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A limited role for apparent competition via granivory in the persistence of a grassland invader
Author(s) -
Radtke Troy M.,
Wilson Scott D.
Publication year - 2015
Publication title -
journal of vegetation science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.1
H-Index - 115
eISSN - 1654-1103
pISSN - 1100-9233
DOI - 10.1111/jvs.12299
Subject(s) - competition (biology) , biology , grassland , biomass (ecology) , ecology , forb , introduced species , invasive species
Aims Apparent competition, in which an invasive plant species promotes herbivory on native plants, has been found in several species‐level studies, but has not been investigated at the community level. We compared native and invaded communities to test two hypotheses: that granivory is higher in invaded than native grassland communities, and that granivory is lower for seeds of invasive than native species. Location Northern edge of the Great Plains, northeast Montana, USA and south Saskatchewan, Canada. Methods We studied the widespread invasive grass Agropyron cristatum , which has established on >10 million ha in North America. We compared low‐biomass, diverse native grassland and high‐biomass, species‐poor Agropyron ‐dominated stands at five sites across 250 km over 2 yr near the northern edge of the Great Plains. Results Granivory in Agropyron stands was nearly double that in native grassland ( P < 0.05), but only in 1 yr (2009). In both years, granivory increased significantly over the growing season and with the cover of forbs. At the two most distal sites, granivory of Agropyron seeds was very similar to the average of all six other plant species examined, suggesting that the invasive species was neither favoured nor disadvantaged by granivory. An exclosure experiment suggested that both birds and small mammals contributed to granivory. As in the community comparison, significant differences among seed species and animal exclosures appeared in 2009 but not 2010. Conclusions Higher granivory in Agropyron stands than native grasslands provided some evidence for apparent competition, but the absence of this effect in 1 yr, combined with the lack of different granivory rates of Agropyron seeds, suggests that granivory‐related apparent competition plays a small role in the persistence of this invader. Instead, granivory is more consistently related to time within the growing season and the cover of forbs, and the persistence of Agropyron may be more strongly related to other factors, especially its relatively great root production.