Simplistic Tradeoffs

"The simplicityof the trade-off concept has made it very popular in consideration of the evolution of life histories (e.g. Smith and Fretwell, 1974) and examples have been found in some instances (e.g. among plants of Silene alba, D. Hanych, pers. comm.). However, it is apparent now that simplistic trade-offs between size and number are often not found (see also Primack, 1978; Hardin, 1984). This does not mean, necessarily, that trade-offs are not important, but it does mean that the scale or conditions in which they may occur need to be examined more thoroughly."

"We found little evidence of trade-offs between mean seed mass and seed numbers within fruits or within whole seed crops."

Michaels et al (1988) made these conclusions after examining average seed mass, total seed production (mass and numbers) and the variation in seed mass in individuals and populations of 39 species. Among-plant variation was significant within 37 of the 39 species. Interestingly, in 29 of these species, within-plant component exceeded the among-plant component of the variation in seeds' mass.

Explanation for lack of tradeoffs between seed size and resource availability?

1. Allometry ["The failure to detect evidence of trade-offs in most cases may be partially attributable to the effects of plant size, which may have such strong effects on seed number that trade-offs maybe evident only when confounding effects of maternal size variation are removed."]


2. A seed's size may respond more to transient signals than to maternal resource availability signals


• seasonal environmental signals--whether it was fertilized early or late in the season


• seeds formed on shaded plants or ramets vs. sunny ones [however, Michaels et al. found that shaded plants of one species produced more variably sized seeds, but found no general relationship between seed mass and the shadiness of the habitat]
  
• fluctuations in resources--was it fertilized during a dry or wet spell