The Sixth Extinction
The unprecedented and rapid ecological changes brought about in the Anthropocene have resulted in the mass extinction of myriad species of plants and animal taxa. The critical pressures include habitat conversion, overconsumption of resources and worldwide introduction of pests and pathogens. Therefore, we are undergoing and causing the extinction of the billion-year history of eukaryotes life on Earth.
Forest of Eastern Hemlock; Source: Flickr
Foundation Species vs Keystone Species
Foundation species differ from keystone species because the former usually occupy low trophic levels whereas the latter is usually apex predators and occupy the highest trophic level.
They are also different from the core species. The foundation species are locally abundant and create locally stable stipulation required for many other species. It also stabilizes the water balance and productivity of an ecosystem.
Wolf as a Keystone Species
The aftermath of Loss of Foundation Species
By its abundance and its structural or functional attributes, foundation species define and create an entire ecosystem or an ecological community. The loss of the foundation species can have a broad array of consequences and dramatically affect our perception of the landscapes for associated biota, ecosystem function and stability.
Importance of Foundation Species
Trees are likely to be the foundation species in most forested ecosystems. The architecture, functional and physiological characteristics define the microclimate of an ecosystem and delineate the forest structure. Meanwhile, the biomass and the foundation species' chemical makeup contribute to the stable functioning and processing of the various ecosystem processes.
Introduction and outbreaks of nonnative pests and pathogens, interruptions of native pests, over-harvesting and high-intensity forestry, and deliberate removal of individual species from forests are few of the various factors governing the loss of foundation tree species throughout the world.
An Example of Loss of Foundation Tree Species- Fall of Foundation Tree: Eastern Hemlock
Hemlock Woolly Adelgid Infection; Source: Pittsburgh Post-Gazette
Eastern Hemlock (Tsuga canadensis) is one of the most long-lived shade-tolerant trees in North America. These trees occur in an almost pure stand with species-poor understories. However, in the south hemlock grows in mixed stands. The combination of deep shade and, acidic and slow decomposing litter upshots in cool, damp microclimate, slower nitrogen cycling rates and nutrient deficit soil. Even though hemlocks have a tremendous whole tree respiration rate in fall and spring, deciduous trees are leafless during summer; hemlocks transpires about 50% of the total water released by deciduous trees.
These characteristics of hemlock with its eminent snow-interception rates, facilitate soil. Moisture levels, stabilize stream base-flows and decreases diel variation in temperatures in streams.
Due to this, streams that flow through hemlock forests support unique assemblages of salamanders, freshwater invertebrates, and fishes intolerant of seasonal drying. These stands also harbour deer and other wildlife.
The population of Eastern Hemlock have declined three times since the Pleistocene glaciation.
The introduction of hemlock woolly adelgid (Adelges tsugae) has been causing the death of a vast number of hemlock population. This rapidly spreading insect kills hemlocks of all sizes and age. Hemlock has no resistance to adelgid and rarely recovers from the attack. Moreover, there is no effective biological or chemical control of the adelgid.
It is also eminent that pre-emptive salvage logging, in which hemlock has fair economic value is cut in anticipation of future infestations. This further contributes to the declining number.
Therefore, hemlock could functionally disappear from easter forests in the next several decades. It is replaced by a range of hardwood species like birch, oaks, and maples which significantly alter the local biota. The decline of hemlock may also result in local loss of its uniquely associated ants and birds. This cascades to cause regional homogenization of floral and faunal assemblages, change soil ecosystem processes and alter hydrological regimes.
The effect of hemlock decline due to adelgid will be extensive on stream ecosystem.
For example, hemlock streams support significantly more taxa of aquatic invertebrates than paired mixed-hardwood stands, and nearly 10% of the taxa are strongly associated with the presence of hemlock (Snyder et al. 2002). Hemlock death may result in a rapid pulse of large amounts of wood that decays more slowly than coarse woody debris from hardwoods. Large hemlock logs in streams retain sediment and organic matter and create novel habitat types. Large hemlock logs are abundant in streams draining forests where hemlock is an important riparian species. Although logs from adelgid-killed hemlocks may persist in streams for decades to centuries, eventually the loss of hemlock will reduce in-stream wood, leading to a decline in sediment retention and productivity.
Furthermore, nitrification rates and nitrogen availability are significantly higher in cut forests than in adelgid-damaged ones. This change might cause an increased threat of nutrient losses and changing food availability in nearby aquatic systems.
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A. Ellison et al., "Loss of foundation species: consequences for the structure and dynamics of forested ecosystems", Frontiers in Ecology and the Environment, vol. 3, no. 9, pp. 479-486, 2005. Available: 10.1890/1540-9295(2005)003[0479:lofscf]2.0.co;2.