Multiple socioecological factors influence timing of natal dispersal in kangaroo rats

Natal dispersal is an important event in the life history of many species. Timing of natal dispersal can influence survivorship and subsequent reproductive success. A variety of individual proximal factors determine if and when offspring disperse from the natal territory by influencing the costs of dispersing and the benefits of delaying dispersal. I examined the influence of multiple factors on dispersal age in the banner-tailed kangaroo rat (Dipodomys spectabilis), a solitary species lacking extreme sex-biased dispersal. I used an information theoretic approach to compare Cox proportional hazards regression models of dispersal age for 121 offspring over a 3-year period consisting of low and high population densities. The top-ranked models indicated that dispersal age was influenced by a combination of socioecological factors related to resource competition, environmental conditions, kin competition, and a lesser extent sex. Circumstances that likely reduced the probability of successful dispersal such as intense resource competition at high population density and being born earlier in the breeding season when environmental conditions were poor lead to longer delays in natal dispersal. Offspring in larger litters also dispersed earlier possibly to avoid competition with kin. Sex was weakly supported in top models but may only influence dispersal age at high population densities. These results suggest that the proximal factors that trigger dispersal are influenced by a combination of multiple effects related to the costs of dispersing and the benefits of remaining at home, even in species that do not form long-term social groups or have extreme sex-biased dispersal.     

Conserved male-specific cuticular hydrocarbon patterns in the trap-jaw ant Odontomachus brunneus

Cuticular hydrocarbons have been identified as the source of sex-recognition signals for many insects, but for social insects, specifically ants, cuticular hydrocarbon profiles of males are often ignored. This study reports male-specific cuticular hydrocarbon patterns for the trap-jaw ant Odontomachus brunneus. Analysis of samples from four Florida populations demonstrated that male-specific overabundance of four hydrocarbons is conserved across populations despite population-level divergence of the remainder of the profile. In addition, hydrocarbon patterns unique to adult males were not present on the cuticle of final instar male larvae, indicating that male-specific profiles arise late in development. The pattern of an abundant subset of conserved cuticular hydrocarbons characteristic of males across divergent populations was compared to earlier findings of the conservation of fertility signals of females across these same populations.     

<Emphasis Type="Italic">Corymbia</Emphasis> phloem phenolics,tannins and terpenoids: interactions with a cerambycid borer

Plant secondary chemistry mediates the ability of herbivores to locate, accept and survive on potential host plants. We examined the relationship between attack by the cerambycid beetle Phoracantha solida and the chemistry of the secondary phloem (inner bark) of two differentially attacked plantation forestry taxa, Corymbia variegata and its hybrid with C. torelliana. We hypothesised that this differential rate of attack may have to do with differences in secondary chemistry between the taxa. We found differences in the bark chemistry of the taxa, both with respect to phenolic compounds and terpenoids. We could detect no difference between bored and non-bored C. variegata trees (the less preferred, but co-evolved host). Hybrid trees were not different in levels of total polyphenols, flavanols or terpenes according to attack status, but acetone extracts were significantly different between bored and non-bored trees. We propose that variations in the bark chemistry explain the differential attack rate between C. variegata and the hybrid hosts.     

A hidden species-area curve

The species-area curve, which describes the relationship between the number of observed species in a geographical region and the area of the region, plays a central role in biogeography. Beyond its scientific importance, the species-area curve is commonly used to assess the loss of species due to habitat loss. When the species-area curve is estimated from spatial samples, the existence of species with low or highly spatially variable abundance exaggerates the true rate at which species accumulate with area. Here, a hidden species-area curve is defined that accounts for this sampling effect and its estimation by maximum likelihood is outlined. Both the species-area curve and the hidden species-area curve are conditioned on the observed species list; thus the analysis does not depend on the total number of observed and unobserved species, that is, species richness. The method is tested by sub-sampling some tropical forest data and found to work well. It is also applied to a classic data set from the deep sea. For these data, accounting for this sampling effect has a large impact.     

Prospective evaluation of designs for analysis of variance without knowledge of effect sizes

Estimation of design power requires knowledge of treatment effect size and error variance, which are often unavailable for ecological studies. In the absence of prior information on these parameters, investigators can compare an alternative to a reference design for the same treatment(s) in terms of its precision at equal sensitivity. This measure of relative performance calculates the fractional error variance allowed of the alternative for it to just match the power of the reference. Although first suggested as a design tool in the 1950s, it has received little analysis and no uptake by environmental scientists or ecologists. We calibrate relative performance against the better known criterion of relative efficiency, in order to reveal its unique advantage in controlling sensitivity when considering the precision of estimates. The two measures differ strongly for designs with low replication. For any given design, relative performance at least doubles with each doubling of effective sample size. We show that relative performance is robustly approximated by the ratio of reference to alternative $\alpha $ quantiles of the $F$ distribution, multiplied by the ratio of alternative to reference effective sample sizes. The proxy is easy to calculate, and consistent with exact measures. Approximate or exact measurement of relative performance serves a useful purpose in enumerating trade-offs between error variance and error degrees of freedom when considering whether to block random variation or to sample from a more or less restricted domain.     

Bridging gaps in demographic analysis with phylogenetic imputation

Phylogenetically informed imputation methods have rarely been applied to estimate missing values in demographic data but may be a powerful tool for reconstructing vital rates of survival, maturation, and fecundity for species of conservation concern. Imputed vital rates could be used to parameterize demographic models to explore how populations respond when vital rates are perturbed. We used standardized vital rate estimates for 50 bird species to assess the use of phylogenetic imputation to fill gaps in demographic data. We calculated imputation accuracy for vital rates of focal species excluded from the data set either singly or in combination and with and without phylogeny, body mass, and life-history trait data. We used imputed vital rates to calculate demographic metrics, including generation time, to validate the use of imputation in demographic analyses. Covariance among vital rates and other trait data provided a strong basis to guide imputation of missing vital rates in birds, even in the absence of phylogenetic information. Mean NRMSE for null and phylogenetic models differed by <0.01 except when no vital rates were available or for vital rates with high phylogenetic signal (Pagel's λ > 0.8). In these cases, including body mass and life-history trait data compensated for lack of phylogenetic information: mean normalized root mean square error (NRMSE) for null and phylogenetic models differed by <0.01 for adult survival and <0.04 for maturation rate. Estimates of demographic metrics were sensitive to the accuracy of imputed vital rates. For example, mean error in generation time doubled in response to inaccurate estimates of maturation time. Accurate demographic data and metrics, such as generation time, are needed to inform conservation planning processes, for example through International Union for Conservation of Nature Red List assessments and population viability analysis. Imputed vital rates could be useful in this context but, as for any estimated model parameters, awareness of the sensitivities of demographic model outputs to the imputed vital rates is essential.     

Preliminary assessment of surface soil lead concentrations in Melbourne,Australia

Urban soils in many cities have been found to be contaminated with lead from past usage of leaded petrol, deteriorating lead-based exterior paints and industrial sources. Currently, the spatial distribution of soil lead concentrations in the Melbourne metropolitan area is unknown. The objective of this study was to perform a preliminary assessment of the spatial distributions of the surface soil lead (Pb) concentrations in the Melbourne metropolitan area, Australia. Fifty-eight surface soil samples were collected at a depth of 0–2 cm along three linear transects oriented across the Melbourne metropolitan area. Surface soil samples were also collected at a higher density in five Melbourne suburbs. Soil cores (0–50 cm) were collected in four locations, soil transects were collected at intervals with distance away from the roadway (0–50 m) in two inner city parks, and one control soil sample was collected in a rural setting. The median soil Pb concentration of the soil transect samples was 173 mg/kg (range 32–710 mg/kg), and the median soil Pb concentration of the five suburbs was 69 mg/kg (range 9–1750 mg/kg). The suburb of Footscray had the highest soil Pb concentration with a median soil Pb concentration of 192 mg/kg (range 40–1750 mg/kg). Soil Pb concentrations were generally higher nearest the centre of the Melbourne metropolitan area and in the west of Melbourne and lower in the outer suburbs to the east and north of the city centre. Soil Pb concentrations decreased with distance from roadways in the two transects taken from urban parks, and soil lead decreased with depth in the four soil cores. The soil Pb concentrations in the Melbourne metropolitan area appear to be lower than soil lead concentrations observed in inner city areas of Sydney New South Wales (NSW) and Newcastle NSW. The spatial extent of the soil Pb hazard remains undefined in portions of the Melbourne metropolitan area.     

Non-indigenous macroalga hosts different epiphytic assemblages to conspecific natives in southeast Australia

Codium fragile ssp. tomentosoides is one of the five most invasive macroalgae worldwide. We compared epiphytic assemblages between native (ssp. tasmanicum and ssp. novae-zelandiae combined) and non-indigenous (ssp. tomentosoides) subspecies of Codium fragile on three rocky intertidal shores in southeast Australia. Twelve species of epiphytes covered up to 20% of the surface of both native and non-indigenous Codium subspecies, but none were unique to either subspecies. For C. fragile subspecies, epiphytic cover declined from the lower (older) parts of the thallus to the branch tips. The abundance and species composition of epiphytes differed significantly between native and non-indigenous C. fragile, but differences varied among rocky shores. Results demonstrate that non-indigenous C. fragile does not play a functionally similar role to closely related native Codium subspecies. Spatial differences in epiphytic assemblages between Codium subspecies among rocky shores demonstrate that effects of non-indigenous species may be strongly location specific. Thus, our study emphasises the need to investigate variation in the effects of invaders across the regional landscape.     

Estimating dispersal potential for marine larvae: dynamic models applied to scleractinian corals

Dispersal influences ecological dynamics, evolution, biogeography, and biodiversity conservation, but models of larval dispersal in marine organisms make simplifying assumptions that are likely to approximate poorly the temporal dynamics of larval survival and capacity for settlement. In particular, larval mortality rates are typically assumed to be constant throughout larval life; and all larvae are frequently assumed to acquire and lose competence at the same time. To improve upon these assumptions, we here develop simple models of dispersal potential that incorporate rates of mortality, and acquisition and loss of settlement competence. We fit these models to empirical competence and survival data for five scleractinian coral species, to test the models' ability to characterize empirical survival and competence patterns, and to estimate the dispersal potential implied by those patterns. The models fit the data well, incorporating qualitative features of competence and survival that traditional approaches to modeling dispersal do not, with important implications for dispersal potential. Most notably, there was high within-cohort variation in the duration of the competent period in all species, and this variation increases both self-recruitment and long-distance dispersal compared with models assuming a fixed competent period. These findings help to explain the seeming paradox of high genetic population structure, coupled with large geographic range size, observed in many coral species. More broadly, our approach offers a way to parsimoniously account for variation in competence dynamics in dispersal models, a phenomenon that our results suggest has important effects on patterns of connectivity in marine metapopulations.