Pollinator Limitation of Plant Reproductive Effort

In many recent studies there have been attempts to describe and explain patterns of resource allocation in plants (e.g., Abrahamson and Gadgil 1973; Newell and Tramer 1978). Nearly all such studies are based on the unstated assumption that the reproductive output of a plant is resource limited. However, if hand-pollinated plants produce more seeds than naturally pollinated controls, then reproduction is being limited, not by energy levels, but by pollinator activity. As part of a study on the population biology of Arisaema triphyllurm, a herbaceous forest perennial (Bierzychudek 1981), I hand pollinated some plants in a natural population. The difference between hand-pollinated and natural seed production was remarkably large. Hand-pollinated plants produced over an order of magnitude more seeds (the mean for medium-sized plants was 43.2 seeds/plant, N = 12) than did controls of similar size (mean = 1.0, N = 20). While there was no relationship between the size of a plant and the number of seeds it produced when naturally pollinated (r = .11, N = 35), this same relationship was significant for hand-pollinated plants (r = .81, N = 33, P < .01), indicating that only the hand-pollinated individuals were resource limited. There is no reason to assume that this level of seed production is unusually low for Arisaema; I observed only slightly higher levels at several other sites and in other years (with a maximum of 10.3 seeds/plant). Arisaema's pollinators (mycetophilid and sciarid flies) seem abundant, if not particularly efficient. Others have observed similar low rates of seed production by Arisaema (Meehan 1886; Sakamoto 1961; Treiber 1980). Is this an isolated case, or do pollinators limit the reproductive output of many plants? The yield of many cultivated crop plants has been found to be pollinator limited (McGregor 1976), but this might be expected when large plantings are made of a single crop species. Information on native species under natural conditions is more difficult o find. Schemske et al. (1978) note that only 33% of naturally pollinated flowers of Eiythronium albidum set seed, compared with 78% of flowers outcrossed by hand. Willson et al. (1979) show that 82.3% of handoutcrossed Phlox divaricata produced some seed, compared to 58% of naturally pollinated plants at the same site. In Costa Rica 29.7% of hand-outcrossed flowers of Combretumfrtuticosurn set fruit, compared to 7% of naturally pollinated flowers (Schemske, in press). Brassavola nodosa, a Central American orchid, also seems to be pollinator limited: 67% of hand-outcrossed flowers set fruit, versus 12% of naturally pollinated flowers (Schemske 1980). Weller (1980) found the fecundity of hand-pollinated Lithospermum caroliniense averaged 17% compared to 9% for naturally pollinated individuals, a significant difference. Finally, Encyclia cordigera, another orchid, may also be pollinator limited: Among hand-pollinated inflorescences, the proportion producing no fruit ranged from 5% to 22%, depending on the identity of the pollen donors. Seventy-eight percent of naturally pollinated inflorescences set no fruit (Janzen et al. 1980). The results of some hand pollination experiments are quite different. Stephenson (1979) found that hand-pollinated Catalpa speciosa produced significantly