Temperature as an Ecological Resource

Ectothermic vertebrates respond to the temperature of theirhabitat in a manner that is remarkably similar to their responseto more traditional ecological resources such as food. We reviewthe response to temperature primarily from literature on fishesin terms of ecological concepts related to niche theory andcompetition. The width of the fundamental thermal niche is about4°C when measured by a mean plus and minus one standarddeviation of the distribution of temperature occupied in a laboratorygradient. Fish of temperate freshwater appear to fall into threethermal guilds along the temperature resource axis —cold,cool, and warm water fishes. Realized thermal niches are similarin central tendency to fundamental niches, but niche width appearsto be more narrow for the realized niche in limited sample data.The success of interference competition for space with preferredtemperature is tied to social dominance in a manner analogousto food competition. Thermal niche shifts in the face of interspecificcompetition for preferred temperature appear supported by onelaboratory study. Exploitation competition in respect to temperatureseems nebulous. If animals successfully compete for their thermalniche, growth and perhaps other measures of fitness are maximized.Cost/benefit models for thermal resources and food resourceslead to similar predictions about resource use. We suggest thatviewing temperature and other niche axes in the way ecologistshave viewed food resources would be useful.     

On the relationship between niche and distribution

Applications of Hutchinson's n -dimensional niche concept are often focused on the role of interspecific competition in shaping species distribution patterns. In this paper, I discuss a variety of factors, in addition to competition, that influence the observed relationship between species distribution and the availability of suitable habitat. In particular, I show that Hutchinson's niche concept can be modified to incorporate the influences of niche width, habitat availability and dispersal, as well as interspecific competition per se . I introduce a simulation model called NICHE that embodies many of Hutchinson's original niche concepts and use this model to predict patterns of species distribution. The model may help to clarify how dispersal, niche size and competition interact, and under what conditions species might be common in unsuitable habitat or absent from suitable habitat. A brief review of the pertinent literature suggests that species are often absent from suitable habitat and present in unsuitable habitat, in ways predicted by theory. However, most tests of niche theory are hampered by inadequate consideration of what does and does not constitute suitable habitat. More conclusive evidence for these predictions will require rigorous determination of habitat suitability under field conditions. I suggest that to do this, ecologists must measure habitat specific demography and quantify how demographic parameters vary in response to temporal and spatial variation in measurable niche dimensions.     

Does foraging adaptation create the positive complexity-stability relationship in realistic food-web structure?

The adaptive food-web hypothesis suggests that an adaptive foraging switch inverses the classically negative complexity-stability relationships of food webs into positive ones, providing a possible resolution for the long-standing paradox of how populations persist in a complex natural food web. However, its applicability to natural ecosystems has been questioned, because the positive relationship does not emerge when a niche model, a realistic "benchmark" of food-web models, is used. I hypothesize that, in the niche model, increasing connectance influences the fraction of basal species to destabilize the system and this masks the inversion of the negative complexity-stability relationship in the presence of adaptive foraging. A model analysis shows that, if this confounding effect is eliminated, then, even in a niche model, a population is more likely to persist in a more complex food web. This result supports the robustness of adaptive food-web hypothesis and reveals the condition in which the hypothesis should be tested.     

Niche differentiation in rainforest ant communities across three continents

A central prediction of niche theory is that biotic communities are structured by niche differentiation arising from competition. To date, there have been numerous studies of niche differentiation in local ant communities, but little attention has been given to the macroecology of niche differentiation, including the extent to which particular biomes show distinctive patterns of niche structure across their global ranges. We investigated patterns of niche differentiation and competition in ant communities in tropical rainforests, using different baits reflecting the natural food spectrum. We examined the extent of temporal and dietary niche differentiation and spatial segregation of ant communities at five rainforest sites in the neotropics, paleotropics, and tropical Australia. Despite high niche overlap, we found significant dietary and temporal niche differentiation in every site. However, there was no spatial segregation among foraging ants at the community level, despite strong competition for preferred food resources. Although sucrose, melezitose, and dead insects attracted most ants, some species preferentially foraged on seeds, living insects, or bird feces. Moreover, most sites harbored more diurnal than nocturnal species. Overall niche differentiation was strongest in the least diverse site, possibly due to its lower number of rare species. Both temporal and dietary differentiation thus had strong effects on the ant assemblages, but their relative importance varied markedly among sites. Our analyses show that patterns of niche differentiation in ant communities are highly idiosyncratic even within a biome, such that a mechanistic understanding of the drivers of niche structure in ant communities remains elusive.     

Applying stable isotopes to examine food‐web structure: an overview of analytical tools

Stable isotope analysis has emerged as one of the primary means for examining the structure and dynamics of food webs, and numerous analytical approaches are now commonly used in the field. Techniques range from simple, qualitative inferences based on the isotopic niche, to Bayesian mixing models that can be used to characterize food‐web structure at multiple hierarchical levels. We provide a comprehensive review of these techniques, and thus a single reference source to help identify the most useful approaches to apply to a given data set. We structure the review around four general questions: (1) what is the trophic position of an organism in a food web?; (2) which resource pools support consumers?; (3) what additional information does relative position of consumers in isotopic space reveal about food‐web structure?; and (4) what is the degree of trophic variability at the intrapopulation level? For each general question, we detail different approaches that have been applied, discussing the strengths and weaknesses of each. We conclude with a set of suggestions that transcend individual analytical approaches, and provide guidance for future applications in the field.     

A thermodynamic perspective on food webs: quantifying entropy production within detrital-based ecosystems

Because ecosystems fit so nicely the framework of a "dissipative system", a better integration of thermodynamic and ecological perspectives could benefit the quantitative analysis of ecosystems. One obstacle is that traditional food web models are solely based upon the principles of mass and energy conservation, while the theory of non-equilibrium thermodynamics principally focuses on the concept of entropy. To properly cast classical food web models within a thermodynamic framework, one requires a proper quantification of the entropy production that accompanies resource processing of the food web. Here we present such a procedure, which emphasizes a rigorous definition of thermodynamic concepts (e.g. thermodynamic gradient, disequilibrium distance, entropy production, physical environment) and their correct translation into ecological terms. Our analysis provides a generic way to assess the thermodynamic operation of a food web: all information on resource processing is condensed into a single resource processing constant. By varying this constant, one can investigate the range of possible food web behavior within a given fixed physical environment. To illustrate the concepts and methods, we apply our analysis to a very simple example ecosystem: the detrital-based food web of marine sediments. We examine whether entropy production maximization has any ecological relevance in terms of food web functioning.     

A study of trophic niche partitioning between larval populations of reproductively isolated whitefish (Coregonus sp.) ecotypes

The abundance, spatio-temporal distribution, and feeding of larvae were compared from two reproductively isolated dwarf and normal ecotypes of whitefish ( Coregonus sp.) to test the hypothesis that larval ecotypes should reduce competition by using different resources. Contrary to a priori expectations, trophic niche partitioning between larval populations was much less pronounced than previously reported for the adult stages of fish ecotypes, presumably due to the lack of competition acting at this stage, and related to non-limiting food resources. Because this study was conducted in a single year and a single lake, we cannot however, strictly rule out the possibility that resource-based competition and resulting niche partitioning may occur at the larval stage in these species complexes. Nevertheless, this suggests for the first time, that resource-based selection at the larval stage may be relatively unimportant compared to that occurring at older life-history stages in driving the divergence and the development of reproductive isolation in sympatric fish ecotypes.     

Current food web models cannot explain the overall topological structure of observed food webs

Topological food webs illustrating “who eats whom” in different systems exhibit similar, non‐random, structures suggesting that general rules govern food web structure. Current food web models correctly predict many measures of food web topology from knowledge of species richness and connectance (fraction of possible predator–prey links that actually occur), together with assumptions about the ecological rules governing “who eats whom”. However, current measures are relatively insensitive to small changes in topology. Improvement of, and discrimination among, current models requires development of new measures of food web structure. Here I examine whether current food web models (cascade, niche, and nested hierarchy models, plus a random null model) can predict a new measure of food web structure, structural stability. Structural stability complements other measures of food web topology because it is sensitive to changes in topology that other measures often miss. The cascade and null models respectively over‐ and underpredict structural stability for a set of 17 high‐quality food webs. While the niche and nested hierarchy models provide unbiased predictions on average, their 95% confidence intervals frequently fail to include the observed data. Observed structural stabilities for all models are overdispersed compared to model predictions, and predicted and observed structural stabilities are uncorrelated, indicating that important sources of variation in structural stability are not captured by the models. Crucially, poor model performance arises because observed variation in structural stability is unrelated to variation in species richness and connectance. In contrast, almost all other measures of food web topology vary with species richness and connectance in natural webs. No model that takes species richness and connectance as the only input parameters can reproduce observed variation in structural stability. Further progress in predicting and explaining food web topology will require fundamentally new models based on different input parameters.     

Pitch the niche – taking responsibility for the concepts we use in ecology and species distribution modelling

In a discussion it is often easier to staunchly reject or offer resolute support for an idea. This third paper on the niche concept aims to develop a balanced argument by exploring general principles for determining an appropriate level for pitching the niche concept that will guide better use and less abuse of niche concepts. To do this we first have to accept that niche concepts are not necessarily essential for ecology. Rather than to improve niche concepts, our aim should then be to pitch the niche in terms of ecology. This aim helps us develop an ‘ultimate goal of the niche’ by which we can evaluate the concepts we use. For species distribution modelling, there has been a focus on the niche as an equilibrium outcome that perhaps has less relevance for disequilibrium situations (e.g. climate change projections). As is the case for much of ecology, more causal explanations of species' distributions use alternative terminologies and less frequently use the word ‘niche’. We suggest that niche concepts that are better aligned with the rest of ecology could arise from taking more responsibility for our own implementations, and by explaining our models with terms other than niche. A general, holistic niche concept promotes this view and promotes practical thinking about what we are modelling and how we interpret those models, which in turn should help inspire and support innovative modelling approaches in species distribution modelling.     

Cryptophytes: An emerging algal group in the rapidly changing Antarctic Peninsula marine environments

The western Antarctic Peninsula (WAP) is a climatically sensitive region where foundational changes at the basis of the food web have been recorded; cryptophytes are gradually outgrowing diatoms together with a decreased size spectrum of the phytoplankton community. Based on a 11-year (2008–2018) in-situ dataset, we demonstrate a strong coupling between biomass accumulation of cryptophytes, summer upper ocean stability, and the mixed layer depth. Our results shed light on the environmental conditions favoring the cryptophyte success in coastal regions of the WAP, especially during situations of shallower mixed layers associated with lower diatom biomass, which evidences a clear competition or niche segregation between diatoms and cryptophytes. We also unravel the cryptophyte photo-physiological niche by exploring its capacity to thrive under high light stress normally found in confined stratified upper layers. Such conditions are becoming more frequent in the Antarctic coastal waters and will likely have significant future implications at various levels of the marine food web. The competitive advantage of cryptophytes in environments with significant light level fluctuations was supported by laboratory experiments that revealed a high flexibility of cryptophytes to grow in different light conditions driven by a fast photo-regulating response. All tested physiological parameters support the hypothesis that cryptophytes are highly flexible regarding their growing light conditions and extremely efficient in rapidly photo-regulating changes to environmental light levels. This plasticity would give them a competitive advantage in exploiting an ecological niche where light levels fluctuate quickly. These findings provide new insights on niche separation between diatoms and cryptophytes, which is vital for a thorough understanding of the WAP marine ecosystem.     

Ontogenetic niche shift, food-web coupling, and alternative stable states

Several recent studies have shown that food web coupling by ontogenetic niche shifts can generate alternative stable states (ASS). However, these studies mainly considered cases where juvenile and adult stages are the top level consumers. The conditions under which ASS occur in more structurally diverse food web configurations have not been explored. In this study, I examine the influence of food-chain length and the trophic positions of juveniles and adults on the existence of ASS. Comprehensive model analysis showed that if both juveniles and adults are top predators, ASS are possible irrespective of their trophic level, because of overcompensation in reproduction and maturation due to strong density dependence, as previously predicted. However, the following potential food-web effects were found: ASS potential (1) disappears if either or both the juveniles and adults have a predator and (2) is once again observed if another predator is added on the stage-specific predator. These mechanisms involve (1) top–down control that relaxes intrastage food competition and (2) top–down cascade that intensifies the intrastage competition, respectively. Furthermore, it was illustrated that the environmental conditions under which ASS occurred varied in complex ways with the coupled food-web configurations. My results provide a novel concept that anthropogenic changes in local community structure (e.g., species extinction and invasion) propagate through space and may cause or prevent regime shifts in broad-scale community structure by altering the resilience to environmental perturbations.     

Trophic niche width,offspring condition and immunity in a raptor species

Strategies developed by organisms to maximize foraging efficiency have a strong influence on fitness. The way in which the range of food resources is exploited has served to classify species, populations and individuals from more specialist (narrow trophic niche) to more generalist (broad trophic niche). Recent studies have provided evidence that many of the considered generalist species/populations are actually composed of different specialist individuals (individual specialization). Even the existence of generalism as an adaptive strategy has been questioned. In this study, we investigated the relationship between trophic niche width, individual quality and offspring viability in a population of common kestrel Falco tinnunculus during 4 years. We showed that the diet of kestrels varied significantly among years and that individuals of better quality fed their offspring with a higher diversity of prey species and a higher amount of food. Moreover, body condition and immune response of nestlings were positively correlated with diversity of prey delivered by parents. Our study suggests that generalism has the potential to increase fitness and that broadening the trophic niche may be an adaptive strategy in unpredictable environments.     

Parasites in food webs: the ultimate missing links

Parasitism is the most common consumer strategy among organisms, yet only recently has there been a call for the inclusion of infectious disease agents in food webs. The value of this effort hinges on whether parasites affect food‐web properties. Increasing evidence suggests that parasites have the potential to uniquely alter food‐web topology in terms of chain length, connectance and robustness. In addition, parasites might affect food‐web stability, interaction strength and energy flow. Food‐web structure also affects infectious disease dynamics because parasites depend on the ecological networks in which they live. Empirically, incorporating parasites into food webs is straightforward. We may start with existing food webs and add parasites as nodes, or we may try to build food webs around systems for which we already have a good understanding of infectious processes. In the future, perhaps researchers will add parasites while they construct food webs. Less clear is how food‐web theory can accommodate parasites. This is a deep and central problem in theoretical biology and applied mathematics. For instance, is representing parasites with complex life cycles as a single node equivalent to representing other species with ontogenetic niche shifts as a single node? Can parasitism fit into fundamental frameworks such as the niche model? Can we integrate infectious disease models into the emerging field of dynamic food‐web modelling? Future progress will benefit from interdisciplinary collaborations between ecologists and infectious disease biologists.     

Niche segregation of coexisting Arctic charr (Salvelinus alpinus) and brown trout (Salmo trutta) constrains food web coupling in subarctic lakes

1. Generalist fish species are recognised as important couplers of benthic and pelagic food‐web compartments in lakes. However, interspecific niche segregation and individual specialisation may limit the potential for generalistic feeding behaviour. 2. We studied summer habitat use, stomach contents and stable isotopic compositions of the generalist feeder Arctic charr coexisting with its common resource competitor brown trout in five subarctic lakes in northern Norway to reveal population‐level and individual‐level niche plasticity. 3. Charr and trout showed partial niche segregation in all five lakes. Charr used all habitat types and a wide variety of invertebrate prey including zooplankton, whereas trout fed mainly on insects in the littoral zone. Hence, charr showed a higher potential to promote habitat and food‐web coupling compared to littoral‐dwelling trout. 4. The level of niche segregation between charr and trout and between pelagic‐caught and littoral‐caught charr depended on the prevailing patterns of interspecific and intraspecific resource competition. The two fish species had partially overlapping trophic niches in one lake where charr numerically dominated the fish community, whereas the most segregated niches occurred in lakes where trout were more abundant. 5. In general, pelagic‐caught charr had substantially narrower dietary and isotopic niches and relied less on littoral carbon sources compared to littoral‐caught conspecifics that included generalist as well as specialised benthivorous and planktivorous individuals. Despite the partially specialised planktivorous niche and thus reduced potential of pelagic‐dwelling charr to promote benthic–pelagic coupling, the isotopic compositions of both charr subpopulations suggested a significant reliance on both littoral and pelagic carbon sources in all five study lakes. 6. Our study demonstrates that both interspecific niche segregation between and individual trophic specialisation within generalist fish species can constrain food‐web coupling and alter energy mobilisation to top consumers in subarctic lakes. Nevertheless, pelagic and littoral habitats and food‐web compartments may still be highly integrated due to the potentially plastic foraging behaviour of top consumers.     

Competition theory,evolution, and the concept of an ecological niche

This article examines some of the main tenets of competition theory in light of the theory of evolution and the concept of an ecological niche. The principle of competitive exclusion and the related assumption that communities exist at competitive equilibrium - fundamental parts of many competition theories and models - may be violated if non-equilibrium conditions exist in natural communities or are incorporated into competition models. Furthermore, these two basic tenets of competition theory are not compatible with the theory of evolution. Variation in ecologically significant environmental factors and non-equilibrium in population numbers should occur in most natural communities, and such changes have important effects on community relations, niche overlap, and the evolution of ecosystems. Ecologists should view competition as a process occurring within a complexdynamic system, and should be wary of theoretical positions built upon simple laboratory experiments or simplistic mathematical models.In considering the relationship between niche overlap and competition, niche overlap should not be taken as a sufficient condition for competition; many factors may prevent or diminish competition between populations with similar resource utilization patterns. The typically opposing forces of intraspecific and interspecific competition need to be simultaneously considered, for it is the balance between them that in large part determines niche boundaries.     

Effect of scavenging on predation in a food web

Scavenging can have important consequences for food web dynamics, for example, it may support additional consumer species and affect predation on live prey. Still, few food web models include scavenging. We develop a dynamic model that includes two facultative scavenger species, which we refer to as the predator or scavenger species according to their natural scavenging propensity, as well as live prey, and a carrion pool to show ramifications of scavenging for predation in simple food webs. Our modeling suggests that the presence of scavengers can both increase and decrease predator kill rates and overall predation in model food webs and the impact varies (in magnitude and direction) with context. In particular, we explore the impact of the amount of dynamics (exploitative competition) allowed in the predator, scavenger, and prey populations as well as the direction and magnitude of interference competition between predators and scavengers. One fundamental prediction is that scavengers most likely increase predator kill rates, especially if there are exploitative feedback effects on the prey or carrion resources like is normally observed in natural systems. Scavengers only have minimal effects on predator kill rate when predator, scavenger, and prey abundances are kept constant by management. In such controlled systems, interference competition can greatly affect the interactions in contrast to more natural systems, with an increase in interference competition leading to a decrease in predator kill rate. Our study adds to studies that show that the presence of predators affects scavenger behavior, vital rates, and food web structure, by showing that scavengers impact predator kill rates through multiple mechanisms, and therefore indicating that scavenging and predation patterns are tightly intertwined. We provide a road map to the different theoretical outcomes and their support from different empirical studies on vertebrate guilds to provide guidance in wildlife management.     

Tolerance niche expansion and potential distribution prediction during Asian openbill bird range expansion

It is prevalent to use ecological niche models in the analysis of species expansion and niche changes. However, it is difficult to estimate the niche when alien species fail to establish in exotic areas. Here, we applied the tolerance niche concept, which means that niche of species can live and grow but preclude a species from establishing self‐sustaining populations, in such fail‐to‐establish events. Taking the rapidly expanded bird, Asian openbill (Anastomus oscitans), as a model species, we investigated niche dynamics and its potential effects on the population by Niche A and ecospat, predicted potential distribution by biomod2. Results showed that niche expansion has occurred in two non‐native populations caused by the tolerance of colder and wetter environments, and potential distribution mainly concentrated on equatorial islands. Our study suggested that the expanded niche belongs to tolerance niche concept according to the populations'' dynamics and GPS tracking evidence. It is essential to consider source populations when we analyze the alien species. We recommended more consideration to the application of tolerance niche in alien species research, and there is still a need for standard measurement frameworks for analyzing the tolerance niche.     

Quantifying Food Web Flows Using Linear Inverse Models

The quantitative mapping of food web flows based on empirical data is a crucial yet difficult task in ecology. The difficulty arises from the under-sampling of food webs, because most data sets are incomplete and uncertain. In this article, we review methods to quantify food web flows based on empirical data using linear inverse models (LIM). The food web in a LIM is described as a linear function of its flows, which are estimated from empirical data by inverse modeling. The under-sampling of food webs implies that infinitely many different solutions exist that are consistent with a given data set. The existing approaches to food web LIM select a single solution from this infinite set by invoking additional assumptions: either a specific selection criterion that has no solid ecological basis is used or the data set is artificially upgraded by assigning fixed values to, for example, physiological parameters. Here, we advance a likelihood approach (LA) that follows a different solution philosophy. Rather than singling out one particular solution, the LA generates a large set of possible solutions from which the marginal probability density function (mPDF) of each flow and correlations between flows can be derived. The LA is exemplified with an example model of a soil food web and is made available in the open-source R-software. Moreover, we show how stoichiometric data, stable isotope signatures, and fatty acid compositions can be included in the LIM to alleviate the under-sampling problem. Overall, LIM prove to be a powerful tool in food web research, which can bridge the gap between empirical data and the analysis of food web structures.     

A general theory of environmental noise in ecological food webs

We examine the effects of environmental noise on populations that are parts of simple two-species food webs. We assume that the species are strongly interacting and that one or the other population is affected by the noise signal. Further assuming that a stable equilibrium with positive population densities exists, we are able to perform a complete frequency analysis of the system. If only one of the populations is subject to noise, the relative noise response by both populations is fully determined by the sign of a single element of the Jacobian matrix. The analysis is readily extended to cases when both species are affected by noise or when the food web has more than two species. The general conclusion about relative responses to noise is then less unambiguous, but the power spectra describing the frequency composition of the population variabilities are nevertheless completely determined. These results are entirely independent on the exact nature of the interaction (i.e., predation, competition, mutualism) between the populations. The results show that the interpretation of the "color" of ecological time series (i.e., the frequency composition of population variability over time) may be complicated by species interactions. The propagation of noise signals through food webs and the importance of web structure for the expected response of all parts of the web to such signals is a challenging field for future studies.     

Does the introduced signal crayfish occupy an equivalent trophic niche to the lost native noble crayfish in boreal lakes?

The introduced North-American signal crayfish (Pacifastacus leniusculus) has become widespread throughout Europe where it has often replaced the native noble crayfish (Astacus astacus). The impact of this replacement on ecosystem processes in boreal lakes is still largely unknown. We compared the trophic niches of these two crayfish species in 16 small to medium sized boreal lakes in southern Finland; eight lakes with noble crayfish and eight lakes where the native crayfish populations had been lost and replaced by signal crayfish. We analysed carbon and nitrogen stable isotopes from samples of the crayfish and their putative food sources, and used stable isotope models to compare trophic niche widths of the two species of crayfish and to quantify the food sources used by them. At species level the signal crayfish exhibited a substantially larger trophic niche than that of the noble crayfish, but within-lake populations of the species did not differ in their niche widths. The isotopic niches of the two species strongly overlapped, and while the estimated proportions of food resources (profundal and littoral macroinvertebrates, terrestrial leaf detritus and macrophytes) used by crayfish varied considerably among individual populations, they did not differ consistently between the species. Our results suggest that, contrary to often expressed concerns, replacement of lost noble crayfish populations by the signal crayfish may not greatly alter the littoral food web structure in boreal lakes.