kind of curves Levins used: trade-off curves and fitness contours. However, fitness contours are not fixed but a function of the resident traits and we only consider those that divide the trait space into potentially successful mutants and mutants which are not able to invade ('invasion boundaries'). The developed approach allows to make a priori predictions about evolutionary endpoints and about their bifurcations. This is illustrated by applying the approach to several examples from the recent literature.

Spatial Flows and the Regulation of Ecosystems

Spatial flows of materials and organisms across ecosystem boundaries are ubiquitous. Understanding the consequences of these flows should be a basic goal of ecosystem science, and yet it has received scant theoretical treatment to date. Here, using a simple, open, nutrient-limited ecosystem model with trophic interactions, we explore theoretically how spatial flows affect the functioning of local ecosystems, how physical mass-balance constraints interact with biological demographic constraints in the regulation of this functioning, and how failure to consider these constraints explicitly can lead to models that are ecologically inconsistent. In particular, we show that standard prey-dependent models for trophic interactions may lead to implausible outcomes when embedded in an ecosystem context with appropriate mass flows and mass-balance constraints. Our analysis emphasizes the need for integration of population, community, and ecosystem perspectives in ecology and the critical consequences of assuming closed versus open systems.

Evolution of mutualism through spatial effects

Mutualism among species is ubiquitous in natural ecosystems but its evolution is not well understood. We provided a simple lattice model to clarify the importance of spatial structure for the evolution of mutualism. We assumed reproductive rates of two species are modified through interaction between species and examine conditions where mutualists of both species, that give some benefit to the other species with their own cost, invade non-mutualists populations. When dispersal of offspring is unlimited, we verified the evolution of mutualism is impossible under any condition. On the other hand, when the dispersal is limited to neighboring lattice sites, mutualists can invade if the ratio of cost to benefit is low and the intrinsic reproductive rate is low in case where the parameter values are symmetric between species. Under the same conditions, non-mutualists cannot invade mutualist populations, that is, the latter are evolutionarily stable. In case of asymmetric parameters, mutualists tend to invade if the average value of costs to two species is low or that of benefits is high, and if the intrinsic reproductive rate is low for one of the two species. A mechanistic explanation of why mutualists increase when the dispersal is limited is given by showing that mutualist pairs of the two species at the same lattice site rapidly increase at the initial phase of the invasion.

Herbivory Promotes Plant Production and Reproduction in Nutrient‐Poor Conditions: Effects of Plant Adaptive Phenology

Many studies have demonstrated positive effects of herbivory on plant performance, and these encompass two categories of effects: enhancement of primary production and enhancement of reproductive success. These positive responses of plants to herbivory have been called "grazing optimization." One possible mechanism of these paradoxical phenomena is the nutrient cycling promoted by herbivory. This article models the nutrient cycling hypothesis and analyzes the evolution of plant production and reproduction enhanced by herbivores, using dynamic optimization of plant phenology. Especially when there is nutrient competition among plant individuals or nutrient transportation by herbivores, we can apply the concept of evolutionary stability for the dynamic optimization. Two types of plant responses, long-term and short-term, are examined. Long-term response is an adaptive response for a given level of herbivory pressure, while short-term response is a nonadaptive one to various levels of herbivory, different from the level to which the plant is adapted. The analysis shows that both long-term and short-term grazing optimizations in primary production can occur under poor nutrient conditions and high nutrient recycling rates. However, grazing optimization in reproduction occurs under the same conditions but requires further conditions. In particular, long-term reproductive grazing optimization occurs only when nutrient competition exists among plant individuals. Accordingly, the present analysis revealed the following points concerning grazing optimization: poor nutrient condition is necessary, nutrient competition between plant individuals can promote optimization, and the native condition of the plant is important in the short-term response.

Consequences of Plant-Herbivore Coevolution on the Dynamics and Functioning of Ecosystems

The potential consequences of plant-herbivore coevolution for ecosystem functioning are investigated using a simple nutrient-limited ecosystem model in which plant and herbivore traits are subject to adaptive dynamics. Although the ecological model is very simple and always reaches a stable equilibrium in the absence of evolution, coevolution can generate a great diversity of dynamical behaviors. The evolutionary dynamics can lead to a stable equilibrium. If the evolution of plants is fast enough, certain values of the trade-off parameters lead to complex evolutionary cycles bounded by physiological constraints. The dynamical behavior of the model is very different when the dynamics of inorganic nutrient is ignored and plant competition is modeled by a logistic growth function. This emphasizes the importance of including explicit nutrient dynamics in studies of plant-herbivore coevolution.