Natural selection by pollinators on floral attractive and defensive traits

 by García et al.

Population of common milkweed (Asclepias syriaca)

Animal-pollinated plants often must both attract pollinators but deter herbivores for successful reproduction. Traits that primarily function as attraction or deterrence/defense can impact animal interactions more broadly than the intended target, so considering both types of traits and interactions can be important for understanding patterns of reproduction and ultimately natural selection. However, phenotypic selection studies that incorporate both floral traits and plant defenses with pollinator activity and herbivore damage remain rare. We studied a population of the common milkweed Asclepias syriaca (Apocynaceae) in Fredericton, New Brunswick, Canada. Public awareness of the milkweed plant has increased in recent years because it is a host plant for the iconic Monarch butterfly. We measured the association of ten traits including attractive and defensive traits and nectar rewards with reproductive success (= phenotypic selection), as well as pollinator activity and herbivore damage. We found selection to increase apparency traits such as plant and inflorescence size, but to decrease petal width in the flowers. We also found selection to increase a defensive trait, floral latex. We found evidence for pollinator-mediated selection but not herbivore-mediated selection on milkweed traits, perhaps because our population is at the northern range edge and experiences less herbivory than populations further south. Interestingly, pollinator-meditated selection did not directly translate to selection via offspring, suggesting that pollinator choice is not always driving floral evolution.

 

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Assessing pollinator assembly and potential across species ranges in the genus Triodanis (Campanulaceae)

Triodanis bee

By Tillotson-Chavez et al.

Flowering plants and the insects that they rely on for pollination continue to be an important topic of study in ecology. Pressures such as habitat destruction can disrupt these important relationships. The impacts of these pressures on plant-pollinator relationships are still understudied for many native plants. Even when pollinators visit flowers, successful transfer of compatible, intraspecific pollen is not always guaranteed, especially when habitat is degraded or destroyed. However, plants have evolved various methods to ensure pollination, such as flowers that are completely self fertile and never open, ensuring that they can reproduce without pollinators. This strategy is “cheaper” than producing resource expensive open flowers with petals and rewards (i.e., nectar) that are attractive to pollinators. The maintenance of these different strategies in a population and how human pressures may influence these reproductive strategies remain questions of interest. We used the native plant species Triodanis, or Venus’ Looking Glass, as a system to investigate these questions. Because these weedy annual plants have both flowers open to pollination (chasmogamous) and  flowers that are completely closed and self-pollinating (cleistogamous)this is an excellent system to investigate  how reproductive strategies influence pollinator relationships.

We specifically examined if variability in production between these two types of flowers impacted the quantity, quality, and diversity of insect pollinators present for species and populations in multiple species in the genus Triodanis. We sampled across the Midwestern US and Texas in different habitats, including those that were very human disturbed and developed (eg., sidewalks, along parking lots, etc), as well as maintained natural areas such as those similar to the Tallgrass Prairie National Preserve. Overall, small solitary native bee species, bee flies, and syrphid or flower fly species were collected across this wide sampling. During feeding or pollen collection, pollinators did not differentiate between the flowers of Triodanis species that occurred together which elucidates a previously undescribed mechanism for hybridization between these species. Overall, pollinator communities (bees and flies) were less diverse and abundant in areas of greater habitat degradation. However, variation in reproductive strategy (proportion of open to closed flowers) did not impact the identity or abundance of pollinators for Triodanis. Our work builds on the sparse natural history of Triodanis and its associated pollinators and emphasizes the importance of habitat quality in maintaining pollinator diversity. Further, we establish a framework for understanding possible relationships between variation in reproductive strategy (i.e., allocation to open flowers) and pollinator community. Future work will examine the plant fitness implications of variation in habitat quality as a driver of pollinator community diversity.

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Interplanted red clover supports pollinator conservation in sweet corn plantings

Bumblebee on red clover

 By Yurchak et al.

Monoculture cropping systems, or those consisting of a single crop, often support significantly fewer pollinators than natural landscapes. This is, in part, due to the low diversity of plants present within these areas. The short bloom period of crops in these systems presents an additional challenge for pollinators, as pollen and nectar are only available for a short time. During periods before and after crop bloom, these habitats are not suitable for foraging bees. Increasing the abundance of flowering plants within crop fields is an often-overlooked practice that may help sustain and enhance pollinator populations. In this study, we investigated the effectiveness of using interplanted red clover to enhance pollinator abundance and richness within sweet corn plantings.

              To accomplish this, we assessed the pollinator community within both the red clover flowers and sweet corn via visual observations of floral visitation and pan traps. Weekly observations of all bees and butterflies visiting red clover flowers were performed throughout the sweet corn growing season. We also observed all insect pollinators visiting sweet corn tassels during the corn flowering period.  The diversity of bee species present within all experimental treatments was further evaluated using pan traps, which were set at multiple dates and varying heights relative to clover and sweet corn flowers throughout the summer.

              Our results provide evidence that interplanting flowering strips into sweet corn fields can increase the diversity and abundance of bees and butterflies within these cropping systems, demonstrating their use for pollinator conservation. However, it is important that careful thought is given to the pest management practices used, as pesticide sprays could prove fatal to foraging pollinators. Thus, this practice may be more favorable to cropping systems utilizing nonconventional pest control methods that reduce pesticide applications and/or pollinator exposure to pesticides.

 

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