Abundance, spatial variance and occupancy: arthropod species distribution in the Azores

1. The positive abundance-occupancy and abundance-variance relationships are two of the most widely documented patterns in population and community ecology. 2. Recently, a general model has been proposed linking the mean abundance, the spatial variance in abundance, and the occupancy of species. A striking feature of this model is that it consists explicitly of the three variables abundance, variance and occupancy, and no extra parameters are involved. However, little is known about how well the model performs. 3. Here, we show that the abundance-variance-occupancy model fits extremely well to data on the abundance, variance and occupancy of a large number of arthropod species in natural forest patches in the Azores, at three spatial extents, and distinguishing between species of different colonization status. Indeed, virtually all variation about the bivariate abundance-occupancy and abundance-variance relationships is effectively explained by the third missing variable (variance in abundance in the case of the abundance-occupancy relationship, and occupancy in the case of the abundance-variance relationship). 4. Introduced species tend to exhibit lower densities, less spatial variance in these densities, and occupy fewer sites than native and endemic species. None the less, they all lie on the same bivariate abundance-occupancy and abundance-variance, and trivariate abundance-variance-occupancy, relationships. 5. Density, spatial variance in density, and occupancy appear to be all the things one needs to know to describe much of the spatial distribution of species.     

Modelling population redistribution in a leaf beetle: an evaluation of alternative dispersal functions

1. Dispersal is a fundamental ecological process, so spatial models require realistic dispersal kernels. We compare five different forms for the dispersal kernel of the tansy beetle Chrysolina graminis moving between patches of its host-plant (tansy Tanacetum vulgare) in a riparian landscape. 2. Multi-patch mark-recapture data were collected every 2 weeks over 2 years within a large network of patches and from 2226 beetles. Dispersal was common (28.4% of 880 recaptures after a fortnight) and was more likely over longer intervals, out of small patches, for females and during flooding. Interpatch movement rates did not differ between years and exhibited no density dependence. Dispersal distances were similar for males and females, in both years and over all intervals, with a median dispersal distance of just 9.8 m, although a maximum of 856 m was recorded. 3. A model of dispersal, where patches competed for dispersers based on their size and distance from the beetle's source patch (scaled by the dispersal kernel) was fitted to the field data with a maximum likelihood procedure and each of five alternative kernels. The best fitting had relatively extended tails of long-distance dispersal, while Gaussian and negative exponential kernels performed worst. 4. The model suggests that females disperse more commonly than males and that both are strongly attracted to large patches but do not differ between years, which are consistent with the empirical results. Model-predicted emigration and immigration rates and dispersal phenologies match those observed, suggesting that the model captured the major drivers of tansy beetle dispersal. 5. Although negative exponential and Gaussian kernels are widely used for their simplicity, we suggest that these should not be the models of automatic choice, and that fat-tailed kernels with relatively higher proportions of long-distance dispersal may be more realistic.     

Testing metapopulation dynamics using genetic, demographic and ecological data

The metapopulation concept is a cornerstone in the recent history of ecology and evolution. However, determining whether a natural system fits a metapopulation model is a complex issue. Extinction-colonization dynamics are indeed often difficult to quantify because species detectability is not always 100%, resulting in an imperfect record of extinctions. Here, we explore whether combining population genetics with demographic and ecological surveys can yield more realistic estimates of metapopulation dynamics. We apply this approach to the freshwater snail Drepanotrema depressissimum in a fragmented landscape of tropical ponds. In addition to studying correlations between genetic diversity and demographical or ecological characteristics, we undertake, for the first time, a detailed search for genetic signatures of extinction-recolonization events using temporal changes in allele frequencies within sites. Surprisingly, genetic data indicate that extinction is much rarer than suggested by demographic surveys. Consequently, this system is better described as a set of populations with different sizes and immigration rates than as a true metapopulation. We identify several cases of apparent extinction owing to nondetection of low-density populations, and of aestivating individuals in desiccated ponds. More generally, we observed a frequent mismatch between genetic and demographical/ecological information at small spatial and temporal scales. We discuss the causes of these discrepancies and show how these two types of data provide complementary information on population dynamics and history, especially when temporal genetic samples are available.     

Resistance to complete submergence in Rumex species with different life histories: the influence of plant size and light

Resistance to complete submergence was tested in three Rumex species that occur in the Dutch river forelands. The species differ in both habitat and life history characteristics. The annual or biennial R. maritimus and the biennial or short lived perennial R. palustris grow on frequently flooded mud flats of low elevation, while the perennial R. thyrsiflorus can be found on dykes and river dunes that are seldom flooded. The flooding characteristics of the habitats of the three species were determined. These data were used to design experiments to determine the survival and biomass development of the three species during submergence and the influence of plant size and light level on these parameters. It was shown in all three species that plants submerged during daytime were much more resistant to flooding than those submerged at night. This is most probably due to the generation of oxygen or carbohydrates by underwater photosynthesis. Mature plants of the three species showed higher survival after submergence than juvenile plants, which might be caused by higher carbohydrate levels in the taproots of mature plants. In addition, the three species clearly differed in survival and biomass development during submergence. Rumex thyrsiflorus , the species least subjected to flooding, is least tolerant to complete submergence. Rumex maritimus , which can avoid the floods by having a short life cycle, is less tolerant to submergence than R. palustris , which has to survive the floods as a vegetative plant. It was noted that some plants that survived the flooding period itself, still died in the following period of drained conditions, possibly due to post-anoxic injury.     

Body size, egg size, and their interspecific relationships with ecological and life history traits in butterflies (Lepidoptera: Papilionoidea, Hesperioidea)

The interspecific relationships between egg size and body size in butterflies (Papilionoidea and Hesperiidae), and between size and egg and larval development time, larval trophic specificity, foodplant structure, climate, and phenology were investigated based on a sample of more than 1180 species. The independent contrasts mediod was used to avoid taxonomy-dependent results. Egg size is allometrically related to adult wing length by a slope of 0.43. Based on a subset of species, fecundity is correlated to adult body size, and there is evidence for a compromise between egg number and egg size (relative to adult size) across species. Butterfly size increases in correlation to the mean annual temperature of me species geographic range, but decreases in relation to increased aridity (or die length of the dry season). Larger butterflies tend to have longer larval development times, use large or structurally complex host plants, and are more likely to lay their eggs in batches, irrespective of climate. Larger eggs tend to develop more slowly, and give rise to larvae with longer developmental periods that will result in larger adults. No evidence was found to support a relationship between butterfly body size and polyphagy. A complex pattern of interrelationships links body size (and egg size) to other traits, although correlations other than mat between egg size and body size are generally low. The results suggest the necessity of separating climate and seasonality into components that are relevant to insect life histories in comparative studies.     

Through a genome, darkly: comparative analysis of plant chromosomal DNA

Plant nuclear genomes encompass a wide range of variation in size and nucleotide composition with diverse arrangements of chromosomal segments, repetitive sequences and distribution of genes. Comparative genomic analysis may be undertaken at different levels of organisation, which are reflected in this review, together with a focus on the genetic and functional significance of the observed variation. Patterns of genome organisation have been revealed which reflect the different underlying mechanisms and constraints driving change. Thus comparative issues of genome size, nucleotide sequence composition and genome heterogeneity are provided as a background to understanding the different levels of segmental and repetitive sequence duplication and distribution of genes. The extent of synteny and collinearity revealed by recent genetic and sequence comparisons is discussed, together with a consideration of problems associated with such analyses. The possible origins and mechanisms of variation in genome size and organisation are covered, including the prevalence of duplication at different levels of organisation. The likely genetic, functional and adaptive consequences of replicated loci are discussed with evidence from comparative studies. The scope for comparative analysis of epigenetic plant genome variation is considered. Finally, opportunities for applying comparative genomics to isolating genes and understanding complex crop genomes are addressed.     

Plant presence and species combination, but not diversity, influence denitrifier activity and the composition of nirK-type denitrifier communities in grassland soil

To explore potential links between plant communities, soil denitrifiers and denitrifier function, the impact of presence, diversity (i.e. species richness) and plant combination on nirK -type denitrifier community composition and on denitrifier activity was studied in artificial grassland plant assemblages over two consecutive years. Mesocosms containing zero, four and eight species and different combinations of two species were set up. Differences in denitrifier community composition were analysed by canonical correspondence analyses following terminal restriction fragment length polymorphism analysis of PCR-amplified nirK gene fragments coding for the copper-containing nitrite reductase. As a measure of denitrifier function, denitrifier enzyme activity (DEA) was determined in the soil samples. The presence as well as the combination of plants and sampling time, but not plant diversity, affected the composition of the nirK -type denitrifier community and DEA. Denitrifier activity significantly increased in the presence of plants, especially when they were growing during summer and autumn. Overall, we found a strong and direct linkage of denitrifier community composition and functioning, but also that plants had additional effects on denitrifier function that could not be solely explained by their effects on nirK -type denitrifier community composition.     

Disentangling determinants of egg size in the Geometridae (Lepidoptera) using an advanced phylogenetic comparative method

We present a phylogenetic comparative study assessing the evolutionary determinants of egg size in the moth family Geometridae. These moths were found to show a strong negative allometric relationship between egg size and maternal body size. Using recently developed comparative methods based on an Ornstein-Uhlenbeck process, we show that maternal body size explains over half the variation in egg size. However, other determinants are less clear: ecological factors, previously hypothesized to affect egg size, were not found to have a considerable influence in the Geometridae. The limited role of such third factors suggests a direct causal link between egg size and body size rather than an indirect correlation mediated by some ecological factors. Notably, no large geometrid species lay small eggs. This pattern suggests that maternal body size poses a physical constraint on egg size, but within these limits, there appears to be a rather invariable selection for larger eggs.     

Repeated evolution of a reproductive polyphenism in plants is strongly associated with bilateral flower symmetry

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