intermediate disturbance hypothesis definition environmental science

Intermediate Disturbance Hypothesis in Ecology: A Literature Review

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Temporal changes in diversity and similarity of a phytoplankton community were investigated in relation to external hydrological disturbance in the Ben Chifley reservoir from September 1998 to January 2002. Species richness varied by a factor of 4–5 at each of three sites studied during the period (n = 53 at each site). Species diversity (measured using Simpson’s D and Shannon–Wiener’s H, based on primarily genus or species number and cell densities) varied by a factor of 8–10, whereas similarity between two consecutive sampling dates (measured using Hurlbert’s index and Pinkham and Pearson’s B) varied by a factor of 10–46. When diversity was measured with H, it had an approximate quadratic (convex) relationship with similarity, as measured with Hurlbert’s index. However, diversity was seldom related to external hydrological disturbance (measured as intensity and variability of daily inflow rates between two consecutive sampling dates). Similarity was significantly and negatively related to disturbance variability. These results suggest that the mechanisms that regulate diversity and similarity may differ from each other, and question the usefulness of observed approximate quadratic relationships between similarity and diversity indices when assessing the effect of disturbance on diversity. Such relationships may therefore not provide support for Connell’s (1978) intermediate disturbance hypothesis.

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Out of the Blue (Ocean)

The adventures of a marine ecologist!

Ecology Explained: The Intermediate Disturbance Hypothesis

The Intermediate Disturbance Hypothesis is one of the staples of ecology, especially marine ecology. The Intermediate Disturbance Hypothesis was first proposed by Connell, a well-known intertidal and general ecologist, in 1978 (See his article “Diversity in Tropical Rain Forests and Coral Reefs”). But what is it exactly? Let me explain!

Let’s start with this handy diagram from Wikipedia.

As you can see, we have Level of Disturbance on the x-axis. That simply describes the level of disturbance present in the system. It could be any sort of disturbance…fires, hurricanes, waves, human or animal trampling, wind, sun, and so on. The level of disturbance increases from left to right. So, the area marked with I has less disturbance than area II.

Species Diversity on the y-axis is more of a general term. Sometimes this is “species richness,” which is a pure count of species present in the ecosystem. Sometimes this is actually referring to “species diversity,” which takes the number of species from species richness and combines it with how the species are distributed in the system. But that’s a subject for another blog post! All you need to know now is that species diversity in the system increases from the bottom to the top.

Now let’s look at those areas marked by Roman numerals.

Area I is first up. In that section, we have a low amount of disturbance, which results in an okay amount of diversity. Why is that?

Ecosystems typically have a successional pathway–a pattern across time of species that are present. Think about a forest directly after a forest fire. Early-regrowth forests are going to look totally different from established forests! Those established forests will most likely have one or more competitive dominants–the species that compete the best. That’s great for the competitive dominants because they do well in those established systems, but it’s not so good for diversity. The competitive dominants compete so well that there isn’t much room for other species.

(Sometimes you will see similar graphs that are a simple bell curve with Area I showing the same low, low diversity that Area III exhibits. This can also happen–if there is no disturbance in the system at all, the species diversity is going to be extremely low. But, from my perception, it’s probably not going to get as bad as Area III. It’s all up to interpretation.)

Area II looks great! There is an intermediate amount of disturbance and the maximum amount of diversity. In Area II, there is enough disturbance in the system to stop the competitive dominants from over-dominating. Organisms earlier in the successional pathway that are poorer competitors (but still play an important role) are able to survive. This results in the maximum amount of species diversity! Hence the name “Intermediate Disturbance Hypothesis!” This area is experiencing an intermediate amount of disturbance, but not enough to push it to Area III.

Area III is not a good place to be. Area III exhibits a very high amount of disturbance and a low amount of diversity. It’s pretty easy to understand why! Think of a coral reef that’s constantly being battered by huge hurricanes or a forest that has repeated, huge fires. The succession pathway barely even gets a chance to begin before another huge disturbance sweeps through. This will result in very low diversity.

So, the Intermediate Disturbance Hypothesis shows us that with an intermediate level of disturbance we can expect a high amount of diversity. With low and high levels of disturbance, not so much!

Do you have a lingering question? Ask it in the comments section and I will be happy to help as best I can!

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9.4: Disturbance

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Ecological Disturbance

Ecological disturbance , is a temporary change in environmental conditions that causes a pronounced change in a community. Disturbances often act quickly and with great effect, to alter the physical structure or arrangement of biotic and abiotic elements. Disturbance plays a significant role in shaping the structure of individual populations and the biodiversity of whole ecosystems.

Minor disturbances include localized wind events, droughts, floods, small wildland fires, and disease outbreaks in plant and animal populations. In contrast, major disturbances include large-scale wind events (such as tropical cyclones), volcanic eruptions, tsunamis, intense forest fires, epidemics, ocean temperature changes stemming from El Niño events or other climate phenomena, and the devastating effects of human impact on the environment (anthropogenic disturbances) such as clear cutting, pollution, or the introduction of invasive species. Not only invasive species can have a profound effect on an ecosystem, but also naturally occurring species can cause disturbance by their behavior.

Disturbance forces can have profound immediate effects on ecosystems and can, accordingly, greatly alter the natural community’s population size or species richness. Because of these and the impacts on populations, disturbance determines the future shifts in dominance, various species successively becoming dominant as their life history characteristics, and associated life-forms, are exhibited over time. The notion of ecological disturbance has deep historical roots in ecological thinking; the first conceptual disturbance-related model in modern ecology was ecological succession , an idea emphasizing the progressive changes in ecosystem structure that follow a disturbance.

The ecological impact of a disturbance is dependent on its intensity and frequency, on the spatial distribution (or the spatial pattern) and size of the disturbed patches, and on the scale (the spatial extent) of the disturbance. Some disturbances may by cyclic. Fire disturbances, for example, occur more often in areas with a higher incidence of lightning and flammable biomass. Other disturbances, such as those caused by humans, invasive species or impact events, can occur anywhere and are not necessarily cyclic. When multiple disturbance events affect the same location in quick succession, this often results in a "compound disturbance," an event which, due to the combination of forces, creates a new situation which is more than the sum of its parts. For example, windstorms followed by fire can create fire temperatures and durations that are not expected in even severe wildfires, and may have surprising effects on post-fire recovery. Diversity is generally low in harsh environments (with frequent or compound disturbances) because of the intolerance of all but opportunistic and highly resistant species to such conditions.

The spatial scale of natural disturbances, which is known to span about 10 orders of magnitude, is also important. For example, a drought that might devastate microbes in a temporary pond would be inconsequential to an elephant. A single tree uprooted by a hurricane is a disaster for the resident ants, but it may become a necessary resource for forest frogs as sufficient water collects around the root cavity. Likewise, while a fire may decimate wildlife populations and scorch large areas of land, the cones of the northern jack pine ( Pinus banksiana ), which are tiny in comparison, require the presence of fire to open ( Figure \(\ PageIndex {1}\) ).

The recovery process for species removed by a disturbance is critically dependent on its dispersal capability and the distance between the disturbed site and surviving source populations. Immediately after a disturbance there is a pulse of recruitment or regrowth under conditions of little competition for space or other resources. These conditions may favor the success of different species than the pre-disturbed community. After the initial pulse, recruitment slows since once an individual species is established it may be very difficult to displace. However, in the absence of further disturbance forces, many communities trend back toward pre-disturbance conditions. Long lived species and those that can regenerate in the presence of their own adults finally become dominant.

Some species are particularly suited for exploiting recently disturbed sites. Vegetation with the potential for rapid growth can quickly take advantage of the lack of competition. In the northeastern United States, shade-intolerant trees like pin cherry and aspen quickly fill in forest gaps created by fire or windstorm (or human disturbance). Silver maple and eastern sycamore are similarly well adapted to floodplains. They are highly tolerant of standing water and will frequently dominate floodplains where other species are periodically wiped out. Species that are well adapted for exploiting disturbance sites are referred to as pioneers. They usually grow and reproduce quickly, but may not be the strongest competitors and are usually replaced by other, more competitively dominant species over time.

The interplay between disturbance and biological processes seems to account for a major portion of the organization and spatial patterning of natural communities. Although disturbances tend to negatively affect populations of resident plants, animals, and other organisms in a given ecosystem, they provide some fugitive species with opportunities to move into and gain footholds in ecosystems whose biological communities once excluded them. This process results in an increase in the biodiversity of the ecosystem.

Intermediate Disturbance Hypothesis

Disturbance variability and species diversity are heavily linked. The success of a wide range of species from all taxonomic groups is closely tied to natural disturbance events such as fire, flooding, and windstorm. Diversity tends to be low in harsh environments because of the intolerance of all but opportunistic and highly resistant species to such conditions. The interplay between disturbance and these biological processes seems to account for a major portion of the organization and spatial patterning of natural communities.

The intermediate disturbance hypothesis (IDH) suggests that local species diversity is maximized when ecological disturbance is neither too rare nor too frequent. At low levels of disturbance, more competitive organisms will push subordinate species to extinction and dominate the ecosystem. At high levels of disturbance, due to frequent forest fires or human impacts like deforestation, all species are at risk of going extinct. According to IDH theory, at intermediate levels of disturbance, diversity is thus maximized because species that thrive at both early and late successional stages can coexist (Figure \(\PageIndex{2}\)). IDH is a nonequilibrium model used to describe the relationship between disturbance and species diversity.

IDH is based on the following premises:

• ecological disturbances have major effects on species richness within the area of disturbance,
• interspecific competition results from one species driving a competitor to extinction and becoming dominant in the ecosystem, and
• moderate ecological scale disturbances prevent interspecific competition.

Disturbances act to disrupt stable ecosystems and clear species' habitat. As a result, disturbances lead to species movement into the newly cleared area. Once an area is cleared there is a progressive increase in species richness and competition between species takes place. Once the conditions that create a disturbance are gone, and competition between species in the formerly disturbed area increases, species richness decreases as competitive exclusion increases. Another explanation is that proposed by Joseph H. Connell who suggested that relatively low disturbance leads to decreased diversity while high disturbance causes an increase in species movement. These proposed relationships lead to the hypothesis that intermediate disturbance levels would be the optimal amount of disorder within an ecosystem.

Another way of thinking about the IDH requires that we consider the types of organisms that could specialize in areas with different levels of disturbance. K-selected species generally demonstrate more competitive traits. Their primary investment of resources is directed towards growth, causing them to dominate stable ecosystems over a long period of time. In contrast, r-selected species colonize open areas quickly and can dominate landscapes that have been recently cleared by disturbance. These characteristics attribute to the species that thrive in habitats with higher and lower amounts of disturbance. Based on the contradictory characteristics of both of these examples, areas of occasional disturbance allow both r and K species to flourish in the same area. If K-selected and r-selected species can live in the same region, species richness can reach its maximum.

• Dial, R.; Roughgarden, J. (1988). "Theory of marine communities: the intermediate disturbance hypothesis". Ecology . 79 (4): 1412–1424.
• Wilkinson, David M. (1999). "The Disturbing History of Intermediate Disturbance". Oikos . 84 (1): 145–7.
• Kricher, John C. (2011). Tropical Ecology . New Jersey, Princeton: Princeton University Press.
• Catford, Jane A.; Daehler, Curtis C.; Murphy, Helen T.; Sheppard, Andy W.; Hardesty, Britta D.; Westcott, David A.; Rejmánek, Marcel; Bellingham, Peter J.; et al. (2012). "The intermediate disturbance hypothesis and plant invasions: Implications for species richness and management". Perspectives in Plant Ecology, Evolution and Systematics . 14 (3): 231–41.
• Vandermeer, John; Boucher, Douglas; Perfecto, Ivette; de la Cerda, Inigo Granzow (1996). "A Theory of Disturbance and Species Diversity: Evidence from Nicaragua After Hurricane Joan". Biotropica . 28 (4): 600–13.
• Connell, J. H. (1978). "Diversity in Tropical Rain Forests and Coral Reefs". Science . 199 (4335): 1302–10.
• Hall, A. R.; Miller, A. D.; Leggett, H. C.; Roxburgh, S. H.; Buckling, A.; Shea, K. (2012). "Diversity-disturbance relationships: Frequency and intensity interact" . Biology Letters . 8 (5): 768–71.

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• 18.2: What are the Effects of Disturbance? from Ecology for All! by Joshua Halpernunder a CC BY-NC-SA license

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• 23 October 2020

Joseph H. Connell (1923–2020)

• Jane Lubchenco 0 &
• Wayne P. Sousa 1

Jane Lubchenco is a professor in the Department of Integrative Biology at Oregon State University in Corvallis. She spent a formative year in Connell’s lab in 1970–71.

You can also search for this author in PubMed   Google Scholar

Wayne P. Sousa is a professor of integrative biology at the University of California, Berkeley. He joined Connell’s lab in 1973, completing both a master’s and a PhD.

You have full access to this article via your institution.

Credit: Tad Theimer

Joseph (Joe) Connell altered both what and how ecologists study. Tree by tree, coral by coral, barnacle by barnacle, he saw patterns and processes across diverse ecosystems. Simply and with incontrovertible evidence, he demonstrated that interactions such as competition and predation could determine where species lived.

Before his classic experiments on Scotland’s rocky shores, field ecology was mainly descriptive, focusing on physical conditions such as temperature or moisture in determining where species lived. Connell, who died last month aged 96, inspired thousands of ecologists to test their hypotheses by manipulating conditions in the field.

Connell established long-term studies of coral reefs at Heron Island in the Great Barrier Reef and of tropical rainforests in Queensland, Australia, that spanned more than three and five decades, respectively. Monitoring revealed the dynamic nature of plant and animal communities that had long been considered stable. He discovered that natural variability in biological interactions and physical factors maintains diversity in these and other endangered ecosystems.

Born in 1923, Connell grew up just outside Pittsburgh, Pennsylvania. When the United States entered the Second World War in 1941, he enlisted in the Army Air Corps and was trained in meteorology. Later, conducting weather surveillance in the Azores — the Portuguese Atlantic archipelago — in support of army operations in Europe, he spent his free time birdwatching and identifying trees. Meeting army recruits who worked as wildlife managers, he realized it was possible to have a career as a biologist. After the war, and a degree in meteorology at the University of Chicago, Illinois, he headed to the University of California, Berkeley, for a master’s in zoology.

Connell produced what he described as a dull, unsatisfying thesis on brush rabbits ( Sylvilagus bachmani ) in the Berkeley Hills. Discouraged by the difficulties of conducting a population study (he trapped only 40 rabbits in 2 years), he adopted a rule of thumb — never again to study anything bigger than his thumb. As a doctoral student at the University of Glasgow, UK, he gleefully discovered what Charles Darwin had found a century before: that thousands of barnacles could easily be studied on the seashore, no traps required.

Connell realized that he could test his hypotheses about what factors determined where on the shore certain species lived by removing, adding or transplanting barnacles and their snail predators. Classic papers ensued, inspiring other ecologists to rethink distribution patterns, and, importantly, to test their ideas with controlled field experiments.

After a postdoc at the Woods Hole Oceanographic Institution in Massachusetts, Connell joined the faculty at the University of California, Santa Barbara, where he remained for the rest of his career. He was curious about processes that affected distribution and abundance, and those that might keep biodiversity high. Shifting to species that live for hundreds or thousands of years on coral reefs and in rainforests, he set up his Australian long-term monitoring studies in 1962 and 1963. Both recorded the demography and interactions of organisms in permanent plots, tracking community dynamics and the impact of disturbances, ranging from fallen trees to cyclones.

Visiting Connell’s sites with him in the 1970s and 1990s, we were impressed with his foresight and inspired by his insights. On the reef, he explained, physical disturbance by large waves associated with recurring cyclones intermittently reduced the cover of dominant species such as staghorn coral ( Acropora aspera ). This prompted recolonization by a diverse assemblage of weaker competitors such as encrusting or mound-like species. Connell coined the term ‘intermediate disturbance hypothesis’ to describe this process.

We strolled through the larger of his rainforest plots, avoiding stinging trees, biting flies, ticks and leeches, and relishing the richness — more than 300 tree species and about 100,000 individual plants. Connell outlined another hypothesis, that forests are more diverse when rarer species such as the conifer Sundacarpus amarus are favoured over more common ones such as the flowering tree Planchonella sp. Patterns of seedling establishment, growth or survival depend on that difference in frequency. Because common species grow more densely than rare ones, they are more vulnerable to specialist herbivores or pathogens.

This pattern of density-dependent predation or infection thins out common species, enabling a richer mix to coexist. It is a central component of the Janzen–Connell hypothesis (independently proposed by US ecologist Daniel Janzen in 1970), which predicts that seedlings are more likely to die under the canopies of their parent trees than farther away, ensuring diversity.

Connell was unfailingly kind, generous and devoted to his family. He never lost his profound curiosity about the natural world or his delight in exploring ideas with students and colleagues. He loved to be challenged and, if proven wrong, he gladly moved on to a new hypothesis or question. He sought truth, not fame. Moreover, he empowered everyone around him to think critically by focusing on ideas and evidence, not personalities. Fortunately for the world, his way of exploring science proved powerful, infectious, fun and enduringly productive.

Nature 586 , 670 (2020)

doi: https://doi.org/10.1038/d41586-020-02990-2

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Anthropogenic disturbances are key to maintaining the biodiversity of grasslands

1 State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau, Institute of Soil and Water Conservation, Northwest A&F University, Yangling, Shaanxi 712100, PR China

2 Institute of Soil and Water Conservation, Chinese Academy of Science and Ministry of Water Resource, Yangling, Shaanxi 712100, China

3 Institute of Grassland Research, Chinese Academy of Agricultural Sciences, Hohhot, Inner Mongolia, 010010, PR China

Robert L. Kallenbach

4 University of Missouri, Division of Plant Sciences, 108 Waters Hall, Columbia, MO 65211, USA

Associated Data

Although anthropogenic disturbances are often perceived as detrimental to plant biodiversity, the relationship between biodiversity and disturbance remains unclear. Opinions diverge on how natural diversity is generated and maintained. We conducted a large-scale investigation of a temperate grassland system in Inner Mongolia and assessed the richness-disturbance relationship using grazing intensity, the primary anthropogenic disturbance in the region. Vascular plant-species richness peaked at an intermediate level of anthropogenic disturbance. Our results support the Intermediate Disturbance Hypothesis, which provides a valid and useful measure of biodiversity at a metacommunity scale, indicating that anthropogenic disturbances are necessary to conserve the biodiversity of grassland systems.

Anthropogenic disturbances often cause habitat loss, ecological fragmentation, and loss of biodiversity 1 . Ecological models and public policies put a heavy emphasis on the negative effects of grazing but often fail to acknowledge the potentially positive effects of grazing on biodiversity in grassland ecosystems. The Intermediate Disturbance Hypothesis (IDH) proposes that within a broad range of environmental disturbance levels, species diversity is maximized at an intermediate level of anthropogenic and natural disturbances, because competitively inferior, disturbance-tolerant species and competitively dominant, disturbance-sensitive species coexist when disturbances are neither too rare nor too frequent 2 , 3 . With low levels of disturbance, richness is predicted to be low because of competitive exclusion. With high levels of disturbance, richness is predicted to be low, because most species cannot tolerate frequent destructive events. With intermediate levels of disturbance, richness is predicted to be high, however, because dominant competitors and rapid colonizers are able to coexist 4 , 5 .

Connell 3 first introduced the IDH with supporting data from tropical rain forests and coral reefs in 1978. Many ecology textbooks feature the IDH. Connell’s original paper on the topic has received more than 4,000 citations in the past 40 years and is still referenced in important scientific papers at an increasing rate, according to ISI Web of Science. The IDH has been extensively validated with both observational and experimental data 6 , 7 , 8 , 9 , 10 . Although the IDH is widely used to explain species diversity patterns, the considerable circumstantial evidence gathered in the past 40 years both for and against the IDH has led to controversy 11 , 12 , 13 , 14 , 15 . Debate about the IDH has led to healthy discussions in the scientific community and prompted investigators to assess experimental designs before collecting data, as Kallenbach 16 proposed, in order to better evaluate scientific models.

An increasing number of critiques reject the IDH as an explanation of spatial patterns of species diversity. Recently, Fox 11 argued that the IDH fails on both empirical and theoretical grounds and so should be absolutely abandoned. In contrast, Huston 15 stated that Fox made fundamental errors, creating oversimplified caricatures of the IDH by ignoring all of the coexistence-promoting mechanisms discussed by Connell 3 . The controversy exemplifies the limits to our ability to predict ecosystem responses to human disturbance. We still do not know whether the IDH represents an important mechanism of species coexistence in the real world. Furthermore, the recent arguments about the IDH 11 , 15 highlight the importance of refining how we develop and test ecological theories.

The IDH model suggests that any community can reach maximum diversity through multiple mechanisms 5 , 12 , which can be described biologically or mathematically and might vary with locality and trophic level 4 , 17 . One general assumption of the IDH is that trade-offs between competitive ability and colonization ability facilitate the disturbance-mediated coexistence of competitively superior species, which are unable to thrive in highly disturbed sites, and colonizer species, which can be outcompeted by competitively superior species in less-disturbed sites 12 , 18 .

Competitively superior and colonizer species can, however, coexist between those extremes, leading to higher biodiversity at intermediate disturbance levels 3 , 12 . Factors such as species’ ability to utilize and partition resources, often reflected in productivity, are involved in the mechanisms that promote species coexistence 12 , 19 . As Kallenbach 20 explained, plants such as tall fescue ( Lolium arundinaceae L.) that form endophytic relationships with fungi are more able to persist in the face of harsh environmental conditions and overgrazing than plants that do not form endophytic relationships. Tall fescue thus gains a competitive advantage in certain ecosystems, which disturbance models must account for. Alternative theoretical models, including the ‘storage effect’ 5 and ‘successional niche’ 21 models, suggest that coexistence can occur when disturbances create spatiotemporal niches in which competitively inferior species gain novel competitive advantages over otherwise competitively dominant species.

Species richness varies with spatial scale 22 , 23 , 24 , 25 , 26 , suggesting that competition and other density-dependent processes might be unimportant to determining patterns of species richness at certain spatial scales. Many reports that are critical of the IDH tested the diversity-disturbance relationships across multiple biomes 27 or, more commonly, at small spatial scales within communities 28 , 29 , 30 . Compared with anthropogenic disturbances, other biotic and abiotic factors that impact diversity at the regional scale change slowly. Sampling vegetation at the regional scale therefore provides an ideal opportunity to test the IDH. The IDH was originally developed for coral reefs and tropical rain forests and was later found to apply to other terrestrial and aquatic (marine) systems with only natural disturbances 31 , 32 , 33 . Some studies have tested the diversity patterns in arid and semi-arid grasslands 27 , 34 , 35 , but often on a large scale over multiple biomes or on a small scale within communities. Few studies have tested the diversity pattern predicted by the IDH at the regional scale.

We surveyed the vascular plant species near 100 local community plots scattered across 62,500 km 2 of arid grasslands in the Inner Mongolian area of northern China ( Supplementary materials , Google Earth KMZ file) 36 and tested whether species diversity displays a normally distributed bell curve to disturbances, as the IDH predicts. The local communities of trophically interacting species occupy discrete resource patches linked by dispersal and can therefore be viewed as a metacommunity 37 , 38 i.e., a set of interacting local communities which are linked by the dispersal of multiple, potentially interacting species. Because the primary anthropogenic disturbance in the region is livestock (i.e., sheep and goat) grazing, we used the grazing intensity within a 5 km radius of each community as a direct measure of anthropogenic disturbance. The grazing intensity was greatest near the most populous towns and least in a natural reserve, where livestock had been excluded for 35 years. The relationship between species richness and disturbance level was unimodal with a peak at an intermediate level of disturbance, supporting the IDH.

Unimodal curves best described the relationships between the vascular plant-species richness and the human disturbance level ( Fig. 1 ). Although we found similar diversity-disturbance trends when we expressed the species diversity using the Shannon-Weiner index and the Simpson index ( Supplementary materials, Figs S1 and S2 ), both the Shannon-Weiner index and the Simpson index had lower R-square values than the richness index. Neither the mean annual temperature nor the precipitation was significantly associated with the species richness in a Poisson regression model. In contrast, both the human population size and the intensity of livestock disturbance were positively and significantly associated with the species richness ( Table 1 ). The species richness peaked at a human population density of 35 individuals per km 2 . A simple quadratic curve ( r 2  = 0.570, p  < 0.001) provided the best fit for the relationship between species richness and the major livestock disturbance in the region, grazing by sheep and goats. The unimodal model revealed that the species richness peaked at the stocking rate of 360 sheep/goats km −2 . Simple unimodal models also provided the best description of the vascular plant-species richness in relation to other livestock-related disturbances, including those from cattle and horses. When the total grazing intensity from all livestock was considered, a simple quadratic curve ( r 2  = 0.604, p  < 0.001) demonstrated that species richness peaked at 480 animals km −2 (see the calculation in the Methods). The livestock disturbance attributed to the sheep and goats was a more accurate predictor of species richness than that attributed to the cattle and horses.

The relationships are best described by non-linear regression (quadratic models, dark grey lines). Grey shade refers to Loess smoothing with 95% confidence intervals.

MAT and MAP stand for mean annual temperature and precipitation, respectively.

In general, the plant species diversity was significantly associated with the human population size and the livestock disturbance but not with the climate ( Table 2 ). The climate data (mean annual temperature and precipitation, MAT and MAP, respectively) collectively explained 17% of the variation in species richness. The climate data combined with the human population size and the livestock disturbance explained 62% of the variation in species richness. Thus, the climate and the level of anthropogenic disturbance, particularly the latter, made a major contribution to the observed variation in species richness. The climate and anthropogenic disturbance level contributed similar fractions to the Shannon-Weiner index and the Simpson index ( Table 2 ).

Overall model significance is: ns (not significant, P  > 0.05), *( P  < 0.05), **( P  < 0.01) or ***( P  < 0.001). The ‘Climate’ models include mean annual temperature (MAT) and mean annual precipitation (MAP). The ‘Disturbance’ models include densities of humans and livestock. The ‘Climate + disturbance’ models include the two sets combined.

In accordance with their life cycles, annual plant species often occupy recently disturbed areas 39 . Therefore, the biomass of the annual vascular plants should reflect the disturbance intensity. We found that species richness peaked at an intermediate level of annual plant biomass ( r 2  = 0.227, p  < 0.001). There was a simple unimodal relationship between the species richness and the biomass of unpalatable species like Artemisia annua L. and A. sieversiana L. ( Fig. 2 ). Both of those species are unpalatable to livestock and tend to dominate the recently disturbed patches of the study region.

Overgrazing by livestock on the grasslands of Inner Mongolia has become an important issue over the past several decades 40 , 41 . We found a unimodal relationship between the vascular plant-species richness and disturbance by herbivores in Inner Mongolian grassland systems, in contrast to other studies that did not observe a diversity peak at intermediate disturbance levels 11 , 42 . Our results support the IDH and reinforce the view that anthropogenic disturbance cannot be abandoned as a regulatory force in species structuring 15 .

Our survey covered multiple levels of a wide disturbance gradient (i.e., grazing intensity) 36 , 43 . In contrast, the narrow range of disturbances examined in many previous studies could have missed the species diversity peak at intermediate disturbance levels by collecting data at disturbance levels either too small or too great to capture the full range of responses. When Connell 3 first proposed the IDH, his evidence came from a Ugandan forest that followed a successional sequence gradient from no disturbance (low diversity) to intensified disturbance (high diversity). Testing the IDH requires a detection system with sufficient sampling to cover both the rising and the falling sections of the curve.

Connell 3 suggested that the IDH “deals only with variations in diversity within local areas”. The “local areas” do not refer to “large-scale geographical gradients such as tropical to temperate differences” or to sites within small communities, but rather to intermediate sized areas on a regional scale. Presumably the 35,200 ha Budongo rainforest 44 that Connell used as reference data could be called a metacommunity, which is composed of local communities 37 , 45 . Unfortunately, studies to test the IDH often make their conclusions based on large-scale sampling, such as sampling on a global scale 27 , or on inadequate, small-scale sampling 11 , 42 , 46 (<100 ha). Hence, results from the IDH model are often contradictory and might be related to the scales used in the various studies.

Although we did not make a scale-related comparison in this study, our results at the metacommunity scale suggest that the results of other studies conducted at global or local scales might be attributed to the size of the sampling area. Diversity-related processes at smaller scales, such as those within a single community, or at larger scales, such as those across biomes, might differ as disturbance mechanisms provide different outcomes as a function of scale 15 , 47 , 48 , 49 . Intra-community observations cannot capture variation in species richness, whereas inter-biome observations cannot detect the richness-disturbance pattern predicted by the IDH. We collected all of our samples under similar climatic and soil conditions. Furthermore, we minimized the impacts of other coexistence mechanisms that factor into larger spatial scales.

The logic of the IDH is not wrong. The supposed empirical failure of the IDH might be due to a failure to consider the conditions under which the IDH is valid. The unimodal (quadratic), intermediate disturbance pattern can only be detected within a specific range of productivity and mortality conditions 15 , 50 . If data are collected along a disturbance gradient from sites that differ greatly in productivity, the effect of the disturbance on diversity might be completely obscured. As the Dynamic Equilibrium Model predicts 51 , the level of productivity would control the effects of disturbance on diversity.

The goal of the IDH is not to demonstrate the long-term stable coexistence of species under the equilibrium conditions of mathematical theory, but rather to explain or predict patterns of species diversity that can be measured. There are so many mechanisms involved in coexistence that it is impossible to reveal whether long-term stable coexistence of species occurs in the real world 15 . The richness-disturbance pattern observed in our study, as predicted by the IDH, suggests that we still need to figure out the underlying mechanisms that contribute to competition, coexistence, and speciation over different spatial and temporal scales.

For appropriate tests of the IDH, it is important to use appropriate measures of disturbance and diversity 9 . Connell did not address how to define or measure disturbance regimes. In the Inner Mongolian grassland ecosystems, domestic livestock grazing is the primary anthropogenic disturbance that shapes species diversity and productivity. The grazing intensity is a direct measure of disturbance and is better than the indirect indices used in most studies, such as those for vegetation cover, canopy height, species density of pioneer trees, heliophilic stems, and basal area 31 , 32 , 52 , 53 , 54 . Frequency, intensity, extent, duration, and time since last disturbance 12 are common terms used to describe an environmental disturbance. Based on our observations, grazing intensity was the dominant anthropogenic disturbance. The sites in our study differed in disturbance intensity but were similar in many other attributes such as climate and soil.

We found that human encroachment favored some species over others; the recently disturbed patches were dominated by annual Artemisia species that livestock find unpalatable. The species diversity within the region was unimodally related to the biomass of the annual Artemisia species, suggesting that those species play a role in species coexistence mechanisms (Figs S3 and S4). The undisturbed sites might have lower diversity because the dominant species inhibit the establishment and succession of less-fit species through competitive exclusion 55 .

The observed, unimodal diversity-disturbance relationship in our study suggests that certain prerequisites of the IDH; such as competitive exclusion, successional stages, and trade-offs between competition and tolerance 3 , 56 ; occur along the grazing gradient in the studied sites. Our findings also indicate that the vegetation in the arid regions of Inner Mongolia is not resistant to grazing, and that the effects of grazing on species diversity vary as a function of the condition of the landscape (i.e., environmental stress).

The human population density explained more of the diversity than the livestock population density. Why that should be the case is unclear, but one possibility is that the human population causes disturbances not only through livestock grazing but also through trampling, fires, digging for medical herbs, the collection of grassland products, and the establishment of camps and roads. All of those activities can cause physical, and hence ecological, disturbances to grassland areas, potentially influencing diversity.

Although the various methods to estimate diversity introduced in the literature might respond differently to disturbances 9 , 42 , we found similar diversity-disturbance relationships for the species richness, Shannon-Weiner, and Simpson indices. The species richness index had the highest R-square values, suggesting that measure is more relevant to the predictions of the IDH than the Shannon-Weiner index, which was in turn better than the Simpson index. Our results suggested that the number of species estimated by the species richness index, rather than dominance estimated by the Shannon-Weiner and Simpon indices, is an appropriate response variable for the system in tests of the IDH. We could not determine the underlying mechanisms to explain the relationship between magnitude of disturbance and specific measures of biodiversity. However, our findings clearly indicate species richness is the important aspect of diversity and it changes in response to disturbance.

Our simple, unimodal, diversity-disturbance models for the typical grassland systems in Inner Mongolia explain how vascular plant-species richness varies with anthropogenic influences on a regional scale. Governmental agencies could use the models generated here to manage the biodiversity of the natural grasslands that cover more than 40% of the region 57 . Consistent with experimental studies of sheep grazing intensity 58 , our results revealed that vascular plant-species richness peaks at a moderate level of grazing intensity (480 sheep km −2 ). Thus the species richness of the grassland ecosystems of Inner Mongolia would be best conserved by properly managed grazing with an acceptable stocking rate rather than by the complete exclusion of livestock. The balanced coexistence of the plants and animals common to the region defines the ideal grassland system. Our results indicate that domesticated livestock grazing has excessively disturbed around 8% of the Inner Mongolian grasslands. Like grassland systems worldwide, those in Inner Mongolia can tolerate grazing disturbance, but only in moderation.

Although several scientists challenge the IDH 11 , 42 , our results suggest that the IDH provides a useful explanation of the response of biodiversity to disturbance at the regional (or metacommunity) scale. The IDH is one potential explanation of an observed unimodal pattern between species richness and disturbance. That does not exclude the involvement of other mechanisms, because climate and anthropogenic disturbances together explained only two-thirds of the variation in diversity in our study. The IDH suggests that disturbance contributes to biodiversity, although it might be difficult to detect the unimodal pattern in improperly scaled communities 31 , 59 . Our results indicate that the IDH is useful, albeit requiring greater precision in its definition. Many of the criticisms of the IDH are misguided, because they do not recognize the underlying logical assumptions of the hypothesis and consequently fail to test the hypothesis appropriately. The IDH holds in nature under specific conditions and thus remains a conceptually useful model. In an increasingly “disturbed” world 60 , 61 , many will find it ironic that the preservation of biodiversity depends on both encouraging and limiting disturbances within ecological thresholds.

The study area was situated in a typical temperate grassland near Xilin Guole (44°56′N, 115°22′E), Inner Mongolia, China. The region’s climate is arid and cold with an average temperature of 1.4 °C and precipitation of 301 mm annually ( Supplementary materials , excel file). Most of the soils in the region are classified as dark chestnut (Mollisols) according to FAO system of soil classification. The study area has a long history of grazing by domestic livestock under nomadic or seminomadic patterns of land use. The types of livestock are sheep, goats, horses, cattle, and occasionally donkeys.

We selected 100 sites and surveyed the richness of vascular plant species within each site during August 2013. The sites were selected in a grid with grid center spacing ~10 km and were owned by herdsmen. The widely distributed sites represent more than 62,500 km 2 of arid grasslands ( Supplementary materials , Google Earth KMZ file). We assessed human disturbance by measuring the intensity of livestock grazing, the primary disturbance in the region. Natural disturbances such as fire and insect damage, especially the former, are not common in the region and were not considered as anthropogenic disturbances. To avoid missing rare species, we chose a ~1 ha sampling area at each of the 100 sites and randomly imposed three 5 × 5 m sampling plots at each sampling area. We identified and recorded all vascular plant species. We set up a 1 × 1 m subplot within each sampling plot and harvested the aboveground biomass. We investigated the intensity of livestock grazing by visiting herdsman and the related government Bureau of Animal Husbandry within a ~5 km radius around the sampling plots. Because different types of livestock consume different amounts of forage, we estimated the total effect of grazing disturbance by a standardized livestock number: total grazing intensity = (number of sheep × 1) + (number of goats × 0.9) + (number of cattle × 6) + (number of horses × 7) + (number of donkeys × 3) 62 , 63 .

Data analysis

Species richness was expressed as the number of vascular plant species present within a plot. Species diversity was expressed by the Shannon-Weiner index [−(Σ p i ln p i )] or by Simpson’s index [1/(Σ p i 2 )], where p i is the proportion of total vascular plant cover contributed by species i . We performed linear and polynomial (quadratic and cubic) regression analyses to determine the best-fit shape of the species richness–disturbance relationship. The parameters of the Poisson regression were estimated using the R package glmmADMB . We also applied Loess smoothing using the R package ggplot2 to assess possible nonlinearity between species richness and disturbance. All statistical analyses were performed using R 3.2.2.

Additional Information

How to cite this article : Yuan, Z. Y. et al. Anthropogenic disturbances are key to maintaining the biodiversity of grasslands. Sci. Rep. 6 , 22132; doi: 10.1038/srep22132 (2016).

Supplementary Material

Acknowledgments.

Thanks to Q Deng and T Li for their assistance with the fieldwork and to YH Li for his assistance with the grazing investigation. Financial support from the National Natural Science Foundation of China (NSFC) (31130008, 31370455 and 31570438) and the Hundred Talents Program of CAS is gratefully acknowledged.

Author Contributions Z.Y.Y. contributed to conceiving the research, performing the analysis, and writing the first draft of the manuscript. Z.Y.Y. and J.F. collected the species richness data. Y.H.L. provided vital data on grazing intensities. R.L.K. contributed substantially to interpreting the analysis and writing the manuscript.

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2.5 Natural Disruptions to Ecosystems

2 min read • december 27, 2022

Joshua Nielsen

Change is a Constant

Our Earth, due to species interference and the life cycles, experiences constant changes. Some are slow and always evolving, some are fast and sporadic, and some are somewhere in the middle. Effects that these processes have can be drastic or negligible to certain ecosystems, and can happen to have short-term or longer-term effects. Manmade, or anthropogenic , disasters may be equal to any natural counterparts.

These alterations in Earth's happenings can be random (like a lightning strike from one storm), seasonal (the months in which hurricanes occur to form a hurricane season), or by episode (like stages of a volcanic eruption). For example, glaciers and climate change are contributing to sea level rise , which increases flooding in coastal cities or communities. These are manmade disasters that can have great effects on ecosystems. Naturally, disasters such as the cold can cause migrations, such as geese in Canada finding similar conditions further south to escape snow and ice storms during winter.

Resistance and Resilience

Resistance is a measurement of how much an ecosystem changes after a disruption (forest fire, invasive species...).  If there is little change, the ecosystem has a high resistance and is considered to be quite stable. That is to say, this ecosystem would bear the brunt of the disaster without experiencing much internal damage.

Resilience is the measure of how quickly the ecosystem can ‘bounce back’ from the disturbance. It is a measure of how quickly an ecosystem can recover and rebuild its environment.

Intermediate Disturbance Hypothesis

Image courtesy of Wikipedia

Maximum species diversity is reached when an ecosystem experiences an intermediate level of disturbance. It is because both early and late succession species are able to survive at the same time. An early succession species (grasses, shrubs) is one that is able to exist in an ecosystem first, requiring fewer nutrients and existing with less interspecies competition. A late succession species is the opposite, and has found the ecosystem more recently. So, an ecosystem with a moderate amount of disturbance (detectable, not destructive!) will allow all succession levels to survive the best they can. 🎥 Watch: AP Environmental Science Streams

Key Terms to Review ( 7 )

Anthropogenic

Early Succession Species

Ecosystem Resilience

Ecosystem Resistance

Late Succession Species

Sea Level Rise

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4.2 The intermediate disturbance hypothesis

The realisation that the frequency of disturbance can influence community structure led to the formulation of the intermediate disturbance hypothesis (IDH) (Connell, 1978; Figure 22 below). The IDH proposes that species diversity is generally maximised if disturbance is neither too rare nor too frequent because species that thrive at both early and late successional stages can coexist.

A hypothetical line graph that shows how species diversity changes with frequency of disturbance. The horizontal axis is labeled ‘frequency of disturbance’ and the vertical axis is labeled ‘species diversity’. There are no quantitative markings on the vertical axis. On the horizontal axis, the extreme left-hand side is labeled ‘low’ and the extreme right hand side ‘high’. The line of the graph starts on the bottom left, increases almost linearly, reaches a peak and then decreases linearly to reach a low value again. So, it is an inverted U-shape. Underneath the graph is a series of three pictures of vegetation. The one on the left which is aligned with the left-hand side of the graph, shows a landscape with mostly grass interspersed with a few small trees. The central picture, which is aligned with the peak on the graph shows a mixture of low vegetation such as grass and shrubs, small trees and larger trees. The third picture which is aligned with the right-hand side of the graph shows mostly large trees. The graph and the pictures together illustrate that at a low frequency of disturbance (LHS of the graph, LH picture), species diversity is low; at an intermediate frequency of disturbance (peak on graph, central picture) diversity is high and reaches a peak; and at a high frequency of disturbance (RHS of graph, RH picture) diversity is once again low.

Connell’s original paper has received more than 4000 citations and is still referenced in important scientific papers. Many studies have empirically validated the IDH, particularly for marine systems. However, there are also an increasing number that show little support and this has led to a great deal of controversy regarding its validity in explaining the relationship between disturbance and species diversity. For example, it can be argued that tropical forests show high diversity even though natural disturbance is minimal.

Whether or not the IDH holds appears to depend, to some degree, on scale (whether on a local or geographical scale) and the type of disturbance. For example, at a small local scale species diversity is often maximised at a high frequency of fire rather than at an intermediate or low frequency.

Give an example where frequent fire is necessary to maintain species diversity.

The Fynbos in the Cape region of South Africa.

However on a larger scale, disturbances of intermediate frequency and/or intensity may generate diversity. For example, a fire of intermediate or mixed severity could increase diversity by generating spatial heterogeneity within a landscape.

This is because a mixed severity fire will result in a complex of patches in a landscape. Patches differ in severity of burn and include unburned patches, low severity burn patches, moderate severity burn patches where perhaps one-third to two-thirds of the vegetation is killed, and high severity burn patches where almost all the vegetation is killed (Figure 23). In this case, the IDH may not hold on a local scale (within patches) because species richness within a small area may decline (for example, in severely burnt areas). However, on a larger landscape scale a fire of intermediate severity may increase species diversity. This is because areas or patches that differ in burn severity, and that are at different stages of post-fire change, would sustain more species with different disturbance sensitivities.

A greyscale image that shows the burn severity. White patches show low severity burn, grey patches show moderate severity burn, and black patches show high severity burn.

Although the IDH is controversial, the idea that landscapes with greater heterogeneity in size, age, and burn severity of post-fire patches support a greater diversity of species, has led some researchers to propose that a diversity of fire regimes across a landscape (named pyrodiversity ) is necessary to maintain biodiversity.