Non‐native insects dominate daytime pollination in a high‐elevation Hawaiian dryland ecosystem

Abstract

PREMISE OF THE STUDY:Over one-third of the native flowering plant species in the Hawaiian Islands are listed as federally threatened or endangered. Lack of sufficient pollination could contribute to reductions in populations, reproduction, and genetic diversity among these species but has been little studied. METHODS:We used systematic observations and manual flower treatments to quantify flower visitation and outcrossing dependency of eight native (including four endangered) plant species in a dryland ecosystem in Hawaii: Argemone glauca, Bidens menziesii, Dubautia linearis, Haplostachys haplostachya, Sida fallax, Silene lanceolata, Stenogyne angustifolia, and Tetramolopium arenarium.KEY RESULTS: During 576.36 h of flower observations, only insects visited the flowers. Out of all recorded flower visits, 85% were performed by non-native species, particularly the honeybee (Apis mellifera) and flies in the family Syrphidae. Some plant species received little visitation (e.g., S. angustifolia received one visit in 120 h of observation), whereas others were visited by a wide diversity of insects. The endangered plant species were visited by fewer visitor taxa than were the common native plant species. For six of the focal plant species, bagging of flowers to exclude pollinators resulted in significant reductions in seed set. CONCLUSIONS:The flower visitor community in this system, although heavily dominated by non-native insects, appears to be facilitating pollination for multiple plant species. Non-native insects may thus be sustaining biotic interactions otherwise threatened with disruption in this island ecosystem. This may be particularly important for the studied endangered plant species, which exhibit fewer partners than the more common plant species.

Invasive predators affect community‐wide pollinator visitation

“…We verified that these insect taxa transported the pollen of the focal plants, as described above. Although flower visitation does not equate to pollination, our prior work with breeding system floral treatments (e.g., bagging of flowers) found that flowers of each of the species in this experiment produced increased seed set when exposed to the visitor community, relative to flowers that were bagged to prevent outcrossing (Aslan et al, 2019); note that this increase was significant for all plant species except S. lanceolata .…”

“…Reproduction of our focal plant species is likely to be negatively affected by decreased pollinator interactions, based on our previous work in the system. Both common and endangered plant species are pollen limited, and flower treatment experiments showed that open‐pollinated flowers produced significantly more seed than bagged flowers for all of our focal plant species except S. lanceolata (Aslan et al, 2019). In general, then, these focal plants have limited autogamy, or self‐fertilization, and are dependent to some degree on pollinators for reproductive fitness.…”

“…During this baseline year, we determined that most flower visitation in the system was performed by generalist insect taxa (e.g., Apis mellifera , Syrphidae, Lasioglossum sp.) that are found globally (Aslan et al, 2019; GBIF, 2020; Ollerton, 2017); we used these baseline data to determine which flower visitors carried pollen and interacted regularly with the reproductive structures of the focal plants, in order to group the visitor taxa and parameterize the tritrophic networks analyzed here (as described in the section on Data synthesis ).…”

“…Rate of visitation was calculated as the product of the frequency of visits by a given visitor group (number of visits per observation period, standardized by the number of open flowers during that period) and the number of flowers visited by each individual during a visit, for each plant; observation effort was used to calculate rate variance for each flowering plant species. To obtain these data, observations followed a systematic protocol combining flower scans and focal individual observations (after Aslan et al, 2014, 2019; Farwig et al, 2017; Renne et al, 2000). Each observation lasted 180 min, divided into 10‐min blocks.…”

Additive and non‐additive responses of seedlings to simulated herbivory and drought

“…Introduced mammals reduce Hawaiian seedling recruitment directly through consumption of flowers, seeds, and seedlings (Pender, Shiels, Bialic‐Murphy, & Mosher, 2013; Shiels & Drake, 2011; Shiels, Pitt, Sugihara, & Witmer, 2014), as well as indirectly through disturbance to the plant‐growth substrate (Cole, Litton, Koontz, & Loh, 2012; Murphy, Inman‐Narahari, Ostertag, & Litton, 2014). Non‐native insects may similarly threaten seedling recruitment indirectly through damage to flowers or through failure to pollinate native plants (Aslan, Shiels, Haines, & Liang, 2019; Aslan, Zavaleta, Tershy, Croll, & Robichaux, 2014). Invasive slugs and snails also target native seedlings directly for consumption in Hawaiian forests (Joe & Daehler, 2008; Shiels, Ennis, & Shiels, 2014).…”