Multiple global-change forces, from habitat alterations to warming, are altering food webs and trophic interaction strengths. Such changes in trophic interactions have important implications, as it is a tenet of ecology that trophic interactions are linked to the functioning and stability of ecosystems. For example, changes in the presence or strength of intraguild predation (IGP), the consumption of a predator by another predator that competes for shared prey, can have cascading effects on the biomasses of species and trophic levels. For this reason, IGP can affect key ecosystem functions at the base of the food web and is of special interest to practitioners of biological pest control. However, the relationship between IGP and ecosystem stability is not yet well understood, especially whether and how IGP might affect the stability of non-adjacent lower trophic levels including primary producers. In this study we simulate the dynamics of a six-species, four-trophic-level food web plus a limiting nutrient to explore the relationship between IGP strength and the temporal variability of species- and trophic group-biomass. By varying the IGP rate given the abundance of the eaten predator, we find that the model food web abruptly shifts between equilibria in which all species maintain either constant biomass or stable limit cycles where all trophic levels exhibit sustained and significant oscillations. While complex feedback in the model creates a divergence between the IGP functional response and the resulting realized IGP strength, both stronger IGP functional responses and stronger realized IGP are associated with a higher likelihood of oscillations. Furthermore, analyses indicate that the strongest consumptive interaction induces the oscillating behavior in an indirect effect initiated by the change in IGP. Overall, these results suggest that as food web structure changes in ecosystems worldwide, strengthening IGP runs the risk of inducing destabilizing effects that extend to the base of food webs, while weakening IGP could confer stability to ecosystem functions such as primary production. Finally, we discuss relevance to management, including the implication that IGP among biological control agents should be minimized to maintain stable crop production. ### Competing Interest Statement The authors have declared no competing interest.

Niche differentiation with respect to light availability as it varies across succession has often been thought to explain tree species coexistence. Demographic light‐related niches represented by growth‐survival and stature‐recruitment trade‐offs and captured by demographic groups (slow, fast, long‐lived pioneers, short‐lived breeders and intermediate) have been shown to accurately represent the biomass dynamics of secondary and old‐growth forests in central Panama in a model. However, whether the simple mechanisms of that well‐parameterized and accurate model are enough to support the long‐term coexistence of demographic groups across these trade‐offs has yet to be tested. Here, we develop a model to test whether stochastic, small‐scale gap disturbances and subsequent competition for light can support the long‐term coexistence of the observed demographic groups in the Barro Colorado Island forest dynamics plot. Specifically, to test whether the demographic differences among species promote coexistence, we compare niche simulation models, parameterized by the different demographic groups, to a variety of neutral models, where the species have the same demographic parameters. Upon exploring the estimated range of possible parameterizations of recruitment (a difficult‐to‐measure parameter), we identify several parameterizations where differences among groups along the growth‐survival and stature‐recruitment trade‐off axes facilitate long‐term coexistence. We find that gap disturbances are essential for these results, indicating that it is the differences in the subsequent competition for light through time that provide the opportunity for stabilizing niche differentiation. Additionally, the parameterizations that generate stable coexistence display successional negative density dependence and realistic within‐patch post‐disturbance forest dynamics. Synthesis . This model‐data integration exercise indicates that small‐scale disturbances and subsequent competition for light may be significant forces for stable diversity maintenance of demographic groups along the growth–survival and stature–recruitment trade‐off axes in a neotropical forest. This result, however, holds only for a subset of the empirically reasonable recruitment parameters, indicating the importance of improving the understanding of recruitment and its demographic trade‐offs for understanding demographic strategy coexistence.

Earth system models (ESMs) have a limited capacity to represent plant functional diversity and shifts in trait distributions. Approaches to improving the representation of this complexity in ESMs include (i) optimality-based approaches that predict trait-environment responses and (ii) explicitly modelling coexistence and community assembly. These approaches are expected to converge only when optimality-based approaches identify competitively dominant strategies, which often differ from strategies that maximize ecosystem functioning or fitness components in monoculture. We used two models, LM3-PPA (a vegetation demographic model designed as an ESM component) and BiomeE (a computationally efficient analog for LM3-PPA), to explore how water limitation affects carbon allocation strategies of canopy trees. We compared competitive allocation strategies and those that maximize biomass or productivity in monoculture. We did not explicitly model coexistence or community assembly. Rather, we used model experiments to identify competitive and maximizing strategies in a two-dimensional trait space under different precipitation and mortality scenarios. At 10 eastern US locations, we simulated historical, wet and dry climate scenarios, novel drought and three different mortality scenarios (low, medium or high sensitivity to water deficit). For each site and scenario, we identified the competitive strategy and three maximizing strategies (maximum biomass, productivity or drought-tolerance). Root: leaf ratios tended to increase and leaf area tended to decrease with increasing water stress (increasing water limitation and its effects on mortality). However, relative to maximizing strategies, competitive strategies shifted towards greater allocation to roots and leaves with increasing water stress. Competitive overinvestments (greater allocation to roots and leaves by competitive strategies compared with maximizing strategies) were robust across different modelling contexts, including vegetation parameter sets (Acer vs. Populus), models (LM3-PPA vs. BiomeE) and uncalibrated vsersus calibrated BiomeE versions. Synthesis: The theoretical prediction that competitive and maximizing allocation strategies differ under water limitation is confirmed for a demographic model designed as an ESM component. Optimality-based trait predictions can simplify representing trait diversity in ESMs but do not always correspond to competitive outcomes. Explicitly modelling coexistence and community assembly in ESMs is challenging but is likely the most general approach to representing trait diversity. A key challenge for Earth system models is improving the representation of plant functional diversity. Experiments with vegetation demographic models show that water limitation leads to divergence between tree allocation strategies that are most competitive versus those that maximize biomass or productivity in monoculture.image

ABSTRACT Secondary tropical forests play an increasingly important role for carbon budgets and biodiversity conservation. Understanding successional trajectories is therefore imperative for guiding forest restoration and climate change mitigation efforts. Forest succession is driven by the demographic strategies – combinations of growth, mortality and recruitment rates – of the tree species in the community. However, our understanding of demographic diversity in tropical tree species stems almost exclusively from old-growth forests. Here, we assembled demographic information from repeated forest inventories along chronosequences in two wet (Costa Rica, Panama) and two dry (Mexico) Neotropical forests to assess whether the range of demographic strategies present in a community shifts across succession. We calculated demographic rates for >500 tree species while controlling for canopy status to compare demographic diversity in early successional (0-30 years), late successional (30-120 years) and old-growth forests. We quantified demographic diversity using two-dimensional hypervolumes of pairs of demographic rates and assessed whether shifts in demographic strategies were caused by intra-specific changes in demographic rates across succession or by species turnover. We expected that demographic strategies would shift from faster life-histories (fast growth, high mortality, high recruitment) in early successional forests to slower life histories (slow growth, low mortality, low recruitment) in old-growth forests and that shifts would be stronger in wet than in dry forests due to more pronounced differences in environmental conditions between early successional and old-growth forests. We also expected that demographic diversity would increase with succession. We found that demographic strategies largely overlapped across successional stages and that early successional stages already covered the full spectrum of demographic strategies found in old-growth forests. An exception was a group of species characterized by exceptionally high mortality rates that was confined to early successional stages in the two wet forests. Demographic diversity did not increase with succession. Our results suggest that current understanding of demographic strategies of tropical tree species, which has been generated mostly from long-term forest monitoring plots in old-growth forests, is largely representative of demographic diversity in general, and that demographic diversity recovers quickly during succession.

Conserving the tree species of the world requires syntheses on which tree species are most vulnerable to pressing threats, such as climate change, invasive pests and pathogens, or selective logging. Here, we review the population and forest dynamics models that, when parameterized with data from population studies, forest inventories, or tree rings, have been used for identifying life-history strategies of species and threat-related changes in population demography and dynamics. The available evidence suggests that slow-growing and/or long-lived species are the most vulnerable. However, a lack of comparative, multi-species studies still challenges more precise predictions of the vulnerability of tree species to threats. Improving data coverage for mortality and recruitment, and accounting for interactions among threats, would greatly advance vulnerability assessments for conservation prioritizations of trees worldwide.

Tropical forest succession and associated changes in community composition are driven by species’ demographic rates, but how demographic strategies shift during succession remains unclear. To identify generalities in demographic trade-offs and successional shifts in demographic strategies, we quantified demographic rates of 787 tree species from two wet and two dry Neotropical forests. Across all forests, we found two demographic trade-offs – the growth–survival and the stature–recruitment trade-off – enabling the data-driven assignment of species to five demographic strategies. Fast species dominated early in succession and were then replaced by long-lived pioneers in three forests. Intermediate and slow species increased in basal area over succession but in contrast to the current conceptual model, long-lived pioneers continued to dominate until the old-growth stage in all forests. The basal area of short-lived breeders was low across all successional stages. These results increase the mechanistic understanding and predictability of Neotropical forest succession.

Tree diameter distributions are important indicators of forest structure, are a principal element of forest carbon stock estimates, and are the outcome of forest demography. Hump-shaped tree diameter distributions, also known as rotated-sigmoid diameter distributions, are characterized by an increased frequency of intermediately-sized trees over what would be extrapolated from the small tree size distribution and are common in temperate forests. One hypothesis to explain hump-shaped tree size distributions is the U-shaped mortality curve, where the mortality rates of the largest canopy trees are higher than those of intermediately-sized canopy trees. However, studies that have directly tested this hypothesis by comparing tree diameter distributions predicted from forest demographic models with empirical diameter distributions are lacking, and how the U-shaped mortality curve influences other aspects of forest structure such as canopy height is not well understood. We used model-data comparisons to test the hypothesis that the U-shaped mortality curve generates the hump-shaped tree diameter distribution. We used two versions of a forest demographic model (the PPA model) to predict tree diameter distributions, one that assumed that tree demographic rates varied only between understory and canopy trees, and a second that assumed tree demographic rates varied among understory trees, small canopy trees, and large canopy trees, consistent with the U-shaped mortality curve. We compared predictions from each form of the model with empirical tree diameter distributions from the Wind River Forest Dynamics plot, a 27.2 ha forest dynamics plot with a hump-shaped diameter distribution. We found that the model that accounted for the higher mortality rates of larger canopy trees generated a hump-shaped distribution, whereas the model that did not account for the higher mortality rates of larger trees was not able to generate a hump-shaped diameter distribution. To understand how the assumptions of forest demographic patterns affected other aspects of forest structure that could be derived from remote sensing, we compared predictions of forest canopy height from each form of the model with measurements of forest canopy height derived from airborne LiDAR. We found that accounting for the higher mortality rates of larger trees became even more critical to accurate prediction of forest canopy height because intermediate-sized trees occupy large areas in the canopy. Our results demonstrate the link between the U-shaped mortality curve and hump-shaped diameter distributions, and suggest that accurate characterization of the demography of large trees may be particularly important for predicting forest canopy structure and carbon stocks.

We developed a demographic vegetation model, BiomeE, to improve the modeling of vegetation dynamics and ecosystem biogeochemical cycles in the NASA Goddard Institute of Space Studies' ModelE Earth system model. This model includes the processes of plant growth, mortality, reproduction, vegetation structural dynamics, and soil carbon and nitrogen storage and transformations. The model combines the plant physiological processes of ModelE's original vegetation model, Ent, with the plant demographic and ecosystem nitrogen processes that have been represented in the Geophysical Fluid Dynamics Laboratory's LM3-PPA. We used nine plant functional types to represent global natural vegetation functional diversity, including trees, shrubs, and grasses, and a new phenology model to simulate vegetation seasonal changes with temperature and precipitation fluctuations. Competition for light and soil resources is individual based, which makes the modeling of transient compositional dynamics and vegetation succession possible. Overall, the BiomeE model simulates, with fidelity comparable to other models, the dynamics of vegetation and soil biogeochemistry, including leaf area index, vegetation structure (e.g., height, tree density, size distribution, and crown organization), and ecosystem carbon and nitrogen storage and fluxes. This model allows ModelE to simulate transient and long-term biogeophysical and biogeochemical feedbacks between the climate system and land ecosystems. Furthermore, BiomeE also allows for the eco-evolutionary modeling of community assemblage in response to past and future climate changes with its individual-based competition and demographic processes.

Ashe juniper (Juniperus ashei Buchholz) and escarpment live oak (Quercus fusiformis Small) are two of the most common woody species of the Edwards Plateau, an arid‐to‐semi‐arid region of nearly 100,000 km2 in central Texas. They have very different hydraulic strategies, yet they experienced similar mortality rates during an extreme drought in 2011. We measured J. ashei and Q. fusiformis sap flow velocities during summer dry periods of 2016 and 2017 at six micro‐sites. Although these were years with more‐or‐less average summer conditions, both included significant dry periods. To estimate the relative rate of decrease in sap velocity for each species during the dry periods, we fit a Bayesian exponential decay model to the sap velocity time series for each tree. We found that Q. fusiformis trees were better able to maintain transpiration than J. ashei during dry periods at five micro‐sites and were comparable at the sixth micro‐site. We conclude that it is likely that Q. fusiformis can better maintain transpiration during dry periods than J. ashei, on average, and that in the case of these two species, greater cavitation resistance may not translate to greater drought resistance.

Abstract Colonial organisms host a large diversity of symbionts (collectively, parasites, mutualists, and commensals) that use vertical transmission (from parent colony to offspring colony) and/or horizontal transmission to disperse between host colonies. The early life of some colonies, characterized by the dispersal and establishment of solitary individuals, may constrain vertical transmission and favor horizontal transmission between large established colonies. We explore this possibility with the miniature cockroach Attaphila fungicola, a symbiont of leaf‐cutter ants and the mutualist fungal gardens they cultivate. The early life of a leaf‐cutter colony is characterized by the dispersal of a female alate (winged “queen”) carrying a fungal pellet, and the subsequent establishment of a foundress (workerless “queen”) raising her incipient fungal garden and colony. Roaches hitchhike on female alates during leaf‐cutter nuptial flights, which strongly suggests that roaches are vertically transmitted to foundresses and their incipient colonies; however, weak compatibility between roaches and incipient gardens may constrain roach vertical transmission. Reciprocally, opportunities for horizontal transmission between large established colonies with abundant fungal gardens may weaken selection against roach‐induced harm (virulence) of incipient gardens. We use a laboratory experiment, behavioral observations, field surveys, and a transmission model to estimate the effect roaches have on the survivorship of incipient gardens and the frequency of roach vertical transmission. Contrary to traditional assumptions, our results indicate that roaches harm incipient gardens and predominantly use horizontal transmission between established leaf‐cutter colonies. Ultimately, “costs of generalism” associated with infecting disparate stages of a host's lifecycle (e.g., incipient vs. established colonies) may constrain the vertical transmission of roaches and a broad range of symbionts.

Tropical forests disappear rapidly because of deforestation, yet they have the potential to regrow naturally on abandoned lands. We analyze how 12 forest attributes recover during secondary succession and how their recovery is interrelated using 77 sites across the tropics. Tropical forests are highly resilient to low-intensity land use; after 20 years, forest attributes attain 78% (33 to 100%) of their old-growth values. Recovery to 90% of old-growth values is fastest for soil (<1 decade) and plant functioning (<2.5 decades), intermediate for structure and species diversity (2.5 to 6 decades), and slowest for biomass and species composition (>12 decades). Network analysis shows three independent clusters of attribute recovery, related to structure, species diversity, and species composition. Secondary forests should be embraced as a low-cost, natural solution for ecosystem restoration, climate change mitigation, and biodiversity conservation.

Trophic cascades - indirect effects of predators that propagate down through food webs - have been extensively documented in many ecosystem types. It has also been shown that predator diversity can mediate these trophic cascades and, separately, that herbivore biomass can influence the stability of primary producers. However, whether predator diversity can cause cascading effects on the stability of lower trophic levels has not yet been studied. We conducted a laboratory microcosm experiment and a field mesocosm experiment manipulating the presence and coexistence of two heteropteran predators and measuring their effects on zooplankton herbivores and phytoplankton basal resources. We predicted that if the predators partitioned their zooplankton prey, for example by size, then the co-presence of the predators would reduce zooplankton prey mass and lead to (1) increased biomass of, and (2) decreased temporal variability of phytoplankton basal resources. We present evidence that the predators partitioned their zooplankton prey, leading to a synergistic suppression of zooplankton. In turn, this enhanced zooplankton suppression led to only a weak, non-significant increase in the central tendency of phytoplankton biomass, but significantly reduced its variability. Our results demonstrate that predator diversity may indirectly stabilize basal resource biomass via a "diversity-stability trophic cascade," seemingly dependent on predator complementarity, even when there is no significant classic trophic cascade altering the central tendency of biomass. Therefore predator diversity, especially if correlated with diversity of prey use, could play a role in regulating ecosystem stability. This link between predator diversity and producer stability has implications for conservation and for potential biological control methods to improve crop yield reliability.

Community composition is a primary determinant of how biodiversity change influences ecosystem functioning and, therefore, the relationship between biodiversity and ecosystem functioning (BEF). We examine the consequences of community composition across six structurally realistic plant community models. We find that a positive correlation between species' functioning in monoculture versus their dominance in mixture with regard to a specific function (the "function-dominance correlation") generates a positive relationship between realised diversity and ecosystem functioning across species richness treatments. However, because realised diversity declines when few species dominate, a positive function-dominance correlation generates a negative relationship between realised diversity and ecosystem functioning within species richness treatments. Removing seed inflow strengthens the link between the function-dominance correlation and BEF relationships across species richness treatments but weakens it within them. These results suggest that changes in species' identities in a local species pool may more strongly affect ecosystem functioning than changes in species richness.

A key question in biology is the predictability of the evolutionary process. If we can correctly predict the outcome of evolution, we may be better equipped to anticipate and manage species’ adaptation to climate change, habitat loss, invasive species, or emerging infectious diseases, as well as improve our basic understanding of the history of life on Earth. In the present article, we ask the questions when, why, and if the outcome of future evolution is predictable. We first define predictable and then discuss two conflicting views: that evolution is inherently unpredictable and that evolution is predictable given the ability to collect the right data. We identify factors that generate unpredictability, the data that might be required to make predictions at some level of precision or at a specific timescale, and the intellectual and translational value of understanding when prediction is or is not possible.

Plants and vegetation play a critical—but largely unpredictable—role in global environmental changes due to the multitude of contributing processes at widely different spatial and temporal scales. In this Perspective, we explore approaches to master this complexity and improve our ability to predict vegetation dynamics by explicitly taking account of principles that constrain plant and ecosystem behaviour: natural selection, self-organization and entropy maximization. These ideas are increasingly being used in vegetation models, but we argue that their full potential has yet to be realized. We demonstrate the power of natural selection-based optimality principles to predict photosynthetic and carbon allocation responses to multiple environmental drivers, as well as how individual plasticity leads to the predictable self-organization of forest canopies. We show how models of natural selection acting on a few key traits can generate realistic plant communities and how entropy maximization can identify the most probable outcomes of community dynamics in space- and time-varying environments. Finally, we present a roadmap indicating how these principles could be combined in a new generation of models with stronger theoretical foundations and an improved capacity to predict complex vegetation responses to environmental change. Integrating natural selection and other organizing principles into next-generation vegetation models could render them more theoretically sound and useful for earth system applications and modelling climate impacts.

Understanding tropical forest dynamics and planning for their sustainable management require efficient, yet accurate, predictions of the joint dynamics of hundreds of tree species. With increasing information on tropical tree life histories, our predictive understanding is no longer limited by species data but by the ability of existing models to make use of it. Using a demographic forest model, we show that the basal area and compositional changes during forest succession in a neotropical forest can be accurately predicted by representing tropical tree diversity (hundreds of species) with only five functional groups spanning two essential trade-offs-the growth-survival and stature-recruitment trade-offs. This data-driven modeling framework substantially improves our ability to predict consequences of anthropogenic impacts on tropical forests.

The classical theory of stomatal optimization stipulates that stomata should act to maximize photosynthesis while minimizing transpiration. This theory, despite its remarkable success in reproducing empirical patterns, does not account for the fact that the available water to plants is dynamically regulated by plants themselves, and that plants compete for water in most locations. Here, we develop an alternative theory in which plants maximize the expected carbon gain under stochastic rainfall in a competitive environment. We further incorporate xylem hydraulic limitation as an additional constraint to transpiration and evaluate its impacts on stomatal optimization by incorporating the direct carbon cost of xylem recovery and the opportunity cost of reduced future photosynthesis as a result of irrecoverable xylem damage. We predict stomatal behaviour to be more conservative with a higher cost induced by xylem damage. By varying the unit carbon cost and extent of xylem recovery, characterizing the direct and opportunity cost of xylem damage, respectively, our model can reproduce several key patterns of stomatal-hydraulic trait covariations. By addressing the key elements of water limitation in plant gas exchange simultaneously, including plants' self-regulation of water availability, competition for water and hydraulic risk, our study provides a comprehensive theoretical basis for understanding stomatal behaviour.

Competition is a major driver of carbon allocation to different plant tissues (e.g., wood, leaves, fine roots), and allocation, in turn, shapes vegetation structure. To improve their modeling of the terrestrial carbon cycle, many Earth system models now incorporate vegetation demographic models (VDMs) that explicitly simulate the processes of individual-based competition for light and soil resources. Here, in order to understand how these competition processes affect predictions of the terrestrial carbon cycle, we simulate forest responses to elevated atmospheric CO2 concentration [CO2] along a nitrogen availability gradient, using a VDM that allows us to compare fixed allocation strategies vs. competitively optimal allocation strategies. Our results show that competitive and fixed strategies predict opposite fractional allocation to fine roots and wood, though they predict similar changes in total net primary production (NPP) along the nitrogen gradient. The competitively optimal allocation strategy predicts decreasing fine root and increasing wood allocation with increasing nitrogen, whereas the fixed strategy predicts the opposite. Although simulated plant biomass at equilibrium increases with nitrogen due to increases in photosynthesis for both allocation strategies, the increase in biomass with nitrogen is much steeper for competitively optimal allocation due to its increased allocation to wood. The qualitatively opposite fractional allocation to fine roots and wood of the two strategies also impacts the effects of elevated [CO2] on plant biomass. Whereas the fixed allocation strategy predicts an increase in plant biomass under elevated [CO2] that is approximately independent of nitrogen availability, competition leads to higher plant biomass response to elevated [CO2] with increasing nitrogen availability. Our results indicate that the VDMs that explicitly include the effects of competition for light and soil resources on allocation may generate significantly different ecosystem-level predictions of carbon storage than those that use fixed strategies.

The combination of individual-based selection with shared access to resources drives individuals to invest more than necessary in taking up their share of resources due to the threat of other individuals doing the same (competitive overinvestments). This evolutionary escalation of investment is common, from deer antlers and peacock feathers to tree height and plant roots. Because plant roots seem to be well intermingled belowground, the simplifying assumption that belowground resources are perfectly well mixed is often made in models—a condition that favours maximal fine-root overinvestments. Here, I develop simple models to investigate the role of space in determining the overlap among individuals belowground and resulting fine-root biomass. Without costs of growing roots through space, evolutionary optimization leads individuals to intermingle their fine roots perfectly and to invest just as much in these roots, whether there are two individuals competing or many. However, if there are any costs of sending roots through soil, investment in fine roots is constrained in amount and spatial extent. Dominant individuals are those that keep their roots in the soil closest to their own stem and the stems of their closest neighbours. These results highlight the importance of space in determining individual strategies as well as competitive networks.