What drives small‐scale spatial patterns in lotic meiofauna communities?

• 1 Lotic meiofaunal communities demonstrate extremely variable dynamics, especially when viewed at small spatial scales (≤ metres). Given the limited amount of research on lotic meiofauna, we chose to organise our discussion of their small‐scale spatial patterns around the dominant factors we believe drive their spatial distributions in streams. We separate scale‐dependent effects that structure lotic meiofauna into biotic factors (e.g. predation, food quantity/quality, dispersal) and abiotic factors (e.g. local flow dynamics and substratum characteristics).
• 2 The impact of predation on the distribution of meiofauna varies with the scale over which predators forage (e.g. fish predation influences meiofauna in different ways and at broader spatial scales than do invertebrate predators), the type of streambed substrata in which the predator‐prey interactions occur, and the dispersal ability of different meiofauna. The latter is greatly influenced by predator and prey (meiofauna) interactions with the flow environment.
• 3 Organic matter influences the small‐scale distribution of meiofauna in streams. Both its quality as food (as indicated by C:N content, ATP content, or microbial biomass) and its spatial distribution on the streambed, influence meiofauna patchiness, community structure and life history characteristics. As a habitat, the structure that organic matter provides (e.g. wood or leaves) can influence predator‐prey interactions, offer materials for case‐building and offer refugia during disturbance events ‐ all of which influence the small‐scale spatial distribution of meiofauna.
• 4 Stream flow influences the distribution of meiofauna at broad scales (10s–100s of metres), primarily because of the high susceptibility of meiofauna to passive drift; small‐scale interactions between flow and substrata are also important, however, particularly at more localised (≤ metre) scales. At both scales, substratum particle size is important to interstitial‐dwelling fauna, influencing the probability of passive drift by meiofauna as well as local microhabitat conditions (e.g. dissolved oxygen; upwelling/downwelling in the hyporheic zone) and, thus, the small‐scale distribution among microhabitats.
• 5 In general, the processes governing the distribution of meiofauna at small scales cannot be separated entirely from those processes working at larger scales. A conceptual diagram is presented illustrating the relative importance of various factors in influencing the spatial patterns of meiofauna and over what scales these factors act.

     

What drives the species richness patterns of non‐volant small mammals along a subtropical elevational gradient?

The biodiversity of non‐volant small mammals along an extensive subtropical elevational gradient was studied for the first time on Gongga Mountain, the highest mountain in Hengduan Mountain ranges in China, located in one of the 25 global biodiversity hotspots. Non‐volant small mammals were replicate sampled in two seasons at eight sampling sites between 1000 and 4200 m elevation on the eastern slope of Gongga Mountain. In all, 726 individual small mammals representing 25 species were documented in 28 800 trap nights. The species richness pattern for non‐volant small mammals along the elevational gradients was hump‐shaped with highest richness at mid‐elevations. However, different richness patterns emerged between endemic and non‐endemic species, between larger‐ranged and smaller‐ranged species and between rodents and insectivores. Temperature, precipitation, plant species richness and geometric constraints (mid‐ domain effect) were most significant in explaining species richness patterns. Based on the analysis of simple ordinary least squares (OLS) and stepwise multiple regressions, the overall richness pattern, as well as the pattern of insectivores, endemic species and larger‐ranged species showed strong correlation with geometric constraint predictions. However, non‐endemic species richness was more strongly correlated with temperature, while rodent richness was correlated with plant species richness. Our study shows that no single key factor can explain all richness patterns of non‐volant small mammals. We need to be cautious in summarizing a general richness pattern of large species groups (e.g. small mammals or mammals) from species in smaller groups having different ecological distributions and life histories. Elevational richness patterns and their driving factors for small mammals are more likely dependent on what kind of species we study.     

What determines species richness of parasitic organisms? A meta‐analysis across animal,plant and fungal hosts

Although a small set of external factors account for much of the spatial variation in plant and animal diversity, the search continues for general drivers of variation in parasite species richness among host species. Qualitative reviews of existing evidence suggest idiosyncrasies and inconsistent predictive power for all proposed determinants of parasite richness. Here, we provide the first quantitative synthesis of the evidence using a meta‐analysis of 62 original studies testing the relationship between parasite richness across animal, plant and fungal hosts, and each of its four most widely used presumed predictors: host body size, host geographical range size, host population density, and latitude. We uncover three universal predictors of parasite richness across host species, namely host body size, geographical range size and population density, applicable regardless of the taxa considered and independently of most aspects of study design. A proper match in the primary studies between the focal predictor and both the spatial scale of study and the level at which parasite species richness was quantified (i.e. within host populations or tallied across a host species' entire range) also affected the magnitude of effect sizes. By contrast, except for a couple of indicative trends in subsets of the full dataset, there was no strong evidence for an effect of latitude on parasite species richness; where found, this effect ran counter to the general latitude gradient in diversity, with parasite species richness tending to be higher further from the equator. Finally, the meta‐analysis also revealed a negative relationship between the magnitude of effect sizes and the year of publication of original studies (i.e. a time‐lag bias). This temporal bias may be due to the increasing use of phylogenetic correction in comparative analyses of parasite richness over time, as this correction yields more conservative effect sizes. Overall, these findings point to common underlying processes of parasite diversification fundamentally different from those controlling the diversity of free‐living organisms.     

Does host‐plant diversity explain species richness in insects? A test using Coccidae (Hemiptera)

1. The megadiverse herbivores and their host plants are a major component of biodiversity, and their interactions have been hypothesised to drive the diversification of both. 2. If plant diversity influences the diversity of insects, there is an expectation that insect species richness will be strongly correlated with host‐plant species richness. This should be observable at two levels (i) more diverse host‐plant groups should harbour more species of insects, and (ii) the species richness of a group of insects should correlate with the richness of the host groups it uses. However, such a correlation is also consistent with a hypothesis of random host use, in which insects encounter and use hosts in proportion to the diversity of host plants. Neither of these expectations has been widely tested. 3. These expectations were tested using data from a species‐rich group of insects – the Coccidae (Hemiptera). 4. Significant positive correlations were found between the species richness of coccid clades (genera) and the species richness of the host‐plant family or families upon which the clades occur. On a global scale, more closely related plant families have more similar communities of coccid genera but the correlation is weak. 5. Random host use could not be rejected for many coccids but randomisation tests and similarity of coccid communities on closely related plant families show that there is non‐random host use in some taxa. Overall, our results support the idea that plant diversity is a driver of species richness of herbivorous insects, probably via escape‐and‐radiate or oscillation‐type processes.     

Do biotic interactions shape both sides of the humped‐back model of species richness in plant communities?

A humped-back relationship between species richness and community biomass has frequently been observed in plant communities, at both local and regional scales, although often improperly called a productivity-diversity relationship. Explanations for this relationship have emphasized the role of competitive exclusion, probably because at the time when the relationship was first examined, competition was considered to be the significant biotic filter structuring plant communities. However, over the last 15 years there has been a renewed interest in facilitation and this research has shown a clear link between the role of facilitation in structuring communities and both community biomass and the severity of the environment. Although facilitation may enlarge the realized niche of species and increase community richness in stressful environments, there has only been one previous attempt to revisit the humped-back model of species richness and to include facilitative processes. However, to date, no model has explored whether biotic interactions can potentially shape both sides of the humped-back model for species richness commonly detected in plant communities. Here, we propose a revision of Grime's original model that incorporates a new understanding of the role of facilitative interactions in plant communities. In this revised model, facilitation promotes diversity at medium to high environmental severity levels, by expanding the realized niche of stress-intolerant competitive species into harsh physical conditions. However, when environmental conditions become extremely severe the positive effects of the benefactors wane (as supported by recent research on facilitative interactions in extremely severe environments) and diversity is reduced. Conversely, with decreasing stress along the biomass gradient, facilitation decreases because stress-intolerant species become able to exist away from the canopy of the stress-tolerant species (as proposed by facilitation theory). At the same time competition increases for stress-tolerant species, reducing diversity in the most benign conditions (as proposed by models of competition theory). In this way our inclusion of facilitation into the classic model of plant species diversity and community biomass generates a more powerful and richer predictive framework for understanding the role of plant interactions in changing diversity. We then use our revised model to explain both the observed discrepancies between natural patterns of species richness and community biomass and the results of experimental studies of the impact of biodiversity on the productivity of herbaceous communities. It is clear that explicit consideration of concurrent changes in stress-tolerant and competitive species enhances our capacity to explain and interpret patterns in plant community diversity with respect to environmental severity.     

Does energy availability influence classical patterns of spatial variation in exotic species richness?

Aim At macroecological scales, exotic species richness is frequently positively correlated with human population density. Such patterns are typically thought to arise because high human densities are associated with increased introduction effort and/or habitat modification and disturbance. Exotic and native species richness are also frequently positively correlated, although the causal mechanisms remain unclear. Energy availability frequently explains much of the variation in species richness and we test whether such species–energy relationships may influence the relationships of exotic species richness with human population density and native species richness. Location Great Britain. Methods We first investigate how spatial variation in the distributions of the 10 exotic bird species is related to energy availability. We then model exotic species richness using native avian species richness, human population density and energy availability as predictors. Species richness is modelled using two sets of models: one assumes independent errors and the other takes spatial correlation into account. Results The probability of each exotic species occurring, in a 10‐km quadrat, increases with energy availability. Exotic species richness is positively correlated with energy availability, human population density and native species richness in univariate tests. When taking energy availability into account, exotic species richness is negligibly influenced by human population density, but remains positively associated with native species richness. Main conclusions We provide one of the few demonstrations that energy availability exerts a strong positive influence on exotic species richness. Within our data, the positive relationship between exotic species richness and human population density probably arises because both variables increase with energy availability, and may be independent of the influence of human density on the probability of establishment. Positive correlations between exotic and native species richness remain when controlling for the influence of energy on species richness. The relevance of such a finding to the debate on the relationship between diversity and invasibility is discussed.     

Does climate determine broad‐scale patterns of species richness? A test of the causal link by natural experiment

Aim Broad‐scale spatial patterns of species richness are very strongly correlated with climatic variables. If there is a causal link, i.e. if climate directly or indirectly determines patterns of richness, then when the climatic variables change, richness should change in the manner that spatial correlations between richness and climate would predict. The present study tests this prediction using seasonal changes in climatic variables and bird richness. Location We used a grid of equal area quadrats (37 000 km²) covering North and Central America as far south as Nicaragua. Methods Summer and winter bird distribution data were drawn from monographs and field guides. Climatic data came from published sources. We also used remotely sensed NDVI (normalized difference vegetation index — a measure of greenness). Results Bird species richness changes temporally (between summer and winter) in a manner that is close to, but statistically distinguishable from, the change one would predict from models relating the spatial variation in richness at a single time to climatic variables. If one further takes into account the seasonal changes in NDVI and within‐season variability of temperature and precipitation, then winter and summer richness follow congruent, statistically indistinguishable patterns. Main conclusions Our results are consistent with the hypothesis that climatic variables (temperature and precipitation) and vegetation cover directly or indirectly influence patterns of bird species richness.     

Do vegetation patch spatial patterns disrupt the spatial organization of plant species?

The mechanisms that structure plant diversity and generate long-range correlated spatial patterns have important implications for the conservation of fragmented landscapes. The ability to disperse and persist influences a plant species’ capacity for spatial organization, which can play a critical role in structuring plant diversity in metacommunities. This study examined the spatial patterns of species diversity within a network of patches in Cabo de Gata Natural Park, southeastern Spain. The objectives were to understand how the spatial heterogeneity of species composition (beta diversity) varies in a structured landscape, and how the long-range spatial autocorrelation of plant species is affected by the spatial configuration of patches.The mechanisms underlying the spatial distribution of plants acted at two scales. Between patches, spatial variation in species distributions was greater than that expected based on spatial randomization, which indicated that movement among patches was restricted. Within patches, diffusion processes reduced spatial variability in species distributions, and the effect was more prominent in large patches. Small patch size negatively influenced the long-range spatial autocorrelation of characteristic species, whereas inter-patch distance had a stronger effect on species frequency than it had on the disruption of spatial organized patterns.The long-range spatial autocorrelation was evaluated based on the dispersal abilities of the species. Among the 106 species evaluated, 39% of the woody species, 17% of the forbs, and 12% of the grasses exhibited disrupted long-range spatial autocorrelation where patches were small. The species that are more vulnerable to the effects of fragmentation tended to be those that have restricted dispersal, such as those that have short-range dispersal (atelechoric), e.g., Phlomis purpurea, Cistus albidus, Teucrium pseudochamaepytis, Brachypodium retusum, and the ballistic species, Genista spartioides. Helianthemum almeriense is another vulnerable species that has actively restricted dispersal (antitelechory), which is common in arid regions. Wind dispersers such as Launaea lanifera were less vulnerable to the effects of fragmentation. Long-distance dispersers whose persistence depends on facilitative interactions with other individuals, e.g., allogamous species such as Thymus hyemalis, Ballota hirsuta, and Anthyllis cytisoides, exhibit disrupted long-range spatial autocorrelation when patch size is reduced.     

What drives the positive correlation between human population density and bird species richness in Australia?

Aim To test six hypotheses that could explain or mediate the positive correlation between human population density (HPD) and bird species richness while controlling for biased sampling effort. These hypotheses were labelled as follows: productivity (net primary productivity, NPP); inherent heterogeneity (diversity of vegetation types); anthropogenic heterogeneity (diversity of land uses); conservation policy (proportion of conservation land); increased productivity (human‐induced productivity increases); and the reduced‐slope hypothesis (which predicts that humans have a negative impact on species numbers across the full range of variation in HPD). Location Australia. Methods All data were collected at a spatial resolution of 1° across mainland Australia. Bird species richness was from 2007 atlas data and random subsampling was used to account for biased sampling effort. HPD was from the 2006 census. All other data were from government produced geographic information system layers. The most important biotic or abiotic factors influencing patterns in both species richness and HPD were assessed using simultaneous autoregressive models and an information theoretic approach. Results NPP appeared to be one of the main factors driving spatial congruence between bird species richness and HPD. Inherent habitat heterogeneity was weakly related to richness and HPD, although an interaction between heterogeneity and NPP indicated that the former may be an important determinant of species richness in low‐productivity regions. There was little evidence that anthropogenic landscape heterogeneity or human‐induced changes in productivity influenced the relationship between species richness and HPD, but conservation policy appeared to act as an important mediating factor and species richness was positively related to the proportion of conservation land only in regions of high HPD. Main conclusions The spatial congruence between bird species richness and HPD occurs because both respond positively to productivity and, in certain circumstances, habitat heterogeneity. Our results suggest that conservation policy could mediate this relationship, but further research is required to determine the importance of conservation reserves in supporting species in regions densely populated by humans.     

What explains patterns of species richness? The relative importance of climatic‐niche evolution,morphological evolution,and ecological limits in salamanders

A major goal of evolutionary biology and ecology is to understand why species richness varies among clades. Previous studies have suggested that variation in richness among clades might be related to variation in rates of morphological evolution among clades (e.g., body size and shape). Other studies have suggested that richness patterns might be related to variation in rates of climatic‐niche evolution. However, few studies, if any, have tested the relative importance of these variables in explaining patterns of richness among clades. Here, we test their relative importance among major clades of Plethodontidae, the most species‐rich family of salamanders. Earlier studies have suggested that climatic‐niche evolution explains patterns of diversification among plethodontid clades, whereas rates of morphological evolution do not. A subsequent study stated that rates of morphological evolution instead explained patterns of species richness among plethodontid clades (along with “ecological limits” on richness of clades, leading to saturation of clades with species, given limited resources). However, they did not consider climatic‐niche evolution. Using phylogenetic multiple regression, we show that rates of climatic‐niche evolution explain most variation in richness among plethodontid clades, whereas rates of morphological evolution do not. We find little evidence that ecological limits explain patterns of richness among plethodontid clades. We also test whether rates of morphological and climatic‐niche evolution are correlated, and find that they are not. Overall, our results help explain richness patterns in a major amphibian group and provide possibly the first test of the relative importance of climatic niches and morphological evolution in explaining diversity patterns.     

Can we predict centres of plant species richness and rarity from environmental variables in sub‐Saharan Africa?

In order to investigate continental-scale patterns of plant species richness and rarity, distribution maps of 3661 plant species were digitized into a one degree grid of sub-Saharan Africa using the WORLDMAP computer programme. Cells with high species richness were also likely to be those containing the greatest number of species of restricted range, but areas such as the South African Cape and the Eastern Arc mountains were found to have more restricted-range species than predicted from their richness scores. The two environmental predictors which had the strongest individual relationships with both species richness and range-size rarity were absolute maximum annual temperature and mean monthly potential evapotranspiration. However, correlative predictive powers of these variables were low, with R =−0.58 and R =−0.54, respectively ( P  < 0.01). Multiple regression also failed to produce a strong explanatory model for observed continental-scale patterns of diversity. Spatial variability analysis showed that this was likely to be because different environmental parameters predicted different centres of richness and rarity. West African species richness was better predicted by absolute maximum annual temperature, whereas East African species richness was better predicted by mean monthly potential evapotranspiration.  © 2003 The Linnean Society of London, Botanical Journal of the Linnean Society , 2003, 142 , 187–197.     

What drives elevational patterns of diversity? A test of geometric constraints,climate and species pool effects for pteridophytes on an elevational gradient in Costa Rica

Aim We studied pteridophyte species richness between 100 m and 3400 m along a Neotropical elevational gradient and tested competing hypotheses for patterns of species richness. Location Elevational transects were situated at Volcán Barva in the Braulio Carrillo National Park and La Selva Biological Station (100–2800 m) and Cerro de la Muerte (2700–3400 m), both on the Atlantic slope of Costa Rica, Central America. Method We analysed species richness on 156 plots of 20 × 20 m and measured temperature and humidity at four elevations (40, 650, 1800 and 2800 m). Species richness patterns were regressed against climatic variables (temperature, humidity, precipitation and actual evapotranspiration), regional species pool, area and predicted species number of a geometric null model (the mid‐domain effect, MDE). Results The species richness of the 484 recorded species showed a hump‐shaped pattern with elevation with a richness peak at mid‐elevations (c. 1700 m). The MDE was the single most powerful explanatory variable in linear regression models, but species richness was also associated strongly with climatic variables, especially humidity and temperature. Area and species pool were associated less strongly with observed richness patterns. Main conclusions Geometric models and climatic models exclusive of geometric constraints explained comparable amounts of the elevational variation in species richness. Discrimination between these two factor complexes is not possible based on model fits. While overall fits of geometric models were high, large‐ and small‐ranged species were explained by geometric models to different extents. Species with narrow elevational ranges clustered at both ends of the gradient to a greater extent than predicted by the MDE null models used here. While geometric models explained much of the pattern in species richness, we cannot rule out the role of climatic factors (or vice versa) because the predicted peak in richness from geometric models, the empirical peak in richness and the overlap in favourable environmental conditions all coincide at middle elevations. Mid‐elevations offer highest humidity and moderate temperatures, whereas at high elevations richness is reduced due to low temperatures, and at low elevations by reduced water availability due to high temperatures.     

Does spatial heterogeneity blur the signature of dispersal syndromes on spatial patterns of woody species? A test in a tropical dry forest

Spatial patterns of adult plants are a consequence of several ecological processes related to seed dispersal and recruitment. Dispersal limitation, mediated by dispersal syndrome, is considered a key factor in the formation of adult plant spatial patterns. Although this initial pattern determined by dispersal has been thoroughly studied, the subsequently modification by the effect of additional ecological factors, such as habitat heterogeneity is less understood. We explored the relative importance of dispersal syndrome and spatial heterogeneity on the realization of spatial patterns of adult trees in an Ecuadorian tropical dry forest. The spatial distribution of 28 species was modeled with four different spatial point processes each: homogeneous Poisson (HPP), inhomogeneous Poisson (IPP), homogeneous Poisson cluster (HPCP), and inhomogeneous Poisson cluster process (IPCP). These models allowed us to discern between effects of random processes, habitat heterogeneity, limited dispersal, and joint effects of habitat heterogeneity and limited dispersal. We employed Akaike's information criterion (AIC) to select the model which best fit the spatial pattern of each species. The best model of each species was used to analyze differences in cluster size and degree of aggregation, between dispersal syndromes. Seventy‐five percent of the species showed inhomogeneous patterns. IPCP yielded the best fit for the spatial distribution of 50% of species in the studied forest and was the prevalent model for the three dispersal syndromes. Thus, the effect of spatial heterogeneity was prevalent in the distribution of most species in this dry tropical forest. Only 21% of species had spatial patterns compatible with random mechanisms associated to limited dispersal around parent sources. Clearly, ignoring habitat heterogeneity could bias the analysis of relationships between dispersal syndrome and species patterns.     

Does allometry of a sexually selected ornamental trait vary with sexual selection intensity? A multi‐species test in damselflies

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Co-variation and indicators of species diversity: Can richness of forest-dwelling species be predicted in northern boreal forests?

Design and establishment of ecologically good networks of conservation areas often requires quick assessments of their biodiversity. Reliable indicators would be useful when doing such assessments. In order to explore the potential indicators for species richness in boreal forests, we studied (1) the co-variation of species richness and composition of species assemblages among beetles, polypores, birds and vascular plants, (2) the relationships between species richness and four boreal forest site types, (3) the relationship between species richness and forest physical structure and (4) the suitability of potential indicator groups within the four taxa to predict the species richness generally. The data show that there are probably not a single taxonomic or forest structural characteristic to be used as a general biodiversity indicator or surrogate for all the species. The correlations in species richness among the four taxa studied were low. However, group-specific indicators were obvious: forest site type was a good surrogate for vascular plant richness, and quantity and quality of dead wood predicted the species richness of polypores. The results support the view that different indicators shall be used for different forest types and taxonomic groups. These indicators should facilitate relatively rapid methods to assess biodiversity patterns at the forest stand level.     

A 10‐year decrease in plant species richness on a neotropical inselberg: detrimental effects of global warming?

The census of vascular plants across a 10-year interval (1995–2005) at the fringe of a neotropical rainforest (Nouragues inselberg, French Guiana, South America) revealed that species richness decreased, both at quadrat scale (2 m²) and at the scale of the inselberg (three transects, embracing the whole variation in community composition). Juvenile stages of all tree and shrub species were most severely affected, without any discrimination between life and growth forms, fruit and dispersion types, or seed sizes. Species turnover in time resulted in a net loss of biodiversity, which was inversely related to species occurrence. The most probable cause of the observed species disappearance is global warming, which severely affected northern South America during the last 50 years (+2 °C), with a concomitant increase in the occurrence of aridity.     

The relative of species pools in determining plant species richness: an alternative explanation of species coexistence?

Explanations of the pattern of species have traditionally relied on small-scale, local processes occurring in ecological time. Differences in species richness have associated with different mechanisms avoiding competition, such as spatiotemporal heterogeneity (weaker competitors may find a more favourable place or time) or environmental stress (competition is assumed to be less intensive under difficult conditions). More recently, large-scale process have been taken into account, raising such questions as: which plant species may potentially grow in a certain community? Are evolutionary processes and species dispersal responsible for the differences between communities? The species-pool theory attempts to answer these general questions, and information about species pools is needed for the design of experiments where the number of species in a community is manipulated.