Species In Natural Communities Research Articles

A bird pollinator shows positive frequency dependence and constancy of species choice in natural plant communities.

Animal pollinators mediate reproduction of many plant species. Foraging theory suggests that animal pollinators exhibit preferences for common plant species in natural communities (positive frequency-dependent foraging) and temporary single-species specialization (flower constancy) during foraging bouts. Positive frequency dependence may favor common plant species; flower constancy may enhance conspecific pollen transfer particularly in rare plant species. Previous experimental studies suggest that avian pollinators are unlikely to exhibit these behaviors. We studied foraging behavior of Cape Sugarbirds (Promerops cafer), the main avian pollinator of many Protea species, using focal-plant and focal-bird sampling, assisted by high-resolution maps of the spatiotemporal distribution of Protea individuals and their flowering status. We found that Sugarbird's visitation preference increased with species' relative floral abundance, and that individual Sugarbirds tended to visit single species in sequence. Flower constancy during foraging bouts was significantly higher than expected from random plant-animal encounters at the scale of pollinator movements. Positive frequency dependence may favor the reproduction of abundant plant species while flower constancy may be particularly important for rare plant species. This first simultaneous study of both behaviors in a natural plant-pollinator system shows that bird pollinators exhibit both types of behavior and, in this way, possibly influence plant community structure.

Dynamics of an experimental microbial invasion

The ecological dynamics underlying species invasions have been a major focus of research in macroorganisms for the last five decades. However, we still know little about the processes behind invasion by unicellular organisms. To expand our knowledge of microbial invasions, we studied the roles of propagule pressure, nutrient supply, and biotic resistance in the invasion success of a freshwater invasive alga, Prymnesium parvum, using microcosms containing natural freshwater microbial assemblages. Microcosms were subjected to a factorial design with two levels of nutrient-induced diversity and three levels of propagule pressure, and incubated for 7 d, during which P. parvum densities and microbial community composition were tracked. Successful invasion occurred in microcosms receiving high propagule pressure whereas nutrients or community diversity played no role in invasion success. Invaded communities experienced distinctive changes in composition compared with communities where the invasion was unsuccessful. Successfully invaded microbial communities had an increased abundance of fungi and ciliates, and decreased abundances of diatoms and cercozoans. Many of these changes mirrored the microbial community changes detected during a natural P. parvum bloom in the source system. This role of propagule pressure is particularly relevant for P. parvum in the reservoir-dominated southern United States because this species can form large, sustained blooms that can generate intense propagule pressures for downstream sites. Human impact and global climate change are currently causing widespread environmental changes in most southern US freshwater systems that may facilitate P. parvum establishment and, when coupled with strong propagule pressure, could put many more systems at risk for invasion.

Mapping diversity indices: not a trivial issue

Summary Mapping diversity indices, that is estimating values in all locations of a given area from some sampled locations, is central to numerous research and applied fields in ecology. Two approaches are used to map diversity indices without including abiotic or biotic variables: (i) the indirect approach, which consists in estimating each individual species distribution over the area, then stacking the distributions of all species to estimate and map a posteriori the diversity index, (ii) the direct approach, which relies on computing a diversity index in each sampled locations and then to interpolate these values to all locations of the studied area for mapping. For both approaches, we document drawbacks from theoretical and practical viewpoints and argue about the need for adequate interpolation methods. First, we point out that the indirect approach is problematic because of the high proportion of rare species in natural communities. This leads to zero‐inflated distributions, which cannot be interpolated using standard statistical approaches. Secondly, the direct approach is inaccurate because diversity indices are not spatially additive, that is the diversity of a studied area (e.g. region) is not the sum of the local diversities. Therefore, the arithmetic variance and some of its derivatives, such as the variogram, are not appropriate to ecologically measure variation in diversity indices. For the direct approach, we propose to consider the β‐diversity, which quantifies diversity variations between locations, by the mean of a β‐gram within the interpolation procedure. We applied this method, as well as the traditional interpolation methods for comparison purposes on different faunistic and floristic data sets collected from scientific surveys. We considered two common diversity indices, the species richness and the Rao's quadratic entropy, knowing that the above issues are true for complementary species diversity indices as well as those dealing with other biodiversity levels such as genetic diversity. We conclude that none of the approaches provided an accurate mapping of diversity indices and that further methodological developments are still needed. We finally discuss lines of research that may resolve this key issue, dealing with conditional simulations and models taking into account biotic and abiotic explanatory variables.

Extreme temperature events alter demographic rates, relative fitness, and community structure.

The frequency and magnitude of extreme events are predicted to increase under future climate change. Despite recent advancements, we still lack a detailed understanding of how changes in the frequency and amplitude of extreme climate events are linked to the temporal and spatial structure of natural communities. To answer this question, we used a combination of laboratory experiments, field experiments, and analysis of multi-year field observations to reveal the effects of extreme high temperature events on the demographic rates and relative dominance of three co-occurrence aphid species which differ in their transmission efficiency of different agricultural pathogens. We then linked the geographical shift in their relative dominance to frequent extreme high temperatures through a meta-analysis. We found that both frequency and amplitude of extreme high temperatures altered demographic rates of species. However, these effects were species-specific. Increasing the frequency and amplitude of extreme temperature events altered which species had the highest fitness. Importantly, this change in relative fitness of species was consistent with significant changes in the relative dominance of species in natural communities in a 1 year long field heating experiment and 6 year long field survey of natural populations. Finally, at a global spatial scale, we found the same relationship between relative abundance of species and frequency of extreme temperatures. Together, our results indicate that changes in frequency and amplitude of extreme high temperatures can alter the temporal and spatial structure of natural communities, and that these changes are driven by asymmetric effects of high temperatures on the demographic rates and fitness of species. They also highlight the importance of understanding how extreme events affect the life-history of species for predicting the impacts of climate change at the individual and community level, and emphasize the importance of using a broad range of approaches when studying climate change.

Alpha and Beta Diversity of Lepidoptera in Eucalyptus Plantations in the Amazonian Region of Brazil

The Lepidoptera are among the insect groups with the greatest species diversity and eco- logical services. The distribution, abundance and variation in time and space of species in natural communities are important. The occurrence of pests (primary pest species) and those species with potential to cause damage (secondary pest species), plus other species, are studied in monitoring programs in forest plantations as groups I, II and III, respectively. The objective of this work was to study alpha and beta diversity patterns, and the effect of native forest in these indexes by monitoring Lepidoptera in eucalyptus plantations in the Amazonian region of Brazil. Surveys were conducted with light traps at 4 sites in this region. Alpha diversity was calculated with the Jackknife first-order proce- dure and beta diversity with the Jaccard index in order to estimate the dissimilarity per point and the effect of the distance between the native forest and the plantations on the Lepidoptera fauna. The richness showed 10 or 11 species of group I; 9 or 10 species of II and 378 to 409 species of III, without differences per group between regions. The curves of accumulated number of primary pest species stabilized between the 15th and 35th samplings and those for secondary pest species stabilized in the range from 20th to 70th samplings, with variations among sites. The estimation of richness for species of group III ranged from 50 to 100 species with a steep slope of the curve until approximately the 60th sampling with a slow increase and a tendency of stabilization in the 4 areas after this date. The primary pest species were the same in all areas and therefore their beta diversity was zero. The dissimilarity of secondary pests was similar between areas ex- cept for Felipe. Group III showed lower dissimilarity between Caracuru and Ponte Maria (14.88%) and highest values between Felipe and Pacanari (21.64%). The lower number of individuals of group I in Felipe and Ponte Maria and lower species richness of group III in Felipe can be explained by the proximity of eucalyptus plantations to the native forest. The knowledge of population dynamics and species richness of Lepidoptera defoliators is important for pest management. These indexes allow a better detection of species with a history of damage and decision-making with the most appropriate preventive measures for each situation.

Sharing of diverse mycorrhizal and root-endophytic fungi among plant species in an oak-dominated cool-temperate forest.

Most terrestrial plants interact with diverse clades of mycorrhizal and root-endophytic fungi in their roots. Through belowground plant–fungal interactions, dominant plants can benefit by interacting with host-specific mutualistic fungi and proliferate in a community based on positive plant–mutualistic fungal feedback. On the other hand, subordinate plant species may persist in the community by sharing other sets (functional groups) of fungal symbionts with each other. Therefore, revealing how diverse clades of root-associated fungi are differentially hosted by dominant and subordinate plant species is essential for understanding plant community structure and dynamics. Based on 454-pyrosequencing, we determined the community composition of root-associated fungi on 36 co-occurring plant species in an oak-dominated forest in northern Japan and statistically evaluated the host preference phenotypes of diverse mycorrhizal and root-endophytic fungi. An analysis of 278 fungal taxa indicated that an ectomycorrhizal basidiomycete fungus in the genus Lactarius and a possibly endophytic ascomycete fungus in the order Helotiales significantly favored the dominant oak (Quercus) species. In contrast, arbuscular mycorrhizal fungi were generally shared among subordinate plant species. Although fungi with host preferences contributed to the compartmentalization of belowground plant–fungal associations, diverse clades of ectomycorrhizal fungi and possible root endophytes were associated not only with the dominant Quercus but also with the remaining plant species. Our findings suggest that dominant-ectomycorrhizal and subordinate plant species can host different subsets of root-associated fungi, and diverse clades of generalist fungi can counterbalance the compartmentalization of plant–fungal associations. Such insights into the overall structure of belowground plant–fungal associations will help us understand the mechanisms that facilitate the coexistence of plant species in natural communities.

A slippery slope: logging alters mass–abundance scaling in ecological communities

Summary Natural ecosystems face ever‐increasing anthropogenic threats from activities such as logging. It is therefore important to: (i) understand anthropogenic impacts on key community characteristics with implications for community structure and function and (ii) identify metrics with mechanistic underpinnings, providing a more functional understanding of anthropogenic impacts on biodiversity. Mass–abundance scaling is the most fundamental property of ecological communities, with implications on energy and resource partitioning and on how species and population traits translate to community structure and function. In habitat patches representing a continuum of logging intensity, I examined the impact of logging on mass–abundance scaling in understorey bird species using mist netting and bird ringing. I used regression quantiles to estimate the slope and intercept of upper bound of the polygonal local‐scale mass–abundance relationship. I show that this slope becomes more negative (and the intercept higher) as logging intensity increases. Logging might therefore significantly alter resource and energy partitioning among species in natural communities. The mechanism underlying this pattern is likely to be an interaction between resource depletion, body size and density compensation. Larger species face more severe resource shortages from depleted resource availability and are expected to decline in abundance following logging, leading to more negative mass–abundance scaling. Density compensation by smaller species following large species decline can ‘push up’ the intercept of the mass–abundance relationship. This might result in both steeper slopes and higher intercepts in logged habitats. Synthesis and applications. This study shows for the first time that anthropogenic habitat change can alter fundamental community properties such as mass–abundance scaling and that this property of ecological communities was better at detecting logging impacts than other standard community measures. Identifying key metrics that provide a functional understanding of anthropogenic impacts on biodiversity is a crucial need, for both the assessment of these impacts and the continued monitoring of habitat change. This study highlights the need to supplement commonly used community descriptions with more mechanistic measures of human impacts on biodiversity.

Habitat heterogeneity promotes the coexistence of exotic seaweeds

Despite the progressive accumulation of exotic species in natural communities, little effort has been devoted to elucidating the mechanisms underpinning the coexistence of invaders in environmentally and biologically heterogeneous systems. The exotic seaweeds, Asparagopsis taxiformis and Caulerpa racemosa, exhibit a segregated distribution on Mediterranean rocky reefs. A. taxiformis dominates assemblages in topographically complex habitats, but is virtually absent on homogenous platforms. In contrast, C. racemosa achieves extensive cover in both types of habitat. We assessed whether differences in their distribution were generated by biotic interactions (between invaders and/or between invaders and natives) or by environmental constraints. Three models were proposed to explain seaweed distribution patterns: (1) invaders inhibit one another; (2) native assemblages, differing between complex and simple habitats, prevent the establishment/spread of one invader, but not that of the other; and (3) environmental conditions regulate the establishment/persistence of the seaweeds in different habitats. We removed the dominant invader and resident assemblages in each type of habitat. Moreover, A. taxiformis thalli were transplanted into the habitat dominated by C. racemosa to establish whether its failure to colonize the simple habitat was due to the lack of propagules or post-recruitment mortality. C. racemosa spread in the complex habitat was not influenced by the removal of resident assemblages, but it was slightly enhanced by A. taxiformis removal. Neither C. racemosa removal nor that of resident assemblages promoted A. taxiformis colonization and survival in simple habitats. Our results suggest that heterogeneity in environmental conditions can promote invader coexistence by mitigating the effects of negative biotic interactions. Therefore, the accumulation of introduced species in native communities does not necessarily imply established invaders fostering further invasion.

Positive diversity-invasibility relationship in species-rich semi-natural grassland at the neighbourhood scale

Attempts to answer the old question of whether high diversity causes high invasion resistance have resulted in an invasion paradox: while large-scale studies often find a positive relationship between diversity and invasibility, small-scale experimental studies often find a negative relationship. Many of the small-scale studies are conducted in artificial communities of even-aged plants. Species in natural communities, however, do not represent one simultaneous cohort and occur at various levels of spatial aggregation at different scales. This study used natural patterns of diversity to assess the relationship between diversity and invasibility within a uniformly managed, semi-natural community. In species-rich grassland, one seed of each of ten species was added to each of 50 contiguous 16 cm(2) quadrats within seven plots (8 × 100 cm). The emergence of these species was recorded in seven control plots, and establishment success was measured in relation to the species diversity of the resident vegetation at two spatial scales, quadrat (64 cm(2)) within plots (800 cm(2)) and between plots within the site (approx. 400 m(2)) over 46 months. Invader success was positively related to resident species diversity and richness over a range of 28-37 species per plot. This relationship emerged 7 months after seed addition and remained over time despite continuous mortality of invaders. Biotic resistance to plant invasion may play only a sub-ordinate role in species-rich, semi-natural grassland. As possible alternative explanations for the positive diversity-invasibility relationship are not clear, it is recommended that future studies elaborate fine-scale environmental heterogeneity in resource supplies or potential resource flows from resident species to seedlings by means of soil biological networks established by arbuscular mycorrhizal fungi.

Bacteriocin-mediated interactions within and between coexisting species.

Bacteriocins are bacteriocidal toxins released by almost all bacteria. They are thought to have a narrow range of killing, but as bacteriocin-mediated interactions have been rarely studied at biologically relevant scales, whether this narrow range of action falls mostly within or mostly between coexisting species in natural communities is an open question with important ecological and evolutionary implications. In a previous study, we systematically sampled Xenorhabdus bacteria along a hillside and found evidence for genotypic variability and bacteriocin-mediated interactions within Xenorhabdus bovienii and X. koppenhoeferi colonies that were collected only a few meters apart. In contrast, colonies that were isolated from the same soil sample were always genetically similar and showed no inhibitions. Here, we conducted pairwise growth-inhibition assays within and between seven X. bovienii and five X. koppenhoeferi colonies that were isolated from different soil samples; all seven X. bovienii colonies and at least three of the X. koppenhoeferi have been distinguished as distinct genotypes based on coarse-grain genomic markers. We found signatures for both conspecific and heterospecific bacteriocin inhibitions in this natural community of Xenorhabdus bacteria, but intraspecific inhibitions were significantly more common than interspecific inhibitions. These results suggest that bacteriocins have a major role in intraspecific competition in nature, but also suggest that bacterocins are important in mediating interspecific interactions among coexisting species in natural communities.

Predation and habitat isolation influence the community composition-area relationship in dytiscid beetles (Coleoptera: Dytiscidae)

A major goal in ecology remains the understanding of patterns in diversity and distributions of species in natural communities. The species-area relationship is an important tool for investigating differences among communities, and may be also influenced by habitat isolation and dominant predator presence. In this two-year study, we evaluated the influences of habitat area, isolation and predation on community composition of dytiscids in two geographical regions dominated by different top predators (large predaceous dragonfly larvae or fish). Contrary to expectations, surface area, isolation and predator presence /absence alone did not significantly influence dytiscid species richness and total abundance, but in association with other environmental variables, such as submerged macrophyte growth forms. Components of habitat heterogeneity likely outweighed effects of area and predation regime on prey species diversity. However, differences in the set of abundant species were best explained by habitat surface area. Thus, in contrast to species diversity, gradients in community composition were not out-weighed by components of heterogeneity. In this study, predator presence was not correlated with habitat isolation. Instead, our results revealed that the effect of predator presence/absence on prey community composition and the resulting set of coexisting species may depend on habitat isolation. Within regions, the effect of large predaceous dragonfly larvae on species composition may depend on pond surface area, whereas the presence of fish influenced species composition alone and in association with area. We advocate that regional differences in environmental gradients be considered when community composition is evaluated.

Australian family ties: does a lack of relatives help invasive plants escape natural enemies?

Invasive plants may initially be released from natural enemies when introduced to new regions, but once established, natural enemies may accumulate. How closely related invasive species are to species in the native recipient community may drive patterns of herbivore and pathogen damage and therefore, may be important in understanding the success of some invasions. We compared herbivore and pathogen damage across a group of invasive species occurring in natural environments on the east coast of Australia. We examined whether the level of damage experienced by the invasive species was associated with the degree of phylogenetic relatedness between these plants and the native plants within the region. We found that phylogenetic distance to the nearest native relative was a good predictor of herbivore and pathogen damage on the invasive plants, explaining nearly 37 % of the variance in leaf damage. Total leaf damage and the variety of damage types declined with increasing phylogenetic distance to the nearest native relative. In addition, as the phylogenetic distance to the nearest native relative increased, invasive species were colonized by fewer functional guilds and the herbivore assemblage was increasingly dominated by generalist species. These results suggest that invasive species that are only distantly related to those in the native invaded community may be released from specialist natural enemies. Our results indicate that the phylogenetic relatedness of invasive plants to species in native communities is a significant predictor of the rate of colonization by the herbivore and pathogen community, and thus a useful tool to assess invasion potential.

Character displacement of song and morphology in African tinkerbirds

Divergence in acoustic signals between populations of animals can lead to species recognition failure, reproductive isolation, and speciation. Character displacement may facilitate coexistence of species in natural communities, yet evidence for character displacement in acoustic signals is scant. Here, we find evidence of character displacement in song as well as body size and bill size of 2 related African tinkerbirds. Playback experiments indicate that related species' songs are perceived differently in sympatry than in allopatry. We suggest character displacement occurs in phenotypic traits facilitating species recognition, which has important implications for understanding the processes that lead to speciation and diversification. Because many of the sites where the 2 species coexist are areas where pristine rainforest has been degraded, results also suggest that anthropogenic pressures resulting from deforestation may be a contributing cause of character displacement in these species.

Interspecific Pollen Transfer: Magnitude, Prevalence and Consequences for Plant Fitness

Interspecific pollen transfer (IPT) is one of the mechanisms underlying potential competition among plants for pollinators, and it refers to movement of pollen between different plant species by pollinators that visit their flowers simultaneously. Two components of IPT, related to each other, are distinguished: (a) heterospecific pollen deposition (HPD) on conspecific stigmas, which may interfere with fertilization by conspecific pollen; and (b) conspecific pollen loss (CPL) on heterospecific flowers, which may reduce the amount of pollen transferred between conspecific flowers. Thus, IPT may lead to reciprocal losses for male and female functions of the plant, with potentially important ecological and evolutionary consequences. In this review, we explore the magnitude and prevalence of IPT, examining documented mechanisms and evaluating such potential ecological and evolutionary consequences. We compiled existing evidence of interspecific pollinator sharing and interspecific pollinator switching between flowers of different species in natural communities. We evaluated the relative importance of both HPD and CPL from studies comparing these variables in pure vs. mixed floral neighborhoods, analyzing evidence for the claim that IPT is an evolutionary force promoting character displacement in habitat affinity, flowering times, and floral morphology. We also examined the findings of hand-pollination experiments carried out to reveal different mechanisms by which heterospecific pollen can affect performance of native pollen. Finally, we review evidence for impacts of alien plant species on native species' reproduction, and briefly comment on risks of crop-to-wild gene flow imposed by the release of genetically modified (transgenic) crops through IPT.

Key issues in achieving an integrative perspective on stress

An integrative perspective on molecular mechanisms of stress resistance requires understanding of these mechanisms not just in vitro or in the model organism in the research laboratory - but in the healthy or diseased human in society,in the cultivated plant or animal in agricultural production,and in populations and species in natural communities and ecosystems. Such understanding involves careful attention to the context in which the organism normally undergoes stress,and appreciation that biological phenomena occur at diverse levels of organization (from molecule to ecosystem). Surprisingly,three issues fundamental to achieving an integrative perspective are presently unresolved: (i) Is variation in lower-level traits (nucleotide sequences, genes, gene products) seldom, commonly, or always consequential for stress resistance? (ii) Does environmental stress reduce or enhance genetic variation, which is the raw material of evolution? (iii) Is the present distribution of organisms along natural gradients of stress largely the result of organisms living where they can, or is adaptive evolution generally sufficient to overcome stress? Effective collaboration among disciplinary specialists and meta-analysis may be helpful in resolving these issues.

Compensatory dynamics are rare in natural ecological communities

In population ecology, there has been a fundamental controversy about the relative importance of competition-driven (density-dependent) population regulation vs. abiotic influences such as temperature and precipitation. The same issue arises at the community level; are population sizes driven primarily by changes in the abundances of cooccurring competitors (i.e., compensatory dynamics), or do most species have a common response to environmental factors? Competitive interactions have had a central place in ecological theory, dating back to Gleason, Volterra, Hutchison and MacArthur, and, more recently, Hubbell's influential unified neutral theory of biodiversity and biogeography. If competitive interactions are important in driving year-to-year fluctuations in abundance, then changes in the abundance of one species should generally be accompanied by compensatory changes in the abundances of others. Thus, one necessary consequence of strong compensatory forces is that, on average, species within communities will covary negatively. Here we use measures of community covariance to assess the prevalence of negative covariance in 41 natural communities comprising different taxa at a range of spatial scales. We found that species in natural communities tended to covary positively rather than negatively, the opposite of what would be expected if compensatory dynamics were important. These findings suggest that abiotic factors such as temperature and precipitation are more important than competitive interactions in driving year-to-year fluctuations in species abundance within communities.

Variability is the nature of the endophyte-grass interaction

Studies of systemic endophyte in grasses have skyrocketed in the past two decades. However, the vast majority of these studies still occur in agroecosystems. We show that ecological and evolutionary concepts derived from these systems may be misleading because they fail to incorporate the enormous variability found in endophyte-host grass interactions in wild grass populations. This variability stems from 1) genetic differences in host plants and endophytes, 2) environmental factors such as light and soil nutrients and moisture, and 3) the tangled web of interacting species in natural communities, such as conspecific and interspecific plants, multiple generalist and specialist herbivore species, and the third trophic level, predators and parasites. Studies of natural populations and communities continue to lag far behind those involving agronomic grass systems. However, we argue that additional studies of infected wild grass populations and communities are essential to advance ecological and evolutionary concepts of endophytegrass interactions. Keywords: endophytes, herbivore resistance, grasses, natural populations and communities, natural enemies, Neotyphodium, pathogens, plant defenses, variability

Ultraviolet and temperature effects on planktonic rotifers and crustaceans in northern temperate lakes

Summary1. Anthropogenic stressors such as climate change, ozone depletion and acidification may act in concert to alter ultraviolet (UV) light and temperature regimes in freshwater ecosystems. These physical and chemical changes will inevitably affect zooplankton community dynamics, but little is known about their relative effects on different species in natural communities. During spring, species that migrate to surface waters to take advantage of warmer temperatures may be especially vulnerable as UV levels can be high.2. The objective of this study was to investigate the in situ effects of UV and temperature on a natural assemblage of planktonic rotifers and crustaceans during the spring. We performed in situ exposure experiments in two lakes with different surface temperatures.3. Exposure to UV had a significant effect on the abundance and/or reproduction of four rotifers: Gastropus spp., Kellicottia bostonensis, Kellicottia longispina, Keratella spp.; two cladocerans: Holopedium gibberum, Daphnia catawba, and one copepod: Leptodiaptomus minutus. Incubation under cooler temperatures had a negative effect on K. longispina and H. gibberum. Temperature and UV had a significant interactive effect on abundance and/or reproduction of L. minutus and Ploesoma truncatum. Our results indicate that changes in underwater UV and temperature can significantly influence the composition of the zooplankton community and ultimately food web dynamics.

Salt marshes: biological controls of food webs in a diminishing environment

This essay reviews two important topics in coastal ecology: the work on the relative role of bottom–up and top–down controls in natural communities and the loss of wetlands worldwide. In salt marshes and other coastal wetlands, bottom–up and top–down mechanisms of control on natural communities are pervasive. Bottom–up effects through nutrient supply may propagate to upper trophic levels via better food quality, or indirectly by altering water and sediment quality. Top–down control by consumers alters lower trophic levels through consumption of primary producers, and indirectly by trophic cascades in which higher predators feed on grazers. The combined forcing of bottom–up and top–down controls govern assemblages of species in natural communities, mediated by physical and biogeochemical factors. Although there is much information about biological controls of coastal food webs, more information is needed. Even more important is that large losses of wetland are occurring along coastlines worldwide due to a variety of economic and social activities including filling, wetland reclamation, and sediment interception. Such losses are of concern because these wetlands provide important functions, including export of energy-rich material to deeper waters, nursery and stock habitats, shoreline stabilization, and intercept land-derived nutrients and contaminants. These important functions justify conservation and restoration efforts; barring such efforts, we will find it increasingly difficult to find coastal wetlands where we can continue to gain further understanding of ecology and biogeochemistry and lack the aesthetic pleasure these wetlands provide to so many of us.

Niche division and abundance: an evolutionary perspective

AbstractIn recent years, biodiversity has become an issue of broad academic interest, and its assessment and maintenance are now recognized as an important area of ecological research. While the concept of biodiversity encompasses, first and foremost, the total number of species co‐occurring in a locality, it has increasingly been realized that information on the relative abundances of co‐occurring species is also required for a better understanding of the patterns and dynamics of biodiversity. In many areas of ecological research, “abundance” constitutes a key variable that characterizes populations and communities. The relative abundances of species in natural communities reflect evolutionary and contemporary processes occurring on different spatiotemporal scales. The idea of niche apportionment has been developed to provide an integrated conceptual framework for the study of species abundance patterns in communities. This article reviews a number of important issues surrounding the concept of niche apportionment, including some aspects that have received very little or no consideration in previous ecological literature. The main emphasis here is on possible evolutionary implications and backgrounds. Further, as a universal factor which affects species abundance in one way or another, body size is highlighted and its relationship with abundance (“density–body‐size relation”) is considered, referring in particular to a recent comprehensive analysis of freshwater benthic data. Consideration of this and other studies has led to the formulation of the biomass equivalence rule, that suggests the independence of the biomass measure of abundance from body size, which strengthens the logical basis of niche apportionment models. It is suggested that, compared with Hubbell's neutral theory of biodiversity, niche apportionment with the biomass equivalence rule represents a conceptually more sound and widely applicable approach to elucidating species abundance patterns.