Biology subjects

Hadley, A. S.

Publications and source records attributed to Hadley, A. S..

3 recordsLinked to original sources

bioRxiv2022

Plant-hummingbird pollination networks exhibit minimal rewiring after experimental removal of a locally abundant plant species

O_LIMutualistic relationships, such as those between plants and pollinators, may be vulnerable to the local extinctions predicted under global environmental change. However, network theory predicts that plant-pollinator networks can withstand species loss if pollinators switch to alternative floral resources (rewiring). Whether rewiring occurs following species loss in natural communities is poorly known because replicated species exclusions are difficult to implement at appropriate spatial scales. C_LIO_LIWe experimentally removed a hummingbird-pollinated plant, Heliconia tortuosa, from within tropical forest fragments to investigate how hummingbirds respond to temporary loss of an abundant resource. Under the rewiring hypothesis, we expected that niche expansions would decrease ecological specialization and reorganize the network structure (i.e., pairwise interactions). C_LIO_LIWe employed a replicated Before-After-Control-Impact experimental design and quantified plant-hummingbird interactions using two parallel sampling methods: observations of hummingbirds visiting focal plants ( camera networks, created from >19,000 observation hours) and pollen collected from individual hummingbirds ( pollen networks, created from >300 pollen samples). To assess hummingbird rewiring, we quantified ecological specialization at the individual, species, and network levels and calculated the amount of network-level interaction turnover (i.e., gain/loss of pairwise interactions). Leveraging our parallel network datasets, we also explored how sampling method influences apparent specialization. C_LIO_LIH. tortuosa removal caused some reorganization of pairwise interactions but did not prompt large changes in specialization, despite the large magnitude of our manipulation (on average, >100 inflorescences removed in treatment areas of >1 ha). Although some individual hummingbirds sampled through time showed modest increases in niche breadth following Heliconia removal (relative to birds that did not experience resource loss), these changes were not reflected in species- and network-level specialization metrics. We also found that camera networks were more specialized than pollen networks, and that correlation between sampling methods was low. C_LIO_LIOur results suggest that animals may not necessarily shift to alternative resources after losing an abundant food resource, even in species thought to be highly opportunistic foragers such as hummingbirds. Given that rewiring contributes to theoretical predictions of network stability, future studies should investigate why pollinators might not expand their diets after a local resource extinction. C_LI

ecology↗

bioRxiv2022

Are tropical hummingbird pollination networks resistant to experimental extirpation of a common flowering plant?

Theory predicts that the structure of plant-pollination networks should withstand disturbance, but experiments testing this prediction remain uncommon. In this study, we simulated the local extinction of a hummingbird-pollinated understory plant, Heliconia tortuosa, from tropical forest fragments using a replicated Before-After-Control-Impact design while quantifying hummingbird abundance and space use (383 hummingbird captures and 72 radio-tagged individuals), floral visitation rates (>19,000 observation hours), and pollination success (529 flowers). We expected that H. tortuosa removal would either result in (i) network collapse, in which hummingbirds vacate fragments and compromise the reproductive success of other flowering plants, or (ii) increased hummingbird reliance on alternative resources (rewiring), leading to sustained fragment use. In our experiment, hummingbird behavior and pollination were remarkably resistant to loss of H. tortuosa, a locally common plant species representing 30-40% of the available nectar resources on average. The exact mechanisms enabling short-term hummingbird persistence after resource removal remain unclear, as we did not discover evidence of rewiring. We hypothesize that physiological adaptations (e.g., torpor and insectivory) may have allowed hummingbird persistence, perhaps alongside high movement ability. With the important caution that short-term experiments may not emulate natural extinction processes, our study provides support for predictions that pollination networks may be robust to plant species loss.

ecology↗

bioRxiv2021

Testing for pollinator recognition in multiple species of Heliconia

Many plants have evolved floral traits that, in effect, filter pollinator communities and promote pollination by efficient pollinators. Most documented pollinator filter traits act to deter or reduce visitation rates by a subsect of the community of floral visitors. However, a recently described pollinator filter termed pollinator recognition (PR) acts at a stage after a pollinator visit. PR was first documented experimentally in Heliconia tortuosa whereby pollen tube germination - a proxy for reproduction - was enhanced following visits from morphologically specialized pollinators, but not generalists. This is thought to promote outcrossing among plants by preferentially investing in reproduction following visits by long-ranging hummingbirds with specialized bill shapes. To date, this plant behavior has only been described in H. tortuosa, but, if widespread, could have important ecological implications; given declines in abundances of specialist pollinators, visits by generalists would not buffer the loss of pollination services to plants with PR. We therefore tested for PR in four taxa spread widely across the Heliconiaceae. We corroborated previous results that visits by long-billed, but not short-billed hummingbirds increased pollen tubes in H. tortuosa with aviary experiments that standardized pollen quality and minimized variation in pollen quantity. Across species, we found great variation in pollen tube responses to experimental treatments. For one species (H. rostrata), we found increased numbers of pollen tubes in those visited by hummingbirds compared to hand pollination alone, regardless of the visiting birds bill morphology, indicating recognition of hummingbirds in general. In other cases, hummingbird visits decreased pollen tube counts compared to hand pollinations alone. Furthermore, our results could not substantiate any specific mechanism for pollinator recognition and highlight the need for further work on the complexities of and variability in reproductive strategies across plant taxa.

ecology↗