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

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) and Chionochloa ( 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 grass Agrostis 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. Within Acaena, 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. Within Chionochloa, common species tended to be stronger competitors than rare species, with the relationship being significant at the low level of fertility. Several of the common Chionochloa species have expanded their geographic ranges relatively recently, consistent with their greater competitive ability. Conversely, rare Chionochloa species occupy specialized habitats that may be interpreted as refuges from competition. Disparate results from Acaena and Chionochloa do 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.

Measuring the effects of invasive species on the demography of a rare endemic plant

“…Yet theory has historically emphasized the importance of competitive interactions in invasions (Parker and Reichard 1998), and when invasive plants appear to suppress native ones competition is often presumed to be the cause. This is particularly likely to be the case for rare and endemic plants, which are often stereotyped as poor competitors, even in the absence of evidence that they necessarily have lower competitive ability than common congeners (Lloyd et al 2002;Brigham 2003). Case studies such as this one suggest more caution in making the oftenunexamined assumption that invasive plant impacts result primarily from resource competition, as well as greater attention to mechanistic studies of native-invasive plant interactions.…”

Seasonal variations in leaf δ13C values: implications for different water-use strategies among species growing on continuous dolomite outcrops in subtropical China

“…Vegetation on rock outcrops is usually dominated by narrow (shallow soil) endemic species, with the coexistence of widespread species (Baskin and Baskin 1988;Walck et al 1999). There has been a growing interest in comparing the competitive ability between narrow endemic and widespread species (Snyder et al 1994;Walck et al 1999;Osunkoya and Swanborough 2001;Lloyd et al 2002;Imbert et al 2012). Some experiments have confirmed a low competitive ability for endemic species (Walck et al 1999;Lloyd et al 2002), while other studies have shown that the endemic has a greater competitive ability than the widespread species (Snyder et al 1994;Osunkoya and Swanborough 2001).…”

Competitive responses of the rare Viola elatior and the common Viola mirabilis

“…Rare species can be inferior competitors (Walck et al 1999;Binney and Bradfield 2000;Lavergne et al 2003;Moora et al 2003), roughly equivalent competitors (Snyder et al 1994;Ru¨nk et al 2004) or even superior competitors (Rabinowitz et al 1984). Lloyd et al (2002) investigated experimentally the largest species sample so far and received mixed results. They concluded that although rare species may have a low competitive ability in certain cases, this should not be assumed to be a uniform cause of rarity.…”