Matthew L. Forister

The Ecology of Individuals: Incidence and Implications of Individual Specialization

Most empirical and theoretical studies of resource use and population dynamics treat conspecific individuals as ecologically equivalent. This simplification is only justified if interindividual niche variation is rare, weak, or has a trivial effect on ecological processes. This article reviews the incidence, degree, causes, and implications of individual‐level niche variation to challenge these simplifications. Evidence for individual specialization is available for 93 species distributed across a broad range of taxonomic groups. Although few studies have quantified the degree to which individuals are specialized relative to their population, between‐individual variation can sometimes comprise the majority of the population’s niche width. The degree of individual specialization varies widely among species and among populations, reflecting a diverse array of physiological, behavioral, and ecological mechanisms that can generate intrapopulation variation. Finally, individual specialization has potentially important ecological, evolutionary, and conservation implications. Theory suggests that niche variation facilitates frequency‐dependent interactions that can profoundly affect the population’s stability, the amount of intraspecific competition, fitness‐function shapes, and the population’s capacity to diversify and speciate rapidly. Our collection of case studies suggests that individual specialization is a widespread but underappreciated phenomenon that poses many important but unanswered questions.

The significance of wing pattern diversity in the Lycaenidae: mate discrimination by two recently diverged species

Closely related species of lycaenid butterflies are determinable, in part, by subtle differences in wing pattern. We found that female wing patterns can act as an effective mate‐recognition signal in some populations of two recently diverged species. In field experiments, we observed that males from a Lycaeides idas population and an alpine population of L. melissa preferentially initiate courtship with conspecific females. A morphometric study indicated that at least two wing pattern elements were important for distinguishing the two species: hindwing spots and orange crescent‐shaped pattern elements called aurorae. We deceived male L. idas into initiating courtship with computer generated paper models of heterospecific females when these pattern elements were manipulated, indicating that the wing pattern elements that define the diversity of this group can be effective mate recognition signals.

Identifying units for conservation using molecular systematics: the cautionary tale of the Karner blue butterfly

The federally endangered North American Karner blue butterfly (Lycaeides melissa samuelis) and the closely related Melissa blue butterfly (L. m. melissa) can be distinguished based on life history and morphology. Western populations of L. m. samuelis share mitochondrial haplotypes with L. m. melissa populations, while eastern populations of L. m. samuelis have divergent haplotypes. Here we test two hypotheses concerning the presence of L. m. melissa mitochondrial haplotypes in western L. m. samuelis populations: (i) mitochondrial introgression has occurred from L. m. melissa populations into western L. m. samuelis populations, or (ii) western populations of the nominal L. m. samuelis are more closely related to L. m. melissa than to eastern L. m. samuelis populations, yet are phenotypically similar to the latter. A Bayesian algorithm was used to cluster 190 L. melissa individuals based on 143 informative amplified fragment length polymorphism (AFLP) loci. This method clearly differentiated L. m. samuelis and L. m. melissa. Thus, genomic divergence was greater between western L. m. samuelis populations and L. m. melissa populations than it was between western and eastern populations of L. m. samuelis. This supports the hypothesis that the presence of L. m. melissa mitochondrial haplotypes in western L. m. samuelis populations is the result of mitochondrial introgression. These data provide valuable information for conservation and management plans for the endangered L. m. samuelis, and illustrate the risks of using data from a single locus for diagnosing significant units of biodiversity for conservation.

Oviposition preference and larval performance within a diverging lineage of lycaenid butterflies

Abstract. 1. The butterfly genus Mitoura in Northern California includes three nominal species associated with four host plants having parapatric or interdigitated ranges. Genetic analyses have shown the taxa to be very closely related, and adults from all host backgrounds will mate and produce viable offspring in the laboratory. Oviposition preference and larval performance were investigated with the aim of testing the hypothesis that variation in these traits can exist in a system in which non‐ecological barriers to gene flow (i.e. geographic barriers and genetic incompatibilities) appear to be minimal.

The genetic architecture of a niche: variation and covariation in host use traits in the Colorado potato beetle

The genetic basis of host plant use by phytophagous insects can provide insight into the evolution of ecological niches, especially phenomena such as specialization and phylogenetic conservatism. We carried out a quantitative genetic analysis of multiple host use traits, estimated on five species of host plants, in the Colorado potato beetle, Leptinotarsa decemlineata (Coleoptera: Chrysomelidae). Mean values of all characters varied among host plants, providing evidence that adaptation to plants may require evolution of both behavioral (preference) and post‐ingestive physiological (performance) characteristics. Significant additive genetic variation was detected for several characters on several hosts, but not in the capacity to use the two major hosts, a pattern that might be caused by directional selection. No negative genetic correlations across hosts were detected for any ‘performance’ traits, i.e. we found no evidence of trade‐offs in fitness on different plants. Larval consumption was positively genetically correlated across host plants, suggesting that diet generalization might evolve as a distinct trait, rather than by independent evolution of feeding responses to each plant species, but several other traits did not show this pattern. We explored genetic correlations among traits expressed on a given plant species, in a first effort to shed light on the number of independent traits that may evolve in response to selection for host–plant utilization. Most traits were not correlated with each other, implying that adaptation to a novel potential host could be a complex, multidimensional ‘character’ that might constrain adaptation and contribute to the pronounced ecological specialization and the phylogenetic niche conservatism that characterize many clades of phytophagous insects.

Foraging ecology of small mammals in semiarid chile: The interplay of biotic and abiotic effects

We report on the foraging behavior of three small-mammal species over 2½ years using experimental foraging trays. Trays provisioned with oat seed were set out in the field at dawn and dusk, and the amount of seed consumed (with re-provisioning) was determined over three consecutive nights and days. Experiments simultaneously evaluated the role of lunar phase, season, and microhabitat (shrub vs. open); additionally, artificial exclosures (∼0.56 ha) were employed to evaluate the influence both of mammalian and avian predators and of a dominant competitor species, the large diurnal degu (Octodon degus). Other principal small mammals included Darwins leaf-eared mouse (Phyllotis darwini) and the olive field mouse (Akodon olivaceus). Trays were enclosed by hardware-cloth cages with either small or large holes in the side to exclude avian consumers and provide selective access to all small mammals (large holes) or to exclude the largest species (small holes). Small-mammal population sizes varied greatly through...

Geological barriers and restricted gene flow in the holarctic skipper Hesperia comma (Hesperiidae)

Patterns of genetic variation within a species may be a consequence of historical factors, such as past fragmentation, as well as current barriers to gene flow. Using sequence data from the mitochondrial cytochrome oxidase subunit II region (COII) and the nuclear gene wingless, we conducted a phylogeographical study of the holarctic skipper Hesperia comma to elucidate patterns of genetic diversity and to infer historical and contemporary processes maintaining genetic variation. One hundred and fifty‐one individuals were sampled from throughout North America and Eurasia, focusing on California and adjacent regions in the western United States where morphological diversity is highest compared to the rest of the range. Analyses of sequence data obtained from both genes revealed a well‐supported division between the Old and New World. Within western North America, wingless shows little geographical structure, while a hierarchical analysis of genetic diversity of COII sequences indicates three major clades: a western clade in Oregon and Northern California, an eastern clade including the Great Basin, Rocky Mountains and British Columbia, and a third clade in southern California. The Sierra Nevada and the Transverse Ranges appear to be the major barriers to gene flow for H. comma in the western United States. Relatively reduced haplotype diversity in Eurasia compared to North America suggests that populations on the two continents have been affected by different historical processes.

Egg Morphology Varies Among Populations and Habitats Along a Suture Zone in the Lycaeides idas-melissa Species Complex (Lepidoptera: Lycaenidae)

Abstract Insect eggs are often characterized by an intricately sculptured external surface, which has been used to a limited extent as a taxonomic character, but not at all in comparisons among populations within a species or among populations of closely related species. We describe egg morphology by using scanning electron microscopy from 12 populations within a species complex of host-specific butterflies in the family Lycaenidae. Variation is found among populations and habitats. Adaptive hypotheses are discussed, and the observed variation is placed within the biogeographical context of the Sierra Nevada Mountains and adjacent regions.

Influence of Host Plant Phenology on Mitoura nelsoni (Lepidoptera: Lycaenidae)

Abstract The influence of host plant phenology on the univoltine specialist lepidopteran herbivore Mitoura nelsoni Boisduval was investigated using incense cedar, Calocedrus decurrens Torrey. The hypothesis that new spring growth is an optimal resource for M. nelsoni was tested by rearing larvae on plants collected along an elevational gradient at two times in the spring (both before and during the typical flight period of M. nelsoni). The oviposition preferences of females were assayed with the same plants. M. nelsoni pupae grew to consistently greater pupal weights when reared on incense cedar branches in the earliest phenological stages (largely consisting of branches taken from trees that had not initiated new growth), although females avoided ovipositing on prenew growth branches. Trees in the earliest phenological stages, which resulted in the highest larval performance, were collected before the typical flight season of M. nelsoni. The phenology of M. nelsoni does not seem to be synchronized to host conditions that are optimal for larval development. These results are discussed within the context of host-associated speciation in the genus Mitoura and temporal isolation that may be an important component of reproductive isolation between M. nelsoni and a closely related species in northern California.

Wing Pattern Variation in the Anise Swallowtail, Papilio zelicaon (Lepidoptera: Papilionidae)

Abstract Papilio zelicaon Lucas has become a common study system, and has been the subject of numerous investigations into ecological variation among populations and regions. Morphological variation has been less intensively studied, with the exception of a dark form occurring on the eastern edge of the animal’s range. In 1968, Remington described populations from the Rocky Mountains and adjacent areas as a distinct species, Papilio gothica. Gothica was not widely adopted, and was subsequently reduced to a junior synonym of P. zelicaon. Using a new wing character not previously considered, we reexamine the evidence for P. gothica, and investigate patterns of morphological variation within the California range of P. zelicaon. Evidence is found supporting significant phenotypic differences within and among three areas: lowland California, montane California, and Colorado. These results are discussed in terms of seasonal and geographic variation, and their bearing on the status of gothica. We conclude that there is significant and patterned phenotypic variation in P. zelicaon, but any taxonomic conclusions at this time would be premature.