Invasion-resistance in experimental grassland communities: species richness or species identity?

The question as to why some communities are more invasible than others has pro-found implications for conservation biology and land management. The theoretical issues involved go right to the heart of our understanding of species coexistence and community assembly. The experiment reported here indicates that for productive, small-scale grassland plots, species identity matters more than species richness in determining both the number of invading species and the total biomass of invasives.     

Why does grassland productivity increase with species richness? Disentangling species richness and composition with tests for overyielding and superyielding in biodiversity experiments.

The causal relationship between the biodiversity of natural and modified environments and their net primary production has been a topic of significant scientific controversy and scrutiny. Early theoretical and empirical results indicated that production was sometimes significantly correlated with species richness when species richness was directly manipulated in experimental systems. Possible mechanisms for this phenomenon include statistical sampling effects, complementary resource use and mutualistic interactions. However, the interpretation of experimental results has sometimes confounded species richness with species composition, and disentangling the effects of species diversity from species identity has proved a formidable challenge. Here, I present a statistical method that is based on simple probability models and does not rely on the species composition of individual plots to distinguish among three phenomena that occur in biodiversity-production experiments: underyielding, overyielding and (a new concept) superyielding. In some cases, distinguishing these phenomena will provide evidence for underlying mechanisms. As a proof-of-concept, I first applied this technique to a simulated dataset, indicating the strengths of the method with both clear and ambiguous cases. I then analysed data from the BIODEPTH experimental biodiversity manipulations. No evidence of either overyielding or superyielding was detected in the BIODEPTH experiment.     

Relationships between species’ floral traits and pollinator visitation in a temperate grassland

Knowledge about plant–plant interactions for pollinator service at the plant community level is still scarce, although such interactions may be important to seed production and hence the population dynamics of individual plant species and the species compositions of communities. An important step towards a better understanding of pollination interactions at the community level is to assess if the variation in floral traits among plant species explain the variation in flower visitation frequency among those species. We investigated the relative importance of various floral traits for the visitation frequency of all insects, and bumblebees and flies separately, to plant species by measuring the visitation frequency to all insect-pollinated species in a community during an entire flowering season. Visitation frequency was identified to be strongly positive related to the visual display area and the date of peak flowering of plant species. Categorical variables, such as flower form and symmetry, were important to the visitation frequency of flies only. We constructed floral similarity measures based on the species’ floral traits and found that the floral similarity for all species’ traits combined and the continuous traits separately were positively related to individual visitation frequency. On the other hand, plant species with similar categorical floral traits did not have similar visitation frequencies. In conclusion, our results show that continuous traits, such as flower size and/or density, are more important for the variation in visitation frequency among plant species than thought earlier. Furthermore, differences in visitation frequency among pollinator groups give a poor support to the expectations derived from the classical pollination syndromes.     

Warmer and richer? Predicting the impact of climate warming on species richness in small temperate waterbodies

Climate change is expected to affect communities worldwide. Many studies focus on responses at the regional level and show an increase in species richness. However, less is known about the consequences of climate change at the local scale (in ecosystems). Small waterbodies, such as ponds, could play an important role for the assessment of the impact of future changes in climate at the local level. We evaluated here the potential changes due to climate warming in the species richness for various groups (plants, snails, beetles, dragonflies, amphibians) across 113 lowland and high altitude ponds in Switzerland. We modelled the relationships between species richness and environmental variables (including temperature) and predicted species richness changes for the end of the century (2090–2100; using the A2 IPCC scenario). Temperature rise could significantly increase pond species richness. For the five taxonomic groups pooled, species richness would potentially increase from 41 to 75 (+83%) in lowland ponds. In presently species‐poor high altitude ponds, the potential increase would be particularly marked, with a proportional increase (+150%; from 14 to 35 species) almost double that in lowland areas. A strong increase in species richness also resulted from models including changes in additional variables, such as landuse or water quality. Future reductions in water quality (e.g. increase in nutrients) may limit the predicted increase in lowland species richness or, conversely, result in a greater increase in species richness in high altitude areas. Nutrient enrichment is shown to affect the taxonomic groups differentially, with plant species richness the most negatively influenced. Climate warming could therefore affect species richness of temperate ponds not only regionally, but also at the local, within ecosystems‐scale; species richness could increase markedly in temperate regions, and especially so at higher altitude.     

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.     

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.     

Small-scale distribution of species richness in a grassland (Bílé Karpaty Mts., Czech Republic)

Variation in the number of species was studied in a subthermophilous grassland at a scale of 0.05 ×0.05 m during a 5-year period. The observed variance of species richness (VSR) was compared with a null model based on random distribution of species over a set of squares. It was found that distribution of species richness had more values than, expected around the mean and less values at the “shoulders”. Both tails fell within the predicted limits. Application of the procedures removing spatial dependence (random shifts, rotation/reflection method byPalmer & van der Maarel 1995) and environmental heterogeneity (patch model byWatkins & Wilson 1992) did not change the observed pattern. Using simulations in which the number of clumps and clumping intensity were manipulated it was found that the effect of the clumped spatial pattern, on VSR results in a wide range of variances. Both variance excess and variance deficit were found more frequently than expected under the null model. To test the effect of the limitation to the number of individuals per square, a null model was developed which included that observed number of plant shoots per square, the observed distribution of the number of shoots belonging to individual species per square and the observed spatial distribution of the shoots. The observed VSR was still lower than that produced by the null model. Therefore, it is concluded that at a scale of 0.05×0.05 m plant species combine in a non-random way in the studied grassland. It is suggested that the shape of left and right “shoulders” of the species richness distribution may be caused by different factors, such as positive and negative covariance between species, respectively. Their simultaneous impact can generate the observed pattern in species richness.     

Variation in species richness on small grassland quadrats: niche structure or small-scale plant mobility?

Abstract. In this study we make clear that the significance of deviations from an expected variance in species richness as demonstrated in alvar grassland, is a function of spatial pattern at the scale richness was measured, i.e. 10 cm². If corrected for spatial dependence, more than half of the significant cases become nonsignificant. As regards cases of significant variance deficit, which has been interpreted as an indication of niche limitation, we suggest that there may be a simple physical limitation to the number of species on the scale of observation in the form of a low number of plant units which can find a place because of the modal plant size of the species involved. In most cases of significant variance deficit the modal size of the plant units involved was probably bigger than average. Insofar as a significant variance deficit can be demonstrated, the resulting species richness distribution curve should be analyzed and we propose a skewness test, enabling us to differentiate between significantly left-skewed curves (‘niche limitation’), significantly right-skewed curves (‘niche facilitation’) and symmetrical curves. We present results obtained with the G-test, a log-likelihood ratio goodness of fit test. Only few cases of significantly left-skewed curves and a majority of symmetrical curves were found. Attempts to demonstrate guild proportionality in grasslands suffer from the heterogeneity of usually distinguished guilds, such as annuals vs. perennials or graminoids vs. dicotyledons. We observe that niche limitation may occur in alvar grassland, but indications for niche facilitation are stronger. Finally, we conclude that deviations in species richness variance are interesting indications of community structure, but only of spatial structure. Niche structure resulting from assembly rules should be investigated through experiments.     

Does land use change affect the interactions between two dry grassland species?

Over the last 20 years, a change in traditional land use practices has taken place in central Germany. Formerly species-rich dry grassland communities have been converted into communities with greatly reduced diversity in many places. Whereas grass species have expanded, several forbs have declined in abundance. For the present study, plant-plant interactions were assessed between the expanding grass Festuca rupicola and the forb Dianthus carthusianorum - two typical, companion grassland species - to ascertain any associated effects of land use change. A competition experiment was set up with replacement design, in which pots were placed in the open field and monitored over two consecutive years. In order to assess the performance of both species in monoculture (D9, F9) and mixture (D3F6, D6F3) the parameters biomass, length of flowering stems, number of flowering stems and number of flowers per individual, or length of panicles, were analyzed. Positive and negative interactions were evaluated using the “Relative Neighbour Effect” index (RNE). To simulate land use by grazing or mowing, individuals in half of the pots were clipped in the first year after sowing. At this time, performance of D. carthusianorum - in terms of length of flowering stems, number of flowering stems and number of flowers - was significantly affected: whereas the first parameter increased, both of the other parameters decreased with increasing presence of the grass species. However, RNE indicated facilitation with a low number of F. rupicola individuals, and competition with a high number of F. rupicola individuals. In contrast, F. rupicola did not show any significant differences in performance with increasing presence of D. carthusianorum, and the RNE switched from neutral interaction to facilitation. In the second year after sowing, the performance of D. carthusianorum in the various treatments did not differ, except for the length of the flowering stems. The simulation of land use practices did not affect the performance of D. carthusianorum, but F. rupicola showed significant differences in biomass and in length of the panicles, as was the case also between monocultures and mixtures. The performance of the grass species was clearly improved in the unclipped treatments. The RNE reflected competition against D. carthusianorum which became reduced in unclipped treatments, while F. rupicola became facilitated. There were no interactions between the different treatments (monocultures and mixtures) and the simulated land use change (clipped and unclipped). Our data suggest that although traditional land use practices, i.e. grazing and mowing, do not affect the development of D. carthusianorum, the abandonment of these practices has improved F. rupicola performance, which has led to the increasingly invasive character of this grass species over the last 20 years.     

Plant composition and diversity at edges in a semi-natural forest–grassland mosaic

Plant Ecology - As key components of landscapes, edges have received considerable scientific attention in anthropogenic ecosystems. However, edges in natural and semi-natural forest–grassland...     

Trait means or variance—What determines plant species' local and regional occurrence in fragmented dry grasslands?

One of the few laws in ecology is that communities consist of few common and many rare taxa. Functional traits may help to identify the underlying mechanisms of this community pattern, since they correlate with different niche dimensions. However, comprehensive studies are missing that investigate the effects of species mean traits (niche position) and intraspecific trait variability (ITV, niche width) on species abundance. In this study, we investigated fragmented dry grasslands to reveal trait‐occurrence relationships in plants at local and regional scales. We predicted that (a) at the local scale, species occurrence is highest for species with intermediate traits, (b) at the regional scale, habitat specialists have a lower species occurrence than generalists, and thus, traits associated with stress‐tolerance have a negative effect on species occurrence, and (c) ITV increases species occurrence irrespective of the scale. We measured three plant functional traits (SLA = specific leaf area, LDMC = leaf dry matter content, plant height) at 21 local dry grassland communities (10 m × 10 m) and analyzed the effect of these traits and their variation on species occurrence. At the local scale, mean LDMC had a positive effect on species occurrence, indicating that stress‐tolerant species are the most abundant rather than species with intermediate traits (hypothesis 1). We found limited support for lower specialist occurrence at the regional scale (hypothesis 2). Further, ITV of LDMC and plant height had a positive effect on local occurrence supporting hypothesis 3. In contrast, at the regional scale, plants with a higher ITV of plant height were less frequent. We found no evidence that the consideration of phylogenetic relationships in our analyses influenced our findings. In conclusion, both species mean traits (in particular LDMC) and ITV were differently related to species occurrence with respect to spatial scale. Therefore, our study underlines the strong scale‐dependency of trait‐abundance relationships.     

Does coevolution promote species richness in parasitic cuckoos?

Why some lineages have diversified into larger numbers of species than others is a fundamental but still relatively poorly understood aspect of the evolutionary process. Coevolution has been recognized as a potentially important engine of speciation, but has rarely been tested in a comparative framework. We use a comparative approach based on a complete phylogeny of all living cuckoos to test whether parasite–host coevolution is associated with patterns of cuckoo species richness. There are no clear differences between parental and parasitic cuckoos in the number of species per genus. However, a cladogenesis test shows that brood parasitism is associated with both significantly higher speciation and extinction rates. Furthermore, subspecies diversification rate estimates were over twice as high in parasitic cuckoos as in parental cuckoos. Among parasitic cuckoos, there is marked variation in the severity of the detrimental effects on host fitness; chicks of some cuckoo species are raised alongside the young of the host and others are more virulent, with the cuckoo chick ejecting or killing the eggs/young of the host. We show that cuckoos with a more virulent parasitic strategy have more recognized subspecies. In addition, cuckoo species with more recognized subspecies have more hosts. These results hold after controlling for confounding geographical effects such as range size and isolation in archipelagos. Although the power of our analyses is limited by the fact that brood parasitism evolved independently only three times in cuckoos, our results suggest that coevolutionary arms races with hosts have contributed to higher speciation and extinction rates in parasitic cuckoos.     

What do we gain from simplicity versus complexity in species distribution models?

Species distribution models (SDMs) are widely used to explain and predict species ranges and environmental niches. They are most commonly constructed by inferring species' occurrence–environment relationships using statistical and machine‐learning methods. The variety of methods that can be used to construct SDMs (e.g. generalized linear/additive models, tree‐based models, maximum entropy, etc.), and the variety of ways that such models can be implemented, permits substantial flexibility in SDM complexity. Building models with an appropriate amount of complexity for the study objectives is critical for robust inference. We characterize complexity as the shape of the inferred occurrence–environment relationships and the number of parameters used to describe them, and search for insights into whether additional complexity is informative or superfluous. By building ‘under fit’ models, having insufficient flexibility to describe observed occurrence–environment relationships, we risk misunderstanding the factors shaping species distributions. By building ‘over fit’ models, with excessive flexibility, we risk inadvertently ascribing pattern to noise or building opaque models. However, model selection can be challenging, especially when comparing models constructed under different modeling approaches. Here we argue for a more pragmatic approach: researchers should constrain the complexity of their models based on study objective, attributes of the data, and an understanding of how these interact with the underlying biological processes. We discuss guidelines for balancing under fitting with over fitting and consequently how complexity affects decisions made during model building. Although some generalities are possible, our discussion reflects differences in opinions that favor simpler versus more complex models. We conclude that combining insights from both simple and complex SDM building approaches best advances our knowledge of current and future species ranges.     

Altitude modifies species richness–nutrient indicator value relationships in a country-wide survey of grassland vegetation

Nutrient enrichment is a threat to botanical diversity in Europe, and its assessment is part of biodiversity monitoring schemes. In Switzerland, this is done by calculating the average nutrient (N) indicator value of the vegetation based on a country-wide systematic vegetation survey. However, it is questionable whether N values indicate eutrophication and resulting species loss equally well across an entire country, which includes wide topographic gradients and distinct biogeographic regions. Here we analyze vascular plant species lists from 415 grassland plots (10 m²) between 365 and 2770 m a.s.l. throughout Switzerland to investigate how the relationship between N value and species richness differs with altitude and among regions. The N value strongly decreased with altitude (piecewise regression: r² = 0.77), particularly between 800 and 2000 m a.s.l., where this decrease was related to a decreasing proportion of fertilized grasslands. In the alpine belt, lower N values were associated with a greater frequency of acidic soils and a restricted species pool. Vascular plant species richness was maximal at intermediate altitude (piecewise regression: r² = 0.33) and intermediate N value (polynomial regression: r² = 0.46). When analyzed separately by altitudinal belt, the relationship between species richness and N value was negative in the lowlands and montane belt but unimodal in the subalpine belt. In the alpine belt, soil pH (R indicator values) explained most of the variation in species richness. Two indices of between-plot diversity (floristic dissimilarity and the contribution of individual plots to total species richness) were negatively related to N values from the lowlands to the subalpine belt but not in the alpine belt. All relationships differed little among the biogeographic regions of Switzerland, but they might be modified by changes in management and by the expansion of common lowland species into mountain grasslands.     

On the brink of extinction? How climate change may affect global chelonian species richness and distribution

Anthropogenic global climate change has already led to alterations in biodiversity patterns by directly and indirectly affecting species distributions. It has been suggested that poikilothermic animals, including reptiles, will be particularly affected by global change and large‐scale reptile declines have already been observed. Currently, half of the world's freshwater turtles and tortoises are considered threatened with extinction, and climate change may exacerbate these declines. In this study, we assess how global chelonian species richness will change in the near future. We use species distribution models developed under current climate conditions for 78% of all extant species and project them onto different Intergovernmental Panel on Climate Change (IPCC) scenarios for 2080. We detect a strong dependence of temperature shaping most species ranges, which coincide with their general temperature‐related physiological traits (i.e., temperature‐dependent sex determination). Furthermore, the extent and distribution of the current bioclimatic niches of most chelonians may change remarkably in the near future, likely leading to a substantial decrease of local species abundance and ultimately a reduction in species richness. Future climatic changes may cause the ranges of 86% of the species to contract, and of these ranges, nearly 12% are predicted to be situated completely outside their currently realized niches. Hence, the interplay of increasing habitat fragmentation and loss due to climatic stress may result in a serious threat for several chelonian species.     

What determines the impact of alien birds and mammals in Europe?

An often-cited reason for studying the process of invasion by alien species is that the understanding sought can be used to mitigate the impacts of the invaders. Here, we present an analysis of the correlates of local impacts of established alien bird and mammal species in Europe, using a recently described metric to quantify impact. Large-bodied, habitat generalist bird and mammal species that are widespread in their native range, have the greatest impacts in their alien European ranges, supporting our hypothesis that surrogates for the breadth and the amount of resources a species uses are good indicators of its impact. However, not all surrogates are equally suitable. Impacts are generally greater for mammal species giving birth to larger litters, but in contrast are greater for bird species laying smaller clutches. There is no effect of diet breadth on impacts in birds or mammals. On average, mammals have higher impacts than birds. However, the relationships between impact and several traits show common slopes for birds and mammals, and relationships between impact and body mass and latitude do not differ between birds and mammals. These results may help to anticipate which species would have large impacts if introduced, and so direct efforts to prevent such introductions.     

Do biogeographic parameters matter? Plant species richness and distribution of macrophytes in relation to area and isolation of ponds in NW Polish agricultural landscape

Effects of pond size and isolation on total vascular plant species richness and number of obligate wetland species were compared. Subsequently, the potential for the presence of spatial patterns in wetland species distribution among ponds in an agricultural landscape was explored. Relationships between species richness and two main biogeographic parameters were analysed using simple and multiple linearised regression models. Spatial patterns were looked for by means of analyses carried out with the R CRAN software (join-count statistics). Simple regression analyses performed on the regional scale (n = 50) revealed the significance of the effect of pond size only (r = 0.46 for total plant species richness and r = 0.28 for wetland species richness vs. pond area). Further analyses conducted on the local scale identified the best multiple regression models in the largest pond cluster (n = 20); the models showed statistical significance of relationships between the species richness and both independent variables (r = 0.80 for total plant species richness and r = 0.70 for wetland species richness vs. pond area and isolation, including mean distance to the nearest ten ponds). Spatial analyses were performed for 26 obligate wetland species selected from 149 species recorded in all the 50 ponds. Exploratory spatial data analysis revealed the presence of significant positive spatial autocorrelation in the distribution of 8 species. In such cases, it is possible to reject the random distribution hypothesis, which justifies exploration of spatial regimes. In practice, correct spatial model specifications may have implications for predicting species occurrences under changing environmental conditions, e.g. changes in the number of ponds.     

Does species diversity limit productivity in natural grassland communities?

Theoretical analyses and experimental studies of synthesized assemblages indicate that under particular circumstances species diversity can enhance community productivity through niche complementarity. It remains unclear whether this process has important effects in mature natural ecosystems where competitive feedbacks and complex environmental influences affect diversity–productivity relationships. In this study, we evaluated diversity–productivity relationships while statistically controlling for environmental influences in 12 natural grassland ecosystems. Because diversity–productivity relationships are conspicuously nonlinear, we developed a nonlinear structural equation modeling (SEM) methodology to separate the effects of diversity on productivity from the effects of productivity on diversity. Meta-analysis was used to summarize the SEM findings across studies. While competitive effects were readily detected, enhancement of production by diversity was not. These results suggest that the influence of small-scale diversity on productivity in mature natural systems is a weak force, both in absolute terms and relative to the effects of other controls on productivity.