Showing papers in "Ecology Letters in 2001"

Quantifying biodiversity: procedures and pitfalls in the measurement and comparison of species richness

Abstract: Species richness is a fundamental measurement of community and regional diversity, and it underlies many ecological models and conservation strategies. In spite of its importance, ecologists have not always appreciated the effects of abundance and sampling effort on richness measures and comparisons. We survey a series of common pitfalls in quantifying and comparing taxon richness. These pitfalls can be largely avoided by using accumulation and rarefaction curves, which may be based on either individuals or samples. These taxon sampling curves contain the basic information for valid richness comparisons, including category‐subcategory ratios (species-to-genus and species-toindividual ratios). Rarefaction methods ‐ both sample-based and individual-based ‐ allow for meaningful standardization and comparison of datasets. Standardizing data sets by area or sampling effort may produce very different results compared to standardizing by number of individuals collected, and it is not always clear which measure of diversity is more appropriate. Asymptotic richness estimators provide lower-bound estimates for taxon-rich groups such as tropical arthropods, in which observed richness rarely reaches an asymptote, despite intensive sampling. Recent examples of diversity studies of tropical trees, stream invertebrates, and herbaceous plants emphasize the importance of carefully quantifying species richness using taxon sampling curves.

Coral bleaching: the winners and the losers

Abstract: Sea surface temperatures were warmer throughout 1998 at Sesoko Island, Japan, than in the 10 preceding years. Temperatures peaked at 2.8 °C above average, resulting in extensive coral bleaching and subsequent coral mortality. Using random quadrat surveys, we quantitatively documented the coral community structure one year before and one year after the bleaching event. The 1998 bleaching event reduced coral species richness by 61% and reduced coral cover by 85%. Colony morphology affected bleaching vulnerability and subsequent coral mortality. Finely branched corals were most susceptible, while massive and encrusting colonies survived. Most heavily impacted were the branched Acropora and pocilloporid corals, some of which showed local extinction. We suggest two hypotheses whose synergistic effect may partially explain observed mortality patterns (i.e. preferential survival of thick-tissued species, and shape-dependent differences in colony mass-transfer efficiency). A community-structural shift occurred on Okinawan reefs, resulting in an increase in the relative abundance of massive and encrusting coral species.

Phenology is a major determinant of tree species range

Abstract: Global warming is expected to have a major impact on plant distributions, an issue of key importance in biological conservation. However, very few models are able to predict species distribution accurately, although we know species respond individually to climate change. Here we show, using a process-based model (PHENOFIT), that tree species distributions can be predicted precisely if the biological processes of survival and reproductive success only are incorporated as a function of phenology. These predictions showed great predictive power when tested against present distributions of two North American species – quaking aspen and sugar maple – indicating that on a broad scale, the fundamental niche of trees coincides with their realized niche. Phenology is shown here to be a major determinant of plant species range and should therefore be used to assess the consequences of global warming on plant distributions, and the spread of alien plant species.

The exploitation of mutualisms

Abstract: Mutualisms (interspecific cooperative interactions) are ubiquitously exploited by organisms that obtain the benefits mutualists offer, while delivering no benefits in return. The natural history of these exploiters is well-described, but relatively little effort has yet been devoted to analysing their ecological or evolutionary significance for mutualism. Exploitation is not a unitary phenomenon, but a set of loosely related phenomena: exploiters may follow mixed strategies or pure strategies at either the species or individual level, may or may not be derived from mutualists, and may or may not inflict significant costs on mutualisms. The evolutionary implications of these different forms of exploitation, especially the threats they pose to the stability of mutualism, have as yet been minimally explored. Studies of this issue are usually framed in terms of a “temptation to defect” that generates a destabilizing conflict of interest between partners. I argue that this idea is in fact rather inappropriate for interpreting most observed forms of exploitation in mutualisms. I suggest several alternative and testable ideas for how mutualism can persist in the face of exploitation.

The carbon–nutrient balance hypothesis: its rise and fall

Abstract: The idea that the concentration of secondary metabolites in plant tissues is controlled by the availability of carbon and nitrogen in the environment has been termed the carbon–nutrient balance hypothesis (CNB). This hypothesis has been invoked both for prediction and for post hoc explanation of the results of hundreds of studies. Although it successfully predicts outcomes in some cases, it fails to such an extent that it cannot any longer be considered useful as a predictive tool. As information from studies has accumulated, many attempts have been made to save CNB, but these have been largely unsuccessful and have managed only to limit its utility. The failure of CNB is rooted in assumptions that are now known to be incorrect and it is time to abandon CNB because continued use of the hypothesis is now hindering understanding of plant–consumer interactions. In its place we propose development of theory with a firm evolutionary basis that is mechanistically sophisticated in terms of plant and herbivore physiology and genetics.

Local adaptation enhances performance of common plant species

Abstract: Geographic variation can lead to the evolution of different local varieties, even in widespread forage plants. We investigated the performance of common forage plants in relation to their genetic diversity and local adaptation at a continental scale using reciprocal transplants at eight field sites across Europe over a 2-year period. The overall performance of the three test species, Trifolium pratense, Dactylis glomerata, Plantago lanceolata, was generally highest for plants replanted at their home site and declined with increasing transplanting distance. The three species differed in the fitness components responsible for the increased overall performance and selection advantage at home sites. In addition to the effects of local adaptation, the majority of measured traits in all three species also showed ecotypic variation. However, no single ecotype of any species was able to outperform the locally adapted strains and do best at all sites, highlighting the importance of maintaining these plant genetic resources.

Protandrous arrival timing to breeding areas: a review

Abstract: Protandry, the earlier arrival of males to breeding areas than females, is a common pattern of sex-biased timing in many animal taxa (e.g. some insects, fish, amphibians, reptiles, birds and mammals). The adaptive significance of protandry is not fully understood and, since the 1970s, at least seven hypotheses for protandry have been proposed. We describe each of these hypotheses and summarize what is known about each. In three of these hypotheses, the relative arrival timing of males and females has no direct fitness consequences for males or females, but selection for different timing in each sex indirectly produces protandry. In the other four hypotheses, the difference between male and female timing has fitness consequences for males or females and selection directly maintains the fitness-maximizing degree of sex-biased timing. The hypotheses are not mutually exclusive, and the degree of multiple mating by males and the occurrence of male territoriality seem to determine the relative importance of each hypothesis. In order to understand the adaptive significance of sex-biased timing, future studies need to consider all the alternatives and to assess the costs and benefits to males of early arrival relative to calendar date, to other males and to females.

Biodiversity may regulate the temporal variability of ecological systems

Abstract: The effect of biodiversity on natural communities has recently emerged as a topic of considerable ecological interest. We review studies that explicitly test whether the number of species in a community (species richness) regulates the temporal variability of aggregate community (total biomass, productivity, nutrient cycling) and population (density, biomass) properties. Theoretical studies predict that community variability should decline with increasing species richness, while population variability should increase. Many, but not all, empirical studies support these expectations. However, a closer look reveals that several empirical studies have either imperfect experimental designs or biased methods of calculating variability. Furthermore, most theoretical studies rely on highly unrealistic assumptions. We conclude that evidence to support the claim that biodiversity regulates temporal variability is accumulating, but not unequivocal. More research, in a broader array of ecosystem types and with careful attention to methodological considerations, is needed before we can make definitive statements regarding richness-variability relationships.

Melanism and disease resistance in insects

Abstract: There is growing evidence that insects in high-density populations invest relatively more in pathogen resistance than those in low-density populations (i.e. density-dependent prophylaxis). Such increases in resistance are often accompanied by cuticular melanism, which is characteristic of the high-density form of many phase polyphenic insects. Both melanism and pathogen resistance involve the prophenoloxidase enzyme system. In this paper the link between resistance, melanism and phenoloxidase activity is examined in Spodoptera larvae. In S. exempta, cuticular melanism was positively correlated with phenoloxidase activity in the cuticle, haemolymph and midgut. Melanic S. exempta larvae were found to melanize a greater proportion of eggs of the ectoparasitoid Euplectrus laphygmae than non-melanic larvae, and melanic S. littoralis were more resistant to the entomopathogenic fungus Beauveria bassiana (in S. exempta the association between melanism and fungal resistance was non-signficant). These results strengthen the link between melanism and disease resistance and implicate the involvement of phenoloxidase.

Experimental support for a resource-based mechanistic model of invasibility

Abstract: Recent theory has suggested a mechanistic relationship between resource availability, competition and invasibility. In a field experiment, in which we manipulated resources and competition, we confirmed that changes in resource availability affected competition intensity, which in turn affected invasibility. We found that fluctuations in resource availability of as short as a few weeks had a large impact on plant invasion success (survival and percentage cover), including up to 1 year following the fluctuations. If resource availability is a primary mechanism controlling invasibility, it may serve as a unifying concept that can integrate earlier ideas regarding invasibility. The results emphasize the important role of history in the invasion process, particularly the occurrence of stochastic, short-lived events that temporarily reduce or suspend competition and increase invasibility. Therefore, it may be very difficult, or even impossible, to reconstruct the ecology of particular invasions after the fact.

Rare Species Loss Alters Ecosystem Function – Invasion Resistance

Abstract: The imminent decline in species diversity coupled with increasing exotic species introductions has provoked investigation into the role of resident diversity in community resistance to exotic species colonization. Here we present the results of a field study using an experimental method in which diversity was altered by removal of less abundant species and the resulting disturbance was controlled for by removal of an equivalent amount of biomass of the most common species from paired plots. Following these manipulations, the exotic grass, Lolium temulentum, was introduced. We found that exotic species establishment was higher in plots in which diversity was successfully reduced by removal treatments and was inversely related to imposed species richness. These results demonstrate that less common species can significantly influence invasion events and highlight the potential role of less common species in the maintenance of ecosystem function.

Genetic differences in growth of an invasive tree species

Abstract: Invasive plants are often more vigorous in their introduced ranges than in their native ranges. This may reflect an innate superiority of plants from some habitats or an escape from their enemies. Another hypothesis proposes that invasive plants evolve increased competitive ability in their introduced range. We present the results of a 14-year common garden experiment with the Chinese Tallow Tree (Sapium sebiferum) from its native range (Asia), place of introduction to North America (Georgia) and areas colonized a century later (Louisiana and Texas). Invasive genotypes, especially those from recently colonized areas, were larger than native genotypes and more likely to produce seeds but had lower quality, poorly defended leaves. Our results demonstrate significant post-invasion genetic differences in an invasive plant species. Post

Contrasting effects of solar UV radiation on dissolved organic sources for bacterial growth

Abstract: The photochemical transformation of dissolved organic matter (DOM) in lakes and oceans has been shown to either reduce or enhance bacterial utilization. We compared the effects of UV radiation on the bacterial use of DOM in a wide range of lakes. Although complex DOM was converted in all irradiated samples into carboxylic acids that are readily utilized by bacteria, irradiation in several lakes resulted in a decreased ability of DOM to support bacterial growth. The effect of irradiation on the ability of DOM to promote bacterial growth was a positive function of the terrestrial humic matter, and a negative function of indigenous algal production. We suggest that the net effect of irradiation is a result of counteracting but concurrent processes rendering DOM either labile or recalcitrant. Humic DOM is predominantly transformed into forms of increased lability, whereas photochemical transformation into compounds of decreased bacterial substrate quality dominates in algal-derived DOM. Hence, solar-induced photochemical reactions interact with microbial degraders in different ways, depending on the origin and nature of the organic matter, affecting the transfer of energy within aquatic food webs, as well as the degradation and preservation of detrital organic matter, in different directions.

Predicting the direction of sexual selection

Abstract: Our current understanding of the operation of sexual selection is predicated on a sex difference in parental investment, which favours one sex becoming limiting and choosy over mates, the other competitive and nonchoosy. This difference is reflected in the operational sex ratio (OSR), the ratio of sexually receptive males to females, considered to be of fundamental importance in predicting the direction of sexual selection. Difficulties in measuring OSR directly have led to the use of the potential reproductive rates (PRR) as a measure of the level of investment in offspring of males and females. Several recent studies have emphasized that other factors, such as variation in mate quality and sex differences in mortality patterns, also influence the direction of sexual selection. However, as yet there has been no attempt to form a comprehensive theory of sex roles. Here we show that neither OSR nor PRR is the most fundamentally important determinant of sex roles, and that they are not interchangeable. Instead, the cost of a single breeding attempt has a strong direct effect on competition and choosiness as well as consistent relationships to both OSR and PRR. Our life history based approach to mate choice also yields simple, testable predictions on lack of choice in either sex and on mutual mate choice.

Stochastic matrix models for conservation and management: A comparative review of methods

Abstract: Stochastic matrix models are frequently used by conservation biologists to measure the viability of species and to explore various management actions. Models are typically parameterized using two or more sets of estimated transition rates between age/size/stage classes. While standard methods exist for analyzing a single set of transition rates, a variety of methods have been employed to analyze multiple sets of transition rates. We review applications of stochastic matrix models to problems in conservation and use simulation studies to compare the performance of different analytic methods currently in use. We find that model conclusions are likely to be robust to the choice of parametric distribution used to model vital rate fluctuations over time. However, conclusions can be highly sensitive to the within-year correlation structure among vital rates, and therefore we suggest using analytical methods that provide a means of conducting a sensitivity analysis with respect to correlation parameters. Our simulation results also suggest that the precision of population viability estimates can be improved by using matrix models that incorporate environmental covariates in conjunction with experiments to estimate transition rates under a range of environmental conditions.

The influence of sociality on the conservation biology of social insects

Abstract: Social insects (ants, bees, wasps and termites) as a group are species rich and ecologically dominant. Many are outstanding “ecological engineers”, or providers of “ecosystem services”, or potential bioindicator species. Few social insects are currently formally classified as Threatened, but this is almost certainly due to a lack of information on population sizes and trends in scarce species. The main influence that sociality has on threats faced by social insects is in reducing effective population sizes, increasing population genetic subdivision and possibly reducing levels of genetic variation relative to solitary species. The main influence that sociality has on threats from social insects is via its role in the ecological success of invasive species, which frequently pose a major hazard to native biotas. In some cases, social features underpinning ecological success in the original range almost certainly contribute to the success of invasive social insects. However, recent studies show or strongly suggest that, in some of the most notoriously invasive populations of ants, bees and wasps, novel social traits have arisen that greatly enhance the rate of spread and ecological competitiveness of these populations. Sociality can therefore represent either a liability or an asset in its contribution to the persistence of social insect populations.

Plants protect their roots by alerting the enemies of grubs

Abstract: Plant roots in the soil are under attack from many soil organisms. Although many ecologists are aware of the presence and importance of natural enemies in the soil that protect the plants from herbivores, the existence and nature of tritrophic interactions are poorly understood. So far, attention has focused on how plants protect their aboveground parts against herbivorous arthropods, either directly or indirectly (i.e. by getting help from the herbivore’s enemies). This article is the first in showing that indirect plant defences also operate underground. We show that the roots of a coniferous plant (Thuja occidentalis) release chemicals upon attack by weevil larvae (Otiorhynchus sulcatus) and that these chemicals thereby attract parasitic nematodes (Heterorhabditis megidis).

Transient dynamics and persistence of ecological systems

Abstract: Using spatially coupled predator–prey systems as an example of a cyclic ecological system where coexistence depends on oscillations, transient dynamics of models where there are no stable persistent solutions are shown to be a reasonable explanation of persistence over ecological time scales. The parameter values leading to transients within the context of a particular model may be far from parameter values that lead to stable solutions, so transients will need to be explicitly considered in model analysis. Since natural systems with many coupled oscillating species are common, and natural communities are often reset by disturbances or seasonality, transients should play a central role in understanding natural systems.

Impacts of landscape structure on butterfly range expansion

Abstract: Since the 1940s, the distributions of several butterfly species have been expanding in northern Europe, probably in response to climate warming. We focus on the speckled wood butterfly Pararge aegeria in order to determine impacts of habitat availability on expansion rates. We analyse observed expansion rates since 1940 and also use a spatially explicit mechanistic model (MIGRATE) to simulate range expansion in two areas of the UK which differ in their distribution of breeding habitat (woodland). Observed and simulated expansion rates were in very close agreement but were 42%–45% slower in an area that had 24% less woodland. Unlike P. aegeria, the majority of butterfly species are not currently expanding, almost certainly because of lack of suitable habitat. Incorporating the spatial distribution of habitat into investigations of range changes is likely to be important in determining those species that can and cannot expand, and for predicting potential future range changes.

Modelling the dynamic spatio-temporal response of predators to transient prey patches in the field

Abstract: The spatio-temporal dynamics of two aphid species (Metopolophium dirhodum and Sitobion arenae) and a generalist predator (Pterostichus melanarius) were observed in a field-scale study using a grid of 256 sampling locations with a 12-m spacing. Using Spatial Analysis by Distance Indices we demonstrate that populations show ephemeral spatial pattern at the field scale. We observed a positive, lagged beetle response to this aphid pattern; Conversely, the aphids displayed a negative, lagged response to beetle spatial pattern. Examination of the local structure of the spatio-temporal dynamics revealed a strong response by the beetle population to aphid patches. The temporal structure of spatial associations between the species shows a strong correspondence with those from a conceptual model of predator-prey spatial interaction. The spatially coupled dynamics were sufficiently strong for the predator to have a negative effect on the intrinsic rate of increase of their prey.

Modelling the adaptive dynamics of traits involved in inter‐ and intraspecific interactions: An assessment of three methods

Abstract: In recent years, three related methods have been used to model the phenotypic dynamics of traits under the influence of natural selection. The first is based on an approximation to quantitative genetic recursion equations for sexual populations. The second is based on evolution in asexual lineages with mutation-generated variation. The third method finds an evolutionarily stable set of phenotypes for species characterized by a given set of fitness functions, assuming that the mode of reproduction places no constraints on the number of distinct types that can be maintained in the population. The three methods share the property that the rate of change of a trait within a homogeneous population is approximately proportional to the individual fitness gradient. The methods differ in assumptions about the potential magnitude of phenotypic differences in mutant forms, and in their assumptions about the probability that invasion or speciation occurs when a species has a stable, yet invadable phenotype. Determining the range of applicability of the different methods is important for assessing the validity of optimization methods in predicting the evolutionary outcome of ecological interactions. Methods based on quantitative genetic models predict that fitness minimizing traits will often be evolutionarily stable over significant time periods, while other approaches suggest this is likely to be rare. A more detailed study of cases of disruptive selection might reveal whether fitness-minimizing traits occur frequently in natural communities.

Constraints on colonization and species richness along a grassland productivity gradient: the role of propagule availability

Abstract: An important goal in ecology is to discern under what habitat conditions community structure is primarily regulated by local ecological interactions and under what conditions community structure is more regulated by the pool of available colonists. I conducted a seed addition experiment in successional grassland to evaluate the relative significance of neighbourhood biotic interactions and propagule availability in regulating plant colonization and species richness along a natural gradient of grassland productivity. In undisturbed field plots, seed additions of 34 species led to an increase in species richness in locations of low productivity, an effect that declined in magnitude as productivity increased. In disturbed plots, seed additions led to a relatively constant increase in species richness at all levels of productivity. The results support the hypothesis that the role of propagule availability in regulating colonization dynamics and species richness declines in significance relative to local-scale competitive interactions as habitat productivity increases.

Amino acid uptake: a widespread ability among boreal forest plants

Abstract: Amino acids constitute a potentially important source of nitrogen for plants in boreal forest ecosystems. Accordingly, it may be suggested that distinct plant species differing abilities to take up amino acids constitutes an important factor in determining plant ecosystem composition. Using GC-MS and isotopically labelled amino acids, we measured the simultaneous uptake of 15 different amino acids by 31 common boreal forest plant species. The results from this study show that all plant species tested, representing a wide variety of plant types, have the ability to take up amino acids from an incubation solution. Furthermore, uptake rates were unrelated to mycorrhizal associations as well as habitat soil amino acid concentrations and plant nitrogen availability dependence as measured by Ellenberg nitrogen indicator values. These results suggest that mycorrhiza is of minor importance for discrete plant amino acid uptake rates and further points out the potential importance of amino acids to plant nitrogen nutrition in boreal forest ecosystems.

Effects of perceived danger on flower choice by bees

Abstract: Studies on animal‐flower interactions have mostly neglected the third trophic level of pollinators’ predators, even though antipredatory behaviour of pollinators may affect patterns of pollinator visitation, pollen transfer and floral traits. In three experiments, it was found that honeybees showed sensitivity to perceived danger at flowers by preferring apparently safe flowers over equally rewarding alternatives harbouring either a dead bee or a spider, and avoiding revisitation of a site where the bees had escaped a simulated predation attempt. These results suggest that bees, like other animals, take antipredatory measures, which may have far reaching effects on animal‐flower interactions.

How large do reserve networks need to be

Abstract: Reserve networks are essential for the long-term persistence of biodiversity. To fulfil this goal, they need not only to represent all species to be conserved but also to be sufficiently large to ensure species’ persistence over time. An extensive literature exists on the required size of individual reserves, but to date there has been little investigation regarding the appropriate size of entire networks. The IUCN’s proposal that 10% of each nation be reserved is often presented as a desirable target, but concerns have been raised that this is insufficient and is dictated primarily by considerations of feasibility and politics. We found that the minimum percentage of area needed to represent all species within a region increases with the number of targeted species, the size of selection units, and the level of species’ endemism. This has important implications for conservation planning. First, no single universal target is appropriate, as ecosystems or nations with higher diversity and/or higher levels of endemism require substantially larger fractions of their areas to be protected. Second, a minimum conservation network sufficient to capture the diversity of vertebrates is not expected to be effective for biodiversity in general. Third, the 10% target proposed by the IUCN is likely to be wholly insufficient, and much larger fractions of area are estimated to be needed, especially in tropical regions.

Climate change, species range limits and body size in marine bivalves

Abstract: We use data on the Pleistocene and modern range limits of Californian marine bivalves to show that species that shifted their geographical ranges in response to Pleistocene climatic fluctuations were preferentially drawn from the large end of the regional body size‐frequency distributions This difference is not due to phylogenetic effects (ie dominance of extralimital species by a few large-bodied clades), differences among major ecological categories (burrowing versus surface-dwelling, or suspension feeding versus non-suspension feeding), or differences in modes of reproduction and larval development In addition, we show that successful invasive species of bivalves in present-day marine habitats also tend to be large-bodied, despite the difference in mechanisms between present-day and Pleistocene range expansions These results indicate that range limits of large-bodied bivalve species are more unstable than smallbodied ones, and that body size and its correlates need to be considered when attempting to predict the responses of marine communities to climate change, biotic interchanges and human-mediated invasions

Fisherian and "good genes" benefits of mate choice: how (not) to distinguish between them

Abstract: “Good genes” models of mate choice are commonly tested by examining whether attractive males sire offspring with improved survival. If offspring do not survive better (or indeed survive less well), but instead inherit the attractiveness of their father, results are typically interpreted to support the Fisherian process, which allows the evolution of preferences for arbitrary traits. Here, I show that the above view is mistaken. Because of life-history trade-offs, an attractive male may perform less well in other components of fitness. A female obtains a “good genes” benefit whenever males show heritable variation in quality, even if high-quality males invest so much in sexual advertisement that attractiveness has no positive correlation with any other life-history trait than male mating success itself. Therefore, a negative correlation between attractiveness and viability does not falsify good genes, if mating with a high-quality male results on average in superior offspring performance (mating success of sons included). The heritable “good genes” benefit can be sustained even if sexually antagonistic genes cause female offspring sired by high-quality males to survive and reproduce less well. Neglecting the component of male mating success from measurements of fitness returns from sons and daughters will bias the advantage of mating with a high-quality male downwards. This result may partly account for the rather weak “good genes” effects found in a recent meta-analysis.

Complementary impact of copepods and cladocerans on phytoplankton

Abstract: The differences in the impact of two major groups of herbivorous zooplankton (Cladocera and Copepoda) on summer phytoplankton in a mesotrophic lake were studied. Field experiments were performed in which phytoplankton were exposed to different densities of two major types of herbivorous zooplankton, cladocerans and copepods. Contrary to expectation, neither of the two zooplankton groups significantly reduced phytoplankton biomass. However, there were strong and contrasting impacts on phytoplankton size structure and on individual taxa. Cladocerans suppressed small phytoplankton, while copepods suppressed large phytoplankton. The unaffected size classes compensated for the loss of those affected by enhanced growth. After contamination of the copepod mesocosms with the cladoceran Daphnia, the combined impact of both zooplankton groups caused a decline in total phytoplankton biomass.