Within-Patch Structure and Dynamics in Pastinaca Sativa and Resource Availability to a Specialized Herbivore

The life—history characteristics of the biennial herb Pastinaca sativa (Umbelliferae) influenced its within—patch structure in size and growth form which, in turn, influenced availability of plants to a specialized herbivore, Depressaria pastinacella (Lepidoptera: Oecophoridae). Of 115 plants tagged in October, 1975, only rosettes with root weights of at least 0.7 g and a total weight of at least 1.1 g flowered the next spring. Plants that grew larger as rosettes produced larger flowering individuals that set more seed. In 4 patches studied in 1976, plants in the interior were significantly smaller (in terms of biomass) and closer to conspecifics than those on the periphery. Larger plants were more susceptible to attack by Depressaria larvae, because unopened umbels, necessary for successful oviposition, were available on these plants over a longer period of time. Pattern of attack within a patch could be predicted from knowledge of plant basal stem diameter, a reliable estimator of availability of unopened umbels. Attack on more isolated plants (no conspecifics within 3 metres) was also significantly related to basal stem diameter. Within a size class, however, plants in patches were more susceptible to attack than isolated plants. The major factor governing dispersion of larvae among isolated plants and within patches was the dispersion of host plants of various sizes and growth forms. Depressaria larvae significantly reduced seed output in size classes in which comparisons were possible. Within—patch dynamics in Pastinaca appears to be governed by life history characteristics, individual plant plasticity and the pattern of attack by Depressaria.