Species pool size and invasibility of island communities: a null model of sampling effects

The success of alien species on oceanic islands is considered to be one of the classic observed patterns in ecology. Explanations for this pattern are based on lower species richness on islands and the lower resistance of species‐poor communities to invaders, but this argument needs re‐examination. The important difference between islands and mainland is in the size of species pools, not in local species richness; invasibility of islands should therefore be addressed in terms of differences in species pools. Here I examine whether differences in species pools can affect invasibility in a lottery model with pools of identical native and exotic species. While in a neutral model with all species identical, invasibility does not depend on the species pool, a model with non‐zero variation in population growth rates predicts higher invasibility of communities of smaller pools. This is because of species sampling; drawing species from larger pools increases the probability that an assemblage will include fast growing species. Such assemblages are more likely to exclude random invaders. This constitutes a mechanism through which smaller species pools (such as those of isolated islands) can directly underlie differences in invasibility.     

A comparative study of the relationship between host size and brood size in Apanteles spp. (Hymenoptera: Braconidae)

ABSTRACT. 1. Current models of insect oviposition predict that clutch size in parasitoids should correlate with host size, with a continuum from solitary species at one end to large gregarious broods at the other. This prediction is tested for the genus Apanteles (sensu lato).
2. The distribution of brood sizes in Apanteles is bimodal, with peaks at one (solitary species) and at about twenty (gregarious species).
3. Brood size of gregarious species correlates with host size, but when a measure of the total volume of a parasitoid brood is plotted against host size, solitary species do not lie on the same regression slope as gregarious species.
4. There is a relative shortage of gregarious species on small hosts, and a relative excess of solitary species on large hosts. Solitary species on large hosts do not fully consume the host resource.
5. The possible role of evolutionary constraints to adaptive progeny allocation in Apanteles is discussed.     

Development, fecundity and survival of the herbivore Lymantria dispar and the number of plant species in its diet

ABSTRACT.

• 1 Larval gypsy moth Lymantria dispar (L.) mortality depends upon the number and type of plant species consumed as well as the sequence in which they are consumed.
• 2 When a two-species diet is composed of a favourable and an unfavourable species, larval mortality is higher on the two-species diet than on the diet of the favourable species alone.
• 3 When a two species diet is composed of a favourable and unfavourable coniferous species, larval mortality is higher on the single conifer species diet than on the two-species diet.
• 4 Two-species diets, in which one species is a conifer, produce individuals with a greater fecundity than diets of either species alone.
• 5 The influence of diets composed of two favourable species, on development, size and fecundity, depends both on the relative quality of each of the two plant species and which of the two is consumed by older instars.

     

不同栖息地状态下物种竞争模式及模拟研究与应用

物种竞争是影响生态系统演化的重要生态过程之一.而物种在受人类影响出现不同程度毁坏的栖息地上的演化又是非常复杂的,因此研究物种演化对栖息地毁坏的响应是非常必要的.在Tilman研究工作的基础上,将竞争系数引入集合种群动力模式,建立了多物种集合种群竞争共存的数学模型,并对5-物种集合种群在不同栖息地状态下的竞争动态进行了计算机模拟研究.结果表明：（1）不同结构的群落（q值不同）,物种之间的竞争排斥作用强度不同,优势物种明显的群落,物种之间的排斥强度大;（2）随着栖息地毁坏程度的增加,对优势物种的负面影响逐渐减小,而对弱势物种的负面影响逐渐增加;（3）随着栖息地恢复幅度的增加,优势物种和弱势物种之间的竞争越强烈,优势物种受到的竞争排斥加大,而弱势物种逐渐变强,出现了强者变弱、弱者变强的格局;（4）物种竞争排斥与共存受迁移扩散能力和竞争能力影响很大,竞争共存的条件是其竞争能力与扩散能力呈非线性负相关关系;（5）竞争共存的物种的强弱序列发生了变化.     

Relative influence of habitat heterogeneity, climate, human disturbance, and spatial structure on vertebrate species richness in Spain

Many factors affect the distribution of species richness. This study examines the relative influence of habitat heterogeneity, climate, human disturbance, and spatial structure on the species-richness distribution of terrestrial vertebrates (amphibians, reptiles, birds and mammals) in mainland Spain. The results indicate that spatial structure and environment exert similar influences on species richness. For all four taxa, species richness increases southward and northward, being lower in the center of the country, when controlled for other variables. This may be the result of a peninsular effect, as found in other studies, and reflect the importance of historical events on species richness in the Iberian Peninsula. Climate is more important than habitat heterogeneity in determining species richness. Temperature is positively correlated with amphibian, reptile, and bird species richness, while mammalian species richness is highest at intermediate temperatures. This effect is stronger in ectotherms than among endotherms, perhaps reflecting physiological differences. Precipitation positively correlates with bird and mammalian species richness, but has no effect on ectotherm species richness. Amphibian species richness increases with altitudinal range, and bird species richness with habitat diversity. Human population density is positively correlated with bird and mammalian species richness, but does not affect ectotherm species richness, while amphibian and bird species richness is highest at moderate levels of human land alteration (farmland). However, unexplained variance remains, and we discuss that the effects of environmental variables on species richness may vary geographically, causing different effects to be obscured on a national scale, diminishing the explanatory power of environmental variables.     

Proportion of exotics and relatedness of host species mediate the positive effect of plant richness on the species richness of fruit flies

1. All else being equal, the greater the local species richness of plants, the greater the number of associated herbivore species. Because most herbivore insects feed on a subset of closely related plant species, plant phylogenetic diversity is expected to play a key role in determining the number of herbivore species. What is not well known, however, is how an increase in the species richness of exotic plants affects the species richness of herbivores. 2. In this study, we used plant–fruit fly interactions to investigate the influence of the proportion and species richness of exotic host plants on the species richness of herbivorous insects. We also tested whether the phylogenetic diversity of host plants increases when the number of exotic plant species increases. 3. We found that the species richness of fruit flies is more accurately predicted by the richness of native host plants than by total plant species richness (including both native and exotic species). The proportion of exotic host species and the phylogenetic diversity of host plants had negative and positive effects, respectively, on the species richness of fruit flies. 4. Our findings suggest that a positive effect of plant richness on herbivore richness occurs only when an increase in plant diversity involves plant species with which native herbivores share some evolutionary history.     

Procedure for separating the selection effect from other effects in diversity–productivity relationship

In a greenhouse pot experiment we cultivated six meadow species in a replacement series design. The plants were grown at two sowing densities in monocultures and all possible species combinations. Our aim was to separate the selection effect from other diversity effects. This distinction is based on the notion that true overyielding is not a consequence of the selection effect. We suggest a hierarchical procedure, which is based on a repeated division of samples into the pots with the most productive species present and missing. Overyielding can be then demonstrated by a positive dependence of productivity on species richness in the subsets with the most productive species present. Although we found a strong dependence of biomass on species richness in the entire data set, the hierarchical method revealed no evidence of overyielding. Above-ground biomass in a monoculture was a good predictor of species success in a species mix.     

Species-pool hypothesis: Limits to its testing

The species richness of a community depends both on the pool of available species and on biotic mechanisms that lead to the exclusion of some of the species from a community. The method suggested byPärtel et al.,Oikos 75: 111–117, 1996 to test the effect of species-pool size on the species richness of a community is discussed in this paper. This method is based on the calculation of a correlation between the actual species richness of a community and the actual species-pool size, and compares the correlation found in the data with that simulated by a null model. In the null model, the species richness has a uniform distribution between zero and the size of the species pool. A correlation significantly higher than that in the null model is interpreted as evidence of the greater role of species pool than of local interactions in formation of community species richness (Zobel,Folia Geobot. 36: 3–8, 2001). It is shown that the interpretation of discrepancies between the null model and reality is difficult or impossible, because: (1) a null model with a uniform distribution of species richness is unrealistic, (2) both models based on the random selection of species from a species pool and models that include competitive interactions in the community predict a higher positive correlation of species richness and size of the species pool than the null model, and (3) local species richness might be affected by species-pool size, but a large species-pool size can also be a result of high local species richness. Caution is urged when interpreting the analyses based on the size of the filtered species-pool size.     

What do the biodiversity experiments tell us about consequences of plant species loss in the real world?

Experiments where the diversity of species assemblage is manipulated are sometimes used to predict the consequences of species loss from real communities. However, their design corresponds to a random selection of the lost species. There are three main factors that limit species richness: harshness of the environment, competitive exclusion, and species pool limitation. Species loss is usually caused by increasing effects of these factors. In the first two cases, the species that are excluded are highly non-random subsets of the potential species set, and consequently, the predictions based on random selection of the lost species might be misleading. The data show that the least productive species are those being recently excluded from temperate grasslands and consequently, species loss is not connected with decline of productivity. The concurrent species loss in many communities, however, means also a reduction of the available diaspore pool on a landscape scale, and could result in increased species pool limitation in other communities.     

The population genetics of host specificity: genetic differentiation in dove lice (Insecta: Phthiraptera)

Some species of parasites occur on a wide range of hosts while others are restricted to one or a few host species. The host specificity of a parasite species is determined, in part, by its ability to disperse between host species. Dispersal limitations can be studied by exploring the genetic structure of parasite populations both within a single species of host and across multiple host species. In this study we examined the genetic structure in the mitochondrial cytochrome oxidase I (COI) gene of two genera of lice (Insecta: Phthiraptera) occurring on multiple sympatric species of doves in southern North and Central America. One genus, Columbicola, is generally less host-specific than the other, Physconelloides. For both genera we identified substantial genetic differentiation between populations of conspecific lice on different host species, generally 10-20% sequence divergence. This level of divergence is in the range of that often observed between species of these two genera. We used nested clade analysis to explore fine scale genetic structure within species of these feather lice. We found that species of Physconelloides exhibited more genetic structure, both among hosts and among geographical localities, than did species of Columbicola. In many cases, single haplotypes within species of Columbicola are distributed on multiple host species. Thus, the population genetic structure of species of Physconelloides reveals evidence of geographical differentiation on top of high host species specificity. Underlying differences in dispersal biology probably explain the differences in population genetic structure that we observed between Columbicola and Physconelloides.     

Why Some Plant Species Are Rare

Biodiversity, including plant species diversity, is threatened worldwide as a result of anthropogenic pressures such as an increase of pollutants and climate change. Rare species in particular are on the verge of becoming extinct. It is still unclear as to why some plant species are rare and others are not. Are they rare due to: intrinsic reasons, dispersal capacity, the effects of management or abiotic circumstances? Habitat preference of rare plant species may play an important role in determining why some species are rare. Based on an extensive data set of soil parameters we investigated if rarity is due to a narrow habitat preference for abiotic soil parameters. For 23 different abiotic soil parameters, of which the most influential were groundwater-table, soil-pH and nutrient-contents, we estimated species responses for common and rare species. Based on the responses per species we calculated the range of occurrence, the range between the 5 and 95 percentile of the response curve giving the habitat preference. Subsequently, we calculated the average response range for common and rare species. In addition, we designed a new graphic in order to provide a better means for presentation of the results. The habitat preferences of rare species for abiotic soil conditions are significantly narrower than for common species. Twenty of the twenty-three abiotic parameters showed on average significantly narrower habitat preferences for rare species than for common species; none of the abiotic parameters showed on average a narrower habitat preference for common species. The results have major implications for the conservation of rare plant species; accordingly management and nature development should be focussed on the maintenance and creation of a broad range of environmental conditions, so that the requirements of rare species are met. The conservation of (abiotic) gradients within ecosystems is particularly important for preserving rare species.     

Does energy availability influence classical patterns of spatial variation in exotic species richness?

Aim At macroecological scales, exotic species richness is frequently positively correlated with human population density. Such patterns are typically thought to arise because high human densities are associated with increased introduction effort and/or habitat modification and disturbance. Exotic and native species richness are also frequently positively correlated, although the causal mechanisms remain unclear. Energy availability frequently explains much of the variation in species richness and we test whether such species–energy relationships may influence the relationships of exotic species richness with human population density and native species richness. Location Great Britain. Methods We first investigate how spatial variation in the distributions of the 10 exotic bird species is related to energy availability. We then model exotic species richness using native avian species richness, human population density and energy availability as predictors. Species richness is modelled using two sets of models: one assumes independent errors and the other takes spatial correlation into account. Results The probability of each exotic species occurring, in a 10‐km quadrat, increases with energy availability. Exotic species richness is positively correlated with energy availability, human population density and native species richness in univariate tests. When taking energy availability into account, exotic species richness is negligibly influenced by human population density, but remains positively associated with native species richness. Main conclusions We provide one of the few demonstrations that energy availability exerts a strong positive influence on exotic species richness. Within our data, the positive relationship between exotic species richness and human population density probably arises because both variables increase with energy availability, and may be independent of the influence of human density on the probability of establishment. Positive correlations between exotic and native species richness remain when controlling for the influence of energy on species richness. The relevance of such a finding to the debate on the relationship between diversity and invasibility is discussed.     

Effect of Hay Transfer on Long-Term Establishment of Vegetation and Grasshoppers on Former Arable Fields

The transfer of seed‐containing hay is a restoration measure for the introduction of plant species of local provenance. We investigated the effect of hay transfer on species richness and on long‐term establishment of target plant and grasshopper species on former arable fields with and without topsoil removal in comparison to reference sites in a nature reserve. Plant species richness, the number of target plant species, and Red List plant species were significantly positively affected by hay transfer, both on the scale of whole restoration fields and on permanent plots of 4 m². Eight years after the start of the restoration, only few of the transferred plant species had disappeared and some target species were newly found. Grasshoppers were affected not by hay transfer but by topsoil removal. The proportion of target grasshopper and plant species and Red List grasshopper species was higher on topsoil removal sites with low standing crop and high cover of bare soil than on sites without soil removal. On topsoil removal sites without hay, however, plant species richness was very low because of the slow natural dispersal of the target species. Vegetation and grasshopper communities still differed between restoration fields and the nature reserve. Nevertheless, our results indicate that the transfer of autochthonous seed‐containing hay is a successful method to establish species‐rich grasslands with a high proportion of target species.     

The classification, evolution and biology of the Costa Rican species of Cryptophion (Hymenoptera: Ichneumonidae)

The six known Costa Rican species of the campoplegine ichneumonid genus Cryptophion Viereck are described and keyed. The distribution of species throughout Costa Rica is detailed based on data gathered by an intensive Malaise trap survey of the ichneumonids of the country. Five new species are recognized: C. espinozai. C. guillermoi, C. manueli. C. moragai and C. tickelli , and a sixth, C. inaequalipes (Cresson) redescribed. The monophyly of the genus is demonstrated and the phylogeny of the Costa Rican species reconstructed. Host relationships have been established for all species in Costa Rica; they develop as koinobiont endoparasitoids of first to third instar larvae of Sphingidae or Saturniidae. Analysis of the host relationships from a phylogenetic perspective suggested that the genus first evolved using macroglossine sphingid larvae feeding on rubiaceous understorey plants as hosts, and subsequently diversified to utilize sphingine sphingids and ceratocampine saturniids feeding on a variety of food-plants. Most species appear to be monophagous and oligophagy is apparently a derived feature of one sister-species pair, C. espinozai and C. manueli . In Santa Rosa National Park, Costa Rica, only a small proportion of the species of Saturniidae and Sphingidae present are used as hosts by Cryptophion species. No one host species is parasitized by more than one Cryptophion species. No species of Cryptophion is known to parasitize more than one host species feeding on any one plant species. The monophagous species C. inaequalipes is only known to develop in its sphingid host when that host is feeding on one of its two alternative food-plants.     

Abundance patterns and coexistence processes in communities of fleas parasitic on small mammals

The abundance of a given species in a community is likely to depend on both the total abundance and diversity of other species making up that community. A large number of co-occurring individuals or co-occurring species may decrease the abundance of any given species via diffuse competition; however, indirect interactions among many co-occurring species can have positive effects on a focal species. The existence of diffuse competition and facilitation remain difficult to demonstrate in natural communities. Here, we use data on communities of fleas ectoparasitic on small mammals from 27 distinct geographical regions to test whether the abundance of any given flea species in a community is affected by either the total abundance of all other co-occurring flea species, or the species richness and/or taxonomic diversity of the flea community. At all scales of analysis, i.e. whether we compared the same flea species on different host species, or different flea species, two consistent results emerged. First, the abundance of a given flea species correlates positively with the total abundance of all other co-occurring flea species in the community. Second, the abundance of any given flea species correlates negatively with either the species richness or taxonomic diversity of the flea community. The results do not support the existence of diffuse competition in these assemblages, because the more individuals of other flea species are present on a host population, the more individuals of the focal species are there as well. Instead, we propose explanations involving either apparent facilitation among flea species via suppression of host immune defenses, or niche filtering processes acting to restrict the taxonomic composition and abundance of flea assemblages.     

A (not-so-radical) solution to the species problem

What are species? One popular answer is that species are individuals. Here I develop another approach to thinking about species, an approach based on the notion of a lineage. A lineage is a sequence of reproducing entities, individuated in terms of its components. I argue that one can conceive of species as groups of lineages, either organism lineages or population lineages. Conceiving of species as groups of lineages resolves the problems that the individual conception of species is supposed to resolve. It has added the virtue of focusing attention on the characteristic of species that is most relevant to understanding their role in evolutionary processes, namely, the lineage structure of species.     

Keystone species and vulnerable species in ecological communities: strong or weak interactors?

The loss of a species from an ecological community can trigger a cascade of secondary extinctions. The probability of secondary extinction to take place and the number of secondary extinctions are likely to depend on the characteristics of the species that is lost--the strength of its interactions with other species--as well as on the distribution of interaction strengths in the whole community. Analysing the effects of species loss in model communities we found that removal of the following species categories triggered, on average, the largest number of secondary extinctions: (a) rare species interacting strongly with many consumers, (b) abundant basal species interacting weakly with their consumers and (c) abundant intermediate species interacting strongly with many resources. We also found that the keystone status of a species with given characteristics was context dependent, that is, dependent on the structure of the community where it was embedded. Species vulnerable to secondary extinctions were mainly species interacting weakly with their resources and species interacting strongly with their consumers.     

How many species are infected with Wolbachia? – a statistical analysis of current data

Wolbachia are intracellular bacteria found in many species of arthropods and nematodes. They manipulate the reproduction of their arthropod hosts in various ways, may play a role in host speciation and have potential applications in biological pest control. Estimates suggest that at least 20% of all insect species are infected with Wolbachia . These estimates result from several Wolbachia screenings in which numerous species were tested for infection; however, tests were mostly performed on only one to two individuals per species. The actual percent of species infected will depend on the distribution of infection frequencies among species. We present a meta-analysis that estimates percentage of infected species based on data on the distribution of infection levels among species. We used a beta-binomial model that describes the distribution of infection frequencies of Wolbachia , shedding light on the overall infection rate as well as on the infection frequency within species. Our main findings are that (1) the proportion of Wolbachia -infected species is estimated to be 66%, and that (2) within species the infection frequency follows a 'most-or-few' infection pattern in a sense that the Wolbachia infection frequency within one species is typically either very high (>90%) or very low (<10%).     

Mechanisms behind positive diversity effects on ecosystem functioning: testing the facilitation and interference hypotheses

Little is known about the mechanisms behind positive effects of species richness on ecosystem functioning. In a previous study that showed a positive effect of aquatic detritivore species richness on leaf litter breakdown (process) rates, we proposed that facilitation and release from intra-specific interference were the two most likely mechanisms. To test the interference hypothesis, we performed an experiment using three densities of each of three detritivore species and found varying effects on leaf breakdown rates across species: one species showed no effect, one a positive, marginally insignificant, effect, and a third species showed a significant, positive effect of decreasing density. The density (interference) effect thus partly explained the results from our previous study. The facilitation hypothesis was tested by sequentially introducing and removing two species. We predicted that, if this hypothesis were true, facilitation would be expressed in higher process rates than when replacing with individuals of the same species. We found that process rate per unit biomass did increase when one species was introduced after the other species, while the opposite sequence did not show any increase. Hence, this result was also confirmative of our previous results. Therefore, we conclude that both intra-specific interference and inter-specific facilitation may explain the positive effect of species richness observed in our system. Since many species exhibit intra-specific interference that inhibits foraging efficiency, this may be a general mechanism generating effects of species richness per se. If facilitation is unidirectional, or if it involves few species, it is more likely to be species specific with species identities being more important than species richness per se. We conclude that species loss may be expected to have negative consequences on ecosystem functioning if anyspecies is lost, with additional effects in the event of losing "facilitator" species.     

The species pool hypothesis: the necessity to shift emphasis

The species pool of a biological community is determined as a group of species that inhabit some area and potentially can be included in a given community. The species pool hypothesis, i.e. the assumption that the size of species pool strongly influences species richness of local community can be confirmed if there is positive linear relationship between these two variables. The results of hypothesis testing however are not obvious. For example, correlation between local richness and species pool size can be caused by their dependence on the third variable--capacity of environment. It seems that in case of decreasing area occupied by local community the environmental conditions become more important than species pool size. If that is true, the influence of species pool on local species richness is not significant. However one can estimate the degree of unsaturation of species pool on the basis of relationships between the number of species in small locations occupied by similar local communities and their species pool. We think, that study of local and regional species richness should shift the emphasis--from the analysis of species pool influence on local community richness to the estimation of historical, ecological and anthropogenic factors in variation of species pool size. The local species richness should be considered rather as a tool (allowing to compare the species capacity of biological communities), than as an object of such study.