The island-mainland species turnover relationship

Many oceanic islands are notable for their high endemism, suggesting that islands may promote unique assembly processes. However, mainland assemblages sometimes harbour comparable levels of endemism, suggesting that island biotas may not be as unique as is often assumed. Here, we test the uniqueness of island biotic assembly by comparing the rate of species turnover among islands and the mainland, after accounting for distance decay and environmental gradients. We modelled species turnover as a function of geographical and environmental distance for mainland (M-M) communities of Anolis lizards and Terrarana frogs, two clades that have diversified extensively on Caribbean islands and the mainland Neotropics. We compared mainland-island (M-I) and island-island (I-I) species turnover with predictions of the M-M model. If island assembly is not unique, then the M-M model should successfully predict M-I and I-I turnover, given geographical and environmental distance. We found that M-I turnover and, to a lesser extent, I-I turnover were significantly higher than predicted for both clades. Thus, in the first quantitative comparison of mainland-island species turnover, we confirm the long-held but untested assumption that island assemblages accumulate biodiversity differently than their mainland counterparts.     

Richness of species and growth-forms within sclerophyll and mesophyll vegetation in eastern Australia

Abstract The patterns in total species richness and in the richness of the dominant growth-forms of vegetation communities of coastal sclerophyll and mesophyll vegetation in eastern Australia are examined. Plant species richness data were obtained from two 500 m² quadrats from 50 sites within a single geographical region north of Sydney, New South Wales. Concentrically nested subquadrats within each quadrat enabled the determination of species-area relationships for total species richness and its components. Three growth-forms were examined (trees, shrubs and ground cover) and patterns in the richness of these components were compared to those exhibited by total species richness. Total species richness was higher in sclerophyll communities on Hawkesbury Sandstone soils than in adjacent mesophyll communities on Narrabeen shales and sandstones. Significant patterns in total species richness within the two soil types were also found. Shrub and ground cover species richness showed strong correlations with total species richness, with higher richness in the sclerophyll communities. However, tree species richness contributed little to the patterns in total species richness. The results of this study suggest that differential patterns in the components of total species richness must be taken into account for effective modelling of natural areas based on species richness and diversity parameters.     

Declines in littoral species richness across both spatial and temporal nutrient gradients: a palaeolimnological study of two taxonomic groups

1. Using a palaeolimnological approach in shallow lakes, we quantified the species richness responses of diatoms and Cladocera to phosphorus enrichment. We also examined differences in species richness responses between littoral and pelagic assemblages of our focal communities. To address both spatial and temporal relationships, our study includes an analysis of both surface sediments from 40 lakes and of a lake sediment record spanning c. 120 years. The objective of our study was to determine whether similar species richness patterns occurred across trophic levels, as well as along spatial and temporal gradients. 2. We found that both diatom and Cladocera species richness estimates significantly declined with increasing phosphorus across space and through time. When the assemblages were subdivided according to known habitat preferences, littoral biodiversity maintained a negative trend, whereas pelagic species richness tended to show no relationship with phosphorus. 3. Negative productivity–diversity patterns have been observed across almost all palaeolimnological studies that span large productivity gradients. This congruence in patterns is most likely due to the similarity in data collection methods and in focal communities studied. The contrasting responses between littoral and pelagic assemblages may be explained by the differences in physical habitat and the pool of taxa in each of these environments. Consistent with the literature, we found statistical support for the idea that littoral diversity declines could be explained by an interaction between macrophytes and nutrients along strong trophic gradients. The general lack of a diversity response in our pelagic assemblages could be attributable to the limited pool of subfossil taxa. The response of the pelagic diatom could also be related to their broad range of nutrient tolerances. 4. The observed negative response of species richness to phosphorus enrichment, particularly in the littoral assemblages, has implications for ecosystems functioning because communities with reduced biodiversity often are less resilient to anthropogenic change.     

The effects of climate change on species composition,succession and phenology: a case study

Climate change and its role in altering biological interactions and the likelihood of invasion by introduced species in marine systems have received increased attention in recent years. It is difficult to forecast how climate change will influence community function or the probability of invasion as it alters multiple marine environmental parameters including rising water temperature, lower salinity and pH. In the present study, we correlate changes in environmental parameters to shifts in species composition in a subtidal community in Newcastle, NH through comparison of two, 3‐year periods separated by 23 years (1979–1981 and 2003–2005). We observed concurrent shifts in climate related factors and in groups of organisms that dominate the marine community when comparing 1979–1981 to 2003–2005. The 1979–1981 community was dominated by perennial species (mussels and barnacles). In contrast, the 2003–2005 community was dominated by annual native and invasive tunicates (sea‐squirts). We also observed a shift in the environmental factors that characterized both communities. Dissolved inorganic nitrogen and phosphate characterized the 1979–1981 community while sea surface temperature, pH, and chlorophyll a characterized the 2003–2005 community. Elongated warmer water temperatures, through the fall and early winter months of the 2000s, extended the growing season of native organisms and facilitated local dominance of invasive species. Additionally, beta‐diversity was greater between 2003–2005 than 1979–1981 and driven by larger numbers of annual species whose life‐history characteristics (e.g., timing and magnitude of recruitment, growth and mortality) are driven by environmental parameters, particularly temperature.     

Genetic diversity of Ranunculus acris L. (Ranunculaceae) populations in relation to species diversity and habitat type in grassland communities

Correlates between genetic diversity at intra- and interpopulation levels and the species diversity in plant communities are rarely investigated. Such correlates may give insights into the effect of local selective forces across different communities on the genetic diversity of local plant populations. This study has employed amplified fragment length polymorphism to assess the genetic diversity within and between 10 populations of Ranunculus acris in relation to the species diversity (richness and evenness) of grassland communities of two different habitat types, 'seminatural' and 'agriculturally improved', located in central Germany. Within-population genetic diversity estimated by Nei's unbiased gene diversity (HE) was high (0.258-0.334), and was not correlated with species richness (Pearson's r = -0.17; P = 0.64) or species evenness (Pearson's r = 0.15; P = 0.68) of the plant communities. However, the genetic differentiation between R. acris populations was significantly correlated with the difference in species evenness (Mantel's r = 0.62, P = 0.02), but not with difference in species richness of plant communities (r = -0.17, P = 0.22). Moreover, we also found that populations of R. acris from the 'seminatural' habitat were genetically different (amova, P < 0.05) from those in 'agriculturally improved' habitats, suggesting that gene flow between these habitat types is limited. The results reported in this study may indicate that habitat characteristics influence the genetic diversity of plant species.     

Testing the intermediate disturbance hypothesis on species diversity in herbaceous plant communities along a human trampling gradient using a 4-year experiment in an old-field

The intermediate disturbance hypothesis predicts unimodal relationships between species diversity and disturbance frequency/intensity. To test this hypothesis, species diversity in herbaceous plant communities along a human trampling gradient was investigated by conducting a 4-year experiment in an old-field. In general, species richness (S), the Shannon–Weiner index (H) from plant cover data and species evenness (J) showed negative linear relationships with trampling frequency, in contrast to the prediction of the intermediate disturbance hypothesis. However, the significant relationships between trampling frequency and species diversity were not observed in the fourth year without J, which showed a unimodal relationship. In all experimental years, the number of new species that colonized the plots after 1year was small under frequent trampling, and the number of species lost from the plots was large under infrequent trampling. The relative number and the relative cover of perennial species increased as trampling frequency increased in the first and second years, but this pattern was not observed in the following years because the dominance of perennials further increased at decreasing frequencies of trampling. The similarity in the species composition and the yearly changes in species dominance indicated that trampling at higher frequencies eliminated more trampling-intolerant species only in the early years of the experiment. These results suggest that trampling mediated early changes in species diversity patterns, but competitive interactions were more important in the later experimental years. The time lag in the effects of trampling and competition appears to be attributable to the infrequent occurrence of unimodal patterns of species diversity.     

Changes in agricultural intensity and river health along a river continuum

1. The impact of agricultural activities on waterways is a global issue, but the magnitude of the problem is often not clearly recognized by landowners, and land and water management agencies. 2. The Pomahaka River in southern New Zealand represents a typical lowland catchment with a long history of agricultural development. Fifteen sites were sampled along a 119-km stretch of the river. Headwater sites were surrounded by low-intensity sheep farming, with high-intensity pasture and dairying occurring in the mid-reach and lower reaches. 3. Water clarity decreased significantly from about 6 m in the headwaters to less than 2 m in the lower reaches. Benthic sediment levels increased significantly downriver, peaking at 35 mg m^??2 below several tributaries with high-intensity agriculture in their catchments. Periphyton levels were also significantly greater in the lower reaches than the headwaters, and coincided with increased nitrogen (DIN) and phosphorus (SRP) concentrations. 4. Macro-invertebrate species richness did not change significantly throughout the river, but species composition did with Ephemeroptera, and to a lesser extent, Plecoptera and Trichoptera dominating the headwater sites (where there was high water clarity, and low nutrient and periphyton levels). Downriver these assemblages were replaced by molluscs, oligochaetes and chironomids. 5. Canonical correspondence analysis indicated that agricultural intensity and physical conditions associated with agriculture activity (e.g. impacted waters, high turbidity and temperature) were strongly associated with the composition of benthic assemblages at differing reaches down the Pomahaka River. 6. The present results indicate that quantifying agricultural intensity within a catchment, particularly relative livestock densities, may provide a useful tool for identifying threshold levels above which river health declines.     

Changes in plant species diversity along a chronosequence of vegetation restoration in the humid evergreen broad-leaved forest in the Rainy Zone of West China

Plant species diversity has been recognized as one of the vital attributes for assessing vegetation restoration. Changes in the diversity may be related to different stages of succession. In this study, 54 sites of humid, evergreen, broad-leaved forest were selected in the Rainy Zone of West China. A chronosequence of the sites was used to study the successive patterns of the diversity in the forest that had undergone natural regeneration for 5 to 350 years and to test the hypothesis that the diversity is maximized in mid-succession. Data were collected simultaneously at different stages of succession, and four α-diversity indices (species richness, Margalef index, Shannon-Wiener index, Pielou Evenness index) and two β-diversity indices (Whittaker index, Sørensen’s index) were calculated for each stratum in each plot. A total of 394 vascular plant species were recorded. From the β-diversity indices, the forest succession may be divided into the early-successional stage (before 50 years), mid-successional stage (from 50 to 300 years), and late-successional stage (after 300 years). In this community, the species diversity and richness were found to be the greatest at the mid-successional stage, followed by the late- and early-successional stages. The results of regression analysis indicated that the richness and Margalef index peaked around the 175th and 165th year, respectively. Shannon-Wiener index values also appeared to follow an approximately humped pattern of succession and were maximal around the 100th year. However, the species evenness did not show any significant relationship with successional age. Our results demonstrate (1) forest restoration is a long-term process and the formation of climax forest requires at least 300 years and (2) the forest has a strong capacity for restoration. Our results also suggest Lindera limprichitii and Machilus pingii as ideal tree species for afforestation because of their wide niche.     

Fine‐scale patterns of species and phylogenetic turnover in a global biodiversity hotspot: Implications for climate change vulnerability

Question: What is the relative importance of environmental and spatial factors for species compositional and phylogenetic turnover? Location: High‐rainfall zone of the Southwest Australian Floristic Region (SWAFR). Methods: Correlates of species compositional turnover were assessed using quadrat‐based floristic data, and establishing relationships with environmental and spatial factors using canonical correspondence analyses and Mantel tests. Between‐quadrat phylogenetic distance measures were computed and examined for correlations with environmental and spatial attributes. Processes structuring pa2t2terns of beta diversity were also evaluated within four broad floristic assemblages defined a priori. Results: Floristic diversity was strongly related to environmental attributes. A low significance of spatial variables on assemblage patterns suggested no evident effect of dispersal limitations. Species compositional turnover was especially high within the swamp and outcrop assemblage. Phylogenetic turnover was closely coupled to species compositional turnover, implying the occurrence of many locally endemic and phylogenetically relict taxa. Beta diversity patterns within assemblages were also significantly correlated with the local environment, and relevant correlates differed between floristic assemblage types. Conclusion: Phylogenetic diversity in the SWAFR high‐rainfall zone is clustered within edaphic microhabitats in a generally subdued landscape. A clustered rather than dispersed distribution of phylogenetic diversity increases the probability of significant plant diversity loss during periods of climate change. Climate change susceptibility of the region's flora is accordingly estimated to be high. We highlight the conservation significance of swamp and outcrops that are characterized by distinct hydrological properties and may provide refugial habitat for plant diversity during periods of moderate climate change.     

Patterns of tree species richness in relation to environment in southeastern New South Wales,Australia

Abstract We present regression models of species richness for total tree species, two growth forms, rainforest trees (broadleaf evergreens) and eucalypts (sclerophylls), and two large subgenera of Eucalyptus. The correlative models are based on a data set of 166 tree species from 7208 plots in an area of southeastern New South Wales, Australia. Eight environmental variables are used to model the patterns of species richness, four continuous variables (mean annual temperature, rainfall, radiation and plot size), plus four categorical factors (topographic position, lithology, soil nutrient level and rainfall seasonality). Generalized linear modelling with curvilinear and interaction terms, is used to derive the models. Each model shows a significant and differing response to the environmental predictors. Maximum species richness of eucalypts occurs at high temperatures, and intermediate rainfall and radiation conditions on ridges with aseasonal rainfall and intermediate nutrient levels. Maximum richness of rainforest species occurs at high temperatures, intermediate rainfall and low radiation in gullies with summer rainfall and high nutrient levels. The eucalypt subgenera models differ in ways consistent with experimental studies of habitat preferences of the subgenera. Curvilinear and interaction terms are necessary for adequate modelling. Patterns of richness vary widely with taxonomic rank and growth form. Any theories of species diversity should be consistent with these correlative models. The models are consistent with an available energy hypothesis based on actual evapotranspiration. We conclude that studies of species richness patterns should include local (e.g. soil nutrients, topographic position) and regional (e.g. mean annual temperature, annual rainfall) environmental variables before invoking concepts such as niche saturation.     

Intermediate disturbance in the ecology of phytoplankton and the maintenance of species diversity: a synthesis

This paper concludes a collection of contributions presented at the 8th Workshop of the International Association of Phytoplankton Taxonomy and Ecology. It derives a consensus as to the virtues and strengths of J. H. Connell's Intermediate Disturbance Hypothesis (IDH), its applicability to phytoplankton ecology and its theoretical and practical weaknesses. The view is expressed that the IDH is too useful a concept to reject and that, as a word model, it provides a powerful link between diversity and disturbance. The more robust investigations that are necessary to consolidate the tenancy of IDH need to concentrate upon the separation and quantification of the stimulus- and response-components of disturbance.     

Plant community dynamics in a semi-arid ecosystem in relation to nutrient addition following a major disturbance

Effects of annual additions of mineral N and P (100 kg ha^–1) on plant species composition and annual aboveground net primary production (ANPP) were investigated during the first three years following disturbance in a semi-arid ecosystem. Additions of N reduced richness of perennial plant species during years 2 and 3, while P reduced the number of perennial species only in year 3. From year 1 to year 2, annual and biennial species richness declined in all treatments while ANPP of annual species increased greatly. Added N increased ANPP of annual species while it decreased ANPP of most perennial species relative to the unfertilized control treatment. Community similarities were higher for the control and native vegetation than for other pairs of treatments using both species presence and plant production data. Nitrogen additions have retarded but not completely arrested secondary succession in this system.     

Correlations of phytoplankton standing crop,species diversity and dominance with physical-chemical data in a coastal salt pond

A set of environmental variables from a coastal salt pond were regressed in a stepwise fashion against the following dependent variables: phytoplankton standing crop, species diversity and dominance. Multiple variable interaction, as measured by multiple correlation coefficients yielded higher values than the coefficients obtained by regressing individual environmental variables against the dependent variables. The seasonal variation in the dependent variables could, therefore, be explained to a greater extent by the effects of multiple variables rather than single limiting factors.Financial support was supplied by a fellowship from the Jessie Smith Noyes Foundation to the senior author.     

Shoot biomass and species richness in relation to some environmental factors in a coastal dune area in The Netherlands

Shoot biomass, species richness and selected environmental factors were studied at Meijendel, a coastal dune area in The Netherlands. The relationship between species richness and shoot biomass of the stand could be best described by a unimodal curve, with the peak at 300 g m^–2. Measures of species richness were positively correlated with soil water, nitrogen and phosphorus.Shoot biomass showed a positive correlation with soil water, nitrogen and humus at low levels of shoot biomass.Nomenclature of phanerogams and plant community types follows Heukels & van Ooststroom (1973), and Westhoff & den Held (1969) respectively.Publication of the Meijendel comité new series: 76.