The fundamentals of the ecosystem health concept have been set already in the eighteenth century with the ideas of the Scottish geologist James Hutton, who started to describe the Earth as an integrated system. Later, the writings about land sickness of the pioneering ecologist Aldo Leopold in 1941 created the first roots of the ecosystem health concept. Since then, the definition of ecosystem health has been constantly evolving toward an integration of more and more human and societal contexts in order to understand what is considered to be healthy. In the United States and in Canada, the concept of ecosystem health has been adopted in legislation and it is part of international political programs, for example, the so-called Rio convention on sustainable development. Therefore, the assessment of ecosystem health does not only take into account ecological components but also requires a comprehension of social, economic, and cultural dimensions. Hence, its definition is more comprehensive than definition of human health. These facts have to be reflected while defining appropriate sets ofindicators that can be applied within respective assessments ofecosystem health.
Suitable indicator sets have to consider both the structure and the function of ecosystems on different spatial and temporal scales. As pointed out in various studies, ecosystems on our planet already are and will continue to be degraded under the pressure of increasing human demands: anthropogenic impacts as desertification, acidification, forest decay, erosion, and eutrophication have become more and more evident during the last decades. There are no more ecosystems on Earth which are not impacted by any human activity. Hence, preventative and restorative strategies are needed in order to achieve the health of regional and global ecosystems on a long-term perspective. With regard to different well-known and established indicators for example species diversity or water quality, many of the ecosystems on our planet can be considered unhealthy. Several of their functions, especially those needed to sustain life of the human community, have become impaired. Consequently, ecosystem health has been changed significantly, that is, because it refers to systems that are manipulated to satisfy human needs. This makes the difference to other related concepts, for example some notions of ecological integrity, which refers to the functioning of ecosystems based on nature-near, self-organized processes.
Generally speaking, an ecosystem is often called healthy if it is stable (respectively resilient) and sustainable in the provision ofgoods and services used by human societies (ecosystem services). This implies that it has the ability to maintain its structure (organization) and function (vigor) over time under external stress (resilience). Further ecological principles and theories like homeo-stasis, diversity, complexity, emergent properties, or hierarchy principles are closely related or included in the concept of ecosystem health as it refers to complex adaptive systems. These systems are characterized by certain dynamics including sudden reconfigurations from one state of organization to another. Some changes can be inherently unpredictable. Both positive and negative feedback processes and cause and effect chains, operating over a range of spatial and temporal scales, predominate these dynamics.
Hence, ecosystem health represents the sustainability of an ecosystem as a whole that needs a minimal external support by management measures. It comprises biophysical, socioeconomic, cultural, and human dimensions of the environment. This notion of ecosystem health is closely interrelated to the ability of ecosystems to provide and sustain ecosystem services.
In the beginning of the article, a short review of the historic development of ecosystem health assessment and indicators is given. Afterwards, different categories and examples of corresponding ecosystem health indicators are presented. This part is supplemented by several examples of commonly applied ecosystem health indicators from lake as well as from terrestrial ecosystems. At the end, some examples of applied methods of ecosystem health assessment are given.
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