A comparison of survivorship and fecundity of four Amaranthaceae species
Studying species within the same family allows determination of life history traits that are most likely to be the result of adaptation of each species to its environment rather than the result of a common shared ancestor. Four species within the Amaranthaceae family were studied in southern Illinois. Amaranthus palmeri and A. rudis are summer annuals typically found as problematic agricultural weeds, Achyranthes japonica and Iresine rhizomatosa are two perennial species that occur in similar habitats but differ in invasiveness. Achryanthes japonica is a non-native, invasive species that is becoming a threat to natural forested areas and is also observed in agricultural field margins. Iresine rhizomatosaalso occurs in forest habitats but is an endangered species in Illinois. The objective of this study was to undertake a comparative life history analysis of these closely related species in habitats where they occur. Fifty seedlings of each species were observed at each of two sites through the 2012 growing season. Seedlings were assigned to life stages (seedling, juvenile, non-flowering adults and flowering adults) based upon node counts and performance recorded. Fecundity was determined by seed counts for the perennial species and from seed weight estimates for the annual species.
Seedlings of the two perennial species exhibited a type II survivorship curve. Both annual species exhibited a type I survivorship curve at one of the two sites, and a type III survivorship curve at the other site, with site differences in survivorship likely reflecting precipitation regime. Sixty percent of seedlings of both Amaranthus species survived through the season with all survivors flowering, a 1:1 sex ratio, and a mean fecundity of 21,274 ± 4,121 seeds per plant for A. palmeri, and 93,725 ± 22,653 seeds per plant for A. rudis. Fifty-seven percent of A. japonica seedlings survived their first season to flower, with adult plant fecundity of 132 ± 20 seeds per plant, whereas for I. rhizomatosa only 38% of seedlings survived and flowered, with adult fecundity of 702 ± 139 seeds per plant. Seed germination rates were low for the annuals (A. rudis seed = 12%, A. palmeri = 29%, under ideal conditions), whereas the perennials had widely divergent germination rates (A. japonica = 98% and I. rhizomatosa < 1%), reflecting their invasive and endangered status, respectively. Thus, seedling survivorship, fecundity, and germination rates of these four Amaranthaceae species help us understand their life history status.