Competitive Abilities of Rare and Common Plants: Comparisons UsingAcaena(Rosaceae) andChionochloa(Poaceae) from New Zealand

Abstract

Abstract:It is commonly suggested that rare plant species are poor competitors. However, the study of rarity and competitive ability has, like much of the research on rare species, been hindered by insufficient sample size, scarce use of experimental methods, and lack of comparison with common species. We used a within‐taxon comparative approach to test for differences in competitive ability between rare and common species in two genera,Acaena( Rosaceae: six rare and four common species) andChionochloa( Poaceae: four rare and five common species). We grew plants in replicated experiments at two levels of soil fertility and with two levels of competition, in monoculture or mixed with the introduced grassAgrostis capillaris. Competitive response was measured as relative yield, relative tiller increase, and change in live‐leaf fraction. Rarity was measured by geographic range size, defined as the number of 10‐km grid squares occupied by each species in New Zealand. The relationship between competitive ability and geographic range size was tested with correlations. WithinAcaena, four of the rare species had high competitive ability, but there was no general relationship between competitive ability and rarity at either high or low levels of fertility. Differences in competitive ability appeared related to differences in ecological distribution rather than to rarity or commonness in a geographic sense. WithinChionochloa, common species tended to be stronger competitors than rare species, with the relationship being significant at the low level of fertility. Several of the commonChionochloaspecies have expanded their geographic ranges relatively recently, consistent with their greater competitive ability. Conversely, rareChionochloaspecies occupy specialized habitats that may be interpreted as refuges from competition. Disparate results fromAcaenaandChionochloado not support the notion that rare species are generally poor competitors. Although rare species may have low competitive ability in some cases, it should not be assumed to be a cause of rarity for all rare species.