Showing papers on "Species richness published in 1993"

Species diversity in ecological communities: historical and geographical perspectives.

Abstract: "Species Diversity in Ecological Communities" looks at biodiversity in its broadest geographical and historical contexts. For many decades, ecologists have tended to study only small areas over short time spans in the belief that diversity is regulated by local ecological interactions. However, to understand fully how communities come to have the diversity they do and to address properly the urgent conservation problems we face, scientists must consider global patterns of species richness and the historical events that shape both regional and local communities. The authors use new theoretical developments, analyses and case studies to explore the large-scale mechanisms that generate and maintain diversity. Case studies of various regions and organisms consider how local and regional processes interact to determine patterns of species richness. The contributors emphasize the fact that ecological processes acting quickly on a local scale do not erase the effects of regional and historical events that occur more slowly and less frequently.

Rare species, the coincidence of diversity hotspots and conservation strategies

Abstract: SPECIES conservation in situ requires networks of protected areas selected for high conservation interest1–3. Throughout most of the world, however, there are neither the resources nor the time to carry out detailed inventories for most taxa2,4 before designating protected areas. Site selection (on grounds other than availability) would be easier and more effective if two things were true: (1) habitats that are species-rich for one taxon are also species-rich for others5; and (2) rare1 species occur in, and therefore benefit from the conservation of, species-rich habitats. Diversity (usually, species richness) and the presence of rare species are the most frequently cited criteria for site selection by conservationists6–8. Here, we use data on British plants and animals held by the Biological Records Centre (BRC)9 and the British Trust for Ornithology (BTO), mapped on a grid of 10 km × 10 km ('10 km squares') to examine the extent to which species-rich areas for different taxa coincide, and whether species-rich areas contain substantial numbers of rare species. The fine scale and high intensity of recording in Britain produces distributional datasets at least as good as and, in most cases, better than those available elsewhere. For Britain at least, we do not find strong support for either proposition. Species-rich areas ('hotspots'10) frequently do not coincide for different taxa, and many rare species do not occur in the most species-rich squares.

Gap analysis: a geographic approach to protection of biological diversity

Abstract: The conventional approach to maintaining biological diversity generally has been to proceed species by species and threat by threat. We suggest that piecemeal approaches are not adequate by themselves to address the accelerating extinction crisis and, furthermore, they contribute to an unpredictable ecological and economic environment. Here, we describe a methodology called Gap Analysis, which identifies the gaps in representation of biological diversity (biodiversity) in areas managed exclusively or primarily for the longterm maintenance of populations of native species and natural ecosystems (hereinafter referred to as biodiversity management areas). Once identified, gaps are filled through new reserve acquisitions or designations, or through changes in management practices. The goal is to ensure that all ecosystems and areas rich in species diversity are represented adequately in biodiversity management areas. We believe this proactive strategy will eliminate the need to list many species as threatened or endangered in the future. Gap Analysis uses vegetation types and vertebrate and butterfly species (and/or other taxa, such as vascular plants, if adequate distributional data are available) as indicators of biodiversity. Maps of existing vegetation are prepared from satellite imagery (LANDSAT) and other sources and entered into a geographic information system (GIS). Because entire states or regions are mapped, the smallest area identified on vegetation maps is 100 ha. Vegetation maps are verified through field checks and examination of aerial photographs. Predicted species distributions are based on existing range maps and other distributional data, combined with information on the habitat affinities of each species. Distribution maps for individual species are overlaid in the GIS to produce maps of species richness, which can be created for any group of species of biological or political interest. An additional GIS layer of land ownership and management status allows identification of gaps in the representation of vegetation types and centers of species richness in biodiversity management areas through a comparison of the vegetation and species richness maps with ownership and management status maps. Underrepresented plant communities (e.g., present on only 1 or 2 biodiversity management areas or with a small total acreage primarily managed for biodiversity) also can be identified in this manner. Realization of the full potential of Gap Analysis requires regionalization of state data bases and use of the data in resource management and planning. Gap Analysis is a powerful and efficient first step toward setting land management priorities. It provides focus, direction, and accountability for conservation efforts. Areas identified as important through Gap Analysis can then be examined more closely for their biological qualities and management needs. As a coarse-filter approach to conservation evaluation, Gap Analysis is not a panacea. Limitations related to minimum mapping unit size (where small habitat patches are missed), failure to distinguish among most seral stages, failure to indicate gradual ecotones, and other factors must be recognized so that Gap Analysis can be supplemented by more intensive inventories. WILDL. MONOGR. 123, 1-41 GAP ANALYSIS: A GEOGRAPHIC APPROACH TO PROTECTION OF BIOLOGICAL DIVERSITY WILDLIFE MONOGRAPHS 6

The Use of Species Accumulation Functions for the Prediction of Species Richness

Abstract: We develop a stochastic theory of the accumula- tion of new species in faunistic orfloristic inventories. Dif- ferential equations for the expected list size and its variance as a function of the time spent collecting are presented and solvedforparticular cases. These particular cases correspond to different models of bow the probability of adding a new species changes with time, the size of the list, the complexity of the area sampled, and other parameters. Examples using field data from butterflies and mammals are discussed, and it is argued that the equations relating sampling effort with size of the list may be useful for conservation purposes be- cause they should lendformality to comparisons among lists and because they may have predictive power by extrapolat- ing the asymptotic size of the lists. The suitability of different models to a variety of field situations is also discussed.

Species Richness of Experimental Productivity Gradients: How Important is Colonization Limitation?

Abstract: The biodiversity of a site should depend on the interplay of local colonization (gain) and extinction (loss) rates, but few theoretical explanations of diversity patterns on productivity gradients have included effects of productivity on colonization. In an 1 -yr study, experimental increases in productivity via nitrogen addition generally led to de- creased species richness in four grassland fields. Decreased diversity in productive plots was caused as much by lower rates of species gain as by greater rates of loss of existing species. Annual grasses and forbs had high gain and loss rates, but these were independent of productivity. In contrast, the rates of gain of perennial grass and forb species declined with productivity, and their rates of loss increased. Species richness was dependent on litter mass and on light penetration, but not on aboveground living plant mass, suggesting that there was no direct effect of productivity on diversity. Diversity in periodically burned prairie, which had low litter mass, was independent of productivity by the 10th and 11th yr of the study. Results suggest that diversity is lower in productive grasslands because accumulated litter, and possibly lower light penetration, inhibit germination and/or survival of seedlings, and thus decrease rates of establishment by new species. Higher productivity also leads to higher rates of loss, presumably via competitive displacement, of existing species. Results do not support the hypothesis that soil N heterogeneity controls diversity on the temporal and spatial scales of this study.

Small-scale plant species turnover in a limestone grassland: the carousel model and some comments on the niche concept.

Abstract: This study reports on small-scale changes in the distribution of plant species in a 2.5 m2 plot of grazed, species- rich Veronica spicata - Avenula pratensis grassland on shal- low, dry, nutrient-poor soil in the Great Alvar area (Stora Alvaret) of southern Oland, southeastern Sweden. Multivari- ate analysis of 0.001 m2 and 0.25 m2 quadrats within the plot showed that there is little floristic variation without any trend in the plot. Average species richness varied little throughout the study period from 1986 to 1991 with 1986 averages of 7.0 on 0.001m2, 16.3 on 0.01 m2, and 26.1 on 0.25 m2. On 0.001 m2 the highest species number found was 12, on 0.01 m2, 27. However, cumulative species richness, i.e. species number in the first year plus new species appearing in later years (aver- aged over 40 quadrats) increased over the same period, on 0.001 m2 from 7.0 in 1986 to 14.9 in 1991, and on 0.01 m2

Climatic gradients in woody plant species richness: towards an explanation based on an analysis of southern Africa's woody flora

Abstract: The distribution of southern Africa's woody flora (N=1372 species) describes a west-to-east pattern of increasing species richness, being lowest in arid to semi- arid areas and highest in mesic to humid areas. Climate accounts for 77.8% (R2; P

Biological diversity of Mexico: origins and distribution.

Abstract: Mexico is among the richest countries in the world in terms of the number of native animal and plant species. Found in a wide variety of habitats--from alpine meadows and tropical forests to vast stretches of desert and isolated pockets of biogeographical uniqueness--these species comprise a fascinating, important, and vastly underutilized biological laboratory. This volume presents a collection of selected papers that explore this marvelous biological abundance. The book is divided into six parts. The first section sets the stage with geological and paleobotanical overviews; the succeeding five sections employ a strong taxonomic base to document species richness, endemism and distribution for animals and plants, followed by reviews of contrasting ecosystems and plants that are closely associated with humans. The last section summarizes the disheartening rate of habitat destruction which threatens to diminish this diversity. In addition to the purely scientific value of this important work, it provides the much-needed basic data that will help conservation policymakers assess and respond to Mexico's ecological evolution.

Global patterns of tree species richness in moist forests : energy-diversity theory does not account for variation in species richness.

Abstract: Energy-diversity theory has gained currency as an explanation for global patterns of species richness. We examine the suggestion of Currie and Paquin (1987; Nature 329: 326-327) that variation in evapotranspiration - a function of moisture availability and temperature that is directly related to plant production - predicts tree species richness in global comparisons. We present contrary evidence: the number of tree species on 26 large (17-7401 kmz) areas of moist temperate forest show continental differences unrelated to geographical patterns in evapotranspiration. Tree species richness of 128 samples of ca 1ha within moist-forest biomes of the world reveal patterns of variation amone continents. and with latitude. that likewise cannot be attributed to eeoeraohi

Latitudinal patterns in European ant assemblages: variation in species richness and body size

Abstract: Using published distributions of 65 species from the British Isles and northern Europe, we show that ant assemblages change with latitude in two ways. First, as commonly found for many types of organisms, the number of ant species decreased significantly with increasing latitude. For Ireland and Great Britain, species richness also increased significantly with region area. Second, although rarely demonstrated for ectotherms, the body size of ant species, as measured by worker length, increased significantly with increasing latitude. We found that this body-size pattern existed in the subfamily Formicinae and, to a lesser extent, in the Myrmicinae, which together comprised 95% of the ant species in our study area. There was a trend for formicines to increase in size with latitude faster than myrmicines. We also show that the pattern of increasing body size was due primarily to the ranges of ant species shifting to higher latitudes as their body sizes increased, with larger formicines becoming less represented at southerly latitudes and larger myrmicines becoming more represented at northerly latitudes. We conclude by discussing five potential mechanisms for generating the observed body-size patterns: the heat-conservation hypothesis, two hypotheses concerning phylogenetic history, the migration-ability hypothesis, and the starvation-resistance hypothesis.

Hymenoptera and biodiversity.

Abstract: Hymenoptera: their diversity, and their impact on the diversity of other organisms intraspecific biodiversity in Hymenoptera: implications for conservation and biological control threats to the diversity of solitary bees in a neotropical dry forest in Central America effects of increasing land utilization on species representation and diversity of aculeate wasps and bees in the semi-arid areas of southern Africa comparison of the arboreal ant mosaic in Ghana, Brazil, Papua New Guinea and Australia - its structure and influence on arthropod diversity bees, pollination systems, and plant diversity diversity of native bees and agro-ecosystems parasitic Hymenoptera, biological control and biodiversity parasitoid webs refuges, host population dynamics and the genesis of parasitoid diversity the role and enhancement of parasitic Hymenoptera biodiversity in agro-ecosystems spatial patterns in the description and richness of the Hymenoptera what does tropical society want from the taxonomist? measuring biodiversity for choosing conservation areas Costa Rica: an example for the study of tropical biodiversity.

The role of seed dispersal in the natural regeneration of rain forest after strip-cutting in the Peruvian Amazon

Abstract: Seed dispersal and forest regeneration were studied on a 30×150 m strip cleared by ‘strip-cutting’, a system of forest management designed for sustained yield (Hartshorn 1989), in high terrace rain forest in the Department of Loreto, Peru. After one year the strip was dominated by seedlings of a few bat- and bird-dispersed pioneer tree species (Cecropia spp., Melastomataceae, and Alchornea triplinervia); stump sprouts from cut trees and saplings that survived the clearing were less numerous. The density of saplings (>2 m in height) surviving the clearing was 903 per hectare; 94% of these survived the subsequent 18 months. About 30% of 417 stumps (>7.5 cm diameter at breast height) resprouted within 3 months, with an additional 10% sprouting in the subsequent 10 months. Sprouting frequency was greater for small stumps than large and varied greatly among plant families. Seed deposition over this year was much lower in the interior of the strip, both in species richness and numbers of seeds, than within the forest; strip edges were intermediate in richness and number. The decline in seed input from forest to edge to strip, both in species and in numbers of seeds, was most pronounced for bird-dispersed taxa (primarily Melastomataceae); bat- and wind- dispersed taxa were more evenly distributed. The similarity in bat species composition between the strip and nearby primary forest was higher than the similarity in bird species composition between these habitats, reflecting a failure of many forest bird species to venture into the strip. The predominance of Cecropia spp. and other pioneers of minimal commercial value in the regeneration question the sustainability of strip-cutting. Subsequent succession and future tree species composition on the cleared strip will depend not only on the survivorship and growth of sprouts, survivors, and seedlings, but also on responses of different seed-dispersing animal taxa to changes in the species composition and structure of the vegetation in the strip.

How many species are there

Abstract: 'How many species are there' is a question receiving more attention from biologists and reasons for this are suggested. Different methods of answering this question are examined and include: counting all species; extrapolations from known faunas and regions; extrapolations from samples; methods using ecological models; censusing taxonomists' views. Most of these methods indicate that global totals of 5 to 15 million species are reasonable. The implications of much higher estimates of 30 million species or more are examined, particularly the question of where these millions of species might be found.

Bat species richness and abundance in tropical rain forest fragments and in agricultural habitats at Los Tuxtlas, Mexico

Abstract: Faced with the rapid and extensive conversion of tropical rain forests to pasture lands and agricultural fields and with the need to preserve the remaining mammalian fauna, it is imperative to determine how the different species that form the mammalian community have responded to the anthropogenic alterations of their natural habitats. To provide data in this direction, we sampled bats in 45 forest islands, in 20 agricultural habitats representing five types of vegetation (cocoa, coffee, mixed, citrus and allspice), in four live-fence sites and in four pasture sites at Los Tuxtlas, Veracruz, Mexico. Sampling effort resulted in the capture of 2587 bats representing 35 species. In forest habitats we detected 32 species. We did not capture any bats at the four pasture sites, but the at the other agricultural habitats studied, we captured 38% of the bats and 77% of the species recorded. Thirty-four percent of the species recorded were present at the live-fence habitats. Isolating distance was an important variable influencing species richness in forests and in agricultural habitats. Only 10% of the species recorded occurred in all the habitats studied, but 77% of the species occurred in a habitat other than rain forest. Recaptures of bats indicated inter habitat movements in the fragmented landscape. We discuss the conservation value for the bat fauna of agricultural islands of vegetation as elements reducing isolating distances among forest fragments.

The species-pool hypothesis and plant community diversity

Abstract: One of the central problems in ecology is to explain species diversity in communities, and a considerable number of hypotheses with this purpose have been formulated during the last three decades. One very influential group of hypotheses, largely emanating from Hutchinson (1959, 1961), seek explanations in terms of mechanisms of coexistence. The question addressed by these hypotheses is: "why are there so many coexisting species?" (implicitly assuming that most potential inhabitants of a certain community are present, or have at least had the opportunity to be present, but may have been subsequently outcompeted). These hypotheses usually predict that species, in order to coexist, in some way must avoid negative effects of competition. A common theme for the wide array of mechanisms suggested to have this, or alike, effects (e.g. Hutchinson 1961, MacArthur 1972, Grubb 1977, Grime 1979, Huston 1979, Tilman 1982, Fagerstrom 1988) is that they are confined to, in a strict sense, "ecological" phenomena, such as spatial or temporal separation of resource use, herbivore-mediated suppression of dominants, or chance effects during recruitment. An alternative (or complementary) view of diversity in communities stresses the importance of historic, or phylogenetic, aspects of species diversification. Present-day diversity is a result of speciation and extinction processes that have occurred over long evolutionary time. In his overview of diversity in land communities, Whittaker (1977) concluded that there is not much evidence, either theoretical or empirical, for the hypothesis that species diversity is in equilibrium. Hence, in Whittakers view, to understand patterns of diversity, large scale aspects of speciation, extinction and biogeography must be accounted for. This line of reasoning can be exemplified by some studies of diversity in tropical rain forests. These communities are well known for their exceptional plant species richness (Gentry 1982), and one general explanation for their high diversity is that tropical forests have accumulated species under stable conditions experienced during long evolutionary time (Stebbins 1974). In a study of tropical tree species diversity, Hubbell and Foster (1986) suggested that tree communities are in a state of non-equilibrium; new species' (either immigrants or newly evolved ones) are capable of invading communites due to a permanent occurrence of "empty sites". Since the risk of species exclusion from a plant community was found to be very small, diversity would be determined by regional species richness and immigration rates. Thus, variation in species diversity among communities, or among guilds within communities, reflect rate-determining mechanisms behind speciation and extinction. High species diversity is expected when the existing speciespool contains many species, and comparatively low species diversity will be found when the species-pool is small. Similar suggestions have been made by several authors, in various contexts, (e.g. Grime 1973, 1979, Cracraft 1985, Hodgson 1986, 1987, Connell and Lowman 1989, Hart 1990, Brooks and McLennan 1991, Cornell and Lawton 1992, Zobel 1992), and Taylor et al. (1990) coined the term "the species-pool hypothesis" for explanations of local diversity by reference to the size of the regional or global pool of species: "All else being equal, the larger the local and/or global area of a habitat type and the older its geological age, the greater the past opportunity for speciation and hence, the greater the number of available species adapted to a particular habitat type". However, if different phylogenetic lineages do not diversify at the same average rate, the age (or size) of a habitat type may be only weakly related to the richness of the species-pool. Furthermore, Zobel (1992) stressed the importance of "evolutionary factors" (i.e. speciation rate) in explanations of species diversity in plant communities, but did not specify what might determine speciation rate per se. Cracraft (1985) approached the diversity problem from a different angle and suggested that community diversity is explicable by the basic processes speciation and extinction; speciation rates are mainly determined by large scale changes in lithospheric complexity, and extinction rates are dependent on "envi-

Changes in chalk-grassland structure and species richness resulting from selective nutrient additions

Abstract: A series of fertilization experiments was carried out over a 5-yr period in a chalk grassland in Limburg (The Netherlands) as part of a study of the maintenance of species richness in species-rich grasslands. Phosphorus and nitrogen were shown to be the most limiting nutrients. Addition of both elements doubled above-ground production, and species rich- ness dropped ca. 50 % in 0.01-m 2 subplots, relative to controls. However, neither the above-ground production nor plant growth-forms were sufficient to explain the observed changes in species richness. Small-scale structural heterogeneity of the vegetation is probably critical for maintaining high levels of richness. Historically, high nitrogen, low phosphorus condi- tions were rarely encountered in the Dutch landscape and few species appear adapted to these conditions. Among the chalk grassland species, Brachypodium pinnatum seems well adapted to these conditions, where it dominates and excludes most other species. A detailed understanding of the small-scale processes responsible for maintenance of species richness is critically important in efforts to maintain the biodiversity of natural ecosystems.

Monitoring seed dispersal at isolated standing trees in tropical pastures: consequences for local species availability

Abstract: The tropical rain forest landscape has been transformed to a mosaic composed of patches of crops, secondary vegetation and remnant forest fragments of different shapes and sizes. Isolation of patches and fragments is a critical issue in the maintenance of local species diversity. In this study we focus on the dispersal of propagules by birds to understand the movement of plants between landscape components. Seed deposition and the behavior of frugivorous birds were monitored at four isolated fig trees (Ficus yoponensis and F. aurea) in man-made pastures. Seed deposition was measured by trapping seeds under canopy trees for six months and by direct observation of bird visits to the four trees for one year. Seed deposition densities were 465,614,632 and 1097 seeds/m2 accumulated over six months under each of the four trees. We recorded 8268 seeds of 107 species under the trees, among them, 6726 seeds (81 %) were of 56 species dispersed by vertebrate frugivores. Seeds of tree species accounted for 26% of the total species. Seventy-three species of birds perched in the observed trees, and 3344 visits were made by 47 species of frugivores. Frugivorous birds occurred in two groups: resident species nesting in the pastures and resident species nesting elsewhere. Propagule exchange between landscape components is clearly influenced by the behavior of these two groups. Structure and dynamics of the landscape depend on plant species availability within the mosaic. This availability is high and suggests possibilities for the management of the local species diversity of tropical rain forests.

Biodiversity of the Pelagic Ocean

Abstract: This paper reviews knowledge of biodiversity in open ocean pelagic communities and discusses the possible causal factors for the patterns. The oceanic pelagic ecosystem is by far the largest on Earth and, although locally its assemblages may be as rich as many terrestrial ecosystems, its global diversity (at both a species and an ecosystem level) is low. There are latitudinal trends in pelagic species diversity similar to those in many terrestrial taxa. High species richness in the oceans, however, tends to be associated with regions of low productivity that lack strong seasonality in the production cycle. The richest zones occur at the boundaries between different types of oceanic water where different faunas are mixed together, but the geographical locations of these boundaries are unstable and shift seasonally by hundreds of kilometers. If high diversity is emphasized in the development of protocols for conservation, then not only will the oceans receive low priority in conservation and resource management, but the regions most important in terms of process will also be overlooked. The scales of oceanic systems are so large that the methodologies developed for terrestrial conservation and resource management are inapplicable. Biodiversity may be regarded as the principal criterion for developing management strategies, yet the links (if any) between the ecological processes in the ocean that play such an important role in global homeostasis remain poorly characterized. The basis for a predictive understanding of the interaction between diversity and ecological process can be greatly enhanced relatively inexpensively by systematically collating existing data and working up extant collections of material

Accumulation of native parasitoid species on introduced herbivores: a comparison of hosts as natives and hosts as invaders

Abstract: Herbivore species newly introduced into foreign locations (hosts as invaders) are often attacked by native parasitoid species. Here we compare the structure and diversity of 87 such parasitoid complexes with those on the same herbivore species in their native regions (hosts as natives). Overall parasitoid attack rates are generally lower on hosts as invaders than on hosts as natives. Also, parasitoid complexes on hosts as invaders are generally less rich and contain a higher proportion of generalists than those on hosts as natives. Overall richness shows a weak tendency to increase with duration in the region of introduction over the first 150 yr, but the ratio of generalists to specialists does not change over this time period. These results, in part, parallel those for herbivore complexes on introduced host plants and suggest that common theoretical principles may apply to both trophic levels. The herbivores were also categorized by level of concealment and taxon (order) to determine whether life-style or phylogeny influenced parasitoid richness in native or foreign locations. No strong influences emerged. Our most novel result is a vulnerability-to-parasitism regression; the numbers of parasitoids attacking host species in invaded regions are correlated with the numbers in native regions. The biological characteristics of the herbivore as well as extrinsic region-specific factors may play important roles in setting parasitoid richness levels on hosts as natives and on hosts as invaders.

Construction and Analysis of a Large Caribbean Food Web

Abstract: We document the construction of a relatively large food web (44 species) from the island of St. Martin in the northern Lesser Antilles, and compare it with patterns observed in other, generally smaller food webs. In constructing this web, we integrate data from a variety of studies, many of which focussed on Anolis lizards and their vertebrate predators. In addition to determining the links between predators and prey, we estimate the frequencies of predation (the link strengths), and find an approximately bell—shaped distribution with a majority of links of intermediate frequencies. Some of the properties of this web contrast strongly with those of webs in the ECOWeB compilation. In particular, our analysis shows this web to possess an unusual richness of intermediate species (relative to top predators or basal species) and of links between those intermediate species. The number and lengths of chains are also unusually high, as is the degree of omnivory. Nor does this web match the predictions of the cascade model, which predicts even higher proportions of intermediate species and links between them, and even more numerous chains. It appears that these and other differences are not due simply to the large number of species involved here, but it is not yet clear whether they should be ascribed to the completeness with which some of the diets are known, to differences between the ways this and other webs were constructed, or to unique ecological conditions on the island of St. Martin.

Species dynamics and nutrient accumulation during early primary succession in coastal sand dunes

Abstract: 1 The present study reports on a primary succession series which started on bare soil on the Dutch island of Schiermonnikoog after the building of a sand dike. Vegetational changes were studied for 18 years by means of permanent transects along a topographic gradient from a moist plain to dry dunes. Soil development and vegetation structure were reconstructed using a chronosequence. A fertilizer experiment was set up in an intermediate successional stage in the plain and on the dune, in order to determine which soil resources limited productivity. 2 Differences in salinity, flooding and moisture content were important determinants of the differences in species composition along the topographic gradient. In addition, year-to-year fluctuations of these factors seem to be responsible for the year-to-year fluctuations in frequency of occurrence of many short-lived species. These factors did not, however, show a consistent long-term trend over time. 3 From soil analyses and the nutrient addition experiment, it is concluded that nitrogen limited above-ground biomass production. Over a period of about 16 years the total amount of nitrogen in the organic layer of the soil increased from 7 to 50 g N m-2 in the plains and from 1 to 15 g N m-2 on the dunes. 4 The accumulation of nitrogen during the successional series is accompanied by an increased biomass, a decreased light penetration to the soil surface, a decreased root/shoot ratio, increasing dominance of tall species, and a decreasing abundance of small, short-lived species. These data suggest that the importance of light competition is increasing during succession. 5 The importance of plant height versus light reduction at the soil surface in determining the outcome of light competition is discussed.

Conservation of allelic richness in wild crop relatives is aided by assessment of genetic markers.

Abstract: Wild crop relatives are an important source of genetic variation for improving domesticated species. Given limited resources, methods for maximizing the genetic diversity of collections of wild relatives are needed to help spread protection over a larger number of populations and species. Simulations were conducted to investigate the optimal strategy of sampling materials from populations of wild relatives, with the objective of maximizing the number of alleles (allelic richness) in collections of fixed size. Two methods, based on assessing populations for variation at marker loci (e.g., allozymes, restriction fragment length polymorphisms), were developed and compared with several methods that are not dependent on markers. Marker-assisted methods yielded higher overall allelic richness in the simulated collections, and they were particularly effective in conserving geographically localized alleles, the class of alleles that is most subject to loss.

Fish Assemblage Recovery Along a Riverine Disturbance Gradient.

Abstract: Artificial fluctuations in streamflow have been documented to alter the composition and structure of stream communities. This study tests the hypothesis that a spatial recovery gradient in fish assemblage structure exists downstream of a hydroelectric dam, and that recovery can be identified by the presence and abundance of species largely restricted to flowing-water habitats (fluvial specialists). A longitudinal gradient of change in a shoreline fish assemblage was quantified in a 66-km reach of a mid-sized, species-rich river (Tallapoosa River, Alabama) with daily flow fluctuations from hydropower generation. The shoreline fish assemblage in a nearby and similar river (Cahaba River, Alabama) was quantified as a regional reference for the occurrence of fish assemblage gradients. Fish were collected with prepositioned area electrofishers in 240 randomly located sampling sites, and physical habitat was quantified. Using distributional and habitat use information, fish species were categorized as fluvial specialists or macrohabitat generalists (species that occur in a wide variety of aquatic systems). Sampled habitats were similar between rivers and along each study reach. The longitudinal pattern of species occurrence and fish abundance was consistent in the free-flowing river. A longitudinal gradient of increasing abundance and richness of only fluvial specialist species existed downstream of the hydroelectric dam. No similar spatial gradient existed for macrohabitat generalists in either river. Although a fish community recovery gradient was identified, a recovery endpoint was not evident because assemblage change was gradual and possibly incomplete. The preservation and management of riverine fish faunas will partly depend on incorporating spatial recovery into decisions about permitting and siting of anthropogenic changes like hydroelectric dams.

Rapoport's rule does not apply to marine teleosts and cannot explain latitudinal gradients in species richness

Abstract: Two methods are used to analyze latitudinal ranges of fish species: means of ranges of all species with a midpoint in the same 5⚬ latitude band plotted against latitude, and means of ranges of all species occurring in the same 5⚬ latitude band plotted against latitude. Application of the first method to data on 894 teleost species from the Indo-Pacific and Atlantic oceans showed that species with midpoints of latitudinal ranges near the equator have, on the average, much greater latitudinal ranges than species with midpoints at higher latitudes. Ranges of species were more or less the same when the second method was used, because of larger ranges and greater species numbers of low-latitude species. Rapoport's rule, according to which high-latitude species have greater latitudinal range than low-latitude species, therefore does not generally apply to marine teleosts. In contrast, the rule applies to North American (720 species) and North European (61 species) freshwater fish above a latitude of approximate...

Vesicular-Arbuscular Mycorrhizal Fungi: A Determinant of Plant Community Structure in Early Succession

Abstract: 1. Infection of plants by vesicular-arbuscular mycorrhizas (VAM) was reduced by application of the contact fungicide iprodione in two early successional plant communities. 2. One community was initiated in 1988 and monitored for four years, the other started in 1990 and monitored for 18 months. 3. In both communities reduction in fungal infection resulted in a lower plant species richness, with a number of species occurring in untreated (control) plots but absent in fungicide-treated plots. These were mostly perennial forbs. 4. It is suggested that VAM fungi are important in structuring early successional communities and that this is brought about by enhanced seedling establishment of forbs when fungi are present

The spatial extent and relative influence of landscape-level factors on wintering bird populations

Abstract: The influences of the landscape matrix (complex of habitats surrounding a study plot) and within-patch vegetation were studied in bird communities wintering in the piedmont of Georgia, USA. Variation at the landscape and within-patch levels was controlled to reduce the likelihood of confounding and spurious relationships. The landscape matrix within 500 m of each study plot was quantified from aerial photographs. Statistical models using landscape matrix and within-patch vegetation variables explained 73–84% of variation in bird abundance and diversity among sites with landscape matrix variables accounting for 30–90% of the variation. Variation in bird species richness and diversity was explained solely by landscape variables. Models for individual species such as Carolina Wrens (Thyrothorus ludovicianus) and Rufous-sided Towhees (Pipilo erythrophthalmus) had r2 > 0.80, with the landscape matrix variables accounting for the majority of this variation. However, other species like Northern Cardinals (Cardinalis cardinalis) and White-throated Sparrows (Zonotrichia albicollis) were most strongly influenced by within-plot vegetation. The landscape influence extended beyond habitats immediately adjacent to the study plots as indicated by significant variables describing variation in more distant habitat patches. These analyses illustrate a technique for comparing the strength of within-patch versus landscape influences and measuring the spatial extent of the landscape influence in fine-grained landscapes. Report No. 3955, Environmental Sciences Division, Oak Ridge National Laboratory.

Hurricanes and coral reefs: The intermediate disturbance hypothesis revisited

Abstract: A review of research on the effects of hurricanes on coral reefs suggests that the intermediate disturbance hypothesis may be applicable to shallow reef zones dominated by branching or foliaceous coral species that are especially susceptible to mechanical damage from storms. Diversity (H') increases because of an increase in evenness following destruction or removal of the species that was monopolizing the space. The intermediate disturbance hypothesis as presented by Connell focuses on changes in number of species, but should be expanded to include diversity (H') and evenness. It should also be modified to incorporate changes in living cover and the time elapsed since disturbances of varying intensities. This hypothesis predicts that when cover is high, diversity will be low. However, research on coral reefs does not consistently demonstrate an inverse correlation of coral diversity, and coral cover. An increase in cover and decrease in diversity with depth would also be expected because deeper reef zones generally experience less disturbance. However, higher diversity (both H' and species richness) is often associated with deeper zones. The effects of hurricanes on coral reefs will depend on the temporal and spatial scales under consideration, the life history characteristics and morphology of the dominant species, the depth of the reef zone, the ecological history of the site, and the influence of any additional natural or human stresses.