The origin of the debate

Building on the ideas of Robert MacArther, E.O. Wilson (The Theory of Island Biogeography, 1967), Harper (1970) framed the seed yield tradeoff concept in 1970, drawing on Erik Pianka's r/K terminology:

Tradeoff 1: Reproductive effort vs. longevity; seeds vs. "persistent vegetative organs"

"There is evidence that plant species differ in the proportion of their net annual assimilated income which is devoted to reproductive effort (2, 3). Crude generalizations about the reproductive effort of different types of plant are summarized in Table 1.

"It appears that species occupying early phases in succession (colonizing species) have a high reproductive effort, usually in the form of a large number of seeds, and a correspondingly high intrinsic rate of natural in- crease. These would be r species in the sense of MacArthur & Wilson (4). Perennials, and particularly woody species with low values of reproductive effort, seem to fit into the category of K species, which have stable habitats in later phases of succession. A greater part of their available energy resource is devoted to persistent vegetative organs; this confers advantages in a long-term intensive struggle for existence in crowded, resource-limited stable environments.

Tradeoff 2: Average seed mass (size) vs. number of seeds per plant

"Just as the resources available to a plant during development are partitioned between seed production and other ends, so the allocation to seeds is itself partitioned between numbers and size. Thus, the number of seeds borne by a plant is determined by 1. the size of the annual assimilated income, 2. the proportion of this devoted to seeds, and 3. the size of the seed units in which it is invested. It is a central thesis of this essay that seed numbers and seed size represent alternative strategies in the disposition of reproductive resources.

Sources:
J. L. Harper, P. H. Lovell, K. G. Moore (1970) The Shapes and Sizes of Seeds Author(s): Source: Annual Review of Ecology and Systematics, Vol. 1 (1970), pp. 327-356
MacArthur, R. and Wilson, E.O. (1967). The Theory of Island Biogeography, Princeton University Press (2001 reprint), ISBN 0-691-08836-5M.
Pianka, E.R. (1970). On r and K selection. American Naturalist 104, 592-597.

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
  

Perennials and seed size

While the main Tradeoff Debate is concerned with the putative negative correlation between plant longevity and yearly seed yield, those familiar with grassland perennial species might be forgiven for suspecting that perennials generally also have smaller seeds than annuals. Certainly, domesticated grains--currently all annuals--have much larger seeds than wild herbaceous perennials. The annual ancestors of some of the annual grains also have relatively large seeds.

However, when hundreds of plant species are compared, perenniality is only a weak predictor of a species' seed size:

• "the difference between the means for annual and perennial herbaceous autotrophs is small enough that herbaceous plants will be treated as a single group for many purposes." [Baker, 1972]
• "In each of the four floras [databases of plants from three continents] , plant perenniality was not associated with significant seed size variation after the addition of plant height or growth form." [Leishman et al., 1988]

Height is a robust predictor of seed size (tall species, in general, produce larger seeds), as is plant form (trees and vines generally have larger seeds than forbs and graminoids), according to Leishman et al. (1988). Perenniality is only a weak predictor...and it is the perennial habit that is associated with larger seed size.

• "Seed size was significantly associated with plant perenniality in the five floras (Fig. 4), with seeds of annual species generally smaller than seeds of perennials." [Leishman et al., p. 525]
• Baker (1972) found significant differences (p<0.001)>

• Mean seed weight of 664 native annual herbs: 5.76

  Mean seed weight of 940 native perennial herbs: 6.35

  Mean seed weight of 441 native shrubs: 7.55

  Mean seed weight of 94 native trees: 9.60

Sources:

Leishman, M. R., Westoby, M., Jurado, E. (1995) Correlates of seed size variation: a comparison among five temperate floras. Journal of Ecology. 83: 517-530

H. G. Baker (1972) Seed Weight in Relation to Environmental Conditions in California. Ecology, Vol. 53, No. 6 (Nov., 1972), pp. 997-1010