Academic literature on the topic 'Trophic interaction strengths'

Food webs (i.e., networks of species and their feeding interactions) share multiple structural features across ecosystems. The factors explaining such similarities are still debated, and the role played by most organismal traits and their intraspecific variation is unknown. Here, we assess how variation in traits controlling predator–prey interactions (e.g., body size) affects food web structure. We show that larger phenotypic variation increases connectivity among predators and their prey as well as total food intake rate. For predators able to eat only a few species (i.e., specialists), low phenotypic variation maximizes intake rates, while the opposite is true for consumers with broader diets (i.e., generalists). We also show that variation sets predator trophic level by determining interaction strengths with prey at different trophic levels. Merging these results, we make two general predictions about the structure of food webs: (i) trophic level should increase with predator connectivity, and (ii) interaction strengths should decrease with prey trophic level. We confirm these predictions empirically using a global dataset of well-resolved food webs. Our results provide understanding of the processes structuring food webs that include functional traits and their naturally occurring variation.

Quantifying species interaction strengths enhances prediction of community dynamics, but variability in the strength of species interactions in space and time complicates accurate prediction. Interaction strengths can vary in response to density, indirect effects, priority effects or a changing environment, but the mechanism(s) causing direction and magnitudes of change are often unclear. We designed an experiment to characterize how environmental factors influence the direction and the strength of priority effects between sessile species. We estimated per capita non-trophic effects of barnacles ( Semibalanus balanoides ) on newly settled germlings of the fucoid, Ascophyllum nodosum , in the presence and absence of consumers in experiments on rocky shores throughout the Gulf of Maine, USA. Per capita effects on germlings varied among environments and barnacle life stages, and these interaction strengths were largely unaltered by changing consumer abundance. Whereas previous evidence shows adult barnacles facilitate fucoids, here, we show that recent settlers and established juveniles initially compete with germlings. As barnacles mature, they switch to become facilitators of fucoids. Consumers caused variable mortality of germlings through time comparable to that from competition. Temporally variable effects of interactors (e.g. S. balanoides ), or spatial variation in their population structure, in different regions differentially affect target populations (e.g. A. nodosum ). This may affect abundance of critical stages and the resilience of target species to environmental change in different geographical regions.

AbstractPredator–prey interaction strengths can be highly context-dependent. In particular, multiple predator effects (MPEs), variations in predator sex and physical habitat characteristics may affect prey consumption rates and thus the persistence of lower trophic groups. Ephemeral wetlands are transient ecosystems in which predatory copepods can be numerically dominant. We examine the interaction strengths of a specialist copepod Paradiaptomus lamellatus towards mosquito prey in the presence of conspecifics using a functional response approach. Further, we examine sex variability in predation rates of P. lamellatus under circadian and surface area variations. Then, we assess the influence of a co-occurring heterospecific predatory copepod, Lovenula raynerae, on total predation rates. We demonstrate MPEs on consumption, with antagonism between conspecific P. lamellatus predatory units evident, irrespective of prey density. Furthermore, we show differences between sexes in interaction strengths, with female P. lamellatus significantly more voracious than males, irrespective of time of day and experimental arena surface area. Predation rates by P. lamellatus were significantly lower than the heterospecific calanoid copepod L. raynerae, whilst heterospecific copepod groups exhibited the greatest predatory impact. Our results provide insights into the predation dynamics by specialist copepods, wherein species density, diversity and sex affect interaction strengths. In turn, this may influence population-level persistence of lower trophic groups under shifting copepod predator composition.

The community matrix measures the direct effect of species on each other in an ecological community. It can be used to determine whether a system is stable (returns to equilibrium after small perturbations of the population abundances), reactive (perturbations are initially amplified before damping out), and to determine the response of any individual species to perturbations of environmental parameters. However, several studies show that small errors in estimating the entries of the community matrix translate into large errors in predicting individual species responses. Here, we ask whether there are properties of complex communities one can still predict using only a crude, order-of-magnitude estimate of the community matrix entries. Using empirical data, randomly generated community matrices, and those generated by the Allometric Trophic Network model, we show that the stability and reactivity properties of systems can be predicted with good accuracy. We also provide theoretical insight into when and why our crude approximations are expected to yield an accurate description of communities. Our results indicate that even rough estimates of interaction strengths can be useful for assessing global properties of large systems.

We measured the strength of direct and indirect interactions in order to develop a standardized estimate of the impact of an invasive snail on its resource and a competitor. The freshwater New Zealand snail, Potamopyrgus antipodarum, an invasive species in the western U.S., is the most abundant benthic macroinvertebrate grazer in several rivers, where it overlaps with several threatened endemic snails. In one watershed, Potamopyrgus coexists with the snail, Pyrgulopsis robusta, which may be affected by resource competition with Potamopyrgus. In field enclosure experiments, we quantified the direct grazing effect of snails on algae and the indirect effects between consumers. Potamopyrgus significantly limited growth of Pyrgulopsis. In contrast, Pyrgulopsis appeared to facilitate growth of the invasive snail (Potamopyrgus). In natural populations, snail densities were positively correlated over five sites, but negatively correlated at two downstream sites. Interaction strengths between snails and algae were equivalent for both snails at both sites, indicating that invasion success could not be attributed to differences in resource acquisition. However, the overall impact of the invader was much higher at the downstream site when both snail abundance and interaction strengths were considered. Negative individual effects of Potamopyrgus at two trophic levels in conjunction with high Potamopyrgus abundance demonstrated a significant impact of the invader in this lotic community.

AbstractThe potential ecosystem effects of fishing for Antarctic toothfish (Dissostichus mawsoni) in the Ross Sea region were investigated. Mixed trophic impact analysis was applied to a model of the Ross Sea foodweb and used to calculate the relative trophic importances of species and trophic groups in the system. The trophic impact of toothfish on medium-sized demersal fish was identified as the strongest top-down interaction in the system based on multiple-step analysis. This suggests a potential for a strong predation-release effect on some piscine prey of toothfish (especially grenadiers and ice-fish on the Ross Sea slope). However, Antarctic toothfish had moderate trophic importance in the Ross Sea foodweb as a whole, and the analysis did not support the hypothesis that changes to toothfish will cascade through the ecosystem by simple trophic effects. Because of limitations of this kind of analysis, cascading effects on the Ross Sea ecosystem due to changes in the abundance of toothfish cannot be ruled out, but for such changes to occur a mechanism other than simple trophic interactions is likely to be involved. Trophic importances were highest in the middle of the foodweb where silverfish and krill are known to have a key role in ecosystem structure and function. The six groups with the highest indices of trophic importance were (in decreasing order): phytoplankton, mesozooplankton, Antarctic silverfish, small demersal fish, Antarctic krill and cephalopods. Crystal krill and small pelagic fish also had high trophic importance in some analyses. Strengths and limitations of this kind of analysis are presented. In particular, it is noted that the analysis only considers trophic interactions at the spatial, temporal and ecological scale of the whole Ross Sea shelf and slope area, averaged over a typical year and in 35 trophic groups. Interference and density-dependent effects were not included in this analysis. Effects at smaller spatial and temporal scales, and effects concerning only parts of populations, were not resolved by the analysis, and this is likely to underestimate the potential risks of fishing to Weddell seals and type-C killer whales.

Global warming and ocean acidification are forecast to exert significant impacts on marine ecosystems, while intensive exploitation of commercial marine species has already caused large-scale reorganizations of biological communities in many of the world’s marine ecosystems. Whilst our understanding on the impact of warming and acidification in isolation on individual species has steadily increased, we still know little on the combined effect of these two global stressors on marine food webs, especially under realistic experimental settings or real-world systems. We particularly lack evidence of how the top of the food web (piscivores and apex predators) will respond to future climate change (ocean warming and acidification) because responses of ecological communities could vary with increasing trophic level. The picture is further complicated by the interaction of global and local stressors that affect our oceans, such as fishing pressure. Accurate predictions of the potential effects of these global and local stressors at ecosystem-levels require a comprehensive understanding of how entire communities of species respond to climate change. Mechanistic insights revealed by a combination of different approaches such as experimental manipulation of food webs, and integrated with ecosystem modelling approaches provide a way forward to improve our understanding of the functioning of future food webs. In this thesis, I show how the combined effect of such global and local stressors could affect a three trophic level temperate marine mesocosm food web and how these outcomes could be translated to predict the response of ecological communities in a four trophic level natural food web. Using a sophisticated mesocosm experiment (elevated pCO2 of approximately 900 ppm and warming of +2.8°C), I first modelled how energy fluxes are likely to change in marine food webs in response to future climate. I experimentally show that the combined stress of acidification and warming could reduce energy flows from the first trophic level (primary producers and detritus) to the second (herbivores) and from the second to the third trophic level (carnivores). Although warming and acidification jointly boosted primary producer biomass, most of it was constrained to the base of the food web as consumers were unable to transfer unpalatable cyanobacterial production up the food web. In contrast, ocean acidification affected the food web positively by increasing the biomass from producers to carnivores. I then developed a unique approach that combines the empirical data on species response to climate change from our mesocosms experiments with historical population data (fisheries biomass and catch data) to predict future changes in a natural food web. I incorporated physiological and behavioural responses (complex species-interactions) of species from primary producers to top predators such as sharks within a time-dynamic integrated ecosystem modelling approach (Ecosim). I show that under continuation of the present-day fishing regime, warming and ocean acidification will benefit most of the higher trophic level community groups (e.g. mammals, birds, demersal finfish). The positive effects of warming and acidification in isolation will likely be reduced under their combined effect (antagonistic interaction) which is likely to be further negated under increased fishing pressure, decreasing the individual biomass of consumers. The total future fisheries biomass, however, will likely still remain high compared to the present-day scenario. This is because unharvested species in present day fishery will likely benefit from decreased competition and an increase in biomass. Nevertheless, ecological indicator such as the Shannon diversity index suggests a trade-off between biomass gain and loss of functional diversity within food webs. The mechanisms behind the increase in biomass at higher trophic level consumers and a decrease in the biomass of lower trophic levels is mostly driven by the increasing top down control by consumers on their prey through increasing trophic interaction strength and a positive response of some of the prey groups under warming irrespective of acidification. I show that in a future food web, temperature-driven changes in direct trophic interactions strength (feeding and competition) will largely determine the direction of biomass change (increase or decrease) of consumers due to higher mean interaction strength (magnitude of change). In contrast, although acidification induces a relatively small increase in trophic interaction strength it shows a much larger change in the percent interactions altered for indirect interactions. Hence, ocean acidification is likely to propagate boosted primary consumer biomass to higher trophic levels. The findings of this thesis reveal that warming in combination with acidification can increase trophic interaction strengths (top down control), resulting in a reorganization of community biomass structure by reducing or increasing the biomass of resources and consumers and a loss of functional diversity within the food web. Also, the degree to which warming and acidification will be beneficial or detrimental to functional groups in future food webs will largely depend on how interaction strengths affects individual consumers or resource groups due to multi-trophic species interaction, the availability of prey resources and the complexity of the food web considered (e.g. three or four trophic level and more diverse ecological communities).
Thesis (Ph.D.) -- University of Adelaide, School of Biological Sciences, 2018

Most climate change predictions focus on the response of individual species to changing local conditions and ignore species interactions, largely due to the lack of a sound theoretical foundation for how interactions are expected to change with climate and how to incorporate them into climate change models. Much of the variability in species interaction strengths may be governed by fundamental constraints on physiological rates, possibly providing a framework for including species interactions into climate change models. Metabolic rates, ingestion rates and many other physiological rates are relatively predictable from body size and body temperature due to constraints imposed by the physical and chemical laws that govern fluid dynamics and the kinetics of biochemical reaction times. My dissertation assesses the usefulness of this framework by exploring the community-level consequences of physiological constraints. In Chapter 2, I incorporated temperature and body size scaling into the biological rate parameters of a series of realistically structured trophic network models. The relative magnitude of the temperature scaling parameters affecting consumer energetic costs (metabolic rates) and energetic gains (ingestion rates) determined how consumer energetic efficiency changed with temperature. I systematically changed consumer energetic efficiency and examined the sensitivity of network stability and species persistence to various temperatures. I found that a species' probability of extinction depended primarily on the effects of organismal physiology (body size and energetic efficiency with respect to temperature) and secondarily on the effects of local food web structure (trophic level and consumer generality). This suggests that physiology is highly influential on the structure and dynamics of ecological communities. If consumer energetic efficiency declined as temperature increased, that is, species did best at lower temperatures, then the simulated networks had greater stability at lower temperatures. The opposite scenario resulted in greater stability at higher temperatures. Thus, much of the community-level response depends on what species energetic efficiencies at the organismal-level really are, which formed the research question for Chapter 3: How does consumer energetic efficiency change with temperature? Existing evidence is scarce but suggestive of decreasing consumer energetic efficiency with increasing temperature. I tested this hypothesis on seven rocky intertidal invertebrate species by measuring the relative temperature scaling of their metabolic and ingestion rates as well as consumer interaction strength under lab conditions. Energetic efficiencies of these rocky intertidal invertebrates declined and species interaction strengths tended to increase with temperature. Thus, in the rocky intertidal, the mechanistic effect of temperature would be to lower community stability at higher temperatures. Chapter 4 tests if the mechanistic effects of temperature on ingestion rates and species interaction strengths seen in the lab are apparent under field conditions. Bruce Menge and I related bio-mimetic estimates of body temperatures to estimates of per capita mussel ingestion rates and species interaction strengths by the ochre sea star Pisaster ochraceus, a keystone predator of the rocky intertidal. We found a strong, positive effect of body temperature on both per capita ingestion rates and interaction strengths. However, the effects of season and the unique way in which P. ochraceus regulates body temperatures were also apparent, leaving room for adaptation and acclimation to partially compensate for the mechanistic constraint of body temperature. Community structure of the rocky intertidal is associated with environmental forcing due to upwelling, which delivers cold, nutrient rich water to the nearshore environment. As upwelling is driven by large-scale atmospheric pressure gradients, climate change has the potential to affect a wide range of significant ecological processes through changes in water temperature. In Chapter 5, my coauthors and I identified long-term trends in the phenology of upwelling events that are consistent with climate change predictions: upwelling events are becoming stronger and longer. As expected, longer upwelling events were related to lower average water temperatures in the rocky intertidal. Furthermore, recruitment rates of barnacles and mussels were associated with the phenology of upwelling events. Thus climate change is altering the mode and the tempo of environmental forcing in nearshore ecosystems, with ramifications for community structure and function. Ongoing, long-term changes in environmental forcing in rocky intertidal ecosystems provide an opportunity to understand how temperature shapes community structure and the ramifications of climate change. My dissertation research demonstrates that the effect of temperature on organismal performance is an important force structuring ecological communities and has potential as a tractable framework for predicting the community level effects of climate change.
Graduation date: 2013
Access restricted to the OSU Community, at author's request, from Nov. 20, 2012 - Nov. 20, 2014

Chapter 7 elucidates the relationships between the structure and functioning of aquatic ecosystems at high altitude through the description of material cycles and food webs. Following the landscape continuum model, material cycling is profoundly influenced by the physical structure of the waterscape (e.g. vegetation cover); as a result a great diversity of energetic pathways characterize high altitude waterscapes, along an autotrophy–heterotrophy gradient. Similarly, high altitude aquatic food webs embrace a great diversity of trophic compartments, feeding strategies, and processes (trophic cascades and terrestrial subsidiarity) that are profoundly shaped by environmental harshness. Harsh conditions also generate stress gradients along which the strength and direction of species interactions (from competition to facilitation) and their functional role (e.g. as ecosystem engineers) are modified. The resulting structural and functional changes affect in turn species coexistence and trigger potential ecosystem shifts.

This chapter addresses the biological processes that influence community composition and dynamics, highlighting the multi-trophic and multi-functional nature of soft-sediment communities. Characterising the biological interactions into competition, predation, adult–juvenile interaction, facilitation, parasitism and disease, the chapter initially focusses on what we know about these processes, their effect on other biological components and their importance in different benthic habitats. It then extends to consider how they, in conjunction with mobility of species, can influence the broader-scale spatial structure and temporal dynamics connecting communities and functions. The chapter recognises the importance of differences in strength of interaction and the potential for weak interactions to affect community dynamics. This is framed around the concept of self-organisation as an emergent characteristic of ecosystem interaction networks and how meta-communities may be constructed.

In all areas of ecology, from studies of individual organisms through populations to communities and ecosystems, there have been huge empirical and theoretical advances over the past several decades. Our guess—a testable hypothesis—is that the worldwide research community of ecologists has grown by roughly an order-of-magnitude since the 1960s, as is of course true for other areas of the life sciences. One consequence is that it is harder to put together a book like the present one. And we find it especially hard when we compare this chapter on community patterns with the corresponding chapter in the second edition of Theoretical Ecology. For the chapters on single populations, for example, there has been growth both in understanding the nonlinear dynamical phenomena that can arise, along with a host of well-designed field and laboratory experiments which illustrate these processes. The narrative, however, retains a unifying central thread, and much of the task of overview and compression lies in choosing good examples from an increasing panoply of choice. For communities, on the other hand, we find so many different yet intersecting areas of growth, many of which have recently produced booklength collections of papers, that the task of choosing which topics to emphasize and which to elide is invidious. The result is necessarily quirky. Without further apology, here is an outline. One broad area of community ecology deals with models for the dynamical behaviour of collections of many interacting species—either within a single trophic level or more generally—essentially as a scale-up of models for single and pairwise- interacting populations. This was the subject of the preceding chapter. Here, we begin by emphasizing the importance of work which views communities from, as it were, a plumber’s perspective, looking at patterns of flow of energy or nutrients or other material. But we then move on quickly to other topics. These include: the network structure of food-webs (connectance, interaction strengths, etc.); what determines species’ richness (niche versus null models); relative abundance of species (observed patterns and suggested causes); succession and disturbance; species–area relations; and scaling laws (with suggested connections among some such laws).

This chapter explores ecological networks and their properties. Ecological networks summarize the many potential interactions between species within a community by representing species as nodes in the network and using links between nodes to represent the interactions between species. The earliest and best-studied ecological networks are food webs that describe who eats whom within a community (trophic links). Most food webs contain a few strong and many weak links between species; the preponderance of weak links promotes food web stability. Body size is a key trait in determining the pattern and strength of trophic interactions in food webs. Mutualistic networks describe the positive interactions between species in a community, where patterns of species associations may be characterized as either “nested” or “modular”. Nestedness may increase stability in mutualistic networks. A major challenge to future research is to incorporate multiple types of species interactions into the same ecological network.

Ecological theory indicates that increasing the number of species, the number of interactions, and the strength of these interactions all tend to make communities less stable. Conversely, stability is enhanced by strong intraspecific density dependence, low connectivity, or weak trophic links. These theoretical predictions are borne out in many fish communities. Species diversity is an important metric for ecological communities. Organizing species into groups according to size, function, or diet composition can reduce the dimensionality of fish community models. Analyses of fish communities from around the world lend support to the prediction of strong compensation within functional groups, with weaker predator–prey links among groups. Size spectra describe the distribution of individuals across size classes irrespective of their species. Qualitative models can be used to assess the indirect effects of species on each other and the overall stability of the community.

Belowground organisms are key components of the trophic structure and they mediate the dynamics of nutrients of all terrestrial ecosystems. The interactions among assemblages of belowground microorganisms and their consumers mediate the cycling of plant-limiting nutrients, influence aboveground plant productivity, affect the course of plant community development, and affect the dynamic stability of aboveground communities following natural and anthropogenic disturbances (Clarholm, 1985; Ingham et al., 1985; Laakso and Setälä, 1999; Naeem et al., 1994; Tilman et al., 1996; Wall and Moore, 1999). The influence of belowground organisms on the aboveground plant community is heightened in systems such as the shortgrass steppe (Blair et al., 2000), given the relatively high percentage of plant production that is diverted belowground through plant roots. Many of the human-induced changes that the shortgrass steppe has been subjected to during the past 150 years fall outside the scope of the natural variations in climate and grazing. This conflict between the natural history of the shortgrass steppe and the more recent human legacy forms the backdrop of this chapter. First we present a detailed description of the belowground food web for the native shortgrass steppe and present its structure in terms of the patterns of trophic interactions, the distribution of biomass, the flow of energy, and the strengths of interactions. Second, we explore how three disturbances—managed grazing, agricultural practices, and climate change (altered precipitation and temperature, and elevated CO2)—have altered the structure of the belowground community. We conclude with a synthesis of the common patterns that we observed in the grassland’s response to these disturbances, and speculate on their consequences. Aboveground plant parts provide from 20% to 40% cover with exposed soil between them (Lauenroth and Milchunas, 1991). Much of the aboveground production remains in place as standing dead, rather than falling to the soil surface as litter. The ratio of shoot production to root production is roughly 1:1, contrasting sharply with forests, where far more production is allocated aboveground (Jackson et al., 1996; Milchunas and Lauenroth, 1993, 2000). Hence, in the shortgrass steppe, plant roots provide the major input of carbon to soil. As such, plant roots are the focal point of biological activity in soils (Coleman et al., 1983).

Highly imperiled Oregon white oak ecosystems are a regional conservation priority of numerous organizations, including Oregon Metro, a regional government serving over one million people in the Portland area. Previously dominant systems in the Pacific Northwest, upland prairie and oak woodlands are now experiencing significant threat, with only 2% remaining in the Willamette Valley in small fragments (Hulse et al. 2002). These fragments are of high conservation value because of the rich biodiversity they support, including rare and endemic species, such as Delphinium leucophaeum (Oregon Department of Agriculture, 2020). Since 2010, Metro scientists and volunteers have collected phenology data on approximately 140 species of forbs and graminoids in regional oak prairie and woodlands. Phenology is the study of life-stage events in plants and animals, such as budbreak and senescence in flowering plants, and widely acknowledged as a sensitive indicator of environmental change (Parmesan 2007). Indeed, shifts in plant phenology have been observed over the last few decades as a result of climate change (Parmesan 2006). In oak systems, these changes have profound implications for plant community composition and diversity, as well as trophic interactions and general ecosystem function (Willis 2008). While the original intent of Metro’s phenology data-collection was to track long-term phenology trends, limitations in data collection methods have made such analysis difficult. Rather, these data are currently used to inform seasonal management decisions on Metro properties, such as when to collect seed for propagation and when to spray herbicide to control invasive species. Metro is now interested in fine-tuning their data-collection methods to better capture long-term phenology trends to guide future conservation strategies. Addressing the regional and global conservation issues of our time will require unprecedented collaboration. Phenology data collected on Metro properties is not only an important asset for Metro’s conservation plan, but holds potential to support broader research on a larger scale. As a leader in urban conservation, Metro is poised to make a meaningful scientific contribution by sharing phenology data with regional and national organizations. Data-sharing will benefit the common goal of conservation and create avenues for collaboration with other scientists and conservation practitioners (Rosemartin 2013). In order to support Metro’s ongoing conservation efforts in Oregon white oak systems, I have implemented a three-part master’s project. Part one of the project examines Metro’s previously collected phenology data, providing descriptive statistics and assessing the strengths and weaknesses of the methods by which the data were collected. Part two makes recommendations for improving future phenology data-collection methods, and includes recommendations for datasharing with regional and national organizations. Part three is a collection of scientific vouchers documenting key plant species in varying phases of phenology for Metro’s teaching herbarium. The purpose of these vouchers is to provide a visual tool for Metro staff and volunteers who rely on plant identification to carry out aspects of their job in plant conservation. Each component of this project addresses specific aspects of Metro’s conservation program, from day-to-day management concerns to long-term scientific inquiry.