Accessory costs of seed production

Abstract

Accessory costs of reproduction are those that are necessary to mature a seed, but that do not involve the direct cost of provisioning the seed itself. This study aims to quantify accessory costs in a range of species, and test whether they decrease as a proportion of total reproductive expenditure with increasing seed mass, as might be expected if economies of scale came into play at larger seed sizes. We also test whether accessory costs varied with growth form, pollination mode, and dispersal mode, with the expectation that biotic pollination and dispersal modes should incur greater costs. Reproductive allocation (dry biomass) over one season, was calculated for 14 diclinous angiosperm species. Accessory costs averaged 73% of total reproductive allocation, with the majority spent on packaging and dispersal. Total accessory costs, packaging and dispersal costs, and costs incurred prior to pollination were proportional to direct costs of reproduction in major axis regressions. However, larger seeded species incurred significantly greater costs associated with aborted seeds and fruits, and matured a smaller proportion of ovules. This is consistent with larger seeded species being more selective of the ovules/embryos matured than small-seeded species. Total accessory costs, and proportion of ovules aborted, were also significantly greater for biotically dispersed species, but only due to an association with larger seed masses. Costs associated with abortions were lower for biotically pollinated species, due to a general trend of more ovules per ovary, resulting in greater cost sharing. This study demonstrates that expenditure on items other than seeds accounts for the majority of reproductive allocation in flowering plants. Yet, far more literature exists on seed mass variation than on investment in accessory structures. We found a proportional relationship between accessory costs and seed mass that warrants further investigation within the context of selection on margin returns on investment.