Influence of environmental covariates on pollinator community occupancy, detection, and richness across urban gardens in Richmond, Virginia (U.S.A.)

Pollination is an essential ecosystem service that supports reproduction
and propagation of most of the world’s flowering plants. The dramatic
decline in pollinators, especially insect pollinators, due to climate
change and pesticide use threatens not only our food supply, but also the
diversity of native plants. Urban areas, if well managed, can serve as
corridors and reserves for pollinator species and benefit agricultural and
natural ecosystems well beyond the urban environment. In this study, we
assessed Mid-Atlantic (U.S.A.)-region urban garden plant-pollinator
interactions, focusing on activity associated with two regionally-native
plants: dense blazing star (Liatris spicata; Asteraceae) and clustered
mountain mint (Pycnanthemum muticum; Lamiaceae). We conducted 350 visual
surveys across 52 gardens and identified 14 taxa in 361 detection events,
with 5 taxa dominating at 331 detections. We built multi-species occupancy
models (MSOMs) in a Bayesian framework using site and survey covariates to
evaluate variables that influenced species occupancy, detection, and
richness. We found little influence of any variables on occupancy, and the
intercept-only models resulted in species-specific occupancy that ranaged
from 0.04 (Halyomorpha halys) to 0.86 (Halictidae). For detection, we
found that plant species and survey start time (or the interaction between
the two) influenced detection of a majority of pollinators at the
community level, while Julian date and urban distance (interaction)
influenced a small number of species. Comparisons between the two plant
species indicated that honey bees (Apis mellifera) and wasps (Vespoidea)
were more likely to be detected on P. muticum compared to L. spicata,
while the reverse was true for A. campestris. All taxa became more
detectable as it became later in the day. A. mellifera and Bombus spp. had
higher detection earlier in the year. Halictidae detections increased
closer to the urban areas, while Bombus spp. detection increased farther
from urban areas. The posterior medians of the number of taxa per site
ranged from 5 – 8 and showed little evidence of differences across sites,
but the composition did vary. The estimated number of taxa occurring
across all sampled sites was 18, indicating that ~25% of taxa present at
our study sites went completely undetected. Our study demonstrates that
MSOMs can be an effective tool for monitoring and investigating the
pollinator community. We were able to estimate occupancy for 14 observed
insect taxa, 9 of which were detected fewer than 8 times. We also
estimated effects of detection covariates that impacted multiple taxa and
provide insight into ways to improve future pollinator monitoring efforts.
These findings further our understanding of how plant species and the
urban setting may variably influence pollinator activity and highlight the
importance of urban gardens in supporting divserse insect communities.