Recent reports of global declines in pollinator species imply an urgent need to assess the abundance of native pollinators and density-dependent benefits for linked plants. In this study, we ...investigated (1) pollinator nest distributions and estimated colony abundances, (2) the relationship between abundances of foraging workers and the number of nests they represent, (3) pollinator foraging ranges, and (4) the relationship between pollinator abundance and plant reproduction. We examined these questions in an alpine ecosystem in the Colorado Rocky Mountains, focusing on four alpine bumble bee species (
Bombus balteatus
,
B. flavifrons
,
B. bifarius
, and
B. sylvicola
), and two host plants that differ in their degrees of pollinator specialization (
Trifolium dasyphyllum
and
T. parryi
).
Using microsatellites, we found that estimated colony abundances among
Bombus
species ranged from ~18 to 78 colonies/0.01 km
2
. The long-tongued species
B. balteatus
was most common, especially high above treeline, but the subalpine species
B. bifarius
was unexpectedly abundant for this elevation range. Nests detected among sampled foragers of each species were correlated with the number of foragers caught. Foraging ranges were smaller than expected for all
Bombus
species, ranging from 25 to 110 m. Fruit set for the specialized plant,
Trifolium parryi
, was positively related to the abundance of its
Bombus
pollinator. In contrast, fruit set for the generalized plant,
T. dasyphyllum
, was related to abundance of all
Bombus
species. Because forager abundance was related to nest abundance of each
Bombus
species and was an equally effective predictor of plant fecundity, forager inventories are probably suitable for assessing the health of outcrossing plant populations. However, nest abundance, rather than forager abundance, better reflects demographic and genetic health in populations of eusocial pollinators such as bumble bees. Development of models incorporating the parameters we have measured here (nest abundance, forager abundance, and foraging distance) could increase the usefulness of foraging worker inventories in monitoring, managing, and conserving pollinator populations.
Ecological partnerships, or mutualisms, are globally widespread, sustaining agriculture and biodiversity. Mutualisms evolve through the matching of functional traits between partners, such as tongue ...length of pollinators and flower tube depth of plants. Long-tongued pollinators specialize on flowers with deep corolla tubes, whereas shorter-tongued pollinators generalize across tube lengths. Losses of functional guilds because of shifts in global climate may disrupt mutualisms and threaten partner species. We found that in two alpine bumble bee species, decreases in tongue length have evolved over 40 years. Co-occurring flowers have not become shallower, nor are small-flowered plants more prolific. We argue that declining floral resources because of warmer summers have favored generalist foraging, leading to a mismatch between shorter-tongued bees and the longer-tubed plants they once pollinated.
Multiple interacting factors drive recent declines in wild and managed bees, threatening their pollination services. Widespread and intensive monitoring could lead to more effective management of ...wild and managed bees. However, tracking their dynamic populations is costly. We tested the effectiveness of an inexpensive, noninvasive and passive acoustic survey technique for monitoring bumble bee behavior and pollination services. First, we assessed the relationship between the first harmonic of the flight buzz (characteristic frequency) and pollinator functional traits that influence pollination success using flight cage experiments and a literature search. We analyzed passive acoustic survey data from three locations on Pennsylvania Mountain, Colorado to estimate bumble bee activity. We developed an algorithm based on Computational Auditory Scene Analysis that identified and quantified the number of buzzes recorded in each location. We then compared visual and acoustic estimates of bumble bee activity. Using pollinator exclusion experiments, we tested the power of buzz density to predict pollination services at the landscape scale for two bumble bee pollinated alpine forbs (Trifolium dasyphyllum and T. parryi). We found that the characteristic frequency was correlated with traits known to affect pollination efficacy, explaining 30-52% of variation in body size and tongue length. Buzz density was highly correlated with visual estimates of bumble bee density (r = 0.97), indicating that acoustic signals are predictive of bumble bee activity. Buzz density predicted seed set in two alpine forbs when bumble bees were permitted access to the flowers, but not when they were excluded from visiting. Our results indicate that acoustic signatures of flight can be deciphered to monitor bee activity and pollination services to bumble bee pollinated plants. We propose that applications of this technique could assist scientists and farmers in rapidly detecting and responding to bee population declines.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Females and males of sexually dimorphic species have distinct resource demands due to differential allocation to reproduction. Sexual allocation theory predicts that functional traits will diverge ...between sexes to support these demands. However, such dimorphism may be masked by the impact of current reproduction on source-sink interactions between vegetative and reproductive organs. We ask whether natural selection has led to genetic dimorphism in homologous physiological traits between sexes of the dioecious willow shrub, Salix glauca. In a common garden experiment we compared physiological responses to drought stress by male and female ramets in the absence of confounding demands from reproductive structures. Ramets experienced similar pre-dawn leaf water status (Ψl) as parental genets in flower within the natural population, indicating that experimental dry-down mirrored environmental conditions in nature. Male and female ramets achieved similar instantaneous water use efficiency, based on the ratio of carbon gain to water loss, under wet and dry conditions. However, female ramets experienced greater water stress (i.e., more negative Ψl) than males under dry conditions. Lower Ψl for female ramets may partly reflect the maintenance of conductance under drought; males, in contrast, maintain Ψl under drought by reducing conductance. Differences between sexes in terms of conductance and leaf water status of the vegetative ramets were absent in a concomitant comparison of parental flowering plants. Our results show (1) genetic divergence in physiology between sexes of S. glauca occurs in the absence of gender-specific reproductive sinks, (2) males are the more physiologically plastic sex with respect to water use, and (3) paradoxically, divergence in water relations between sexes is not detectable at sexual maturity under natural conditions.
Partner abundance affects costs and benefits in obligate mutualisms, but its role in facultative partnerships is less clear. We address this gap in a pollination web consisting of two clovers (
...Trifolium
) that differ in specialization on a bumble bee pollinator
Bombus balteatus
. We examine how pollination niche breadth affects plant responses to pollinator abundance, comparing early-flowering (specialized) and late-flowering (generalized) cohorts of
T. parryi
and early
T. parryi
to
T. dasyphyllum
, a pollination generalist. Co-pollinators disrupt the link between
B. balteatus
visitation and pollination rate for both clovers. Only for early-flowering
T. parryi
do visitation, pollination, and seed set increase with density of
B. balteatus
. Bumble bee density also alters timing of seed germination in
T. parryi
, with seeds from plants receiving augmented
B. balteatus
germinating sooner than seeds of open-pollinated counterparts. Benefits saturate at intermediate bumble bee densities. Despite strong effects of
B. balteatus
density on individual plant fitness components, population models suggest little impact of
B. balteatus
density on λ in
T. parryi
or
T. dasyphyllum
. Findings show that functional redundancy in a pollinator guild mediates host-plant responses to partner density. Unexpected effects of pollinator density on life history schedule have implications for recruitment under pollinator decline.
• The blue light photoreceptor phototropin-1 has been shown to enhance fitness in Arabidosis thaliana under field conditions. Here, we ask whether performance consequences of phototropin-1 reflect ...its impact on root growth and drought tolerance. • We used a PHOT1-GFP gene construct to test whether phototropin-1 abundance in roots is highest at shallow soil depths where light penetration is greatest. We then compared root growth efficiency and size at maturity between individuals with and without functional phototropin-1. Comparisons were made under wet and dry conditions to assess the impact of phototropin-1 on drought tolerance. • Phototropin-1 was most abundant in upper root regions and its impact on root growth efficiency decreased with soil depth. Roots of plants with functional phototropin-1 made fewer random turns and traveled further for a given length (higher efficiency) than roots of phot1 mutants. In dry (but not wet) soil, enhancement of root growth efficiency by phototropin-1 increased plant size at maturity. • Results indicate that phototropin-1 enhances performance under drought by mediating plastic increases in root growth efficiency near the soil surface.
In a controlled environment, we artificially induced drought during flowering of Epilobium angustifolium, an animal-pollinated plant. Leaf water potential (ψ1) and floral traits were monitored over a ...12-d period of soil moisture depletion. Soil moisture depletion induced drought stress over time, as revealed by significant treatment x day interactions for predawn and midday ψ1. Nectar volume and flower size showed significant negative responses to drought stress, but nectar sugar concentration did not vary between treatments. Floral traits were more buffered from drought than leaf water potentials. We used path analysis to examine direct and indirect effects of ψ1on floral traits for plants in well-watered (control) vs. drought treatments. According to the best-fit path models, midday ψ1has significant positive effects on flower size and nectar volume in both environments. However, for controls midday ψ1also had a significant negative effect on nectar sugar concentration. Results indicate that traits influencing floral attractiveness to pollinators in E. angustifolium vary with plant water status, such that pollinator-mediated selection could indirectly target physiological or biochemical controls on ψ1. Moreover, under mesic conditions selection for greater nectar sugar reward may be constrained by the antagonistic effects of plant water status on nectar volume and sugar concentration.
Flowers of the alpine skypilot, Polemonium viscosum, are attacked by nectar thieving ants of Formica neorufibarbus gelida. Ants exert selection on flower scent, size and shape in skypilots by ...damaging the pistils. Here, I report on the frequency and nature of contact between ants and pollen-bearing anthers and determine the consequences of such contact for pollen performance and pollen donor paternity. In laboratory trials, ants entered flowers with full intact anthers and emasculated (female) flowers equivalently. Similarly, flower visitation rates of ants foraging naturally were not affected by the frequency of male phase flowers per plant. Ants actively interacted with the pollen-bearing anthers during 21% of flower visits, on average. The rate at which such interactions occurred was predicted by the proportion of flowers in the male phase, under a random foraging model. The effect of ants on pollen fertility was tested experimentally by enclosing ants in male-phase flowers on intact inflorescences. Adjacent control flowers were left un-occupied. Pollen from flowers with a history of ant occupancy had significantly lower germination on virgin recipient stigmas than pollen from unoccupied control flowers. With hand-pollination, sufficient pollen was transferred from ant-occupied flowers to saturate seed set. However, a model based on the relationship between seed set and compatible pollen delivery by natural pollinators indicated that ant damage to pollen should reduce paternity accruing per flower visit by 20-26% on average, in nature. Results support the hypothesis that in P. viscosum, selection on floral traits by nectar-thieving ants operates through male as well as female function.
In a recent paper, we reported on the evolution of shorter tongues in two alpine bumble bee species in response to climate-induced flower deficits. De Keyzer et al. concede that tongue lengths have ...decreased but criticize the level of support for our claims. Here, we address the alternative mechanisms they proposed, highlight evidence presented in the supplementary material, and elaborate on the support for our claims in the literature. De Keyzer et al.’s criticisms reflect concerns about the misrepresentation of our work in the popular press. To clarify, we do not imply that evolutionary rescue is necessarily a prudent conservation strategy; we illustrate that remote bumble bee populations buffered from other environmental stressors have undergone an adaptive evolutionary response to dwindling resources under climate change.
Abstract
Restoration of glades over the past 30 years, involving removal of woody cover and re‐establishment of herbaceous plant communities, has created an archipelago of habitat patches varying in ...age, size, and isolation. Within these glades, habitat varies in quality from edge to core. We investigated impacts of within‐ and among‐glade variation on the frequency of root colonization by arbuscular mycorrhizal fungi (
AMF
) for two host species,
Schizachyrium scoparium
and
Rudbeckia missouriensis
, and the plant community at large. We also conducted a soil analysis to explore the role of nutrient resources in mediating putative habitat effects. We used Akaike's information criterion (
AIC
) to evaluate a set of a priori models that assessed effects of glade size, isolation, age, and edge effects on
AMF
colonization. In community samples,
AMF
colonization increased while variation in
AMF
colonization decreased with restoration age. Edge effects also reduced plant community
AMF
colonization.
AMF
colonization was higher in
R. missouriensis
than in
S. scoparium
, but increased for both species in core habitat. Glade size had no direct effect on
AMF
at the plant scale, but indirectly affected overall
AMF
occupancy at the landscape scale via its geometric relationship to glade edge:core ratio. Hosts in small glades that are dominated by edge exhibit lower occupancy rates. The first two axes in a principal components analysis (
PCA
) combining soil nutrient variables explained 74% of sample variance:
PC
1 correlated positively with organic matter (
OM
), total nitrogen (N), calcium (Ca), magnesium (Mg), and potassium (K) and decreased from the edge to the core of older glades.
PC
2, which correlated positively with
pH
and negatively with available P, increased with time since restoration. While results are consistent with a role of soil properties in explaining edge and age effects, the two
PCA
dimensions themselves are weakly correlated with
AMF
colonization. Overall, our findings suggest that environmental filters associated with woody encroachment limit the colonization of otherwise suitable herbaceous hosts and create a landscape mosaic with larger, older glades serving as hotspots or reservoirs of
AMF
root colonization and smaller, newly restored glades as cold spots or potential sinks.
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