Migratory bird species that feed on air-borne insects are experiencing widespread regional declines, but these remain poorly understood. Agricultural intensification in the breeding range is often regarded as one of the main drivers of these declines. Here, we tested the hypothesis that body mass in breeding individuals should reflect habitat quality in an aerial insectivore, the tree swallow (Tachycineta bicolor), along a gradient of agricultural intensity. Our dataset was collected over 7 years (2005-2011) and included 2918 swallow captures and 1483 broods. Analyses revealed a substantial decline of the population over the course of the study (-19% occupancy rate), mirrored by decreasing body mass. This trend was especially severe in females, representing a total loss of 8% of their mass. Reproductive success was negatively influenced by intensive agriculture, but did not decrease over time. Interestingly, variation in body mass was independent of breeding habitat quality, leading us to suggest that this decline in body mass may result from carry-over effects from non-breeding areas and affect population dynamics through reduced survival. This work contributes to the growing body of evidence suggesting that declines in migratory aerial insectivores are driven by multiple, complex factors requiring better knowledge of year-round habitat use.
In many parts of the world, farmland bird species are declining at faster rates than other birds. For aerial insectivores, this decline has been related to a parallel reduction in the abundance of their invertebrate prey in agricultural landscapes. While the effects of agricultural intensification (AI) on arthropod communities at the landscape level have been substantially studied in recent years, seasonal variation in these impacts has not been investigated. To assess the contention that intensive cultures negatively impact food resources for aerial insectivorous birds, we analyzed the spatiotemporal distribution patterns of Diptera, the main food resource for breeding tree swallows Tachycineta bicolor), across a gradient of AI in southeastern Quebec, Canada. Linear mixed models computed from a data set of 5000 samples comprising >150,000 dipterans collected over three years (2006-2008) suggest that both Diptera abundance and biomass varied greatly during swallow breeding season, following a quadratic curve. Globally, AI had a negative effect on Diptera abundance (but not biomass), but year-by-year analyses showed that in one of three years (2008), dipterans were more abundant in agro-intensive landscapes. Analyses also revealed a significant interaction between the moment in the season and AI: In early June, Diptera abundances were similar regardless of the landscape, but differences increased as the season progressed, with highly intensive landscapes harboring fewer prey, possibly creating an "ecological trap" for aerial insectivores. While global trends in our results are in agreement with expectations (negative impact of Al on insect abundance), strong discrepancies in 2008 highlight the difficulty of predicting the abundance of insect communities. Our study indicates that predicting the effects of AI may prove more challenging than generally assumed, even when large data sets are collected, and that temporal variation within a season is important to take into consideration. While further work is required to assess the direct impacts of these seasonal trends in Diptera abundance on bird breeding success and post-fledging survival, management strategies in agricultural landscapes may need to consider the phenology of breeding birds and their dipteran prey in order to mitigate the potentially negative effects of AI late in the breeding season.
The light brown apple moth, Epiphyas postvittana (Walker) (Lepidoptera: Tortricidae), is a horticultural pest of Australia and New Zealand that has more recently invaded Hawaii, Europe, and California. A 2,216-bp region of the mitochondrial genome containing the cytochrome oxidase I and II genes was sequenced from 752 individuals. Haplotype network analyses revealed a major split between a predominantly Western Australian clade and all other samples, suggestive of either a deep genetic divergence or a cryptic species. Nucleotide and haplotype diversity were highest in the country of origin, Australia, and in New Zealand populations, with evidence of haplotype sharing between New Zealand and Tasmania. Nucleotide and haplotype diversity were higher in California than within the British Isles or Hawaii. From the total of 96 haplotypes, seven were found in California, of which four were private. Within California, there have been at least two introductions; based on genetic diversity we were unable to assign a likely source for a single moth found and eradicated in Los Angeles in 2007; however, our data suggest it is unlikely that Hawaii and the British Isles are sources of the major E. postvittana population found throughout the rest of the state since 2006.
Dispersal is a major force in shaping the genetic structure and dynamics of species; thus, its understanding is critical in formulating appropriate conservation strategies. In many species, sexes do not face the same evolutionary pressures, and consequently dispersal is often asymmetrical between males and females. This is well documented in birds and mammals but has seldom been investigated in other taxa, including reptiles and, more specifically, nonmarine chelonians. In these species, nest-site fidelity observations are frequent but still remain to be associated with natal homing. Here, we tested for sex-biased dispersal in the radiated tortoise (Astrochelys radiata) from southern Madagascar. Using data from 13 microsatellite markers, we investigated patterns of relatedness between sexes in 2 populations. All Mantel tests indicated significant isolation by distance at the individual level in females but not in males. Furthermore, spatial autocorrelation analyses and 2 analytical approaches designed to assess general trends in sex-specific dispersal also supported male-biased dispersal. On the other hand, comparisons of overall genetic structure among sampling sites did not provide conclusive support for greater philopatry in females, but these tests may have low statistical power because of methodological and biological constraints. Radiated tortoises appear to be both polyandrous and polygynous, and evolutionary processes that may lead to a sex bias in dispersal are discussed with respect to tortoise breeding biology. Female natal homing is hypothesized as a key trait explaining greater female philopatry in A. radiata. These findings highlight the necessity of additional research on natal homing in tortoises, a behavioral trait with direct implications for conservation.
The likely future extinction of various species will result in a decline of two quantities: species richness and phylogenetic diversity (PD, or evolutionary history). Under a simple stochastic model of extinction, we can estimate the expected loss of these quantities under two conservation strategies: An egalitarian approach, which reduces the extinction risk of all species, and a targeted approach that concentrates conservation effort on the most endangered taxa. For two such strategies that are constrained to experience the same expected loss of species richness, we ask which strategy results in a greater expected loss of PD. Using mathematical analysis and simulation, we describe how the strategy (egalitarian versus targeted) that minimizes the expected loss of PD depends on the distribution of endangered status across the tips of the tree, and the interaction of this status with the branch lengths. For a particular data set consisting of a phylogenetic tree of 62 lemur species, with extinction risks estimated from the IUCN Red List, we show that both strategies are virtually equivalent, though randomizing these extinction risks across the tip taxa can cause either strategy to outperform the other. In the second part of the paper, we describe an algorithm to determine how extreme the loss of PD for a given decline in species richness can be. We illustrate the use of this algorithm on the lemur tree.
Although several methods are available to study the extent of isolation by distance (IBD) among natural populations, comparatively few exist to detect the presence of sharp genetic breaks in genetic distance datasets. In recent years, Monmoniers maximum-difference algorithm has been increasingly used by population geneticists. However, this method does not provide means to measure the statistical significance of such barriers, nor to determine their relative contribution to population differentiation with respect to IBD. Here, we propose an approach to assess the significance of genetic boundaries. The method is based on the calculation of a multiple regression from distance matrices, where binary matrices represent putative genetic barriers to test, in addition to geographic and genetic distances. Simulation results suggest that this method reliably detects the presence of genetic barriers, even in situations where IBD is also significant. We also illustrate the methodology by analyzing previously published datasets. Conclusions about the importance of genetic barriers can be misleading if one does not take into consideration their relative contribution to the overall genetic structure of species.
Evaluating the permeability of potential barriers to movement, dispersal and gene exchanges can help describe spreading patterns of wildlife diseases. Here, we used landscape genetics methods to assess the genetic structure of the striped skunk (Mephitis mephitis), which is a frequent vector of rabies, a lethal zoonosis of great concern for public health. Our main objective was to identify landscape elements shaping the genetic structure of this species in Southern Québec, Canada, in an area where the raccoon rabies variant has been detected. We hypothesised that geographic distance and landscape barriers, such as highways and major rivers, would modulate genetic structure. We genotyped a total of 289 individuals sampled across a large area (22,000 km²) at nice microsatellite loci. Genetic structure analyses identified a single genetic cluster in the study area. Major rivers and highways, however, influenced the genetic relatedness among sampled individuals. Sex-specific analyses revealed that rivers significantly limited dispersal only for females while highways only had marginal effects. Rivers and highways did not significantly affect male dispersal. These results support the contention that female skunks are more philopatric than males. Overall, our results suggest that the effects of major rivers and highways on dispersal are sex-specific and rather weak and are thus unlikely to prevent the spread of rabies within and among striped skunk populations.
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