Morph-specific differences in reproductive success in the distylous Primula veris in a context of habitat fragmentation

Heterostylous self-incompatible plant species are particularly sensitive to habitat fragmentation and to disruption of pollination processes because of the need of intermorph cross-pollination for producing seeds. Heterostyly is characterized by sexual polymorphism through the occurrence of two (distyly) or three (tristyly) morph types that differ in floral traits (style length and anther position). We examined whether the long-styled (pin) and short-styled (thrum) morph types show differences in reproductive components and responses to habitat fragmentation in the distylous, self-incompatible perennial herb Primula veris. We documented reproductive components for pin and thrum individuals and their relationships with population size, plant density and morph ratio (pin frequency), in nine populations from Flanders (northern Belgium) located in fragmented habitats of the intensively used agricultural landscape. Seed abortion increased in small populations as a result of inbreeding depression. Fruit set increased with plant density. Seed set was positively related to pin proportion. Seed set was higher for pin than thrum in small populations, but lower in large populations. Two hypotheses can be considered to explain these morph-specific differences: a pollen transfer asymmetry, and a reproductive advantage for the partially self-compatible pin morph. Morph types appear to respond differently to habitat fragmentation constraints. A floral morph type showing partial self-compatibility may be favored in populations under pollination failure, because it can increase reproductive success and mating opportunities through intramorph crosses.     

The estimation of population size,migration rates and survival in a stratified population

Summary Estimates of survival, migration rates, and population size are developed for a triple catch marking experiment onn (n>-2) areas with migration among all areas and death in all areas occurring, but no recruitment (birth). This repressents the extension to three sampling times of the method ofChapman andJunge (1956) for estimates in a stratified population. The method is further extented to allow for ‘losses on capture’.     

Architecture,seasonal growth and interference in three grass species with different flowering phenologies in a tropical savanna

In a field experiment we studied the relationships between architecture, seasonal aerial biomass growth and interference from neighbours in three savanna grass species differing in time of flowering: a precocious species (Elyonurus adustus), an early species (Leptocoryphium lanatum) and a late species (Andropogon semiberbis). To detect the neighbour interference upon architecture and seasonal regrowth, we measured the leaf interception of a plane at different heights, when the species grew alone, in pairs and in groups of three. Although the three species differed widely in the spatial and temporal patterns of occupation of above-ground space, important levels of interference among neighbouring plants were detected. The species differ in: a) the pattern of space occupation when growing alone; b) the magnitude of the interference effect by the companion species; c) the change in pattern as a consequence of interference; d) their reaction to fire.The interference is not symmetric and it is not related to phenological similarities based on flowering season.L. lanatum was both the most impaired and the least impairing of the three species and the opposite is valid forA. semiberbis. The basal species (E. adustus andL. lanatum) showed a higher potential to produce leaf surface during the growth season than the tall species (A. semiberbis), but the stronger interference from the latter tended to equate their growth when the species grew in mixtures.In all three species interference led to a reduction of the growing period. Based on the analysis of growth at the various heights and the architectural peculiarities of the species we concluded that neighbour interference is probably reducing both tillering and leafing in the basal species but only culm elongation and leafing in the erect species.The species also differed in their reaction to fire.A. semiberbis andE. adustus showed a pulse of regrowth after the passing of fire which is missing inL. lanatum. In all cases the neighbour interference affected both the intensity of growth and the length of the growing period, but it did not affect either the height of the plants nor the reproductive phenology of these three grass species.     

A Comparison of Population Size Estimators under the Truncated Count Model With and Without Allowance for Contaminations

The purpose of the study is to estimate the population size under a homogeneous truncated count model and under model contaminations via the Horvitz‐Thompson approach on the basis of a count capture‐recapture experiment. The proposed estimator is based on a mixture of zero‐truncated Poisson distributions. The benefit of using the proposed model is statistical inference of the long‐tailed or skewed distributions and the concavity of the likelihood function with strong results available on the nonparametric maximum likelihood estimator (NPMLE). The results of comparisons, for finding the appropriate estimator among McKendrick's, Mantel‐Haenszel's, Zelterman's, Chao's, the maximum likelihood, and the proposed methods in a simulation study, reveal that under model contaminations the proposed estimator provides the best choice according to its smallest bias and smallest mean square error for a situation of sufficiently large population sizes and the further results show that the proposed estimator performs well even for a homogeneous situation. The empirical examples, containing the cholera epidemic in India based on homogeneity and the heroin user data in Bangkok 2002 based on heterogeneity, are fitted with an excellent goodness‐of‐fit of the models and the confidence interval estimations may also be of considerable interest. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The Knox method and other tests for space-time interaction

The Knox method, as well as other tests for space-time interaction, are biased when there are geographical population shifts, i.e., when there are different percent population growths in different regions. In this paper, the size of the population shift bias is investigated for the Knox test, and it is shown that it can be a considerable problem. A Monte Carlo method for constructing unbiased space-time interaction tests is then presented and illustrated on the Knox test as well as for a combined Knox test. Practical implications are discussed in terms of the interpretation of past results and the design of future studies.     

A goodness-of-fit test for capture-recapture model Mt under closure

A new, fully efficient goodness-of-fit test for the time-specific closed-population capture-recapture model Mt is presented. This test is based on the residual distribution of the capture history data given the maximum likelihood parameter estimates under model Mt, is partitioned into informative components, and is based on chi-square statistics. Comparison of this test with Leslie's test (Leslie, 1958, Journal of Animal Ecology 27, 84-86) for model Mt, using Monte Carlo simulations, shows the new test generally outperforms Leslie's test. The new test is frequently computable when Leslie's test is not, has Type I error rates that are closer to nominal error rates than Leslie's test, and is sensitive to behavioral variation and heterogeneity in capture probabilities. Leslie's test is not sensitive to behavioral variation in capture probabilities but, when computable, has greater power to detect heterogeneity than the new test.     

The rainbow bridge: Hamiltonian limits and resonance in predator-prey dynamics

 In the presence of seasonal forcing, the intricate topology of non-integrable Hamiltonian predator-prey models is shown to exercise profound effects on the dynamics and bifurcation structure of more realistic schemes which do not admit a Hamiltonian formulation. The demonstration of this fact is accomplished by writing the more general models as perturbations of a Hamiltonian limit, ℋ, in which are contained infinite numbers of periodic, quasiperiodic and chaotic motions. From ℋ, there emanates a surface, Γ, of Nejmark-Sacker bifurcations whereby the annual oscillations induced by seasonality are destabilized. Connecting Γ and ℋ is a bridge of resonance horns within which invariant motions of the Hamiltonian case persist. The boundaries of the resonance horns are curves of tangent (saddle-node) bifurcations corresponding to subharmonics of the yearly cycle. Associated with each horn is a rotation number which determines the dominant frequency, or “color”, of attractors within the horn. When viewed through the necessarily coarse filter of ecological data acquisition, and regardless of their detailed topology, these attractors are often indistinguishable from multi-annual cycles. Because the tips of the horns line up monotonically along Γ, it further follows that the distribution of observable periods in systems subject to fluctuating parameter values induced, for example, by year-to-year variations in the climate, will often exhibit a discernible central tendency. In short, the bifurcation structure is consistent with the observation of multi-annual cycles in Nature. Fundamentally, this is a consequence of the fact that the bridge between ℋ and Γ is a rainbow bridge. While the present analysis is principally concerned with the two species case (one predator and one prey), Hamiltonian limits are also observed in other ecological contexts: 2n-species (n predators, n prey) systems and periodically-forced three level food chain models. Hamiltonian limits may thus be common in models involving the destruction of one species by another. Given the oft-commented upon structural instability of Hamiltonian systems and the corresponding lack of regard in which they are held as useful caricatures of ecological interactions, the pivotal role assigned here to Hamiltonian limits constitutes a qualitative break with the conventional wisdom. Received: 2 November 1998     

Does immune function influence population fluctuations and level of parasitism in the cyclic geometrid moth?

Populations of the autumnal moth, Epirrita autumnata, exhibit cycles with high amplitudes in northernmost Europe, culminating in devastating outbreak densities at favourable sites. Parasitism by hymenopteran parasitoids has been hypothesised to operate with a delayed density dependence capable of producing the observed dynamics. It has also been hypothesised that insects in crowded conditions invest greatly in their immunity as a counter-measure to increased risk of parasitism and pathogen infections. Furthermore, inducible plant defences consequent to grazing by herbivorous insects may be linked to the performance of parasitoids and pathogens through increased immunocompetence of the herbivore feeding on the foliage, in which the defence induction has taken place. At ten sampling sites, we quantified larval abundance, outbreak status and percentage larval parasitism during an extended peak phase of a population cycle. These population level covariates, together with an individual pupal mass, were used to explain differences in the immune defence, measured as an encapsulation reaction to artificial antigen. We also conducted a field study for an investigation of the susceptibility of autumnal moth pupae to naturally occurring pupal parasitoids. We did not find obvious differences between the encapsulation rate of autumnal moths originating from the sites with different past and current larval densities and risks for parasitism. The best ranked statistical models included pupal mass and outbreak status as explanatory variables, although both showed only slight effects on the encapsulation rate. The host resistance test revealed positive relationships between the encapsulation rate, body size and percentage parasitism of the exposed pupae, indicating that pupal parasitoids chose, and/or survived better, in large host individuals irrespective of their encapsulation ability. Thus, our results do not provide support for the hypothesis that variation in the immune function drives or modulates population cycles of autumnal moths. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorised users.     

Variation in northern bobwhite demography along two temporal scales

Quantification and understanding of demographic variation across intra- and inter-annual temporal scales can benefit from the development of theoretical models of evolution and applied conservation of species. We used long-term survey data for northern bobwhites (Colinus virginianus) collected at the northern and southern extent of its geographic range to develop matrix population models which would allow investigation of intra- and inter-annual patterns in bobwhite population dynamics. We first evaluated intra-annual patterns in the importance of a seasonal demographic rate to asymptotic population growth rate with prospective perturbation analysis (elasticity analysis). We then conducted retrospective analysis (life table response experiments) of inter-annual patterns in the contribution of observed changes in demography to the observed change in population growth rate. Survival in the earliest age class during the nonbreeding season had the greatest potential influence in both the northern and southern populations. Examination of inter-annual variation in demography indicated that variation in nonbreeding season survival in the earliest age class contributed the most to observed changes in population growth rate in the northern population. In contrast, changes in fertility in the earliest age class in the southern population had the greatest influence on changes in population growth rate. Prospective elasticity analyses highlight the similarities in bobwhite demography throughout different parts of its geographic range, while retrospective life table response experiments revealed important patterns in the temporal differences of bobwhite life history at the northern and southern extent of its geographic range.     

Highly Variable Polymorphism of the α-Amylase Gene Family in <Emphasis Type="Italic">Litopenaeus</Emphasis><Emphasis Type="Italic">vannamei</Emphasis> (Crustacea Decapoda)

-Amylase from the tropical shrimp Litopenaeus vannamei presents a high degree of polymorphism and at least eight different electromorphs are detected by electrophoresis. Based on nucleotide sequences, three cDNAs have been previously characterized. In this paper we report on the organization and the evolution of corresponding -amylase genes, determined after PCR amplification. Three AMY genes have been characterized, spanning over 3.3 kb and encoding mature proteins of 495 amino acids (aa), which are all expressed in the digestive gland. The existence of nine short introns, ranging from 86 to 454 bp, located at the same positions for each of the different genes, and presenting no similarity between them, is reported. Between 11 and 15% of changes are observed in the coding aa sequences of genes II and III compared to the gene I sequence respectively. One 5 putative promoter sequence has been sequenced and shows no classical TATA box upstream to the coding sequence. Based on the intron size difference, a single PCR (producing the S–R fragments) allows the separation of a partial gene I (750 bp), corresponding to cDNA 20, from the others (650–680 bp). Sequencing different S–R PCR fragments from one shrimp shows at least eight different haplotypes. A complex microsatellite repeat is present in intron 6 of gene II. Using size and sequence differences in this repeated portion, it is possible to characterize two gene subfamilies (IIa and IIb) encoding previously described cDNAs 28 and 37, respectively. For the gene II family, two to four alleles are present in one shrimp corresponding to these two genes. Within the Panama natural population, 35 different alleles are shown at this locus. Regarding -amylase gene structure in the shrimp, many recombinants are present from a set of individuals and constitute an important mechanism of evolution of -amylase function. Accession numbers: AJ132379, L. vannamei -amylase gene I; AJ133526, gene II; AJ133119, gene III