Distribution Ecology: Variation in Plant Recruitment over a Gradient in Relation to Insect Seed Predation

Abstract

Abstract. Although predispersal seed predation by insects is common, no test exists of its effect on plant recruitment. This study examines seed predation in the population dynamics of a native, temperate shrub, Haplopappus squarrosus H. and A. (Asteraceae), over an elevational gradient in the coastal sage scrub vegetation of San Diego County, California USA. Frequency and abundance of H. squarrosus increase from coast to mountains. Expected abundance, based on flowers initiated, was highest at the coast and lowest in the interior, the opposite of the observed adult plant distribution. Overall flower and seed predation by insects was high (44-73%) and was greatest at the coast. Insect exclusion experiments at sites along the gradient had three main results. (1) Seedling recruitment was proportional to the number of viable seeds after seed predation. The exclusion plots had significantly higher numbers of seedlings established at all sites than did control plots; the greatest increase was at the coast. (2) Seedling survivorship was independent of both seedling and adult densities. (3) Juvenile recruitment was proportional to seedling establishment. Predation by insects prior to release of seeds played a critical role in the population recruitment of H. squarrosus within and among sites along the gradient. Survivorship of established plants, in addition, was as high at the coast as it was in the interior. These results suggest that indigenous insect seed predators can be a major force controlling the dynamics of a native plant species over its natural distributional range.

Variation in Herbivore-Mediated Indirect Effects of an Invasive Plant on a Native Plant

“…An herbivore on an isolated host plant must survey a larger area to find a second suitable host and, therefore, may be influenced to a greater extent by floral display, compared to an herbivore within a patch of many host plants. Further studies that integrate the effects of individual plant traits and ecological context on levels of pre-dispersal seed predation are needed (e.g., Louda 1982Louda , 1983Leimu et al 2002) to improve our ability to predict the extensive spatial and temporal variation in seed loss and its demographic consequences.…”

No interaction between competition and herbivory in limiting introduced Cirsium vulgare rosette growth and reproduction

“…Competition for limited resources, such as nutrients, water and light, often reduces plant performance (Tilman 1982;Goldberg and Novoplansky 1997). Tissue loss to herbivores also can reduce plant survival, growth, or reproduction and decrease plant population density (Louda 1982;Crawley 1983;Louda and Potvin 1995;Maron and Vila 2001;Levine et al 2004;Maron and Crone 2006). However, much less is known about the potential interactive effects of competition and herbivory on plant performance, especially for introduced plants.…”

Experimental evidence for insect impact on populations of short-lived, perennial plants, and its application in restoration ecology

“…First, insects feeding on the developing inflorescences limited both the number of viable seeds and the number of seedlings recruited by both species at all study sites (Louda 1982a(Louda ,b, 1983. Second, the distribution of the inland species, scaly goldenbush, was compressed and displaced away from the coast by more intense predispersal seed predation coastally (Louda 1982b). Third, the distribution of coastal goldenbush was also compressed, but it was displaced toward the coast by the combination of intense seed reduction by insects all along the gradient and by higher consumption of seedlings by vertebrate herbivores in the inland portion of an environmental gradient (Louda 1983).…”

“…Predispersal seed predation limits viable seed and seedling establishment at all sites for both species (Louda 1982a(Louda ,b, 1983. Interestingly, the replacement of the two species along the gradient is best explained by differential predation pressure along the gradient: much higher seed losses for H. squarrosa caused by inflorescence-feeding insects near the coast (Louda 1982b) plus significantly more intense seedling losses for I. veneta caused by vertebrate herbivores inland (Louda 1983). …”