Asymmetric habitat isolation and sexual isolation predicted by the cost of migration and hybridization generate novel signatures of reinforcing selection

Reinforcement is an evolutionary process whereby increased prezygotic reproductive isolation evolves in response to the cost of hybridization. Despite theory predicting that multiple prezygotic barriers can evolve via reinforcement, most empirical studies examine a single barrier. We test novel predictions for the reinforcement of both habitat isolation and sexual isolation between ecologically divergent lineages under asymmetric migration: the lineage that emigrates more should evolve stronger habitat isolation due to the lower fitness of immigrants in the alternative habitat, while the lineage that receives more immigrants should exhibit stronger sexual isolation due to the lower fitness of hybrids. We found both signatures of reinforcement in two sympatric sister species of gall wasps that are host specific to the southern live oaks, Quercus virginiana and Q. geminata, respectively. Specifically, we observed stronger habitat isolation in the species with higher emigration rates, Belonocnema treatae, and stronger sexual isolation in the species facing more immigrants, B. fossoria. In contrast, comparisons of both species to a third, allopatric, species showed that B. kinseyi exhibited both lower habitat isolation and sexual isolation than the sympatric species, consistent with the classic predictions of reinforcement. Our study provides a rare examination of the interplay of ecology and geography in the evolution of multiple reproductive barriers to gene flow. Given that asymmetric migration between ecologically divergent lineages increasingly appears to be the rule rather than the exception, concomitant asymmetries in the strength of habitat and sexual isolation could be more widespread than currently understood.

How much does habitat isolation drive forest bird morphology?

Rapid environmental change caused by humans has become a major concern for wildlife conservationists. But phenotypic and evolutionary responses of species to such change may often be swift enough to prevent their collapse. Several North American bird species living in boreal forests now have more pointed wings (a proxy for sustained ight efficiency), than they had a century ago. This remarkable pattern has been hypothesized as resulting from selective pressures favoring colonization of isolated habitat. Additionally, aerodynamics predict that heavier birds can achieve faster ight, a further advantage for exploring isolated habitats. We tested whether birds establishing territories in isolated areas have more pointed wings and are heavier than con-specifics found in more densely populated areas. Wing shapes of wild-caught adults from 21 passerine bird species did not generally support this prediction. However individuals with large body mass relative to their species were found more frequently in isolated habitats. Our results offer partial support for the isolation hypothesis at the landscape scale. We encourage further work at coarser, regional, scales to further examine whether wing shape and body mass evolutionarily respond to habitat isolation.

Habitat fragmentation shapes natal dispersal and sociality in an Afrotropical cooperative breeder

It remains poorly understood how effects of anthropogenic activity, such as large-scale habitat fragmentation, impact sociality in animals. In cooperatively breeding species, groups are mostly formed through delayed offspring dispersal, and habitat fragmentation can affect this process in two opposite directions. Increased habitat isolation may increase dispersal costs, promoting delayed dispersal. Alternatively, reduced patch size and quality may decrease benefits of philopatry, promoting dispersal. Here, we test both predictions in a cooperatively breeding bird (placid greenbul, Phyllastrephus placidus ) from an Afrotropical cloud forest archipelago. Males born in fragmented forest dispersed about 1 year earlier than those born in continuous forest. Contrary to females, males also started to reproduce earlier and mostly settled within their natal patch. Females only rarely delayed their dispersal for more than 1 year, both in fragmented and continuous forests. Our results suggest that early male dispersal and reproduction is jointly driven by a decrease in the value of the natal territory and an increase in local breeding opportunities in fragmented forest. While plasticity in dispersal strategies of cooperative breeders in response to anthropogenic change is believed to optimize reproduction-survival trade-offs, to what extent it shapes the ability of species to respond to rapid environmental change remains to be studied.

Interaction of habitat isolation and seasonality: impact of fragmentation in dynamic landscapes on local and regional population variability in meta-food chains

While habitat loss is a known key driver of biodiversity decline, the effect of habitat fragmentation ‘per se’ is far less clear. Environmental perturbations, e.g. due to seasonality, potentially interact with effects of habitat fragmentation, thus making the latter even more difficult to assess. In order to analyze how the dynamic stability of local and metapopulations is affected by habitat fragmentation, we simulate a meta-food chain with three species on complex networks of habitat patches and evaluate the average variability of the local populations and the metapopulations as well as the level of synchronization between the patches. To account for environmental perturbations, we contrast simulations of static landscapes with simulations of dynamic landscapes, where 30 percent of the patches periodically become unavailable as habitats. We find that it depends on the parameterization of the food chain whether higher dispersal rates in less fragmented landscapes synchronize the dynamics or not. Even if the dynamics synchronize, local population variability can decrease due to indirect effects of dispersal mortality, thereby stabilizing metapopulation dynamics and reducing the risk of extinction. In dynamic landscapes, periodic external perturbations often fully synchronize the dynamics even if they act on a much slower time scale than the ecological interactions. Furthermore, they not only increase the variability of local and metapopulations, but also mostly overrule the effects of habitat fragmentation. This may explain why in nature effects of habitat fragmentation are often small and inconclusive.

Characterization and Analysis of the Full-Length Transcriptomes of Multiple Organs in Pseudotaxus chienii (W.C.Cheng) W.C.Cheng

Pseudotaxus chienii, a rare tertiary relict species with economic and ecological value, is a representative of the monotypic genus Pseudotaxus that is endemic to China. P. chienii can adapt well to habitat isolation and ecological heterogeneity under a variety of climate and soil conditions, and is able to survive in harsh environments. However, little is known about the molecular and genetic resources of this long-lived conifer. Herein, we sequenced the transcriptomes of four organs of P. chienii using the PacBio Isoform Sequencing and Illumina RNA Sequencing platforms. Based on the PacBio Iso-Seq data, we obtained 44,896, 58,082, 50,485, and 67,638 full-length unigenes from the root, stem, leaf, and strobilus, respectively, with a mean length of 2692 bp, and a mean N50 length of 3010.75 bp. We then comprehensively annotated these unigenes. The number of organ-specific expressed unigenes ranged from 4393 in leaf to 9124 in strobilus, suggesting their special roles in physiological processes, organ development, and adaptability in the different four organs. A total of 16,562 differentially expressed genes (DEGs) were identified among the four organs and clustered into six subclusters. The gene families related to biotic/abiotic factors, including the TPS, CYP450, and HSP families, were characterized. The expression levels of most DEGs in the phenylpropanoid biosynthesis pathway and plant–pathogen interactions were higher in the root than in the three other organs, suggesting that root constitutes the main organ of defensive compound synthesis and accumulation and has a stronger ability to respond to stress. The sequences were analyzed to predict transcription factors, long non-coding RNAs, and alternative splicing events. The expression levels of most DEGs of C2H2, C3H, bHLH, and bZIP families in the root and stem were higher than those in the leaf and strobilus, indicating that these TFs may play a crucial role in the survival of the root and stem. These results comprise the first comprehensive gene expression profiles obtained for different organs of P. chienii. Our findings will facilitate further studies on the functional genomics, adaptive evolution, and phylogeny of P. chienii, and lay the foundation for the development of conservation strategies for this endangered conifer.

Implications of fragile landscapes for endemic pollinators in an eastern afro-montane biodiversity hotspot of Kenya

This study was carried out to determine how disturbances could shape the natural occurrence of African meliponine bee species in different ecological habitats of Taita hills of Kenya and how this could lead to changes in their diversity. Habitats sampled were indigenous forests, exotic forests, mixed highland forests, Acacia dominated bush lands, mixed deciduous woodlands and grasslands which were further categorized as either fragmented or un-fragmented habitats based on levels of disturbance. The study sites were chosen based on various features such as, forest fragment size, level of forest fragment isolation, forest fragment age, and level of degradation. In all study locations, meliponine bee species were sampled using the conventional complementary method, belt transect (direct observation of nesting colonies synonymous to a visual census) from the months of March to September 2014 (combining both the long rainy season and dry season). Nesting colonies of the 4 meliponine bee species, namely, Hypotrigona gribodoi, Meliponula ferruginea (black), Hypotrigona ruspolii and Plebeina hildebrandti, were surveyed following a successive gradient. In each study site (25 ha), 20 linear transects of 250 m × 20 m each were established using a global positioning system (GPS) receiver to mark coordinates with relation to habitat type. Data such as nesting site/substrate, GPS coordinates of nest, and names of nesting trees were recorded. A total of four species depicted by the Renyi diversity profile was recorded in five out of the six main habitat types surveyed and a further extrapolation with Shannon index (EH) also predicted the highest species richness of 4.24 in a deciduous habitat type with more trees and vegetation and the lowest species of 1.01 in isolated forest patches habitat type. These meliponine bee species (Hypotrigona gribodoi, Hypotrigona ruspolii, Meliponula ferruginea (black) and Plebeina hildebrandti) were observed to be unevenly distributed across all six sampled habitats, further indicating that mixed deciduous habitat proved to be more diverse than Acacia dominated bush lands, grasslands and exotic forest strips which have all undergone varying levels of fragmentation. This has revealed that unprecedented conversions of natural habitats leading to fragmentations of such habitats is a key driving factor causing increased habitat isolation and vulnerability in this afro-montane region which may potentially distort local assemblages of native pollinators such as meliponine bee species. Keywords: Meliponine bees, habitat fragmentation, diversity, Eastern Afro Montane

Drivers of Functional Composition of Bird Assemblages in Green Spaces of a Neotropical City: A Case Study From Merida, Mexico

Given current urbanization trends, understanding the factors that affect local biodiversity is paramount for designing sound management practices. Existing evidence suggests that the assembly of urban communities is influenced by the environmental filtering of organisms based on their traits. Here, we investigate how environmental characteristics including isolation measurements affect the functional composition of avian assemblages in green spaces of Merida, Mexico, a Neotropical city. We sampled 22 sites, analyzed point-count data collected during fall migration, and characterized the habitat with regard to floristic and structural vegetation attributes, vegetation cover within green spaces, urban infrastructure, and isolation. We assessed the relationship between habitat descriptors and bird functional traits using RLQ and fourth-corner tests and compared trait–environment associations between resident and wintering species. Our results showed that functional composition of resident bird assemblages was linked to the environmental characteristics of the site, while the functional composition of wintering species was not. In particular, the degree of isolation revealed to be an important determinant of trait composition. Plant species richness, particularly native tree and shrub species, were critical for the functional composition of resident birds in green spaces. Our findings suggested shifts in body mass from less to more isolated green spaces. Specifically, we observed that large-bodied species predominated in isolated green spaces. This information is useful given the predicted increases in habitat isolation and transformation of green spaces due to urbanization.

A proposal for practical and effective biological corridors to connect protected areas in northwest Costa Rica

Habitat loss and increases in habitat isolation are causing animal population reductions and extirpations in forested areas of the world. This problem extends to protected areas, which, while often well-conserved, can be too small and isolated to maintain species that exist at low densities and require large contiguous areas of habitat (e.g. some large mammals). Costa Rica has been at the forefront of tropical forest conservation and a large proportion of the country’s land area is currently under some form of protection. One such area is the northwest portion of Costa Rica, which is an extremely biodiverse region with several noteworthy national and privately-owned protected areas. However, each protected area is an isolated island in a sea of deforestation. Within Costa Rica’s existing framework of biological corridors, we propose four sub-corridors as targets for restoration and full protection. These sub-corridors would link five major protected areas in northwest Costa Rica, with all of them linking to larger protected areas in the central portion of the country, while impacting a small number of people who reside within the corridors. After natural or active reforestation of the corridors, the result would be a contiguous protected area of 348,000 ha. The proposed sub-corridors would represent a 3.7% increase in protected area size in the region and only 0.2% of Costa Rica’s total land area. Using the jaguar (Panthera onca) as a model umbrella species, we estimated that each current isolated protected area could support between 8–104 individuals. Assuming lack of dispersal between protected areas (distance between each ranges from 8.1 to 24.9 km), these population sizes are unlikely to be viable in the long term. However, the combined protected areas, connected by biological sub-corridors, could support about 250 jaguars, a population size with a higher probability of surviving. Our study shows that focusing conservation efforts on a relatively small area of Costa Rica could create a large protected area derived from numerous small isolated preserves.

The biggest losers: habitat isolation deconstructs complex food webs from top to bottom

Habitat fragmentation threatens global biodiversity. To date, there is only limited understanding of how the different aspects of habitat fragmentation (habitat loss, number of fragments and isolation) affect species diversity within complex ecological networks such as food webs. Here, we present a dynamic and spatially explicit food web model which integrates complex food web dynamics at the local scale and species-specific dispersal dynamics at the landscape scale, allowing us to study the interplay of local and spatial processes in metacommunities. We here explore how the number of habitat patches, i.e. the number of fragments, and an increase of habitat isolation affect the species diversity patterns of complex food webs ( α -, β -, γ -diversities). We specifically test whether there is a trophic dependency in the effect of these two factors on species diversity. In our model, habitat isolation is the main driver causing species loss and diversity decline. Our results emphasize that large-bodied consumer species at high trophic positions go extinct faster than smaller species at lower trophic levels, despite being superior dispersers that connect fragmented landscapes better. We attribute the loss of top species to a combined effect of higher biomass loss during dispersal with increasing habitat isolation in general, and the associated energy limitation in highly fragmented landscapes, preventing higher trophic levels to persist. To maintain trophic-complex and species-rich communities calls for effective conservation planning which considers the interdependence of trophic and spatial dynamics as well as the spatial context of a landscape and its energy availability.