Population Structure Plays a Key Role in Community Stability

The relationship between ecosystem complexity and stability remains unresolved and a mechanistic explanation for the stunning levels of biodiversity observed in communities and ecosystems is still lacking. Recent work has shown that differences in the foraging capacity and predation risk of juveniles versus adults within populations result in larger, more complex communities than predicted by unstructured models. Here, we develop a general framework to integrate population structure into community stability analyses and show that stage-asymmetric interactions are key to stability. Specifically, while cross-stage predator-prey interactions enhance stability, competition across different stages destabilises the community. Our results offer new insights into the stability-diversity paradox, emphasising the critical role of population structure in ecological resilience, an often neglected feature of natural systems.

Theoretical studies of the relationship between ecosystem complexity and stability usually conclude that systems with more species, more interspecific interactions per species (connectance), or stronger interactions are not as likely to be stable as systems with fewer of these attributes (Gardner and Ashby 1970; May 1972, 1974, 1979; DeAngelis 1975; Gilpin 1975; Pimm 1979a, 1979b, 1980). Yet, in one of the few empirical investigations of this problem, McNaughton (1977) concluded that increased complexity stabilized certain ecosystem properties; more precisely, that a large mammalian grazer changed total green plant biomass less in more diverse than in less diverse grassland plots. We try to resolve this apparent contradiction between theory and empiricism by investigating, in model grazing systems, the relationship between complexity and the lack of change in plant biomass (which we call biomass stability) following the removal of an herbivore. We established a set of structured food web models composed of one herbivore and n plant species. The models were based on the familiar Lotka-Volterra equations, and we selected their parameters over intervals designed to be biologically sensible and also to reflect the pattern of interactions in the food web. Only those models with a locally stable equilibrium involving species with positive biomasses were retained for further analysis. From each model of this subset, the herbivore was removed and the resultant change in total plant biomass followed until a new stable equilibrium was achieved. Relative biomass stability was calculated from the relative change in the total plant biomasses of the two equilibria. Clearly, a ratio near unity indicates biomass stability, while a large ratio indicates that biomass has increased considerably, following removal of the herbivore. We modeled webs of varying complexity as measured by: (1) the number of species; (2) the number of competitive interactions between plant species; and (3) species diversity (a measure combining the number of species and their relative abundances), and related these features to biomass stability. Our conclusion is that increased complexity can enhance biomass stability, even

Adaptive foragers and community ecology: linking individuals to communities and ecosystems

Dispersal abilities play a crucial role in shaping the extent of population genetic structure, with more mobile species being panmictic over large geographical ranges and less mobile ones organized in metapopulations exchanging migrants to different degrees. In turn, population structure directly influences the coalescence pattern of the sampled lineages, but the consequences on the estimated variation of the effective population size (Ne ) over time obtained by means of unstructured demographic models remain poorly understood. However, this knowledge is crucial for biologically interpreting the observed Ne trajectory and further devising conservation strategies in endangered species. Here we investigated the demographic history of four shark species (Carharhinus melanopterus, Carharhinus limbatus, Carharhinus amblyrhynchos, Galeocerdo cuvier) with different degrees of endangered status and life history traits related to dispersal distributed in the Indo-Pacific and sampled off New Caledonia. We compared several evolutionary scenarios representing both structured (metapopulation) and unstructured models and then inferred the Ne variation through time. By performing extensive coalescent simulations, we provided a general framework relating the underlying population structure and the observed Ne dynamics. On this basis, we concluded that the recent decline observed in three out of the four considered species when assuming unstructured demographic models can be explained by the presence of population structure. Furthermore, we also demonstrated the limits of the inferences based on the sole site frequency spectrum and warn that statistics based on linkage disequilibrium will be needed to exclude recent demographic events affecting meta-populations.

Unbalanced N and P input has substantially altered the relative importance of N and P limitation in grassland ecosystems, which resulted in profound impacts on species nutrient cycling, community structure, and ecosystem stability. However, the underlying species-specific nutrient use strategy and stoichiometric homeostasis in driving community structure and stability changes remain unclear. A split-plot N and P addition experiment (main-plot: 0, 25, 50, and 100 kgN hm-2 a-1; subplot: 0, 20, 40, and 80 kgP2O5 hm-2 a-1) was conducted during 2017-2019 in two typical grasslands (perennial grass and perennial forb) communities in the Loess Plateau. The stoichiometric homeostasis of 10 main component species, species dominance, stability changes, and their contribution to community stability were investigated. Perennial legume and perennial clonal species tend to perform higher stoichiometric homeostasis than non-clonal and annual forb. Large shifts in species with high homeostasis vs. low homeostasis caused by N and P addition showed consistently profound impacts on community homeostasis and stability in both communities. In both two communities, species dominance performed significantly positive relationships with homeostasis under no N and P addition. P alone or combined with 25 kgN hm-2 a-1 addition strengthened species dominance-homeostasis relationship and increased community homeostasis due to increased perennial legumes. Under 50 and 100 kgN hm-2 a-1 combined with P addition, species dominance-homeostasis relationships were weakened, and community homeostasis decreased significantly in both communities, which was due to that increased annual and non-clonal forb suppressed perennial legume and clonal species. Our results demonstrated that trait-based classifications of species-level homeostasis offer a reliable tool in predicting species performance and community stability under N and P addition, and conserving species with high homeostasis is important to enhance semiarid grassland ecosystem function stability on the Loess Plateau.

The relationship between species diversity and ecosystem functioning has been debated for decades, especially in relation to the "macroscopic" realm (higher plants and metazoans). Although there is emerging consensus that diversity enhances productivity and stability in communities of higher organisms; however, we still do not know whether these relationships apply also for communities of unicellular organisms, such as phytoplankton, which contribute approximately 50% to the global primary production. We show here that phytoplankton resource use, and thus carbon fixation, is directly linked to the diversity of phytoplankton communities. Datasets from freshwater and brackish habitats show that diversity is the best predictor for resource use efficiency of phytoplankton communities across considerable environmental gradients. Furthermore, we show that the diversity requirement for stable ecosystem functioning scales with the nutrient level (total phosphorus), as evidenced by the opposing effects of diversity (negative) and resource level (positive) on the variability of both resource use and community composition. Our analyses of large-scale observational data are consistent with experimental and model studies demonstrating causal effects of microbial diversity on functional properties at the system level. Our findings point at potential linkages between eutrophication and pollution-mediated loss of phytoplankton diversity. Factors reducing phytoplankton diversity may have direct detrimental effects on the amount and predictability of aquatic primary production.

To begin identifying what behavioral details might be needed to characterize community dynamics and stability, we examined the effect of prey behavioral responses to predation risk on community dynamics and stability. We considered the case of prey altering their foraging effort to trade off energy gain and predation risk. We used state-dependent dynamic optimization to calculate the optimal trade-off for four models of prey behaviorally responding to predation risk. We consider a fixed behavior model in which prey use constant levels of foraging effort and three flexible behavior models in which prey change their foraging effort according to their physiological state and their perceived level of predation risk. Flexible behavior was destabilizing at the community level as evidenced by higher predator-prey oscillations and lower community persistence times. The mechanisms by which prey estimated predation risk also affected community stability. We found that community dynamics resulting from prey with flexible behavior and fixed perception of risk approximated community dynamics resulting from prey with flexible behavior and perfect information about predation risk, however neither approximated the community dynamics resulting from prey with flexible behavior and flexible perception of risk. Thus, whether it might be possible to abstract complex behavior with simpler rules when modeling community dynamics depends on the prey's behavioral mechanisms, which are empirically poorly known.

The impact of nitrogen enrichment on grassland ecosystem stability depends on nitrogen addition level

Prey responses to predation risk can affect multiple phenotypic traits that have significant implications for population‐to‐ecosystem‐level properties. The general stress paradigm (hereafterGSP) proposes integrating the principles of ecological stoichiometry and prey physiological plasticity in response to predation risk to provide an explicit mechanistic framework to evaluate the direct influence of predation risk on ecosystem functioning.However, theoretical attempts evaluating theGSPhave focused on only one component of phenotypic plasticity, neglecting the fact that multiple phenotypic responses can affect nutrient demand and consumption, and therefore, the overall animal effect on nutrient cycling rates and stoichiometry.In this study, we used a dynamic state variable model to examine how variation in predation and starvation risk on a landscape of patch types that vary in food quality affects the behavioural and physiological phenotypic responses of prey and the linked nutrient and carbon recycling.We found that the stress imposed by predation risk promotes prey carbon limitation, which leads to increased nutrient release and carbon uptake due to a stoichiometric mismatch between an animal's demand and food stoichiometry. Such effects can be altered by prey shifting spatially to carbon‐ or nutrient‐rich patches. However, the abilities of prey to cope with new physiological demands can be affected by absolute stress level or the predation risk distribution on a spatially implicit landscape of stoichiometric‐divergent patch type availability. Overall, starvation risk can act as a selective force favouring individuals that maximize their growth, thus reducing their nutrient release, but increasing their exposure to predation risk.Our results provide an important connection between predator‐induced stress ecology and ecosystem functioning, linking organism nutrition and ecological stoichiometry to fitness outcomes, thus improving our understanding of how non‐consumptive predator effects can scale up to ecosystems.Aplain language summaryis available for this article.

Atmospheric nitrogen (N) deposition includes inorganic N (IN) and organic N (ON). IN enrichment trends to reduce species richness greater than ON, likely lowering ecosystem stability, as species richness and ecosystem stability are usually positively related. However, previous field experiments evaluating N deposition effects on ecosystem stability used either IN or ON additions, likely biasing results. We assessed the effects of IN:ON ratios (0:10, 3:7, 5:5, 7:3, and 10:0) at 10 g N m-2 yr-1 on the temporal stability of plant community productivity in a temperate meadow grassland using 6-year (2017-2022) data from a long-term N addition experiment established in 2014. Species richness, species asynchrony, population stability, and dominant species stability were investigated to explore mechanisms underlying community stability changes. We found that IN:ON ratio showed no significant effect on community stability, although all N addition significantly reduced community stability (averaged 26.7% reduction). However, IN decreased species richness more than ON (54.1% reduction in 10:0 versus 31.8% reduction in 0:10). IN:ON ratio showed no significant effect on species asynchrony, population stability or dominant species stability. Species asynchrony and dominant species stability were both positively related to community stability, while population stability showed no significant association. It implies that species asynchrony and dominant species stability maintained community stability across IN:ON ratios. Overall, our findings suggest that, despite IN reducing species richness greater than ON, it may be reasonable to assess N deposition effects on ecosystem stability using either IN or ON addition.

Organisms with complex life cycles commonly exhibit adaptive plasticity in the timing of transitions between life stages. While the threat of predation is predicted to induce earlier transitions, empirical support has been equivocal. When predation risk affects both the propensity to transition to the next life stage and the ability to reach the energetic thresholds necessary to complete the transition, only those individuals in the best physiological condition may be able to accelerate development and emerge earlier. To test this hypothesis, we followed uniquely marked dragonfly larvae (Pachydiplax longipennis) through emergence in pools where we factorially manipulated the presence of a large heterospecific predator (Anax junius) and cannibalism risk via conspecific size variation. Consistent with our hypothesis, high-condition larvae were more likely to emerge in the presence of the heterospecific predator than in its absence, and low-condition larvae were more likely to emerge in its absence than in its presence. Moreover, high-condition larvae emerged earlier when cannibalism risk was high than when it was low. Predation risk therefore has condition-dependent effects on emergence. As predation risk frequently affects resource accumulation, similar mechanisms across taxacould commonly underlie the incongruence between empirical results and theoretical expectations for predator-induced life-history variation.

Multiple prey traits, multiple predators: keys to understanding complex community dynamics

Prey organisms are confronted with time and resource allocation trade-offs. Time allocation trade-offs partition time, for example, between foraging effort to acquire resources and behavioral defense. Resource allocation trade-offs partition the acquired resources between multiple traits, such as growth or morphological defense. We develop a mathematical model for prey organisms that comprise time and resource allocation trade-offs for multiple defense traits. Fitness is determined by growth and survival during ontogeny. We determine optimal defense strategies for environments that differ in their resource abundance, predation risk, and defense effectiveness. We compare the results with results of simplified models where single defense traits are optimized. Our results indicate that selection acts in favor of integrated traits. The selective advantage of expressing multiple defense traits is most pronounced at intermediate environmental conditions. Optimizing single traits generally leads to a more pronounced response of the defense traits, which implies that studying single traits leads to an overestimation of their response to predation. Behavioral defense and morphological defense compensate for and augment each other depending on predator densities and the effectiveness of the defense mechanisms. In the presence of time constraints, the model shows peak investment into morphological and behavioral defense at intermediate resource levels.

Fertilization and grazing are two common anthropogenic disturbances that can lead to unprecedented changes in biodiversity and ecological stability of grassland ecosystems. A few studies, however, have explored the effects of fertilization and grazing on community stability and the underlying mechanisms. We conducted a six-year field experiment to assess the influence of nitrogen (N) fertilization and grazing on the community stability in a long-term enclosure and grazing grassland ecosystems on the Loess Plateau. A structural equation modeling method was used to evaluate how fertilization and grazing altered community stability. Our results indicated that the community stability decreased in the enclosure and grazing grassland ecosystems with the addition of N. The community stability began to decline significantly at 4.68 and 9.36 N g m−2 year−1 for the grazing and enclosure grassland ecosystems, respectively. We also found that the addition of N reduced the community stability through decreasing species richness, but a long-term enclosure can alleviate its negative effect. Overall, species diversity can be a useful predictor of the stability of ecosystems confronted with disturbances. Also, our results showed that long-term enclosure was an effective grassland management practice to ensure community stability on the Loess Plateau of China.

Effects of plant diversity on primary productivity and community stability along soil water and salinity gradients