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Environmental Heterogeneity, Species Diversity and Co-Existence at Different Spatial Scales

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Environmental Heterogeneity, Species Diversity and Co-Existence at Different Spatial Scales Journal of Vegetation Science 21: 796–801, 2010 DOI: 10.1111/j.1654-1103.2010.01185.x & 2010 International Association for Vegetation Science FORUM Environmental heterogeneity, species diversity and co-existence at different spatial scales Riin Tamme, Inga Hiiesalu, Lauri Laanisto, Robert Szava-Kovats & Meelis Pa¨rtel Abstract individual plants, which forage through the patches, The positive relationship between spatial environ- species diversity can be either positively or nega- mental heterogeneity and species diversity is a tively affected by a change in the mean of an widely accepted concept, generally associated with environmental factor. (3) Heterogeneity as a sepa- niche limitation. However, niche limitation cannot rate niche axis: the ability of species to tolerate account for negative heterogeneity–diversity rela- heterogeneity at spatial scales smaller than plant tionships (HDR) revealed in several case studies. size varies, affecting HDR. We conclude that pro- Here we explore how HDR varies at different spatial cesses other than niche limitation can affect the scales and provide novel theories for small-scale relationship between heterogeneity and diversity. species co-existence that explain both positive and Keywords: Community ecology; Grain; Meta-analysis; negative HDR. At large spatial scales of heteroge- Niche limitation; Small and large scale; Spatial neity (e.g. landscape level), different communities heterogeneity. co-exist, promoting large regional species pool size and resulting in positive HDR. At smaller scales Abbreviations: HDR 5 heterogeneity–diversity rela- within communities, species co-existence can be tionship. enhanced by increasing the number of different patches, as predicted by the niche limitation theory, Introduction or alternatively, restrained by heterogeneity. We conducted meta-regressions for experimental and The relationship between spatial environmental observational HDR studies, and found that negative heterogeneity and species diversity (HDR) is one of HDRs are significantly more common at smaller the principal concepts of community ecology, but spatial scales. We propose three theories to account has not yet been studied thoroughly (Wilson 2000). for niche limitation at small spatial scales. (1) The term ‘‘heterogeneity’’ is rarely defined precisely, Microfragmentation theory: with increasing spatial leading to additional ambiguity (Sparrow 1999; heterogeneity, large homogeneous patches lose Lundholm 2009). According to Ettema & Wardle areaandbecomeisolated,whichinturnrestrainsthe (2002), ‘‘spatial heterogeneity is variability in spatial establishment of new plant individuals and popula- structure, such that spatial distributions are not tions, thus reducing species richness. (2) Heteroge- uniform or random, but aggregated (patchy, neity confounded by mean: when heterogeneity clumped).’’ Although it is widely thought that het- occurs at spatial scales smaller than the size of erogeneous environments should maintain more species than homogeneous ones, empirical evidence of the true trend of HDR is surprisingly scarce. In a Tamme, R. (corresponding author, [email protected]), thorough review of the literature of the past century, Hiiesalu, I. ([email protected]), Laanisto, L. (lauri. Lundholm (2009) found only 41 observational and [email protected]), Szava-Kovats, R. (robert.szava-kovats @ut.ee) & Pa¨rtel, M. ([email protected]): Institute of 11 experimental case studies. Contrary to conven- Ecology and Earth Sciences, University of Tartu, Lai 40, tional theory, negative HDR relationships have also 51005, Estonia. been reported. Laanisto, L.: Museum of Natural Sciences, CSIC, The premise of positive HDR is based on clas- Department of Biodiversity and Evolutionary Biology, sical niche theory, which expects species to prefer C/Gutierrez Abascal 2, ES-28006, Madrid, Spain; La- particular resource conditions, and habitat hetero- boratorio Internacional en Cambio Global, UC-CSIC, geneity to allow species co-existence. It is possible to Departemento de Ecologı´a, Facultad de Ciencias Biolo´- distinguish a-andb-niches (Pickett & Bazzaz 1978; gicas, PUC, Alameda 340, PC 6513677, Santiago de Ackerly & Cornwell 2007). a-niches are components Chile, Chile. of the species niche associated with factors occurring Environmental heterogeneity, species diversity and spatial scale 797 at small (within community) scales, whereas (Wilson 2000). Since biotic processes function b-niches refer to the species responses to factors mostly at small spatial scales, biotic heterogeneity occurring at large spatial scales (Pickett & Bazzaz might be more relevant to species co-existence. 1978; Ackerly & Cornwell 2007). Spatial hetero- However, most case studies consider only abiotic geneity occurs simultaneously at different scales. At heterogeneity at relatively large scales. larger spatial scales, heterogeneity is expressed Lundholm (2009) suggests that new theories are mainly as gradients of environmental factors. In- needed to account for the diverse results from HDR dividual patches within communities, which can studies, and that this topic remains unresolved. influence species co-existence, can be distinguished Here, we explore how different HDR theories are at smaller scales (Ettema & Wardle 2002). ‘‘Small supported using a meta-regression of HDR and scale’’ usually refers to within-community or spatial scale (grain instead of focal scale), propose a-scale, whereas ‘‘large scale’’ deals with inter-com- novel theories to describe both positive and negative munities or b-scale processes. The border between HDR with respect to species co-existence, and small and large spatial scale varies among studies, provide insights for future research. because of different community structures, and remains ambiguous. Species distribute themselves along environ- Meta-Analysis and the Effect of mental gradients or habitat types according to their Spatial Scale b-niche, resulting in co-existence of communities, rather than species. For species co-existence, the HDR is expected to be positive at larger spatial a-niche is more important (Silvertown 2004). scales due to habitat selection (i.e. b-niche differ- Additionally, studies at large spatial scales might entiation). At smaller scales the limitation of encompass different vegetation types (e.g. grass- a-niches would be supported if HDR is also posi- lands and forests), which should be analysed tive. However, non-existent or even negative HDR separately. For example, grasses and trees probably at smaller spatial scales demands alternative the- respond to heterogeneity at different spatial scales, ories to explain how species co-existence is inhibited since their root systems exhibit spatial differences by heterogeneity (negative HDR at small scale). We (Schenk & Jackson 2002). This difference also leads used meta-analyses of the experimental and ob- to diverse associations between heterogeneity and servational studies collected by Lundholm (2009) to vegetation type (Pa¨rtel et al. 2008). test this idea. We applied meta-regressions with Lundholm (2009) distinguishes different spatial mixed effects (unrestricted maximum likelihood scale parameters: extent (size of total area within model) between HDR pattern and grain (Borenstein which samples are located); grain (scale at which et al. 2009). All analyses were performed using heterogeneity is measured, i.e. the patch size, gen- Comprehensive Meta-Analysis v. 2 (Biostat Inc.). erally deemed the average spatial distance between Experimental and observational studies were neighbouring environmental measurements); and analysed separately. To quantify HDR, different focal scale (scale at which species diversity is mea- parameters were designated as effect size. Standard sured, e.g. vegetation sample plot size). Lundholm difference in diversity means (between homo- (2009) studied how HDR might vary according to geneous and heterogeneous treatments) served as focal scale. Although grain and focal scale are cor- effect size for experimental studies and Fisher’s Z related in some studies, this correlation is not (transformed from correlation coefficient in order to universal. We suggest that grain is more important obtain normal distribution) for observational stu- for species co-existence than other scale parameters. dies. If studies provided no correlation coefficient, Species respond to environmentally distinct patches P- and t-values were transformed to Fisher’s Z. rather than abstract heterogeneity indices. Also, the Only a single measure of heterogeneity and species factors causing heterogeneity vary with spatial scale diversity was used for each experiment or descrip- (Pickett et al. 2000; Ettema & Wardle 2002). Abiotic tion, since different measures are usually correlated heterogeneity is caused mainly by climate, local to- and can bias statistical tests. If a study was con- pography or parent material (Hodge 2005), and thus ducted at different sites (observational studies) or occurs at relatively large spatial scales (Pickett et al. varied in configuration of heterogeneity (experi- 2000). Biotic heterogeneity is typically caused by soil mental studies), one data point from each site or microorganisms (Ettema & Wardle 2002) or herbi- different experimental heterogeneity configuration vores (e.g. Augustine & Frank 2001), or through was used. Thus, some studies provided several in- patchy resource uptake and release by plants dependent data points. 798 Tamme, Riin et al. (a) (b) 3.5 2.0 3.0 1.5 2.5 2.0 1.0 1.5 1.0 0.5 0.5 Fisher's
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