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Indirect effects of abiotic factors on swift populations could be mediated either by their host plant or through predators
and pathogens. Input of sea spray is higher at the upwind sites (Barbour et al. 1973). One consequence of higher salt
spray loads could be higher soil phosphorous levels. A possible mechanism by which phosphorus could drastically alter
allocation patterns in the plant has been demonstrated by Dinkelaker et al. (1989) in a congeneric species, Lupinus albus.
Under conditions of low phosphorus, this plant secretes large quantities of citrate (up to 23% of its dry matter production)
from proteoid roots. The citrate enhances the solubility of phosphorus, making it more available to the plant. Thus, plants
that have more phosphorous available could have much more fixed carbon to allocate to growth rather than to acquiring
nutrients since they would not need to secrete as much citrate to get their required phosphorous. If a similar mechanism is
present in L. arboreus, this would almost certainly alter plant chemistry between sites with different levels of phosphorous in
ways that could affect individual growth rates and survival of the swift caterpillars by changing the nutritional quality of their
food. Differential g_:owth and survival rates could in turn determine the density of caterpillars in the stems and roots of
plants.
Abiotic factors, such as soil properties and moisture, could contribute to differences in the nematode populations
between the two sites. Heterorhabditis sp. are probably vulnerable to desiccation (Kaya 1990), hence in the dry summer,
slight differences in soil moisture between sites may be quite important to their survival.
DISCUSSION
The picture sketched by our preliminary information leads to several hypotheses concerning the differences in the
impact of herbivore on its host plant among our different sites. Understanding the mechanism by which plants are "protected"
from herbivory at sites with low herbivory as compared to sites with high herbivory will allow us to assess the
relative importance of the top-down effects of natural enemies and the bottom-up effects of plant chemistry (including
nutrition). The simplest hypotheses implicate a single abiotic factor or an abiotic factor mediated by either the nematode or
the lupine. These include differences in oviposition due to wind, differences in levels of predatory nematodes due to differences
in soil chemistry, and differences in the secondary chemistry of the lupine due to differences in nutrient levels. While
each of these may play a role, it seems unlikely that any of them are sufficient alone.
An additional hypothesis involves the interaction of plant chemistry and predation in explaining the observed pattern.
The hypothesis is that the different nutrient levels at some sites allow plants to be better defended against herbivory, or to be
more vigorous and better able to withstand herbivory (Price 1991), both of which would reduce the impact of the herbivore
enough that the plants would be able to persist at that site. Then, since the plants persist at that site, rather then undergoing
local extinction, there is a constant supply of the swift for the nematode to prey upon. At sites that experience local extinction
of lupine, the nematode could have no host for a period of time and go extinct at that site. Since it is not a very mobile
predator, it may not be able to recolonize rapidly when the lupine and the swift reappear. Thus, the nematode reinforces the
pattern caused by the differences in herbivore "resistance" between the sites by reducing swift populations where the lupines
usually persist, but not where they experience high mortality.
Experimental manipulations in the field of natural enemies and conditions affecting plant chemistry will be needed to
assess the relative roles of top-down control and bottom-up protection in controlling herbivore populations. The third
alternative, as we have mentioned, is the action of abiotic factors directly on the herbivore populations; this possibility should
also be examined by looking at egg deposition and the desiccation of early instars at the different sites.
SUMMARY
Mature bush lupine, Lupinus arboreus, die after heavy stem and root boring by caterpillars of the ghost moth or
swift, Hepialus californicus. Mortality rates of these plants can be extremely high in some stands and quite low in other
stands in close proximity. This system is ideal for testing for top-down versus bottom-up control of herbivore populations by
examining the differences between sites where there is heavy herbivory and sites where there is not heavy herbivory. Nutrient
levels that are likely to affect lupine plant allocation patterns probably vary over the area of interest. Population levels of
an entomopathogenic nematode that prey upon the swift moth also vary over the reserve. It is hypothesized that both plant
chemistry variation and the nematode natural enemy play an important role in determining the level of herbivore attack.
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