Summary1. Shifts in the spatial and temporal patterns of flowering could affect the resources available to pollinators, and such shifts might become more common as climate change progresses. 2. As mid-summer temperatures have warmed, we found that a montane meadow ecosystem in the southern Rocky Mountains of the United States exhibits a trend toward a bimodal distribution of flower abundance, characterized by a mid-season reduction in total flower number, instead of a broad, unimodal flowering peak lasting most of the summer season. 3. We examined the shapes of community-level flowering curves in this system and found that the typical unimodal peak results from a pattern of complementary peaks in flowering among three distinct meadow types (dry, mesic and wet) within the larger ecosystem. However, high mid-summer temperatures were associated with divergent shifts in the flowering curves of these individual meadow types. Specifically, warmer summers appeared to cause increasing bimodality in mesic habitats, and a longer interval between early and late flowering peaks in wet and dry habitats. 4. Together, these habitat-specific shifts produced a longer mid-season valley in floral abundance across the larger ecosystem in warmer years. Because of these warming-induced changes in flowering patterns, and the significant increase in summer temperatures in our study area, there has been a trend toward non-normality of flowering curves over the period . This trend reflects increasing bimodality in total community-wide flowering. 5. The resulting longer periods of low flowering abundance in the middle of the summer season could negatively affect pollinators that are active throughout the season, and shifts in flowering peaks within habitats might create mismatches between floral resources and demand by pollinators with limited foraging ranges. 6. Synthesis. Early-season climate conditions are getting warmer and drier in the high altitudes of the southern Rocky Mountains. We present evidence that this climate change is disrupting flowering phenology within and among different moisture habitats in a sub-alpine meadow ecosystem, causing a mid-season decline in floral resources that might negatively affect mutualists, especially pollinators. Our findings suggest that climate change can have complex effects on phenology at small spatial scales, depending on patch-level habitat differences.
Pollinator-mediated reproductive isolation is often a principal factor in determining the rate of hybridization between plant species.Pollinator preference and constancy can reduce interspecific pollen transfer between otherwise interfertile, coflowering species.The importance of this ethological isolation can be assessed by comparing the strength of preference and constancy of pollinators in contact sites that differ in the frequency of hybrid individuals. We observed visitation by hummingbirds and hawkmoths in natural single-species patches and artificial mixed-species arrays in two Ipomopsis aggregata/I. tenuituba contact sites-one with few hybrids, and one in which hybrids are abundant. Pollinator preference and constancy were stronger at the low-frequency hybrid site, especially for hawkmoths (Hyles lineata). Hawkmoths at the low-frequency hybrid site showed significant preference and constancy for I. tenuituba, while at the high-frequency site hawkmoths visited both species equally. One hypothesis that might explain these differences in hawkmoth foraging is that warmer nights at the low-frequency hybrid site allow for nocturnal foraging where the light-colored corollas of I. tenuituba have a visibility advantage. These differences in hawkmoth behavior might in turn affect hummingbirds differently at the two sites, through changes in nectar resources, leading to greater pollinator-mediated isolation at the low-frequency hybrid site. Our results suggest that differences in pollinator behaviors between sites can have both direct and indirect effects on hybridization rates between plant species.
Summary• Rates of hybridization vary among angiosperm taxa. Among-taxon variation in hybridization rate has been used to compare the importance of pre-and postzygotic reproductive isolating mechanisms. Variation in rates of hybridization within a single-species pair would suggest that local conditions also affect reproductive isolation within a single taxonomic context.• In this study, contact sites of Ipomopsis aggregata-Ipomopsis tenuituba were surveyed for variation in frequency of hybrids, and spatial structure. Floral morphology was used to identify parent species and hybrids in seven contact sites in the western Rocky Mountains, USA.• Contact sites varied widely in elevational range, the degree to which morphological variation was clinal rather than mosaic and the frequency of hybrids. Two sites provided a strong contrast between a clinal, unimodal site and a mosaic, bimodal site.• This natural variation among contact sites of the same species pair provides an opportunity to assess the effect of local ecological conditions and spatial structure of parent populations on reproductive isolation, while controlling for between-taxon variation.
Various models purporting to explain natural hybrid zones make different assumptions about the fitness of hybrids. One class of models assumes that hybrids have intrinsically low fitness due to genetic incompatibilities, whereas other models allow hybrid fitness to vary across natural environments. We used the intrinsic rate of increase to assess lifetime fitness of hybrids between two species of montane plants Ipomopsis aggregata and Ipomopsis tenuituba planted as seed into multiple field environments. Because fitness is predicted to depend upon genetic composition of the hybrids, we included F1 hybrids, F2 hybrids, and backcrosses in our field tests. The F2 hybrids had female fitness as high, or higher, than expected under an additive model of fitness. These results run counter to any model of hybrid zone dynamics that relies solely on intrinsic nuclear genetic incompatibilities. Instead, we found that selection was environmentally dependent. In this hybrid zone, cytoplasmic effects and genotype-by-environment interactions appear more important in lowering hybrid fitness than do intrinsic genomic incompatibilities between nuclear genes. K E Y W O R D S : Fitness, hybridization, natural selection, reproductive isolation.
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