LB 25-266 - Grass/forb ratios in prairie restorations alter Lepidoptera richness and network composition

Background/Question/Methods:

Prairie restoration projects typically focus on plant establishment. Because of this, important animal and insect groups are hardly considered. Flowering plant species support many insect communities, including those in decline, through mutualistic relationships providing insects with a source of food, pollen, and nectar. Butterflies (Insecta: Lepidoptera) heavily utilize these forbs and add to the species diversity of a prairie, and include species that are threatened or rare.

Using a field site in Central Iowa with 55 30 x 30 plots containing 11 grass to forb ratios in seed mixes (n = 5), data collection was conducted biweekly from June 2021 through September 2021 by visual identification and counting of butterflies and visited plants in each plot. Data was used to develop plant-pollinator networks to test whether interactions were altered by grass/forb ratios. Network measures such as nestedness (BITMATNEST test), connectivity, betweenness, degree centrality, modularity, and interaction evenness were quantified. Butterfly networks were separated by season to compare the effect of early, mid, and late flowering species had on butterfly visitation.

Results/Conclusions:

Danaus plexippus was observed to be the most dominant butterfly in the summer and visited the most forb species. Butterfly species richness showed a hump-shaped relationship with grass/forb across plots (P > 0.05). These results indicate that richness was highest in plots with roughly equal grass to forb ratios. Network measures differed across seasons. None of the BINMATNEST nestedness values exceeded 2 indicating low nestedness for all seasons, which suggests forb species are visited by generalist and specialist species. Checkerboard scores showed lowest scores (< 1) in the fall season suggesting very little competition between butterflies and plants during this season. The season with the most meaningful modularity value was the spring season with a value of 0.46 indicating a stronger community structure than the other seasons. Symphyotrichum pilosum had the highest hub score ( >1) in the Fall season meaning this plant species has the most potential of becoming a keystone species within this restored prairie site.

Our results indicate that prairie restorations should include a mix of both grasses and early, mid, and late flowering forbs to sustain butterfly populations.