Biological Factors

Primary Question (chapter 17): How have biological agents including insects and disease-causing organisms influenced the overall health of the South’s forests, and how will they likely affect it in the future?

Related Question (chapter 3): What are the likely effects of expanding human populations, urbanization, and infrastructure development on wildlife and their habitats?

Ecological systems are often restructured from within. For example, natural succession favors the replacement of pioneer species with shade-tolerant species. Insect populations can shift from endemic to epidemic status and quickly kill a substantial share of overstory trees. Plant diseases reduce vigor or cause localized mortality and selectively remove certain plant species from the forest. Regeneration and growth that follow mortality can change the species composition of the subsequent forest.

While pathogens, insects, and mortality are natural components of forested ecosystems, land use and management can alter their effects. Introduced exotic species, which are prevalent in the South, can completely displace native plant and animal species, thereby restructuring forest ecosystems. Cultivating tree species beyond their natural range and altering forest composition can predispose forests to insect and disease outbreaks. Conversely, management strategies and direct intervention can mitigate the spread of and damage caused by these outbreaks.

The Assessment evaluated the dynamics of several biological agents that have had or will likely have a substantial impact on forest ecosystems or the values that people derive from them (chapter 17). We found that:

• In general, the dynamics of native disease agents and native insects are heavily influenced by the availability and condition of host materials. For those disease agents and insects that target pine species, the concentration of pine forests in some subregions has increased the availability and contiguity of host material (see fig. 11). Mortality caused by insects and diseases, especially southern pine beetle and fusiform rust, could increase commensurate with the expected increase in pine plantations, but this will depend on the degree of management.

• Exotic plants (trees, shrubs, and vines) and animals (birds and mammals) are having large impacts on forested ecosystems. Expanding urban areas will increase the impact of domestic animals on wildlife (chapter 3).

• Exotic pathogens and insects generally have a greater potential to restructure forested ecosystems, especially hardwood forests, than do native species because natural enemies of exotic species are absent or control methods are ineffective at broad scales.

• The impacts of exotic pests are expected to grow as a conservation concern in the South. Multiple exotic insects, such as gypsy moths, and exotic diseases such as dogwood anthracnose, will especially influence hardwood forest types in the northern portions of the region. An insect, the balsam woolly adelgid, interacting with environmental stressors is causing tree mortality and altering habitat values in the rare spruce-fir ecosystem of the South.