An ecosystem disturbance can be natural or human induced stress. An example of a natural disturbance is a hurricane or a tornado. An example of a human-induced or anthropogenic disturbance is tillage or pesticide application. Redundancy in ecosystem structure and function often infers stability on a system. For instance, if there is more than one redundant population of microbes that convert ammonium to nitrate and a disturbance wipes out one population, that function nitrification will continue to be performed by the remaining populations.
Because agroecosystems have reduced structural and functional diversity, they have less resilience than natural systems Gleissman, The expected outputs from the system yield cannot be sustained without human inputs, therefore humans are a integral part of agroecosystems. A concept related to ecosystem stability is the Intermediate Disturbance Hypothesis, which states that the highest levels of diversity are supported at intermediate levels of disturbance frequency or intensity.
Functioning ecosystems provide humans with essential services, such as clean water and protection from disease. Protecting these services requires managing ecological thresholds.
One step is recognizing things such as damming, intensive farming and grazing can produce complex shifts in nearby ecosystems, especially when human changes are accompanied by unexpected natural events, such as extreme climate events. Thresholds can be managed by supporting stable ecosystems, which are less prone to collapse. The two key components of ecosystem stability are resilience and resistance. Resistance is an ecosystem's ability to remain stable when confronted with a disturbance.
Resilience is the speed at which an ecosystem recovers from a disturbance. For example, resistance refers to a forest's ability to withstand a windstorm; resilience refers to how quickly felled trees would grow back.
Given enough disturbance, systems can cross an ecological threshold that is difficult to reverse. In stable systems, if a windstorm blows down a forest, it recovers after the storm. But if the forest is compromised by invasive pests, and the windstorm is followed by a severe fire and a heavy rainstorm that washes away the soil, a non-forest ecosystem would likely take its place. What can we do to prevent undesirable threshold changes in ecosystems?
Or, put another way, what can we do to increase the resistance and resilience of ecosystems? This is an active topic of research at the Cary Institute; some ideas are emerging. First is to maintain a diversity of plants and animals in an ecosystem. 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.
Ecology Letters 9 , — Fischer J. Compensatory dynamics in zooplankton community responses to acidification: Measurement and mechanisms. Ecological Applications 11 , — Hooper, D. Effects of biodiversity on ecosystem functioning: A consensus of current knowledge and needs for future research. Ecological Monographs 75 , 3—35 Ives, A. Stability and diversity of ecosystems. Science , 58—62 Jactel, H. A test of the biodiversity-stability theory: Meta-analysis of tree species diversity effects on insect pest infestations, and re-examination of responsible factors.
Forest Diversity and Function , — McGrady-Steed, J. Biodiversity regulates ecosystem predictability. Naeem, S. Biodiversity enhances ecosystem reliability.
Declining biodiversity can alter the performance of ecosystems. Sala, O. Global biodiversity scenarios for the year Science , — Schindler, D. Experimental perturbations of whole lakes as tests of hypotheses concerning ecosystem structure and function. Oikos 57 , 25—41 Stork, N.
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