Showing papers on "Species richness published in 1996"

Productivity and sustainability influenced by biodiversity in grassland ecosystems

Abstract: THE functioning and sustainability of ecosystems may depend on their biological diversity1–8. Elton's9 hypothesis that more diverse ecosystems are more stable has received much attention1,3,6,7,10–14, but Darwin's proposal6,15 that more diverse plant communities are more productive, and the related conjectures4,5,16,17 that they have lower nutrient losses and more sustainable soils, are less well studied4–6,8,17,18. Here we use a well-replicated field experiment, in which species diversity was directly controlled, to show that ecosystem productivity in 147 grassland plots increased significantly with plant biodiversity. Moreover, the main limiting nutrient, soil mineral nitrogen, was utilized more completely when there was a greater diversity of species, leading to lower leaching loss of nitrogen from these ecosystems. Similarly, in nearby native grassland, plant productivity and soil nitrogen utilization increased with increasing plant species richness. This supports the diversity–productivity and diversity–sustainability hypotheses. Our results demonstrate that the loss of species threatens ecosystem functioning and sustainability.

High level of genetic differentiation for allelic richness among populations of the argan tree [Argania spinosa (L.) Skeels] endemic to Morocco.

Abstract: Genetic diversity at nine isozyme loci was surveyed in an endangered tree species, the argan tree, endemic to south-western Morocco. The species is highly diverse (3.6 alleles/locus) with populations strongly differentiated from each other (FST=0.25). This example is used to illustrate a method for standardizing measures of allelic richness in samples of unequal sample sizes, which was developed for the estimation of the number of species and relies on the technique of rarefaction. In addition, it is shown that the measure of subdivision, θST, obtained when allelic richness is used in place ofh (Nei's index of diversity), is much larger than the FST [e.g. θST(40)=0.52, where (40) indicates the specified sample used to estimate the allelic richness]. This suggests that rare alleles (which strongly influence measures of allelic richness) have a more scattered distribution than more frequent ones, a result which raises special conservation issues for the argan tree.

Land Use and Avian Species Diversity Along an Urban Gradient

Abstract: I examined the distribution and abundance of bird species across an urban gradient, and concomitant changes in community structure, by censusing summer resident bird populations at six sites in Santa Clara County, California (all former oak woodlands). These sites represented a gradient of urban land use that ranged from relatively undisturbed to highly developed, and included a biological preserve, recreational area, golf course, residential neighborhood, office park, and business district. The composition of the bird community shifted from predominantly native species in the undisturbed area to invasive and exotic species in the business district. Species richness, Shannon diversity, and bird biomass peaked at moderately disturbed sites. One or more species reached maximal densities in each of the sites, and some species were restricted to a given site. The predevelopment bird species (assumed to be those found at the most undisturbed site) dropped out gradually as the sites became more urban. These patterns were significantly related to shifts in habitat structure that occurred along the gradient, as determined by canonical correspondence analysis (CCA) using the environmental variables of percent land covered by pavement, buildings, lawn, grasslands, and trees or shrubs. I compared each formal site to four additional sites with similar levels of development within a two-county area to verify that the bird communities at the formal study sites were rep- resentative of their land use category.

Statistics and partitioning of species diversity, and similarity among multiple communities

Abstract: Species richness, Shannon information, and Simpson diversity are the three most commonly used nonparametric measures of species diversity. The sampling bias and variance of these measures differ greatly. Species richness may be seriously underestimated for even very large samples from a speciose community. The bias in species richness and Shannon information depend on unknown parameters of the species abundance distribution. An unbiased estimator exists only for Simpson diversity. Each of these measures is concave, so that the total diversity in a pooled set of communities exceeds (or equals) the average diversity within communities. The total diversity in a set of communities can therefore be partitioned into positive, additive components within and among communities, corresponding to α- and β-diversity. Partitioning Simpson diversity corresponds to an analysis of variance. The proportion of the total diversity found within communities provides a natural measure of similarity among multiple communities. The expected similarity among multiple random samples from the same community depends on the number of samples and on the underlying measure of diversity.

A consumer's guide to evenness indices

Abstract: Many indices have been proposed for measuring species evenness in ecological communities, but there is no consensus on which are best. We assemble criteria for an appropriate index for the evenness of a biological sample. The most important criterion is that evenness should be independent of species richness. Twelve previously proposed indices and variants are considered, and two apparently new indices. Four indices are recommended as joint best buys: A) If symmetry between minor and abundant species is not important, or if it is required that the index be less affected by minor species: 1) If it is essential that the index be able to reach a minimum of zero with any particular number of species, or if the shape of the index response to an evenness gradient is important: E 1/D (based on a common form of Simpson's index). 2) If good mid-range behaviour is desired: E' (proposed by Camargo). B) If equal sensitivity to minor and abundant species is required: I) If the shape of the index response to an evenness gradient is not important, the clear winner is: E Q (a new index). 2) If the shape is important: E var (another new index). The overall recommendation for general use is E war .

Species loss in fragments of tropical rain forest: a review of the evidence.

Abstract: A review of the literature shows that in nearly all cases tropical rain forest fragmentation has led to a local loss of species. Isolated fragments suffer reductions in species richness with time after excision from continuous forest, and small fragments often have fewer species recorded for the same effort of observation than large fragments or areas of continuous forest. Birds have been the most frequently studied taxonomic group with respect to the effects of tropical forest fragmentation. The mechanisms of fragmentation-related extinction include the deleterious effects of human disturbance during and after deforestation, the reduction of population sizes, the reduction of immigration rates, forest edge effects, changes in community structure (second- and higher-order effects) and the immigration of exotic species. The relative importance of these mechanisms remains obscure. Animals that are large, sparsely or patchily distributed, or very specialized and intolerant of the vegetation surrounding fragments, are particularly prone to local extinction. The large number of indigenous species that are very sparsely distributed and intolerant of conditions outside the forest make evergreen tropical rain forest particularly susceptible to species loss through fragmentation. Much more research is needed to study what is probably the major threat to global biodiversity.

Insects in fragmented forests: a functional approach

Abstract: Insects are highly susceptible to the adverse effects of forest fragmentation. It is now beyond any doubt that fragmentation-induced changes in abundance and species richness occur in many insect groups. However, the study of insects in fragmented forests is still in its infancy and lacks real direction. Simple empirical studies are not answering the questions we most want to answer about fragmented systems. Are we in the midst of a mass-extinction crisis? What is the functional significance of the immense insect biodiversity? Does biodiversity loss affect ecosystem functioning? A more focused, functional approach to the study of forest fragmentation is required to move beyond the description of pattern and to determine how changes in insect communities affect ecosystem processes in fragmented forests.

A Comparison of Richness Hotspots, Rarity Hotspots, and Complementary Areas for Conserving Diversity of British Birds

Abstract: Biodiversity conservation requires efficient methods for choosing priority areas for in situ conservation management. We compared three quantitative methods for choosing 5% (an arbitrary figure) of all the 10 × 10 km grid cells in Britain to represent the diversity of breeding birds: (1) hotspots of richness, which selects the areas richest in species; (2) hotspots of range-size rarity (narrow endemism), which selects areas richest in those species with the most restricted ranges; and (3) sets of complementary areas, which selects areas with the greatest combined species richness. Our results show that richness hotspots contained the highest number of species-in-grid-cell records (with many representations of the more widespread species), whereas the method of complementary areas obtained the lowest number. However, whereas richness hotspots included representation of 89% of British species of breeding birds, and rarity hotspots included 98%, the areas chosen using complementarity represented all the species, where possible, at least six times over. The method of complementary areas was also well suited to supplementing the existing conservation network. For example, starting with grid cells with over 50% area cover by existing “Sites of Special Scientific Interest,” we searched for a set of areas that could complete the representation of all the most threatened birds in Britain, the Red Data species. The method of complementary areas distinguishes between irreplaceable and flexible areas, which helps planners by providing alternatives for negotiation. This method can also show which particular species justify the choice of each area. Yet the complementary areas method will not be fully able to select the best areas for conservation management until we achieve integration of some of the more important factors affecting viability, threat, and cost.

Host-Dependent Sporulation and Species Diversity of Arbuscular Mycorrhizal Fungi in a Mown Grassland

Abstract: 1 In laboratory microcosm experiments, co-occurring plant species were found to support very different rates of sporulation of arbuscular mycorrhizal (AM) fungi. These differences were not affected by the time of harvest, suggesting that they reflect host-dependent differences in fungal growth rates, rather than host-dependent timing of sporulation. 2 Spore counts in field soil and estimates from sorghum trap cultures showed that the association of AM fungi with particular host plants in the field was positively correlated with the sporulation rates observed on those hosts in the microcosm experiments. 3 The AM fungal species richness observed at the field site was high relative to estimates made in previous studies. 23 distinct species of AM fungi were found, seven of which have not been previously described. 4 The host-dependence of the relative growth rates of fungal populations may play an important role in the maintenance of fungal species diversity.

Biodiversity : a biology of numbers and difference

Abstract: What is biodiversity? Part 1 Measuring biodiversity: genetics of biological diversity - from varieties to species comparing character diversity among biotas species richness - measure and measurement measuring relations among biodiversity, ecological function and functional diversity diversity and higher-levels of organization. Part 2 Patterns in biodiversity: spatial and temporal patterns of genetic diversity within species spatial patterns in taxonomic diversity temporal changes in biodiversity - detecting patterns and identifying causes spatial and temporal patterns in functional diversity. Part 3 Conservation and management: does biodiversity matter? - evaluating the case for conserving species identifying priorities for the conservation of biodiversity - systematic biological criteria within a socio-political framework managing biodiversity biodiversity and global change.

Tropical forest structure and composition on a large-scale altitudinal gradient in costa rica

Abstract: 1 Forest inventory data were collected in 1988-89 from permanent plots in undisturbed tropical forest along an altitudinal transect on the northern slope of Volcan Barva, Costa Rica. Plot altitude ranged from 30 m at the base to 2600 m near the summit. 2 A total of 14 plots with a total area of 23.4 hectares was censused; all stems > 10 cm d.b.h. were tagged, identified, mapped, and measured in diameter and height. 3 Altogether 11 478 live stems > 10 cm d.b.h. were encountered in the 1988-89 census, representing 561 species in 91 families. 4 Canopy height was greatest at 300 m, decreasing both above and below that altitude, and reaching a minimum at the summit. Mean stem diameter remained constant from the base of the gradient to 1500 m a.s.l., increasing slightly at higher altitudes. Largediameter trees were least abundant at middle altitudes. Basal area was greatest near the summit, exceeding 40 m2 ha' in the two highest plots. 5 Diversity was highest at 300 m, with 149 species and 55 families per hectare. There was a progressive decrease both above and below this altitude in species richness, species diversity, number of families, and the number of species per family. Diversity was lowest at the summit. 6 Five life-forms were recorded: dicot trees comprised 78.0% of stems; palms 14.9%; tree ferns 5.2%; lianas 1.5%; and hemi-epiphytes 0.4%. Life-form distribution varied markedly over the gradient. 7 Species composition varied continuously with altitude, as shown by a detrended correspondence analysis ordination of data from 375 20-m x 20-m subplots. There were no discontinuities, nor evidence of discrete floristic zones. 8 No species was distributed over the entire 2600-m range of altitudes. The species with the greatest amplitude, Ardisia palmana, occurred over 75% of the gradient (a range of around 2000 m). A total of 203 species were recorded from only a single 1-ha plot. 9 The altitudinal range of species did not vary with altitude: the range of lowland species was similar to that of montane species. Species of high-diversity assemblages (encountered at low altitude) were similar in altitudinal niche breadth to species of low-diversity assemblages (found at high altitude).

Designing a Cost‐Effective Invertebrate Survey: A Test of Methods for Rapid Assessment of Biodiversity

Abstract: We investigated three procedures that may lead to rapid and accurate assessment of epigaeic arthropod biodiversity. They are: (1) the identification of taxa whose diversity is correlated with that of others: (2) the identification of times and methods of sampling that produce estimates of diversity representative of more intensive sampling; and (3) the use of morphospecies inventories generated by non-specialists. Ants, beetles, and spiders were sampled from four forest types, in three seasons, using two collecting methods: pitfall trapping and extraction from litter. Specimens were sorted by a non-specialist to morphospecies and by specialist taxonomists to species. Richness (α-diversity) and turnover (β-diversity) were compared for different sampling regimes using morphospecies and species inventories. We found no significant positive correlations between ant, beetle, and spider species richness but there was a strong negative correlation between ant and beetle richness. For beetles alone, richness within the families Carabidae, Scarabaeidae, and Pselaphidae (i.e, avoiding taxonomically problematic families) was significantly correlated with richness within all other families. Assessment of turnover revealed that: (1) the four forest types contained significantly different assemblages of ants and beetles but not spiders and 92) the four forests were less clearly discriminated using species from the three beetle families Carabidae, Scarabaeidae, and Pselaphidae when compared to species from all beetle families pooled. Analyses of single sampling periods and methods revealed that summer and spring pitfall samples were most representative of more intensive sampling. That is: (1) the richness of ants and beetles in these samples was significantly positively correlated with the richness of all other samples and 92) turnover of beetles and ants among the four forests revealed by summer pitfall samples was similar to turnover using all samples. The three beetle surrogate families recorded by pitfall samples in spring, and to a lesser extent summer, showed significant correlations in richness with all other beetle species recorded in the same samples. However, the assessment of turnover was less accurate when only surrogate families were used. The most accurate and cost-effective assessment of turnover was generated by a summer pitfall sample in which data for ants, carabid, and scarab beetles were combined and analyzed as a single data set. Results were largely consistent regardless of whether species or morphospecies were used, which suggests that monitoring and assessment of terrestrial invertebrate biodiversity may be achieved by the careful use of morphospecies. Our results also suggest those invertebrate taxa, sampling methods, and sampling periods that yield the most consistent and reliable assessment of epigaeic invertebrate biodiversity in Australian temperate hardwood forests. However, empirical studies that follow the protocols discussed in this paper are urgently required in different environments. These studies may point the way to more representative monitoring and assessment of terrestrial biodiversity.

Species-area and species-individual relationships for tropical trees : a comparison of three 50-ha plots

Abstract: 1 Species-accumulation curves for woody plants were calculated in three tropical forests, based on fully mapped 50-ha plots in wet, old-growth forest in Peninsular Malaysia, in moist, old-growth forest in central Panama, and in dry, previously logged forest in southern India. A total of 610 000 stems were identified to species and mapped to < Im accuracy. Mean species number and stem number were calculated in quadrats as small as 5 m x 5 m to as large as 1000 m x 500 m, for a variety of stem sizes above 10 mm in diameter. Species-area curves were generated by plotting species number as a function of quadrat size; species-individual curves were generated from the same data, but using stem number as the independent variable rather than area. 2 Species-area curves had different forms for stems of different diameters, but species-individual curves were nearly independent of diameter class. With < 10(4) stems, species-individual curves were concave downward on log-log plots, with curves from different forests diverging, but beyond about 104 stems, the log-log curves became nearly linear, with all three sites having a similar slope. This indicates an asymptotic difference in richness between forests: the Malaysian site had 2.7 times as many species as Panama, which in turn was 3.3 times as rich as India. 3 Other details of the species-accumulation relationship were remarkably similar between the three sites. Rectangular quadrats had 5-27% more species than square quadrats of the same area, with longer and narrower quadrats increasingly diverse. Random samples of stems drawn from the entire 50 ha had 10-30% more species than square quadrats with the same number of stems. At both Pasoh and BCI, but not Mudumalai. species richness was slightly higher among intermediate-sized stems (50-100mm in diameter) than in either smaller or larger sizes, These patterns reflect aggregated distributions of individual species, plus weak density-dependent forces that tend to smooth the species abundance distribution and 'loosen' aggregations as stems grow. 4 The results provide support for the view that within each tree community, many species have their abundance and distribution guided more by random drift than deterministic interactions. The drift model predicts that the species-accumulation curve will have a declining slope on a log-log plot, reaching a slope of O.1 in about 50 ha. No other model of community structure can make such a precise prediction. 5 The results demonstrate that diversity studies based on different stem diameters can be compared by sampling identical numbers of stems. Moreover, they indicate that stem counts < 1000 in tropical forests will underestimate the percentage difference in species richness between two diverse sites. Fortunately, standard diversity indices (Fisher's sc, Shannon-Wiener) captured diversity differences in small stem samples more effectively than raw species richness, but both were sample size dependent. Two nonparametric richness estimators (Chao. jackknife) performed poorly, greatly underestimating true species richness.

The species pool and its relation to species richness : evidence from Estonian plant communities

Abstract: Two types of species pool are distinguished. The regional species pool is defined as the set of species, occurring in a certain region (here: Estonia) which are capable of coexisting in a target community. The actual species pool is defined as the set of species present in a community. Field data from 14 different vegetation types in Estonia were used. The regional pool was compiled by including from the regional flora (1) all species for which the Ellenberg indicator values did not differ more than 1.5 relative units from the community mean and (2) all indifferent species. The actual pool was compiled by careful field observations. The aim of the paper is to test the validity of two null hypotheses about the species pool. HO 1 postulates that any size of the actual species pool is equally probable in the interval between zero and the size of the regional species pool. HO 2 postulates that any value of species richness per unit area (1 m 2 ) is equally probable in the interval between zero and the size of the actual species pool. To test the strengths of the relationships Monte Carlo modelling was used. It was shown that the relation between variables was stronger than proposed by the null models (P = 0.041 for HO 1 and P = 0.002 for HO 2 ). Consequently, the size of the actual species pool is largely determined by the regional species pool, and the species richness per 1 m 2 is largely determined by the actual pool. The results are discussed in the framework of coexistence theory. The size of the regional pool is determined by evolutionary (speciation) and historical (large-scale migration) processes. The size of the actual pool depends on local-scale migration, which can be a function of isolation, successional stage, local management history, etc.

Zooplankton egg banks as biotic reservoirs in changing environments

Abstract: The long-lived diapausing eggs of zooplankton constitute an ecological and evolutionary reservoir that can impact the rate and direction of population, community, and ecosystem response to environmental change. Viable diapausing eggs are often extremely abundant and can survive in aquatic sediments for decades or longer. As the mean environment changes, the frequency of extreme conditions will likely increase, and species with prolonged diapause will be able to survive extreme years of no recruitment, whereas species lacking an egg bank will not. An altered environment may change which species have poor recruitment and even, through effects on thermal diapause cues, which species produce an egg bank. The interaction between environmental variation and generation overlap (produced by prolonged diapause) results in the maintenance of biotic diversity (both species richness and genetic variation), which forms the foundation for response to future environmental change.

Fishery Recovery in a Coral‐reef Marine Park and Its Effect on the Adjacent Fishery

Abstract: Numbers offish and their wet weights were estimated in Kenyan coral-reef lagoons on seven reefs over 6 years. Two sites were protected from fishing for over 20 years, whereas the other five sites were heavily fished in recent years. A heavily fished site was converted into a marine park (Mombasa Marine National Park, approximately 10 km2 no fishing allowed), and the number of fishers allowed was slowly decreased between August 1991 and August 1992. The area adjacent to the park was converted into a marine reserve (only fishing traps, lines, and gill nets allowed) that provided fishing grounds for fishers excluded from the park. Data from a fish-landing site adjacent to the newly created marine park were collected for 3 years and analyzed to determine the effect of the park’s creation on fish catches. Results suggest that fishing in the reserve reduced fish wet weight by about a factor of 10 and reduced fish numbers and species richness by a factor of two. Both field studies and landing data suggest harvesting at a bionomic equilibrium. For example, approximately 65% of the landing site’s fishing grounds were protected with the creation of the park, and 65% of the fishers quit the studied landing, leaving nearly the same density of fishers in the remaining area (∼ 12 fishers/km2). Further, fishers using pull seines were excluded from the reserve, and their numbers were replaced by fishers using other gear (mostly basket traps). Although the overall catch per unit effort increased by about 110% after the park’s creation, the total fish landed decreased by 35% and the catch per unit effort decreased toward the end of the study period despite increasing fish abundance in the park. Although establishment of small parks elsewhere have increased the total catch, the large park we studied did not; one reason may have been the lower ratio of edge to park area of the large park. Alternatively, the park’s edge may have provided a good fishing area, so fishing effort may have been highest along the park’s edge. Consequently, a barrier may have been created that restricted fish dispersal to most of the reserve. Therefore, the area that had an increased catch was small (< 1 to 2 km from the edge) and could not compensate for the lost fishing area. Most fish species within the park showed recovery after fisher exclusion. Total fish wet weights 3 years after the fishers’ exclusion were 25% below the older marine parks. Poor recovery of the herbivoroas parrot and surgeonfish can account for much of this shortfall. Competition for resources with sea urchins appear to be slowing recovery of these two groups. A study site 2.5 km from the park’s southern boundary, in the reserve section of the protected area, showed no changes in fish abundance over the study period, despite changing gear regulations.

The diversity, abundance and biomass of termites under differing levels of disturbance in the Mbalmayo Forest Reserve, southern Cameroon

Abstract: This paper presents data on the abundance, biomass and species richness of termites in the Mbalmayo Forest Reserve, southern Cameroon. Five plots of differing disturbance level (near primary forest, old secondary forest, young plantation, weeded Chromolaena fallow, and completely cleared forest) were sampled for termites in two successive years (July 1992 and July 1993, giving a total of ten sampling areas, plus one in the completely cleared plot in November 1992). A stratified sampling regime of soil pits, wood samples, mound samples and soil scrape samples was used. Estimated abundance and biomass were extremely high in the near primary and old secondary plot (maximum estimated abundance, old secondary sampling area 1, 10488 m -2 , maximum biomass density, near primary sampling area 1,123.2 g m -2 ). In all cases termite abundance was highly clumped. Disturbance had apparently little effect on termite abundances and biomass in forested plots, but there were clear reductions in abundance and biomass in the cleared plots. In the completely cleared plot, abundance and biomass fell sharply from year 1 to year 2, presumably because colonies left after clearance had dried out and died. There were large differences in the taxonomic-, nesting- and feeding-group composition of the plots, with soil feeding termites being especially strongly affected by disturbance. The sources of error associated with this sampling programme are discussed. It is argued that the near primary forest plot may have higher microhabitat and concomitant termite assemblage heterogeneity than the more disturbed plots. In most cases over 90 % of the overall abundance in a sampling area was due to a few species; extinction, and will make areas. This may be due to the unpredictable dynamics of colony foundation and extinction, and will make estimations of the effect of termites on overall ecosystem processes (from abundance and biomass data alone) problematical. There is no evidence of immigration of savanna termites into the forest reserve, and thus cleared areas have depauperate forest assemblages. It is concluded that this is the most accurate estimation of termite assemblage parameters yet attempted.

Effect of Landscape Structure on Parasitoid Diversity and Parasitism in Agroecosystems

Abstract: The armyworm (Pseudaletia unipuncta) was used as a model host insect to explore the influence of agricultural landscape structure at two spatial scales on larval parasitoid species richness and rates of larval parasitism in southern Michigan First, within fields, we compared parasitoid communities in maize fields near, and distant from, a hedge- row edge Second, we replicated these studies within a complex landscape (agricultural fields of small size embedded in a landscape with abundant hedgerows and woodlots) vs a simple landscape (agricultural fields of large size embedded in a landscape with few hedgerows and woodlots) The structural differences between the simple and complex agricultural landscapes were characterized by analysis of aerial photographs and digital land-use data After landscape analysis, three maize fields from each area were selected for the experimental studies The complex landscape contained fields that were 75% smaller, had 63% more perimeter of wooded field edge per hectare of field area, and had 81% more field edge in wide hedgerow than fields in the simple landscape Fields in the simple landscape had 74% and 53% more field edge in herbaceous roadside and crop-to-crop interfaces, respectively, than did fields in the complex landscape In the six selected maize fields, third and fifth instar P unipuncta were released indi- vidually onto maize plants 5 m and 90 m from a hedgerow edge Larvae were recovered after 4-5 d and reared in the laboratory to record parasitoid emergence Parasitoid species diversity was similar in both landscape types (simple landscape: four species; complex landscape: five species) Mean percentage parasitism was significantly higher in the complex landscape than in the simple landscape (13 1% vs 24%) but was not affected by the location within fields (near hedgerows vs distant from hedgerows) in either landscape type

The Vegetation of Restored and Natural Prairie Wetlands

Abstract: Thousands of wetland restorations have been done in the glaciated mid-continent of the United States. Wetlands in this region revegetate by natural recolonization after hydrology is restored. The floristic composition of the vegetation and seed banks of 10 restored wetlands in northern Iowa were compared to those of 10 adjacent natural wetlands to test the hypothesis that communities rapidly develop through natural recolonization. Restoration programs in the prairie pothole region assume that the efficient-community hypothesis is true: all plant species that can become established and survive under the environmental conditions found at a site will eventually be found growing there and/or will be found in its seed bank. Three years after restoration, natural wetlands had a mean of 46 species compared to 27 species for restored wetlands. Some guilds of species have significantly fewer (e.g., sedge meadow) or more (e.g., submersed aquatics) species in restored than natural wetlands. The distribution and abundance of most species at different elevations were significantly different in natural and restored wetlands. The seed banks of restored wetlands contained fewer species and fewer seeds than those of natural wetlands. There were, however, some similarities between the vegetation of restored and natural wetlands. Emergent species richness in restored wetlands was generally similar to that in natural wetlands, although there were fewer shallow emergent species in restored wetlands. The seed banks of restored wetlands, however, were not similar to those of natural wetlands in composition, mean species richness, or mean total seed density. Submersed aquatic, wet prairie, and sedge meadow species were not present in the seed banks of restored wetlands. These patterns of recolonization seem related to dispersal ability, indicating the efficient-community hypothesis cannot be completely accepted as a basis for restorations in the prairie pothole region.

Effects of bison grazing, fire, and topography on floristic diversity in tallgrass prairie.

Abstract: Grazed and ungrazed sites subjected to different fire frequencies were sampled on the Konza Prairie Research Natural Area in northeast Kansas after 4 years of bison grazing (1987-1991). The objective was to study effects of bison grazing on plant species composition and diversity components (plant species richness, equitability, and spatial heterogeneity) in sites of contrasting fire frequency. Cover and frequency of cool-season graminoids (e.g. Poa pratensis L., Agropyron smithii Rydb., Carex spp.) and some fortes (e.g. Aster ericoides [A. Gray] Howell, and Oxalis stricta L.) were consistently higher in sites grazed by bison than in ungrazed exclosures, whereas the dominant warm-season grasses (Andropogon gerardii Vitman, Sorghastrum nutans [L.] Nash, Panicum virgatum L., Schizachyrium scoparium [Michx.] Nash) and other forbs (e.g. Solidago missouriensis Nutt.) decreased in response to bison. Plant species diversity (H') and spatial heterogeneity in all areas sampled were significantly increased by bison. Increased heterogeneity and mean species richness in grazed prairie (40 species per sample site) compared to ungrazed prairie (29 species per site) were likely a result of greater microsite diversity generated by bison, whereas preferential grazing of the dominant grasses and concomitant increases in subordinate species resulted in an increase in equitability of species abundances. Species/area relationships indicated greater effects of bison on plant species richness with increasing sample area. Increases in plant diversity components associated with bison grazing were generally greater in annually burned than in 4-year burned sites. Effects of ungulate grazers on floristic diversity have important implications given recent evidence that plant species diversity and the compositional and production stability of grassland plant communities are positively related.

Biodiversity and plant productivity in a model assemblage of plant species

Abstract: We examined productivity as a function of biotic diversity. We manipulated plant species richness as an experimental factor to determine if productivity (net above ground primary productivity or NPP) is affected by changes in plant diversity (species richness). We constructed 164 assemblages that varied in species richness and measured their biomass at the end of one growing season. The plants were drawn from a pool of 16 species of self-pollinating annual herbs common to English weedy fields. On average, species-poor assemblages were less productive. Results also showed, however, that species-poor assemblages had wider ranges of possible productivities than more diverse assemblages.

The Effects of Land Use on the Structure of Ground‐Foraging Ant Communities in the Argentine Chaco

Abstract: We examined the responses of a ground-foraging ant community to a gradient of land-use intensity in a grazing agroecosystem in the Chaco of northern Argentina. The gradient extended from a highly degraded condition characteristic of traditional grazing practices, through an area of less severe disturbance where grazing was less concentrated, to two areas in which grazing had been managed for 3 and 18 yr, respectively. Ground cover changed along this gradient from bare to litter-covered, ground-layer vegetation changed from sparse to a structurally complex mixture of grasses and forbs, and canopy cover increased in areas of intermediate grazing intensity and then decreased. Community diversity varied among the sites depending on both season and scale of analysis. Site-scale ant species richness was slightly higher in sites of intermediate disturbance in the summer- wet season but was much greater in the least disturbed site in the winter-dry season. The same dry-season pattern was evident in both species richness and diversity at the scale of transects within sites, whereas species richness at the scale of individual traps within transects was significantly lower at sites of intermediate disturbance than at either highly restored or highly degraded sites. Abundances of individual ant species and functional groups also changed along the land-use gradient. Litter-inhabiting cryptic species and spe- cialized predators responded positively to grazing management, whereas opportunists and the hot-climate specialist Forelius nigriventris were prevalent in highly disturbed areas. Other functional groups exhibited redundancy and species turnover along the gradient. Detrended correspondence analysis (DCA) revealed that the ant faunas at the extremes of the land-use gradient were more similar than expected. We hypothesize that the interaction of local-scale habitat features with historical and biogeographic influences may determine the responses of this ant community to land use, and that highly degraded areas may have conservation value because they are regional sources of arid-adapted ants.

What factors influence the diversity of saproxylic beetles? A multiscaled study from a spruce forest in southern Norway

Abstract: The diversity in different groups of obligate saproxylic beetles was related to ecological variables at three levels of spatial scale in mature spruce-dominated forest. The variables were connected to: (i) decaying wood, (ii) wood-inhabiting fungi, (iii) the level of disturbance, (iv) landscape ecology, and (v) vegetational structure. Several strong relationships were found at medium (1 km2) and large scales (4 km2), while only weak relationships were found at a small scale (0.16 ha; 1 ha=104 m2). This may be explained by the local variations in habitat parameters and the high mobilities of many beetle species. Factors connected to decaying wood and wood-inhabiting fungi were clearly the most important factors at all scale levels. In particular, the variables diversity of dead tree parts, number of dead trees of large diameter and number of polypore fungi species increased the species richness of many groups and increased the abundance of many species. Eight species were absent below a certain density of decaying wood per 1 or 4 km2. Former extensive cutting was a negative factor at large scale, probably because of decreasing recolonization with increasing distance to the source habitats. Thinning reduced the diversity of species associated with birch. The development of guidelines favouring the diversity of saproxylic beetles are discussed below.

Is the humped relationship between species richness and biomass an artefact due to plot size

Abstract: If plants vary in size, then the use of small quadrats of fixed size will show maximum species richness at intermediate biomass. Fewer plants are found in plots of fixed size both if plants increase in size (and biomass therefore increases), or if vegetation becomes sparse (biomass decreases).The supposed response of species richness to biomass is therefore produced as an artefact of varying plant number. Grime derived a humped relationship between species richness and biomass from studies using relatively small quadrats of 50 cm x 50 cm, or 1/4 m2 (AlMufti et al. 1977; Grime 1979), and similar patterns have been found in many later studies on terrestrial vegetation (e.g. Moore & Keddy 1989; Wisheu & Keddy 1989; Wheeler & Shaw 1991; Garcia et al. 1993; Rosenzweig & Abramsky 1993; Abrams 1995). The ecological reason usually proposed to explain the humped relationship is that stress reduces diversity at both extremes (harsh environments at low biomass and intensive competition at high biomass, Grime 1979; Abrams 1995), although other explanations have been proposed (Rosenzweig & Abramsky 1993). In this paper I show how a humped response can be produced without assuming any biological interactions.

Effects of sampling area and subsampling procedure on comparisons of taxa richness among streams

Abstract: the biotic integrity of communities. In all cases, we make the implicit and sometimes brash assumption that we can really measure the number of taxa in a community. Although measuring taxa richness might appear straightforward, accurate measurement has been extraordinarily difficult; and despite years of effort, no universally accepted methods for its measurement have emerged. The essential problem is that we can never completely census a taxonomic assemblage or entire community; we rely instead on estimates that describe some portion of the real taxa richness of an assemblage. The problem of knowing what percent of the taxa present have been collected is exacerbated when investigators fail to explicitly define their universe of interest (i.e., the spatial bounds of the community or communities in question). Comparisons of taxa richness among studies that used different sampling and subsampling methods are especially difficult and should be viewed skeptically. The difficulty of obtaining accurate measurements of richness is due to the collector's curve

Variation in the composition and structure of tropical savannas as a function of rainfall and soil texture along a large‐scale climatic gradient in the Northern Territory, Australia

Abstract: . Variation in structural and compositional attributes of tropical savannas are described in relation to variation in annual rainfall and soil texture along a subcontinental-scale gradient of rainfall in the wet-dry tropics of the Northern Territory, Australia. Rainfall varies along the gradient from over 1500 mm p.a. in the Darwin region (c. 12° S) to less than 500 mm in the Tennant Creek region (c. 18° S). Soils are patchy, and sands, loams and clays may occur in all major districts within the region. We utilized a large data set (1657 quadrats ° 291 woody species; with numerous measured and derived sample variables) covering an area of 0.5 million km2. Correlations between floristic composition of woody species and environmental variables were assessed using DCA ordination and vector fitting of environmental variables. Vectors of annual rainfall and soil texture were highly correlated with variation in species composition. Multiple regression analyses incorporating linear and quadratic components of mean annual rainfall and topsoil clay content were performed on three structural attributes (tree height, tree cover, tree basal area) and two compositional attributes (woody species richness, deciduous tree species richness). Tree height declined with decreasing rainfall; cover, basal area, woody species richness and deciduous species richness all declined with decreasing rainfall and increasing soil clay content. Regression models accounted for between 17% and 45% of the variation in the data sets. Variation in other factors such as soil depth, landscape position and recent land-use practices (for which there were no data on an individual quadrat basis) are likely to have contributed to the large residual variation in the data set.