Ecological Archives E088-126-A3

Kerri M. Crawford, Gregory M. Crutsinger, and Nathan J. Sanders. 2007. Host-plant genotypic diversity mediates the distribution of an ecosystem engineer. Ecology 88:2114–2120.

Appendix C. The effects of soil nutrient availability and patch size and isolation on the abundance of bunch galls.

Factors other than host-plant genotype and patch-level genotypic diversity may also affect the abundance and occurrence of the goldenrod bunch gall midge. Resource availability influences the distribution of herbivores in general and gall-inducing species in particular by increasing the nutritional quality of plants and/or decreasing secondary metabolite production (Herms and Mattson 1992). Fertilization experiments have shown that the abundance of galling species is higher in fertilized than unfertilized plots in other systems (e.g., Stiling and Moon 2005). Thus, Solidago occurring in nitrogen-rich environments may be more susceptible to galling than Solidago in nitrogen-poor sites. Finally, the spatial distribution of host plants across a landscape may influence the distribution of galling species (McGeoch and Price 2004). Plants growing near other galled plants might have a higher probability of being galled than host plants that are far from one another because galling arthropods may be dispersal limited (Cronin et al. 2001, McGeoch and Price 2004).

To examine how soil nutrient availability affects rates of galling, we took advantage of an ongoing experiment at the study site that manipulates soil-nitrogen availability at three levels in old-field vegetation dominated by S. altissima: (1) addition of nitrogen (applied each May as urea fertilizer, at a rate of 10 g/m2), (2) addition of carbon (applied as sucrose at a rate of 167 g/m2 in May and again in August), and (3) unmanipulated controls. Applications of sucrose, which is ~46% C in a molecular form readily available to microbes, results in immobilization of plant-available N in the soil solution (Zink and Allen 1998). When we censused galls in this experiment (August 2005), the treatments had been maintained for two seasons, and nitrogen resin bags indicated strong treatment effects on soil-nitrogen availability (Sanders et al., unpublished manuscript). In each of 36 plots (n = 12 per nutrient treatment), we randomly placed a 1-m2 quadrat and counted the total number of Solidago ramets and the proportion of those ramets that were galled by R. solidaginis. We used a one-way ANOVA to examine whether the arcsine-transformed percent of ramets galled depended on soil nitrogen treatments (nitrogen added, carbon added, and control).

We used observational data to explore how patch size and isolation might influence rates of galling. We identified 20 distinct patches of Solidago in July 2005 in a neighboring old field. In each patch, we counted the total number of Solidago ramets and the proportion of ramets galled. We estimated the area of each patch as [(Length of the patch) ÷ 2] × (Width of the patch) × π. Isolation was estimated as the average distance from the edge of the focal patch to the closest Solidago patches in four cardinal directions. We used a series of linear regressions to examine the relationship between the arcsine-transformed percent of ramets galled and each of the following variables: patch area, minimum isolation distance (or distance to the nearest neighboring patch), and mean isolation distance (mean distance of the four nearest patches in the cardinal directions).

In contrast to host-plant genotype and patch-level genotypic diversity, we found no effect of soil N treatments on galling rates (F2,33 = 0.29, P = 0.75), nor did we find an effect of patch size (R2 = 0.01, P = 0.62), minimum patch isolation (R2 = 0.11, P = 0.13), or average patch isolation (R2 = 0.12, P = 0.12) on galling rates.

Contrary to previous research (Stiling and Moon 2005, Cronin et al. 2001, McGeoch and Price 2004), we found no effect of soil nitrogen availability or spatial distribution of patches (patch size and isolation) on gall abundance or occurrence. It may be that the gallers control the amount of nitrogen in the leaf tissue, mediating the effect of nitrogen addition and reduction (Hartley and Lawton 1992). The simplest explanation, though, is that the gallers respond more to some other plant trait. Other studies have suggested that galling arthropods may be dispersal limited, creating patches of colonized and uncolonized host plants (Cronin et al. 2001, McGeoch and Price 2004). If the gallers are dispersal limited and occur in low abundances, we would expect larger patches that are closer to other colonized patches to have the highest percentage of stems galled. Perhaps galling midges are not dispersal limited, at least at the scale of this study. It could also be that they occur in such high abundances, with ~50% of the ramets galled (G. M. Crutsinger, unpublished data) at the field site, dispersal limitation does not affect their distribution at the scale we examined. Nevertheless, these results indicate that host-plant genotype and population-level genotypic diversity of host plants have an over-riding influence on the abundance and distribution of this galling species. 

LITERATURE CITED

Cronin, J. T., K. Hyland, and W. G. Abrahamson. 2001. The pattern, rate, and range of within-patch movement of a stem-galling fly. Ecological Entomology 26:16–24.

Hartley, S. E., and J. H. Lawton. 1992. Host-plant manipulation by gall-insects - a test of the nutrition hypothesis. Journal of Animal Ecology 61:113–119.

Herms, D. A., and W. J. Mattson. 1992. The dilemma of plants - to grow or defend. Quarterly Review of Biology 67:478–478.

McGeoch, M. A., and P. W. Price. 2004. Spatial abundance structures in an assemblage of gall-forming sawflies. Journal of Animal Ecology 73:506–516.

Stiling, P., and D. C. Moon. 2005. Quality or quantity: the direct and indirect effects of host plants on herbivores and their natural enemies. Oecologia 142:413–420.

Zink, T. A., and M. F. Allen. 1998. The effects of organic amendments on the restoration of a disturbed coastal sage scrub habitat. Restoration Ecology 6:52–58.



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