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between the forest ecosystem and the lake ecosystem may be relatively easy, but is the young forest on the left a different ecosystem from the older forest on the right?

      Distinguishing ecosystems is also difficult because ecologists think about ecosystems at a variety of spatial scales. A pool of water that collects in a hole in an old tree and is home to some algae and invertebrates can be considered an ecosystem. At the other extreme, ecosystems are sometimes defined on the basis of the movements of wide‐ranging animals. When biologists speak of the Serengeti Ecosystem they are referring to an area of almost 27,000 km² defined in large part by the habitat needs of a migratory wildebeest population (Rentsch and Packer 2015). At the largest known scale, the Earth’s entire biosphere can be considered an ecosystem.

The key thing to understand is that the term “ecosystem” is a conceptual tool that makes it easier for us to organize our understanding about ecological interactions and to communicate about it. For the purposes of this book we can think of ecosystems at a spatial scale that is easy to detect from an airplane, typically from a fraction of a hectare to a few hundred hectares. We can draw the boundaries between adjacent ecosystems where they will separate distinct sets of species. Conservationists then give priority to maintaining examples of the different types of ecosystems thus delineated; we cannot realistically expect to protect every individual ecosystem.

      Classifying Ecosystems

Just as it can be difficult to delineate particular ecosystems on the ground, it is also difficult to classify them into different types once they are delineated (Whittaker 1973). How do we decide if two different forests are the same type of forest ecosystem? Although there are several quantitative methods for assessing similarity of community composition, there is no standard level of similarity used to decide whether two ecosystems are of the same type (Table 4.1). Despite the lack of universal standards, significant progress has been made for some countries (e.g. Australia, Canada, United Kingdom, and United States) and regions (Latin America and the Caribbean) on developing vegetation classification schemes that are effectively terrestrial ecosystem classification systems (USNVC.org; Faber‐Langendoen et al. 2014). As depicted in Table 4.2, ecosystem classification is usually approached hierarchically. For example, at the highest level we could separate terrestrial and aquatic ecosystems; at a lower level freshwater, marine, and estuarine ecosystems; then freshwater ecosystems into lakes and rivers; and so on. However, there is no universally accepted system for doing this analogous to the kingdom‐phylum‐class‐order‐family‐genus‐species system.

Table 4.1 Relative abundance of species (percentages) in three hypothetical ecosystems. Based on the limited data presented, most ecologists would probably classify A and B as belonging to one type of ecosystem and C to a different type. Note that the similarity index (which has a range of 0 to 1) is much higher between A and B than between B and C or A and C. However, there is no standard level of similarity used to determine if two ecosystems are of the same type. (See Magurran 2004 for calculation of the Morisita–Horn similarity index, used here, and others)

Ecosystem	A	B	C
Black oak	40	30	10
White pine	30	40	10
Red maple	20	10	10
Yellow birch	10	20	70
Similarity index	A vs B 0.96B vs C 0.54A vs C 0.40

Table 4.2 This example depicts how one type of forest nests within the levels of the International Vegetation Classification hierarchy for terrestrial vegetation.

      Sources: NatureServe Explorer and The U.S National Vegetation Classification

Class	1: Forest & Woodland
Subclass	1.B: Temperate & Boreal Forest & Woodland
Formation	1.B.2: Cool Temperate Forest & Woodland
Division	1.B.2.Na: Eastern North American Forest & Woodland
Macrogroup	Appalachian‐Interior‐Northeastern Mesic Forest
Group	Appalachian‐Allegheny Northern Hardwood ‐ Conifer Forest
Alliance	Central & Southern Appalachian Rich Northern Hardwood Forest
Association	Sugar Maple ‐ Yellow Birch ‐ Black Cherry Forest

      Geography also needs to be considered when classifying ecosystems. Two alkaline eutrophic lakes that share a very similar biota would probably be considered the same type of ecosystem even if they are hundreds of kilometers apart and on either side of a mountain range. On the other hand, if the mountain range was a geographic barrier for many species and the two lakes had quite different biotas we might decide that they are different types of ecosystems.

How can we recognize both the basic physical similarity of the two alkaline eutrophic lakes and the biological differences that occur because of their geographic separation? One approach involves dividing the world into regions based on biologically meaningful patterns that shape the distribution and abundance of species such as climatic zones, mountain ranges, oceans that isolate terrestrial biota, or continents that isolate marine biota. There are many examples of such maps and they use a variety of criteria and names such as ecoregions, ecoclimatic zones, biogeographic provinces, and biophysical regions (Bailey 1996 , 2005 ; Loveland and Merchant 2005). One set of maps originally created by World Wildlife Fund‐US and The Nature Conservancy delineated 867 terrestrial ecoregions (Olson et al. 2001, see Fig. 4.2 for latest version), 426 freshwater ecoregions (feow.org; Abell et al. 2008), and 232 coastal marine ecoregions (Spalding et al. 2007). By using such maps we can recognize the differences that exist between the two lakes because they are in different ecological regions, but we could still recognize their basic similarity by calling them both alkaline eutrophic lakes.

Figure 4.2 This map depicts the Earth’s terrestrial ecoregions; see text about analogous maps for freshwater and coastal ecoregions and Dinerstein et al. (2017). An interactive version of this map is available at http://ecoregions2017.appspot.com.

      From a conservation perspective we could largely avoid the issue by organizing conservation efforts for each ecological region. However, conservation efforts are usually

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