PARENT SESSION
Oral Session #3: Plant Population Ecology. Presiding: N. Fetcher.
Monday, August 6, 2001. 8:00 AM to 11:30 AM. Hall of Ideas L&M.

Life history evolution in variable environments: Cole's Paradox revisited.

EVANS, MARGARET¹, VENABLE, D.¹, ¹

ABSTRACT- Why some organisms reproduce once and subsequently die (e. g. annuals) whereas others reproduce more than once (e. g. perennials) has been a central question in life history theory since Cole (1954; Cole's Paradox). Seed dormancy in annual plants and iteroparity in perennials have each been described as bet-hedging strategies for reducing fitness variance, but seed dormancy has been missing from most analyses comparing the annual and perennial habit. We investigated the conditions under which bet-hedging via seed dormancy vs. iteroparity is more advantageous, using stochastic simulations of annual and perennial life histories with seed dormancy. First, we examined the long-term stochastic growth rates of a series of density-independent models grading from perennial to annual as a function of changes in the mean and variance of seed survival, juvenile survival, fecundity, and adult survival. Second, we subjected a lottery model of a perennial life history with seed dormancy to invasibility analysis, to evaluate the evolutionary stable strategies (ESS) for germination fraction and adult survival under various regimes of demographic stochasticity. In general, increasing the average or variance of seed or juvenile mortality favors iteroparity, whereas increasing the average or variance of adult mortality favors the seed-banking annual habit. Exceptions to this pattern are discussed in light of our research on transitions between the perennial and annual habit in Oenothera Sect. Anogra (Onagraceae).

KEY WORDS: Cole's Paradox, bet-hedging, seed dormancy, stochastic simulation