Positive interactions have been shown to drive ecosystem infrastructure, especially in harsh habitats such as deserts and other arid ecosystems. Pollination is a common positive interaction integral for all angiosperms, though the mechanisms vary widely. Exozoochoric pollination may be either generalized or specialized, though the mechanisms distinguishing between differing classes of flying pollinators (e.g. avian and insect) are not well understood. In shrub-dominated ecosystems, plant size and flower output are both indicators of pollination frequency by flying pollinators, though the allocation of these two structures are usually negatively correlated. We examined the influence of avian pollination on a primarily bee-pollinated foundational cactus species while controlling for individual level showiness (expressed as plant size, number of flowers, and floral syndromes) in a desert habitat. We hypothesized that avian pollinators would prefer mid-level cactus height and flower production. We observed in-field cactus individuals of varying sizes, manipulating the number of flowers (zero, fifteen, and thirty) using motion sensing camera traps (N=81), 1.5 hour continuous footage (N=40), and 1 hour focal observations (N=81) over twenty-four days in May and June of 2019 (peak migration season). We performed the experiment first with mimic fabric flowers, then repeated with real flowers upon blooming.
Results/Conclusions
Trochilids (Hummingbirds) were the sole avian visitor to cacti engaging in pollination behaviors. This family made up 70% of birds species interacting with a focal plant (n=88), and 50% of all interactions were pollination events (n=44). We found that birds attempted to pollinate cacti with mimic flowers (M = 0.77, SD = 0.94) significantly more frequently than cacti with real flowers (M = 0.00, SD = 0.00), the latter having zero instances of attempted pollination (though birds were witnessed to pollinate real flowers outside of surveys); t(29) = -3.43, p = 0.002. However, density of flowers (0, 15, or 30 flowers) did not impact pollination attempts among mimic-display cacti; F(2, 57) = 1.16, MSE = 1.02, p = 0.322. Additionally, we found that cactus physical structures (a proxy for size) and reproductive structures were negatively correlated among multiple metrics. These findings suggest some level of communication between multiple phyla (plants, insects, and birds) by which at least birds benefit. Further study is needed to understand what mechanisms, whether floral, avian, insect, or some combination of the three, are influencing this foraging behavior.