Evenness–invasibility relationships differ between two extinction scenarios in tallgrass prairie

To develop a better understanding of how biodiversity loss and productivity are related, we need to consider ecologically realistic rarity (i.e. reduced evenness and increased dominance) and extinction (i.e. reduced richness) scenarios. Furthermore, we need to identify and better understand the factors that influence species and community yielding behaviors because the general conditions for overyielding are the same as those for coexistence. We established experimental tallgrass prairie plots in Iowa to determine how two ecologically realistic rarity–extinction scenarios influenced aboveground net primary productivity (ANPP) and disassembly. Equal‐mass seedlings of six tallgrass prairie species were transplanted into field plots to establish realistic declining species evenness (high, medium, low) and richness (4, 1) treatments. Across declining evenness treatments, the relative abundance of the ubiquitous tall species Andropogon gerardii increased, the relative abundance of the tall species Salvia azurea was constant, and the relative abundance of two short (dissimilar height scenario) or two tall species (tall scenario) decreased. Monocultures of Andropogon represented a continuation of this trend until there was complete dominance by Andropogon and extinction of all other species. Our treatments also allowed us to test if variation in plant height contributes to the complementarity effect. Niche partitioning in plant height was not positively related to complementarity. The effects of declining species evenness and richness on the diversity–productivity relationship were different for these two ecologically realistic rarity–extinction scenarios. Specifically, as diversity declined across treatments, ANPP and the selection effects decreased in tall communities, but not in dissimilar communities. Additionally, differences between these two scenarios revealed that decreased species yielding behavior is associated with two tallgrass prairie extinction risk factors, rarity and short height. The differences between these scenarios demonstrate the importance of incorporating the known patterns of diversity declines into future studies.

Keywords: Rotational grazing, full protection, continuous grazing species richness, species diversity, soil seed bank, Bayesian methods, Salsola vermiculata, seed longevity, rangeland management, Syria. Rangelands represent 70% of the semi-arid and arid Mediterranean land mass. It is a natural habitat for millions of people whose livelihood depends on animal husbandry. The revolutionary developments in the animal husbandry and veterinary medicines resulted in exponential increases in human and livestock population living on and from dry lands. To respond to population growth, urbanization and transportation means and expanded road networks, land reform and rural development policies forced nomads to adopt semi to fully sedentary lifestyles with disintegrated traditional community structures and organizational frameworks. Under these demographic changes coupled with national and international border crossing restrictions, there was an escalation in opportunistic cultivation, and excessive exploitation of the scarce and slowly renewable vegetation cover resulting in a steep decline in the primary production components of the rangeland ecosystems. In an attempt to stop and reverse the degradation process, large-scale re-vegetation programs based on transplanting and reseeding with perennial shrubs, resting and grazing management systems were devised and implemented. This study aimed to evaluate the impacts of the rehabilitation programs on the above-ground vegetation cover and soil seed bank replenishment in the Syrian rangelands. The underlying assumption of the rehabilitation program is that with a well-established perennial plant cover and proper grazing management, top soil is stabilized, soil moisture, nutrients and seed bank are replenished, organic matter is accumulated and microorganisms' activity is promoted resulting in greater abundance, species richness and diversity of annuals. To test the above hypothesis, field and controlled environment based studies were carried out with quantitative data collection and processing on plant species abundance, richness and diversity of above-ground vegetation and soil seed bank for fully protected rotationally and continuously grazed areas of 10 rangeland sites in northern Syria for three consecutive seasons. In addition to the use of frequentist statistical approaches for species diversity estimation in the studies, the Bayesian method was explored. Moreover, the crucial issues of seed quality in re-vegetation were tackled through a study on seed viability and longevity in Salsola vermiculata L., which is a highly palatable, well adapted and widely used species in the arid Mediterranean rangeland rehabilitation programs. The major findings are indicated below. Above ground vegetation cover The vegetation cover data analysis using ANOVA showed that overall plant density consistently declined from full protection to rotational and then continuous grazing in 9 out of the 10 sites studied, whereas the trends of change in species richness and diversity were not consistent. Pair-wise comparison showed that plant density, species richness and diversity were lowest for the overall plant population under rotational grazing in which plant density of perennial grasses was highest. This suggests that rotational grazing has reshaped the composition of the plant communities under the study areas by promoting the perennial grasses that are more arid rangeland adapted and ecosystem significant. Higher plant density under rotational grazing and similarity in species diversity under the three grazing treatments for perennial grasses imply that a longer period of rehabilitation and/or probably incorporating inter-seasonal rotational grazing and variation in herbivore types into the current intra-seasonal rotational grazing system will be required to cross the line of no return in plant community composition optimization through the prevalent arid Mediterranean rangelands rehabilitation programs. Soil seed bank assessment The soil seed bank data analysis using ANOVA showed no significant differences in the overall physical and germinable soil seed bank size and diversity along the grazing gradient. However, there was a significant grazing-by-site interaction for both and a significant grazing-by-year interaction for germinable seed bank size showing that the change in seed bank size is moderated by physical and environmental characteristics and human-induced disturbances. Continuous grazing treatments for some sites were located near agglomerations of people and animals, main roads and water points. Under such conditions the more disturbance-adapted ephemerons and non-palatable plants with limited constraints for seed setting dominated and this resulted in a larger soil seed bank under continuous compared to rotational and full protection grazing treatments. For the more human-induced disturbance distanced sites, the soil seed bank was at larger or similar under full and/or rotational compared to continuous grazing. Results from pair-wise comparisons showed a simultaneous decline and surge in physical and germinable soil seed bank size of annuals and those of perennials under the grazing treatments over sites. This suggests relative differences in root competition and gap exploitation characteristics among plant functional groups; these differences could be considered indicative to rangeland status and a guide to vary herbivores in order to maintain optimum plant species diversity in the target rangelands. Soil seed banks with no seeds of perennial grasses generated 208 seedlings m-2 of germinable soil seed bank under continuous grazing. This is probably due to seed setting failure resulting from overgrazing compensated by vegetative reproduction. The widely used phanerophytes in the rangeland rehabilitation program had a physical soil seed bank of 59.7 to 119 seed m-2 and a zero germinable one. This shows high complementarity between physical and germinable seed testing methods for rangeland monitoring. Similarity indices High Morisita-Horn and Sørensen similarities were recorded between the quadrat and point intercept measurements of the above ground vegetation and with each of physical and germinable soil seed banks. However, the similarity indices of the above ground vegetation measurements were higher with the germinable soil seed bank than with the physical soil seed bank. This suggests that the germinable soil seed bank is more suitable for monitoring arid Mediterranean rangeland than the physical soil seed bank. Correlation coefficients Based on the coefficients of determination (CDs), the variation in plant density and seed bank size accounted for a significant portion of the variations in spcies richness but not of the diversity indices. However, plant density and species diversity consistently and significantly declined during the season with the lowest mean annual precipitation showing the crucial role of precipitation in the dynamics of the yet active soil seed banks of the study areas. The CDs for the germinable soil seed bank size tested under optimum soil moisture with species richness were also significant throughout the grazing treatments reflecting the dependence of seed bank dynamics on soil moisture. For the physical soil seed bank (PSSB), CDs of its size with species richness were only significant under rotational grazing implying positive impacts of grazing management on soil seed bank replenishment. Morover, the CDs of PSSB richness with the diversity indices were only significant for Singletons but not for Shannon and Simpson. This is attributable to the fact that the Singleton index is more sensitive to rare species than Shannon and Simpson. The non-significant correlations between plant density and species diversity reflect a need to incorporate inter-seasonal rotational grazing and herbivore variation to the current intra-seasonal rotational grazing for greater plant community integrity. Phytogeographic analysis Using two above and two underground vegetation data collection methods, a total of 137 species, including 102 annuals and 35 perennials from 36 families of 11 chorotypes, were recorded. The number of species recorded were 56, 66, 68 and 98 from physical seed extraction, point intercept, quadrat and grow out test, respectively. These results showed the superiority of the growing out test over the three other methods. With its easiness and relative flexibility of application in terms of time and space, the grow out test seems to be the best method for arid Mediterranean rangelands monitoring and assessment of rehabilitation impacts in which the frequency of good rainy season is one out of four years. Conclusions  The rotational grazing component of the rangeland rehabilitation program resulted in a change in plant community composition shown by an increase in low proportional abundance perennial grasses with greater arid Mediterranean rangeland adaptation.  Continuous grazing reduced plant density but not richness and diversity, indicating that maximum diversity is not a sign of rangeland health and integrity. This also suggests that inter-seasonal rotational grazing and herbivore variation could probably improve the shaping effects of grazing on the arid rangeland rehabilitation programs.  Capturing more species and higher similarity indices with the above ground measurements, the simple and flexible, germinable soil seed bank test seems to be a good monitoring and evaluation method for arid Mediterranean rangelands. However, results from the tedious and less accurate physical seed extraction method could be crucial to capture the species with long seed dormancy.  Larger Bayesian estimates of diversity, smaller standard errors, lower p-values and more significance of differences in diversity between grazing treatments compared to the frequentist approaches were observed, thus indicating clear merits for the approach in estimating diversity.  The seed longevity study showed that under relatively higher seed moisture content, longevity increased suggesting that desiccation susceptibility is probably the cause of short seed longevity of Salsola vermiculata L. Moreover, drying and packaging alone increased longevity, thus providing a simple, cost-effective and environmentally friendly method for rangeland rehabilitation programs.

ABSTRACTUnderstanding how the characteristics of native plant communities influence invasion is a pressing question, with implications for theory and management. For decades, the primary native community characteristic used in tests of biotic resistance was species richness. However, previous studies have demonstrated that evolutionary history and functional traits shape the invasion process, as ecological theory predicts. Theoretically, restoration projects would benefit from designing seed mixtures around maximizing resistance to invasion. However, there is little empirical evidence on the importance of evolutionary diversity for management and still less guidance for practitioners on effective application of ecological theories. We empirically tested how several native community characteristics (phylogenetic diversity, functional diversity, phylogenetic relatedness, and mean trait values) affected the survival of three introduced invasive species. We explored this question in experimentally restored 15‐species prairie plots with three levels of phylogenetic diversity and two levels of functional diversity. Our experiment also included monocultures of all native species, which were also experimentally invaded. We found evidence that phylogenetic diversity conferred biotic resistance against one invasive species, contributing to reduced biomass in models explaining up to 10% of variance. Tall species better suppressed invaders, with height explaining up to 27% of variation in invader biomass. Surprisingly, we found patterns in leaf and seed traits linked to invasion resistance which were associated with both conservative and resource‐acquisitive strategies. We also found evidence in both the diversity and monoculture plots that invaders were more successful with more closely related native species. Taken together, our results indicate that invasion resistance emerges from nuanced interactions between phylogenetic diversity, functional traits, and community composition, rather than from any single community characteristic. Our results underscore the complexity of biotic resistance and suggest that practitioners should prioritize phylogenetic diversity and strategic species selection when designing restoration plantings to enhance invasion resistance.

Is community persistence related to diversity? A test with prairie species in a long-term experiment

Summary1. The majority of experiments examining effects of species diversity on ecosystem functioning have randomly manipulated species richness. More recent studies demonstrate that realistic species losses have dramatically different effects on ecosystem functioning than those of randomized losses, but these results are based primarily on microcosm experiments or modelling efforts.2. We conducted a field‐based experiment directly comparing the consequences of realistic and randomized plant species losses on invasion resistance and productivity in a native‐dominated serpentine grassland in California, USA. The realistic species loss order was based on nested subset analysis of long‐term presence/absence data from our research site and reflects differing species sensitivities to prolonged drought.3. Biomass of exotic invasive plant species was inversely related to native species richness in the realistic loss order. In contrast, invader biomass was consistently low across species richness levels in the randomized species loss order, with no effect of native species richness on invader biomass among randomized assemblages. Although total above‐ground plant biomass increased with soil depth (a proxy for resource availability) in both realistic and randomized assemblages, soil depth influenced invader biomass only in the randomized assemblages.4. Synthesis. Our results illustrate that the functional consequences of realistic species losses can differ distinctly from those of randomized species losses and that incorporation of realistic species loss scenarios can increase the relevance of experiments linking biodiversity and ecosystem functioning to conservation in the face of anthropogenic global change.

Summary 1. We related above-ground biomass allocation to light interception by trees and lianas growing in three tropical rain forest stands that were 0.5, 2 and 3-year-old regeneration stages after slash and burn agriculture. 2. Stem height and diameter, leaf angle, the vertical distribution of total above-ground biomass and leaf longevity were measured in individuals of three short-lived pioneers (SLP), four later successional species (LS) and three lianas (L). Daily light capture per individual ( Φ d ) was calculated with a canopy model. Mean daily light interception per unit leaf area ( Φ area ), leaf mass ( Φ leaf mass ) and above-ground mass ( Φ mass ) were used as measures of instantaneous efficiency of biomass use for light capture. 3. With increasing stand age, vegetation height and leaf area index increased while light at the forest floor declined from 34 to 5%. The SLP, Trema micanthra and Ochroma pyramidale , dominated the canopy early in succession and became three times taller than the other species. SLP had lower leaf mass fractions and leaf area ratios than the other groups and this difference increased with stand age. 4. Over time, the SLP intercepted increasingly more light per unit leaf mass than the other species. Lianas, which in the earliest stage were self-supporting and started climbing later on, gradually became taller at a given mass and diameter than the trees. Yet, they were not more efficient than trees in light interception. 5. SLP had at least three-fold shorter leaf life spans than LS and lianas. Consequently, total light interception calculated over the mean life span of leaves ( Φ leaf mass total = Φ area × SLA death leaves × leaf longevity) was considerably lower for the SLP than for the other groups. 6. Synthesis . We suggest that early dominance in secondary forest is associated with a high rate of leaf turnover which in turn causes inefficient long-term use of biomass for light capture, whereas persistence in the shade is associated with long leaf life spans. This analysis shows how inherent tradeoffs in crown and leaf traits drive long-term competition for light, and it presents a conceptual tool to explain why early dominants are not also the long-term dominants.

The number of species is known to decrease from the humid tropics towards drier and colder climates, but how species richness varies along environmental and spatial gradients within the tropical rain forests is not clear. We inventoried 214 transects of 0.25 ha to document species diversity patterns in an example plant group (ferns and lycophytes) across non‐inundated rain forests of western and central Amazonia, and assessed how well these conformed with proposed hypotheses about species richness. The observed number of species varied between 6 and 71 per transect. The effective number of species (emphasising the degree of unevenness in species abundances) varied between 1.02 and 8.60, and diversity profiles revealed considerable differences among transects in community structure. Although the density of individuals varied over almost two orders of magnitude, species diversity was better explained by other variables. In particular, within‐transect species diversity increased substantially with increasing soil cation concentration. It also increased with soil aluminium concentration, heterogeneity in soil chemistry, annual rainfall and dry season rainfall, and was higher in western than in central Amazonia. Multiple regression models explained up to 70% of the variance in species diversity, but the relationships between species diversity and the environmental gradients became progressively weaker as species abundances were given more weight in the calculation of diversity. Our results conformed to the proposal that site productivity promotes species diversity. This seemed to arise from larger species pools on more fertile soils and in wetter climates, even when it could be expected that the older and more widespread infertile soils would have provided more opportunities for speciation.

Understanding temporal effects of fire frequency on plant species diversity and vegetation structure is critical for managing tallgrass prairie (TGP), which occupies a mid-continental longitudinal precipitation and productivity gradient. Eastern TGP has contributed little information toward understanding whether vegetation-fire interactions are uniform or change across this biome. We resampled 34 fire-managed mid- and late-successional ungrazed TGP remnants occurring across a dry to wet-mesic moisture gradient in the Chicago region of Illinois, USA. We compared hypotheses that burning acts either as a stabilizing force or causes change in diversity and structure, depending upon fire frequency and successional stage. Based on western TGP, we expected a unimodal species richness distribution across a cover-productivity gradient, variable functional group responses to fire frequency, and a negative relationship between fire frequency and species richness. Species diversity was unimodal across the cover gradient and was more strongly humpbacked in stands with greater fire frequency. In support of a stabilizing hypothesis, temporal similarity of late-successional vegetation had a logarithmic relationship with increasing fire frequency, while richness and evenness remained stable. Temporal similarity within mid-successional stands was not correlated with fire frequency, while richness increased and evenness decreased over time. Functional group responses to fire frequency were variable. Summer forb richness increased under high fire frequency, while C4 grasses, spring forbs, and nitrogen-fixing species decreased with fire exclusion. On mesic and wet-mesic sites, vegetation structure measured by the ratio of woody to graminoid species was negatively correlated with abundance of forbs and with fire frequency. Our findings that species richness responds unimodally to an environmental-productivity gradient, and that fire exclusion increases woody vegetation and leads to loss of C4 and N-fixing species, suggest that these processes are uniform across the TGP biome and not affected by its rainfall-productivity gradient. However, increasing fire frequency in eastern TGP appears to increase richness of summer forbs and stabilize late-successional vegetation in the absence of grazing, and these processes may differ across the longitudinal axis of TGP. Managing species diversity in ungrazed eastern TGP may be dependent upon high fire frequency that removes woody vegetation and prevents biomass accumulation.

Relationships between local annual immigration and extinction rates of plant species and total species richness were determined from long-term data in permanent plots in tallgrass and shortgrass prairies in Kansas. Combining plots resulted in higher equilibrium numbers of species as predicted from immigration and extinction rates. Immigration and extinction rates also increased with scale. Extinction rates are higher because the regional scale supports more rare species which, in turn, have high probabilities of extinction. We also tested the hypotheses that extinction rates would be higher on burned versus unburned grasslands, and that immigration rates would be higher on grazed versus ungrazed grasslands. Extinction rates were positively correlated with the number of species at a site, and this relationship was not altered by burning or grazing. Immigration rates were variable, but were sometimes positively correlated with growing season precipitation. Immigration rates decreased in years sites were burned. Therefore, after fire, the number of species going locally extinct was still dependent on earlier species richness, but the number of species added to the site was reduced. Variances in immigration and extinction rates were high, therefore, confident predictions regarding the effects of burning or grazing regimes on species richness could not be made. Variance in rates of immigration and extinction results in a range of values within which the equlibrium number of species fluctuates randomly.

Summary1. A critical need in conservation biology is to determine which species are most vulnerable to extinction. Freshwater mussels (Bivalvia: Unionacea) are one of the most imperilled faunal groups globally. Freshwater mussel larvae are ectoparasites on fish and depend on the movement of their hosts to maintain connectivity among local populations in a metapopulation.2. I calculated local colonisation and extinction rates for 16 mussel species from 14 local populations in the Red River drainage of Oklahoma and Texas, U.S. I used general linear models and AIC comparisons to determine which mussel life history traits best predicted local colonisation and extinction rates.3. Traits related to larval dispersal ability (host infection mode, whether a mussel species was a host generalist or specialist) were the best predictors of local colonisation.4. Traits related to local population size (regional abundance, time spent brooding) were the best predictors of local extinction. The group of fish species used as hosts by mussels also predicted local extinction and was probably related to habitat fragmentation and host dispersal abilities.5. Overall, local extinction rates exceeded local colonisation rates, indicating that local populations are becoming increasingly isolated and suffering an ‘extinction debt’. This study demonstrates that analysis of species traits can be used to predict local colonisation and extinction patterns and provide insight into the long‐term persistence of populations.

During community assembly, early arriving exotic species might suppress other species to a greater extent than do native species. Because most exotics were intentionally introduced, we hypothesize there was human selection on regeneration traits during introduction. This could have occurred at the across- or within-species level (e.g. during cultivar development). We tested these predictions by seeding a single species that was either native, exotic 'wild-type' (from their native range), or exotic 'cultivated' using 28 grassland species in a glasshouse experiment. Priority effects were assessed by measuring species' effect on establishment of species from a seed mix added 21 d later. Exotic species had higher germination and earlier emergence dates than native species, and differences were found in both 'wild' and 'cultivated' exotics. Exotic species reduced biomass and species diversity of later arriving species much more than native species, regardless of seed source. Results indicate that in situations in which priority effects are likely to be strong, effects will be greater when an exotic species arrives first than when a native species arrives first; and this difference is not merely a result of exotic species cultivation, but might be a general native-exotic difference that deserves further study.

The islands of the tropical South Pacific (TSP) are considered biodiversity hotspots. However, the biota of this region has received limited scientific attention and very little is known about its diversity, ecology and evolution. In this thesis we investigate some of the ecological and evolutionary processes in the TSP, focussing on lowland rain forests. We use molecular techniques to investigate evolutionary processes and vegetation surveys to study species diversity patterns and ecological processes. Chapter 1 reviews molecular, distributional and geographic evidence for dispersal versus vicariance explanations for the diversity and distribution of the TSP biota. Most islands of the TSP are geologically young (less than 40 million years old) and of oceanic origin, so most (if not all) of the biota on islands in the TSP arrived through long-distance dispersal events. This view is strongly corroborated by genetic data from published studies. Molecular studies also suggest two major source areas. One is located in the northwest, which includes Malesia and Southeast Asia, while the other is in the southwest and includes New Caledonia, Australia and New Zealand. We argue that local extinctions have occurred in source and stepping stone areas, creating sources of error for the interpretation of distribution and molecular data. In Chapter 2 we use allozyme data to investigate the question how Pacific cycads (Cycas, subsection Rumphiae) colonised the Pacific. We show that they colonised the Pacific and East Africa by long-distance dispersal, probably through floating seeds from a Malesian source area. Allozymes and morphological data provide support for two major groups within subsection Rumphiae and reveal close relationships between the extant species, suggesting very recent and/or ongoing dispersal events. Cycads are an example of recent diversification in a lineage with a long fossil record. The podocarp genus Dacrydium is another lineage with a long fossil record and in chapter 3 we investigate the colonisation and speciation processes in this lineage using allozymes and trnL-trnF plastid sequences. Our results suggest that the Pacific species of Dacrydium arrived recently (within the last 10 million years) in the TSP but are inconclusive about the source area of the genus. Combined molecular and ecological data suggest the occurrence of both allopatric and sympatric speciation in the Pacific radiation in this genus. Allozyme data also demonstrate the occurrence of hybridisation between two New Caledonian species. Our findings suggest that hybridisation and sympatric speciation may have played an important role in the evolution of the biota in the TSP. In chapter 4 we attempt to untangle the disparate forces driving alpha species diversity, forest structure and species composition in old-growth lowland tropical rainforest by assessing the tree species composition of twelve 1 ha vegetation count plots on 13 islands between New Guinea and Samoa. Using simplifications of a model based on biogeographic and ecological disturbance theory, we show that species diversity and richness are mainly influenced by size and area of an island, while endemism is mostly determined by isolation and area. High cyclone frequency is shown to increase the density of stems (with dbh > 10 cm). Correlations between the abundance of widespread canopy tree taxa and cyclone frequency suggest that cyclones affect species composition by increasing the abundance of cyclone-resistant species. However, floristic similarities show that geographic distance also affects species composition. It therefore appears that, for lowland rain forests in the TSP, biogeography is the major driver of species diversity and endemism and that disturbance is the major driver of forest structure, while both biogeography and disturbance affect the species composition. In chapter 5 we test the ability of NDVI (Normalised Difference Vegetation Index, a remotely sensed index of productivity) data and leaf samples as covariates of alpha species diversity using twelve vegetation count plots. NDVI performed poorly in estimating species diversity and species richness. However, the cost- and time-efficiency associated with remotely sensed data shows the potential of these methods, but only if accurate methods to estimate species richness are found. Species richness and species diversity estimates obtained from leaf litter samples correlate reasonably well with similar estimates obtained from count plots and are more than 30% cheaper and about 10% faster to obtain. If travel can be avoided through collaboration, leaf litter-based estimates of diversity could be obtained at about 5% the cost and in about half the time compared to count plots. Therefore the analysis of leaf litter is potentially a suitable and efficient method to obtain rapid estimates of species diversity in count plots. The final chapter discusses the roles of ecological and evolutionary processes in the TSP. While research to date has been scarce, especially on ecological processes acting on large scales, data show that the effects of ecology, evolution and biogeography are interlinked during the colonisation, establishment and subsequent evolution of taxa and biomes in the TSP.

Summary1. In relatively fertile ecosystems, such as the tallgrass prairie, local species diversity is largely controlled by the competitive effects of community dominants. Despite the relative importance of soil resources in shaping competitive outcomes, we have a limited understanding of the ways by which plants partition below‐ground space and resources while competing, and thus, how these interactions feedback to affect local diversity.2. We experimentally tested whether potential rooting depth affected plant species diversity and composition by seeding 36 tallgrass prairie species into replicated, bare‐ground plots in which soil depth was manipulated to produce shallow‐ (20 cm), medium‐ (42 cm) and deep‐soil treatments all within one soil type. Because root architecture and foraging strategies differ among species, we hypothesized that soil depth alone could affect plant richness, diversity and community composition.3. After 3 years, richness (S) significantly increased with soil depth (P < 0.0001), but there was no significant change in species diversity (P > 0.1) or composition (multi‐response permutation procedure,P > 0.2). The lack of a depth effect upon diversity resulted from the opposing effect of increasing soil depth enhancing S, but decreasing evenness.4. Species presence among depth treatments was strongly nested, with species found in shallow soils reflecting a subset of the species found in the medium‐depth treatment, and the species found within the medium‐depth treatment reflecting a subset of those found in the deepest soils.5. All depth treatments contained the same dominant grasses, thus differences inSresulted from the nested loss of forbs. Conversely, increasing soil depth added sets of new species, but the specific identity of the species present appeared interchangeable among replicates of a given depth.6. Synthesis. Our results provide the first field‐based experimental evidence that altering soil depth alters species occurrence and diversity in predictable ways in seeded tallgrass prairie. Our results have important theoretical implications for understanding the processes promoting plant co‐occurrence in grasslands, and generate testable hypotheses concerning the conditions under which root‐niche partitioning is probably important for maintaining local richness in grasslands. Future work is needed to elucidate the generality and mechanistic basis of our results.

Recent multi-habitat studies across a range of spatial scales have shown that species-rich habitats are often highly invasible by exotic species. The primary measures of ‘invasion’ in these and other studies are invader richness and the absolute cover or biomass of invaders. We argue that the relative biomass or cover of invaders (dominance) is an important but overlooked measure of plant invasion. We re-analyzed data presented in five previous studies to evaluate whether exotic relative abundance is positively correlated with native richness. There were either no relationships or negative relationships between native richness and relative exotic cover calculated from three spatial scales (1, 1000 and 4000 m2). Thus while the original studies reported high exotic richness or absolute cover in habitats rich in native species, native richness did not predict the degree to which exotics had become dominant or abundant relative to natives. Absolute measures of exotic cover reported in the original studies underestimated relative exotic cover in habitats with low native species richness. High exotic dominance in areas of low native richness may indicate that exotic richness and dominance are controlled by different factors. We conclude that it is useful for researchers to measure both invader richness and invader dominance when trying to understand the environmental factors that are associated with plant invasions.

Understanding determinants of local species diversity remains central to developing plans to preserve biodiversity. In the continental United States, climate, grazing by large mammals, fire, and topography are important ecosystem drivers that structure North American tallgrass prairie, with major impacts on plant community composition and vegetation structure. Frequency of fire and grazing by bison (Bos bison), through effects on plant community composition and altered spatial and structural heterogeneity of veg- etation in tallgrass prairie, may act as bottom-up processes that modulate insect community species richness. As previously seen for plant species richness, I hypothesized that grazing had more impact than fire frequency in determining species richness of insect herbivore communities. I examined this prediction with grasshoppers at Konza Prairie, a representative tallgrass prairie site in which fire frequency and bison grazing are manipulated over long terms with landscape-level treatments. Topographic position (upland vs. lowland) and fire frequency (1-, 2-, 4-year intervals, and unburned) did not significantly influence grasshopper species richness or indices of diversity, while grazing had significant effects. On average, I found ;45% more grasshopper species and significantly increased values of Shannon H9 diversity at sites with bison grazing. Species abundances were more equally distributed (Shannon's Evenness Index) in grazed sites as well. No significant interactions among burning and grazing treatments explained variation in grasshopper species diversity. Grasshopper species richness respond- ed positively to increased heterogeneity in vegetation structure and plant species richness, and negatively to average canopy height and total grass biomass. Variation in forb biomass did not influence grasshopper species richness. A significant positive relationship between grasshopper species richness and overall grasshopper density was observed. Species rich- ness increased marginally as watershed area of treatments in grazed areas increased, but not in ungrazed areas. Disturbance from ecosystem drivers operating at watershed spatial scales exhibits strong effects on local arthropod species diversity, acting indirectly by mediating changes in the spatial heterogeneity of local vegetation structure and plant species diversity.