Appendix A. Natural enemies of EPN at Bodega Marine Reserve.
We explored the possibility that the trophic cascade in which EPN suppressed root feeding caterpillars and protected bush lupine in shrublands actually had an additional link in which nematophagous enemies suppressed the EPN. Nematophagous organisms could explain why root feeding caterpillars killed bush lupine at some sites but not at others (Strong et al. 1995, 1996). We rejected the hypothesis that natural enemies of the EPN were more potent at sites with lower incidence of the EPN (Jaffee et al. 1996). Using most probable number assays of fungal propagules in samples of soil, in four seasons of the year, we found that the most common natural enemies of nematodes in lupine rhizospheres at BMR are 12 species of nematode trapping fungi. These fungi are potent parasites of Heterorhabditis marelatus and other EPN species.
While two of these fungal species showed weak specificity to the six study sites, the other 10 did not vary among sites. Neither field abundances nor diversity of the fungi varied among the sites in a way consistent with a trophic cascade in which the fungi suppressed the EPN. There was actually a weak trend to the contrary, and sites with high EPN incidence actually had more fungi. While propagule numbers of some of the 12 species of nematophagous fungi were very high in some rhizospheres, fungal propagules in the four seasonal samples never varied among sites in correlation with the incidence of H. marelatus. Pasteurization (gentle heating of field soil) significantly increased the survival of the EPN relative to survival in a comparable raw soil subsample lupine rhizosphere. Pasteurization killed virtually all animals as well as greatly reduced fungi and bacteria in the soil samples. While fugal suppressiveness was very high in some rhizospheres, the sites did not differ significantly in suppressiveness. Finally, we found high incidences of root knot nematode in the lupine rhizospheres, but these were not suppressive to H. marelatus.
In another study motivated by the idea of a trophic cascade, we found extremely strong bottom up effects but weak top-down effects on EPN populations. Caterpillars added to soil samples produced huge numbers of EPN, which in turn produced very large numbers of four species of nematophagous fungi. The greatest increase was ca. 10× in spores of Arthrobotrys oligospora, which captures EPN extremely efficiently with networks of adhesive mycelia in soil. These strong bottom-up enhancements of nematode-trapping fungi were not matched by top-down suppression of EPN. The high densities of fungi trapped fewer than 30% of dispersing H. marelatus (Jaffee and Strong 2005), and had much less effect upon H. marelatus survival through time than soil dryness (Preisser and Strong 2004). Subsequent work has reinforced the conclusion that nematophagous fungi cannot kill enough of the hundreds of thousands of IJ that emerge from a host cadaver to affect the trophic cascade from EPN to root feeding insects, to bush lupine (Jaffee et al. 2007). While a few mites, enchytraeids, and collembolans were found, the majority of rhizosphere soil samples contained none.
To test whether natural enemies of EPN were responsible for varying patterns of incidence, we conducted three separate experiments over a 24 month period between 1995 and 1996. In each one, we compared survival of EPN between raw (soil with enemies) and pasteurized (soil with enemies removed) or some combination thereof. We report the results of these experiments below.
Experiment 1: In 1995, we collected a 300 cc samples from 25 bushes at each of six sites. We then divided each sample into three subsamples of 100 cc each. Sample a was un-manipulated, sample b was heated at 60 °C for 2 days and then cooled, and sample c was half raw and half heated. After rehydrating each sample to 20% moisture by volume, we then added 100 IJ. We subsequently added six Galleria larvae to each sample and counted IJ inside of infected cadavers. We use the log ratio of as an index of suppression. Positive values of the ratio indicate lack of suppression and negative values suggest that soils are suppressive. A one-way ANOVA among sites shows lack of a significant difference among the sites (F5, 144 = 1.67, P = 0.15).
Experiment 2: In this experiment, we collected soil from two high (Mussel point and Cove) and two low incidence (Upper Draw and Bayshore) sites. At each site, we collected soil from the rhizosphere of 24 bushes. Within sites, we also collected paired samples from grassland two feet from each bush. We divided each of these 200 cc of soil into two, one of which remained raw and the other pasteurized. We conducted a two-way ANOVA looking at the effects of site and rhizosphere on the log ratio of . There was no effect of site (F3, 184 = 1.2732, P = 0.29) rhizosphere (F1,184 = 0.31, P = 0.58) or an interaction (F3,184 = 1.49, P = 0.22) between the two.
LITERATURE CITED
Jaffee, B. A., D. R. Strong, and A. E. Muldoon. 1996. Nematode-trapping fungi of a natural shrubland: Tests for food chain involvement. Mycologia 88:554–564.
Jaffee, B. A. and D. R. Strong. 2005. Strong bottom-up and weak top-down effects in soil: nematode-parasitized insects and nematode-trapping fungi. Soil Biology & Biochemistry 37:1011–1021.
Jaffee, B. A., J. L. Bastow, and D. R. Strong. 2007. Suppression of nematodes in a coastal grassland soil. Biology and Fertility of Soils 44:19–26.
Preisser, E. L., and D. R. Strong. 2004. Climate affects predator control of an herbivore outbreak. American Naturalist 163:754–762.
Strong, D. R., J. L. Maron, P. G. Connors, A. V. Whipple, S. Harrison, and R. L. Jefferies. 1995. High mortality, fluctuation in numbers, and heavy subterranean insect herbivory in bush lupine, Lupinus arboreus. Oecologia 104:85–92.
Strong, D. R., H. K. Kaya, A. V. Whipple, A. L. Child, S. Kraig, M. Bondonno, K. Dyer, and J. L. Maron. 1996. Entomopathogenic nematodes: Natural enemies of root-feeding caterpillars on bush lupine. Oecologia 108:167–173.