Many studies have documented the negative impact of invasive species on populations, communities, and ecosystems, although most have focused solely on antagonistic rather than mutualistic interactions. For mutualistic interactions, such as pollination, a key to understanding their impacts is how invasive species interact with native species and alter interaction networks. Chapter 1 explores the impacts of invasive species on islands, particularly in regard to plants, pollinators, and how these exotic species attach to existing pollination interaction networks. Island pollination networks differ from mainland counterparts in several important characteristics, including fewer species, more connectance, and increased vulnerability to both invasion and extinction. A progression of invasion has been previously proposed, through which supergeneralist native species facilitate the entry of new exotic species, then are eventually replaced by a few supergeneralist invader species that ultimately dominate the interaction networks. As a result, highly-linked exotic supergeneralists become central nodes in the networks, thus altering network topology and community structure and functioning. Here, I evaluate the evidence for (1) native supergeneralists that provide attachment points for exotic species, (2) exotic supergeneralists that are potentially replacing the function of native species, and (3) the consequences for the replacement of native species with exotics. Both native and exotic supergeneralist species are found on islands, which may therefore represent different points along the invasion trajectory, with consequent concerns for future conservation.
Chapter 2 utilizes a long-term series of observed floral visitations to break apart the potential differences between plants and pollinators as invaders of a community. When plants are introduced into a new environment, their reproductive success can be limited by the lack of a suitable pollinator. If there is no suitable native pollinator, the success of exotic plants may depend on the presence of exotic pollinators, a situation mirrored for exotic plant visitors. Yet, rarely are the distinct roles for native and non-native species of both plants and pollinators examined in the same community. This study examines the role of exotic plants and insects in floral visitation networks in Hawaii, in simple ecological communities with a depauperate native pollinator fauna. On the island of Hawaii, in sites that differed with respect to the presence of exotic plants, floral visitors were observed and quantified across multiple years and seasons. Where exotic plants were present, exotic insects were observed to visit both native and exotic plant species, while native insects rarely utilized exotic plant resources. Additionally, the majority of floral visitors comprised exotic bees and syrphid flies. In contrast, where the vegetation was dominated by native plants, native bees were major visitors. Thus, the impact of exotic plants and insect visitors on visitation networks was non-symmetrical. Exotic plants relied upon exotic insect taxa, while exotic insect taxa were able to utilize both native and exotic plants. This study demonstrates that the role of mutualistic interactions on the success and impact of invasive species cannot be predicted by looking at isolated interactions, but must also consider the context of the interactions.
Chapter 3 evaluates how these floral visitation networks vary over time. Pollination systems provide important ecosystem services in both natural and managed ecosystems, but their future ecological stability is uncertain as a result of global change, including the impacts of invasive species, habitat loss, and a changing climate. Understanding how these systems vary naturally through time, including intra-annually, can provide critical context for evaluating future change, as well as elucidating the complexity of interspecific interactions in the community. This study examines temporal variation in floral visitation networks in a tropical system in Hawaii characterized by both native and non-native pollinators and plants, and less seasonal variation than in temperate regions. The three most common floral visitors exhibited unique seasonal visitation patterns. In the presence of only native plant species, both the exotic honeybee Apis mellifera and the endemic Hylaeus bees had similar seasonal variation in floral foraging. However, when the vegetation was a mix of native and exotic species, Apis visitation tracked the peak blooming of exotic plants while Hylaeus only visited native plants, leading to seasonal variation in resource partitioning. In contrast, visitation by the invasive yellowjacket Vespula pensylvanica consistently peaked during the fall, unrelated to plant blooming cycles. Thus, even in a system with minimal seasonal climate variation, there were marked differences in the patterns of pollination interactions between seasons, suggesting that intra-annual variation must be considered in predictions for stability of pollination networks in a changing world.
Finally, Chapter 4 reviews and assesses the range of conservation threats to these Hawaiian pollination systems. Pollination interactions worldwide are facing a wide variety of threats, including habitat loss/change, agricultural intensification, pesticide/herbicide use, invasive species, parasites/disease, and global climate change. Pollination networks in Hawaii are of special concern, because of the unique nature of Hawaii’s terrestrial biota, including both plants and pollinators. As the sites from this study were located within a protected national park, the most likely sources for their endangerment are exotic/invasive species, the introduction and spread of parasites/disease, and the slow but potentially devastating effects of climate change. Hawaiian ecosystems, and these sites in particular, are additionally subject to the changes and hazards associated with a zone of active geologic activity. In this chapter, I address specifically both the rising global threats of parasites/disease and climate change and the unique local dangers of active volcanoes for Hawaiian pollination interactions. The variety and magnitude of potential effects provide a wealth of opportunities for future research utilizing existing network data to evaluate how these factors operate both independently and interactively to create change.