Diversity-disturbance relationships: frequency and intensity interact

An influential ecological theory, the intermediate disturbance hypothesis (IDH), predicts that intermediate levels of disturbance will maximize species diversity. Empirical studies, however, have described a wide variety of diversity-disturbance relationships (DDRs). Using experimental populations of microbes, we show that the form of the DDR depends on an interaction between disturbance frequency and intensity. We find that diversity shows a monotonically increasing, unimodal or flat relationship with disturbance, depending on the values of the disturbance aspects considered. These results confirm recent theoretical predictions, and potentially reconcile the conflicting body of empirical evidence on DDRs.     

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.     

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.     

Comparing Neutral and Trade-off Community Models in Shaping the Community Biomass-Diversity Relationship Under Different Disturbance Levels

Among numerous mechanisms shaping the unimodal relationship between diversity and community biomass, the trade-off model of “CRS” theory is the most famous one. However, recent researches indicate that this relationship may also emerge under the neutral model where all species are identical with each other. By using an individual-based spatially-explicit model, we evaluated the underlying mechanisms shaping this curve for both models under different disturbance levels. We found unimodal relationships emerged for both models at low and medium disturbance levels; the richness for the trade-off community was lower than the neutral community for most of the environment severity levels, especially at the benign environment due to the strong competitive exclusions among species. Whereas under high disturbance level, the positive relationships emerged for both models; both communities had similar richness with their curves nearly overlapped with each other, that is, because the high disturbance intensity strongly decreased the competitive exclusions within the trade-off community. Our results indicate that although the underlying mechanisms are totally different, both models will produce the similar relationship between diversity and community biomass under different disturbance levels.     

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.     

Evolutionary coexistence in a fluctuating environment by specialization on resource level

Microbial communities in fluctuating environments, such as oceans or the human gut, contain a wealth of diversity. This diversity contributes to the stability of communities and the functions they have in their hosts and ecosystems. To improve stability and increase production of beneficial compounds, we need to understand the underlying mechanisms causing this diversity. When nutrient levels fluctuate over time, one possibly relevant mechanism is coexistence between specialists on low and specialists on high nutrient levels. The relevance of this process is supported by the observations of coexistence in the laboratory, and by simple models, which show that negative frequency dependence of two such specialists can stabilize coexistence. However, as microbial populations are often large and fast growing, they evolve rapidly. Our aim is to determine what happens when species can evolve; whether evolutionary branching can create diversity or whether evolution will destabilize coexistence. We derive an analytical expression of the invasion fitness in fluctuating environments and use adaptive dynamics techniques to find that evolutionarily stable coexistence requires a special type of trade-off between growth at low and high nutrients. We do not find support for the necessary evolutionary trade-off in data available for the bacterium Escherichia coli and the yeast Saccharomyces cerevisiae on glucose. However, this type of data is scarce and might exist for other species or in different conditions. Moreover, we do find evidence for evolutionarily stable coexistence of the two species together. Since we find this coexistence in the scarce data that are available, we predict that specialization on resource level is a relevant mechanism for species diversity in microbial communities in fluctuating environments in natural settings.     

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.     

Effects of disturbance on species diversity: a multitrophic perspective

Models of the effects of disturbance on ecological communities have largely considered communities of competing species at a single trophic level. In contrast, most real communities have multiple interacting trophic levels. I explored several versions of simple single- and multitrophic models to determine whether predictions of the intermediate disturbance hypothesis (IDH), derived from considering only a single trophic level, apply to multitrophic situations. The IDH was predicted by models of competing species at a single trophic level but did not hold in many situations with more natural trophic structure. In general, basal species in a food web tended to follow the IDH, whereas competitors at top trophic levels did not. Additional analyses indicated that outside immigration interacted with trophic structure to produce widely differing predictions about the consequences of disturbance and that density-dependent disturbance events could recapture the IDH in some multiple trophic level situations. Model predictions matched the results of empirical studies to date: the IDH has generally been supported for species competing for nondynamic basal resources but not for mobile aquatic invertebrates at higher trophic levels. The model analysis also verified basic predictions of verbal models addressing the effects of physical stress. Three different aspects of disturbance and their contributions to species coexistence were identified: changes in average mortality rates, changes in temporal variability, and changes in spatial heterogeneity. The results indicate that the IDH should be applied with caution to real multitrophic communities.     

Issues affecting the measurement of disturbance response patterns in herbaceous vegetation – A test of the intermediate disturbance hypothesis

The 'intermediate disturbance hypothesis' (IDH) predicts maximum species diversity at intermediate levels of disturbance. Disturbance levels are measured by intensity or frequency of disturbance or by the time since disturbance. The IDH has been tested over a wide range of studies and communities with results either accepting or rejecting the hypothesis. The objectives of this study were to determine how observed disturbance response patterns for the same herbaceous plant community are influenced by modelling techniques, particularly in relation to the expression of disturbance gradients, choice of species diversity indices, and time of assessment since disturbance. Response patterns were examined using a multi-factorial disturbance experiment involving grazing (absent and present); fire (absent and present); soil cultivation (none, 5 and 20 cm); and amendment (none, fertiliser, and fertiliser plus clover seeds). Generalized linear models with a b-spline function were used to define response patterns for five different disturbance gradients over a 24-month period using three indices of diversity. Seven basic disturbance response models were recognized ranging from the classical IDH bell-shaped pattern through increasing or decreasing trends, to a no-change model with increasing levels of disturbance. Only 6.7% of all models were consistent with the IDH. The no-change model was found in nearly half the cases investigated and increased in occurrence with time since disturbance. The choice of the disturbance intensity gradient, the species diversity index used and the time of assessment after disturbance significantly influenced the frequency of occurrence of the disturbance response models observed. Consequently, the responses of vegetation to disturbance gradients show many patterns depending on how they are defined and modelled rather than the simple bell-shaped curve as predicted by the IDH, but the ecological mechanisms supporting some of these patterns need to be further investigated.     

The interaction between nutrient availability and disturbance frequency on the diversity of benthic marine communities on the north-east coast of England

1. Several theoretical models predict under what conditions maximum species diversity can be maintained, and they are often used to develop effective ecosystem management plans. 2. Two models that are currently used to predict patterns of species diversity were empirically tested in marine subtidal benthic communities of different successional stages. 3. The two models were: the interactive effects of nutrient availability and disturbance frequency proposed by Kondoh (2001; Proceedings of the Royal Society London B, 268, 269-271), and the intermediate disturbance hypothesis (IDH) proposed by Connell (1978; Science, 199, 1302-1310). 4. Interactive effects were found to be transient and only occurred in the older communities, while the unimodal pattern suggested by the IDH was not supported in either successional stage. 5. It is concluded that these models are very general and thus lack sufficient explanatory power. Both models require a number of specific prerequisites for maximum diversity to be found, and though applicable in many different ecosystems they need to be refined as tools in order that they can be effectively used in habitat management plans.     

Scaling Disturbance Instead of Richness to Better Understand Anthropogenic Impacts on Biodiversity

A primary impediment to understanding how species diversity and anthropogenic disturbance are related is that both diversity and disturbance can depend on the scales at which they are sampled. While the scale dependence of diversity estimation has received substantial attention, the scale dependence of disturbance estimation has been essentially overlooked. Here, we break from conventional examination of the diversity-disturbance relationship by holding the area over which species richness is estimated constant and instead manipulating the area over which human disturbance is measured. In the boreal forest ecoregion of Alberta, Canada, we test the dependence of species richness on disturbance scale, the scale-dependence of the intermediate disturbance hypothesis, and the consistency of these patterns in native versus exotic species and among human disturbance types. We related field observed species richness in 1 ha surveys of 372 boreal vascular plant communities to remotely sensed measures of human disturbance extent at two survey scales: local (1 ha) and landscape (18 km²). Supporting the intermediate disturbance hypothesis, species richness-disturbance relationships were quadratic at both local and landscape scales of disturbance measurement. This suggests the shape of richness-disturbance relationships is independent of the scale at which disturbance is assessed, despite that local diversity is influenced by disturbance at different scales by different mechanisms, such as direct removal of individuals (local) or indirect alteration of propagule supply (landscape). By contrast, predictions of species richness did depend on scale of disturbance measurement: with high local disturbance richness was double that under high landscape disturbance.     

Coupling effects of altitude and human disturbance on landscape and plant diversity in the vicinity of mountain villages of Beijing, China

Cone-cores discarded by Eurasian red squirrels were used to study the habitat selection of Korean pine-seeds hoarding, in forest patch Nos. 16 and 19 in Liangshui Nature Reserve, China. Ten transects with a total length of 15 km were uniformly set, and data from 343 valid samples were collected in a 369 hm² area. One hundred and eighty four were hoarding samples which were determined according to the cluster analysis based on the number of the cone-cores, while the other 159 were control samples. The principal component analysis, using 11 habitat factors, suggested that the distance from Korean pine forest, forest type, number of Korean pine seedlings, density and type of bush significantly influenced the habitat selection of hoarding by Eurasian red squirrels. The results of Bailey’s method indicated that the squirrels showed (1) preference for natural coniferous forest, natural fir and spruce forest and planted spruce forest; (2) avoidance of planted Korean pine forest and planted larch forest; and (3) random use of natural Korean pine forest. Moreover the distance from the Korean pines in the range of 150–600 m showed no effect on the habitat selection of hoarding by the Eurasian red squirrels. More than 50% of the cone-cores were discarded in either fringe or gap of the Korean pine forest with more cone-cores found at <300 m than at 300 m away (One-Way ANOVA; df = 3, 183, F = 5.76, p = 0.0009). This demonstrated that the Eurasian red squirrels could take the cone-cores out of the Korean pine forest. The density of bushes in samples of hoarding area was significantly lower than that in control samples (Kruskal–Wallis test; df = 1, χ² = 83.99, p < 0.0001). The number of the Korean pine seedlings in samples of hoarding area was significantly higher than that in the control samples (Kruskal–Wallis test; df = 1, χ² = 104.13, p < 0.0001). This illustrated that the hoarding habitat favoured the germination of the Korean pine seedlings. In conclusion the behavior of hoarding Korean pine seeds by the Eurasian red squirrels can promote the regeneration and dispersal of the Korean pines.     

Coupling effects of altitude and human disturbance on landscape and plant diversity in the vicinity of mountain villages of Beijing, China

Villages represent the source of human disturbance in mountain regions, but how they alter surrounding landscapes and further affect plant diversity distribution along altitudinal gradient is still not well documented. Although the unimodal pattern along latitudinal gradient and the hypothesis of immediate disturbance (IDH) have been supported in many studies, their coupling effects on plant diversity distribution have been given less attention. In this paper, the coupling effects surrounding mountain villages were detected: (1) altitude determined the mountain landscape at regional scale, while human disturbance altered landscape fragmentation and diversity at local scale surrounding villages. (2) With the reducing human disturbance away from villages, plant diversity decreased, then increased, and finally decreased in different land uses. The plant diversity of shrubs reached the lowest. With the increasing altitude, plant diversity of the forest represented the unimodal trend, but other land use types had different properties. (3) The mechanism of the coupling effects is that the combination of topographic and soil factors determines plant diversity distribution at both landscape and plant community levels surrounding villages.     

The response of benthic rhizopods to sediment disturbance does not support the intermediate disturbance hypothesis

Disturbance is one of the mechanisms which counteract competitive exclusion of populations in resource-limited communities, thereby facilitating coexistence and maintaining community species diversity. The intermediate disturbance hypothesis predicts maximum diversity at intermediate disturbance intensities and frequencies. This paper reports results of an experimental test of this hypothesis using a coastal benthic community of rhizopods (Protozoa: Rhizopoda), and experimental sediment resuspension as a simulated natural disturbance. We carried out two experiments of 5 d duration which focussed on the effects of resuspension intensity and frequency, respectively, on the abundance, species richness and on the Shannon-Weaver diversity index of rhizopod communities in surface sediments of natural sediment cores from the coastal southern Baltic. Care was taken to adjust the experimental treatments to the natural disturbance regime in this area.
Twenty-four and 28 rhizopod species were present during the intensity and frequency experiment, respectively. Small bacterivorous rhizopods of the Vannellidae, Cochliopodidae, Paramoebidae and Rhizopoda incertae sedis dominated the communities during both experiments. Rhizopod abundance, species richness and diversity increased towards the end of the intensity experiment, but they did not show effects of disturbance intensity. Similarly, no effects of disturbance frequency were found during the frequency experiment. Our results indicate that coexistence and community diversity maintenance in benthic rhizopod communities, and probably in benthic heterotrophic protistan communities in general, may rely on different mechanisms than intermediate disturbance, such as trophic niche separation and high rates of dispersal and colonisation.     

Plant species richness and community productivity: why the mechanism that promotes coexistence matters

This paper stresses that the mechanism of coexistence is the key to understanding the relationship between species richness and community productivity. Using model plant communities, we explored two general kinds of mechanisms based on resource heterogeneity and recruitment limitation, with and without any trade-off between reproductive and competitive abilities. We generated different levels of species richness by changing model parameters, in particular the number of species in the regional pool, the degree of recruitment limitation, and the level of heterogeneity. Different diversity–productivity patterns are obtained with different coexistence mechanisms, indicating there is no reason to expect any general relationship between species richness and productivity. We discuss these results in the context of the within-site and across-site aspects of the relationship between species richness and productivity. Furthermore, we extend these results to hypothesize the relationship between species richness and productivity for other coexistence mechanisms not explicitly considered here.     

陆生食肉动物竞争与共存研究概述

陆生食肉动物(食肉目哺乳动物, 以下简称食肉动物)作为食物链与营养级的高位物种对维持生态系统结构与功能稳定性起到重要作用。过度人类干扰已在全球范围内造成食肉动物种群数量剧烈下降和栖息地质量显著退化, 探究食肉动物的区域共存机制对理解生物群落构建、濒危物种保护与管理具有重要意义。本文通过梳理100余篇有关食肉动物在空间、时间和营养3个生态位维度上相互作用的研究, 分析了体型大小、猎物组成、种群结构、环境差异、人类干扰和气候变化等因素对食肉动物种间关系和区域共存的影响, 并对今后食肉动物区域共存研究中亟需解决的问题进行了展望。食肉动物通过生态位分离达到共存并没有单一的理论解释, 猎物、栖息地和人类干扰等因素可以调节食肉动物相互作用关系并直接或间接地影响共存, 共同适应在食肉动物区域共存中具重要作用。食肉动物区域共存是经过长期演化形成的相对稳定状态, 需要以动态的眼光去审视。要明晰生态位重叠与区域共存机制的区别与联系, 在理解生态位分离的基础上, 结合生活史、家域和行为等对食肉动物共存进行综合分析。     

Exposure of non-target vertebrates to second-generation rodenticides in Britain, with particular reference to the polecat

The Intermediate Disturbance Hypothesis [IDH] and the Gradual Climate Change Hypothesis [GCC] offer intuitively appealing, verbal non-equilibrium explanations to species coexistence in competitive communities, but so far they lack a solid theoretical background and a proper experimental methodology. To make them testable and comparable on a solid methodological basis, they should be formulated as well-defined non- equilibrium community dynamical models. We suggest that this is possible, if explicit assumptions on the spatiotemporal structure of the environment and the pattern-generating mechanisms of the species assemblage in question are given. In the framework of a non-spatial population dynamical model we show that disturbance and climate change effects can be safely distinguished, and the "intermediate" level of external effects leading to maximum community diversity can be quantified. Based on the information statistical analysis of field data and simulation results, we explain why it is necessary to consider simultaneously the spatiotemporal patterns of the vegetation, the abiotic environment and the disturbances in order to predict the consequences of external effects regarding community diversity.     

Predicting Plant Diversity Response to Disturbance: Applicability of the Intermediate Disturbance Hypothesis and Mass Ratio Hypothesis

Predicting the relationships between disturbance, biodiversity and productivity of ecosystems continue to preoccupy ecologists and resource managers. Two hypotheses underpin many of the discussions. The Intermediate Disturbance Hypothesis (IDH), which proposes that biodiversity peaks at intermediate levels of disturbance, is often extended to predict that productivity follows the same response pattern. The Mass Ratio Hypothesis (MRH) proposes that the biological traits of the dominant species are the critical drivers of ecosystem function (e.g., productivity) and that these species increase in biomass rapidly after disturbance then stabilize. As a consequence, species diversity first peaks then declines after disturbance as a few species dominate the site. Both provide a conceptual link among disturbance, species diversity and productivity (an index of ecosystem function). We assessed the current state of empirical support for these two hypotheses with a literature survey and determined if their conformance is related to ecosystem type or site productivity. Conformance of IDH reported in past reviews (considering all ecosystems) ranged from 16 to 21%. This contrasts with our finding that in terrestrial ecosystems conformance to IDH was 46% (22 of 48 studies), 17% studies reported non-compliance, and 23% reported inconclusive results. Most studies explained their results with respect to IDH or MRH. Only two studies were specifically designed to test the validity of IDH or MRH. We conclude that (i) the IDH is mostly applicable to predict species diversity response to disturbance in upland sites of medium to high productivity and the MRH is applicable to organic sites of low productivity; (ii) there is a critical need for more studies specifically designed to test these hypotheses in natural ecosystems using common protocols; and (iii) enhanced understanding of these models will add value in refining management policies and in the selection of meaningful diversity indicators of sustainability.     

The effects of species pool,dispersal and competition on the diversity–productivity relationship

Aim The diversity–productivity relationship is a controversial issue in ecology. Diversity is sometimes seen to increase with productivity but a unimodal relationship has often been reported. Competitive exclusion was cited initially to account for the decrease of diversity at high productivity. Subsequently, the roles of evolutionary history (species pool size) and dispersal rate have been acknowledged. We explore how the effects of species pool, dispersal and competition combine to produce different diversity–productivity relationships. Methods We use a series of simulations with a spatially explicit, individual‐based model. Following empirical expectations, we used four scenarios to characterize species pool size along the productivity gradient (uniformly low and high, linear increase and unimodal). Similarly, the dispersal rate varied along the productivity gradient (uniformly low and high, and unimodal). We considered both neutral communities and communities with competitive exclusion. Results and main conclusions Our model predicts that competitive interactions will result in unimodal diversity–productivity relationships. The model often predicts unimodal patterns in neutral communities as well, although the decline in richness at high productivity is less than in competing communities. A positive diversity–productivity relationship is simulated for neutral communities when the species pool size increases with productivity and the dispersal rate is high. This scenario is probably more widespread in nature than the others since positive diversity–productivity relationships have been observed more frequently than previously expected, especially in the tropics and for woody species. Our simulated effects of species pool, dispersal and competition on diversity patterns can be linked to empirical observations to uncover mechanisms behind the diversity–productivity relationship.     

Alpine vascular plant species richness: the importance of daily maximum temperature and pH

Species richness in the alpine zone varies dramatically when communities are compared. We explored (i) which stress and disturbance factors were highly correlated with species richness, (ii) whether the intermediate stress hypothesis (ISH) and the intermediate disturbance hypothesis (IDH) can be applied to alpine ecosystems, and (iii) whether standing crop can be used as an easily measurable surrogate for causal factors determining species richness in the alpine zone. Species numbers and standing crop were determined in 14 alpine plant communities in the Swiss Alps. To quantify the stress and disturbance factors in each community, air temperature, relative air humidity, wind speed, global radiation, UV-B radiation, length of the growing season, soil suction, pH, main soil nutrients, waterlogging, soil movement, number of avalanches, level of denudation, winter dieback, herbivory, wind damage, and days with frost were measured or observed. The present study revealed that 82% of the variance in␣vascular species richness among sites could be explained by just two abiotic factors, daily maximum temperature and soil pH. Daily maximum temperature and pH affect species richness both directly and via their effects on other environmental variables. Some stress and disturbance factors were related to species richness in a monotonic way, others in an unimodal way. Monotonic relationships suggest that the harsher the environment is, the fewer species can survive in such habitats. In cases of unimodal relationships (ISH and IDH) species richness decreases at both ends of the gradients due to the harsh environment and/or the interaction of other environmental factors. Competition and disturbance seemed only to play a secondary role in the form of fine-tuning species richness in specific communities. Thus, we concluded that neither the ISH nor the IDH can be considered useful conceptual models for the alpine zone. Furthermore, we found that standing crop can be used as an easily measurable surrogate for causal factors determining species richness in the alpine zone, even though there is no direct causality.