SPATIAL PATTERNING IN PLANTS - OPPOSING EFFECTS OF HERBIVORY AND COMPETITION

(1) A field experiment was conducted to determine how the spatial distribution of Poa annua influenced the ability of Senecio vulgaris to establish in experimental plots. The relative effects of slug herbivores, dead individuals of Poa annua and live individuals of Poa annua on Senecio establishment were also investigated. (2) Senecio exhibited a higher rate of population growth when planted amidst clumped Poa than when planted amidst a random distribution of Poa. This change in population growth is due to increased survival of Senecio seedlings which emerge in areas having a low density of dead Poa. (3) The advantage associated with a clumped distribution of Poa was opposed by two other consequences of spatial distribution: herbivores consumed more Senecio seedlings, and intraspecific competition among Senecio seedlings was greater, where the grass was clumped than where it was randomly distributed. (4) Despite a number of direct effects, higher-order interactions and indirect effects, the net result of spatial patchiness is easily understood in this system because seedling suppression by dead Poa has overriding importance.