ABSTRACT
The ecological co‐dependency between plants and hummingbirds is a classic example of a mutualistic interaction: hummingbirds rely on floral nectar to fuel their rapid metabolisms, and more ...than 7000 plant species rely on hummingbirds for pollination. However, threats to hummingbirds are mounting, with 10% of 366 species considered globally threatened and 60% in decline. Despite the important ecological implications of these population declines, no recent review has examined plant–hummingbird interactions in the wider context of their evolution, ecology, and conservation. To provide this overview, we (i) assess the extent to which plants and hummingbirds have coevolved over millions of years, (ii) examine the mechanisms underlying plant–hummingbird interaction frequencies and hummingbird specialization, (iii) explore the factors driving the decline of hummingbird populations, and (iv) map out directions for future research and conservation. We find that, despite close associations between plants and hummingbirds, acquiring evidence for coevolution (versus one‐sided adaptation) is difficult because data on fitness outcomes for both partners are required. Thus, linking plant–hummingbird interactions to plant reproduction is not only a major avenue for future coevolutionary work, but also for studies of interaction networks, which rarely incorporate pollinator effectiveness. Nevertheless, over the past decade, a growing body of literature on plant–hummingbird networks suggests that hummingbirds form relationships with plants primarily based on overlapping phenologies and trait‐matching between bill length and flower length. On the other hand, species‐level specialization appears to depend primarily on local community context, such as hummingbird abundance and nectar availability. Finally, although hummingbirds are commonly viewed as resilient opportunists that thrive in brushy habitats, we find that range size and forest dependency are key predictors of hummingbird extinction risk. A critical direction for future research is to examine how potential stressors – such as habitat loss and fragmentation, climate change, and introduction of non‐native plants – may interact to affect hummingbirds and the plants they pollinate.
Mutualistic 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. We 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 behavioural flexibility would allow hummingbirds to use alternative resources, leading to decreased ecological specialization and reorganization of the network structure (i.e. pairwise interactions). Alternatively, morphological or behavioural constraints-such as trait-matching or interspecific competition-might limit the extent to which hummingbirds alter their foraging behaviour. We employed a replicated Before-After-Control-Impact experimental design and quantified plant-hummingbird interactions using two parallel sampling methods: pollen collected from individual hummingbirds ('pollen networks', created from >300 pollen samples) and observations of hummingbirds visiting focal plants ('camera networks', created from >19,000 observation hours). To assess the extent of rewiring, we quantified ecological specialization at the individual, species and network levels and examined interaction turnover (i.e. gain/loss of pairwise interactions). H. 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 exclusion 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. Our results suggest that, at least over short time-scales, 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.
Theory suggests that species loss can trigger a coextinction cascade within a community, leading to declines in ecosystem function. However, experiments testing this prediction for plant–pollinator ...interactions 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 (BACI) design while quantifying hummingbird abundance and space use (383 hummingbird captures and 36 radio‐tagged individuals), flower visitation rates (> 19 000 observation hours), and pollination success (529 flowers). We expected that H. tortuosa removal would either result in 1) coextinction, in which hummingbirds vacate fragments and compromise the reproductive success of the remaining flowering plants, or 2) increased hummingbird reliance on alternative floral resources, leading to sustained fragment use (persistence). In our experiment, hummingbird behavior and plant–hummingbird interactions were remarkably resistant to loss of H. tortuosa, a locally common plant species representing at least 30–40% of the available nectar resources on average. However, we did not discover evidence that hummingbirds increasingly used alternative floral resources to cope with this temporary resource loss. With the important caution that short‐term experiments may not emulate natural extinction processes, our study suggests that plant–pollinator interactions may be able to withstand single‐species plant losses, but the exact mechanisms enabling hummingbird persistence after resource removal require further research.
Context
Climate and landscape structure are predicted to affect pollination and plant reproduction but few studies have tested how these stressors interact.
Objectives
Using a 9-year dataset we ...tested whether climate interacts synergistically with forest loss and fragmentation to affect pollination and subsequent reproduction in a tropical understory herb,
Heliconia tortuosa
. We hypothesized that hot and/or dry conditions might amplify effects of habitat loss and fragmentation, leading to declines in plant reproduction.
Methods
We collected data on pollen tubes, fruit, and seeds of
H. tortuosa
in a mensurative experiment representing gradients in forest amount and patch size (N = 40 focal-patch landscapes). We modeled these reproductive metrics as a function of landscape composition, configuration, precipitation, temperature, and statistical interactions among these variables.
Results
We found little support for synergistic landscape and climate effects. However,
H. tortuosa
reproduction decreased in wet years, small patches, and, counterintuitively, in landscapes with high forest cover. We speculate that reproduction improved in deforested landscapes because the lack of surrounding forest constrains hummingbird movement, and therefore visitation is focused on plants that remain.
Conclusion
Although we did not detect synergistic effects, we emphasize the importance of additive effects of climate and landscape structure on plant reproduction. Wet years, small patches, and heavily forested landscapes appear to have negative consequences for
Heliconia
reproduction. Decline in this common species is likely to have cascading consequences for hummingbird pollinators, and subsequently other hummingbird-dependent plant species.
Research hypotheses have been a cornerstone of science since before Galileo. Many have argued that hypotheses (1) encourage discovery of mechanisms, and (2) reduce bias—both features that should ...increase transferability and reproducibility. However, we are entering a new era of big data and highly predictive models where some argue the hypothesis is outmoded. We hypothesized that hypothesis use has declined in ecology and evolution since the 1990s, given the substantial advancement of tools further facilitating descriptive, correlative research. Alternatively, hypothesis use may have become more frequent due to the strong recommendation by some journals and funding agencies that submissions have hypothesis statements. Using a detailed literature analysis (N = 268 articles), we found prevalence of hypotheses in eco–evo research is very low (6.7%–26%) and static from 1990–2015, a pattern mirrored in an extensive literature search (N = 302,558 articles). Our literature review also indicates that neither grant success nor citation rates were related to the inclusion of hypotheses, which may provide disincentive for hypothesis formulation. Here, we review common justifications for avoiding hypotheses and present new arguments based on benefits to the individual researcher. We argue that stating multiple alternative hypotheses increases research clarity and precision, and is more likely to address the mechanisms for observed patterns in nature. Although hypotheses are not always necessary, we expect their continued and increased use will help our fields move toward greater understanding, reproducibility, prediction, and effective conservation of nature.
We use a quantitative literature review to show that use of a priori hypotheses is still rare in the fields of ecology and evolution. We provide suggestions about the group and individual‐level benefits of hypothesis use.
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