Widespread ecological damage can occur when an insect or disease becomes invasive. The introduction of an invasive forest pest can cause a decline in the biodiversity and health of a forest ecosystem and a large reduction in available wood fibre within a forest (NRCan, 2014). To make matters worse, since invasive pests often do not have natural predators in natural forests outside of their native range, they can spread rapidly and uninhibited to cause extensive amounts of ecological damage.
The ecology of an insect or disease determines the direct ecological impact it will have on a tree. Some forest pests feed on a tree’s leaves, reducing the health of its canopy. Other species feed on the tissues beneath a tree’s bark, reducing the tree’s ability to transport water and nutrients. Invasive insects and diseases can cause trees to develop one or more of a number of unhealthy conditions, such as leaf blight, root rot, volume loss, dieback, deformity, growth loss, cankers, or death (NRCan, 2014). While some trees can be healthy enough to resist or even recover from the attack of an invasive insect or disease, others, usually older or weakened trees, cannot survive the attack and will decline in health and eventually die (NRCan, 2014).
The negative impacts incurred by invasive species are not limited to the tree they infest; they have the potential to cause widespread impacts on the health of the whole ecosystem. The loss of trees in Canada’s forests can reduce habitat for native animals and insects, create canopy gaps altering the microclimate of the forest, make forests vulnerable to subsequent invasive species, and can reduce the overall biodiversity within forest ecosystems. Specifically, populations of native species that have specialized interactions with the threatened host, such as terrestrial arthropod species with a high level of association with ash, might be at increased risk (Gandhi and Herms 2010). Ecological impacts caused by introduced species are hard to predict, and even harder to assign a dollar value to. Healthy, functioning ecosystems provide a wide range of ‘ecological services’ such as air and water purification, erosion control, carbon sequestration, nutrient cycling, wildlife habitat, and climate regulation. Troy and Bagstad (2009) estimate that these ecosystem services in southern Ontario provide billions of dollars of economic benefits (Ontario’s Biodiversity Strategy (MNR, 2011)).
Forest pest infestations can also have social impacts on traditional Aboriginal activities associated with a healthy forest. Many First Nations people rely on natural forests for their livelihoods and for maintaining their culture. Therefore, a less resilient forest ecosystem could negatively impact the traditions and activities of these communities. Further, the aesthetic and spiritual values that various people associate with forests and natural ecosystems could be impacted as a result of invasive forest pests damaging natural areas (FIAS, NRCan, 2013).