Top-down and bottom-up interactions in arthropod communities
My research has examined how spatial and temporal variation in “bottom-up” (resource related) factors modify predator-prey interactions (“top-down” effects). Most recently, I have worked in tidal wetlands examining the interactions between the dominant salt marsh plant, Spartina alterniflora, the numerically dominant herbivores of these plants, delphacid planthoppers and mirid bugs, and their most common natural enemies, hunting and sheet-weaving spiders.
Our research found that variation in habitat structure and plant quality, commonly referred to as “bottom-up” factors (as they originate from the base of a food web), can not only influence the population dynamics of herbivorous insects directly, but can also alter the effect of natural enemies. For example, highly structured or complex habitats, by virtue of high litter accumulation, promote the aggregation of spiders which in turn suppress herbivore populations. Moreover, these direct and indirect effects change over time, both within a season and across years.
- Gratton, C. and Denno, R.F. 2003, Oecologia. Temporal shifts in top-down and bottom-up effects on an herbivore population. DOI 10.1007/s00442-002-1137-8 at http://link.springer.de/link/service/journals/00442/contents/02/01137/.
- Denno, R.F., C. Gratton, H. Döebel, D. L. Finke. In press, Ecology. Risk of predation influences the relative strength of bottom-up and top-down effects in a herbivorous insect community.
- Denno, R.F., C. Gratton, M.A. Peterson, G.A. Langellotto, D. L. Finke, A. F. Huberty. 2002. Bottom-up forces mediate natural-enemy impact in a phytophagous insect community. Ecology 83 (5): 1443-1458
This research was funded in part by the National Science Foundation.
Invasive species and restoration ecology
One of the most serious threats to biodiversity is the introduction of non-native species to an environment. I have been interested in examining the effects of invasive plants on native arthropod food webs. In a recent project, I examined the impact of the invasive wetland plantPhragmites australis on the structure and function of a native arthropod community in a brackish wetland. Phragmites has invaded this southern New Jersey wetland (Alloway Creek Watershed, Salem Co.) and displaced much of the native vegetation, marsh that has historically been dominated by Spartina.
In 1996, a restoration program was initiated whereby Phragmites was actively removed by glyphosate application. By 2001, native plants had become re-established on much of the restoration site. Our study further showed that much of arthropod diversity had also become re-established over this 4 year period. Moreover, using stable isotopes of C and N as tracers of energy flow and feeding linkages, I found that feeding relationships among arthropods in the recently restored habitats were also similar to those in reference sites. These findings suggest that not only is arthropod diversity and community structure re-established following the removal of an invasive plant, but trophic linkages between the resources and consumers are also re-established. This study is examined the ecological interconnections between invaded and natural habitats. In particular, to what extent do adjacent habitats serve as sources or sinks for natural enemies and herbivores.
This research was funded by NOAA / New Jersey Sea Grant (Marsh Ecology Research Program).
Host-specialization in phytophagous insects.
Most herbivorous insects feed on a very select group of host plants, often restricted to several related species or genera. I have been interested in the ecological aspects of host plant specialization and in understanding the factors promote narrow diet breadth in herbivores. One hypothesis suggests that natural selection favors herbivores feeding on plants where mortality from natural enemies is minimized. To test this hypothesis I experimentally examined the interactions between a host-specific leafmining fly, Liriomyza helianthi(Agromyzidae), its host plant, Heliathus annuus (common sunflower) and the assemblage of parasitoids attacking the leafminer larvae. I found that use of novel host plants indeed resulted in lower mortality from natural enemies (in support of the enemy-free space hypothesis). Yet, the relative benefit of using novel plants depended on the dominant life history strategy of parasitoids that were present in any given year. When generalists such as Diglyphus (Eulophidae) were common, there was little advantage of feeding on novel plants.
Gratton, C. and S. C. Welter. 1999. Does enemy-free space exist? Experimental host-shifts of an herbivorous fly. Ecology. 80:773-785
Gratton, C. and S. C. Welter. 1998. Oviposition preference and larval performance of Liriomyza helianthi (Diptera: Agromyzidae) on normal and novel host plants. Environmental Entomology 27: 926-935
This research was funded in part by a Committee on Research Grant (UC Berkeley).