Larry Venable, Department of Ecology and Evolutionary Biology, University of Arizona, began long-term studies of winter annuals at the Desert Laboratory in 1982 with particular emphasis on population ecology.
Annual plant species comprise 50 percent of local floras in the Sonoran Desert, and of these about 60 percent to 80 percent are winter annuals. Seeds of winter annuals germinate during cool-season rains, and, if the seedlings survive, the plants flower between February and April and die when hot weather arrives in May. When abundant, winter annuals help sustain populations of rabbits, tortoises, and other browsing animals. In good years, after abundant winter rains, spring-blooming annuals add millions of seeds to the soil seed bank, renewing the major food source for certain species of ants, rodents, and birds. Not all years are good years, of course, and after very dry winters, winter annuals are present only as dormant seeds in the soil.
The work of Larry Venable and his graduate students has illuminated the relation between winter annuals and climatic variability. Not all seeds germinate even in good years; rather, a fraction of seeds remains dormant, thus preventing extinction in case no seedlings survive long enough to flower and disperse seeds. As the graph above shows, species of winter annuals differ in their specific response to climatic variability. Those that vary greatly in reproductive success allow smaller fractions of seeds to germinate at any one time. Those that are more certain of reproductive success take greater risks and germinate a higher proportion of seeds. Differences in bet-hedging behavior allow many different species of winter annuals to co-exist by providing a mechanism for sharing the same environment through time.