No abstract

Since the publication of the theory of island biogeography, ecologists have postulated that fragmentation of continuous habitat presents a prominent threat to species diversity. However, negative fragmentation effects may be artifacts; the result of species diversity declining with habitat loss, and habitat loss correlating positively with degree of fragmentation. In this study, we used butterfly assemblages on islands of Lake of the Woods, Ontario, Canada to decouple habitat fragmentation from habitat loss and test two competing hypotheses: (1) the island effect hypothesis, which predicts that decreasing fragment size and increasing fragment isolation reduces species diversity beyond the effects of habitat loss, and (2) the habitat amount hypothesis, which negates fragmentation effects and predicts that only total habitat area determines the diversity of species persisting on fragmented landscapes. Using eight independent size classes of islands (ranging from 0.1 to 8.0 ha) that varied in number of islands while holding total area constant, species diversity comparisons, species accumulation curves, and species-area relationship extrapolations demonstrated that smaller insular habitats contained at least as many butterfly species as continuous habitat. However, when highly mobile species occurring on islands without their larval food plants were excluded from analyses, island effects on potentially reproducing species became apparent. Similarily, generalized linear models suggested that effects of island isolation and vascular plant richness on insular butterfly richness were confounded by species of high mobility. We conclude that inter-fragment movements of highly mobile species may obscure important fragmentation effects on potentially reproducing populations, questioning support for the habitat amount hypothesis.

Mayr, 1963; Nosil, 2012). When genetic divergence has a strong spatial component, causes are generally attributed to spatial variation in evolutionary processes, such as gene flow, genetic drift,

Aim: Investigate relationships between fragmentation and species diversity in the context of the theory of island biogeography, sample-area effect, and habitat diversity hypothesis.Location: Lake of the Woods, Canada. Taxon: Vascular plantsMethods: Vascular plant species diversity was inventoried on 30 islands, organized into two island sets. Each island set contained four size classes that varied in degree of fragmentation while controlling for the sample-area effect (small island set: 8 × 0.1-ha, 4 × 0.2-ha, 2 × 0.4-ha, and 1 × 0.8-ha islands; large island set: identical pattern utilizing 1.0-ha to 8.0-ha islands). Fragmentation effects were then examined using SLOSS-based analyses, addressing whether single large or several small islands contained more species/habitats: (a) direct comparisons of species and habitat richness across size classes; (b) extrapolations of species-area relationships; and (c) analyses of species and habitat accumulation curves. Multigroup path analysis was next used to quantify effects of habitat diversity, island area, and isolation on species richness for both island sets. Finally, pairwise and multiple-site dissimilarity was estimated for both species and habitats across 0.1ha and 1.0-ha islands to investigate whether: (a) variation in species composition was related to habitat composition; and (b) species dissimilarity increased with inter-island distance.Results: SLOSS-based analyses indicated that several small islands contained more species than single large islands in both island sets. This pattern was also observed for habitats, but only in the small islands set. Path analysis suggested that island area had significant direct and indirect (mediated by habitat diversity) effects on species richness. Habitat diversity and island isolation had significant positive and negative effects on species richness, respectively, independent of island area. Species and habitat dissimilarities were significantly related across 0.1-ha but not 1.0-ha islands, and showed no relationship to inter-island distance. Main conclusions:The overall positive relationship between fragmentation and species richness may be attributed to greater habitat diversity and increased species dissimilarity across smaller islands relative to larger islands. However, negative isolation effects indicate that landscape configuration is still an important conservation consideration. These results each align with different predictions of the theory of island biogeography, sample-area effect, and habitat diversity hypothesis, questioning the exclusivity of these theoretical frameworks. K E Y W O R D S habitat amount hypothesis, habitat diversity, habitat fragmentation, island biogeography, sample-area effect, SLOSS, small island effect, species-area relationship, structural equation model, vascular plants Author contributions: All authors collectively conceived of the study design; I.D.A. completed data collection and species identification with assistance from Z.G.M.; Z.G.M. completed the analysis and led the writing wi...

Butterflies are widely invoked as model organisms in studies of metapopulation and dispersal processes. Integral to such investigations are understandings of perceptual range; the maximum distance at which organisms are able to detect patches of suitable habitat. To infer perceptual range, researchers have released butterflies at varying distances from habitat patches and observed their subsequent flight behaviors. It is often assumed that butterflies rely on visual senses for habitat detection; however, this assumption has not been explicitly investigated. Here, we assess the extent and sensory determinants of perceptual range for the great spangled fritillary (Speyeria cybele (Fabricius, 1775)) and Atlantis fritillary (Speyeria atlantis (W.H. Edwards, 1862)). This was achieved by experimentally releasing butterflies over open water at various distances from a lake island, representing an isolated habitat patch in a dichotomous habitat-matrix landscape. To infer whether butterflies rely on vision for habitat detection, we exposed a subset of butterflies to a series of intense light flashes before release to induce flash blindness (bleaching of photoreceptive rhodopsins) without affecting olfaction. Flashed individuals were 30.1 times less likely to successfully navigate to the target island after release, suggesting butterflies rely primarily on visual senses to navigate fragmented landscapes. For unflashed butterflies, the likelihood of successful navigation decreased by a factor of 2.1 for every 10 m increase in release distance. However, no specific distance threshold for perceptual range was observed. We therefore suggest that perceptual range is best viewed as a continuum of probabilities (targeting ability), reflecting the likelihood of habitat detection across a range of distances.

Diverse geographic, environmental, and ecological factors affect gene flow and adaptive genomic variation within species. With recent advances in landscape ecological modelling and high‐throughput DNA sequencing, it is now possible to effectively quantify and partition their relative contributions. Here, we use landscape genomics to identify determinants of genomic differentiation in the forest tent caterpillar, Malacosoma disstria, a widespread and irruptive pest of numerous deciduous tree species in North America. We collected larvae from multiple populations across Eastern Canada, where the species experiences a diversity of environmental gradients and feeds on a number of different host tree species, including trembling aspen (Populus tremuloides), sugar maple (Acer saccharum), red oak (Quercus rubra), and white birch (Betula papyrifera). Using a combination of reciprocal causal modelling (RCM) and distance‐based redundancy analyses (dbRDA), we show that differentiation of thousands of genome‐wide single nucleotide polymorphisms (SNPs) among individuals is best explained by a combination of isolation by distance, isolation by environment (spatial variation in summer temperatures and length of the growing season), and differences in host association. Configuration of suitable habitat inferred from ecological niche models was not significantly related to genomic differentiation, suggesting that M. disstria dispersal is agnostic with respect to habitat quality. Although population structure was not discretely related to host association, our modelling framework provides the first molecular evidence of host‐associated differentiation in M. disstria, congruent with previous documentation of reduced growth and survival of larvae moved between natal host species. We conclude that ecologically mediated selection is contributing to variation within M. disstria, and that divergent adaptation related to both environmental conditions and host association should be considered in ongoing research and management of this important forest pest.

The island species area relationship (ISAR) is an important tool for measuring variation in species diversity in variety of insular systems, from true‐island archipelagoes to fragmented terrestrial landscapes. However, it suffers from several limitations. For example, due to the sample‐area effect, positive relationships between species and area cannot be directly interpreted as evidence for deterministic effects of area per se. Additionally, richness‐based analyses may obscure species‐level responses to area and isolation that may better inform conservation practice. Here, we use random placement models to control for variation in abundance, occupancy and richness associated with the sample‐area effect, allowing deterministic effects of area and isolation, and how they vary with species' functional traits, to be resolved using linear mixed effects models. We demonstrate the utility of this approach using a butterfly assemblage persisting on a naturally fragmented landscape of lake islands. The ISAR did not significantly deviate from random placement in relation to island area, isolation or habitat diversity, supporting stochastic assembly consistent with the sample‐area effect. Such inferences support the habitat amount hypothesis, which prioritizes preserving the maximum amount of habitat irrespective of its degree of fragmentation. However, species‐level analyses demonstrated that species' abundances were significantly lower on both smaller and more isolated islands than what is predicted by the sample‐area effect. Moreover, effects of area per se were significantly greater for smaller, less mobile and rare species. Species' occurrences also significantly deviated from predictions of the sample‐area effect in relation to island isolation. Thus, our approach illustrates that richness‐based analyses not only result in incorrect inferences on mechanisms underlying ISARs, but also obscure important effects of area per se and isolation on individual species that vary with functional traits. We therefore suggest that these effects should not be solely inferred from richness‐based analyses, but rather evaluated on a species‐by‐species basis.

Recent advances in both genomics and ecological modelling present new, multidisciplinary opportunities for resolving species boundaries and understanding the mechanisms that maintain their integrity in regions of contact. Here, we use a combination of high-throughput DNA sequencing and ecological niche modelling to resolve species boundaries and niche divergence within the Speyeria atlantis-hesperis (Lepidoptera: Nymphalidae) complex, a confusing group of North American butterflies. This complex is notorious for its muddled species delimitations, morphological ambiguity, and extensive mitonuclear discordance. Our admixture and multispecies coalescent-based analyses of single nucleotide polymorphisms identified substantial divergences between S. atlantis and S. hesperis in areas of contact, as well as between distinct northern and southern lineages within S. hesperis. Our results also provide evidence of past introgression relating to another species, S. zerene, which previous work has shown to be more distantly related to the S. atlantis-hesperis complex. We then used ecological models to predict habitat suitability for each of the three recovered genomic lineages in the S. atlantis-hesperis complex and assessed their pairwise niche divergence. These analyses resolved that these three lineages are significantly diverged in their respective niches and are not separated by discontinuities in suitable habitat that might present barriers to gene flow. We therefore infer that ecologically-mediated selection resulting in disparate habitat associations is a principal mechanism reinforcing their genomic integrity. Overall, our results unambiguously support significant evolutionary and ecological divergence between the northern and southern lineages of S. hesperis, sufficient to recognize the southern evolutionary lineage as a distinct species, called S. nausicaa based on taxonomic priority.