Species turnover differentiates diversity–disturbance relationships between aboveground vegetation and soil seedbank

An understanding of relationships between species diversity and disturbance gradient is important to comprehend the role of disturbances in the structure of plant communities. Although some studies have demonstrated incongruence in diversity–disturbance relationships (hereafter DDRs) between aboveground vegetation and soil seedbank, the process that causes the difference remains unclear. This incongruence between the two DDRs could result in the decrease in the source of recovery of aboveground vegetation following disturbances being overlooked when only aboveground vegetation is surveyed. Here, we verified a process that species turnover across the disturbance gradient causes the incongruence. Based on a vegetation and seedbank survey, we examined DDRs and species turnover of aboveground vegetation and seedbank along disturbance duration (i.e., excluding years of ungulate grazing). The degree of species turnover was considerably greater in aboveground vegetation than in seedbank; thus, the degree of species turnover along a disturbance gradient caused the difference in DDR between aboveground vegetation and seedbank.

     

Disturbance,competition and the maintenance of clonal diversity in Daphnia pulex

Laboratory microcosm experiments tested the intermediate disturbance hypothesis, which states that the highest level of diversity (e.g. species diversity) will be maintained at intermediate scales of disturbance. The effects of disturbance on the maintenance of clonal diversity and on competitive interactions among clones of the obligately parthenogenetic freshwater cladoceran, Daphnia pulex were examined. No significant effect of disturbance size (i.e. dilution volume) on clonal diversity was noted. However, frequency of disturbance had a pronounced effect on clonal diversity, with the highest clonal diversity maintained at low to intermediate disturbance frequencies. Competitive hierarchies among clones were often invariant within a given experiment. Generally, one or two clones dominated, with several less abundant clones persisting throughout an experiment. Results suggest that low to intermediate disturbances could be important in the maintenance of genetic variation in natural populations (i.e. through pre-emption of competitive exclusion between genotypes). This could have a direct bearing on the maintenance of both intra- and interspecific diversity.     

不同强度旅游干扰对黄山松群落物种多样性的影响

研究天柱山国家森林公园不同强度旅游干扰下黄山松群落的物种多样性,用物种丰富度指数S、Simpson指数D、ShannonWiener指数日和Pialou均匀度J比较分析了不同层次物种多样性变化。结果表明：乔木层中,低度干扰下多样性指数较大;其他层次中,中度干扰下多样性指数最高。整个群落分析表明,低度干扰下几种多样性指数分别为38、3．0976、0．9326、0．7515;中度干扰下,多样性指数最高,分别为44、3．2519、0．9421、O．8594,其中灌木层和草本层所起的作用较大;高度干扰下,多样性指数有很大程度的下降,分别为34、3．0095、0．9289、0．7834。利用群落优势度（C）衡量了不同干扰下黄山松群落的稳定性。结果发现中度干扰下的群落稳定性最低。表明中度干扰能一定程度增加群落的物种多样性,但可能会导致群落稳定性降低。     

Moving from pattern to process: coexistence mechanisms under intermediate disturbance regimes

Coexistence mechanisms that require environmental variation to operate contribute importantly to the maintenance of biodiversity. One famous hypothesis of diversity maintenance under disturbance is the intermediate disturbance hypothesis (IDH). The IDH proposes patterns of peaked diversity under intermediate disturbance regimes, based on a tension between competitively superior species and species which can rapidly colonize following disturbance. We review the literature, and describe recent research that suggests that more than one underlying mechanism can generate this unimodal diversity pattern in disturbed environments. Several exciting emerging research areas are identified, including interactions between disturbance types, operation of the IDH in multi‐trophic systems, and changes in disturbance regimes. However, empirical work is still focussed on describing the IDH pattern, with little emphasis on identifying its mechanistic basis. We discuss how to extend methods for identifying different coexistence mechanisms, developed in the theoretical literature, to experimental research. In an attempt to operationalize these various ideas we outline a hypothetical IDH research programme. A solid understanding of the life history attributes of the component species and their responses to disturbance will facilitate identification of the coexistence mechanism(s) underlying the IDH pattern, and provide a framework by which empirical and theoretical results can be more fully integrated.     

高原鼠兔干扰对高寒草甸β多样性的影响

β多样性反映生物群落沿某一环境梯度的物种周转速率, 该研究尝试采用β多样性揭示植物群落随小型啮齿草食动物干扰梯度变化的生态过程。该研究利用野外随机样地的采集数据, 分析了高原鼠兔(Ochotona curzoniae)不同干扰强度下Whittaker指数的变化特征, 并利用群落二元丰富度的方差分解法, 确定了单个物种(SCBD)和单个干扰位点(LCBD)对β多样性的贡献。主要结果: 随高原鼠兔干扰强度增加, 植物群落内物种周转速率呈先增加后降低的趋势; 占据位点数居中的物种对区域内的β多样性贡献较大, 其中冰草(Agropyron cristatum)、臭蒿(Artemisia hedinii)、小花草玉梅(Anemone rivularis var. flore-minore)等单个物种对整个区域内β多样性的贡献最为突出; 整个区域内干扰位点T₀ (高原鼠兔干扰强度为0)对区域β多样性贡献值最大, LCBD值和该位点的群落丰富度呈显著负相关关系, 但与高原鼠兔干扰强度无显著关联。说明重点保护LCBD值高的干扰位点所在的高寒草甸, 以及SCBD值较高的冰草、臭蒿、小花草玉梅, 对保护高原鼠兔存在时高寒草甸植物群落多样性具有重要意义。     

The intermediate disturbance hypothesis does not explain fire and diversity pattern in fynbos

The intermediate disturbance hypothesis is a widely accepted generalization regarding patterns of species diversity, but may not hold true where fire is the disturbance. In the Mediterranean-climate shrublands of South Africa, called fynbos, fire is the most importance disturbance and a controlling factor in community dynamics. The intermediate disturbance hypothesis states that diversity will be highest at sites that have had an intermediate frequency of disturbance and will be lower at sites that have experienced very high or very low disturbance frequencies. Measures of diversity are sensitive to scale; therefore, we compared species richness for three fire regimes in South African mountain fynbos to test the intermediate disturbance hypothesis over different spatial scales from 1 m² to 0.1 hectares. Species diversity response to fire frequency was highly scale-dependent, but the relationship between species diversity and disturbance frequency was opposite that predicted by the intermediate disturbance hypothesis. At the largest spatial scales, species diversity was highest at the least frequently burned sites (40 years between fires) and lowest at the sites of moderate (15 to 26 years between fires) and high fire frequency (alternating four and six year fire cycle). Community heterogeneity, measured both as the slope of the species-area curve for a site and as the mean dissimilarity in species composition among subplots within a site, correlated with species diversity at the largest spatial scales. Community heterogeneity was highest at the least frequently burned sites and lowest at the sites that experienced an intermediate fire frequency.     

Disturbance–diversity models: what do they really predict and how are they tested?

The intermediate disturbance hypothesis (IDH) and the dynamic equilibrium model (DEM) are influential theories in ecology. The IDH predicts large species numbers at intermediate levels of disturbance and the DEM predicts that the effect of disturbance depends on the level of productivity. However, various indices of diversity are considered more commonly than the predicted number of species in tests of the hypotheses. This issue reaches beyond the scientific community as the predictions of the IDH and the DEM are used in the management of national parks and reserves. In order to compare responses with disturbance among measures of biodiversity, we used two different approaches of mathematical modelling and conducted an extensive meta-analysis. Two-thirds of the surveyed studies present different results for different diversity measures. Accordingly, the meta-analysis showed a narrow range of negative quadratic regression components for richness, but not evenness. Also, the two models support the IDH and the DEM, respectively, when biodiversity is measured as species richness, but predict evenness to increase with increasing disturbance, for all levels of productivity. Consequently, studies that use compound indices of diversity should present logical arguments, a priori, to why a specific index of diversity should peak in response to disturbance.     

The effects of vertical mixing on a phytoplankton community: a modelling approach to the intermediate disturbance hypothesis

1.  Connell's (1978) intermediate disturbance hypothesis (IDH) has been proposed as one explanation of why diversity is often highest at intermediate levels of disturbance. We used a model phytoplankton responses to environmental change (PROTECH) to investigate the validity of this hypothesis.
2. In a simulated phytoplankton assemblage of eight species, we found that the relationship between the increased intensity of a single forcing event and diversity was described by a positively skewed curve.
3. A progressive increase in forcing frequency introduced a sharp decrease in diversity at a threshold frequency. However, the highest diversity values were found at an intermediate frequency of disturbance.
4. We described the shape of this breakpoint response as like a 'cliff' and reconcile it with multiple stable-point theory. It is argued that the IDH should possibly be represented by this 'cliff' relationship, which may be applied to (or encourage the re-examination of) many previous studies.     

Diversity and Disturbances in the Antarctic Megabenthos: Feasible versus Theoretical Disturbance Ranges

The intermediate disturbance hypothesis (IDH) predicts a hump-shaped relationship between regional diversity and the disturbance rate. We tested the IDH for the megabenthos inhabiting the Antarctic sea floor, which is disturbed by iceberg scouring. We used models based on the empirical knowledge of succession to calculate the IDH curve for this system and to extrapolate the presently observable range of the IDH curve to higher and lower disturbance rates. Although the hump-shaped relationship has been found for a purely theoretical (extremely large) disturbance range, within the feasible disturbance range (assumed as realistic in the Antarctic region under climate change), the regional diversity of successional stages due to iceberg scouring strongly decreases with lower disturbance rates but levels off only slowly with higher disturbance rates. The reason is the unevenness in the lifetimes of the successional stages, in that early stages are short-lived whereas late stages are long-lived. With such unevenness, increasing disturbances support the early stages without jeopardizing the later ones. Additionally, we converted this regional diversity of stages to the regional diversity of taxa using a transformation formula based on empirical knowledge of the number and mean abundance of taxa in the particular stages. Our results suggest that a decrease in iceberg scouring due to climate change would be more detrimental to the diversity of the Antarctic megabenthos than an increase.     

The intermediate disturbance hypothesis applies to tropical forests, but disturbance contributes little to tree diversity

The intermediate disturbance hypothesis (IDH) predicts local species diversity to be maximal at an intermediate level of disturbance. Developed to explain species maintenance and diversity patterns in species-rich ecosystems such as tropical forests, tests of IDH in tropical forest remain scarce, small-scale and contentious. We use an unprecedented large-scale dataset (2504 one-hectare plots and 331 567 trees) to examine whether IDH explains tree diversity variation within wet, moist and dry tropical forests, and we analyse the underlying mechanism by determining responses within functional species groups. We find that disturbance explains more variation in diversity of dry than wet tropical forests. Pioneer species numbers increase with disturbance, shade-tolerant species decrease and intermediate species are indifferent. While diversity indeed peaks at intermediate disturbance levels little variation is explained outside dry forests, and disturbance is less important for species richness patterns in wet tropical rain forests than previously thought.     

The interplay of stress and mowing disturbance for the intensity and importance of plant interactions in dry calcareous grasslands

Background and Aims There is still debate regarding the direction and strength of plant interactions under intermediate to high levels of stress. Furthermore, little is known on how disturbance may interact with physical stress in unproductive environments, although recent theory and models have shown that this interplay may induce a collapse of plant interactions and diversity. The few studies assessing such questions have considered the intensity of biotic interactions but not their importance, although this latter concept has been shown to be very useful for understanding the role of interactions in plant communities. The objective of this study was to assess the interplay between stress and disturbance for plant interactions in dry calcareous grasslands. Methods A field experiment was set up in the Dordogne, southern France, where the importance and intensity of biotic interactions undergone by four species were measured along a water stress gradient, and with and without mowing disturbance. Key Results The importance and intensity of interactions varied in a very similar way along treatments. Under undisturbed conditions, plant interactions switched from competition to neutral with increasing water stress for three of the four species, whereas the fourth species was not subject to any significant biotic interaction along the gradient. Responses to disturbance were more species-specific; for two species, competition disappeared with mowing in the wettest conditions, whereas for the two other species, competition switched to facilitation with mowing. Finally, there were no significant interactions for any species in the disturbed and driest conditions. Conclusions At very high levels of stress, plant performances become too weak to allow either competition or facilitation and disturbance may accelerate the collapse of interactions in dry conditions. The results suggest that the importance and direction of interactions are more likely to be positively related in stressful environments.     

The effect of colonization and competition processes on the relation between disturbance and diversity in plant communities

Many theoretical and field studies have emphasized the impact of disturbance in the dynamics and diversity of sessile organism communities. This view is best reflected by the Intermediate Disturbance Hypothesis (IDH), which states that a maximum of diversity is found in ecosystems or communities experiencing intermediate disturbance regimes or at an intermediate stage of development since the last major disturbance event. Although theoretical models based on competitive interactions tend to validate this hypothesis, a recent meta-analysis of field experiments revealed that the mono-modal relationship between disturbance and diversity might not be a general pattern. In this article, we investigate the relationship between disturbance and diversity through the study of patch models, combining two types of competitive interactions: with or without competitive hierarchy, with two mechanisms influencing colonization: negative frequency dependence in colonization rates and immigration. These combinations led to various disturbance-diversity patterns. In the model without competitive hierarchy (founder effect model), a decreasing relationship appeared to be the rule as mentioned in previous studies. In the model with competitive hierarchy, the IDH pattern was obtained for low frequency dependence and low immigration. Nevertheless, high negative frequency dependence in colonization rates led to a decreasing relationship between disturbance and diversity. In contrast, high immigration led to an increasing relationship. The coexistence window (the range of disturbance intensity allowing coexistence) was the widest for intermediate immigration rates. For random species assemblages, patterns with multiple peaks were also possible. These results highlight the fact that the mono-modal IDH pattern should not be considered a rule. Competition and colonization mechanisms have a profound impact on the relationship between disturbance and diversity.     

Effects of different traveling disturbances on the species diversity in Pinus taiwanensis communities

The effects of different traveling disturbances on the different layers of the Pinus taiwanensis community in Tianzhu Mountain National Forest Park were studied using the diversity indices of species richness index (S), Simpson index (D), Shannon-Wiener index (H) and Pielou evenness index (J). The results showed that the diversity indices of the arbor layer in low disturbance were the highest. Whereas, the diversity indices of the other layers in intermediate disturbance were the highest. The values of S, D, H, and J in low disturbance were 38, 3.0976, 0.9326, and 0.7515. The values of the indices in intermediate disturbance were 44, 3.2519, 0.9421, and 0.8594, higher than those in low disturbed forest, especially in the shrub and herb layers. In the highly disturbed forest, the diversity indices were 34, 3.0095, 0.9289, and 0.7834, lower than those in the intermediately disturbed forest. By comparing the stability of differently disturbed forests with the community dominance index (C), it was clear that the C of the intermediately disturbed forests was the lowest among the three differently disturbed forests. It is proposed that the intermediately disturbed forests can improve the species diversity, but maybe decrease the community stability.     

Effects of different traveling disturbances on the species diversity in Pinus taiwanensis communities

The effects of different traveling disturbances on the different layers of the Pinus taiwanensis community in Tianzhu Mountain National Forest Park were studied using the diversity indices of species richness index (S), Simpson index (D), Shannon-Wiener index (H) and Pielou evenness index (J). The results showed that the diversity indices of the arbor layer in low disturbance were the highest. Whereas, the diversity indices of the other layers in intermediate disturbance were the highest. The values of S, D, H, and J in low disturbance were 38, 3.0976, 0.9326, and 0.7515. The values of the indices in intermediate disturbance were 44, 3.2519, 0.9421, and 0.8594, higher than those in low disturbed forest, especially in the shrub and herb layers. In the highly disturbed forest, the diversity indices were 34, 3.0095, 0.9289, and 0.7834, lower than those in the intermediately disturbed forest. By comparing the stability of differently disturbed forests with the community dominance index (C), it was clear that the C of the intermediately disturbed forests was the lowest among the three differently disturbed forests. It is proposed that the intermediately disturbed forests can improve the species diversity, but maybe decrease the community stability.     

Species' traits predict the effects of disturbance and productivity on diversity

Disturbance is an important factor influencing diversity patterns. Ecological theory predicts that diversity peaks at intermediate levels of disturbance, but this pattern is not present in a majority of empirical tests and can be influenced by the level of ecosystem productivity. We experimentally tested the effects of disturbance on diversity and show that species' autecological traits and community relations predicted species loss. We found that – alone or in concert – increasing disturbance intensity or frequency, or decreasing productivity, reduced diversity. Our species did not exhibit a clear competition-colonization trade-off, and intrinsic growth rate was a more important predictor of response to disturbance and productivity than measures of competitive ability. Furthermore, competitive ability was more important in predicting responses when, in addition to killing individuals, disturbance returned nutrients to the ecosystem. Our results demonstrate that species' traits can help resolve conflicting patterns in the response of diversity to disturbance and productivity.     

Impact of pocket gopher disturbance on plant species diversity in a shortgrass prairie community

Summary We examined the impact of pocket gopher disturbances on the dynamics of a shortgrass prairie community. Through their burrowing activity, pocket gophers (Thomomys bottae) cast up mounds of soil which both kill existing vegetation and create sites for colonization by competitively-inferior plant species. Three major patterns emerge from these disturbances: First, we show that 10 of the most common herbaceous perennial dicots benefit from pocket gopher disturbance; that is, a greater proportion of seedlings are found in the open space created by pocket gopher disturbance than would be expected based on the availability of disturbed habitat. Additionally, these seedlings exhibited higher growth rates than adjacent seedlings of the same species growing in undisturbed habitat. Second, we tested two predictions of the Intermediate Disturbance Hypothesis and found that species diversity was greatest for plots characterized by disturbances of intermediate age. However, we did not detect significant differences in diversity between plots characterized by intermediate and high levels of disturbance, indicating that many species are adapted to or at least tolerant of high levels of disturbance. Third, we noted that the abundance of grasses decreased with increasing disturbance, while the abundance of dicots increased with increasing disturbance.     

Reciprocal relationships and potential feedbacks between biodiversity and disturbance

Two major foci of ecological research involve reciprocal views of the relationship between biodiversity and disturbance: disturbance determines community diversity or diversity determines realized disturbance severity. Here, we present an initial attempt to synthesize these two approaches in order to understand whether feedbacks occur, and what their effects on patterns of diversity might be. Our review of published experiments shows that (i) disturbance severity can be both a cause and a consequence of local diversity in a wide range of ecosystems and (ii) shapes of the unidirectional relationships between diversity and disturbance can be quite variable. To explore how feedbacks between diversity and disturbance might operate to alter expected patterns of diversity in nature, we develop and then evaluate a conceptual model that decomposes the relationships into component parts, considering sequentially the effect of diversity on disturbance severity, and the effect of realized disturbance on diversity loss, subsequent recruitment, and competitive exclusion. Our model suggests that feedbacks can increase mean values of richness, decrease variability, and alter the patterns of correlation between diversity and disturbance in nature. We close by offering ideas for future research to help fill gaps in our understanding of reciprocal relationships among ecological variables like diversity and disturbance.     

Experimental test of the intermediate disturbance hypothesis: frequency effects of emersion on fouling communities

The validity of predictions derived from the intermediate disturbance hypothesis (IDH) was tested in situ by manipulating mussel dominated Western Baltic fouling communities. Assemblages of two different successional stages, 3 and 12 months old, underwent a 3-month period of disturbance treatment in terms of various frequencies of emersion. Emersion frequency levels ranged from 1×15 to 48×15 min emersion day⁻¹. The study on the 3-month-old communities was repeated in 2 subsequent study years. Species richness, evenness and diversity (Shannon index) were recorded to measure the effects of frequency treatments on community structure.The IDH was confirmed in the first year, when diversity was found to be a unimodal function of the applied emersion frequency gradient. Diversity-disturbance relationships were inverse unimodal or non-significant in the second year, which was true for both successional stages. This ambiguous picture partially confirms the validity of the mechanisms proposed by the IDH, but also shows that their forcing can be masked by fluctuations in environmental parameters, such as climatic conditions. Diversity increased again under severe disturbance conditions, due to a disturbance-induced change in community structure, namely the shift from mussel to algal dominance. This is a new aspect in the discussion concerning disturbance-diversity relationships.     

A Field Test of the Intermediate Disturbance Hypothesis in the Soft‐Bottom Intertidal

The intermediate disturbance hypothesis has had success in explaining changes in local diversity in many habitats. Recent laboratory work has shown that disturbance may act to increase diversity in soft‐bottom marine communities as predicted by the hypothesis. In this paper, we present the results of a field experiment which tested the impacts of physical disturbance on soft‐bottom, intertidal macrobenthic communities. Five disturbance treatments were used, differing in the frequency of applied disturbance events; the highest frequency treatments were dug every week, lowest every eight weeks. The experiment was run for a total of 25 weeks over the winter of 1997/98. Our experiment controlled for differences in recovery time after disturbances. Abundances of Pygospio elegans, Macoma balthica, Hydrobia ulvae and Streblospio benedicti were all significantly reduced in high disturbance treatments, as was the total number of species. No species showed significant increases in abundance in disturbed treatments, and there was no evidence of an increase in diversity in any treatments. These results are discussed in the context of the intermediate disturbance hypothesis. Our results suggest that the interspecific competitive effects postulated by the hypothesis are not important in structuring this low diversity, intertidal community. However, unequivocally rejecting the hypothesis is difficult because it contains many ambiguities, and acts more as a conceptual model than as a falsifiable hypothesis.     

Parasites mediate the relationship between host diversity and disturbance frequency

Patterns of community and population diversity are likely to be dependent on interactions between ecological variables. Here we address how two important ecological variables – extrinsic periodic mortality events (disturbances) and the presence of obligate‐killing parasites – interact to affect the diversity of niche‐specialist genotypes in laboratory populations of the bacterium Pseudomonas fluorescens. Consistent with previous studies, diversity was maximized at intermediate frequencies of disturbance in the absence of parasitic bacteriophages (phages). By contrast, no relationship was found between diversity and disturbance frequency in the presence of phage. The results can be explained in part by differential effects of phage on bacterial densities, and hence resource competition, under different disturbance regimes.