Showing papers on "Biodiversity published in 1999"

Post-glacial re-colonization of European biota

Abstract: Population structure is the result of both present processes and past history. Molecular markers are proving of great value in describing the former, and it is important to similarly determine the latter in order to understand their respective contributions. The study of palaeo-climates has also advanced significantly, and in particular that of the Pleistocene ice ages, which modified species ranges considerably. The last ice age and rapid post-glacial colonization of Europe is summarized. Possible population genetic consequences of expansion northward from southern refugia, and those of remaining in these mountainous regions are discussed. A series of recent case studies are detailed where DNA sequence information has been used to describe species genetic variation and subdivision across Europe. These include a grasshopper, the hedgehog, oak trees, the common beech, the black alder, the brown bear, newts, shrews, water vole, silver fir and house mice. These molecular data confirm southern peninsulas of Europe as major ice age refugia, and in most cases demonstrate that genetically distinct taxa emerged from them. They can thus define genomic differences and so greatly augment previous fossil data. The refugial genomes contributed differently in various species to the re-colonization of Europe, with three broad patterns described as paradigms—«grasshopper», «hedgehog» and «bear». These different expansion patterns produced clusters of hybrid zones where they made contact, and it is argued that many species genomes may be further cryptically subdivided. A reduction in diversity from southern to northern Europe in the extent of allelic variation and species subdivision is seen; this is attributed to rapid expansion northward and the varied topography of southern refugia allowing populations to diverge through several ice ages. The differences in DNA sequence indicate that some species have been diverging in refugial regions for a few ice ages at most, whilst distinct lineages in other species suggest much more ancient separation.

Biodiversity and ecosystem productivity in a fluctuating environment: The insurance hypothesis

Abstract: Although the effect of biodiversity on ecosystem functioning has become a major focus in ecology, its significance in a fluctuating environment is still poorly understood. According to the insurance hypothesis, biodiversity insures ecosystems against declines in their functioning because many species provide greater guarantees that some will maintain functioning even if others fail. Here we examine this hypothesis theoretically. We develop a general stochastic dynamic model to assess the effects of species richness on the expected temporal mean and variance of ecosystem processes such as productivity, based on individual species’ productivity responses to environmental fluctuations. Our model shows two major insurance effects of species richness on ecosystem productivity: (i) a buffering effect, i.e., a reduction in the temporal variance of productivity, and (ii) a performance-enhancing effect, i.e., an increase in the temporal mean of productivity. The strength of these insurance effects is determined by three factors: (i) the way ecosystem productivity is determined by individual species responses to environmental fluctuations, (ii) the degree of asynchronicity of these responses, and (iii) the detailed form of these responses. In particular, the greater the variance of the species responses, the lower the species richness at which the temporal mean of the ecosystem process saturates and the ecosystem becomes redundant. These results provide a strong theoretical foundation for the insurance hypothesis, which proves to be a fundamental principle for understanding the long-term effects of biodiversity on ecosystem processes.

Global patterns of plant invasions and the concept of invasibility

Abstract: With a simple model, I show that comparisons of invasibility between regions are impossible to make unless one can control for all of the variables besides invasibility that influence exotic richness, including the rates of immigration of species and the characteristics of the invading species themselves. Using data from the literature for 184 sites around the world, I found that nature reserves had one-half of the exotic fraction of sites outside reserves, and island sites had nearly three times the exotic fraction of mainland sites. However, the exotic fraction and the number of exotics were also dependent on site area, and this had to be taken into account to make valid comparisons between sites. The number of native species was used as a surrogate for site area and habitat diversity. Nearly 70% of the variation in the number of exotic species was accounted for by a multiple regression containing the following predictors: the number of native species, whether the site was an island or on the mainland, and whether or not it was a nature reserve. After controlling for scale, there were significant differences among biomes, but not continents, in their level of invasion. Multiple biome regions and temperate agricultural or urban sites were among the most invaded biomes, and deserts and savannas were among the least. However, there was considerable within-group variation in the mean degree of invasion. Scale-controlled analysis also showed that the New World is significantly more invaded than the Old World, but only when site native richness (probably a surrogate for habitat diversity) is factored out. Contrary to expectation, communities richer in native species had more, not fewer, exotics. For mainland sites, the degree of invasion increased with latitude, but there was no such relationship for islands. Although islands are more invaded than mainland sites, this is apparently not because of low native species richness, as the islands in this data set were no less rich in native species than were mainland sites of similar area. The number of exotic species in nature reserves increases with the number of visitors. However, it is difficult to draw conclusions about relative invasibility, invasion potential, or the roles of dispersal and disturbance from any of these results. Most of the observed patterns here and in the literature could potentially be explained by differences between regions in species properties, ecosystem properties, or propagule pressure.

The ecological consequences of changes in biodiversity: a search for general principles101

Abstract: This paper uses theory and experiments to explore the effects of diversity on stability, productivity, and susceptibility to invasion. A model of resource competition predicts that increases in diversity cause com- munity stability to increase, but population stability to decrease. These opposite effects are, to a great extent, explained by how temporal variances in species abundances scale with mean abundance, and by the differential impact of this scaling on population vs. community stability. Community stability also depends on a negative covariance effect (competitive compensation) and on overyielding (ecosystem productivity increasing with diversity). A long-term study in Minnesota grasslands supports these predictions. Models of competition predict, and field experiments confirm, that greater plant diversity leads to greater primary productivity. This diversity-productivity relationship results both from the greater chance that a more productive species would be present at higher diversity (the sampling effect) and from the better ''coverage'' of habitat heterogeneity caused by the broader range of species traits in a more diverse community (the niche differentiation effect). Both effects cause more complete utilization of limiting resources at higher diversity, which increases resource retention, further increasing productivity. Finally, lower levels of available limiting resources at higher diversity are predicted to decrease the susceptibility of an ecosystem to invasion, supporting the diversity-invasibility hypothesis. This mechanism provides rules for community assembly and invasion resistance. In total, biodiversity should be added to species composition, disturbance, nutrient supply, and climate as a major controller of population and ecosystem dynamics and structure. By their increasingly great directional impacts on all of these controllers, humans are likely to cause major long-term changes in the functioning of ecosystems worldwide. A better understanding of these ecosystem changes is needed if ecologists are to provide society with the knowledge essential for wise management of the earth and its biological resources.

Plant diversity and productivity experiments in european grasslands

Abstract: At eight European field sites, the impact of loss of plant diversity on primary productivity was simulated by synthesizing grassland communities with different numbers of plant species. Results differed in detail at each location, but there was an overall log-linear reduction of average aboveground biomass with loss of species. For a given number of species, communities with fewer functional groups were less productive. These diversity effects occurred along with differences associated with species composition and geographic location. Niche complementarity and positive species interactions appear to play a role in generating diversity-productivity relationships within sites in addition to sampling from the species pool.

The Relationship Between Productivity and Species Richness

Abstract: ▪ Abstract Recent overviews have suggested that the relationship between species richness and productivity (rate of conversion of resources to biomass per unit area per unit time) is unimodal (hump-shaped). Most agree that productivity affects species richness at large scales, but unanimity is less regarding underlying mechanisms. Recent studies have examined the possibility that variation in species richness within communities may influence productivity, leading to an exploration of the relative effect of alterations in species number per se as contrasted to the addition of productive species. Reviews of the literature concerning deserts, boreal forests, tropical forests, lakes, and wetlands lead to the conclusion that extant data are insufficient to conclusively resolve the relationship between diversity and productivity, or that patterns are variable with mechanisms equally varied and complex. A more comprehensive survey of the ecological literature uncovered approximately 200 relationships, of which 3...

Global environmental impacts of agricultural expansion: The need for sustainable and efficient practices

Abstract: The recent intensification of agriculture, and the prospects of future intensification, will have major detrimental impacts on the nonagricultural terrestrial and aquatic ecosystems of the world. The doubling of agricultural food production during the past 35 years was associated with a 6.87-fold increase in nitrogen fertilization, a 3.48-fold increase in phosphorus fertilization, a 1.68-fold increase in the amount of irrigated cropland, and a 1.1-fold increase in land in cultivation. Based on a simple linear extension of past trends, the anticipated next doubling of global food production would be associated with approximately 3-fold increases in nitrogen and phosphorus fertilization rates, a doubling of the irrigated land area, and an 18% increase in cropland. These projected changes would have dramatic impacts on the diversity, composition, and functioning of the remaining natural ecosystems of the world, and on their ability to provide society with a variety of essential ecosystem services. The largest impacts would be on freshwater and marine ecosystems, which would be greatly eutrophied by high rates of nitrogen and phosphorus release from agricultural fields. Aquatic nutrient eutrophication can lead to loss of biodiversity, outbreaks of nuisance species, shifts in the structure of food chains, and impairment of fisheries. Because of aerial redistribution of various forms of nitrogen, agricultural intensification also would eutrophy many natural terrestrial ecosystems and contribute to atmospheric accumulation of greenhouse gases. These detrimental environmental impacts of agriculture can be minimized only if there is much more efficient use and recycling of nitrogen and phosphorus in agroecosystems.

Biodiversity of floodplain river ecosystems: ecotones and connectivity1

Abstract: A high level of spatio-temporal heterogeneity makes riverine floodplains among the most species-rich environments known. Fluvial dynamics from flooding play a major role in maintaining a diversity of lentic, lotic and semi-aquatic habitat types, each represented by a diversity of successional stages. Ecotones (transition zones between adjacent patches) and connectivity (the strength of interactions across ecotones) are structural and functional elements that result from and contribute to the spatio-temporal dynamics of riverine ecosystems. In floodplain rivers, ecotones and their adjoining patches are arrayed in hierarchical series across a range of scales. At a coarse scale of resolution, fringing floodplains are themselves complex ecotones between river channels and uplands. At finer scales, patches of various types and sizes form habitat and microhabitat diversity patterns. A broad spatio-temporal perspective, including patterns and processes across scales, is needed in order to gain insight into riverine biodiversity. We propose a hierarchical framework for examining diversity patterns in floodplain rivers. Various river management schemes disrupt the interactions that structure ecotones and alter the connectivity across transition zones. Such disruptions occur both within and between hierarchical levels, invariably leading to reductions in biodiversity. Species richness data from the connected and disconnected floodplains of the Austrian Danube illustrate this clearly. In much of the world, species-rich riverine/floodplain environments exist only as isolated fragments across the landscape. In many large rivers, these islands of biodiversity are endangered ecosystems. The fluvial dynamics that formed them have been severely altered. Without ecologically sound restoration of disturbance regimes and connectivity, these remnants of biodiversity will proceed on unidirectional trajectories toward senescence, without rejuvenation. Principles of ecosystem management are necessary to sustain biodiversity in fragmented riverine floodplains. Copyright © 1999 John Wiley & Sons, Ltd.

Biodiversity of plankton by species oscillations and chaos

Abstract: Biodiversity has both fascinated and puzzled biologists1. In aquatic ecosystems, the biodiversity puzzle is particularly troublesome, and known as the ‘paradox of the plankton’2. Competition theory predicts that, at equilibrium, the number of coexisting species cannot exceed the number of limiting resources3,4,5,6. For phytoplankton, only a few resources are potentially limiting: nitrogen, phosphorus, silicon, iron, light, inorganic carbon, and sometimes a few trace metals or vitamins. However, in natural waters dozens of phytoplankton species coexist2. Here we offer a solution to the plankton paradox. First, we show that resource competition models6,7,8,9,10 can generate oscillations and chaos when species compete for three or more resources. Second, we show that these oscillations and chaotic fluctuations in species abundances allow the coexistence of many species on a handful of resources. This model of planktonic biodiversity may be broadly applicable to the biodiversity of many ecosystems.

Effects of plant species richness on invasion dynamics, disease outbreaks, insect abundances and diversity

Abstract: Declining biodiversity represents one of the most dramatic and irreversible aspects of anthropogenic global change, yet the ecological implications of this change are poorly understood. Recent studies have shown that biodiversity loss of basal species, such as autotrophs or plants, affects fundamental ecosystem processes such as nutrient dynamics and autotrophic production. Ecological theory predicts that changes induced by the loss of biodiversity at the base of an ecosystem should impact the entire system. Here we show that experimental reductions in grassland plant richness increase ecosystem vulnerability to invasions by plant species, enhance the spread of plant fungal diseases, and alter the richness and structure of insect communities. These results suggest that the loss of basal species may have profound effects on the integrity and functioning of ecosystems.

Biodiversity Hotspots in the Mediterranean Basin: Setting Global Conservation Priorities

Abstract: Due to the current species extinction crisis, there is an urgent need to identify the most threatened areas of exceptionally high biodiversity and rates of endemism (i.e., "hotspots"; Mittermeier et al. 1998; Myers 1988; Reid 1998). Conservation strategies represent a crucial issue in the mediterranean biome because this area, which represents only 2% of the world's surface, houses 20% of the world's total floristic richness (Medail & Quezel 1997). Myers initially (1988, 1990) defined 14 hotspots in the tropical biome and four in mediterranean bioclimates (southwestern Australia, Cape Region of South Africa, California, and part of Chile). Like the four other mediterranean areas, the Mediterranean Basin is one of the world's major centers for plant diversity, where 10% of the world's higher plants can be found in an area representing only 1.6% of the Earth's surface (Medail & Quezel 1997). The prominent role played by these areas as reservoirs for plant biodiversity has been emphasized by Myers (1990). He hesitated, however, to group the whole Mediterranean Basin into one single hotspot because it covers such a large surface area, and insufficient data were available for certain regions. In this context, Medail and Quezel (1997) performed a global survey of plant richness and endemism to more precisely define hotspots in the Mediterranean Basin; they identified 10 hotspots. Three main approaches, however, have been taken in recent studies performed by international conservation organizations to define priority conservation areas in the Mediterranean Basin.

Environmental warming alters food-web structure and ecosystem function

Abstract: We know little about how ecosystems of different complexity will respond to global warming1,2,3,4,5. Microcosms permit experimental control over species composition and rates of environmental change. Here we show using microcosm experiments that extinction risk in warming environments depends on trophic position but remains unaffected by biodiversity. Warmed communities disproportionately lose top predators and herbivores, and become increasingly dominated by autotrophs and bacterivores. Changes in the relative distribution of organisms among trophically defined functional groups lead to differences in ecosystem function beyond those expected from temperature-dependent physiological rates. Diverse communities retain more species than depauperate ones, as predicted by the insurance hypothesis, which suggests that high biodiversity buffers against the effects of environmental variation because tolerant species are more likely to be found6,7. Studies of single trophic levels clearly show that warming can affect the distribution and abundance of species2,4,5, but complex responses generated in entire food webs greatly complicate inferences based on single functional groups.

Species diversity and invasion resistance in a marine ecosystem

Abstract: Theory predicts that systems that are more diverse should be more resistant to exotic species, but experimental tests are needed to verify this. In experimental communities of sessile marine invertebrates, increased species richness significantly decreased invasion success, apparently because species-rich communities more completely and efficiently used available space, the limiting resource in this system. Declining biodiversity thus facilitates invasion in this system, potentially accelerating the loss of biodiversity and the homogenization of the world's biota.

Effects of habitat isolation on pollinator communities and seed set

Abstract: Destruction and fragmentation of natural habitats is the major reason for the decreasing biodiversity in the agricultural landscape. Loss of populations may negatively affect biotic interactions and ecosystem stability. Here we tested the hypothesis that habitat fragmentation affects bee populations and thereby disrupts plant-pollinator interactions. We experimentally established small ”habitat islands” of two self-incompatible, annual crucifers on eight calcareous grasslands and in the intensively managed agricultural landscape at increasing distances (up to 1000 m) from these species-rich grasslands to measure effects of isolation on both pollinator guilds and seed set, independently from patch size and density, resource availability and genetic erosion of plant populations. Each habitat island consisted of four pots each with one plant of mustard (Sinapis arvensis) and radish (Raphanus sativus). Increasing isolation of the small habitat islands resulted in both decreased abundance and species richness of flower-visiting bees (Hymenoptera: Apoidea). Mean body size of flower-visiting wild bees was larger on isolated than on nonisolated habitat islands emphasizing the positive correlation of body size and foraging distance. Abundance of flower-visiting honeybees depended on the distance from the nearest apiary. Abundance of other flower visitors such as hover flies did not change with increasing isolation. Number of seeds per fruit and per plant decreased significantly with increasing distance from the nearest grassland for both mustard and radish. Mean seed set per plant was halved at a distance of approximately 1000 m for mustard and at 250 m for radish. In accordance with expectations, seed set per plant was positively correlated with the number of flower-visiting bees. We found no evidence for resource limitation in the case of mustard and only marginal effects for radish. We conclude that habitat connectivity is essential to maintain not only abundant and diverse bee communities, but also plant-pollinator interactions in economically important crops and endangered wild plants.

Fungal endophyte symbiosis and plant diversity in successional fields.

Abstract: Increasing evidence suggests that microbial interactions are important determinants of plant biodiversity. The hypothesis that fungal endophyte symbiosis reduces diversity in successional fields was tested by manipulating infection of tall fescue, the most abundant perennial grass in the eastern United States. Over a 4-year period, species richness declined and tall fescue dominance increased in infected plots relative to uninfected plots without differences in total productivity. A host-specific endophyte, with negligible biomass, altered plant community structure in this long-term field experiment and may be reducing plant diversity throughout its expanding range.

Terrestrial ecoregions of North America : a conservation assessment

Abstract: A comprehensive assessment of biodiversity in the US and Canada The authors, scientists with the World Wildlife Fund, use a rigorous ecoregion approach, rather than the less relevant state-by-state approach Six chapters present the rationale for the approach, defining the ecoregions and analyzing the threats to each From this the authors develop a conservation agenda and set out recommendations for preserving and restoring biodiversity to the continent Thirteen colour maps provide essential information about each ecoregion and its biodiversity

Birds and butterflies along an urban gradient: surrogate taxa for assessing biodiversity?

Abstract: This study examines whether birds and butterflies may be used as surrogates for one another in assessing biodiversity at the community level. To do this, I compared the distribution and abundance of bird and butterfly species across an urban gradient by surveying six sites near Palo Alto, California, USA (all former oak woodlands) to see if these taxa have responded similarly to urbanization. The sites represent a gradient of urban land use ranging from relatively undisturbed to highly developed and include nature preserves, recreational areas, golf courses, residential neighborhoods, office parks, and business districts. At the community level, the two taxa displayed similar patterns across the gradient: species richness and Shannon diversity peak at intermediate levels of development, and the oak-woodland species gradually drop out at more developed sites. These measures are highly correlated between the two groups. The two taxa differed in their patterns of total abundance, however. Butterfly abundance...

Spatial scaling laws yield a synthetic theory of biodiversity

Abstract: Ecologists still search for common principles that predict well-known responses of biological diversity to different factors. Such factors include the number of available niches in space, productivity, area, species' body size and habitat fragmentation. Here we show that all these patterns can arise from simple constraints on how organisms acquire resources in space. We use spatial scaling laws to describe how species of different sizes find food in patches of varying size and resource concentration. We then derive a mathematical rule for the minimum similarity in size of species that share these resources. This packing rule yields a theory of species diversity that predicts relations between diversity and productivity more effectively than previous models. Size and diversity patterns for locally coexisting East African grazing mammals and North American savanna plants strongly support these predictions. The theory also predicts relations between diversity and area and between diversity and habitat fragmentation. Thus, spatial scaling laws provide potentially unifying first principles that may explain many important patterns of species diversity.

On the causes of gradients in tropical tree diversity

Abstract: 1 The number of woody species in tropical forests tends to increase with precipitation, forest stature, soil fertility, rate of canopy turnover and time since catastrophic disturbance, and decrease with seasonality, latitude, altitude, and diameter at breast height (d.b.h.). 2 A model is presented to account for these trends. Novel hypotheses include how increased rainfall and substrate fertility, and decreased seasonality, might (i) increase attacks by natural enemies, and thus the overall level of density-dependent plant mortality; (ii) increase shade tolerance, canopy turnover, and stem density of the species-rich understorey; and (iii) increase reliance on relatively sedentary forest-interior birds for seed dispersal, fostering high rates of speciation in understorey genera. 3 High rainfall and low seasonality in the tropics favour two key groups of natural plant enemies – insects and fungi – that are directly responsible for promoting high rates of density-dependent plant mortality. Lower rainfall, greater seasonality, soil infertility, or unfavourable rooting conditions favour greater allocation to anti-herbivore defences, and thus lead to lower rates of such mortality and thence to lower tree diversity. The increased number of individuals on rainier sites is a minor contributor to increased tree diversity, accounting for only about 17% of the 8.3-fold increase with rainfall in the lowland Neotropics. 4 Predictions of the model are consistent with many ecological patterns of variation in tropical tree diversity within regions, and may help explain the decrease in tree diversity with elevation and the accompanying decrease in horizontal patchiness (within-habitat β diversity). 5 Random drift over evolutionary time in the relative effectiveness of density-dependent control of individual tree species by specialized natural enemies may better account for the observed distribution of tropical tree abundance than a random walk of species abundance through ecological time.

Genes in the Field : On-Farm Conservation of Crop Diversity

Abstract: Introduction and Overview * The issues of in situ conservation of crop genetic resources * Population Biology and Social Science: The Genetic Structure of Crop Landraces and the Challenge to Conserve Them in situ on Farms - Case Studies * Barley Landraces From the Fertile Crescent: a lesson for Plant Breeders * The Barleys of Ethiopia. Traditional Management of Seed and Genetic Diversity: What is a Landrace?- Keeping Diversity Alive: an Ethiopian Perspective * Policy and Institutional Issues: Optimal Genetic Resource Conservation: in situ and ex situ * The Cultures of the Seed in the Peruvian Andes * On-Farm Conservation of Crop Diversity: Policy and Institutional Lessons From Southern Africa * In Situ Conservation and Intellectual Property Rights * Farmer Decision-Making and Genetic Diversity: Linking Multidiciplinary Research to Implementation On-Farm.

Handbook of Functional Plant Ecology

Abstract: Methods in comparative functional ecology. Part 1 The role of structure and growth form in plant performance: opportunistic growth and desiccation tolerance - the ecological success of poikilohydrous autotrophs ecological significance of inherent variation in relative growth rate architecture, ecology and evolution of plant crowns the structure and function of root systems. Part 2 Physiological ecology: water relations and hydraulic architecture exploitation of changing environments influence of light on leaf longevity and photosynthetic physiology - ecological observations and molecular possibilities acquisition, use and loss of nutrients. Part 3 Habitats and plant distribution: functional attributes in Mediterranean-type ecosystems: plant survival in arid environments tropical forests - diversity and function of dominant life-forms plant diversity in tropical forests arctic ecology plant life in Antarctica. Part 4 Populations and communities: ecology of reproduction -mating systems and pollination seed and seedling ecology facilitation in plant communities plant interactions -competition plant-herbivore interactions - beyond a binary vision biological diversity and functioning of ecosystems. Part 5 New approaches: resistance to air pollutions - from cell to community flux control at the ecosystem level canopy photosynthesis modelling ecological applications of remote sensing at multiple scales generalizations in functional plant ecology - the species sampling problem, plant ecology strategy schemes and phylogeny epilogue.