Graduation Year

2005

Document Type

Thesis

Degree

M.S.

Degree Granting Department

Biology

Major Professor

Peter Stiling, Ph.D.

Committee Member

Susan Bell, Ph.D.

Committee Member

Gary Huxel, Ph.D.

Keywords

gall makers, sap suckers, Borrichia frutescens, fertilizer, sticky traps, parasitoids, herbivory

Abstract

I compared the strength of bottom- up and top-down effects on insect herbivores along a natural salinity gradient in salt marsh communities in West - Central, Florida. I used a 2x2 factorial design with plots divided into four different treatments: 1) fertilizer applied to increase plant quality 2) sticky traps added to remove natural enemies (parasitoids) 3) fertilizer applied and sticky traps added and 4) control plots. These plots were placed on 7 different sites containing the salt marsh plant Borrichia frutescens along a natural stress salinity gradient. In each plot I determined the abundance of the sap sucker Pissonotus quadripustulatus, the gall maker Asphondylia borrichiae, spiders and the number of chewed leaves and bored stems. I also recorded leaf area, plant density, plant height and foliar nitrogen. Plants in fertilized plots exhibited increased height, density and leaf area. In fertilized plots, galls were more abundant than in nonfertilized plots (approximately 63% increase), and the same pattern was observed for Pissonotus (55% increase). For chewed leaves and spiders there were non significant increases on fertilized as compared to unfertilized plots. There were no significant effects of natural enemy (parasitoid) removal. Gall density and Pissonotus density were both significantly more abundant in the sites with lower soil salinity, but there was no interaction of either treatment with salinity level. My results suggest that in this system bottom up effects are stronger than top down effects but there is no change in the strength of bottom up and or top down effects along an environmental salinity gradient.

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