The effect of climate on masting in the European larch and on its specific seed predators.

Masting is the intermittent production of large seed crops by a population of plants. Two main hypotheses have been proposed to explain masting. Variations in seed crop may result from stochastic climate factors (temperature, rainfall, etc.), and/or masting may be a plant evolutionary strategy to avoid specific seed predators. To determine the effect of climate on the annual pattern of cone production in the European larch (Larix decidua), we analyzed larch cone production from 1975 to 2005 at 20 sites in the French Alps, ranging from 1,300 to 2,100 m a.s.l. (on average 17 years per site were sampled). We examined the effects of mast seeding on the predation of larch cones by the dominant specific pre-dispersal seed predators, cone fly Strobilomyia spp. Larch cone production varied across the years and was spatially synchronized throughout the region. We constructed two models to explain seed production, one for sites at low (<1,800 m) and one for sites at high (> or =1,800 m) altitudes, using partial least squares (PLS) regressions to detect across a large number of climate indices (306) the factors which best explain cone production. Monthly indices were more accurate descriptors than 4-month period indices. The predation rate was lower in high cone production years that followed low production years, supporting the predator satiation hypothesis. However, variable cone production explained only a small part of predation rates (45 and 25% at low and high altitudes, respectively). Predation was also directly affected by climate conditions. PLS regressions taking into account both cone production and climate factors accounted for as much as 68 and 82% of the predation rate variation at low and high altitudes, respectively. This study contributes to a better understanding of how climate factors differently affect the members of an interacting community.

Linking patterns and processes of species diversification in the cone flies Strobilomyia (Diptera: Anthomyiidae).

Phytophagous insects provide useful models for the study of ecological speciation. Much attention has been paid to host shifts, whereas situations where closely related lineages of insects use the same plant during different time periods have been relatively neglected in previous studies of insect diversification. Flies of the genus Strobilomyia are major pests of conifers in Eurasia and North America. They are specialized feeders in cones and seeds of Abies (fir), Larix (larch) ,and Picea (spruce). This close association is accompanied by a large number of sympatric Strobilomyia species coexisting within each tree genus. We constructed a molecular phylogeny with a 1320 base-pair fragment of mitochondrial DNA that demonstrated contrasting patterns of speciation in larch cone flies, as opposed to spruce and fir cone flies; this despite their comparable geographic distributions and similar resource quality of the host. Species diversity is the highest on larch, and speciation is primarily driven by within-host phenological shifts, followed by allopatric speciation during geographical expansion. By contrast, fewer species exploit spruce and fir, and within-host phenological shifts did not occur. This study illustrates within-host adaptive radiation through phenological shifts, a neglected mode of sympatric speciation.