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.     

Does disturbance regime change community assembly of angiosperm plant communities in the boreal forest?

Aims To examine if and how species and phylogenetic diversity change in relation to disturbance, we conducted a review of ecological literature by testing the consistency of the relationship between phylogenetic diversity and disturbance and compared taxonomic groups, type of disturbance and ecosystem/habitat context. We provide a case study of the phylogenetic diversity–disturbance relationship in angiosperm plant communities of a boreal forest region, compared with types of natural and anthropogenic disturbances and plant growth forms.Methods Using a large-scale sampling plot network along a complete (0–100%) anthropogenic disturbance gradient in the boreal biome, we compared the changes of angiosperm plant community structure and composition across plots. We estimated natural disturbance with historical records of major fires. We then calculated phylogenetic diversity indexes and determined species richness in order to compare linear and polynomial trends along disturbance gradients. We also compared the changes of community structure for different types of anthropogenic disturbances and examined how the relationships between species and phylogenetic diversity and disturbance regimes vary among three different life forms (i.e. forbs, graminoids and woody plants).Important findings Phylogenetic diversity was inconsistently related to disturbance in previous studies, regardless of taxon, disturbance type or ecosystem context. In the understudied boreal ecosystem, angiosperm plant communities varied greatly in species richness and phylogenetic diversity along anthropogenic disturbance gradients and among different disturbance types. In general, a quadratic curve described the relationship between species richness and anthropogenic disturbance, with the highest richness at intermediate anthropogenic disturbance levels. However, phylogenetic diversity was not related to disturbance in any consistent manner and species richness was not correlated with phylogenetic diversity. Phylogenetic relatedness was also inconsistent across plant growth forms and different anthropogenic disturbance types. Unlike the inconsistent patterns observed for anthropogenic disturbance, community assembly among localities varying in time since natural disturbance exhibited a distinct signature of phylogenetic relatedness, although those trends varied among plant growth forms.     

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.     

Seasonal reversals of upland-riparian diversity gradients in the Sonoran Desert

Flood disturbance and water resource availability vary sharply over time and space along arid‐region rivers and can interact in complex fashion to shape diversity patterns. Plant diversity showed spatial patterning along a topogradient from the floodplain of the San Pedro River (Arizona, USA) to the arid upland, but the patterns shifted temporally as the suite of limiting factors changed. During two of three sampling times, spatial diversity patterns were shaped primarily by gradients of water availability, the regional limiting factor. In the summer dry season, microscale diversity (species richness per 1 m²) and mesoscale diversity (cumulative species and functional types in 20, 1‐m² plots) of herbaceous plants decreased along the topogradient from floodplain to upland, reflecting the greater water availability on the low surfaces. During a summer wet season with moderate rains and flooding, diversity increased in all hydrogeomorphic zones (floodplain, terrace, upland), but the spatial pattern along the topogradient persisted. Following a very wet winter, patterns along the topogradient reversed: scour from large floods limited diversity on the floodplain and competitive exclusion limited the diversity on undisturbed river terrace, while abundant rains allowed for high microscale diversity in the upland. Disturbance and resource availability thus interacted to influence plant species diversity in a fashion consistent with the dynamic‐equilibrium model of species diversity. In contrast to the microscale patterns, mesoscale diversity of species and functional types remained high in the floodplain during all sampling times, with 58% more plant species and 90% more functional types sampled in low floodplain than arid upland for the year as a whole. Species with a wide range of moisture and temperature affinities were present in the floodplain, and seasonal turnover of species was high in this zone. The floodplain zone of a perennial to intermittent‐flow river thus had greater plant diversity than arid Sonoran Desert upland, as measured at temporal scales that capture seasonal variance in resource and disturbance pulses and at spatial scales that capture the environmental heterogeneity of floodplains. Although periodically limited by intense flood disturbance, diversity remains high in the floodplain because of the combination of moderate resource levels (groundwater, seasonal flood water) and persistent effects of flood disturbance (high spatial heterogeneity, absence of competitive exclusion), in concert with the same climatic factors that produce seasonally high diversity in the region (temporally variable pulses of rainfall).     

Wildfire severity reduces richness and alters composition of soil fungal communities in boreal forests of western Canada

Wildfire is the dominant disturbance in boreal forests and fire activity is increasing in these regions. Soil fungal communities are important for plant growth and nutrient cycling postfire but there is little understanding of how fires impact fungal communities across landscapes, fire severity gradients, and stand types in boreal forests. Understanding relationships between fungal community composition, particularly mycorrhizas, and understory plant composition is therefore important in predicting how future fire regimes may affect vegetation. We used an extreme wildfire event in boreal forests of Canada's Northwest Territories to test drivers of fungal communities and assess relationships with plant communities. We sampled soils from 39 plots 1 year after fire and 8 unburned plots. High‐throughput sequencing (MiSeq, ITS) revealed 2,034 fungal operational taxonomic units. We found soil pH and fire severity (proportion soil organic layer combusted), and interactions between these drivers were important for fungal community structure (composition, richness, diversity, functional groups). Where fire severity was low, samples with low pH had higher total fungal, mycorrhizal, and saprotroph richness compared to where severity was high. Increased fire severity caused declines in richness of total fungi, mycorrhizas, and saprotrophs, and declines in diversity of total fungi and mycorrhizas. The importance of stand age (a surrogate for fire return interval) for fungal composition suggests we could detect long‐term successional patterns even after fire. Mycorrhizal and plant community composition, richness, and diversity were weakly but significantly correlated. These weak relationships and the distribution of fungi across plots suggest that the underlying driver of fungal community structure is pH, which is modified by fire severity. This study shows the importance of edaphic factors in determining fungal community structure at large scales, but suggests these patterns are mediated by interactions between fire and forest stand composition.     

Plant species richness,identity and productivity differentially influence key groups of microbes in grassland soils of contrasting fertility

The abundance of microbes in soil is thought to be strongly influenced by plant productivity rather than by plant species richness per se. However, whether this holds true for different microbial groups and under different soil conditions is unresolved. We tested how plant species richness, identity and biomass influence the abundances of arbuscular mycorrhizal fungi (AMF), saprophytic bacteria and fungi, and actinomycetes, in model plant communities in soil of low and high fertility using phospholipid fatty acid analysis. Abundances of saprophytic fungi and bacteria were driven by larger plant biomass in high diversity treatments. In contrast, increased AMF abundance with larger plant species richness was not explained by plant biomass, but responded to plant species identity and was stimulated by Anthoxantum odoratum. Our results indicate that the abundance of saprophytic soil microbes is influenced more by resource quantity, as driven by plant production, while AMF respond more strongly to resource composition, driven by variation in plant species richness and identity. This suggests that AMF abundance in soil is more sensitive to changes in plant species diversity per se and plant species composition than are abundances of saprophytic microbes.     

Productivity-diversity relationships in lake plankton communities

One of the most intriguing environmental gradients connected with variation in diversity is ecosystem productivity. The role of diversity in ecosystems is pivotal, because species richness can be both a cause and a consequence of primary production. However, the mechanisms behind the varying productivity-diversity relationships (PDR) remain poorly understood. Moreover, large-scale studies on PDR across taxa are urgently needed. Here, we examined the relationships between resource supply and phyto-, bacterio-, and zooplankton richness in 100 small boreal lakes. We studied the PDR locally within the drainage systems and regionally across the systems. Second, we studied the relationships between resource availability, species richness, biomass and resource ratio (N:P) in phytoplankton communities using Structural Equation Modeling (SEM) for testing the multivariate hypothesis of PDR. At the local scale, the PDR showed variable patterns ranging from positive linear and unimodal to negative linear relationships for all planktonic groups. At the regional scale, PDRs were significantly linear and positive for phyto- and zooplankton. Phytoplankton richness and the amount of chlorophyll a showed a positive linear relationship indicating that communities consisting of higher number of species were able to produce higher levels of biomass. According to the SEM, phytoplankton biomass was largely related to resource availability, yet there was a pathway via community richness. Finally, we found that species richness at all trophic levels was correlated with several environmental factors, and was also related to richness at the other trophic levels. This study showed that the PDRs in freshwaters show scale-dependency. We also documented that the PDR complies with the multivariate model showing that plant biomass is not mirroring merely the resource availability, but is also influenced by richness. This highlights the need for conserving diversity in order to maintain ecosystem processes in freshwaters.     

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.     

Functional dependence underlies a positive plant-grasshopper richness relationship

A central focus of ecology is identifying the factors that shape spatial patterns of species diversity and this is particularly relevant in an era of global change. Positive relationships between plant and consumer diversity are common, but could be driven by direct responses of each trophic level to underlying environmental gradients, or indirectly where changes in environmental conditions propagate through food webs. Here we use structural equation modeling to examine the relative importance of soil resource availability and disturbance (fire) in mediating relationships between plant and grasshopper richness in insular grasslands. We found a positive relationship between plant and consumer richness that became stronger after accounting for disturbance, despite unique responses of plants and consumers to the two environmental gradients. Plant richness responded to an underlying gradient in soil resource availability. Time since the last fire had a direct positive effect on grasshopper richness but had no effect on plant richness. This work supports that plant and consumer richness are functionally linked, rather than having similar responses to environmental gradients. By disentangling the direct and indirect processes underlying a positive relationship between plant and consumer diversity in a natural system that spans multiple environmental gradients, we demonstrate the importance of investigating biodiversity through explicit multivariate models.     

火烧强度对兴安落叶松群落叶片功能性状及功能多样性的影响

林火是北方针叶林的重要生态因子,直接影响火烧迹地物种多样性及功能多样性,进而影响森林群落的演替.以牙克石地区火后自然恢复12年的兴安落叶松群落为研究对象,在群落尺度上分析火烧迹地土壤养分含量、叶片功能性状、物种多样性和功能多样性在不同火烧强度(轻、中和重度火烧)下的变化规律.结果表明: 火烧显著降低了土壤全氮含量,对土壤全磷含量无显著影响.轻、中度火烧有助于维持群落较高的物种多样性和功能多样性;中度火烧显著增加了群落的物种多样性,物种丰富度指数、Shannon指数、Simpson指数和Pielou均匀度指数均在中度火烧时有最大值;火烧降低了群落的功能丰富度和功能离散度指数,二者分别在中、轻度火烧时最大(除未过火外),而群落的功能均匀度和二次熵指数在林火干扰后增加,轻度火烧后最大.随火烧强度增加,叶干物质含量、叶组织密度和叶磷含量总体呈显著增大趋势,而比叶面积、叶含水量、叶氮含量和叶N∶P则表现出未过火>中度火烧>轻度火烧>重度火烧的规律,叶片厚度呈先增大后减小的变化规律.火烧强度对森林群落的叶片功能性状和功能多样性均有显著影响,且适度的火干扰对森林群落的恢复具有促进作用.     

The interplay of nurse and target plant traits influences magnitude and direction of facilitative interactions under different combinations of stress and disturbance intensities in Andean dry grassland

Aims Facilitation is a key process in vegetation dynamics, driving the response to natural and anthropogenic pressures. In harsh-grazed systems, palatable plants mainly survive when nested under unpalatable tussocks and shrubs. The magnitude and direction of positive interactions are driven by resource availability, extent of herbivory and type of nurse species. We hypothesized that different combinations of disturbance and environmental stress affect community composition in the dry Puna (southern Peruvian Andes) by modifying nurse types and plant interactions in magnitude and specific associations. We investigated whether different combinations of stress and disturbance influence species richness, type and frequency of occurrence of nurse and beneficiary species and magnitude and patterns of plant interactions; whether nurse species influence these interactions and target species change their interactions under different combinations of stress and disturbance and whether plant functional traits differ in the studied communities and influence the pattern of spatial interactions.Methods We selected three plant communities subject to different precipitation and management regimes: in each we laid a number of transects proportional to its extension. Data collected include species presence/absence, type of spatial interactions with nurse species and functional traits. We calculated species richness and rarefaction patterns, described the patterns of plant–plant spatial interactions and investigated the associations between nurse and other species in the three communities using indicator species analysis (ISA). We performed ISA and correlation analysis to investigate whether plant functional traits influenced facilitative interactions.Important findings We found that different combinations of stress and disturbance shaped a complex set of responses, including changes in the nurse species set. Nurse composition influenced magnitude and direction of plant interactions under different stress intensities. Heavy disturbance increased the relative importance of facilitation, even if the overall number of facilitated species decreased. Under equivalent disturbance regimes, increased abiotic stress led to a greater importance of facilitation. Different combinations of stress and disturbance affected the community assemblage also by changing the behaviour of some non-nurse species. Both heavy disturbance and strong stress led to a decrease of trait states; with certain combinations of stress and disturbance, preferential distribution of these states was observed. We also found that plant traits were of key importance in determining facilitative interactions. Some traits were mainly associated with one type of spatial interaction: plant architecture, life cycle and root type influenced the type of interaction between nurses and beneficiaries under different combinations of stress and disturbance. Our results also demonstrate that in plant interaction research the object of observations (species per se, species percentage, etc.) might influence outputs, and to effectively assess the impact of different stress and disturbance intensities on plant interactions it is necessary to work at the community level to consider the whole species pool.     

Spatial and temporal patterns of species richness in a riparian landscape

Aim To test for control of vascular plant species richness in the riparian corridor by exploring three contrasting (although not mutually exclusive) hypotheses: (1) longitudinal patterns in riparian plant species richness are governed by local, river‐related processes independent of the regional species richness, (2) riparian plant species richness is controlled by dispersal along the river (longitudinal control), and (3) the variation in riparian plant species richness mirrors variation in regional richness (lateral control). Location The riparian zones of the free‐flowing Vindel River and its surrounding river valley, northern Sweden. Methods We used data from three surveys, undertaken at 10‐year intervals, of riparian reaches (200‐m stretches of riverbank) spanning the entire river. In addition, we surveyed species richness of vascular plants in the uplands adjacent to the river in 3.75‐km² large plots along the same regional gradient. We explored the relationship between riparian and upland flora, and various environmental variables. We also evaluated temporal variation in downstream patterns of the riparian flora. Results Our results suggest that local species richness in boreal rivers is mainly a result of local, river‐related processes and dispersal along the corridor. The strongest correlation between species richness and the environment was a negative one between species number and soil pH, but pH varied within a narrow range. We did not find evidence for a correlation between species richness on regional and local scales. We found that the local patterns of species richness for naturally occurring vascular plants were temporally variable, probably in response to large‐scale disturbance caused by extreme floods. Most previous studies have found a unimodal pattern of species richness with peaks in the middle reaches of a river. In contrast, on two of three occasions corresponding to major flooding events, we found that the distribution of species richness of naturally occurring vascular plants resembled that of regional diversity: a monotonic decrease from headwater to coast. We also found high floristic similarity between the riparian corridor and the surrounding landscape. Main conclusions These results suggest that local processes control patterns of riparian species richness, but that species composition is also highly dependent on the regional species pool. We argue that inter‐annual variation in flood disturbance is probably the most important factor producing temporal variability of longitudinal species richness patterns.     

The response of plant diversity to ecosystem retrogression: evidence from contrasting long-term chronosequences

Following catastrophic disturbances, succession and vegetation development occur, but in the prolonged absence of these disturbances a decline (retrogressive) phase follows in which nutrient availability and tree biomass declines considerably. We measured plant diversity across six long-term chronosequences that each included retrogressive stages in Australia, New Zealand, Alaska, Hawaii and Sweden. In contrast to theories predicting negative or hump-shaped responses of tree diversity to biomass or soil fertility, tree species richness often peaked coincidentally with tree basal area (a surrogate of tree biomass), and declined during retrogression. Similar patterns were found regardless of whether or not species richness estimates were rarefraction-adjusted to correct for variation in stem densities across plots. The Shannon-Weiner diversity index sometimes showed the same pattern, but in two chronosequences was least when tree basal area peaked; this was driven by the domination of total basal area by single tree species in both cases. The decline in tree diversity during retrogression was often associated with reduced relative amounts of total phosphorus in soil. In contrast, total vascular plant species richness often increased during retrogression. These results demonstrate that forests with high tree diversity and biomass do not persist indefinitely in the long-term absence of catastrophic disturbance, and that similar patterns occur across the boreal, temperate and subtropical zones.     

Frequent fire promotes diversity and cover of biological soil crusts in a derived temperate grassland

The intermediate disturbance hypothesis (IDH) predicts that species diversity is maximized at moderate disturbance levels. This model is often applied to grassy ecosystems, where disturbance can be important for maintaining vascular plant composition and diversity. However, effects of disturbance type and frequency on cover and diversity of non-vascular plants comprising biological soil crusts are poorly known, despite their potentially important role in ecosystem function. We established replicated disturbance regimes of different type (fire vs. mowing) and frequency (2, 4, 8 yearly and unburnt) in a high-quality, representative Themeda australis–Poa sieberiana derived grassland in south-eastern Australia. Effects on soil crust bryophytes and lichens (hereafter cryptogams) were measured after 12 years. Consistent with expectations under IDH, cryptogam richness and abundance declined under no disturbance, likely due to competitive exclusion by vascular plants as well as high soil turnover by soil invertebrates beneath thick grass. Disturbance type was also significant, with burning enhancing richness and abundance more than mowing. Contrary to expectations, however, cryptogam richness increased most dramatically under our most frequent and recent (2 year) burning regime, even when changes in abundance were accounted for by rarefaction analysis. Thus, from the perspective of cryptogams, 2-year burning was not an adequately severe disturbance regime to reduce diversity, highlighting the difficulty associated with expression of disturbance gradients in the application of IDH. Indeed, significant correlations with grassland structure suggest that cryptogam abundance and diversity in this relatively mesic (600 mm annual rainfall) grassland is maximised by frequent fires that reduce vegetation and litter cover, providing light, open areas and stable soil surfaces for colonisation. This contrasts with detrimental effects of 2-year burning on native perennial grasses, indicating that this proliferation of cryptogams has potentially high functional significance for situations where vegetation cover is depleted, particularly for reducing soil erosion.     

Influence of experimental soil disturbances on the diversity of plants in agricultural grasslands

Aims and Methods Disturbance is supposed to play an important role for biodiversity and ecosystem stability as described by the intermediate disturbance hypothesis (IDH), which predicts highest species richness at intermediate levels of disturbances. In this study, we tested the effects of artificial soil disturbances on diversity of annual and perennial vascular plants and bryophytes in a field experiment in 86 agricultural grasslands differing in land use in two regions of Germany. On each grassland, we implemented four treatments: three treatments differing in application time of soil disturbances and one control. One year after experimental disturbance, we recorded vegetation and measured biomass productivity and bare ground. We analysed the disturbance response taking effects of region and land-use-accompanied disturbance regimes into account.Important findings Region and land-use type strongly determined plant species richness. Experimental disturbances had small positive effects on the species richness of annuals, but none on perennials or bryophytes. Bare ground was positively related to species richness of bryophytes. However, exceeding the creation of 12% bare ground further disturbance had a detrimental effect on bryophyte species richness, which corresponds to the IDH. As biomass productivity was unaffected by disturbance our results indicate that the disturbance effect on species richness of annuals was not due to decreased overall productivity, but rather due to short-term lowered inter- and intraspecific competition at the newly created microsites. Generally, our results highlight the importance of soil disturbances for species richness of annual plants and bryophytes in agricultural grasslands. However, most grasslands were disturbed naturally or by land-use practices and our additional experimental soil disturbances only had a small short-term effect. Overall, total plant diversity in grasslands seemed to be more limited by the availability of propagules rather than by suitable microsites for germination. Thus, nature conservation efforts to increase grassland diversity should focus on overcoming propagule limitation, for instance by additional sowing of seeds, while the creation of additional open patches by disturbance might only be appropriate where natural disturbances are scarce.     

Effects of belowground resource use comlementarity on invasion of constructed grassland plant communities

At two field sites that differed in fertility, we investigated how species richness, functional group diversity, and species composition of constructed plant communities influenced invasion. Grassland communities were constructed to be either functionally diverse or functionally simple based on belowground resource use patterns of constituent species. Communities were also constructed with different numbers of species (two or five) to examine interactions between species richness, functional diversity and invasion resistance. We hypothesized that communities with more complementary belowground resource use (i.e., more species rich and more functionally diverse communities) would be less easily invaded than communities with greater degrees of belowground resource use overlap. Two contrasting invasive species were introduced: an early-season, shallow rooting annual grass, Bromus hordeaceus (soft chess), and a late-season, deep rooting annual forb, Centaurea solstitialis (yellow starthistle). Invader responses to species richness and functional diversity treatments differed between sites. In general, the more similar the patterns of belowground resource use between residents of the plant community and the invader, the poorer the invader’s performance. Complementarity or overlap of resource use among species in the constructed communities appeared to affect invader success less than complementarity or overlap of resource use between the invader and the species present in the community.     

Structural and taxonomic diversity predict above-ground biomass better than functional measures of maximum height in mixed-species forests

Aims: Mixed-species forests are known to be highly productive systems because of their high species diversity, including taxonomic diversity (species richness) and structural diversity. Recent empirical evidence also points to plant maximum height, as a functional trait that potentially drives forest above-ground biomass (AGB). However, the interrelations between these biotic variables are complex, and it is not always predictable if structural diversity attributes or functional metrics of plant maximum height would act as the most important determinant of stand biomass. Here we evaluated the relative importance of structural diversity attributes and functional metrics of plant maximum height (Hmax) in predicting and mediating AGB response to variation in species richness in mixed-species forests, while also accounting for fine-scale environmental variation. Location: Northern Benin. Methods: We used forest inventory data from mixed-species stands of native and exotic species. We quantified structural diversity as coefficient of variation of tree diameter at breast height (CVdbh) and of height (CVHt). For plant Hmax, we computed three metrics: functional range (FRHmax), functional divergence (FDHmax) and community-weighted mean (CWMHmax). We used topographical variables such as elevation and slope to account for possible environmental effects. Simple and multiple mixed-effects models, and structural equation models were performed to assess the direct and indirect links of AGB with species richness through structural diversity attributes and functional metrics of plant Hmax. Results: Species richness and CVdbh were positively related to AGB, while functional metrics of plant Hmax were not. Structural equation models revealed that species richness influenced AGB indirectly via CVdbh, which alone strongly promoted AGB. Elevation only had a positive direct effect on AGB. While increasing species richness enhanced CVdbh and functional measures of plant Hmax, there was no support for the latter mediating the effects of species richness on AGB. Conclusion: Structural diversity has a significant advantage in predicting and mediating the positive effect of species richness on AGB more so than functional measures of plant Hmax. We argue that structural diversity acts as a mechanism for the species richness–AGB relationship, and that maintaining high structural diversity would enhance biomass in mixed-species forests.     

Seed arrival, ecological filters, and plant species richness: a meta-analysis

Theoretical models predict that effects of dispersal on local biodiversity are influenced by the size and composition of the species pool, as well as ecological filters that limit local species membership. We tested these predictions by conducting a meta-analysis of 28 studies encompassing 62 experiments examining effects of propagule supply (seed arrival) on plant species richness under contrasting intensities of ecological filters (owing to disturbance and resource availability). Seed arrival increased local species richness in a wide range of communities (forest, grassland, montane, savanna, wetland), resulting in a positive mean effect size across experiments. Mean effect size was 70% higher in disturbed relative to undisturbed communities, suggesting that disturbance increases recruitment opportunities for immigrating species. In contrast, effect size was not significantly influenced by nutrient or water availability. Among seed-addition experiments, effect size was positively correlated with species and functional diversity within the pool of added seeds (species evenness and seed-size diversity), primarily in disturbed communities. Our analysis provides experimental support for the general hypothesis that species pools and local environmental heterogeneity interactively structure plant communities. We highlight empirical gaps that can be addressed by future experiments and discuss implications for community assembly, species coexistence, and the maintenance of biodiversity.     

Effects of plant diversity, plant productivity and habitat parameters on arthropod abundance in montane European grasslands

Arthropod abundance has been hypothesized to be correlated with plant diversity but the results of previous studies have been equivocal. In contrast, plant productivity, vegetation structure, abiotic site conditions, and the physical disturbance of habitats, are factors that interact with plant diversity, and that have been shown to influence arthropod abundance. We studied the combined effect of plant species diversity, productivity and site characteristics on arthropod abundance in 71 managed grasslands in central Germany using multivariate statistics. For each site we determined plant species cover, plant community biomass (productivity), macro- and micronutrients in the soil, and characterized the location of sites with respect to orographic parameters as well as the current and historic management regimes. Arthropods were sampled using a suction sampler and classified a priori into functional groups (FGs). We found that arthropod abundance was not correlated with plant species richness, effective diversity or Camargo's evenness, even when influences of environmental variables were taken into account. In contrast, plant community composition was highly correlated with arthropod abundances. Plant community productivity influenced arthropod abundance but explained only a small proportion of the variance. The abundances of the different arthropod FGs were influenced differentially by agricultural management, soil characteristics, vegetation structure and by interactions between different FGs of arthropods. Herbivores, carnivores and detritivores reacted differently to variation in environmental variables in a manner consistent with their feeding mode. Our results show that in natural grassland systems arthropod abundance is not a simple function of plant species richness, and they emphasize the important role of plant community composition for the abundance patterns of the arthropod assemblages.     

Evaluating the role of fire disturbance in structuring small reptile communities in temperate forests

Fire is an integral disturbance shaping forest community dynamics over large scales. However, understanding the relationship between fire induced habitat disturbance and biodiversity remain equivocal. Ecological theories including the intermediate disturbance hypothesis (IDH) and the habitat accommodation model (HAM) offer predictive frameworks that could explain faunal responses to fire disturbances. We used an 80 year post-fire chronosequence to investigate small reptile community responses to fires in temperate forests across 74 sites. First, we evaluated if changes in species richness, abundance and evenness post-fire followed trends of prior predictions, including the IDH. Second, using competing models of fine scale habitat elements we evaluated the specific ways which fire influenced small reptiles. Third, we evaluated support for the HAM by examining compositional changes of reptile community post-fire. Relative abundance was positively correlated to age post-fire while richness and evenness showed no associations. The abundance trend was as expected based on the prior prediction of sustained population increase post-disturbance, but the trend for richness contradicted the prediction of highest diversity at intermediate levels of disturbance (according to IDH). Abundance changes were driven mainly by changes in overstorey, ground layer, and shelter, while richness and evenness did not associate with any vegetation parameter. Community composition was not strongly correlated to age since fire, thus support for the HAM was weak. Overall, in this ecosystem, frequent fire disturbances can be detrimental to small reptiles. Future studies utilizing approaches based on species traits could enhance our understanding of biodiversity patterns post-disturbance.