Status and structure of the griffon vulture (Gyps fulvus) population in Crete

Griffon vulture (Gyps fulvus) population surveys were conducted during 1996–2002 in the island of Crete (Greece) to document population status and structure. Fieldwork was carried out during the breeding period when birds could be monitored in their colonies. Total population size was estimated at 379 individuals (range = 341–417) with adult birds comprising 63%. The breeding population was estimated at 141 pairs, which were distributed on an average in 23 colonies per year (range = 16–30) while the mean number of breeding pairs that laid eggs was 98 (range= 64–126). Crete thus supports the largest insular population of the species in the world and hosts 70–80% of the breeding population of the species in Greece. Population density was estimated at 6.9 individuals/100 km², 2.6 breeding pairs/100 km² and 1.8 nesting pairs/100 km². The average home range of an occupied colony (i.e., breeding group) was estimated at ca. 204 km² producing a theoretical foraging range of 8 km radius around the breeding cliff. No trends in the total number of individuals and breeding pairs appeared to exist, although significant differences in population size of individual colonies occurred between the years. The majority of the population was concentrated in small-sized colonies, which showed a low occupancy rate. The number of abandoned sites and the colonization of new ones could represent a shift of breeding pairs to alternative colonies provoked by local food abundance and conspesific attraction.     

The Role of Density Regulation in Extinction Processes and Population Viability Analysis

We review the role of density dependence in the stochastic extinction of populations and the role density dependence has played in population viability analysis (PVA) case studies. In total, 32 approaches have been used to model density regulation in theoretical or applied extinction models, 29 of them are mathematical functions of density dependence, and one approach uses empirical relationships between density and survival, reproduction, or growth rates. In addition, quasi-extinction levels are sometimes applied as a substitute for density dependence at low population size. Density dependence further has been modelled via explicit individual spacing behaviour and/or dispersal. We briefly summarise the features of density dependence available in standard PVA software, provide summary statistics about the use of density dependence in PVA case studies, and discuss the effects of density dependence on extinction probability. The introduction of an upper limit for population size has the effect that the probability of ultimate extinction becomes 1. Mean time to extinction increases with carrying capacity if populations start at high density, but carrying capacity often does not have any effect if populations start at low numbers. In contrast, the Allee effect is usually strong when populations start at low densities but has only a limited influence on persistence when populations start at high numbers. Contrary to previous opinions, other forms of density dependence may lead to increased or decreased persistence, depending on the type and strength of density dependence, the degree of environmental variability, and the growth rate. Furthermore, effects may be reversed for different quasi-extinction levels, making the use of arbitrary quasi-extinction levels problematic. Few systematic comparisons of the effects on persistence between different models of density dependence are available. These effects can be strikingly different among models. Our understanding of the effects of density dependence on extinction of metapopulations is rudimentary, but even opposite effects of density dependence can occur when metapopulations and single populations are contrasted. We argue that spatially explicit models hold particular promise for analysing the effects of density dependence on population viability provided a good knowledge of the biology of the species under consideration exists. Since the results of PVAs may critically depend on the way density dependence is modelled, combined efforts to advance statistical methods, field sampling, and modelling are urgently needed to elucidate the relationships between density, vital rates, and extinction probability.     

Interpatch movement of the red milkweed beetle,Tetraopes tetraophthalmus: individual responses to patch size and isolation

Individual movement patterns and the effects of host plant patch size and isolation on patch occupancy were examined for red milkweed beetles, Tetraopes tetraophthalmus, residing in a heterogeneous landscape. Male beetles were found to move both more often and farther between host plant patches than female beetles, and this difference affected the patterns of patch occupancy observed. Overall, unoccupied milkweed patches were smaller and more isolated than patches occupied by beetles. Patches uninhabited by females tended to be more isolated, but not necessarily smaller, than patches with female beetles, indicating that females may be affected more by patch isolation than patch size. Presence of male beetles on patches showed a stronger response to patch size than to patch isolation. Differences in movement between males and females illustrate the need for demographically based dispersal data. Comparisons of Tetraopes interpatch movement patterns between landscapes composed of patches of different size revealed that landscapes with overall smaller patches may have greater rates of interpatch movement.     

Buccal swabs allow efficient and reliable microsatellite genotyping in amphibians

Buccal swabs have recently been used as a minimally invasive sampling method in genetic studies of wild populations, including amphibian species. Yet it is not known to date what is the level of reliability for microsatellite genotypes obtained using such samples. Allelic dropout and false alleles may affect the genotyping derived from buccal samples. Here we quantified the success of microsatellite amplification and the rates of genotyping errors using buccal swabs in two amphibian species, the Alpine newt Triturus alpestris and the Green tree frog Hyla arborea, and we estimated two important parameters for downstream analyses, namely the number of repetitions required to achieve typing reliability and the probability of identity among genotypes. Amplification success was high, and only one locus tested required two to three repetitions to achieve reliable genotypes, showing that buccal swabbing is a very efficient approach allowing good quality DNA retrieval. This sampling method which allows avoiding the controversial toe-clipping will likely prove very useful in the context of amphibian conservation.     

Patch dynamics of a native grass in relation to the spread of invasive smooth brome (Bromus inermis)

The effects of invasive species on the patch dynamics (establishment, growth, and local extinction) of native species are not well studied, owing to the need for relatively fine-scale data on the distribution of species. Within the prairie pothole region of the United States and Canada, the grass, Bromus inermis (smooth brome) has become established by invading disturbed prairies, and through repeated introductions for soil retention and animal graze. In this study, the impact of smooth brome on the patch dynamics of a dominant native grass species, Spartina pectinata (prairie cordgrass), was assessed using fine-scale (sub-meter) mapping of the distribution of cordgrass and brome in three prairie fragments from 2000 to 2006. Using GIS spatial analyses, we determined that cordgrass patch growth was two times greater in areas not invaded by smooth brome versus areas that were heavily infested with smooth brome. Among sites and time periods, there was a consistent significant negative relationship between the amount of smooth brome surrounding a patch of cordgrass and the growth of that cordgrass patch. The probability of establishment of a new patch of cordgrass averaged 1.3 times higher in areas of low brome coverage (<25%) than areas of high brome coverage (>75%). Conversely, existing cordgrass patches were 7.8 times more likely to go extinct in areas of high than low brome coverage. This is one of only a few field studies to provide evidence of the negative impact of smooth brome on native flora and hopefully will serve as justification for the development of a formal management plan to limit the distribution of this species in tallgrass prairie ecosystems.     

Predicting extinctions: interspecific competition,predation and population variability in experimental Daphnia populations

We examine how interspecific competition and two types of size-selective predation affect population density, variability and persistence in laboratory cultures of two species of Daphnia, D. magna and D. longispina. When both species were analysed together, and for D. longispina alone, there were weak negative relationships between mean population density and population variability. Interspecific competition resulted in lower population densities and higher population variability. Extinct populations had lower densities and were also more variable than persisting ones. There was still an effect of population variability on extinction probability after the effect of density on population variability had been accounted for. Hence, the effects of population density and variability on population persistence were partly independent of each other. The effects of size-selective predation on population persistence were more species-specific and not directly related to density or variability. Since the effects of species interactions on persistence were large, we suggest that it is likely that population vulnerability analyses not incorporating effects of interspecific interactions are often misleading.     

The influence of habitat heterogeneity on host-pathogen population dynamics

The influence of spatial heterogeneity on the population dynamics of a naturally occurring invertebrate host-pathogen system was experimentally investigated. At ten week intervals over a two year period, I quantified the spatial distribution of natural populations of the terrestrial isopod crustacean Porcellio scaber infected with the isopod iridescent virus (IIV). During the seasonally dry periods of summer and early fall in central California, isopod populations were highly aggregated and the degree of patchiness and distance between inhabited patches was greatest. Coincident with increased patchiness and patch spacing was an increase in isopod density within patches. During the wet seasons of winter and spring, isopod population patchiness, inter-patch spacing, and within-patch density was low. Seasonal changes in virus prevalence were negatively correlated with within-patch density, patchiness, and inter-patch spacing. The influence of the spatial distribution of isopods on virus prevalence was also tested in field experiments. The virus was introduced into arrays of artificial habitat patches colonized by isopods in which interpatch distance was varied. The prevalence of resulting infections was monitored at weekly intervals. In addition, dispersal rates between artificial patches and natural patches were quantified and compared. The results showed that isopods in treatments with the smallest inter-patch spacing had the highest virus prevalence, with generally lower prevalence among isopods in more widely spaced patches. The spacing of experimental patches significantly affected virus prevalence, although the experiments did not resolve a clear relationship between patch spacing and virus prevalence. Rates of dispersal between patches decreased with increased patch spacing, and these rates did not differ significantly from dispersal between natural patches. The results suggest that rates of dispersal between isopod subpopulations may be an important component of the infection dynamics in this system. I discuss the consequences of these findings for host-pathogen dynamics in fragmented habitats, and for other ecological interactions in spatially heterogeneous habitats.     

Multiscale determinants of tree frog (Hyla arborea L.) calling ponds in western Switzerland

A tree frog (Hyla arborea L., 1758) metapopulation in western Switzerland was studied during spring 2001. All potential calling ponds in an area of 350km² were searched for tree frog calling males. Twenty-nine out of 111 ponds sheltered between 1 and 250 callers. Most ponds were occupied by less than 12 males. Pond parameters were measured at three different levels using field analysis and a Geographical Information System (GIS). The first level was water chemistry and pond-associated measures. The second level was the surrounding land use in a 30m buffer around the pond. The third level consisted of landscape indices on a broader scale (up to 2km). Logistic regression was applied to identify parameters that can predict the presence of calling males in a pond. Response variable was the presence or absence of callers. Four significant parameters allowed us to explain about 40% of the total deviance of the observed occupational pattern. Urbanization around the pond had a highly negative impact on the probability of presence of calling males. Hours of direct sunlight on the pond was positively correlated with callers. Higher water conductivity was associated with a lesser probability of species presence. Finally, the further the closest two-lane road, the higher the probability of callers presence. Our results show that presence or absence of callers is influenced by factors acting at various geographical scales.     

Habitat requirements and local persistence of three damselfly species (odonata: coenagrionidae)

Habitat requirements and population persistence were investigated in three damselfly species, all coastal plain pond specialists: Enallagma recurvatum, E. laterale, and E. pictum. Because of geographic restriction, two are of special concern to conservation, E. recurvatum and E. laterale. We surveyed more than 70 ponds on Cape Cod, Massachusetts, and collected adult presence-absence data during the summers of 1999 and 2000. We achieved a detection rate approaching 100% for each species by visiting each pond up to three times. We looked for relationships between the presence of each damselfly species and presence of specific aquatic vegetation, the presence of the other Enallagma species, and the number of ponds within various distances of the 72 surveyed ponds. Using stepwise logistic regression, we found the following significant associations: E. recurvatum with the rush Juncus militaris; E. laterale with water lilies (Nuphar variegatum and Brasenia schreberi) the damselfly E. pictum, and the number of ponds within 2 km; and E. pictum with the water lily Nymphaea odorata, the damselfly E. laterale, and the number of ponds within 1.5 km and 2.5 km. Presence-absence data were used to calculate turnover and local extinction rates for each species between the two years. E. recurvatum's turnover and local extinction rates (33.3% and 41% respectively) were much higher than either E. laterale (9.8%, 11.5%) or E. pictum (7.7%, 10%). These results suggest that E. recurvatum occurs in a metapopulation, and that patch colonization rates might be important to local population persistence.     

The population of the hypogeous fungus<Emphasis Type="Italic"> Tuber aestivum</Emphasis> syn.<Emphasis Type="Italic"> T. uncinatum</Emphasis> on the island of Gotland

The aim of our study was to examine the genetic variation within Tuber aestivum on the Baltic island of Gotland, Sweden. Variation in such a limited geographical area should help illuminate the dispersal abilities of T. aestivum. Knowledge of the genetic variation in this northern outpost could also be useful in the selection of inoculum for the establishment of truffle orchards. Genetic structure and homogeneity of the population were studied using principal component and parsimony analyses of randomly amplified polymorphic DNA data. Our inventories showed that T. aestivum is abundantly distributed in suitable habitats on Gotland. The genetic variation observed suggests sexual reproduction and slow dispersal on the island. It is possible that the present population was established from one introduction, which may be due to ability to survive in this habitat rather than to rare colonising events. The T. aestivum population on Gotland may be an ecotype adapted to the climate and soil conditions on the island.     

Minimum recruitment frequency in plants with episodic recruitment

There is concern about the lack of recruitment of Acacia trees in the Negev desert of Israel. We have developed three models to estimate the frequency of recruitment necessary for long-term population survival (i.e. positive average population growth for 1,000 years and <10% probability of extinction). Two models assume purely episodic recruitment based on the general notion that recruitment in arid environments is highly episodic. They differ in that the deterministic model investigates average dynamics while the stochastic model does not. Studies indicating that recruitment episodes in arid environments have been overemphasized motivated the development of the third model. This semi-stochastic model simulates a mixture of continuous and episodic recruitment. Model analysis was done analytically for the deterministic model and via running model simulations for the stochastic and semi-stochastic models. The deterministic and stochastic models predict that, on average, 2.2 and 3.7 recruitment events per century, respectively, are necessary to sustain the population. According to the semi-stochastic model, 1.6 large recruitment events per century and an annual probability of 50% that a small recruitment event occurs are needed. A consequence of purely episodic recruitment is that all recruitment episodes produce extremely large numbers of recruits (i.e. at odds with field observations), an evaluation that holds even when considering that rare events must be large. Thus, the semi-stochastic model appears to be the most realistic model. Comparing the prediction of the semi-stochastic model to field observations in the Negev desert shows that the absence of observations of extremely large recruitment events is no reason for concern. However, the almost complete absence of small recruitment events is a serious reason for concern. The lack of recruitment may be due to decreased densities of large mammalian herbivores and might be further exacerbated by possible changes in climate, both in terms of average precipitation and the temporal distribution of rain.     

Restricted dispersal in a flying beetle assessed by telemetry

Many insects living in ancient trees are assumed to be threatened as a result of habitat loss and fragmentation. It is generally expected that species in habitats with low temporal variability in carrying capacity have lower degree of dispersal in comparison to those in more ephemeral habitats. As hollow trees are long-lived, species in that habitat are expected to be sensitive to habitat fragmentation, due to a low capacity to establish new populations far from present ones. Using radio telemetry, we studied the dispersal for a beetle, Osmoderma eremita, living in hollow trees. O. eremita exhibited philopatry and only dispersed over short ranges. About 82–88% of the adults remained in the tree where they were caught. All observed dispersal movements ended up in nearby hollow trees and 62% in the neighbouring hollow tree. These results corroborate the suggestion that habitat fragmentation may be detrimental to insects living in temporally stable but spatially variable habitats. In order to preserve such species, we propose that conservation efforts should be focused on maintaining or increasing the number of suitable trees in and near presently occupied stands.     

Dispersal-mediated selection on plant height in an autochorously dispersed herb

Dispersal ability is an important fitness component in most plant species. Therefore, some phenotypic traits can be selected due to their effect on dispersal. In this study I determine the potential for dispersal-mediated selection on plant height in an autochorous plant, Erysimum mediohispanicum (Brassicaceae). Selection was quantified by selection gradients, structural equation modeling and generalized additive models. I detected significant dispersal-mediated linear selection gradient on plant height, taller plants dispersing seeds farther. Nevertheless, the generalized additive models suggest that the selection on stalk height was non linear. Indeed, it detected a threshold in the effect of stalk height on dispersal ability; plants shorter than that threshold had an extremely short dispersal, whereas plants taller than that threshold dispersed the seeds very far. Furthermore, the structural equation modeling showed that stalk height indirectly affected dispersal distance through its significant effect on one reproduction-related fitness component, taller plants having greater fecundity. Selection on E. mediohispanicum stalk height occurs through two simultaneous paths, one via producing many seeds and the other through increasing probability of dispersing them far away.     

Evidence for differential maternal allocation to eggs in relation to manipulated male attractiveness in the pied flycatcher (Ficedula hypoleuca)

We present evidence of differential maternal allocation to eggs in response to manipulated male attractiveness in the migratory pied flycatcher (Ficedula hypoleuca). We manipulated the size of a male secondary sexual trait, the white forehead patch, right after male arrival to the breeding area and before female arrival. Patch size was (1) enlarged to the maximum observed in the population, (2) reduced by 40% or (3) kept constant by painting with indelible felt markers over the natural feather patch. Male behaviour was affected by the experimental manipulation, as individuals with enlarged patches performed more approaches to the nestbox in response to song playback during the territory occupation and nest-site presentation phases. Females paired with males with reduced forehead patches laid significantly smaller eggs than those paired with males in the control and enlarged-patch treatments. Laying date and clutch size did not differ among the experimental groups. We discuss that manipulations of ornaments designed to study differential allocation at laying should reduce as well as enlarge their expression.     

Globalization and fruitfly invasion and expansion: the medfly paradigm

The phytophagous insects of the Tephritidae family commonly referred to as “true fruit flies” offer different case histories of successful invasions. Mankind has played an important role in altering the distributions of some of the more polyphagous and oligophagous species. However, the question arises why only a few species have become major invaders. The understanding of traits underlying adaptation in different environments is a major topic in invasion biology. Being generalists or specialists, along the K–r gradient of the growth curve, make a difference in term of food resources exploitation and interspecies competition and displacement. The species of the genus Ceratitis are good examples of r-strategists. The genetic and biological data of the most notorious Ceratitis species, the Mediterranean fruit fly Ceratitis capitata (medfly), are reviewed to investigate the traits and behaviours that make the medfly an important invader. It can be learnt from medfly, that invasions in a modern global trade network tend to be due to multiple introductions. This fact allows a maintenance or enhancement of genetic variability in the adventive populations, which in turn increases their potential invasiveness. Our current knowledge of the medfly genome opens the way for future studies on functional genomics.     

Scaling up tests on virulence of the cassava green mite fungal pathogen <Emphasis Type="Italic">Neozygites tanajoae</Emphasis> (Entomophthorales: Neozygitaceae) under controlled conditions: first observations at the population level

Virulence of entomopathogens is often measured at the individual level using a single host individual or a group of host individuals. To what extent these virulence assessments reflect the impact of an entomopathogen on their host in the field remains largely untested, however. A methodology was developed to induce epizootics of the cassava green mite fungal pathogen Neozygites tanajoae under controlled conditions to evaluate population-level virulence of two (one Beninese and one Brazilian) isolates of the entomopathogen—which had shown similar individual-level virulence but different field impacts. In unrepeated separate experiments we inoculated mite-infested potted cassava plants with either 50 or 25 live mites (high and low inoculum) previously exposed to spores of N. tanajoae and monitored the development of fungal infections for each isolate under the same conditions. Both isolates caused mite infections and an associated decline in host mite populations relative to the control (without fungus) in all experiments, but prevalence of the fungus varied with isolate and increased with inoculum density. Peak infection levels were 90% for the Beninese isolate and 36% for the Brazilian isolate at high inoculum density, and respectively 17% and 25% at low inoculum density. We also measured dispersal from inoculated plants and found that spore dispersal increased with host infection levels, independent of host densities, whereas mite dispersal varied between isolates. These results demonstrate that epizootiology of N. tanajoae can be studied under controlled conditions and suggest that virulence tests at the population level may help to better predict performance of fungal isolates than individual-level tests.