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While one of the most striking features of our planet is its great variety of life, studies show that ongoing biodiversity loss could reduce the productivity of ecosystems by as much as 50%. However, evidence comes largely from experiments that have used highly simplified communities with on average seven species, all from a single trophic group. In contrast, natural communities have dozens, if not hundreds, of species spanning a variety of trophic levels. Would this additional complexity alter our conclusions about the functional consequences of diversity loss? This chapter reviews five hypotheses about how the fluxes of energy and matter through food-webs might depend on the diversity of species interacting within, as well as across trophic levels. After outlining the empirical support for or against each hypothesis, this chapter discusses several avenues of research that may prove useful as ecologists move towards a food web perspective on biodiversity and ecosystem functioning

This chapter summarizes what the authors consider to be the key and general organizing principles of kelp forests and coral reefs along gradients of oceanography, latitude, and effects of fishing and resource use on these ecosystems. The general structure of these food webs is described as well as the effects of top-down versus bottom-up controls and the prevalence of trophic cascades. Human effects and recommendations for management are presented.

The chapter summarizes the current understanding of the kelp forest ecosystems of Alaska by focusing on the key role that otters, killer whales, and man have played in the ecological organization of this ecosystem. The role of kelp, the effect of anti-predator chemistry, and sea urchin interactions are discussed. The history of the ecosystem as well as the science is covered and includes a discussion of the debate on the role of whaling in influencing otter–killer whale interactions, which may be further changing the ecology of this ecosystem.

This chapter uses simple theory and experiments to address the fundamental question of what determines the biomass (abundance) of different trophic levels (plants, herbivores, carnivores) in a community. Theory predicts joint control of trophic-level abundance by bottom-up effects (resources) and top-down effects (predation), with the relative strengths of top-down and bottom-up effects depending on the number of trophic levels and species composition within a trophic level. Observations and experiments support these predictions. Trophic cascades provide evidence for the importance of top-down processes, but the existence of a trophic cascade says little about the relative importance of predator limitation versus resource limitation. Cascading effects result from either the consumptive or non-consumptive effects of predators, or both. Natural systems contain as few as three and as many as six trophic levels, but what determines this number is unknown. Evidence suggests that both productivity and ecosystem size, perhaps in combination, are the primary factors.

Studies on streams provide some of the most detailed and best resolved webs yet available, and provide a means to test food web theory. These systems can be used to test patterns in connectance webs and the effect of sampling on the patterns evident. The role of body-size in stream webs is reviewed. This chapter deals with attempts to quantify food webs by measures of energy flow and interaction strengths, and to characterize the circumstances leading to the presence or absence of trophic cascades in food webs. Streams are strongly linked through their food webs with surrounding systems, such as ground water and flood plains, and such cross-system subsidies is considered. The effect of natural and anthropogenic disturbances on food web attributes is addressed. Finally, the relationship between biodiversity/ecosystem functions in stream webs is addressed in the context of environmental stress.

Birds function within their ecosystems in ways that include many direct and indirect chains of trophic interaction. In some interaction chains, birds exert top-down effects in “trophic cascades.” These occur when a predatory species directly reduces its prey abundance and consequently indirectly releases suppression of species at lower trophic levels. Many bird species initiate trophic cascades in terrestrial and aquatic ecosystems, and in natural and agro-ecosystems. Services provided by birds through trophic cascades benefit humans primarily through pest control in natural forests, forestry plantations, fruit orchards, and a variety of crop-based agro-ecosystems. Cascade strength may be affected by predator prey specificity, redundancy, species diversity, and productivity. Some birds influence ecosystem function both through either bottom up interactions or through intermediate trophic positions within interaction networks. These effects can also reverberate through trophic webs. Global change, including anthropogenic perturbation, may lead to a loss of ecosystem services, including seed dispersal and pollination services by birds. These losses may cause declines keystone or foundation plant species, resulting in losses in biodiversity and ecosystem integrity. We conclude with suggestions for future research needs on bird ecosystem services, particularly in light of global change.

This chapter on food web interactions connects the organisms and their interactions with the abiotic frame and provides a helicopter perspective on the function of freshwater ecosystems. Initially, the theoretical basis for an ecosystem approach is outlined, including food web theory, the bottom-up and top-down concepts and how these have evolved in concert with empirical advances. Specifically, the concepts of cascading trophic interactions and alternative stable states are discussed both from a theoretical and empirical viewpoint, as well as in both benthic and pelagic habitats. This chapter links all components, from microbes to vertebrates, to temporal and spatial changes in abiotic features leading to successional patterns in populations and communities.

Food webs are theoretical abstractions of the complex linkages and interactions that are thought to occur in nature. Although few real food webs have actually been characterized scientifically, there is a large body of literature on the processes that contribute towards complexity and stability in webs. Food webs are generally thought of as ‘what eats what’ webs, but parasites are not usually incorporated into webs even though parasitism is a feeding strategy shared by a majority of species on earth (70%). This chapter examines major ideas on the roles of parasites in food webs, starting with Elton’s (1927) idea that parasites are analogous to predators. It describes some general patterns of parasite web structure (e.g., inverted pyramid of numbers and body size hypotheses) using both available published data and data from studies on food webs in freshwater streams in New Jersey.

This chapter uses the community model to repeat many of the classic impact calculations of a single stock on the entire community. Here, a focus is the appearance of trophic cascades initiated by the removal of large predators. When a component of an ecosystem is perturbed, the effects are not isolated to the component itself but cascade through the ecosystem. Perturbations are mainly propagated through the predator–prey interactions. The chapter also considers the trade-offs between a forage fishery and a consumer fishery, and the extension of the maximum sustainable yield (MSY) concept to the community, before finally returning to the single-stock aspects.

This chapter considers the theoretical equilibrium consequences of ratio-dependent and prey-dependent views and compares them with evidence from nature. It provides evidence, mostly from lakes and marine systems, regarding the responses of food chains to enrichment. It discusses a number of theoretical observations such as the trophic cascade pattern. It also looks into how food webs based on donor control, a case of ratio dependence, are shown to be much more stable than those based on the Lotka-Volterra model, and explains the persistence of complex food webs in nature.

This chapter begins with a viral video about a trophic cascade initiated by the return of wolves to Yellowstone National Park. It then challenges the narrative of that video. How strong is the evidence for the trophic cascade that has been claimed to exist in Yellowstone? A survey of the relevant literature suggests that the matter is far from settled. But the absence of a scientific consensus is not reflected in the popular press. Analysis of a random sample of newspaper articles about wolf reintroduction shows that a simplistic version of the scientific story is reported far more often than the more complex, but more accurate, tale of an unresolved hypothesis. A particular study on wolf-mediated effects on grizzly bears, via elk and berries, is examined in more depth. Ultimately, the chapter makes the case that the more nuanced story is not only more factually accurate, it also tells an essential truth about the nature of ecology.

Parasites have important and unique impacts on marine food webs. By infecting taxa across all trophic levels, parasites affect both bottom-up and top-down processes in marine systems. When host densities are high enough, parasites can regulate or even decimate their populations, causing regime shifts in marine systems. As consumers and resources, parasites are enmeshed in food webs in ways that are different from free-living species. Their unique lifestyle renders parasites more susceptible to perturbations than their free-living hosts. As a result, parasites serve as useful indicators of ecosystem integrity. A theory for how food webs affect parasites will help us better understand why a particular infectious disease has become problematic, give insight into how restoration might reduce a costly marine disease, or let us use parasites as indicators to follow changes in food-web complexity.

This chapter reviews how marine ecosystems respond to parasites. Evidence from several marine ecosystems shows that parasites can wield control over ecosystem structure, function, and dynamics by regulating host density and phenotype. Like predators, parasites can generate or modify trophic cascades, regulate important foundational species and ecosystem engineers, and mediate species coexistence by affecting competitive outcomes. Sometimes the parasites have clear positive impacts within ecosystems, such as increasing species diversity or maintaining ecosystem stability. Other times, parasites may have destabilizing effects that signal an ecosystem out of balance. But it is now clear that some (but not all) parasites can have strong and, at times, predictable effects, and should thus be incorporated into food web and ecosystem models

This chapter examines the predation and trophic cascades in kelp communities, focusing on the relationships between sea urchins and their predators. It is often assumed that the natural state of kelp forests is tight trophic control, such that when top predators are reduced in abundances by humans, there is a “relaxation in top-down control” on sea urchins and a subsequent decline in macroalgal abundance, often referred to as a trophic cascade. There are two types of trophic cascade: species-level and community-level. In a species-level cascade, changes in predator numbers affect one or a few plant species in a compartment of the food web. Community-level cascades may apply to any multilevel linear food web interaction and alter the distribution of plant biomass through an entire system.

The consequences of saltmarsh destruction and restoration are brought to bear in the story of Pine Creek Marsh. Pine Creek Marsh was cut off from tidal influence, which led to a cascade of plant and animal disappearances and unintended problems, including flooding, for the surrounding human community. The subsequent restoration of tidal influence corrected many issues, reversed vegetation changes, and served as a model for ecosystem repair along the coastline. The mystery of marsh die-off along the New England coast provides another opportunity to examine salt marsh ecology and trophic cascades as scientists study the interactions among cordgrass, crabs, and crab predators. The impacts of climate change are made apparent as sea level rises and scientists seek to develop models for future flood prediction, carbon storage, and marsh protection.

This chapter examines the basic ecology of streams in the Toolik Region. It provides information gathered from monitoring year-to-year variability and to identify long-term changes in the ecology of streams. It looks into the importance of understanding the role nutrients have in limiting and structuring stream ecosystems. It describes the ecological characteristics of the five types of arctic streams in the Toolik region, including biogeochemistry, primary producers, microheterotrophs, and secondary producers. It enumerates details of three stream-fertilization experiments that provide the basis of past and current research. It combines the research results under six themes: resource limitation; recovery from nutrient enrichment; trophic cascades; hyporheic processes; disturbance as a driver; and how warming leads to changes in permafrost and thermokarst.

Rewilding is an optimistic environmental agenda to reverse the loss of biodiversity and reconnect society with nature. This chapter explores Britain’s ecological history, back to the Last Interglacial before the arrival of modern humans, when the climate was similar to today, to analyse how conservationists can learn from the past to rewild the ecosystems of the present and prepare for an uncertain future. Because there is no single point in history that should or could be recreated, rewilding focuses on re-establishing naturally dynamic ecological processes that, through an appropriate sequence of species reintroductions, attempts to move the ecosystem towards a more appropriately biodiverse and functional state. A state that is self-sustaining in the present climate, and that projected for the near future. Specifically, this chapter explores a rewilding solution to conservation challenges associated with over-grazing, limited germination niche availability, and river dynamics: the reintroduction of wolves, wild boar, and beaver respectively. This sequence of reintroductions is suggested to be complimentary, each altering ecosystem dynamics to facilitate the return of the next. Evidence indicates wolves will reduce deer abundance and re-distribute browsing intensity promoting tree regeneration, particularly in riparian areas, increasing woodland availability to the more habitat-dependent wild boar and beaver. An important message behind rewilding is that a rich biodiversity with all guilds well represented, including the ones that polarize public opinion, such as large predators, are important components of ecosystem service rich and self-sustaining ecosystems, particularly in core areas.

Sea otters are the only fully marine-living mustelid and the smallest extant marine mammal. They have the highest mass-specific metabolic rate of any marine mammal, which coupled with the lack of blubber for insulator and energy storage, relegates them to operating as an extreme income strategist, and appears to have led to a life history tactic in which pregnancy rate is fixed while reproductive success varies with the mother’s body condition at the time of birth, which triggers a decision immediately post-partum to care for or abandon her pup. When resources are limiting, sea otters assume highly individualized diets, which are inherited matrilineally. Sea otters exert strong limiting influences on their macroinvertebrate prey, leading to far reaching indirect effects on the structure and function of coastal marine ecosystems. This chapter reviews and synthesizes the interplay between sea otter energetics and life histories, diet and foraging behaviour, and ecosystem influences.

It would seem to be like searching for a needle in a haystack. One hundred million years on, strata containing recognizable fossils of multicellular creatures will extend through a 640-million-year span in time, and will have piled up, in total, to many kilometres in thickness. Somewhere in those endless stretches of rock might be the layer in which traces of humanity may be preserved, awaiting discovery by our curious visitors. Would they happen upon this by chance? Or would they be led to it by following a trail of clues, much as a detective is led to the perpetrator of a crime by the ripple effects of the act itself: the wealth gone astray, the scattered victims, the damaged property, the spree in Monte Carlo. In the case of humanity, there have been victims, and damage, and stolen wealth. It has been a quite singular felony. It will leave echoes, collateral changes, that may act, in the far future, as signposts. Some of these may give clear directions to a heist that was quite out of the ordinary. On a planetary scale, in fact. But first, how thick a section of strata should we take, as the geological target to be searched for, analysed, interpreted for signs of a vanished civilization? One might start by taking ten thousand years’ worth. To us, that represents a gigantic stretch of time (just imagine trying to peer ten thousand years into the future). Geologically, it barely counts. There are a hundred such intervals in a million-year span, and a million-year span represents the small change of geological history. Nonetheless, there are good reasons for choosing this duration, for it represents the span during which human activities can be said to have left a detectable imprint upon the geological record—an imprint beyond the odd vanishingly rare bone of an obscure bipedal hominid. Ten thousand years ago, half of the large mammals of the Earth abruptly disappeared, and it seems increasingly likely that this disappearance was mainly the result of hunting by humans.

The effects of global warming, habitat loss, and alien species are pushing extinction rates to new heights. We are currently in the midst of a mass extinction event, which began in the Pleistocene and has accelerated in recent centuries, with massive impacts on ecosystem structure and function. Palaeoecological and historical records can contribute to understanding how ecosystems may have functioned if megafauna had survived, providing an ecological basis for re-wilding projects. Palaeoecology and palaeontology can identify the former ranges of currently rare species, and can be used to study the interacting effects of climate, fire, and megafauna on ecosystem structure and function, thereby providing guidance for restoration and habitat management. Re-wilding can contribute to the maintenance and restoration of ecosystem services, providing alternatives to landscapes that are increasingly transformed, industrialized, or abandoned.