Exotic species: species that have established populations outside of their native range via direct transport by humans.
Exotic species have been implicated in the decline and extinction of many native plants and animals and pose a significant threat to biological diversity.
Invasive exotic species in Vermont are likely to affect bird populations via changes in habitat structure or food availability.
Some species will benefit from insect and pathogen invasions, while others will suffer.
- Chestnut blight and Dutch elm disease benefitted bird species that favored the more open, shrubby conditions created as the canopy opened. Bird species associated with conditions found in the interior of mature, closed-canopy forests declined in abundance
- Some species show positive short-term responses to gypsy moth defoliations whereas others show short-term declines
- Species associated with closed-canopy conditions may decline as overstory tree species die and create canopy gaps: Eastern hemlock facing infestation by hemlock woolly adelgid; American beech, suffering from beech bark disease; and ash, currently at risk from the invasive emerald ash borer.
- Species that use gaps or edges may benefit.
- Over time, forest succession may gradually erase many of these short-term changes. The arrival of new plant species will likely produce a similar pattern of winners and losers.
Bird populations are somewhat resilient to exotic invasive plants, because most do not depend upon a single plant species to provide food or habitat. Again, some species will benefit from changes in vegetation structure due to exotic plants, and others will not.
However, a laissez-faire approach to exotics also carries many potential risks.
- Whether observed short-term responses can be extrapolated remains an open question.
- (need long-term response studies)
- Bird assemblages occupying uncommon or restricted habitats within Vermont may prove to be more sensitive to invasives.
- The demise of spruce and fir forests in the southern Appalachians due to Balsam woolly adelgid has led many bird populations to the brink of extirpation.
- Simultaneous arrival of many exotic species can cause synergistic effects, resulting in an “invasional meltdown”.
- Even if bird populations in Vermont prove resilient, plants and insects will likely not.
The scope of the problem can seem daunting.
Public support for invasive-species control programs is critical, as are efforts to convince legislators of the importance of enacting laws that will reduce the number of future ecological invasions.
At smaller spatial scales the choices that individual landowners make – from landscaping to their source for their firewood – may prove critical.
- Yards landscaped with native plants instead of exotics support a greater richness and diversity of bird species.
- Avoiding gardens full of exotic plants may also help avoid accidentally introducing the next invasive species.
- Avoid planting species listed on the state’s watch list, including Amur maple, Norway maple, and burning bush.