While most research on the relationship between ecosystem diversity and stability has focused on species richness, it is variation in species composition that provides the mechanistic basis to explain the relationship between species richness and ecosystem functioning. Species differ from one another in their resource use, environmental tolerances, and interactions with other species, such that species composition has a major influence on ecosystem functioning and stability. The traits that characterize the ecological function of a species are termed functional traits, and species that share similar suites of traits are often categorized together into functional groups.
When species from different functional groups occur together, they can exhibit complementary resource-use, meaning that they use different resources or use the same resources at different times. For example, two animal predators may consume different prey items, so they are less likely to compete with one another, allowing higher total biomass of predators in the system.
In the case of plants, all species may utilize the same suite of resources space, light, water, soil nutrients, etc. Increasing species diversity can influence ecosystem functions — such as productivity — by increasing the likelihood that species will use complementary resources and can also increase the likelihood that a particularly productive or efficient species is present in the community.
While primary production is the ecosystem function most referred to in this article, other ecosystem functions, such as decomposition and nutrient turnover, are also influenced by species diversity and particular species traits. Stability can be defined at the ecosystem level — for example, a rancher might be interested in the ability of a grassland ecosystem to maintain primary production for cattle forage across several years that may vary in their average temperature and precipitation.
Figure 1 shows how having multiple species present in a plant community can stabilize ecosystem processes if species vary in their responses to environmental fluctuations such that an increased abundance of one species can compensate for the decreased abundance of another.
Biologically diverse communities are also more likely to contain species that confer resilience to that ecosystem because as a community accumulates species, there is a higher chance of any one of them having traits that enable them to adapt to a changing environment.
Such species could buffer the system against the loss of other species. In this situation, species identity — and particular species traits — are the driving force stabilizing the system rather than species richness per se see Figure 2. Figure 1: Conceptual diagram showing how increasing diversity can stabilize ecosystem functioning Each rectangle represents a plant community containing individuals of either blue or green species and the total number of individuals corresponds to the productivity of the ecosystem.
Green species increase in abundance in warm years, whereas blue species increase in abundance in cold years such that a community containing only blue or green species will fluctuate in biomass when there is interannual climate variability. In contrast, in the community containing both green and blue individuals, the decrease in one species is compensated for by an increase in the other species, thus creating stability in ecosystem productivity between years.
Note also that, on average, the diverse community exhibits higher productivity than either single-species community. This pattern could occur if blue or green species are active at slightly different times, such that competition between the two species is reduced. This difference in when species are active leads to complimentary resource utilization and can increase total productivity of the ecosystem. In contrast, if stability is defined at the species level, then more diverse assemblages can actually have lower species-level stability.
This is because there is a limit to the number of individuals that can be packed into a particular community, such that as the number of species in the community goes up, the average population sizes of the species in the community goes down. For example, in Figure 2, each of the simple communities can only contain three individuals, so as the number of species in the community goes up, the probability of having a large number of individuals of any given species goes down.
The smaller the population size of a particular species, the more likely it is to go extinct locally, due to random — stochastic — fluctuations, so at higher species richness levels there should be a greater risk of local extinctions.
Thus, if stability is defined in terms of maintaining specific populations or species in a community, then increasing diversity in randomly assembled communities should confer a greater chance of destabilizing the system. Figure 2: Conceptual model illustrating the insurance hypothesis Simple communities are represented by a box; in this case, these communities are so small that they can only contain 3 individuals.
For example, this could be the case for a small pocket of soil on a rocky hillslope. Looking at all possible combinations of communities containing 1, 2 or 3 species, we see that, as the number of species goes up, the probability of containing the blue species also goes up.
Thus, if hillslopes in this region were to experience a prolonged drought, the more diverse communities would be more likely to maintain primary productivity, because of the increased probability of having the blue species present. A wealth of research into the relationships among diversity, stability, and ecosystem functioning has been conducted in recent years reviewed by Balvanera et al.
The first experiments to measure the relationship between diversity and stability manipulated diversity in aquatic microcosms — miniature experimental ecosystems — containing four or more trophic levels, including primary producers, primary and secondary consumers, and decomposers McGrady-Steed et al.
These experiments found that species diversity conferred spatial and temporal stability on several ecosystem functions. Stability was conferred by species richness, both within and among functional groups Wardle et al. When there is more than one species with a similar ecological role in a system, they are sometimes considered "functionally redundant.
More recently, scientists have examined the importance of plant diversity for ecosystem stability in terrestrial ecosystems, especially grasslands where the dominant vegetation lies low to the ground and is easy to manipulate experimentally.
In , David Tilman and colleagues established experimental plots in the Cedar Creek Ecosystem Science Reserve, each 9 x 9 m in size Figure 3A , and seeded them with 1, 2, 4, 8 or 16 species drawn randomly from a pool of 18 possible perennial plant species Tilman et al.
Plots were weeded to prevent new species invasion and ecosystem stability was measured as the stability of primary production over time. Over the ten years that data were collected, there was significant interannual variation in climate, and the researchers found that more diverse plots had more stable production over time Figure 3B.
In contrast, population stability declined in more diverse plots Figure 3C. These experimental findings are consistent with the theory described in the prior section, predicting that increasing species diversity would be positively correlated with increasing stability at the ecosystem-level and negatively correlated with species-level stability due to declining population sizes of individual species.
Figure 3: A biodiversity experiment at the Cedar Creek Ecosystem Science Reserve a demonstrates the relationship between the number of planted species and ecosystem stability b or species stability c.
All rights reserved. Experiments manipulating diversity have been criticized because of their small spatial and short time scales, so what happens in naturally assembled communities at larger spatial scales over longer time scales?
In a year study of naturally assembled Inner Mongolia grassland vegetation, Bai et al. They found that while the abundance of individual species fluctuated, species within particular functional groups tended to respond differently such that a decrease in the abundance of one species was compensated for by an increase in the abundance of another.
This compensation stabilized the biomass productivity of the whole community in a fluctuating environment see Figure 1. These findings demonstrate that local species richness — both within and among functional groups — confers stability on ecosystem processes in naturally assembled communities. Experiments in aquatic ecosystems have also shown that large-scale processes play a significant role in stabilizing ecosystems.
A whole-lake acidification experiment in Canada found that although species diversity declined as a result of acidification, species composition changed significantly and ecosystem function was maintained Schindler This suggests that given sufficient time and appropriate dispersal mechanisms, new species can colonize communities from the regional species pool and compensate for those species that are locally lost Fischer et al.
This observation emphasizes the importance of maintaining connectivity among natural habitats as they experience environmental changes. Evidence from multiple ecosystems at a variety of temporal and spatial scales, suggests that biological diversity acts to stabilize ecosystem functioning in the face of environmental fluctuation. Variation among species in their response to such fluctuation is an essential requirement for ecosystem stability, as is the presence of species that can compensate for the function of species that are lost.
While much of the evidence presented here has focused on the consequences of changes in species diversity on primary production in natural ecosystems, recent research has found similar relationships between species diversity and ecosystem productivity in human-managed ecosystems e.
Bai, Y. Ecosystem stability and compensatory effects in the Inner Mongolia grassland. Nature , — Balvanera, P. Quantifying the evidence for biodiversity effects on ecosystem functioning and services. These services are essential for human well-being. However, at present there are few studies that link changes in biodiversity with changes in ecosystem functioning to changes in human well-being.
Protecting the Catskill watersheds that provide drinking water for New York City is one case where safeguarding ecosystem services paid a dividend of several billion dollars. Further work that demonstrates the links between biodiversity, regulating and supporting services , and human well-being is needed to show this vital but often unappreciated value of biodiversity C4, C7, C Species composition matters as much or more than species richness when it comes to ecosystem services.
Ecosystem functioning, and hence ecosystem services, at any given moment in time is strongly influenced by the ecological characteristics of the most abundant species, not by the number of species. The relative importance of a species to ecosystem functioning is determined by its traits and its relative abundance. Thus conserving or restoring the composition of biological communities , rather than simply maximizing species numbers, is critical to maintaining ecosystem services C Local or functional extinction, or the reduction of populations to the point that they no longer contribute to ecosystem functioning, can have dramatic impacts on ecosystem services.
Local extinctions the loss of a species from a local area and functional extinctions the reduction of a species such that it no longer plays a significant role in ecosystem function have received little attention compared with global extinctions loss of all individuals of a species from its entire range. Loss of ecosystem functions, and the services derived from them, however, occurs long before global extinction. Often, when the functioning of a local ecosystem has been pushed beyond a certain limit by direct or indirect biodiversity alterations, the ecosystem-service losses may persist for a very long time C Changes in biotic interactions among species—predation, parasitism, competition, and facilitation—can lead to disproportionately large, irreversible, and often negative alterations of ecosystem processes.
In addition to direct interactions, such as predation, parasitism, or facilitation, the maintenance of ecosystem processes depends on indirect interactions as well, such as a predator preying on a dominant competitor such that the dominant is suppressed, which permits subordinate species to coexist.
Interactions with important consequences for ecosystem services include pollination; links between plants and soil communities , including mycorrhizal fungi and nitrogen-fixing microorganisms; links between plants and herbivores and seed dispersers; interactions involving organisms that modify habitat conditions beavers that build ponds, for instance, or tussock grasses that increase fire frequency ; and indirect interactions involving more than two species such as top predators, parasites, or pathogens that control herbivores and thus avoid overgrazing of plants or algal communities C Many changes in ecosystem services are brought about by the removal or introduction of organisms in ecosystems that disrupt biotic interactions or ecosystem processes.
Because the network of interactions among species and the network of linkages among ecosystem processes are complex, the impacts of either the removal of existing species or the introduction of new species are difficult to anticipate C See Table 1. Table 1. As in terrestrial and aquatic communities , the loss of individual species involved in key interactions in marine ecosystems can also influence ecosystem processes and the provisioning of ecological services.
For example, coral reefs and the ecosystem services they provide are directly dependent on the maintenance of some key interactions between animals and algae. As one of the most species-rich communities on Earth, coral reefs are responsible for maintaining a vast storehouse of genetic and biological diversity. Substantial ecosystem services are provided by coral reefs—such as habitat construction, nurseries, and spawning grounds for fish; nutrient cycling and carbon and nitrogen fixing in nutrient - poor environments; and wave buffering and sediment stabilization.
The total economic value of reefs and associated services is estimated as hundreds of millions of dollars. Yet all coral reefs are dependent on a single key biotic interaction: symbiosis with algae.
Biodiversity affects key ecosystem processes in terrestrial ecosystems such as biomass production , nutrient and water cycling, and soil formation and retention—all of which govern and ensure supporting services high certainty. The relationship between biodiversity and supporting ecosystem services depends on composition, relative abundance, functional diversity , and, to a lesser extent, taxonomic diversity.
If multiple dimensions of biodiversity are driven to very low levels, especially trophic or functional diversity within an ecosystem, both the level and stability for instance, biological insurance of supportive services may decrease CF2 , C Region-to-region differences in ecosystem processes are driven mostly by climate, resource availability, disturbance, and other extrinsic factors and not by differences in species richness high certainty.
In natural ecosystems , the effects of abiotic and land use drivers on ecosystem services are usually more important than changes in species richness. Plant productivity , nutrient retention, and resistance to invasions and diseases sometimes grow with increasing species numbers in experimental ecosystems that have been reduced to low levels of biodiversity.
In natural ecosystems, however, these direct effects of increasing species richness are usually overridden by the effects of climate, resource availability, or disturbance regime C Even if losses of biodiversity have small short-term impacts on ecosystem function, such losses may reduce the capacity of ecosystems for adjustment to changing environments that is, ecosystem stability or resilience, resistance, and biological insurance high certainty.
The loss of multiple components of biodiversity, especially functional and ecosystem diversity at the landscape level, will lead to lowered ecosystem stability high certainty. Although the stability of an ecosystem depends to a large extent on the characteristics of the dominant species such as life span, growth rate, or regeneration strategy , less abundant species also contribute to the long-term preservation of ecosystem functioning.
As tragically illustrated by social conflict and humanitarian crisis over droughts, floods, and other ecosystem collapses, stability of ecosystems underpins most components of human well-being , including health , security, satisfactory social relations, and freedom of choice and action C6 ; see also Key Question 2.
The preservation of the number, types, and relative abundance of resident species can enhance invasion resistance in a wide range of natural and semi-natural ecosystems medium certainty. Although areas of high species richness such as biodiversity hot spots are more susceptible to invasion than species- poor areas, within a given habitat the preservation of its natural species pool appears to increase its resistance to invasions by non-native species. This is also supported by evidence from several marine ecosystems, where decreases in the richness of native taxa were correlated with increased survival and percent cover of invading species C Pollination is essential for the provision of plant-derived ecosystem services , yet there have been worldwide declines in pollinator diversity medium certainty.
Many fruits and vegetables require pollinators, thus pollination services are critical to the production of a considerable portion of the vitamins and minerals in the human diet.
Although there is no assessment at the continental level, documented declines in more-restricted geographical areas include mammals lemurs and bats, for example and birds hummingbirds and sunbirds, for instance , bumblebees in Britain and Germany, honeybees in the United States and some European countries, and butterflies in Europe.
The causes of these declines are multiple, but habitat destruction and the use of pesticide are especially important. Estimates of the global annual monetary value of pollination vary widely, but they are in the order of hundreds of billions of dollars C Biodiversity influences climate at local, regional, and global scales, thus changes in land use and land cover that affect biodiversity can affect climate.
The important components of biodiversity include plant functional diversity and the type and distribution of habitats across landscapes. For example, forests have higher evapotranspiration than other ecosystems, such as grasslands, because of their deeper roots and greater leaf area. Thus forests have a net moistening effect on the atmosphere and become a moisture source for downwind ecosystems. In addition to biodiversity within habitats, the diversity of habitats in a landscape exerts additional impacts on climate across multiple scales.
This air is replaced by cooler moister air that flows laterally from adjacent patches advection. Effects of biodiversity on ecosystem functioning: a consensus of current knowledge Ecological Monographs. By: D. Hooper , F. Chapin III , J. Ewel , A. Hector , P. Inchausti , S. Lavorel , J. Lawton , D. Lodge , M. Loreau , S. Naeem , B. Schmid , H. SetSlS , A.
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