The emerald ash borer (EAB), Agrilus planipennis, is an invasive insect native to northeastern Asia. This metallic green beetle was first identified in North America near Detroit, Michigan, in 2002, though evidence suggests it arrived earlier, possibly in the early 1990s, likely transported via untreated wooden shipping materials. In its native range, the EAB typically does not cause significant harm to ash trees. However, in North America, it poses a significant threat to all 16 native species of ash trees (Fraxinus spp.) across the continent.
Direct Impact on Ash Trees
The primary ecological effect of the emerald ash borer is the widespread mortality it inflicts upon ash trees. Adult female EAB lay their eggs in crevices within the bark of ash trees, typically from June through September. Upon hatching, the larvae immediately burrow beneath the bark.
EAB larvae feed on the phloem and outer sapwood, which are the tree’s vascular tissues located just beneath the bark. This feeding creates winding galleries that disrupt the tree’s ability to transport water, nutrients, and sugars from its roots to its canopy. This disruption effectively starves the tree.
Infested ash trees often show signs of canopy dieback within two years, with tree death typically occurring within three to four years of initial infestation. In areas with severe EAB infestations, over 99% of ash trees may die. This widespread mortality has already resulted in the loss of tens to hundreds of millions of ash trees across North America, and it continues to threaten the estimated 8.7 billion ash trees throughout the continent.
Changes in Forest Structure and Composition
The large-scale death of ash trees fundamentally alters the physical structure of affected forests. As ash trees die and fall, they create significant canopy gaps. These openings allow more sunlight to reach the forest floor than before.
This increase in light availability profoundly affects the understory plant communities. Native plant species that prefer shaded conditions may decline, while light-loving native species could proliferate. However, the disturbed conditions and increased light often favor the rapid growth and spread of invasive plant species. These non-native plants can quickly colonize the newly opened spaces, outcompeting native tree seedlings and other desirable understory vegetation.
The widespread ash mortality also leads to a shift in forest composition over time. With ash trees removed from the ecosystem, other tree species, both native and sometimes invasive, will begin to fill the void. This can result in forests becoming dominated by species that were previously less common, potentially reducing the overall diversity of tree species and altering the long-term successional pathways of these forest ecosystems.
Impact on Dependent Species
The extensive loss of ash trees creates ripple effects that extend to numerous species reliant on these trees. Many insects, for instance, have evolved to feed specifically on ash foliage, wood, or other parts. The disappearance of their primary food source can lead to significant population declines for these specialist herbivores.
Beyond direct consumption, ash trees provide habitat for a variety of organisms. Birds that nest in ash cavities or forage for insects on ash bark, such as certain woodpecker species, may experience reduced nesting opportunities and food availability. The loss of ash trees also impacts various fungi and microorganisms that play roles in nutrient cycling and decomposition, as they specifically break down ash wood or form symbiotic relationships with ash roots.
These impacts can alter local food webs, shifting predator-prey dynamics and potentially leading to declines in populations of species higher up the trophic levels. The overall biodiversity of affected areas can diminish as species dependent on ash trees struggle to find alternative resources or habitats.
Broader Ecological Consequences
The large-scale mortality of ash trees instigates systemic changes that affect fundamental ecosystem processes. One significant consequence involves nutrient cycling. The rapid decomposition of millions of dead ash trees releases a substantial pulse of nutrients into the soil. This sudden influx can alter soil nutrient availability, potentially leading to nutrient leaching or changes in microbial communities.
Forest hydrology also experiences modifications from the widespread ash loss. Reduced canopy cover means less interception of rainfall and decreased transpiration. This can result in increased soil moisture, altered runoff patterns, and potentially higher water tables in some areas, impacting streamflow and local water dynamics.
The accumulation of dead ash wood increases the amount of combustible material in affected forests, elevating the risk and intensity of wildfires. The loss of tree cover also leaves soils more exposed to the elements, potentially increasing erosion, particularly on slopes or along riparian zones. These disturbed sites become more vulnerable to the establishment and spread of invasive plant species, which can further degrade habitat quality and ecosystem function.
The broad ecological consequences also extend to ecosystem services. The capacity of forests to sequester carbon from the atmosphere, for example, is diminished with the widespread death of ash trees. Air quality can also be affected as the reduced tree canopy provides fewer benefits in filtering pollutants.