Diversity and community biomass depend on dispersal and disturbance in microalgal communities

The evidence for species diversity effects on ecosystem functions is mainly based on studies not explicitly addressing local or regional processes regulating coexistence or the importance of community structure in terms of species evenness. In experimental communities of marine benthic microalgae, we altered the successional stages and thus the strength of local species interactions by manipulating rates of dispersal and disturbance. The treatments altered realized species richness, evenness and community biomass. For species richness, dispersal mattered only at high disturbance rates; when opening new space, dispersal led to maximized richness at intermediate dispersal rates. Evenness, in contrast, decreased with dispersal at low or no disturbance, i.e. at late successional stages. Community biomass showed a non-linear hump-shaped response to increasing dispersal at all disturbance levels. We found a positive correlation between richness and biomass at early succession, and a strong negative correlation between evenness and biomass at late succession. In early succession both community biomass and richness depend directly on dispersal from the regional pool, whereas the late successional pattern shows that if interactions allow the most productive species to become dominant, diverting resources from this species (i.e. higher evenness) reduces production. Our study emphasizes the difference in biodiversity–function relationships over time, as different mechanisms contribute to the regulation of richness and evenness in early and late successional stages.     

An assessment of the dependence of evenness indices on species richness

Evenness is the component of species diversity that was built to be mathematically independent from species richness. Yet, earlier work questioned the generality of this independence. However, this earlier work involved only a few relative abundance distributions (RADs), very limited gradients of species richness, or evenness indices, such as Pielou's index, that were not defined in relation to a precise, coherent set of axioms. We show that for very different theoretical RADs, a relationship between evenness indices and species richness does exist and is strong-at least for some RADs and/or when species richness is under 20. Furthermore, this relationship is mostly negative, which contradicts some previous studies. We also show that, for the very uneven RADs we tested, evenness indices depend even more on species richness than diversity indices do. Finally, we discuss the philosophy behind the analysis of evenness-richness relationships within and between RADs, as well as the interest of correcting evenness indices for species richness when species richness varies, especially with many values under 20.     

Temporal patterns of diversity, abundance and evenness for invertebrate communities from coastal freshwater and brackish water rock pools

Aquatic invertebrate data were collected from 49 erosional, Jamaican,rock pools between 1989 and 1998 and used to describe temporal patterns ofspecies diversity. This unique series of pools on the north coast of Jamaica,classified as either brackish (31) or freshwater (18), was used to determinehowdiversity changes over time, whether there was a difference between poolclassifications, and the impacts of environmental variables. Mean communitymetrics (richness, diversity, evenness, abundance) were not significantlydifferent between freshwater and brackish pools. However, there weresignificantdifferences among the eight sampling dates and differences over time dependedonpool classification. Measures of diversity for freshwater pools were relativelyconstant over time, implying little change at the community level. Brackishpools showed significant differences over time in species richness, totalabundance, and evenness implying that community composition and structure werenot static but changed in response to either environmental or biotic changes(possibly initiated by environmental change).Some temporal changes in community metrics could be linked to temporal changesin environmental variables. In brackish water pools, a significant increase inpool salinity between January 1991 and January 1992 corresponded to an increasein species richness, likely due to an increase in marine fauna. Similarly,changes in abundance and evenness corresponded to changes in temperature,dissolved oxygen, and pH. In addition, physicochemical variables used in thisstudy were shown to affect community metrics and those relationships dependedonpool classification. Most relationships between community metrics andenvironmental variables were negative with the exception of Simpson's diversityindex for which positive relationships were found. This may indicate that, aspool conditions become less favorable, a few species flourish and dominate thecommunity.     

Consistent Effects of Biodiversity on Ecosystem Functioning Under Varying Density and Evenness

Biodiversity experiments typically vary only species richness and composition, yet the generality of their results relies on consistent effects of these factors even under varying starting conditions of density and evenness. We tested this assumption in a factorial species richness x density x evenness experiment using a pool of 60 common grassland species divided into four functional groups (grasses, legumes, tall herbs and short herbs). Richness varied from 1, 2, 4, 8 to 16 species, total planting density was 1,000 or 2,000 seeds/m², and species were sown in even or uneven proportions, where one functional group was made dominant. Aboveground plant biomass increased linearly with the logarithm of species richness in all density and evenness treatments during all three years of the experiment. This was due to a convergence of realized density and evenness within species richness levels, although functional groups which were initially made dominant retained their dominance. Between species richness levels, realized density increased, and realized evenness decreased with species richness. Thus, more individuals could coexist if they belonged to different species. Within species richness levels, higher biomass values were correlated with lower density, suggesting an underlying thinning process. However, communities with low realized evenness also had low biomass values; thus high biomass did not result from species dominance. So-called complementarity and selection effects were similar across density and evenness treatments, indicating that the mechanisms underpinning the biodiversity effects were not altered. Species richness was the dominant driver of aboveground biomass, irrespective of variations in total densities and species abundance distributions at the start of the experiment; rejecting the hypothesis that initial differences in species abundance distributions might lead to different “stable states” in community structure or biomass. Thus, results from previous biodiversity experiments that only manipulated species richness and composition should be quite robust and broadly generalizable.     

放牧干扰下高寒草甸物种多样性指数评价与选择

在青藏高原东北缘高寒草甸设置6个放牧强度样地,连续4年研究10个多样性指数(Richness和Abundance 2个实测指数,优势度指数、均匀度指数、丰富度指数和综合指数各2个)对放牧强度和年限影响植物群落的解释能力.结果表明: 相对于重要值,利用多度计算的多样性指数对放牧干扰更敏感.优势度指数(Berger-Parker、Dominance)与放牧强度和年限均无关,不能将放牧干扰对群落优势种的影响有效区分.均匀度指数(Equitability、Evenness)均与放牧强度无关,但Evenness指数与放牧年限呈显著负相关,不受偶见种影响且与物种多度的变异系数呈显著正相关,在基于时间尺度的均匀度比较中可以选择Evenness指数.丰富度指数(Menhinick、Margalef)均与放牧年限无关,但Margalef指数与放牧强度呈显著正相关,且不受偶见种影响.综合指数(Shannon、Simpson)均与放牧强度无关,但Shannon指数与物种丰富度和多度呈显著正相关,且随放牧年限增加而显著升高,不受偶见种影响,Shannon指数可用于在长时间尺度下比较物种多样性变化.在所有多样性指数中,只有实测物种丰富度和多度均与放牧强度呈显著负相关,与放牧年限呈显著正相关,且不受偶见种影响,故实测物种丰富度和多度相结合可作为放牧干扰下多样性比较的首选指标.此外,多样性指数选择须考虑放牧干扰的强度与时间特征、多样性组分和研究目的.     

Species richness and evenness respond in a different manner to propagule density in developing prairie microcosm communities

Diversity has two basic components: richness, or number of species in a given area, and evenness, or how relative abundance or biomass is distributed among species. Previously, we found that richness and evenness can be negatively related across plant communities and that evenness can account for more variation in Shannon’s diversity index (H′) than richness, which suggests that relationships among diversity components can be complex. Non-positive relationships between evenness and richness could arise due to the effects of migration rate or local species interactions, and relationships could vary depending on how these two processes structure local communities. Here we test whether diversity components are equally or differentially affected over time by changes in seed density (and associated effects on established plant density and competition) in greenhouse communities during the very early stages of community establishment. In our greenhouse experiment, we seeded prairie microcosms filled with bare field soil at three densities with draws from a mix of 22 grass and forb species to test if increased competition intensity or seedling density would affect the relationships among diversity components during early community establishment. Increased seed density treatments caused diversity components to respond in a different manner and to have different relationships with time. Richness increased linearly with seed density early in the experiment when seedling emergence was high, but was unrelated to density later in the experiment. Evenness decreased log-linearly with seed densities on all sampling dates due to a greater dominance by Rudbeckia hirta with higher densities. Early in the experiment, diversity indices weakly reflected differences in richness, but later, after the competitive effects of Rudbeckia hirta became more intense, diversity indices more strongly reflected differences in evenness. This suggests that species evenness and diversity indices do not always positively covary with richness. Based on these results, we suggest that evenness and richness can be influenced by different processes, with richness being more influenced by the number of emerging seedlings and evenness more by species interactions like competition. These results suggest that both diversity components should be measured in plant diversity studies whenever it is possible.     

Species evenness and productivity in experimental plant communities

In nature, plant biomass is not evenly distributed across species, and naturally uncommon species may differ from common species in the probability of loss from the community. Understanding relationships between evenness and productivity is therefore critical to understanding changes in ecosystem functioning as species are lost from communities. We examined data from a large multi-site grassland experiment (BIODEPTH) for relationships between evenness of species composition (proportional abundance of biomass) and total biomass of communities. For plots which started with the same and even species composition, but which diverged in evenness over time, those with lower evenness had a significantly greater biomass. The relationship between evenness and biomass across all plots was also negative. However, for communities where the most common species represented one of the three largest species in monoculture at that site (inclusion of a large dominant species), the relationship was neutral. Path analyses indicated that three paths contributed to this negative relationship. First, higher species richness decreased evenness, but increased biomass (primarily through an increase in maximum plant size). Contrary to predictions, maximum plant size had either no effect on evenness, or a positive effect (in year 3 plots with a large dominant species), thereby reducing this relationship. In year 2, large variation among species in plant size (as measured in monoculture) both decreased evenness and increased biomass, thus increasing the strength of the negative relationship between evenness and biomass. However, the former effect was only found in plots with a large dominant species, the latter only in plots without a large dominant species. When species richness, maximum plant size, and variation in size were accounted for, in year 2 evenness positively affected biomass in plots that included a large dominant species. Our results are consistent with the view that naturally uncommon species may be unaffected by (or even benefit from) the presence of a large naturally common species, and that uncommon plants may have little ability to increase productivity in the absence of such a species. We conclude that the observed negative relationship between evenness and biomass resulted from multiple direct and indirect effects, the relative strength of which depended in part on the presence of large dominant species.     

Density may alter diversity–productivity relationships in experimental plant communities

Contemporary biodiversity experiments, in which plant species richness is manipulated and aboveground productivity of the system measured, generally demonstrate that lowering plant species richness reduces productivity. However, we propose that community density may in part compensate for this reduction of productivity at low diversity. We conducted a factorial experiment in which plant functional group richness was held constant at three, while plant species richness increased from three to six to 12 species and community density from 440 to 1050 to 2525 seedlings m⁻². Response variables included density, evenness and above- and belowground biomass at harvest. The density gradient converged slightly during the course of the experiment due to about 10% mortality at the highest sowing density. Evenness measured in terms of aboveground biomass at harvest significantly declined with density, but the effect was weak. Overall, aboveground, belowground and total biomass increased significantly with species richness and community density. However, a significant interaction between species richness and community density occurred for both total and aboveground biomass, indicating that the diversity–productivity relationship was flatter at higher than at lower density. Thus, high species richness enabled low-density communities to reach productivity levels otherwise seen only at high density. The relative contributions of the three functional groups C₃, C₄ and nitrogen-fixers to aboveground biomass were less influenced by community density at high than at low species richness. We interpret the interaction effects between community density and species richness on community biomass by expanding findings about constant yield and size variation from monocultures to plant mixtures.     

Invasion of the cosmopolitan species Echinochloa colona into herbaceous vegetation of a tropical wetland system

The negative effects of alien plant species on ecosystem structure and functions are increasingly recognised, and efforts to control these species are vital to restore degraded ecosystems and preserve biodiversity. However, we lack a full understanding of factors that determine alien species invasions along spatial gradients in herbaceous vegetation of tropical systems. We therefore examined the effects of community properties, environmental variables and human-related disturbance factors on the invasion of the alien grass Echinochloa colona (L.) Link at small- and large scales in the Kilombero Valley wetland, Tanzania. Generalized additive mixed models showed that E. colona abundance on a small scale was negatively related to above-ground biomass and evenness of resident species, whereas E. colona abundance was positively related to grazing intensity. On a large scale, biomass (negatively related to E. colona abundance) and distance to river (positive) were important in explaining E. colona abundance. These findings support the assertion that different factors may contribute to the invasion of alien plant species at different spatial scales, as also reported in many temperate systems. Overall, our results show that successful invasion of alien species is a function of plant community properties, human-related disturbance and favourable environmental conditions. Effective management strategies should consider mitigations that can increase the biomass and evenness of native species and a reduction of grazing pressure to restore the wetland and conserve biodiversity.     

A Comparative Study of the Far Eastern Seas and the Northern Pacific Ocean Based on Integral Parameters of Net Zooplankton in the Epipelagic Layer

Integral parameters of zooplankton community, including species diversity and its components were compared between the Chukchi Sea, Bering Sea, Sea of Okhotsk, Sea of Japan, and adjacent Pacific waters based on the data obtained by standard Juday net with a mouth area of 0.1 m² during the large-scale surveys conducted by the Pacific Fisheries Research Center (TINRO Center) in 1984–2013. These parameters were calculated for the total surveyed area of approximately 7.0 million km² and separately for each of the considered water bodies. In Pacific waters, species richness is higher than that in all the seas, while the concentration of individuals (expressed in terms of abundance, ind./m³) and evenness of their distribution over species were lower. The only sea with a larger mean size of organisms compared to the ocean is the Bering Sea. A lower species diversity than in the ocean has been recorded only from the Chukchi Sea; a lower density (in terms of biomass, g/m³) was determined only from the Sea of Japan. Among the four seas, the Chukchi Sea ranks first in terms of biomass and abundance of zooplankton, second in species evenness, third in the mean size of individuals, and last in species richness and diversity. The Bering Sea ranks first in terms of mean size of plankton organisms, second in species richness, diversity, and biomass, third in abundance, and last in species evenness. The Sea of Okhotsk ranks second in terms of mean size of individuals, last in their abundance, and third in the other parameters. The Sea of Japan ranks first in terms of species richness, evenness, and diversity, second in abundance, and last in mean size of zooplankton organisms, and, therefore, their biomass. The biomass of zooplankton, in accordance with the concentration of nutrients, increases in the southto-north direction (while its absolute abundance depends largely on the size of the body of water). The mean size of organisms increases in the same direction; the evenness of their distribution over species increases in the reverse direction (with the exception of both parameters for the Chukchi Sea). The rank of a water body for its biodiversity coincides with the species richness rank. The latter increases from north to south (except for the Okhotsk Sea), but greatly depends on the surveyed area and, even more, on the surveyed volume of water. A study of the literature data found some unexpected statistically significant relationships of the integral parameters of zooplankton with those of pelagic and bottom macrofauna, as well as with the parameters of zooplankton production, on the size of the considered bodies of water. The causes and the biological meanings of most of these relationships still do not have any rational interpretation. Their testing at other spatial scales will be continued in future works.     

Relationship Between Aboveground Biomass and Multiple Measures of Biodiversity in Subtropical Forest of Puerto Rico

Anthropogenic activities have accelerated the rate of global loss of biodiversity, making it more important than ever to understand the structure of biodiversity hotspots. One current focus is the relationship between species richness and aboveground biomass (AGB) in a variety of ecosystems. Nonetheless, species diversity, evenness, rarity, or dominance represent other critical attributes of biodiversity and may have associations with AGB that are markedly different than that of species richness. Using data from large trees in four environmentally similar sites in the Luquillo Experimental Forest of Puerto Rico, we determined the shape and strength of relationships between each of five measures of biodiversity (i.e., species richness, Simpson's diversity, Simpson's evenness, rarity, and dominance) and AGB. We quantified these measures of biodiversity using either proportional biomass or proportional abundance as weighting factors. Three of the four sites had a unimodal relationship between species richness and AGB, with only the most mature site evincing a positive, linear relationship. The differences between the mature site and the other sites, as well as the differences between our richness–AGB relationships and those found at other forest sites, highlight the crucial role that prior land use and severe storms have on this forest community. Although the shape and strength of relationships differed greatly among measures of biodiversity and among sites, the strongest relationships within each site were always those involving richness or evenness.     

Experimental Manipulation of Grassland Plant Diversity Induces Complex Shifts in Aboveground Arthropod Diversity

Changes in producer diversity cause multiple changes in consumer communities through various mechanisms. However, past analyses investigating the relationship between plant diversity and arthropod consumers focused only on few aspects of arthropod diversity, e.g. species richness and abundance. Yet, shifts in understudied facets of arthropod diversity like relative abundances or species dominance may have strong effects on arthropod-mediated ecosystem functions. Here we analyze the relationship between plant species richness and arthropod diversity using four complementary diversity indices, namely: abundance, species richness, evenness (equitability of the abundance distribution) and dominance (relative abundance of the dominant species). Along an experimental gradient of plant species richness (1, 2, 4, 8, 16 and 60 plant species), we sampled herbivorous and carnivorous arthropods using pitfall traps and suction sampling during a whole vegetation period. We tested whether plant species richness affects consumer diversity directly (i), or indirectly through increased productivity (ii). Further, we tested the impact of plant community composition on arthropod diversity by testing for the effects of plant functional groups (iii). Abundance and species richness of both herbivores and carnivores increased with increasing plant species richness, but the underlying mechanisms differed between the two trophic groups. While higher species richness in herbivores was caused by an increase in resource diversity, carnivore richness was driven by plant productivity. Evenness of herbivore communities did not change along the gradient in plant species richness, whereas evenness of carnivores declined. The abundance of dominant herbivore species showed no response to changes in plant species richness, but the dominant carnivores were more abundant in species-rich plant communities. The functional composition of plant communities had small impacts on herbivore communities, whereas carnivore communities were affected by forbs of small stature, grasses and legumes. Contrasting patterns in the abundance of dominant species imply different levels of resource specialization for dominant herbivores (narrow food spectrum) and carnivores (broad food spectrum). That in turn could heavily affect ecosystem functions mediated by herbivorous and carnivorous arthropods, such as herbivory or biological pest control.     

Effects of plant diversity, plant productivity and habitat parameters on arthropod abundance in montane European grasslands

Arthropod abundance has been hypothesized to be correlated with plant diversity but the results of previous studies have been equivocal. In contrast, plant productivity, vegetation structure, abiotic site conditions, and the physical disturbance of habitats, are factors that interact with plant diversity, and that have been shown to influence arthropod abundance. We studied the combined effect of plant species diversity, productivity and site characteristics on arthropod abundance in 71 managed grasslands in central Germany using multivariate statistics. For each site we determined plant species cover, plant community biomass (productivity), macro- and micronutrients in the soil, and characterized the location of sites with respect to orographic parameters as well as the current and historic management regimes. Arthropods were sampled using a suction sampler and classified a priori into functional groups (FGs). We found that arthropod abundance was not correlated with plant species richness, effective diversity or Camargo's evenness, even when influences of environmental variables were taken into account. In contrast, plant community composition was highly correlated with arthropod abundances. Plant community productivity influenced arthropod abundance but explained only a small proportion of the variance. The abundances of the different arthropod FGs were influenced differentially by agricultural management, soil characteristics, vegetation structure and by interactions between different FGs of arthropods. Herbivores, carnivores and detritivores reacted differently to variation in environmental variables in a manner consistent with their feeding mode. Our results show that in natural grassland systems arthropod abundance is not a simple function of plant species richness, and they emphasize the important role of plant community composition for the abundance patterns of the arthropod assemblages.     

Relationships between arthropod richness,evenness, and diversity are altered by complementarity among plant genotypes

Biodiversity is quantified via richness (e.g., the number of species), evenness (the relative abundance distribution of those species), or proportional diversity (a combination of richness and evenness, such as the Shannon index, H′). While empirical studies show no consistent relationship between these aspects of biodiversity within communities, the mechanisms leading to inconsistent relationships have received little attention. Here, using common evening primrose (Oenothera biennis) and its associated arthropod community, we show that relationships between arthropod richness, evenness, and proportional diversity are altered by plant genotypic richness. Arthropod richness increased with O. biennis genotypic richness due to an abundance-driven accumulation of species in response to greater plant biomass. Arthropod evenness and proportional diversity decreased with plant genotypic richness due to a nonadditive increase in abundance of a dominant arthropod, the generalist florivore/omnivore Plagiognathas politus (Miridae). The greater quantity of flowers and buds produced in polycultures—which resulted from positive complementarity among O. biennis genotypes—increased the abundance of this dominant insect. Using choice bioassays, we show that floral quality did not change in plant genotypic mixtures. These results elucidate mechanisms for how plant genotypic richness can modify relationships between arthropod richness, evenness, and proportional diversity. More broadly, our results suggest that trophic interactions may be a previously underappreciated factor controlling relationships between these different aspects of biodiversity.     

Effects of plant species richness and evenness on soil microbial community diversity and function

Understanding the links between plant diversity and soil communities is critical to disentangling the mechanisms by which plant communities modulate ecosystem function. Experimental plant communities varying in species richness, evenness, and density were established using a response surface design and soil community properties including bacterial and archaeal abundance, richness, and evenness were measured. The potential to perform a representative soil ecosystem function, oxidation of ammonium to nitrite, was measured via archaeal and bacterial amoA genes. Structural equation modeling was used to explore the direct and indirect effects of the plant community on soil diversity and potential function. Plant communities influenced archaea and bacteria via different pathways. Species richness and evenness had significant direct effects on soil microbial community structure, but the mechanisms driving these effects did not include either root biomass or the pools of carbon and nitrogen available to the soil microbial community. Species richness had direct positive effects on archaeal amoA prevalence, but only indirect impacts on bacterial communities through modulation of plant evenness. Increased plant evenness increased bacterial abundance which in turn increased bacterial amoA abundance. These results suggest that plant community evenness may have a strong impact on some aspects of soil ecosystem function. We show that a more even plant community increased bacterial abundance, which then increased the potential for bacterial nitrification. A more even plant community also increased total dissolved nitrogen in the soil, which decreased the potential for archaeal nitrification. The role of plant evenness in structuring the soil community suggests mechanisms including complementarity in root exudate profiles or root foraging patterns.     

Can we tell how a community was constructed? A comparison of five evenness indices for their ability to identify theoretical models of community construction

Evenness indices provide a simple measure of community structure. It might therefore be possible to use them to identify the process by which a community has been assembled. To test whether this is practicable, we constructed simulated community samples using five stochastic models of community construction, proposed by Tokeshi. We then examined the ability of five evenness indices to identify the model under which the community samples were produced. Each model produced samples with a range of evenness values, but mean evenness differed between the models. The dominance decay (DD) model produced community samples with the greatest evenness, followed by MacArthur fraction (MF). Evenness was lowest under the dominance pre-emption (DP) model. The differences between models were quite consistent across indices and consistent between 5-species and 15-species communities. Samples produced under the DD and MF models varied least in evenness between 5-species and 15-species samples, and those produced under the random assortment (RA) model varied most, irrespective of the evenness index used. Evenness varied considerably between replicate samples, as expected with stochastic processes. In 5-species communities, the greatest robustness in evenness across replicates was seen using indices O or E'. In 15-species communities, O and E(Q) were the most robust. The index best able to identify the model which had generated the sample differed between models and with species richness. If the model of interest is not known in advance, the best index for identifying the generating model is E(var) for communities of more than 10 species for RF, RA and DP models and O for other communities. The number of samples required in a data set before it could be effectively identified was, for example, more than 30 for 5-species samples produced under the random fraction (RF) model, and 3 for 15-species RF samples. In contrast, a 5-species DD data set could be effectively identified when it contained 15 or more samples. We conclude that evenness can be used to identify the process by which the community has been constructed, out of the five models considered here. The best evenness index for doing this varies with the species richness and the evenness of the community but we can suggest the use of O without knowing the model because its the more stable one against species richness and the more robust and unbiased against simulated samples and it encompass a good discriminating power between the models in most of the cases studied.     

The decoupling of abundance and species richness in lizard communities

1. Patterns of species richness often correlate strongly with measures of energy. The more individuals hypothesis (MIH) proposes that this relationship is facilitated by greater resources supporting larger populations, which are less likely to become extinct. Hence, the MIH predicts that community abundance and species richness will be positively related. 2. Recently, Buckley & Jetz (2010, Journal of Animal Ecology, 79, 358-365) documented a decoupling of community abundance and species richness in lizard communities in south-west United States, such that richer communities did not contain more individuals. They predicted, as a consequence of the mechanisms driving the decoupling, a more even distribution of species abundances in species-rich communities, evidenced by a positive relationship between species evenness and species richness. 3. We found a similar decoupling of the relationship between abundance and species richness for lizard communities in semi-arid south-eastern Australia. However, we note that a positive relationship between evenness and richness is expected because of the nature of the indices used. We illustrate this mathematically and empirically using data from both sets of lizard communities. When we used a measure of evenness, which is robust to species richness, there was no relationship between evenness and richness in either data set. 4. For lizard communities in both Australia and the United States, species dominance decreased as species richness increased. Further, with the iterative removal of the first, second and third most dominant species from each community, the relationship between abundance and species richness became increasingly more positive. 5. Our data support the contention that species richness in lizard communities is not directly related to the number of individuals an environment can support. We propose an alternative hypothesis regarding how the decoupling of abundance and richness is accommodated; namely, an inverse relationship between species dominance and species richness, possibly because of ecological release.     

The impact of density-independent mortality on species coexistence: an experimental test with zooplankton

Mortality (e.g. predation, disturbance) is often thought to lower the intensity of interspecific competition and thereby promote the coexistence of competing species. However, surprisingly few tests of this idea exist, especially for metazoans feeding on a self-renewing resource. Here we examined the effect of density-independent mortality on the coexistence of four species of pond zooplankton (microcrustacean grazers) in a series of laboratory microcosms. Across the experimental mortality gradient, consumer biomass decreased and resource abundance increased with an increase in mortality. Thus, the treatments resulted in an increase in resource availability per consumer (one measure of reduced competitive intensity). There was no significant effect of mortality treatment on species relative abundances or species evenness, and the zooplankter Diaphanosoma dominated community biomass at all mortality levels. Mortality rate did have a marginally significant effect on species richness (p<0.07), but richness did not increase monotonically with mortality level. Instead, richness tended to be highest in the low- and no-mortality treatments.