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Interactions between genotype and environment determine the underlying constitutive (background-level) resistance of individual plants to herbivory. Resistance, however, may change in response to herbivory (induced resistance). This study was designed to investigate temporal fluctuations in resistance, and to test the relative influences of constitutive and browsing-induced resistance on patterns of mammalian herbivory. A natural population of creosotebush, Larrea tridentata, browsed by black-tailed jackrabbits, Lepus californicus, served as the study system.

Creosotebushes varied considerably in their resistance to jackrabbit browsing. Some individual plants were fed upon lightly and infrequently, while others had up to 90% of their living branches clipped during a single month, and were browsed several times per year. Resistance levels of individual shrubs were similar between seasons, and over time scales >2 yr. Shrubs with a history of heavy browsing (low constitutive resistance) were more likely to be browsed again than were individuals with a history of light browsing (high constitutive resistance). In an experiment designed to test jackrabbit response to constitutive resistance and recent browsing damage (artificially clipped vs. unclipped), the importance of both factors was confirmed. Jackrabbits browsed more heavily on plants with low than with high constitutive resistance, but less heavily on clipped shrubs than on controls. The enhanced resistance of clipped shrubs is interpreted as an induced response by individuals to herbivory, and may be responsible for variation in browsing levels of individual shrubs over time scales of 1-2 yr.