Papers by Jessica Forrest
The spring phenology of co-occurring trees and understory herbaceous plants (spring ephemerals) i... more The spring phenology of co-occurring trees and understory herbaceous plants (spring ephemerals) in northeastern hardwood forests is thought to be regulated by different environmental drivers. Previous work at a single site has suggested that global climate change could reduce photosynthetic rates and reproductive success of spring ephemerals through advancements in the timing of canopy leaf-out relative to the timing of understory growth. However, phenological responses to warming vary interspecifically and geographically, making generalizations difficult. To answer more generally whether tree leaf-out and flowering of spring ephemerals have advanced their phenologies at different rates with respect to increasing temperature, we examined the phenological responses to warming of 22 species of trees and spring ephemerals (12,777 observations from 1,534 sites) in the Northeastern US using data from the USA-National Phenology Network for the past 13 (2009-2021) years.
Ecology, 2018
Evolutionary ecologists seek to explain the processes that maintain variation within populations.... more Evolutionary ecologists seek to explain the processes that maintain variation within populations. In plants, petal color variation can affect pollinator visitation, environmental tolerance, and herbivore deterrence. Variation in sexual organs may similarly affect plant performance. Withinpopulation variation in pollen color, as occurs in the eastern North American spring ephemeral Erythronium americanum, provides an excellent opportunity to investigate the maintenance of variation in this trait. Although the red/yellow pollen-color polymorphism of E. americanum is widely recognized, it has been poorly documented. Our goals were thus (1) to determine the geographic distribution of the color morphs, and (2) to test the effects of pollen color on components of pollen performance. Data provided by citizen scientists indicated that populations range from monomorphic red, to polymorphic, to monomorphic yellow, but there was no detectable geographic pattern in morph distribution, suggesting morph occurrence cannot be explained by a broad-scale ecological cline. In field experiments, we found no effect of pollen color on the probability of predation by the pollen-feeding beetle Asclera ruficollis, on the ability of pollen to tolerate UV-B radiation, or on siring success (as measured by the fruit set of handpollinated flowers). Pollinators, however, exhibited site-specific pollen-color preferences, suggesting they may act as agents of selection on this trait, and, depending on the constancy of their preferences, could contribute to the maintenance of variation. Collectively, our results eliminate some hypothesized ecological effects of pollen color in E. americanum, and identify effects of pollen color on pollinator attraction as a promising direction for future investigation.
The American Naturalist, 2016
Many specialist herbivores eat foods that are apparently low quality. The compensatory benefits o... more Many specialist herbivores eat foods that are apparently low quality. The compensatory benefits of a poor diet may include protection from natural enemies. Several bee lineages specialize on pollen of the plant family Asteraceae, which is known to be a poor-quality food. Here we tested the hypothesis that specialization on Asteraceae pollen protects bees from parasitism. We compared rates of brood parasitism by Sapyga wasps on Asteraceae-specialist, Fabeae-specialist, and other species of Osmia bees in the field over several years and sites and found that Asteraceae-specialist species were parasitized significantly less frequently than other species. We then tested the effect of Asteraceae pollen on parasites by raising Sapyga larvae on three pollen mixtures: Asteraceae, Fabeae, and generalist (a mix of primarily non-Asteraceae pollens). Survival of parasite larvae was significantly reduced on Asteraceae provisions. Our results suggest that specialization on low-quality pollen may evolve because it helps protect bees from natural enemies.
The American Naturalist
Organisms must often make developmental decisions without complete information about future condi... more Organisms must often make developmental decisions without complete information about future conditions. This uncertaintyfor example, about the duration of conditions favorable for growthcan favor bet-hedging strategies. Here, we investigated the causes of life cycle variation in Osmia iridis, a bee exhibiting a possible bethedging strategy with co-occurring 1-and 2-year life cycles. One-year bees reach adulthood quickly but die if they fail to complete pupation before winter; 2-year bees adopt a low-risk, low-reward strategy of postponing pupation until the second summer. We reared larval bees in incubators in various experimental conditions and found that warmer-but not longer-summers and early birthdates increased the frequency of 1-year life cycles. Using in situ temperature measurements and developmental trajectories of laboratory-and field-reared bees, we estimated degree-days required to reach adulthood in a single year. Local long-term (1950-2015) climate records reveal that this heat requirement is met in only ∼7% of summers, suggesting that the observed distribution of life cycles is adaptive. Warming summers will likely decrease average generation times in these populations. Nevertheless, survival of bees attempting 1-year life cycles-particularly those developing from late-laid eggs-will be !100%; consequently, we expect the life cycle polymorphism to persist.
Journal of Applied Ecology, 2015
1. Land-use change frequently reduces local species diversity. Species losses will often result i... more 1. Land-use change frequently reduces local species diversity. Species losses will often result in loss of trait diversity, with likely consequences for community functioning. However, the converse need not be generally true: management approaches that succeed in retaining species richness could nevertheless fail to maintain trait diversity. We evaluated this possibility using bee communities in a California agroecosystem. 2. We examined among site patterns in bee species diversity and functional-trait diversity in a landscape composed of a mosaic of semi-natural habitat, organic farms and conventional farms. We sampled bees from all three habitat types and compiled a data base of life-history ('functional') traits for each species. 3. Although species richness was higher on organic farms than conventional farms, functional diversity was lower in both farm types than in natural habitat. Nesting location (below-ground vs. above-ground) was the primary trait contributing to differences in functional diversity between farms and natural habitat, reflecting observed differences in availability of nesting substrates among habitat types. Other traits, including phenology and sociality, were also associated with species' occurrences or dominance in particular site types. These patterns suggest that management practices common to all farms act as environmental filters that cause similarly low functional diversity in their bee communities. 4. Synthesis and applications. Although our results support the value of organic farming in maintaining abundant and species-rich bee communities, components of bee functional diversity are not well supported in farmed landscapes, regardless of farming practice. Maintenance of natural habitat, and/or the addition of natural habitat elements to farms, is therefore important for the retention of functionally diverse bee assemblages in agroecosystems.
Journal of Pollination Ecology
Bilateral symmetry has evolved from radial symmetry in several floral lineages, and multiple hypo... more Bilateral symmetry has evolved from radial symmetry in several floral lineages, and multiple hypotheses have been proposed to account for the success of this floral plan. One of these hypotheses posits that bilateral symmetry (or, more generally, a reduced number of planes of floral symmetry) allows for more precise pollen placement on pollinators. Greater precision would maximize the efficacy of pollen transfer to conspecifics, while minimizing reproductive interference amongst plant species. Despite the intuitiveness of this hypothesis, it has little experimental support. Here, we tested whether a reduction in the number of floral planes of symmetry (as in the transition from radial to bilateral symmetry) increases the potential precision of pollen placement. We analyzed video recordings of bumblebees (Bombus impatiens) visiting artificial flowers to determine whether consistency in flower entry angle differed between radial (round) and disymmetric (rectangular) “flowers”. We obse...
Journal of Pollination Ecology
Current Opinion in Insect Science
The Quarterly Review of Biology, 2015
Background/Question/Methods Climate change is expected to alter patterns of species co-occurrence... more Background/Question/Methods Climate change is expected to alter patterns of species co-occurrence, in both space and time. Species-specific shifts in reproductive phenology may produce changes in the assemblages of plant species in flower at any given time during the growing season. Temporal overlap in the flowering periods (co-flowering) of animal-pollinated species may be an important determinant of reproductive success, if competitive or facilitative interactions between plant species affect pollinator services. We used a unique 33-year dataset on flowering phenology in subalpine meadow communities to determine whether interannual variation in snowmelt date caused predictable changes in co-flowering patterns. We focussed on the flowering period of Lathyrus leucanthus (Fabaceae), a bee-pollinated species that flowers in early summer and that is well represented in the dataset. Results/Conclusions We found that the composition of the assemblage of plants with flowering periods over...
Background/Question/Methods: In dioecious plant species (those with separate sexes), it is freque... more Background/Question/Methods: In dioecious plant species (those with separate sexes), it is frequently observed that males flower earlier in the season than females. However, the generality of this phenomenon has not been quantified, and the reasons for it have not been fully explained. The conventional hypothesis is that female plants require more time for resource acquisition prior to reproduction. An alternate hypothesis is that sexual selection for access to high-quality ovules favors early-flowering males. First, to quantify the prevalence of male precedence (protandry), I compiled published data on flowering phenology of 29 dioecious taxa and compared standardized flowering curves of males and females. I also assembled data on the relationship between plant size and time of flowering onset. Then, to test the role of sexual selection in driving early male flowering, I constructed a model of the evolution of flowering time in a simulated dioecious plant population in which timing...
Background/Question/Methods: Many flowers require animal-mediated pollination in order to reprodu... more Background/Question/Methods: Many flowers require animal-mediated pollination in order to reproduce and in many cases the pollinators require plant resources for their reproduction. While several studies have determined that bee communities, the most important group of pollinators, are not predictable over time and space, few studies report on the predictability of flowers on a community level. It is also not known whether the predictability of flowering is a function of community type. In this analysis, we attempt to quantify the consistency of flower communities on both a monthly and a seasonal basis in several plant community types. Plant phenology data were collected for thirty-five years at the Rocky Mountain Biological Laboratory in Colorado, twenty years in the Santa Catalina Mountains of Arizona and nine years on the Sevilleta National Wildlife Refuge in New Mexico. We calculated turnover rates of species in bloom from one season to the next (eg. spring to spring) and freque...
Oikos, 2014
ABSTRACT Climate change can affect plant–pollinator interactions in a variety of ways, but much o... more ABSTRACT Climate change can affect plant–pollinator interactions in a variety of ways, but much of the research attention has focused on whether independent shifts in phenology will alter temporal overlap between plants and pollinators. Here I review the research on plant–pollinator mismatch, assessing the potential for observational and experimental approaches to address particular aspects of the problem. Recent, primarily observational studies suggest that phenologies of co-occurring plants and pollinators tend to respond similarly to environmental cues, but that nevertheless, certain pairs of interacting species are showing independent shifts in phenology. Only in a few cases, however, have these independent shifts been shown to affect population vital rates (specifically, seed production by plants) but this largely reflects a lack of research. Compared to the few long-term studies of pollination in natural plant populations, experimental manipulations of phenology have yielded relatively optimistic conclusions about effects of phenological shifts on plant reproduction, and I discuss how issues of scale and frequency-dependence in pollinator behaviour affect the interpretation of these ‘temporal transplant’ experiments. Comparable research on the impacts of mismatch on pollinator populations is so far lacking, but both observational studies and focused experiments have the potential to improve our forecasts of pollinator responses to changing phenologies. Finally, while there is now evidence that plant–pollinator mismatch can affect seed production by plants, it is still unclear whether this phenological impact will be the primary way in which climate change affects plant–pollinator interactions. It would be useful to test the direct effects of changing climate on pollinator population persistence, and to compare the importance of phenological mismatch with other threats to pollination.
Evolutionary Ecology, 2015
ABSTRACT Plants might be under selection for both attracting efficient pollinators and deterring ... more ABSTRACT Plants might be under selection for both attracting efficient pollinators and deterring wasteful visitors. Particular floral traits can act as exploitation barriers by discouraging the unwelcome visitors. In the genus Penstemon, evolutionary shifts from insect pollination to more efficient hummingbird pollination have occurred repeatedly, resulting in the convergent evolution of floral traits commonly present in hummingbird-pollinated flowers. Two of these traits, a reduced or reflexed lower petal lip and a narrow corolla, were found in a previous flight-cage study to affect floral handling time by bumble bees, therefore potentially acting as “anti-bee” traits affecting preference. To test whether these traits do reduce bumble bee visitation in natural populations, we manipulated these two traits in flowers of bee-pollinated Penstemon strictus to resemble hummingbird-adapted close relatives and measured the preferences of free-foraging bees. Constricted corollas strongly deterred bee visitation in general, and particularly reduced visits by small bumble bees, resulting in immediate specialization to larger, longer-tongued bumble bees. Bees were also deterred—albeit less strongly—by lipless flowers. However, we found no evidence that lip removal and corolla constriction interact to further affect bee preference. We conclude that narrow corolla tubes and reduced lips in hummingbird-pollinated penstemons function as exploitation barriers that reduce bee access to nectaries or increase handling time.
Journal of evolutionary biology, 2015
Variation among the leaves, flowers or fruit produced by a plant is often regarded as a nuisance ... more Variation among the leaves, flowers or fruit produced by a plant is often regarded as a nuisance to the experimenter and an impediment to selection. Here, we suggest that within-plant variation can drive selection on other plant-level traits. We examine within-plant variation in floral sex allocation and in fruit set and predict that such variation generates variation in male success among plants, thereby driving selection on flowering time. We tested this prediction in a simulation model estimating selection on flowering time through male fitness when floral sex allocation and/or fruit set vary directionally among flowers on plants. We parameterized the model through a quantitative literature survey of within-plant change in sex allocation. As predicted, within-plant variation in floral sex allocation and in fruit set probability can generate selection on flowering time through male fitness. Declining fruit set from first to last flowers on plants, as occurs in many species, select...
The American naturalist, 2014
It is frequently observed that males of dioecious plant species flower earlier in the season than... more It is frequently observed that males of dioecious plant species flower earlier in the season than females, although the generality of this pattern has not been quantified. One hypothesis for earlier male flowering is that females require more time for resource acquisition before reproduction; another is that selection for access to unfertilized ovules favors early-flowering males. Here I show that protandry is indeed the usual pattern in dioecious plants--males typically initiate flowering before females--and I propose a new hypothesis to explain this pattern. In many natural plant populations, individuals that begin flowering early are larger and--in the case of females or hermaphrodites--therefore more fecund. When this population-level seasonal decline in size is included in simulations of flowering time evolution in a dioecious plant, males evolve earlier flowering onset than females. Correlations between size (or condition) and reproductive phenology are widespread and likely c...
ABSTRACT Background/Question/Methods Bees are key pollinators in natural and managed ecosystems t... more ABSTRACT Background/Question/Methods Bees are key pollinators in natural and managed ecosystems throughout the world and in many regions are threatened by anthropogenic landuse change. Their sensitivity to landuse change is in part determined by ecological and life history traits, which define their need for different essential resources, such as nesting sites and forage. These resources may be differentially distributed throughout the landscape such that trait-based bee guilds respond differentially to landuse change. The result may be shifts in biodiversity, community composition, and pollination function. To test for differential responses of bee guilds to landuse change, we surveyed bee communities throughout the growing season at semi-natural habitats and adjacent farm sites in a California agricultural landscape. We assigned all bee species to different ecological guilds based on specific traits such as nesting substrate, trophic specialization and sociality. We then used ANOVA to test for changes in the relative abundances of different guilds according to landuse. To explore how nesting and floral resources interact with life history traits to differentially affect bee responses, we applied a spatially explicit model that coded the landscape based on suitability for nesting and forage resource for each bee species to predict relative abundance of different bee guilds across the landscape. Results/Conclusions Species traits strongly affected their sensitivity to landuse change. In particular, bees that depended on wood for nesting substrate were relatively less abundant than ground-nesting species at agricultural sites than at adjacent natural habitats (F2,13 = 17.92, P < 0.001) . The landscape model revealed how the distributions of essential resource can provide a mechanism for bee responses to landuse change. The model predictions matched observed abundances of wood-dependent species throughout the landscape (R2 = 0.36, P = 0.01); however, it was less predictive of ground-nesting species (R2 = 0.11, P = 0.20). The results highlight the importance of species traits and resources distributions in filtering ecological communities among habitats including those modified by anthropogenic disturbance
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Papers by Jessica Forrest