Nectar Standing Crops in Delphinium Nelsonii Flowers: Spatial Autocorrelation among Plants?

Several aspects of nectarivore foraging behavior have been interpreted as responses to spatial reward patchiness of the kind documented for Delphinium nelsonii floral nectar by Pleasants and Zimmerman (1979). Working with this same species over 3 yr, however, we were unable to detect substantial pattern in nectar standing volumes, either through contingency analyses or spatial autocorrelation. Although spatial outocorrelations between rewards of neighboring plants were positive in 5 or 6 samples examined, only one value was statistically significant. Spatial autocorrelations over longer distances were erratic. We used computer simulations of nectarivores foraging in a large plant population to explore factors that promote reward patchiness. Simulations suggest that moderate patchiness will develop at all but extremely low or high flower visitation rates. Rates were intermediate at our sites and those of Pleasants and Zimmerman, however, so visitation intensity does not seem to explain the discrepancy between our results and theirs. On the other hand, reward patchiness in simulations declined substantially as nectarivores exhibited less area—restricted foraging. Hummingbirds were important visitors at our sites, and fly farther between plants than the bumble bees that predominated at Pleasant's and Zimmerman's sites. Finally, simulations suggest that spatial patchiness is lower when interplant coefficient of variation in nectar production rate is large, as in our populations.