Animal pee in the sea: consumer‐mediated nutrient dynamics in the world's changing oceans

Humans have drastically altered the abundance of animals in marine ecosystems via exploitation. Reduced abundance can destabilize food webs, leading to cascading indirect effects that dramatically reorganize community structure and shift ecosystem function. However, the additional implications of these top‐down changes for biogeochemical cycles via consumer‐mediated nutrient dynamics (CND) are often overlooked in marine systems, particularly in coastal areas. Here, we review research that underscores the importance of this bottom‐up control at local, regional, and global scales in coastal marine ecosystems, and the potential implications of anthropogenic change to fundamentally alter these processes. We focus attention on the two primary ways consumers affect nutrient dynamics, with emphasis on implications for the nutrient capacity of ecosystems: (1) the storage and retention of nutrients in biomass, and (2) the supply of nutrients via excretion and egestion. Nutrient storage in consumer biomass may be especially important in many marine ecosystems because consumers, as opposed to producers, often dominate organismal biomass. As for nutrient supply, we emphasize how consumers enhance primary production through both press and pulse dynamics. Looking forward, we explore the importance of CDN for improving theory (e.g., ecological stoichiometry, metabolic theory, and biodiversity–ecosystem function relationships), all in the context of global environmental change. Increasing research focus on CND will likely transform our perspectives on how consumers affect the functioning of marine ecosystems.     

Spatial stoichiometry: cross‐ecosystem material flows and their impact on recipient ecosystems and organisms

Cross‐ecosystem material flows, in the form of inorganic nutrients, detritus and organisms, spatially connect ecosystems and impact food web dynamics. To date research on material flows has focused on the impact of the quantity of these flows and largely ignored their elemental composition, or quality. However, the ratios of elements like carbon, nitrogen and phosphorus can influence the impact material flows have on food web interactions through stoichiometric mismatches between resources and consumers. The type and movement of materials likely vary in their ability to change stoichiometric constraints within the recipient ecosystem and materials may undergo changes in their own stoichiometry during transport. In this literature review we evaluate the importance of cross‐ecosystem material flows within the framework of ecological stoichiometry. We explore how movement in space and time impacts the stoichiometry of material flow, as these transformations are essential to consider when assessing the ability of these flows to impact food web productivity and ecosystem functioning. Our review suggests that stoichiometry of cross‐ecosystem material flows are highly dynamic and undergo changes during transport across the landscape or from human influence. These material flows can impact recipient organisms if they change stoichiometry of the abiotic medium, or provide resources that have a different stoichiometry to in situ resources. They might also alter consumer excretion rates, in turn altering the availability of nutrients in the recipient ecosystem. These alterations in stoichiometric constraints of recipient organisms can have cascading trophic effects and shape food web dynamics. We highlight significant gaps in the literature and suggest new avenues for research that explore how cross‐ecosystem material flows impact recipient ecosystems when considering differences in stoichiometric quality, their movement through the landscape and across ecosystem boundaries, and the nutritional constraints of the recipient organisms.     

Habitat coupling in lake ecosystems

Lakes are complex ecosystems composed of distinct habitats coupled by biological, physical and chemical processes. While the ecological and evolutionary characteristics of aquatic organisms reflect habitat coupling in lakes, aquatic ecology has largely studied pelagic, benthic and riparian habitats in isolation from each other. Here, we summarize several ecological and evolutionary patterns that highlight the importance of habitat coupling and discuss their implications for understanding ecosystem processes in lakes. We pay special attention to fishes because they play particularly important roles as habitat couplers as a result of their high mobility and flexible foraging tactics that lead to inter-habitat omnivory. Habitat coupling has important consequences for nutrient cycling, predator-prey interactions, and food web structure and stability. For example, nutrient excretion by benthivorous consumers can account for a substantial fraction of inputs to pelagic nutrient cycles. Benthic resources also subsidize carnivore populations that have important predatory effects on plankton communities. These benthic subsidies stabilize population dynamics of pelagic carnivores and intensify the strength of their interactions with planktonic food webs. Furthermore, anthropogenic disturbances such as eutrophication, habitat modification, and exotic species introductions may severely alter habitat connections and, therefore, the fundamental flows of nutrients and energy in lake ecosystems.     

Consistent nutrient storage and supply mediated by diverse fish communities in coral reef ecosystems

Corals thrive in low nutrient environments and the conservation of these globally imperiled ecosystems is largely dependent on mitigating the effects of anthropogenic nutrient enrichment. However, to better understand the implications of anthropogenic nutrients requires a heightened understanding of baseline nutrient dynamics within these ecosystems. Here, we provide a novel perspective on coral reef nutrient dynamics by examining the role of fish communities in the supply and storage of nitrogen (N) and phosphorus (P). We quantified fish‐mediated nutrient storage and supply for 144 species and modeled these data onto 172 fish communities (71 729 individual fish), in four types of coral reefs, as well as seagrass and mangrove ecosystems, throughout the Northern Antilles. Fish communities supplied and stored large quantities of nutrients, with rates varying among ecosystem types. The size structure and diversity of the fish communities best predicted N and P supply and storage and N : P supply, suggesting that alterations to fish communities (e.g., overfishing) will have important implications for nutrient dynamics in these systems. The stoichiometric ratio (N : P) for storage in fish mass (~8 : 1) and supply (~20 : 1) was notably consistent across the four coral reef types (but not seagrass or mangrove ecosystems). Published nutrient enrichment studies on corals show that deviations from this N : P supply ratio may be associated with poor coral fitness, providing qualitative support for the hypothesis that corals and their symbionts may be adapted to specific ratios of nutrient supply. Consumer nutrient stoichiometry provides a baseline from which to better understand nutrient dynamics in coral reef and other coastal ecosystems, information that is greatly needed if we are to implement more effective measures to ensure the future health of the world's oceans.     

The evolution of ecosystem processes: growth rate and elemental stoichiometry of a key herbivore in temperate and arctic habitats

Understanding the reciprocal interactions between the evolved characteristics of species and the environment in which each species is embedded is a major priority for evolutionary ecology. Here we use the perspective of ecological stoichiometry to test the hypothesis that natural selection on body growth rate affects consumer body stoichiometry. As body elemental composition (nitrogen, phosphorus) of consumers influences nutrient cycling and trophic dynamics in food webs, such differences should also affect biogeochemical processes and trophic dynamics. Consistent with the growth rate hypothesis, body growth rate and phosphorus content of individuals of the Daphnia pulex species complex were lower in Wisconsin compared to Alaska, where the brevity of the growing season places a premium on growth rate. Consistent with stoichiometric theory, we also show that, relative to animals sampled in Wisconsin, animals sampled in Alaska were poor recyclers of P and suffered greater declines in growth when fed low‐quality, P‐deficient food. These results highlight the importance of evolutionary context in establishing the reciprocal relationships between single species and ecosystem processes such as trophic dynamics and consumer‐driven nutrient recycling.     

Elements of disease in a changing world: modelling feedbacks between infectious disease and ecosystems

An overlooked effect of ecosystem eutrophication is the potential to alter disease dynamics in primary producers, inducing disease‐mediated feedbacks that alter net primary productivity and elemental recycling. Models in disease ecology rarely track organisms past death, yet death from infection can alter important ecosystem processes including elemental recycling rates and nutrient supply to living hosts. In contrast, models in ecosystem ecology rarely track disease dynamics, yet elemental nutrient pools (e.g. nitrogen, phosphorus) can regulate important disease processes including pathogen reproduction and transmission. Thus, both disease and ecosystem ecology stand to grow as fields by exploring questions that arise at their intersection. However, we currently lack a framework explicitly linking these disciplines. We developed a stoichiometric model using elemental currencies to track primary producer biomass (carbon) in vegetation and soil pools, and to track prevalence and the basic reproduction number (R₀) of a directly transmitted pathogen. This model, parameterised for a deciduous forest, demonstrates that anthropogenic nutrient supply can interact with disease to qualitatively alter both ecosystem and disease dynamics. Using this element‐focused approach, we identify knowledge gaps and generate predictions about the impact of anthropogenic nutrient supply rates on infectious disease and feedbacks to ecosystem carbon and nutrient cycling.     

Carrion cycling in food webs: comparisons among terrestrial and marine ecosystems

In light of current global changes to ecosystem function (e.g. climate change, trophic downgrading, and invasive species), there has been a recent surge of interest in exploring differences in nutrient cycling among ecosystem types. In particular, a growing awareness has emerged concerning the importance of scavenging in food web dynamics, although no studies have focused specifically on exploring differences in carrion consumption between aquatic and terrestrial ecosystems. In this forum we synthesize the scavenging literature to elucidate differences in scavenging dynamics between terrestrial and marine ecosystems, and identify areas where future research is needed to more clearly understand the role of carrion consumption in maintaining ecosystem function within each of these environments. Although scavenging plays a similar functional role in terrestrial and aquatic food webs, here we suggest that several fundamental differences exist in scavenging dynamics among these ecosystem types due to the unique selection pressures imposed by the physical properties of water and air. In particular, the movement of carcasses in marine ecosystems (e.g. wave action, upwelling, and sinking) diffuses biological activity associated with scavenging and decomposition across large, three‐dimensional spatial scales, creating a unique spatial disconnect between the processes of production, scavenging, and decomposition, which in contrast are tightly linked in terrestrial ecosystems. Moreover, the limited role of bacteria and temporal stability of environmental conditions on the sea floor appears to have facilitated the evolution of a much more diverse community of macrofauna that relies on carrion for a higher portion of its nutrient consumption than is present in terrestrial ecosystems. Our observations are further discussed as they pertain to the potential impacts of climate change and trophic downgrading (i.e. removal of apex consumers from ecosystems) on scavenging dynamics within marine and terrestrial ecosystems.     

Vectors with autonomy: what distinguishes animal‐mediated nutrient transport from abiotic vectors?

Animal movements are important drivers of nutrient redistribution that can affect primary productivity and biodiversity across various spatial scales. Recent work indicates that incorporating these movements into ecosystem models can enhance our ability to predict the spatio‐temporal distribution of nutrients. However, the role of animal behaviour in animal‐mediated nutrient transport (i.e. active subsidies) remains under‐explored. Here we review the current literature on active subsidies to show how the behaviour of active subsidy agents makes them both ecologically important and qualitatively distinct from abiotic processes (i.e. passive subsidies). We first propose that animal movement patterns can create similar ecological effects (i.e. press and pulse disturbances) in recipient ecosystems, which can be equal in magnitude to or greater than those of passive subsidies. We then highlight three key behavioural features distinguishing active subsidies. First, organisms can transport nutrients counter‐directionally to abiotic forces and potential energy gradients (e.g. upstream). Second, unlike passive subsidies, organisms respond to the patterns of nutrients that they generate. Third, animal agents interact with each other. The latter two features can form positive‐ or negative‐feedback loops, creating patterns in space or time that can reinforce nutrient hotspots in places of mass aggregations and/or create lasting impacts within ecosystems. Because human‐driven changes can affect both the space‐use of active subsidy species and their composition at both population (i.e. individual variation) and community levels (i.e. species interactions), predicting patterns in nutrient flows under future modified environmental conditions depends on understanding the behavioural mechanisms that underlie active subsidies and variation among agents' contributions. We conclude by advocating for the integration of animal behaviour, animal movement data, and individual variation into future conservation efforts in order to provide more accurate and realistic assessments of changing ecosystem function.     

Stoichiometric flexibility as a regulator of carbon and nutrient cycling in terrestrial ecosystems under change

Ecosystems across the biosphere are subject to rapid changes in elemental balance and climatic regimes. A major force structuring ecological responses to these perturbations lies in the stoichiometric flexibility of systems - the ability to adjust their elemental balance whilst maintaining function. The potential for stoichiometric flexibility underscores the utility of the application of a framework highlighting the constraints and consequences of elemental mass balance and energy cycling in biological systems to address global change phenomena. Improvement in the modeling of ecological responses to disturbance requires the consideration of the stoichiometric flexibility of systems within and across relevant scales. Although a multitude of global change studies over various spatial and temporal scales exist, the explicit consideration of the role played by stoichiometric flexibility in linking micro-scale to macro-scale biogeochemical processes in terrestrial ecosystems remains relatively unexplored. Focusing on terrestrial systems under change, we discuss the mechanisms by which stoichiometric flexibility might be expressed and connected from organisms to ecosystems. We suggest that the transition from the expression of stoichiometric flexibility within individuals to the community and ecosystem scales is a key mechanism regulating the extent to which environmental perturbation may alter ecosystem carbon and nutrient cycling dynamics.     

Predator control of ecosystem nutrient dynamics

Predators are predominantly valued for their ability to control prey, as indicators of high levels of biodiversity and as tourism attractions. This view, however, is incomplete because it does not acknowledge that predators may play a significant role in the delivery of critical life‐support services such as ecosystem nutrient cycling. New research is beginning to show that predator effects on nutrient cycling are ubiquitous. These effects emerge from direct nutrient excretion, egestion or translocation within and across ecosystem boundaries after prey consumption, and from indirect effects mediated by predator interactions with prey. Depending on their behavioural ecology, predators can create heterogeneous or homogeneous nutrient distributions across natural landscapes. Because predator species are disproportionately vulnerable to elimination from ecosystems, we stand to lose much more from their disappearance than their simple charismatic attractiveness.     

Stoichiometric distribution models: ecological stoichiometry at the landscape extent

Human activities are altering the fundamental geography of biogeochemicals. Yet we lack an understanding of how the spatial patterns in organismal stoichiometry affect biogeochemical processes and the tools to predict the impacts of global changes on biogeochemical processes. In this contribution we develop stoichiometric distribution models (StDMs), which allow us to map spatial structure in resource elemental composition across a landscape and evaluate spatial responses of consumers. We parameterise StDMs for a consumer‐resource (moose‐white birch) system and demonstrate that we can develop predictive models of resource stoichiometry across a landscape and that such models could improve our predictions of consumer space use. With results from our study system application, we argue that explicit consideration of the spatial patterns in organismal elemental composition may uncover emergent individual, population, community and ecosystem properties that are not revealed at the local extents routinely used in ecological stoichiometry. We discuss perspectives for further developments and application of StDMs to advance three emerging frameworks for spatial ecosystem ecology in an era of global change; meta‐ecosystem theory, macroecological stoichiometry and remotely sensed biogeochemistry. Progress on these emerging frameworks will allow for the integration of ecological stoichiometry and individual space use and fitness.     

Tannins in nutrient dynamics of forest ecosystems - a review

Tannins make up a significant portion of forest carbon pools and foliage and bark may contain up to 40% tannin. Like many other plant secondary compounds, tannins were believed to function primarily as herbivore deterrents. However, recent evidence casts doubts on their universal effectiveness against herbivory. Alternatively, tannins may play an important role in plant–plant and plant–litter–soil interactions. The convergent evolution of tannin-rich plant communities on highly acidic and infertile soils throughout the world, and the intraspecific variation in tannin concentrations along edaphic gradients suggests that tannins can affect nutrient cycles. This paper reviews nutrient dynamics in forest ecosystems in relation to tannins. Tannins comprise a complex class of organic compounds whose concentration and chemistry differ greatly both among and within plant species. Because the function and reactivity of tannins are strongly controlled by their chemical structure, the effects of tannins on forest ecosystem processes are expected to vary widely. Tannins can affect nutrient cycling by hindering decomposition rates, complexing proteins, inducing toxicity to microbial populations and inhibiting enzyme activities. As a result, tannins may reduce nutrient losses in infertile ecosystems and may alter N cycling to enhance the level of organic versus mineral N forms. The ecological consequences of elevated tannin levels may include allelopathic responses, changes in soil quality and reduced ecosystem productivity. These effects may alter or control successional pathways. While a great deal of research has addressed tannins and their role in nutrient dynamics, there are many facets of tannin biogeochemistry that are not known. This lack of information hinders a complete synthesis of tannin effects on forest ecosystem processes and nutrient cycling. Areas of study that would help clarify the role of tannins in forest ecosystems include improved characterization and quantification techniques, enhanced understanding of structure-activity relationships, investigation of the fate of tannins in soil, further determination of the influence of environmental factors on plant tannin production and decomposition, and additional information on the effects of tannins on soil organisms.     

Adaptive differences in plant physiology and ecosystem paradoxes: insights from metabolic scaling theory

The link between variation in species‐specific plant traits, larger scale patterns of productivity, and other ecosystem processes is an important focus for global change research. Understanding such linkages requires synthesis of evolutionary, biogeograpahic, and biogeochemical approaches to ecological research. Recent observations reveal several apparently paradoxical patterns across ecosystems. When compared with warmer low latitudes, ecosystems from cold northerly latitudes are described by (1) a greater temperature normalized instantaneous flux of CO₂ and energy; and (2) similar annual values of gross primary production (GPP), and possibly net primary production. Recently, several authors attributed constancy in GPP to historical and abiotic factors. Here, we show that metabolic scaling theory can be used to provide an alternative ‘biotically driven’ hypothesis. The model provides a baseline for understanding how potentially adaptive variation in plant size and traits associated with metabolism and biomass production in differing biomes can influence whole‐ecosystem processes. The implication is that one cannot extrapolate leaf/lab/forest level functional responses to the globe without considering evolutionary and geographic variation in traits associated with metabolism. We test one key implication of this model – that directional and adaptive changes in metabolic and stoichiometric traits of autotrophs may mediate patterns of plant growth across broad temperature gradients. In support of our model, on average, mass‐corrected whole‐plant growth rates are not related to differences in growing season temperature or latitude. Further, we show how these changes in autotrophic physiology and nutrient content across gradients may have important implications for understanding: (i) the origin of paradoxical ecosystem behavior; (ii) the potential efficiency of whole‐ecosystem carbon dynamics as measured by the quotient of system capacities for respiration, R, and assimilation, A; and (iii) the origin of several ‘ecosystem constants’– attributes of ecological systems that apparently do not vary with temperature (and thus with latitude). Together, these results highlight the potential critical importance of community ecology and functional evolutionary/physiological ecology for understanding the role of the biosphere within the integrated earth system.     

Nutrient flows between ecosystems can destabilize simple food chains

Dispersal of organisms has large effects on the dynamics and stability of populations and communities. However, current metacommunity theory largely ignores how the flows of limiting nutrients across ecosystems can influence communities. We studied a meta-ecosystem model where two autotroph-consumer communities are spatially coupled through the diffusion of the limiting nutrient. We analyzed regional and local stability, as well as spatial and temporal synchrony to elucidate the impacts of nutrient recycling and diffusion on trophic dynamics. We show that nutrient diffusion is capable of inducing asynchronous local destabilization of biotic compartments through a diffusion-induced spatiotemporal bifurcation. Nutrient recycling interacts with nutrient diffusion and influences the susceptibility of the meta-ecosystem to diffusion-induced instabilities. This interaction between nutrient recycling and transport is further shown to depend on ecosystem enrichment. It more generally emphasizes the importance of meta-ecosystem theory for predicting species persistence and distribution in managed ecosystems.     

Emergence of nutrient co‐limitation through movement in stoichiometric meta‐ecosystems

Evidence that ecosystems and primary producers are limited in their productivity by multiple nutrients has caused the traditional nutrient limitation framework to include multiple limiting nutrients. The models built to mimic these responses have invoked local mechanisms at the level of the primary producers. In this paper, we explore an alternative explanation for the emergence of co‐limitation by developing a simple, stoichiometrically explicit meta‐ecosystem model with two limiting nutrients, autotrophs and herbivores. Our results show that differences in movement rates for the nutrients, autotrophs and herbivores can allow for nutrient co‐limitation in biomass response to emerge despite no local mechanisms of nutrient co‐limitation. Furthermore, our results provide an explanation to why autotrophs show positive growth responses to nutrients despite ‘nominal’ top‐down control by herbivores. These results suggest that spatial processes can be mechanisms for nutrient co‐limitation at local and regional scales, and can help explain anomalous results in the co‐limitation literature.     

Ecological stoichiometry of indirect grazer effects on periphyton nutrient content

Ecological stoichiometry has been successful in enhancing our understanding of trophic interactions between consumer and prey species. Consumer and prey dynamics have been shown to depend on the nutrient composition of the prey relative to the nutrient demand of the consumer. Since most experiments on this topic used a single consumer species, little is known about the validity of stoichiometric constraints on trophic interactions across consumers and ecosystems. We conducted a quantitative meta-analysis on grazer–periphyton experiments to test (1) if benthic grazers have consistent effects on the nutrient composition of their prey, and (2) whether these effects can be aligned to the nutrient stoichiometry of grazer and periphyton, other environmental factors, or experimental constraints. Grazers significantly lowered periphyton C:N and C:P ratios, indicating higher N- and P-content of grazed periphyton across studies. Grazer presence on average increased periphyton N:P ratios, but across studies the effect size did not differ significantly from zero. The sign and strength of grazer effects on periphyton nutrient ratios was strongly dependent on the nutrient content of grazers and their food, but also on grazer biomass, the amount of biomass removal and water column nutrients. Grazer with low P-content tended to reduce periphyton P-content, whereas grazers with high P-content increased periphyton P-content. This result suggests that low grazer P-content can be an indication of physiological P-limitation rather than a result of having relatively low and fixed P-requirements. At the across-system scale of this meta-analysis, predictions from stoichiometric theory are corroborated, but the plasticity of the consumer nutrient composition has to be acknowledged. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Cross‐ecosystem effects of a large terrestrial herbivore on stream ecosystem functioning

Large terrestrial consumers have direct and indirect effects on forest ecosystem function, but few studies have investigated the impacts of terrestrial consumers on freshwater ecosystems. In the Cape Breton Highlands of Nova Scotia, browsing by hyper‐abundant moose following a spruce budworm outbreak has transformed boreal forest into grasslands. We conducted a field study to investigate the potential for cross‐ecosystem effects of hyper‐abundant moose following budworm outbreak on small boreal stream ecosystem structure and function. With our field study, we tested the prediction that watersheds with higher levels of moose‐mediated grasslands in their sub‐basin would have higher stream temperatures, total nitrogen, electrical conductivity, periphyton biomass and macroinvertebrate abundances. While our data supported several of our predictions pertaining to moose impacts on the abiotic variables (i.e. temperature range, total nitrogen, electrical conductivity) we found evidence of variable moose impacts on the benthic community. Specifically, we observed lower relative abundance of predatory invertebrates in streams with high moose impacts compared to streams with low moose impacts in their watersheds but no evidence of moose impacts on the relative abundance of shredders, filterers, gatherers, and grazers. This empirical study fills a key gap in our understanding of spatial ecosystem ecology by providing insight into the effects of large terrestrial consumers across ecosystem boundaries with potential implications for landscape‐scale management of hyper‐abundant ungulates. Given the broad availability and improvement in remote sensing technology, the novel integration of remote sensing and field studies employed here may provide a roadmap for future studies of meta‐ecosystem dynamics.     

Assessing the role of large herbivores in the structuring and functioning of freshwater and marine angiosperm ecosystems

While large herbivores can have strong impacts on terrestrial ecosystems, much less is known of their role in aquatic systems. We reviewed the literature to determine: 1) which large herbivores (> 10 kg) have a (semi‐)aquatic lifestyle and are important consumers of submerged vascular plants, 2) their impact on submerged plant abundance and species composition, and 3) their ecosystem functions. We grouped herbivores according to diet, habitat selection and movement ecology: 1) Fully aquatic species, either resident or migratory (manatees, dugongs, turtles), 2) Semi‐aquatic species that live both in water and on land, either resident or migratory (swans), 3) Resident semi‐aquatic species that live in water and forage mainly on land (hippopotamuses, beavers, capybara), 4) Resident terrestrial species with relatively large home ranges that frequent aquatic habitats (cervids, water buffalo, lowland tapir). Fully aquatic species and swans have the strongest impact on submerged plant abundance and species composition. They may maintain grazing lawns. Because they sometimes target belowground parts, their activity can result in local collapse of plant beds. Semi‐aquatic species and turtles serve as important aquatic–terrestrial linkages, by transporting nutrients across ecosystem boundaries. Hippopotamuses and beavers are important geomorphological engineers, capable of altering the land and hydrology at landscape scales. Migratory species and terrestrial species with large home ranges are potentially important dispersal vectors of plant propagules and nutrients. Clearly, large aquatic herbivores have strong impacts on associated species and can be critical ecosystem engineers of aquatic systems, with the ability to modify direct and indirect functional pathways in ecosystems. While global populations of large aquatic herbivores are declining, some show remarkable local recoveries with dramatic consequences for the systems they inhabit. A better understanding of these functional roles will help set priorities for the effective management of large aquatic herbivores along with the plant habitats they rely on.     

How lichens impact on terrestrial community and ecosystem properties

Lichens occur in most terrestrial ecosystems; they are often present as minor contributors, but in some forests, drylands and tundras they can make up most of the ground layer biomass. As such, lichens dominate approximately 8% of the Earth's land surface. Despite their potential importance in driving ecosystem biogeochemistry, the influence of lichens on community processes and ecosystem functioning have attracted relatively little attention. Here, we review the role of lichens in terrestrial ecosystems and draw attention to the important, but often overlooked role of lichens as determinants of ecological processes. We start by assessing characteristics that vary among lichens and that may be important in determining their ecological role; these include their growth form, the types of photobionts that they contain, their key functional traits, their water‐holding capacity, their colour, and the levels of secondary compounds in their thalli. We then assess how these differences among lichens influence their impacts on ecosystem and community processes. As such, we consider the consequences of these differences for determining the impacts of lichens on ecosystem nutrient inputs and fluxes, on the loss of mass and nutrients during lichen thallus decomposition, and on the role of lichenivorous invertebrates in moderating decomposition. We then consider how differences among lichens impact on their interactions with consumer organisms that utilize lichen thalli, and that range in size from microfauna (for which the primary role of lichens is habitat provision) to large mammals (for which lichens are primarily a food source). We then address how differences among lichens impact on plants, through for example increasing nutrient inputs and availability during primary succession, and serving as a filter for plant seedling establishment. Finally we identify areas in need of further work for better understanding the role of lichens in terrestrial ecosystems. These include understanding how the high intraspecific trait variation that characterizes many lichens impacts on community assembly processes and ecosystem functioning, how multiple species mixtures of lichens affect the key community‐ and ecosystem‐level processes that they drive, the extent to which lichens in early succession influence vascular plant succession and ecosystem development in the longer term, and how global change drivers may impact on ecosystem functioning through altering the functional composition of lichen communities.