Graduate Thesis Or Dissertation
 

Can Pollination Networks Withstand Species Loss? An Examination of Plant–Hummingbird Interactions and Their Responses to Experimental Removal of a Flowering Plant Species

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  • Global environmental change is causing local extinctions of species. When species depend on one another, as in the mutualistic relationship between plants and pollinators, loss of one interaction partner may cause cascading effects within the community – such as additional extinctions and reduced pollination services. Network theory provides a way to conceptualize biotic interactions among species and has led to predictions about how single species loss affects the broader community (i.e., the interaction network). Many theoretical studies suggest that mutualistic networks are relatively robust, such that loss of one species should not trigger a coextinction cascade. Robustness is expected to occur due to the nested structure of interactions and the ability of animals to switch food resources. However, well-replicated field experiments that evaluate the robustness of mutualistic networks in natural communities remain uncommon. In this dissertation, I addressed this knowledge gap by examining how tropical plant–pollinator networks respond to the loss of a locally abundant interaction partner, a scenario that might occur with continued environmental change (as introduced in Chapter 1). First, I reviewed the evolutionary and ecological literature about hummingbirds more broadly (Chapter 2), in part to understand how hummingbirds might be affected by major anthropogenic threats (climate change and habitat loss) and to identify general mechanisms structuring interactions between flowering plants and hummingbirds. I then investigated how plant–hummingbird interactions change following temporary removal of an understory plant species, Heliconia tortuosa, using a replicated species removal experiment within forest fragments in southern Costa Rica (Chapters 3 and 4). Finally, I summarized key findings and lessons from the removal experiment and highlight some future research directions related to robustness in plant–pollinator networks (Chapter 5). In Chapter 2, I conducted a wide-ranging synthesis of literature related to hummingbird co-evolution, ecology, and conservation literature. This chapter represents the first comprehensive review of plant–hummingbird interactions in more than 20 years, thus incorporating many research insights associated with the development of network theory. As part of this review, I identified several key factors that structure pairwise interactions: hummingbird foraging strategy, inter- and intra-specific competition between hummingbirds, and morphological trait-matching (i.e., between bill shape and flower shape). I also reviewed how climate change and habitat loss might affect hummingbird populations and plant–hummingbird interactions as a whole. For instance, local extinctions of floral resources may suddenly occur due to extreme weather events (e.g., hurricanes, drought) or arise over longer time scales through spatial and temporal mismatches between interaction partners. For instance, climate change can separate species in space by inducing range shifts, or in time by altering flowering phenology. Habitat loss and fragmentation can also alter species composition via filtering effects on the hummingbird community assemblage, with potential cascading consequences for plant populations (and eventually hummingbird populations as well). Therefore, a key direction for future research is understanding how plant–hummingbird interactions change following the loss of certain floral resources. In Chapter 3, I examined whether hummingbirds persisted in forest fragments following experimental H. tortuosa extirpation. I also investigated whether this persistence led to uninterrupted pollination services for other plant species; both requirements are necessary to prevent a coextinction cascade over time. To complement the experiment, I also used a space-for-time approach that examined hummingbird responses (space use, floral visitation rates) and plant pollination success across a natural gradient in H. tortuosa density. I hypothesized that H. tortuosa declines 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, leading to sustained fragment use. In the removal experiment, hummingbird persistence and plant pollination success were remarkably resistant to loss of H. tortuosa, a locally common plant species representing >40% of the nectar resources available to hummingbirds, on average. However, using the space-for-time approach, I found that naturally low H. tortuosa densities were associated with reduced floral visitation rates and decreased pollination. The exact mechanisms enabling short-term hummingbird persistence after experimental resource removal remained unclear, as I did not discover evidence that hummingbirds increased their use of alternative resources. In Chapter 4, I further assessed the extent to which hummingbirds exhibited behavioral foraging flexibility following experimental removal of H. tortuosa, this time leveraging a network approach to examine pollinator specialization. I relied on two parallel sampling methods: observations of hummingbirds visiting focal plants (‘visitation networks’) and pollen grains collected from individual hummingbirds (‘pollen networks’). I measured ecological specialization at the individual, species, and network levels and quantified the amount of network-level interaction turnover over time (i.e., gain/loss of pairwise interactions). H. tortuosa removal appeared to cause higher levels of interaction turnover, and some individual birds sampled through time showed a modest increase in niche breadth following H. tortuosa removal – but only relative to birds that did not experience resource loss. Moreover, this pattern only applied to a subset of individuals, and I observed minimal changes in specialization when examining species- and network-level metrics. Collectively, the results from Chapters 3-4 provide support for robustness of plant–pollinator networks following species loss, at least for the short duration of the experiment (four days). However, based on two lines of evidence, I expect that coextinction cascades may still occur as a result of processes operating at longer times scales, such as those associated with climate change and habitat loss (e.g., range shifts, phenological mismatch, filtering of community assemblage). First, the space-for-time approach revealed that high H. tortuosa densities can facilitate hummingbird visitation and, subsequently, pollination (Chapter 3). Second, I found limited evidence that hummingbirds expanded their diets to include alternative floral resources, at least over short time scales (Chapter 4). If behavioral flexibility was constrained by general factors known to structure plant–hummingbird interactions (e.g., morphological trait-matching and/or competition), then rewiring may also be restricted across longer time periods. Elucidating the mechanisms of network robustness and further investigating how changes in pollinator behavior influence plant reproduction will be key next steps to understanding the extent to which plant–pollinator interaction networks can withstand global environmental change.
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  • Chapter 2 (published version in Biological Reviews)
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  • 2022-05-31 to 2023-07-02

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