Avian community response across a gradient of native to invasive riparian vegetation in northwest New Mexico

Background/Question/Methods

In riparian areas of the arid and semi-arid western North America, tamarisk (Tamrix spp.) and Russian olive (Elaeagnus angustifolia) threaten native cottonwood (Populus fremontii) and willow (Salix spp.) habitats. These native woodlands are known to maintain among the highest avian densities and species richness totals in temperate North America through habitat heterogeneity that provides nesting substrates, cavities, and food resources for many species relative to the surrounding landscape. Although resource managers are actively managing these invasive species, the effects of invasive vegetation on bird communities are site and species specific. Why some species, guilds, and families do poorly and others are either unaffected or thrive in the face of invasion is poorly understood. Utilizing a unique gradient of native to invasive riparian vegetation on the San Juan and Animas Rivers in northwest New Mexico, we used point count surveys, vegetation assessments, and prey surveys to address how avian communities, species, and guilds respond to invasive vegetation, and identify which life history characteristics make them predisposed to avoidance or attraction to invasive vegetation types.  

Results/Conclusions

Overall, sites containing predominantly native vegetation had higher species richness and diversity than predominantly invasive sites. Our results suggest that responses to the vegetation gradient are species specific. We found that occupancy likelihood for omnivores, certain members of the aerial forager guild, and Emberizidae family members increased with invasion. Alternatively, cavity nesting species, frugivorous species, and many insectivorous species decreased with increased proportions of invasive species. Avian prey diversity and evenness showed a negative relationship with increased proportion of invasive vegetation. Our data suggest that the relative abundance of invertebrate orders per dominant vegetation type may be somewhat controlling insectivore abundance. For example, Hymenoptera, a dominant food source of Yellow-breasted chats (Icteria virens), were more abundant in Russian olive, where chats were higher in abundance. Similarly, Hemiptera were more abundant in cottonwood, where Yellow warblers (Setophaga petechia) were more likely to feed. Plecoptera and Trichoptera were strongly related to native dominant vegetation and avian insectivore occupancy. The unique approach of exploring invasion on a gradient scale paired with avian life history characteristics and avian prey abundance measures have provided an interesting and hopefully applicable approach to conservation of avian species.