Understanding a native invader: Implications of Tanglehead (Heteropogon contortus) on plants and birds in desert grasslands

Background/Question/Methods:

Most invasive plants are non-native; however, native plants may become invasive and result in similar ecosystem changes.  Tanglehead (Heteropogon contortus) is a native bunchgrass that has increased in distribution and dominance in south Texas grasslands within the last 15 years, behaving like an invasive species.  We sought to determine if increased dominance by tanglehead resulted in changes to the vegetation and bird communities, as has been documented with many non-native invasive plants.  We selected plots along a gradient of tanglehead dominance (0-80%, n = 70) in south Texas and sampled vegetation and avian communities in winter and summer 2010-2011.  We sampled vegetation structure and composition in 0.25-m2 quadrats (n = 17/plot) and used a Wiens pole to quantify thickness (i.e., number of contacts with the pole in 10-cm height increments).  We used distance sampling along 250-m transects in winter and at points in  the summer to estimate avian richness and density.

Results/Conclusions:

We documented increased vegetation height and thickness with increased tanglehead; vegetation thickness >40-cm tall increased by 3-4 contacts/10 cm between the extremes of the dominance gradient in summer and winter 2010 (95% CI = 2-5 contacts/10 cm).  Richness of native grasses decreased by 2 species (CI = 1-3 species) along the gradient of tanglehead dominance in winter 2010 and cover of forbs and native grasses decreased by 32% and 16% (CI = 6-26% and 18-47%) in summer 2010.  We expected that these changes in vegetation structure would affect avifauna; however, we did not detect differences in density of Northern Bobwhite (Colinus virginianus), a ground-dwelling permanent resident that depends on seeds and insects.  Moreover, avian richness increased by 2 species (CI = 0-4 and 0-5 species) along the gradient of tanglehead dominance in summer and winter 2010.  Avian species may be able to persist because areas invaded by  tanglehead still have patches of native plants present; these sandier areas may be uninhabitable by tanglehead, but still allow birds to find appropriate habitat conditions.  However, because this increase in dominance is relatively recent, tanglehead should continue to be monitored to understand the effects fully.