Exploring top-down and bottom-up interactions between Echinacea angustifolia and its specialist ant-tended aphid

Background/Question/Methods

Due to their complex web of direct and indirect interactions, plant-aphid systems provide a valuable opportunity for studying trophic interactions in disrupted ecosystems. Conversion of the North American tallgrass prairie has dramatically reduced and fragmented native habitat and disrupted networks of pollination and herbivory. The prairie perennial Echinacea angustifolia is a model system for studying population-level effects of habitat fragmentation. Its most common herbivore is a specialist aphid (Aphis echinaceae) that is tended by ants and feeds exclusively on E. angustifolia. My research explores top-down and bottom-up interactions between E. angustifolia and Aphis echinaceae in order to evaluate consequences of habitat fragmentation for plant and aphid population dynamics. I used a combination of experimental and observational approaches to investigate: 1) effects of aphid infestation on plant growth, senescence, and damage from other herbivores; 2) relationships between seasonal peaks of aphid infestation and host flowering over time and across populations, and 3) factors influencing aphid distribution in a plant population within and across seasons, including plant size, flowering, and aphid dispersal distance.

Results/Conclusions

Aphid infestation did not significantly alter plant performance in the following year for plants with manipulated or naturally-occurring aphid infestation. Manipulated aphid infestation accelerated leaf senescence and decreased foliar herbivore damage; however, the relationship with foliar herbivore damage was the opposite over five years in plants with naturally-occurring aphid infestation. I found that the peak of winged aphids roughly coincided with peak flowering in multiple populations and years, shifting when a spring burn delayed peak flowering. Aphids dispersed short distances within a population and dispersed further to flowering plants than to non-flowering plants. Aphid distribution in 2012 corresponded to early-season clusters of aphid infestation from 2011. These results suggest that 1) aphid population dynamics are closely tied to host phenology, and 2) fragmentation of host plant populations may limit dispersal of Aphis echinaceae across prairie remnants, leading to loss of aphids in some populations and heavy aphid infestation in others. Considering Echinacea is a long-lived perennial, multiple years of aphid infestation may induce costs that were not apparent in this one-year experiment. Loss of aphids in a plant population may increase vulnerability to other herbivores. I will also discuss limitations of manipulative experiments in inferring top-down effects in natural populations.