W. Scott Armbruster

Improving assessment and modelling of climate change impacts on global terrestrial biodiversity

Understanding how species and ecosystems respond to climate change has become a major focus of ecology and conservation biology. Modelling approaches provide important tools for making future projections, but current models of the climate-biosphere interface remain overly simplistic, undermining the credibility of projections. We identify five ways in which substantial advances could be made in the next few years: (i) improving the accessibility and efficiency of biodiversity monitoring data, (ii) quantifying the main determinants of the sensitivity of species to climate change, (iii) incorporating community dynamics into projections of biodiversity responses, (iv) accounting for the influence of evolutionary processes on the response of species to climate change, and (v) improving the biophysical rule sets that define functional groupings of species in global models.

Evolution of plant pollination systems : hypotheses and tests with the neotropical vine Dalechampia

The results of pollination and mating‐system studies were integrated with a phylogenetic study of 40 Neotropical species of Dalechampia L. (Euphorbiaceae) to reconstruct the history of evolutionary change in pollination systems. The results of this analysis were treated as a hypothesis and tested for circularity problems and robustness in the face of changes in the data set. The historical hypothesis was used to make specific predictions about details of pollination ecology and reward biochemistry; the predictions were supported by independent observations. I conclude that pollination systems in Dalechampia have been evolutionarily labile, relative to most morphological features, with repeated parallelisms and reversals. Transitions among the three pollination systems evolved by Dalechampia (pollination by resin‐collecting bees, fragrance‐collecting male euglossine bees, and pollen‐collecting bees) have been facilitated by biochemical exaptation (preadaptation). Pollination by male euglossine bees is relatively rare in the genus but has originated independently three to four times. In contrast, pollination by resin‐collecting female bees is very common, but has originated only once. Eighty‐six to 97% of transitions between pollination systems involved an intermediate phase during which both old and new pollinators were effective, but 3 to 14% of transitions may have been “instantaneous,” lacking the intermediate phase. Clades of species secreting resin rewards are about 10 times as species rich as clades of species secreting fragrance rewards; circumstantial evidence suggests that different extinction rates may be responsible for this difference. Relatively allogamous (cross‐pollinating) species have evolved from more autogamous (self‐pollinating) species up to 13 times, and autogamous species have evolved from more allogamous ones up to 11 times. Species occurring in disturbed habitats are facultatively autogamous, whereas species of undisturbed habitats are often highly allogamous.

Switch from specialized to generalized pollination

The once prevalent view that the evolution of extreme ecological specialization is accompanied by a loss of potential for adapting to new conditions, and thus is irreversible, has been challenged by several recent examples,,. However, we know of no modern phylogenetic studies showing reversal in pollination relationships from extreme specialization to generalization, although such reversals are theoretically expected,. Here we present molecular phylogenetic evidence for an evolutionary shift in Dalechampia (Euphorbiaceae) vines from a highly specialized relationship (pollination by one or a few animal species,) with resin-collecting bees to generalized pollination by a variety of pollen-feeding insects. This shift was associated with dispersal from Africa to Madagascar, where the specific resin-collecting pollinators are absent. These results show that plants dispersing beyond the range of their specific pollinators may succeed by evolving more generalized pollination systems.

Associations between floral specialization and species diversity: cause, effect, or correlation?

It has been proposed frequently, from Darwin’s time onwards, that specialized pollination increases speciation rates and thus the diversity of plant species (i.e. clade species richness). We suggest here that the correlation between clade species richness and floral specialization is real, but that clade species richness is frequently the cause, not the result of floral specialization. We urge a broader, variance-partitioning perspective for assessing the causes of this correlation by suggesting four models of how the diversity-specialization correlation might come about: (1) floral specialization promotes initial reproductive isolation (“Initial-RI” model), (2) floral specialization promotes reinforcement of reproductive isolation upon secondary contact (“Reinforcement” model), (3) floral specialization reduces the extinction rate by promoting tighter species packing (“Extinction” model), (4) floral specialization is the result of high clade species richness, which increases the number of related species in communities, and thus selects for floral character displacement (“Character-Displacement” model). These hypotheses are evaluated by comparing the relationships between species richness, speciation mechanisms, and pollination precision, accuracy, and specialization in the broader literature and, more specifically, in four study systems: Dalechampia (Euphorbiaceae), Collinsia (Plantaginaceae), Burmeistera (Campanulaceae), and Stylidium (Stylidiaceae). These systems provide stronger support for the character-displacement hypothesis, wherein local species diversity drives the evolution of specialized pollination. Although the two reproductive-isolation hypotheses may hold for plants like orchids, with extremely precise pollination systems, the reproductive character-displacement hypothesis seems likely to be more important for plant groups with less precise pollination systems.

Multilevel Comparative Analysis of the Morphology, Function, and Evolution of Dalechampia Blossoms

Morphological and ecological data from Dalechampia flowers were subjected to analysis of covariance and component regression analysis, a technique that allows separate estimation of within— and among—group relationships. They yielded results consistent with hypotheses stating that bee—flower relationships are influenced by several morphological parameters of the blossoms (pseudanthial inflorescences). The following correlations emerged: (1) the area of the gland that secretes the pollinator reward (and hence the amount of reward) and the size of the largest bees that visit the flowers; (2) the distance between gland and stigmas and the size of the smallest floral visitor contacting the stigmas; and (3) the distance between gland and anthers and the size of the smallest floral visitor contacting the anthers. Together these variables appear to determine which subset of the bee fauna will be effective in pollination. The distance between the anthers and stigma was correlated with the rate of autopollination ...

Evolvability and genetic constraint in Dalechampia blossoms: components of variance and measures of evolvability

Abstract Many evolutionary arguments are based on the assumption that quantitative characters are highly evolvable entities that can be rapidly moulded by changing selection pressures. The empirical evaluation of this assumption depends on having an operational measure of evolvability that reflects the ability of a trait to respond to a given external selection pressure. We suggest short‐term evolvability be measured as expected proportional response in a trait to a unit strength of directional selection, where strength of selection is defined independently of character variation and in units of the strength of selection on fitness itself. We show that the additive genetic variance scaled by the square of the trait mean, IA, is such a measure. The heritability, h2, does not measure evolvability in this sense. Based on a diallel analysis, we use IA to assess the evolvability of floral characters in a population of the neotropical vine Dalechampia scandens (Euphorbiaceae). Although we are able to demonstrate that there is additive genetic variation in a number of floral traits, we also find that most of the traits are not expected to change by more than a fraction of a percent per generation. We provide evidence that the degree of among‐population divergence of traits is related to their predicted evolvabilities, but not to their heritabilities.

PATTERNS OF CHARACTER DIVERGENCE AND THE EVOLUTION OF REPRODUCTIVE ECOTYPES OF DALECHAMPIA SCANDENS (EUPHORBIACEAE)

The variation of four floral characters (resin‐gland area, gland‐stigma distance, gland‐anther distance, and anther‐stigma distance) was analyzed across 15 populations of Dalechampia scandens occurring sympatrically, in various combinations, with five other congeners. Univariate and multivariate analysis of variance and a posteriori comparison tests indicate that there are significant statistical patterns of character divergence away from sympatric congeners for three of the floral characters. These characters, which on the basis of common garden studies appear to be under genetic control, may not vary independently; i.e., genetic control may be overlapping. The characters appear to be functionally related. Populations of Dalechampia scandens occurring sympatrically with congeners possessing relatively large resin glands, large gland‐stigma distances and large anther‐stigma distances (e.g., D. dioscoreifolia and D. affinis) have significantly smaller resin glands, gland‐stigma distances and anther‐stigma distances than do populations occurring sympatrically with congeners with relatively small resin glands, gland‐stigma distances, and anther‐stigma distances (e.g., D. cissifolia, D. heteromorpha and D. schottii). Populations of D. scandens not sympatric with other Dalechampia species generally have intermediately sized structures. The pattern of bidirectional divergence is consistent with the evolutionary scenario that selection against interspecific pollination has resulted in local ecotypic differentiation and character displacement in populations sympatric with ecologically similar congeners.

Local temperatures inferred from plant communities suggest strong spatial buffering of climate warming across Northern Europe

Recent studies from mountainous areas of small spatial extent (<2500 km(2) ) suggest that fine-grained thermal variability over tens or hundreds of metres exceeds much of the climate warming expected for the coming decades. Such variability in temperature provides buffering to mitigate climate-change impacts. Is this local spatial buffering restricted to topographically complex terrains? To answer this, we here study fine-grained thermal variability across a 2500-km wide latitudinal gradient in Northern Europe encompassing a large array of topographic complexities. We first combined plant community data, Ellenberg temperature indicator values, locally measured temperatures (LmT) and globally interpolated temperatures (GiT) in a modelling framework to infer biologically relevant temperature conditions from plant assemblages within <1000-m(2) units (community-inferred temperatures: CiT). We then assessed: (1) CiT range (thermal variability) within 1-km(2) units; (2) the relationship between CiT range and topographically and geographically derived predictors at 1-km resolution; and (3) whether spatial turnover in CiT is greater than spatial turnover in GiT within 100-km(2) units. Ellenberg temperature indicator values in combination with plant assemblages explained 46-72% of variation in LmT and 92-96% of variation in GiT during the growing season (June, July, August). Growing-season CiT range within 1-km(2) units peaked at 60-65°N and increased with terrain roughness, averaging 1.97 °C (SD = 0.84 °C) and 2.68 °C (SD = 1.26 °C) within the flattest and roughest units respectively. Complex interactions between topography-related variables and latitude explained 35% of variation in growing-season CiT range when accounting for sampling effort and residual spatial autocorrelation. Spatial turnover in growing-season CiT within 100-km(2) units was, on average, 1.8 times greater (0.32 °C km(-1) ) than spatial turnover in growing-season GiT (0.18 °C km(-1) ). We conclude that thermal variability within 1-km(2) units strongly increases local spatial buffering of future climate warming across Northern Europe, even in the flattest terrains.

Partitioning and Sharing of Pollinators by Four Sympatric Species of Dalechampia (Euphorbiaceae) in Panama

Repartition des especes dans la zone du canal de Panama. Morphologie des fleurs, pollinisation

PHENOTYPIC SELECTION ON DALECHAMPIA BLOSSOMS: HONEST SIGNALING AFFECTS POLLINATION SUCCESS

Pollinators may choose which flowers to visit by direct assessment of rewards or by indirect assessment of “honest” advertisements or other traits correlated with the quantity or quality of reward. We wished to know whether selection generated by pollinators acted directly or indirectly on floral rewards, on advertisement traits, and/or traits affecting pollinator efficiency (their fit with flowers) in Dalechampia vines (Euphorbiaceae) and whether the advertisement (bract size) was correlated honestly with reward amount (measured by resin-gland area). In Gabon we studied bee visitation and pollen arrival rates to blossoms of D. ipomoeifolia Benth. and found that, despite the apparent visibility of the resin reward (and its volume), the strongest bee-mediated natural selection acted directly on bract size rather than gland area. Blossoms with larger bracts were visited more often by the only pollinators, female Heriades nr. spiniscutis (Apoidea: Megachilidae), and these blossoms received more pollen on their stigmas. Blossoms with larger resin glands were also visited more often and received more pollen overall, but this effect disappeared when bract size (which was phenotypically correlated with gland size) was controlled for statistically. These observational data were confirmed by experimental reduction of bract size, which significantly decreased pollen arrival rates. Thus, the bees appear to rely on the “honest” correlation between advertisement and reward in choosing the best blossoms to visit, and this behavior generates direct selection for larger bracts and indirect selection for larger resin glands. Bees visiting blossoms with larger separation between the gland and stigmas contacted the stigmas less frequently, and such blossoms received less pollen on their stigmas. Because gland area, bract size, and gland–stigma separation are positively correlated phenotypically, response to selection for larger bracts may be limited in this population by conflicting selection against large gland–stigma separation.