We are currently living through the sixth great extinction crisis in the Earth’s history. Ecologists and evolutionary biologists have documented that the unprecedented and rapidly accelerating loss of non-human species is the direct result of human activities: habitat conversion, over-consumption of resources, and worldwide introductions -- both deliberate and accidental -- of pests and pathogens. At the same time, ecologists are working to discover whether particular species provide unique services or if their functions can be carried out by other species. A number of species have been identified as foundation species -- species that create habitats for themselves and a wide range of other species, stabilize the environment, and uniquely provide or control ecosystem processes such as productivity and fluxes of energy, nutrients, and water.
Long-term research at the Harvard Forest (HFR) Long Term Ecological Research site in Massachusetts is revealing the foundational role of a long-lived tree species, eastern hemlock (Tsuga canadensis). This research focuses on changes in forest dynamics and ecosystem processes as hemlock declines and dies from the impact of the exotic insect, the hemlock woolly adelgid (HWA; Adelges tsugae). Data from field plots established in the mid-1990s in southern New England show that hemlock mortality is progressive and extensive after multiple years of HWA infestation (Figure 1). Hemlock does not re-establish following adelgid-induced mortality, but rather is replaced by early successional hardwood trees, including birch (Betula species) and red maples (Acer rubrum). The loss of hemlock leads to dramatic changes in the terrestrial and aquatic enivironments they define. Notable changes include: the corresponding loss of birds uniquely associated with these deep woods; changes in distribution and abundance of ants; homogenization of local flora and fauna; increased rates of decomposition of leaf-litter and export of carbon dioxide to the atmosphere; and changes in the chemistry, flow rates, and water quality of headwater streams.
Interestingly, hemlock populations across North America experienced a similar decline ~5,500 years ago (Figure 2). HFR scientists are studying this decline by examining tree pollen that is deposited continuously in sediments at the bottom of lakes. These studies have revealed that the decline was caused by climatic change and defoliating insects. Hemlock eventually recovered from this decline, but the recovery took nearly 2,000 years.
At the same time, Harvard Forest researchers have led the development of large-scale experimental methods that can be used to explore the roles that foundation species play in forest ecosystems. These methods are being applied by researchers around the world to systems ranging from coastal mangrove forests in Florida, cottonwood forests in the southwest United States, oak forests in southeastern Massachusetts, Douglas fir forests in northwestern Canada, and both managed and unmanaged Eucalyptus forests in Australia. All of these studies examine the short and long-term changes in ecosystem structure and function following their loss. Long-term approaches that include exploring past and current declines of foundation trees will improve predictions of the types and magnitudes of forest changes that are expected as the climate changes and these changes interact with multiple stressors.