Response of Avian Communities to Disturbance by an Exotic Insect in Spruce-Fir Forests of the Southern Appalachians

Abstract

The avian community of montane coniferous forests changed substantially over 21 years in response to the introduction of the balsam woolly adelgid insect ( Adelges piceae ) and the resulting death of endemic Fraser fir trees ( Abies fraseri ). These relict forests are at the southern limit of their distribution on the highest ridges of the southern Appalachian Mountains. We combined a historical study at Mount Collins in the Great Smoky Mountains with a geographic comparison of sites within five southern Appalachian mountain ranges variably affected by the adelgid. At Mount Collins, fir was virtually eliminated and canopy cover reduced to half its previous level. Long-term data from Mount Collins showed that the combined density for all breeding birds also declined by half. Of the common territorial species present in 1974, 10 of 11 declined, 6 by more than 50%. Some species are near local extinction. Canopy-and subcanopy-foraging species declined more than near-ground and trunk-foraging species. In addition, invasions of birds characteristic of open and disturbed forests have diluted the boreal character of the avifauna. These changes are consistent with two other long-term studies and are not explained by regional population trends. In the other southern Appalachian mountain ranges, where habitat is not as extensive, the adelgid invasion resulted in greater declines in avian abundance, stronger effects on sensitive species, and more pronounced invasions by successional species. Sensitive species in the southern Appalachian studies were also strongly affected in other studies of forest disturbance by fire and logging. The extent of relatively pristine spruce-fir forest and the population pools in other forest types in the Great Smoky Mountains appear to have buffered the effects of forest decline on the bird community. Respuesta de Comunidades de Aves a Perturbaciones de un Insecto Exótico en Bosques de Abeto en el Sur de los Apalaches Resumen: La comunidad de aves de bosques coníferos de montaña cambió sustancialmente a lo largo de 21 años en respuesta a la introducción del insecto adélgido lanudo Aldeges piceae y a la resultante muerte de árboles de Abetos Fraser ( Abies fraseri ). Estos vestigios de bosques se encuentran al limite sureño de su distribución en las montañas mas altas de los Apalaches. Combinamos un estudio histórico del monte Collins en las montañas Great Smoky con una comparación geográfica de sitios entre cinco cordilleras de los Apalaches afectados en forma variable por el adélgido. En el monte Collins, los abetos fueron virtualmente eliminados y la covertura del dosel reducida a la mitad de su nivel previo. Datos de largo plazo del monte Collins mostraron que la densidad combinada para todas las especies de aves anidando también disminuyó a la mitad.De las especies comunes en el territorio en 1974, 10 de 11 disminuyeron, seis en mas de un 50%. Algunas especies se encuentran cerca de su extinción local. Especies forrajeras de dosel y sub-dosel disminuyeron mas que aquellas especies forrajera...

Resistance of a sub-arctic bird community to severe forest damage caused by geometrid moth outbreaks

“…These results correspond to those of Rabenold et al (1998), who found that bird species foraging mainly in the canopy showed the strongest declines following mortality of Fraser fir (Abies fraseri) caused by the balsam wooly adelgid (Adelges piceae). However, it is noteworthy that the species showing the clearest negative response to forest damage, namely the willow warbler, mainly forages for insects among the foliage of trees.…”

“…due to rapid growth of tree saplings or invasion by tall shrub species) (Bell and Whitmore 1997;Canterbury and Blockstein 1997;Rabenold et al 1998;Gale et al 2001;Becker et al 2008). This general lack of positive responses is quite surprising, since studies in other systems have repeatedly found bird species associated with open or early successional habitats to increase in forest affected by insect outbreaks.…”

“…Bird species which are associated with closed-canopy forest have typically been found to undergo population declines in areas experiencing outbreak-induced mortality of trees, while species that prefer clearings and disturbed habitats have usually been found to increase (Rabenold et al 1998;Gale et al 2001;Becker et al 2008). Many species of birds forage, seek shelter from predators and build their nests in trees or among understorey vegetation.…”

The natives are restless, but not often and mostly when disturbed

“…In Texas, Chinese tallow (Sapium sebiferum) invades grassland partly by virtue of shading native grassland plants, and the native hackberry (Celtis laevigata) follows in its wake (Siemann and Rogers 2003). In the high elevations of the southern Appalachians, elimination of Fraser fir (Abies fraseri ) by the Asian balsam woolly adelgid (Adelges piceae) has created conditions favorable to invasion by native mountain ash (Sorbus americana) and fire cherry (Prunus pensylvanica) (Rabenold et al 1998). Finally, native ragweed (Ambrosia artemisiifolia) is favored by mound-building by the red imported fire ant (Solenopsis invicta) in South Carolina (Seaman and Marino 2003).…”

Ecological effects of invasive alien insects

“…The disturbance of Fraser fir forests by the balsam woolly aphid has had a detrimental effect on local birds, 10 out of 11 species declining, and six species by more than 50% (Rabenold et al 1998). Similarly, the decline of eastern hemlock due to A. tsugae in North America strongly affects bird species composition (Tingley et al 2002) and also has an effect on salamander populations (Brooks 2001), and on deer survival through modifications in forest microclimates (Lishawa et al 2007).…”