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Foundation species build and define ecosystems and maintain a wide range of services: clean air and water, wood products and wildlife, and solitude and inspiration. This chapter discusses the challenges to ascertain whether a species is foundational in any ecosystem, and then identifies the hemlock as a foundation species, including its nature and role in the ecosystem.

The eastern hemlock is identified as a foundation species—a species that creates its own ecosystem and is intimately linked to the majority of other species in the system. This chapter discusses the characteristics of the eastern hemlock as a foundation species. Studies conducted have shown that the loss of hemlock in hemlock-dominated systems, whether from adelgid or logging, results in both short-term and long-term effects to associated plant and animals species along with shifts in ecological processes.

Birds function within their ecosystems in ways that include many direct and indirect chains of trophic interaction. In some interaction chains, birds exert top-down effects in “trophic cascades.” These occur when a predatory species directly reduces its prey abundance and consequently indirectly releases suppression of species at lower trophic levels. Many bird species initiate trophic cascades in terrestrial and aquatic ecosystems, and in natural and agro-ecosystems. Services provided by birds through trophic cascades benefit humans primarily through pest control in natural forests, forestry plantations, fruit orchards, and a variety of crop-based agro-ecosystems. Cascade strength may be affected by predator prey specificity, redundancy, species diversity, and productivity. Some birds influence ecosystem function both through either bottom up interactions or through intermediate trophic positions within interaction networks. These effects can also reverberate through trophic webs. Global change, including anthropogenic perturbation, may lead to a loss of ecosystem services, including seed dispersal and pollination services by birds. These losses may cause declines keystone or foundation plant species, resulting in losses in biodiversity and ecosystem integrity. We conclude with suggestions for future research needs on bird ecosystem services, particularly in light of global change.

Disease outbreaks have driven declines in some threatened species on land, including amphibians, bats, and birds. In the oceans, infectious diseases can also drive declines and are a potential agent of community change and threat to marine biodiversity. Recent disease-driven mass mortalities have affected a range of marine biota. This chapter outlines four case histories of disease outbreaks that affect marine communities and, in multiple cases, contribute to endangerment or listing as endangered species. The case histories include disease impacts to foundation species of corals and seagrasses, herbivores like abalone, and keystone predators like sea stars. Multi-host diseases in all these cases have contributed to extreme population declines. Infectious diseases pose the greatest threat to marine communities when they reduce foundation species like corals and seagrasses. In turn, disruption of marine communities may impact the services they provide to humans and other organisms through pathogen biofiltration. The authors suggest that marine disease impacts may be slowed by protecting the natural services provided by intact ecosystems.

This chapter reviews how marine ecosystems respond to parasites. Evidence from several marine ecosystems shows that parasites can wield control over ecosystem structure, function, and dynamics by regulating host density and phenotype. Like predators, parasites can generate or modify trophic cascades, regulate important foundational species and ecosystem engineers, and mediate species coexistence by affecting competitive outcomes. Sometimes the parasites have clear positive impacts within ecosystems, such as increasing species diversity or maintaining ecosystem stability. Other times, parasites may have destabilizing effects that signal an ecosystem out of balance. But it is now clear that some (but not all) parasites can have strong and, at times, predictable effects, and should thus be incorporated into food web and ecosystem models