On classifying interactions between populations

Summary The classification of interspecific interactions can have an important impact on ecologists' world views. Previous classifications have often been incomplete, have suffered from ambiguously defined categories, and/or have wrongly equated categories of population level effects with particular mechanisms of interaction. I use several simple mathematical models to argue that effects on short-term population growth rate, long term population size, and short term relative fitness of interactants may differ qualitatively. Equating all (--) effects with competition and all (+-) effects with predation may have caused ecologists to ignore a variety of potentially important interaction mechanisms. Failure to define the type of effect used in classifying interactions has led to confusion about the nature of interactions; several controversies regarding competition have apparently been caused or exaccerbated by problems with definition or clasification. In applying classification schemes, ecologists should realize that the classification of an interaction between two populations may change with the sizes of those populations or of other populations with which they interact.     

Scaling the consequences of interactions between invaders from the individual to the population level

The impact of human‐induced stressors, such as invasive species, is often measured at the organismal level, but is much less commonly scaled up to the population level. Interactions with invasive species represent an increasingly common source of stressor in many habitats. However, due to the increasing abundance of invasive species around the globe, invasive species now commonly cause stresses not only for native species in invaded areas, but also for other invasive species. I examine the European green crab Carcinus maenas, an invasive species along the northeast coast of North America, which is known to be negatively impacted in this invaded region by interactions with the invasive Asian shore crab Hemigrapsus sanguineus. Asian shore crabs are known to negatively impact green crabs via two mechanisms: by directly preying on green crab juveniles and by indirectly reducing green crab fecundity via interference (and potentially exploitative) competition that alters green crab diets. I used life‐table analyses to scale these two mechanistic stressors up to the population level in order to examine their relative impacts on green crab populations. I demonstrate that lost fecundity has larger impacts on per capita population growth rates, but that both predation and lost fecundity are capable of reducing population growth sufficiently to produce the declines in green crab populations that have been observed in areas where these two species overlap. By scaling up the impacts of one invader on a second invader, I have demonstrated that multiple documented interactions between these species are capable of having population‐level impacts and that both may be contributing to the decline of European green crabs in their invaded range on the east coast of North America.     

Non-protein amino acids: plant, soil and ecosystem interactions

Non-protein amino acids are a significant store of organic nitrogen in many ecosystems, but there is a lack of knowledge relating to them. Research has indicated that they play important roles as metabolites, as allelopthic chemicals, in nutrient acquisition, in signalling and in stress response. They are also thought to be responsible for significant medical issues in both invertebrate and vertebrate animals. This review attempts to appraise the literature related to non-protein amino acids, both in terms of their metabolism, plant?Csoil interactions and at the level of the ecosystem, where they are seen as significant drivers of structure and function. Finally, important areas for future research are discussed.     

Warming weakens facilitative interactions between decomposers and detritivores,and modifies freshwater ecosystem functioning

Warming is among the major drivers of changes in biotic interactions and, in turn, ecosystem functioning. The decomposition process occurs in a chain of facilitative interactions between detritivores and microorganisms. It remains unclear, however, what effect warming may have on the interrelations between detritivores and microorganisms, and the consequences for the functioning of natural freshwater ecosystems. To address these gaps, we performed a field experiment using tank bromeliads and their associated aquatic fauna. We manipulated the presence of bacteria and detritivorous macroinvertebrates (control, “bacteria,” and “bacteria + macroinvertebrates”) under ambient and warming scenarios, and analyzed the effects on the microorganisms and ecosystem functioning (detritus mass loss, colored dissolved organic matter, and nitrogen flux). We applied antibiotic solution to eliminate or reduce bacteria from control bromeliads. After 60 days incubation, bacterial density was higher in the presence than in the absence of macroinvertebrates. In the absence of macroinvertebrates, temperature did not influence bacterial density. However, in the presence of macroinvertebrates, bacterial density decreased by 54% with warming. The magnitude of the effects of organisms on ecosystem functioning was higher in the combined presence of bacteria and macroinvertebrates. However, warming reduced the overall positive effects of detritivores on bacterial density, which in turn, cascaded down to ecosystem functioning by decreasing decomposition and nitrogen flux. These results show the existence of facilitative mechanisms between bacteria and detritivores in the decomposition process, which might collapse due to warming. Detritivores seem to contribute to nutrient cycling as they facilitate bacterial populations, probably by increasing nutrient input (feces) in the ecosystem. However, increased temperature mitigated these beneficial effects. Our results add to a growing research body that shows that warming can affect the structure of aquatic communities, and highlight the importance of considering the interactive effects between facilitation and climatic drivers on the functioning of freshwater ecosystems.     

From individual interactions to population dynamics: individual resource partitioning simulation exposes the causes of nonlinear intra-specific competition

Intra-specific competition defines the relationship between population density and the performance of individual organisms (R-function). Observation of this relationship in nature shows it to be frequently nonlinear, and it has been argued, on intuitive grounds, that this nonlinearity is due to the type of competition (scramble or contest) being expressed. Here, we use an individual-based simulation model to investigate the effects of three resource partitioning schemes, representing different types of competition, on the form of the R-function. Results indicate that all resource partitioning schemes can give rise to concave or convex functions depending on the balance between maximum individual birth rate, maintenance cost, and demand for resources. Given high growth rates and maintenance costs, contest competitors tend to exhibit less concavity than scramblers. Therefore, population stability can be strongly affected by the strategy of resource partitioning. Life histories and environmental conditions that encourage the homogeneous distribution of resources among individuals lead to complex and unstable dynamics. Stable dynamics is fostered by heterogeneous resource distribution, which could result from such things as social hierarchies, individual and environmental variability, and large, indivisible resource packets.     

Genetic structure of an endangered raptor at individual and population levels

The Finnish population of White-tailed Eagle (Haliaeetus albicilla) has gone through two major demographic bottlenecks during the last two centuries. Strong conservation measures have allowed the population to recover, but despite the rapid population growth during recent years the species is still classified as endangered. We studied the genetic population structure at both individual and population levels in an attempt to recognize the processes shaping it. We used 9 microsatellite loci and 473 base pair fragment of the mitochondrial DNA control region on samples collected between the years 2003 and 2007 (N = 489). We found a clear isolation by distance pattern at fine scale (i.e. individual level) which is most likely a result of species’ philopatric behaviour. Although we did not find signs of the recent bottlenecks, we did find evidence of an ancient bottleneck that has occurred most likely over 21,000 years ago, long before the genetic divergence of the two present Finnish subpopulations (one along the Baltic Sea coast line and another in Lapland and easternmost Finland). We conclude that the present population structure is mainly a consequence of the species philopatric behaviour over a long time period instead of recent population bottlenecks. Based on our results, the Finnish population seems to have ongoing immigration from neighbouring populations. Hence, even though the population has recovered mainly through local growth, our results suggest that gene flow from genetically differentiated populations have had an impact as well.     

Trophic interactions and the relationship between species diversity and ecosystem stability

Several theoretical studies propose that biodiversity buffers ecosystem functioning against environmental fluctuations, but virtually all of these studies concern a single trophic level, the primary producers. Changes in biodiversity also affect ecosystem processes through trophic interactions. Therefore, it is important to understand how trophic interactions affect the relationship between biodiversity and the stability of ecosystem processes. Here we present two models to investigate this issue in ecosystems with two trophic levels. The first is an analytically tractable symmetrical plant-herbivore model under random environmental fluctuations, while the second is a mechanistic ecosystem model under periodic environmental fluctuations. Our analysis shows that when diversity affects net species interaction strength, species interactions--both competition among plants and plant-herbivore interactions--have a strong impact on the relationships between diversity and the temporal variability of total biomass of the various trophic levels. More intense plant competition leads to a stronger decrease or a lower increase in variability of total plant biomass, but plant-herbivore interactions always have a destabilizing effect on total plant biomass. Despite the complexity generated by trophic interactions, biodiversity should still act as biological insurance for ecosystem processes, except when mean trophic interaction strength increases strongly with diversity.     

Climate change, biotic interactions and ecosystem services

Climate change is real. The wrangling debates are over, and we now need to move onto a predictive ecology that will allow managers of landscapes and policy makers to adapt to the likely changes in biodiversity over the coming decades. There is ample evidence that ecological responses are already occurring at the individual species (population) level. The challenge is how to synthesize the growing list of such observations with a coherent body of theory that will enable us to predict where and when changes will occur, what the consequences might be for the conservation and sustainable use of biodiversity and what we might do practically in order to maintain those systems in as good condition as possible. It is thus necessary to investigate the effects of climate change at the ecosystem level and to consider novel emergent ecosystems composed of new species assemblages arising from differential rates of range shifts of species. Here, we present current knowledge on the effects of climate change on biotic interactions and ecosystem services supply, and summarize the papers included in this volume. We discuss how resilient ecosystems are in the face of the multiple components that characterize climate change, and suggest which current ecological theories may be used as a starting point to predict ecosystem-level effects of climate change.     

Behavioural interactions between ecosystem engineers control community species richness

Behavioural interactions between ecosystem engineers may strongly influence community structure. We tested whether an invasive ecosystem engineer, the alga Caulerpa taxifolia , indirectly facilitated community diversity by modifying the behaviour of a native ecosystem engineer, the clam Anadara trapezia , in southeastern Australia . In this study, clams in Caulerpa -invaded sediments partially unburied themselves, extending >30% of their shell surface above the sediment, providing rare, hard substrata for colonization. Consequently, clams in Caulerpa had significantly higher diversity and abundance of epibiota compared with clams in unvegetated sediments. To isolate the role of clam burial depth from direct habitat influences or differential predation by habitat, we manipulated clam burial depth, predator exposure and habitat ( Caulerpa or unvegetated) in an orthogonal experiment. Burial depth overwhelmingly influenced epibiont species richness and abundance, resulting in a behaviourally mediated facilitation cascade. That Caulerpa controls epibiont communities by altering Anadara burial depths illustrates that even subtle behavioural responses of one ecosystem engineer to another can drive extensive community-wide facilitation.     

Aggregation and emergence in hierarchically organized systems: population dynamics

The aim of this work is to present aggregation methods of hierarchically organized systems allowing one to replace the initial micro-system by a macro-system described by a few global variables. We also study the relations between the fast micro-dynamics and the slow macro-dynamics which can produce global properties. Emergence corresponds to a bottom-up coupling that is the result effected by a micro-level at a macro-level. As an example, we present prey-predator models with different time scales in an heterogeneous environment. A fast time scale is associated to the migration process on spatial patches and a slow time scale is associated to growth and interactions between the populations. Preys must go on spatial patches where resources are located and where predators can attack them. The efficiency of the predators to catch preys is patch dependent. Perturbation methods allow us to aggregate the initial system of differential equations for the patch sub-populations into a macro-system of two differential equations governing the total population densities. We study the case of density independent and density dependent migrations. In the latter case, we show that different functional responses can emerge in the macro prey-predator model as a result of the coupling between the slow and fast systems.     

Variation in nectar volume and composition of Impatiens capensis at the individual,plant, and population levels

Although the volume and chemical composition of nectars are known to vary among plant species and to affect pollinator response to plants, relatively little is known of the variation in volume, and sugar and amino acid composition within species. We collected nectar from Impatiens capensis in a nested design: three flowers from each of three plants from each of three populations. This design enabled us to quantify variation within individual plants, among plants within populations, and among populations. Using high performance liquid chromatography, we analyzed the sugar and amino composition of the 27 flowers. Analysis of variance showed that none of the parameters (volume, concentrations of three sugars and 24 amino compounds) varied within individuals. Variation in nectar volume was not significant among plants but was nearly significant among populations. Of the three sugars detected (sucrose, glucose, and fructose), the only significant variation was that of sucrose among populations. Concentrations of 12 amino compounds varied significantly at the plant level while 7 amino compounds varied among populations. The results indicate that: (1) pooling of nectar samples from flowers of individual plants can be an acceptable methodology for those seeking to understand within species variation; (2) amino compounds appear to vary more than either volumes or sugar concentrations; (3) future studies should assess how much of the observed variation is due to genetic versus environmental differences; (4) additional studies should examine the geographic variation in nectar parameters and pollinators of I. capensis in order to assess the role different pollinators play in shaping nectar composition.     

Nontrophic interactions, biodiversity, and ecosystem functioning: an interaction web model

Research into the relationship between biodiversity and ecosystem functioning has mainly focused on the effects of species diversity on ecosystem properties in plant communities and, more recently, in food webs. Although there is growing recognition of the significance of nontrophic interactions in ecology, these interactions are still poorly studied theoretically, and their impact on biodiversity and ecosystem functioning is largely unknown. Existing models of mutualism usually consider only one type of species interaction and do not satisfy mass balance constraints. Here, we present a model of an interaction web that includes both trophic and nontrophic interactions and that respects the principle of mass conservation. Nontrophic interactions are represented in the form of interaction modifications. We use this model to study the relationship between biodiversity and ecosystem properties that emerges from the assembly of entire interaction webs. We show that ecosystem properties such as biomass and production depend not only on species diversity but also on species interactions, in particular on the connectance and magnitude of nontrophic interactions, and that the nature, prevalence, and strength of species interactions in turn depend on species diversity. Nontrophic interactions alter the shape of the relationship between biodiversity and biomass and can profoundly influence ecosystem processes.     

Inferring infection hazard in wildlife populations by linking data across individual and population scales

Our ability to infer unobservable disease‐dynamic processes such as force of infection (infection hazard for susceptible hosts) has transformed our understanding of disease transmission mechanisms and capacity to predict disease dynamics. Conventional methods for inferring FOI estimate a time‐averaged value and are based on population‐level processes. Because many pathogens exhibit epidemic cycling and FOI is the result of processes acting across the scales of individuals and populations, a flexible framework that extends to epidemic dynamics and links within‐host processes to FOI is needed. Specifically, within‐host antibody kinetics in wildlife hosts can be short‐lived and produce patterns that are repeatable across individuals, suggesting individual‐level antibody concentrations could be used to infer time since infection and hence FOI. Using simulations and case studies (influenza A in lesser snow geese and Yersinia pestis in coyotes), we argue that with careful experimental and surveillance design, the population‐level FOI signal can be recovered from individual‐level antibody kinetics, despite substantial individual‐level variation. In addition to improving inference, the cross‐scale quantitative antibody approach we describe can reveal insights into drivers of individual‐based variation in disease response, and the role of poorly understood processes such as secondary infections, in population‐level dynamics of disease.     

Microbial diversity, producer-decomposer interactions and ecosystem processes: a theoretical model

Interactions between the diversity of primary producers and that of decomposers--the two key functional groups that form the basis of all ecosystems--might have major consequences on the functioning of depauperate ecosystems. I present a simple ecosystem model in which primary producers (plants) and decomposers (microbes) are linked through material cycling. The model considers a diversity of plant organic compounds and a diversity of microbial species. Nutrient recycling efficiency from organic compounds to decomposers is then the key parameter that controls ecosystem processes (primary productivity, secondary productivity, producer biomass and decomposer biomass). The model predicts that microbial diversity has a positive effect on nutrient recycling efficiency and ecosystem processes through either greater intensity of microbial exploitation of organic compounds or functional niche complementarity, much like in plants. Microbial niche breadth and overlap should not affect ecosystem processes unless they increase the number of organic compounds that are decomposed. In contrast, the model predicts that plant organic compound diversity can only have a negative effect or, at best, no effect on ecosystem processes, at least in a constant environment. This creates a tension between the effects of plant diversity and microbial diversity on ecosystem functioning, which may explain some recent experimental results.