The Escape Hypothesis: A Test Involving a Temperate, Annual Grass

Abstract

The escape hypothesis predicts a greater probability of survival for seeds that are dispersed farther away from an isolated parent plant. This hypothesis was examined for Panicum miliaceum L., an annual grass that grows as an agrestal weed or as a crop. The patterns of seeds and mature offspring surrounding each of twenty parent plants were mapped. For those cases involving a large sample size, the average distance of the mature offspring from the parent plant was significantly (p <0.05) greater than the mean dispersal distance of the seeds. When the slope of the semi-log plot of seed density against dispersal distance was compared with the equivalent slope for mature offspring density, the former was found to be steeper (more negative) than the latter in seven of twenty cases. Again, these were the cases with the most completely described seed and mature offspring patterns. These results support the escape hypothesis. They were interpreted to be the result of density dependent mortality. The evidence in the literature against the escape hypothesis was found to be weak and sometimes irrelevant. The implications of density independent mortality in parent offspring relationships are discussed.