Linking the influence and dependence of people on biodiversity across scales

Abstract

Biodiversity enhances many of nature's benefits to people, including the regulation of climate and the production of wood in forests, livestock forage in grasslands and fish in aquatic ecosystems. Yet people are now driving the sixth mass extinction event in Earth's history. Human dependence and influence on biodiversity have mainly been studied separately and at contrasting scales of space and time, but new multiscale knowledge is beginning to link these relationships. Biodiversity loss substantially diminishes several ecosystem services by altering ecosystem functioning and stability, especially at the large temporal and spatial scales that are most relevant for policy and conservation.

Figure 1. The influence and dependence of people on biodiversity.

a, People influence biodiversity directly by changing land-use, climate and biogeochemical cycles, as well as by introducing species — actions known collectively as anthropogenic drivers. At the global scale, these activities are driving the sixth mass extinction in the history of life on Earth. At the local scale, species losses decrease ecosystem functioning (for example, ecosystem productivity and resource uptake) and stability (the invariability of ecosystem productivity across a period of years). At the intermediate scales such as landscapes or regions, changes in ecosystem functioning can alter the supply of ecosystem services, including the production of wood in forests, livestock forage in grasslands and fish in aquatic ecosystems. It is important to build multiscale knowledge at the intersections of the numerous components of the system. Various system components are positioned in a gradient that spans the social (yellow) to ecological (blue) ends of a socio–ecological continuum. Dashed arrows indicate other important relationships that are beyond the scope of this Review. b, At present, there are mismatches in the spatial and temporal scales at which the relationships between anthropogenic drivers, biodiversity, ecosystem functioning and ecosystem services are best understood. This makes it a challenge to link the cascading effects of human activities on biodiversity, ecosystems and ecosystem services. Furthermore, the scales at which knowledge is available for some of the relationships do not yet align with the scales at which policies and other decisions (orange circle) are often made. Relationships are positioned at the approximate scales at which they are currently best understood.

Figure 2. The influence of anthropogenic drivers on ecosystems through effects on the richness and types of species.

a, Most biodiversity experiments consider the dependence of ecosystem functioning on the random loss of species, finding that the decrease in ecosystem functioning per lost species becomes larger as species richness declines (black line). By contrast, non-random changes in ecosystem functioning and species richness (dashed lines) that result from anthropogenic drivers such as herbivore invasion, nutrient enrichment or habitat fragmentation also include shifts in the composition and traits of species that are most vulnerable or favoured, which can reinforce or offset the effects of changes in species richness. The grey region indicates potential variation in ecosystem functioning due to changes in species composition at a particular level of species richness. b–d, Positive (blue arrows) and negative (red arrows) influences of specific anthropogenic drivers of biodiversity change (dark grey ovals) are presented in the style of a structural equation model. Curved dashed arrows indicate indirect effects of specific drivers on ecosystem functioning, and horizontal solid arrows show direct effects on ecosystem functioning that are independent of changes in the composition or richness of species. Non-horizontal solid arrows represent the component relationships of indirect effects. b, Invasion by a herbivorous species has a direct negative effect on plant productivity. However, such invasion can indirectly increase plant productivity by increasing plant species richness, and these positive effects are enhanced when favoured plants contribute substantially to plant productivity. The asterisk indicates a hypothesized relationship. c, Nutrient enrichment has a direct positive effect on plant productivity. But it can also decrease plant productivity indirectly by decreasing plant species richness, and this negative effect is reinforced when the most vulnerable plants contribute substantially to plant productivity. d, Habitat fragmentation has a direct negative effect on arthropod biomass. It can also decrease arthropod biomass indirectly by decreasing arthropod species richness, but this negative effect is offset by shifts in species composition when the most vulnerable arthropods make only a small contribution to arthropod biomass.

Figure 3. Temporal and spatial insurance effects enhance and stabilize ecosystem productivity.

In a hypothetical example, various plant species (coloured distributions) are most productive (row 1, left) at different levels of the environmental factor soil moisture (dry, red; wet, blue). As soil moisture changes with time (row 1, middle) and space (row 1, right), communities are dominated by the species that is most productive under the particular conditions of each community. In this case, communities that consist of two dissimilar species (row 3) or many species (row 4) are expected to be more productive and to vary less in productivity in time and space than communities with only two similar species (row 2). (Productivity levels in space (rows 2–4, right) are indicated by a gradient from black (high) to white (low).) These performance-enhancing and stabilizing temporal and spatial insurance effects can be thought of as a combination of selection and complementarity effects because they emerge when species have complementary traits and dominate where and when species are most fit.

Figure 4. Complementary approaches for understanding the ecosystem consequences of human-driven biodiversity change.

Four main approaches to understanding the effects of anthropogenic biodiversity change on ecosystems are shown (blue). Each approach has certain strengths and weaknesses (green) and enriches the others in several ways, and it is the combination of their results that best informs policy and decision-making at the scales at which populations and species are changed, and at which ecosystem services are delivered (yellow). The simultaneous use of all four approaches is therefore crucial for improving our knowledge of socio–ecological systems and to inform policy and decision-making.