Florally rich habitats reduce insect pollination and the reproductive success of isolated plants

Abstract

Landscape heterogeneity in floral communities has the potential to modify pollinator behavior. Pollinator foraging varies with the diversity, abundance, and spatial configuration of floral resources. However, the implications of this variation for pollen transfer and ultimately the reproductive success of insect pollinated plants remains unclear, especially for species which are rare or isolated in the landscape. We used a landscape‐scale experiment, coupled with microsatellite genotyping, to explore how the floral richness of habitats affected pollinator behavior and pollination effectiveness. Small arrays of the partially self‐compatible plant Californian poppy (Eschscholzia californica) were introduced across a landscape gradient to simulate rare, spatially isolated populations. The effects on pollinator activity, outcrossing, and plant reproduction were measured. In florally rich habitats, we found reduced pollen movement between plants, leading to fewer long‐distance pollination events, lower plant outcrossing, and a higher incidence of pollen limitation. This pattern indicates a potential reduction in per capita pollinator visitation, as suggested by the lower activity densities and richness of pollinators observed within florally rich habitats. In addition, seed production reduced by a factor of 1.8 in plants within florally rich habitats and progeny germination reduced by a factor of 1.2. We show this to be a consequence of self‐fertilization within the partially self‐compatible plant, E. californica. These findings indicate that locally rare plants are at a competitive disadvantage within florally rich habitats because neighboring plant species disrupt conspecific mating by co‐opting pollinators. Ultimately, this Allee effect may play an important role in determining the long‐term persistence of rarer plants in the landscape, both in terms of seed production and viability. Community context therefore requires consideration when designing and implementing conservation management for plants which are comparatively rare in the landscape.

Pollinator-Mediated Gene Flow Connects Green Roof Populations Across the Urban Matrix: A Paternity Analysis of the Self-Compatible Forb Penstemon hirsutus

“…We found that nearly half of the offspring generation resulted from self-fertilization but also that about a quarter resulted from long-distance pollen movement. These results are consistent with expectations for small, fragmented populations within an inhospitable matrix (Hendrix and Kyhl, 2000;Trapnell and Hamrick, 2005;Evans et al, 2017). Small population sizes, such as those typically found in cities, can lead to longer and more pollinator foraging bouts within both individuals and populations, which can result in higher rates of geitonogamy and inbreeding (Severns, 2003).…”

The relationship between pollinator community and pollination services is mediated by floral abundance in urban landscapes

“…In this study, we add to the body of literature suggesting that insect pollination is an essential part of successful plant reproduction in urban systems (Verboven et al 2014, Leong et al 2014, Lowenstein et al 2015, Potter and LeBuhn 2015. Furthermore, while resource-driven foraging interactions have been documented to impact pollinator visitation in simulated models (Essenberg 2012), grasslands (Totland andMatthews 1998floodplains (Ebeling et al 2008), and rural agricultural systems (Veddeler et al 2006;Jha and Vandermeer 2009;Evans et al 2017;Xie et al 2019), our study is the first to document this interaction mediating plant reproduction within urban landscapes. Specifically, we show that when urban garden floral density is high, increasing pollinator richness (and not pollinator abundance) increases crop reproductive success.…”

“…In addition to landscape factors, local floral diversity, abundance, and composition can mediate pollination through impacts on pollinator densities (Steffan-Dewenter and Westphal 2008;Williams et al 2015), foraging dynamics (Kunin and Iwasa 1996;Kunin 1993), visitation rates (Van Nuland et al 2013, Veddeler et al 2006, and pollen deposition (Lortie and Aarssen 1999;Evans et al 2017). Classic research in foraging biology has shown that, when a limited number of foragers spread out in a high resource patch, it can reduce per-plant visitation, something called the "dilution effect" (Root and Kareiva 1984;Yamamura 1999).…”

Pollinator effectiveness in a composite: a specialist bee pollinates more florets but does not move pollen farther than other visitors

“…Thus, our estimates of effective pollen movements likely reflect true pollen transport distances, regardless of compatibility. In other forb species, pollen transport distances are much shorter than 16 m (Evans et al., ; Thavornkanlapachai et al., ; Valverde et al., ), with pollen sometimes moving <1 m between mates (Evans et al., , Valverde et al., ).…”