PARENT SESSION
Oral Session #64: Plant Population Ecology.
Presiding: S. Collins
Wednesday, August 7. 1:00 PM to 4:45 PM. Gila Meeting Room, TCC.
Life history evolution in evening primroses: evidence from experimental and natural populations.
Evans, Margaret*,1, Venable, D.1, McPherson, Guy2, 1 Department of Ecology and Evolutionary Biology, Tucson, AZ2 School of Renewable Natural Resources, Tucson, AZ
ABSTRACT- Understanding the conditions under which (iteroparous) perennial vs. (semelparous) annual life histories are favored has long been a pursuit in evolutionary ecology (Cole's Paradox, from Cole 1954). Charnov and Schaffer (1973) predicted that the perennial habit should be favored when the ratio of adult and juvenile survival is greater than the difference between annual and perennial fecundity. We tested this prediction with data from experimental and natural populations of two closely related, allopatric species of evening primroses (Oenothera, Onagraceae). In a reciprocal common garden experiment, annuals greatly outperformed perennials in both the annual's and the perennial's habitat. Average fecundity of annuals was much greater than perennials, by 24,839 and 12,058 seeds per plant, in the annual's and perennial's habitat, respectively. Conditions in the common gardens were maintained such that juvenile survival of both species was maximal (100%, both habitats), yet adult survival of perennials was minimal (0% and 1% in the annual's and the perennial's habitat, respectively). In contrast to the common garden experiment, perennials in natural populations outperformed annuals in two winter-spring seasons, including one exceptionally dry and one wet season. Perennials had higher average fecundity than annuals in both the dry and wet years (by 231 and 272 seeds per plant, respectively), and higher juvenile survival in the dry year (41% vs. 4%). We reconcile these data within a bet-hedging framework, and in the context of a trade-off between stress tolerance and potentially high growth rate and fecundity. We suggest that stress-tolerant vegetative traits function as a conservative bet-hedging strategy in the perennial, whereas seed dormancy probably functions in a similar manner in the annual. This is a new view of the advantages of perennial vs. annual strategies.
KEY WORDS: bet-hedging, life history evolution, Oenothera, semelparity vs. iteroparity