Urban domestic gardens (VI): environmental correlates of invertebrate species richness

Domestic gardens associated with residential zones form a major component of undeveloped land in towns and cities. Such gardens may play a vital role in maintaining biodiversity in urban areas, but explanations for the variation in the richness of species assemblages in gardens are lacking. We report the results from a case study of 12 invertebrate groups in 61 domestic gardens in the city of Sheffield, UK. The mean number of species within a taxon, recorded per garden, was no greater than 3, 10, and 20 species in litter, pitfall trap and Malaise trap samples, respectively. Relatively speciose groups exhibited high turnover between gardens, with typically 50% of the group occurring only once. In contrast, several species-poor taxa were virtually ubiquitous. Species richness was analysed by multiple regression and hierarchical tree analysis in relation to garden and landscape variables. In general, the two methods of analysis corroborated one another. In total, 22 explanatory variables entered into regression models, although 12 of them only did so once. The amount of variation in species richness explained in models was generally quite high, with the factors involved operating over a range of scales. However, the patterns that emerged were not consistent across taxa. The most important predictors of species richness, of relevance to land use planners, were components of garden vegetation, especially the abundance of trees. Likely reasons for inconsistencies in the relationships are discussed in the context of sampling and species biology.     

Grain-dependent relationships between plant productivity and invertebrate species richness and biomass in calcareous grasslands

The relationships among productivity, species richness and consumer biomass are of fundamental importance for understanding determinants of biodiversity. These relationships may depend on grain size. We examined the relationships between productivity (above-ground phytomass) and plant species richness and between productivity and species richness and biomass of gastropods and grasshoppers using sampling units of different sizes (0.5, 2.75 and 23 m²) in nutrient-poor, calcareous grasslands in north-western Switzerland in two successive years. Species richness of forbs had a unimodal relationship with productivity in sampling units of 0.5 m² and was negatively correlated with productivity at the other two plot sizes in one year. In the other year, forb species richness tended to decrease with productivity in sampling units of 23 m². No similar relationship was found for grasses. Gastropod biomass had a unimodal relationship with productivity at 0.5 m² in the first year. Grasshopper species richness was correlated with forb species richness at plot sizes of 2.75 and 23 m². This study demonstrates that patterns detected between productivity and diversity and between productivity and biomass of consumers depend on the grain size used in the investigation and vary among years.Die Zusammenhänge zwischen Produktivität, Artenreichtum und Biomasse von Konsumenten sind wichtig, um zu verstehen, was Biodiversität beeinflußt. Diese Zusammenhänge können von der Größe der Untersuchungsfläche abhängig sein. Wir untersuchten während zwei aufeinanderfolgenden Jahren die Zusammenhänge zwischen Produktivität (oberirdische Pflanzenbiomasse) und Artenreichtum von Gefäßpflanzen, sowie zwischen Produktivität und Artenreichtum und Biomasse von Schnecken und Heuschrecken bezüglich dreier räumlicher Skalen (0,5, 2,75 und 23 m²) in Kalkmagerrasen in der Nordwestschweiz. Der Zusammenhang zwischen dem Artenreichtum von Kräutern und der Produktivität war unimodal in Flächeneinheiten von 0,5 m² und negativ in Flächeneinheiten von 2,75 und 23 m² im ersten Jahr und war tendenziell negativ in Flächeneinheiten von 23 m² im zweiten Jahr, während kein solcher Zusammenhang bei Gräsern gefunden wurde. Der Zusammenhang zwischen Produktivität und Biomasse von Schnecken war unimodal in Flächeneinheiten von 0,5 m² im ersten Jahr. Außerdem bestand ein Zusammenhang zwischen dem Artenreichtum von Kräutern und Heuschrecken in Flächeneinheiten von 2,75 und 23 m². Diese Arbeit zeigt, daß Zusammenhänge zwischen Produktivität und Diversität sowie zwischen Produktivität und Biomasse von Konsumenten von der Größe der Untersuchungsfläche abhängen und zwischen Jahren variieren.     

Effects of small mammal and invertebrate herbivory on plant species richness and abundance in tallgrass prairie

Summary A factorial field experiment was designed to test the effects of small mammals and above- and below-ground invertebrates on plant species richness and composition in native tallgrass prairie at Konza Prairie Research Natural Area, northeast Kansas. Over a 4-year period, Microtus ochrogaster densities were maintained by live-trapping in fenced plots, and invertebrate levels were reduced using the pesticides carbaryl for above-ground invertebrates and an organophosphate (isofenphos) for belowground invertebrates. ANOVA according to a split-plot design of plant species biomass data harvested in 1984 and 1986 revealed few significant effects of either small mammal densities or pesticide application. Of 54 species harvested from both sample dates, only 10 were significantly affected by either treatment. Analysis of species richness according to 8 life-form classes provided a clearer pattern of response than did biomass either by species or life-form class. For example, numbers of C4 grasses were reduced by increasing small mammal densities, whereas numbers of C4 annual forbs were lowest when above-ground herbivory was reduced. While consumers have been shown to have strong effects on successional communities, the few significant results observed in this study suggests that the manipulated levels of small mammals and insects had few effects on a mature tallgrass prairie.Deceased May, 1986     

Variation in plant species richness in and adjacent to 64 ponds in SE Norwegian agricultural landscapes

All vascular plants, classified by life and growth form into six groups, four groups of hydrophytes (lemnids, nymphaeids, elodeids and isoetids), helophytes, and terrestrial species of pond margins, were inventoried in 64 SE Norwegian agricultural landscape ponds and their adjacent margins. The study sites varied considerably with respect to species richness; 0–4 for each hydrophyte group, 0–9 for helophytes, and 13–77 for terrestrial species. A total of 56 explanatory variables were recorded for each pond and adjacent margin to explain the observed richness variability.     

Environmental correlates of species and genetic richness in lungless salamanders (family plethodontidae)

Biological diversity is distributed across the planet in non-random, organised ways. At the species level, numerous environmental variables have been proposed to explain this non-random distribution with available energy and habitat heterogeneity receiving the most empirical support. With regard to genetic organisation, environmental stress and habitat heterogeneity have been widely supported. However, few studies have addressed if these two scales of biological organisation are structured via similar processes. Here, we tested whether or not the distributional organisation of genetic and species richness were driven by similar environmental variables for salamanders of the family Plethodontidae across North America. In general, we found that those environmental variables related to energy, particularly energy made accessible to salamanders via the actions of available moisture, were the primary determinants of both genetic and species richness. This finding is consistent with both the “more individuals hypothesis” of species richness and neutral-theory expectations for genetic richness. Additionally, greater habitat heterogeneity, as measured by increased topographic variance, was of secondary importance in positively influencing species richness, although its effects on genetic richness were far more variable. In total, our results suggest that both of these scales of biological organisation are influenced by similar environmental variables, even though increased genetic richness at the population-level does not always translate into greater species richness.     

Environmental correlates of vegetation patterns and species richness in the northern Grampians,Victoria

Abstract Plant species cover-abundance and density data were collected for 94 sample plots across a gradient from rocky uplands to sandy outwash plains in the northern part of Grampians (Gariwerd) National Park in western Victoria. Detrended correspondence analysis (DCA) was used to identify dominant gradients in species composition. A range of static (e.g. substrate type, soil depth, microclimate indicators) and dynamic (e.g. elapsed time since last fire) environmental variables were measured. Correlations were sought between these variables and vegetation patterns including those for richness (R) and Shannon-Weiner diversity (H′). The dominant gradient of vegetation change identified by DCA separated rocky sites and sites near ephemeral streams, from well-drained, sandy sites. Secondary gradients identified time since last fire as important for sandy sites, and altitude and aspect-related microclimate for rocky sites. Diversity was highest in the first 2 years after fire but showed no further decline in older sites. Overall, R and H' were negatively correlated with soil nutrient concentrations. On sandy sites R was high, but was low on rocky sites and near streams. Within the rocky sites, R was highest on cool, moist south and east slopes, and lowest on hot, dry north and west slopes. Explanations of diversity patterns based on inhibition of competitive exclusion due to stress and recurrent disturbance best fit the results presented here.     

Imprints of glacial history and current environment on correlations between endemic plant and invertebrate species richness

Aim We evaluate how closely diversity patterns of endemic species of vascular plants, beetles, butterflies, molluscs and spiders are correlated with each other, and to what extent similar environmental requirements or survival in common glacial refugia and comparable dispersal limitations account for their existing congruence. Location Austria. Methods We calculated pairwise correlations among species numbers of the five taxonomic groups in 1405 cells of a 3′ × 5′ raster (c. 35 km²) using the raw data as well as the residuals of regression models that accounted for: (1) environmental variables, (2) environmental variables and the occurrence of potential refugia during the Last Glacial Maximum, or (3) environmental variables, refugia and spatial filters. Results Pairwise cross‐taxonomic group Spearman’s rank correlations in the raw data were significantly positive in most cases, but only moderate (0.3 < ρ < 0.5) to weak (ρ < 0.3) throughout. Correlations were closest between plants and beetles, plants and butterflies, and plants and snails, respectively, whereas the distribution of endemic spiders was largely uncorrelated with those of the other groups. Environmental variables explained only a moderate proportion of the variance in endemic richness patterns, and the response of individual groups to environmental gradients was only partly consistent. The inclusion of refugium locations and the spatial filters increased the goodness of model fit for all five taxonomic groups. Moreover, removing the effects of environmental conditions reduced congruence in endemic richness patterns to a lesser extent than did filtering the influence of refugium locations and spatial autocorrelation, except for spiders, which are probably the least dispersal‐limited of the five groups. Main conclusions The moderate to weak congruence of endemic richness patterns clearly limits the usefulness of a surrogacy approach for designating areas for the protection of regional endemics. On the other hand, our results suggest that dispersal limitations still shape the distributions of many endemic plant, snail, beetle and butterfly species, even at the regional scale; that is, survival in shared refugia and subsequent restricted spread retain a detectable signal in existing correlations. Concentrating conservation efforts on well‐known Pleistocene refugia hence appears to be a reasonable first step towards a strategy for protecting regional endemics of at least the less mobile invertebrate groups.     

Environmental correlates of species richness of European springtails (Hexapoda: Collembola)

Our knowledge about environmental correlates of the spatial distribution of animal species stems mostly from the study of well known vertebrate and a few invertebrate taxa. The poor spatial resolution of faunistic data and undersampling prohibit detailed spatial modeling for the vast majority of arthropods. However, many such models are necessary for a comparative approach to the impact of environmental factors on the spatial distribution of species of different taxa. Here we use recent compilations of species richness of 35 European countries and larger islands and linear spatial autocorrelation modeling to infer the influence of area and environmental variables on the number of springtail (Collembola) species in Europe. We show that area, winter length and annual temperature difference are major predictors of species richness. We also detected a significant negative longitudinal gradient in the number of springtail species towards Eastern Europe that might be caused by postglacial colonization. In turn, environmental heterogeneity and vascular plant species richness did not significantly contribute to model performance. Contrary to theoretical expectations, climate and longitude corrected species–area relationships of Collembola did not significantly differ between islands and mainlands.     

Grassland plant and invertebrate species richness increases from mowing are mediated by impacts on soil chemistry

Pasture and improved grasslands are commonly managed by a combination of artificial fertilisation and biomass removal, but a deeper understanding of how management options interact over the long-term are required to improve sustainability. Studies of multi-trophic responses to these options can provide important insights for biodiversity and soil management, particularly when they cover long time periods. In this study, we provide a novel perspective on long-term experimental field studies of grassland management by examining the direct and indirect effects of N fertilisation and mowing (with biomass retention and removal) on above-ground biodiversity, below-ground soil chemistry and their interactions. Our experimental treatments were applied annually from 1994 in medium to high soil fertility conditions on a non-native pastoral farm in New Zealand, and analysis of data to 2013 show that in general, plants and soil properties did not respond to N fertiliser treatments. In response to mowing regimes, soil properties exhibited subtle, but annually varying changes mostly related to biomass retention or removal, and plant richness was consistently higher under all mowing treatments. The management regime with the greatest gains in diversity also depended on year of study. We further analysed the indirect effects of mowing treatments on plant and arthropod richness via soil properties using structural equation modelling, and found that the impact of mowing is likely to be mediated by soil chemistry changes. In particular, the direct positive impact of mowing on plant richness may be offset by changes to soil properties, depending on whether biomass is retained or removed. We suggest that management regime effects on soil chemistry may limit plant composition changes to those species able to take advantage of altered conditions. These findings suggest that management to improve grassland diversity and soil conditions should consider the abiotic history and conditions of the site.     

Habitat patchiness and plant species richness

The pattern of woody species richness decline with a decrease in woody vegetation cover was studied within a tallgrass prairie. The decline in species richness is highly non-linear, with a well-defined threshold below which species richness collapses. This relationship can be understood after considering information on how landscape structure changes with woody vegetation cover, and how species richness is related to landscape structure.     

Effects of woody plant species richness on mammal species richness in southern Africa

Aim To determine the relationship between the species richness of woody plants and that of mammals after accounting for the effect of environmental variables. Location Southern Africa, including Namibia, South Africa, Lesotho, Swaziland, Botswana, Zimbabwe, and part of Mozambique. Methods We used a comprehensive dataset including the species richness of mammals and of woody plants and environmental variables for 118 quadrats (each of 25,000 km²) across southern Africa, and used structural equation models (SEMs) and spatial regressions to examine the relationship between the species richness of woody plants and of mammal trophic guilds (herbivores, insectivores, carni/omnivores) and habitat guilds (aquatic/fossorial, ground‐living, climbers, aerial), after controlling for environment. We compared the results of SEMs with those of single‐predictor regressions (without controlling for environment) and of spatial regressions (controlling for both environment and residual spatial autocorrelation). Results The geographical variation of mammal species richness in southern Africa was strongly and positively related to that of woody plant species richness, and this relationship held for most mammal guilds even when the influence of environment and spatial autocorrelation had been accounted for. However, the effect of woody plant species richness on the richness of aquatic/fossorial species almost disappeared after controlling for environment, suggesting that the congruence in species richness patterns between these two groups results from similar responses to the same environmental variables. For many mammal guilds, the relative role of environmental predictors as measured by standardized partial regression coefficients changed depending on whether non‐spatial single‐predictor regressions, non‐spatial SEMs, or spatial regressions were used. Main conclusions Woody plants are important determinants of the species richness of most mammal guilds in southern Africa, even when controlling for environment and residual spatial autocorrelation. Environmental correlates with animal species richness as measured by simple correlations or single‐predictor regressions might not always reflect direct effects; they might, at least to some degree, result from indirect effects via woody plants. Interpretations of the strength of the effect of environmental variables on mammal species richness in southern Africa depend largely on whether spatial or non‐spatial models are used. We therefore stress the need for caution when interpreting environmental ‘effects’ on broad‐scale patterns of species richness if spatial and non‐spatial methods yield contrasting results.     

Environmental correlates of leguminosae species richness in Mexico: Quantifying the contributions of energy and environmental seasonality

Explaining species richness patterns is a central issue in ecology, but a general explanation remains elusive. Environmental conditions have been proposed to be important drivers of these patterns, but we still need to better understand the relative contribution of environmental factors. Here, we aim at testing two environmental hypotheses for richness gradients: energy availability and environmental seasonality using diversity patterns of the family Leguminosae across Mexico. We compiled a data base of 502 species and 32,962 records. After dividing Mexico into 100 × 100 km grid cells, we constructed a map of variation in species richness that accounts for heterogeneity in sampling effort. We found the cells with the highest species richness of legumes are in the Neotropical region of Pacific coastal and southern Mexico, where the legume family dominates the tropical rain forests and seasonally dry tropical forests. Regression models show that energy and seasonality predictors can explain 25% and 49% of the variation in richness, respectively. Spatial autocorrelation analysis showed that richness has a strong spatial structure, but that most of this structure disappears when both energy and seasonality are used to account for richness gradient. Our study demonstrates multiple environmental conditions contribute complementarily to explain diversity gradients. Moreover, it shows that in some regions, environmental seasonality can be more important than energy availability, contradicting studies at coarser spatial scales. More basic taxonomic and floristic work is needed to help describe patterns of diversity for many groups to allow for testing the underlying mechanisms responsible for diversity gradients. Abstract in Spanish is available with online material.     

Higher zooplankton species richness associated with an invertebrate top predator

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Plant species richness in a natural Argentinian matorral shrub-land correlates negatively with levels of plant phosphorus

The aim of this study was to ascertain whether there is a relationship between plant species richness and plant-available N, P and water in an environment subject to little anthropogenic disturbance. To accomplish this we studied the vegetation in matorral shrub-lands in northern Patagonia, Argentina. Due to the variation in slope, precipitation and aspect between the sites water status was determined using the ¹²C/¹³C fraction, ??¹³C, to investigate whether this was a confounding factor. The numbers of herb, shrub, liana and tree species were determined at 20 sites along an estimated precipitation gradient. Leaf P and N content and the ??¹³C of Berberis buxifolia were determined, as well as the soil P and N content at the different sites. A negative correlation was found between species richness and Berberis buxifolia foliar P concentration (52% of the species richness variation was accounted for), and a positive correlation was found between plant species richness and Berberis buxifolia foliar N: P ratios (54% of the species richness variation was accounted for). The relationship between species richness and foliar P was seen when all layers of vegetation were included (trees, lianas, shrubs and herbs). Foliar N showed no correlation with species richness, while soil extractable NH₄ showed a weak positive correlation with the number of shrub layer species (lianas, shrubs and trees). The species richness of the shrub layer increased with decreasing values of ??¹³C. Low soil P availability thus affects local species richness in the matorral shrub-lands of Patagonia in Argentina although the growth of vegetation in the area has been shown to be limited by N. We suggest that low P levels increase plant species richness because low soil P concentration is associated with a high P partitioning and high potential for niche separation.     

Dependence of broad-scale geographical variation in fleshy-fruited plant species richness on disperser bird species richness

Aim We analysed the interdependence of avian frugivore‐ and fruited plant‐species richness at the scale of major river basins across Europe, taking into account several environmental factors along different spatial gradients. Location Continental Europe and the British Isles. Methods We focused on wintering birds and autumn/winter fruiting plants, and used major river basins as geographical units and Structural Equation Modelling as the principal analytical tool. Results The statistical influence of disperser species richness on fleshy‐fruited plant species richness is roughly double that of the reverse. Broad‐scale variation in frugivore richness is more dependent on environmental factors than on fruited plant richness. However, the influence of disperser richness on plant richness is four times higher than the influence of environmental factors. Environmental influences on both birds and plants are greater than purely spatial influences. Main conclusions Our results are interpreted as indicating that biotic dispersal of fruits strongly affects broad‐scale geographical trends of fleshy‐fruited plant species richness, whereas richness of fruited plants moderately affects frugivore richness.     

Environmental gradients shift the direction of the relationship between native and alien plant species richness

Aim

To assess how environmental, biotic and anthropogenic factors shape native–alien plant species richness relationships across a heterogeneous landscape.

Location

Banks Peninsula, New Zealand.

Methods

We integrated a comprehensive floristic survey of over 1200 systematically located 6 × 6 m plots, with corresponding climate, environmental and anthropogenic data. General linear models examined variation in native and alien plant species richness across the entire landscape, between native‐ and alien‐dominated plots, and within separate elevational bands.

Results

Across all plots, there was a significant negative correlation between native and alien species richness, but this relationship differed within subsets of the data: the correlation was positive in alien‐dominated plots but negative in native‐dominated plots. Within separate elevational bands, native and alien species richness were positively correlated at lower elevations, but negatively correlated at higher elevations. Alien species richness tended to be high across the elevation gradient but peaked in warmer, mid‐ to low‐elevation sites, while native species richness increased linearly with elevation. The negative relationship between native and alien species richness in native‐dominated communities reflected a land‐use gradient with low native and high alien richness in more heavily modified native‐dominated vegetation. In contrast, native and alien richness were positively correlated in very heavily modified alien‐dominated plots, most likely due to covariation along a gradient of management intensity.

Main conclusions

Both positive and negative native–alien richness relationships can occur across the same landscape, depending on the plant community and the underlying human and environmental gradients examined. Human habitat modification, which is often confounded with environmental variation, can result in high alien and low native species richness in areas still dominated by native species. In the most heavily human modified areas, dominated by alien species, both native and alien species may be responding to similar underlying gradients.

     

Indicators of species richness at the local scale in an alpine region: a comparative approach between plant and invertebrate taxa

Studies investigating congruent variations in species richness patterns in alpine habitats are scarce. We investigated the potential of using the indicator taxa approach for species richness in alpine habitats of the Scandes (Norway). In four areas, we investigated seven functional and taxonomic terrestrial groups of organisms and evaluated their contribution to the species diversity. The function of each group as a surrogate for the overall species diversity or for the diversity of another taxon was analysed. Three groups of invertebrates (spiders without Lycosids, Lycosids only, and ground beetles), three groups of plants (shrubs, graminoids, and herbs), and lichens were used for a cross-taxon analysis of species diversity. Congruence in species richness was restricted to several significant results, with vascular plants and spiders (Araneae) being best suited as surrogate taxa in alpine habitats of the Scandes. In the cross-taxon analyses they showed strongest significant positive correlations, covering the total species richness of the alpine habitats best. Species counts in one group account for up to 70% of the variation in total species richness. We found only limited evidence for an ideal, efficient biodiversity indicator taxon that could be applied without restrictions at different alpine habitats in low and middle alpine areas. Thus, our results suggest that it is very important to use more than one taxon as indicator for species richness in terrestrial alpine habitats. This should facilitate future conservation planning in alpine areas.     

Environmental correlates of mammal species richness in South America: effects of spatial structure, taxonomy and geographic range

Although some consensus exists regarding the positive synergism between energy and heterogeneity in increasing species diversity, the role of environmental variability remains controversial. We examine how these factors interact to explain spatial variation in mammal species richness in South America. After taking into account the effects of spatial autocorrelation and area, elevation variability and energy mainly drive spatial variation in mammal species richness. The effect of environmental variability is less important. When different taxonomic groups of mammals are analyzed separately, three ways emerge whereby energy and heterogeneity interact to promote species richness. Heterogeneity may have no effect on species richness, habitat heterogeneity and energy availability contribute independently to species richness, or heterogeneity increases in importance with an increase in energy availability. The partition of species into range size quartiles shows that habitat heterogeneity and temporal instability in the resource supply account for the species richness pattern in the narrowest- ranging species. Habitat heterogeneity is significant also for intermediate ranging species but not for the widest-ranging species. Energy alone drives the species richness pattern in the latter species. The interplay between ecology and biogeographic history may ultimately explain these differences given that narrow- and wide-ranging species show distinct biogeographic patterns, and different taxonomic groups also unequally represent them.