Extinction-colonization dynamics and host-plant choice in butterfly metapopulations.

Species living in highly fragmented landscapes often occur as metapopulations with frequent population turnover. Turnover rate is known to depend on ecological factors, such as population size and connectivity, but it may also be influenced by the phenotypic and genotypic composition of populations. The Glanville fritillary butterfly (Melitaea cinxia) in Finland uses two host-plant species that vary in their relative abundances among distinct habitat patches (dry meadows) in a large network of approximately 1,700 patches. We found no effect of host species use on local extinction. In contrast, population establishment was strongly influenced by the match between the host species composition of an empty habitat patch and the relative host use by larvae in previous years in the habitat patches that were well connected to the target patch. This "colonization effect" could be due to spatially variable plant acceptability or resistance or to spatially variable insect oviposition preference or larval performance. We show that spatial variation in adult oviposition preference occurs at the relevant spatial scale and that the other possible causes of the colonization effect can be discounted. We conclude that the colonization effect is generated by host preference influencing the movement patterns of ovipositing females. Migrant females with dissimilar host preferences have different perceptions of relative patch quality, which influences their likelihood of colonizing patches with particular host composition.

The difficulty of deducing behavior from resource use: an example from hilltopping in checkerspot butterflies.

Resource preferences of individuals cannot be deduced from patterns of resource use at the population level. For example, a particular pattern of resource use can be produced by a variety of preference distributions in an animal population. The strength of the association between resource preference (a behavioral parameter) and resource use (an ecological parameter) detemines the extent to which natural selection acting on resource use translates into selection acting on preference. However, the mechanism underlying the evolution of resource use is the evolution of preference (unless the resource itself is evolving). Therefore, a clear distinction between ecological and behavioral parameters is necessary for any evolutionary prediction. We apply these principles to the use of a particular spatial resource, hilltops, by the butterfly Euphydryas editha. We distinguish between the occupation of a hilltop location (resource use) and preference for these locations, or "hilltopping" behavior. Previous authors estimated that the mating success of male E. editha on a hilltop was lower than that on the slopes below. In consequence, hilltopping by these males was labeled "maladaptive." We argue that the evidence presented by these authors fails to demonstrate (1) that hilltopping behavior occurred, (2) that, if it did occur, it was variable among individuals, and (3) that, if variation of hilltopping was present, it was associated with mating success. Thus, discussion of the evolutionary significance of intrapopulation variation of hilltopping behavior is premature. In our own study of a different population of the same species, we failed to find evidence of hilltopping.

Quinolizidine alkaloids obtained byPedicularis semibarbata (Scrophulariaceae) fromLupinus fulcratus (Leguminosae) fail to influence the specialist herbivoreEuphydryas editha (Lepidoptera).

Pedicularis semibarbata is apparently an obligate hemiparasite of coniferous trees. It is also a facultative parasite ofLupinus fulcratus from which we find that it obtains quinolizidine alkaloids, principally α-isolupanine. As a result, a single population ofP. semibarbata contains both alkaloidrich and alkaloid-free plants. The butterflyEuphydryas editha naturally oviposits on both plant types. This butterfly population, which is the principal herbivore attackingP. semibarbata at this site, is known to contain two morphs. Individuals of a specialist morph discriminate when ovipositing among individualP. semibarbata plants and produce offspring that survive better on accepted than on rejected plants. Those of a generalist morph accept allP. semibarbata plants and produce offspring that survive equally well on plants accepted or rejected by the discriminating morph. Because of the existence of this complex variation among the butterflies, the presence of naturally laid eggs on alkaloid-containing plants still leaves the possibility that the alkaloids may defend the plants against the specialist morph. In experiments on both oviposition preference and larval performance in early instars, we failed to detect any correlation between alkaloid content of a plant and either its acceptability to or suitability for the discriminating morph of the insect. Alkaloid presence in the host-plant population, achieved through root parasitism, is currently neither subject to strong insect-mediated selection nor a major cause of selection on the insects.

HERITABILITY OF OVIPOSITION PREFERENCE AND ITS RELATIONSHIP TO OFFSPRING PERFORMANCE WITHIN A SINGLE INSECT POPULATION.

Within a population of the butterfly Euphydryas editha that oviposits predominantly on two host species, heritable variation in postalighting oviposition preference was found. In a separate experiment, oviposition preference of adult females was found to be correlated with offspring performance (growth). There was a significant tendency for offspring to perform better on the host species that their female parent preferred. Analysis of the data showed that no single factor, neither maternal preference nor the host species on which the offspring were raised, accounted for any significant variation in larval performance. However, the effect of the interaction between host species and maternal preference on offspring performance was highly significant. These findings imply specialization in both oviposition preference and offspring performance by individuals within a single population. With present evidence, this preference-performance correlation is likely to be genetic. However, as in previous studies, other interpretations cannot be excluded.

INCORPORATION OF A EUROPEAN WEED INTO THE DIET OF A NORTH AMERICAN HERBIVORE.

Populations of the butterfly Euphydryas editha living within a 30 times 100-km region on the eastern slope of the Sierra Nevada range were compared for oviposition preference and ability of larvae to grow and survive on two host plants, Collinsia parviflora and Plantago lanceolata. Since its introduction approximately 100 years ago, P. lanceolata has been incorporated in the diet of E. editha in one of the study populations. The populations differed in oviposition preference; only the population that uses P. lanceolata contains some individuals that prefer P. lanceolata. Larvae from two populations, one using both P. lanceolata and C. parviflora, the other using only C. parviflora, were not found to differ in relative abilities to grow or survive on P. lanceolata. The potential for E. editha to use P. lanceolata appears in populations that have had no prior exposure to this plant, while oviposition preference for this plant has evolved in the population in which the plant now grows.

Extinction, reduction, stability and increase: The responses of checkerspot butterfly (Euphydryas) populations to the California drought.

The California drought of 1975-77 has been correlated with unusual size changes in populations of two species of Euphydryas butterflies. Several populations became extinct, some were dramatically reduced, others remained stable and at least one increased. These differences in population dynamic response are not concordant with predictions made earlier that populations with heavy density-dependent mortality should be more stable in the face of drought than unregulated populations. The different responses are related to the fine details of the relationships between the insects and their host plants, relationships which are variable between populations. Revised predictions are given in the light of better knowledge of the variability and complexity of these insect-host relationships. The diversity of responses underlines the dangers of generalizing about "the ecology" of a taxonomic species. The extinctions support the view that such events are frequent and significant in the biology of populations.

Complex Components of Habitat Suitability within a Butterfly Colony.

The microdistribution of adult Euphydryas editha changes from year to year, and the colony is subdivided into three populations that fluctuate independently in size. These observations are attributed largely to fluctuations in time and space of three complex larval resources associated with the availability of food. This cotnplexity also entails selection pressure favoring the observed low dispersal tendency of adults.

Butterfly feeding on lycopsid.

Larvae of Euptychia westwoodi feed on Selaginella, a lycopsid. This is the first feeding record in the Satyrinae outside the monocotyledons and one of the few records of a butterfly feeding on other than a seed plant. Clues to possible evolutionary origins of this habit are found in the oviposition behavior of other Euptychia species.