Twenty-Fourth Annual Meeting and Symposium of the Desert Tortoise Council, March 5-8, 1999
Abstracts

Coexisting species are likely to have both negative and positive effects on each other, and the resultant of both types of interactions may contribute to long-term interspecific dynamics and species coexistence. Using the close association of annual plants with a desert shrub (Ambrosia dumosa) in the northern Mojave Desert of California as a test system, we identified and quantified negative and positive effects of annuals on shrubs and shrubs on annuals. A separation of negative and positive effects was achieved using an experimental design which included reciprocal removals of neighbors and simulations of physical effects of neighbors on water availability using artificial structures. Assessments of performance of shrubs and annuals showed that neighboring shrubs and annuals compete for resources but also facilitate each other. Even though positive and negative effects were acting simultaneously, the relative importance of positive and negative effects shifted during the growing season. Annual plants benefited from the presence of shrubs to the largest extent early in the growing season, while the negative effect of annuals on shrubs declined as senescence of the annuals ensued later in the season. Positive and negative effects among associated plant species are likely to vary among seasons. Therefore the outcomes of interactions will change among years and neighbors may be favored or depressed to varying extent in different years, thereby influencing long-term coexistence. The investigation of a shrub-annual association over four consecutive years showed that bi-directional positive and negative effects between shrubs and annuals varied strongly from year to year. Since these opposing effects also varied independently among years, the magnitude of resulting net effects changed in time as well.

Based on theoretical considerations it has been predicted that positive net interactions (facilitation) will be more prominent in years of low resource availability (e.g., low rainfall). Negative net interactions (interference) are expected to be of greater importance in years with high resource availability. Such a predicted trend was not found for the effects of shrubs on annuals. Shrubs facilitated annuals to the largest extent in intermediate years, in extremely wet and dry years positive net effects were comparatively smaller. The effect of annuals on shrubs followed the prediction more closely; annuals interfered with shrubs increasingly from dry to wet years. Overall however, annuals benefited from shrubs most of the time and annuals always had negative effects on shrubs. Annuals seem to act as "shrub parasites". It remains to be tested whether this disproportion in the interaction is caused by recent changes in the annual plant community due to invasion by non-native plants. Today, non-native plants contribute more than 3/4 of the biomass within the investigated annual plant community. Long-term changes in the interaction of shrubs with annuals need to be monitored in order to assess possible changes in desert shrub communities.