Direct and plant trait‐mediated effects of the local environmental context on butterfly oviposition patterns

Abstract

Variation in the intensity of plant–animal interactions over different spatial scales is widespread and might strongly influence fitness and trait selection in plants. Differences in traits among plant individuals have been shown to influence variation in interaction intensities within populations, while differences in environmental factors and community composition are shown to be important for variation over larger scales. However, little is still known about the relative importance of the local environmental context vs. plant traits for the outcome of interactions within plant populations. We investigated how oviposition by the seed‐predator butterfly Phengaris alcon on its host plant Gentiana pneumonanthe was related to host plant traits and to local environmental variation, as well as how oviposition patterns translated into effects on host plant fruit set. We considered the local environmental context in terms of height of the surrounding vegetation and abundance of the butterfly's second host, Myrmica ants. The probability of oviposition was higher in plants that were surrounded by lower vegetation, and both the probability of oviposition and the number of eggs increased in early‐flowering and tall plants with many flowers in the three study populations. Flowering phenology, shoot height and flower production were, in turn, related to higher surrounding vegetation. Myrmica abundance was correlated with vegetation height, but had no effect on oviposition patterns. Oviposition and subsequent seed predation by the caterpillars strongly reduced host plant fruit set. Our results show that plant–animal interactions are context‐dependent not only because the context influences the abundance or the behavior of the animal interactor, but also because it influences the expression of plant traits that affect the outcome of the interaction. The results also demonstrate that heterogeneity in environmental conditions at a very local scale can be important for the outcomes of interactions.

Resource overlap and dilution effects shape host plant use in a myrmecophilous butterfly

“…We found that both the probability of P. alcon butterflies laying at least one egg on a host plant and the number of eggs increased with an earlier phenology of its host plant G. pneumonanthe . This agrees with previous studies showing that insects usually prefer plants with a particular phenology (Elzinga et al., ), and more specifically, that antagonists preferentially attack early‐flowering plants (König, Wiklund, & Ehrlén, ; Parachnowitsch & Caruso, ; Valdés & Ehrlén, , ). The timing of butterfly oviposition relative to plant phenology is likely to vary among years, depending on temporal variation in both plant and butterfly phenology.…”

“…Higher temperatures associated with climate change are expected to cause significant advances in the phenology of many organisms (Parmesan, ), and the synchrony of plant–insect interactions, like the one examined in this study, might be affected if interacting species respond differently to warming. Anyway, our results, together with previous studies (Valdés & Ehrlén, , ) provide convincing evidence of the important role of the timing of the plant resource (i.e. host plant phenology) relative to the oviposition period of P. alcon .…”

“…Spatial overlap between the distribution of host plants and host ant nests at larger scales has previously been shown to increase the density and population persistence of the congeneric species P. arion (Casacci et al., ; Thomas, Simcox, & Clarke, ). Other studies with Phengaris butterflies have demonstrated effects of host plant phenology (Thomas & Elmes, ; Valdés & Ehrlén, , ), Myrmica presence (Van Dyck et al., ; Wynhoff & van Langevelde, ; Wynhoff et al., ) or both of these factors (Casacci et al., ) for oviposition site selection. Our study suggests that the spatial overlap between these two resources also at the scale of the host plant individual is associated with butterfly oviposition preference, and that the effects of plant phenology and ant abundance are interactive.…”

“…Each stage corresponds to a time interval of flower development, with higher values indicating a more advanced floral development at the day of recording, that is an earlier flowering phenology. According to the observed average duration of the stages, a one‐unit increase in this phenology measure roughly corresponds to 1 week earlier development (Valdés & Ehrlén, , ). The number of eggs was counted on the whole shoot.…”

“…Female butterflies oviposit preferentially on buds in early phenological stages, probably because these constitute higher‐quality food for the caterpillars; the developing seeds might become tougher, drier and more difficult to digest as buds develop into flowers and fruits (Arnaldo, Gonzalez, Oliveira, van Langevelde, & Wynhoff, ). In some species, host plant flowering time determines oviposition and individuals flowering early in the season receive more eggs because the presence of the preferred bud developmental stages overlaps more with the period of butterfly oviposition in early‐ than in late‐flowering plants (Valdés & Ehrlén, ). Moreover, the mobility of caterpillars after leaving their host plants is low (Hayes, ), and further development is possible only if the foraging range of suitable host ants is sufficiently close to the host plant individual (usually a few metres).…”