Document: STE-3-48-15

What is the role of herbivores in relationships between plant population size and reproduction?

HENDRIX, S.D.* and C.K.LEWIS

University of Iowa, Iowa City, IA 52242 ¹

Abstract:
Reductions in plant population size due to habitat loss may have negative consequences for reproduction because of changes in plant-animal interactions. In this two-year study using the tall grass prairie shrub, Amorpha canescens (Fabaceae), we 1) examined the effects of plant population size on flowering phenology and reproduction, and 2) assessed the impact of floral herbivores on reproduction. We measured flowering phenologies, reproduction, herbivore species richness, and herbivore abundance in roadside remnants and patches on preserves ranging in size from 2-30,000 flowering ramets. In both years, small populations had shorter flowering periods and started significantly later than large populations. The size of racemes did not vary significantly with population size, site type (preserve or roadside), or between years. Percent fruit set was related to population size in 1996 when the flowering period was relatively short (6-42 days), but in 1997, when the flowering period was longer (12-55 days) population size and percent fruit set were related only within roadside populations. The number of undamaged fruits per raceme showed a positive trend with population size in 1996, but not in 1997. In both years the species richness of herbivores at a site was significantly related to population size, as was the abundance of the bruchid Acanthoscelides submuticus and an unidentified lepidopteran webber. A composite measure of the abundance of the three most common floral herbivores, however, was only weakly related to percent fruit set/raceme in 1996 and was not related to it in 1997. These results indicate that factors other than floral herbivory drive much of the interaction between population size and plant reproduction. Indirect evidence suggests that differences in pollinator service may be important in determining plant reproduction. Very small populations (<20 ramets) had consistently low reproduction across years even though herbivory was also low. Intermediate populations (50-200 ramets), had large increases in reproduction from 1996 to 1997 with the longer flowering season, but did not experience any release from herbivory. Lastly, large patches (>200 ramets) had lower percent fruit set in 1997 than 1996 despite the longer flowering season, indicating pollinator limitation.

Keywords:

This abstract is being presented at: 3:30 PM in session:
PLANT-ANIMAL INTERACTIONS