CHAPTER 4 Ecosystem Diversity
Flying over the landscape in an airplane you see patterns: dark green patches that are forests, a distant white line of snow‐capped mountains, blue patches and ribbons that are lakes and rivers, brown patches that are tilled fields, grey splotches that are urban areas, and so on. These are the coarse manifestations of an enormously complicated web of ecological interactions, a myriad of species interacting with one another and their physical environment. Despite this complexity, all is not chaos. There are patterns; some are so obvious that they can be seen from far above the Earth, and some are so subtle that we have little awareness or understanding of them. These patterns of interactions are the basis for ecosystems, and they are fundamental to the goal of maintaining biodiversity.
What Is an Ecosystem?
It is easy to define an ecosystem conceptually. It is a group of interacting organisms (usually called a community) and the physical environment they inhabit. It is much harder to delineate ecosystems in the real world – to decide where one ecosystem ends and another begins – because the web of interactions does not have clean breaks (Fig. 4.1). Most ecologists would say that a forest and an adjacent lake are different ecosystems because the assemblages of organisms inhabiting them are almost completely different and have relatively few direct interactions. This said, there are some interactions across the shoreline. Frogs leave the forest to lay their eggs in the lake and later small frogs hop into the forest. Wind moves huge numbers of autumn leaves into the lake, where they decompose. A bear visits the lake shore to catch fish and later defecates those digested fish back in the forest. These interactions in aggregate can be quite important. Conversely, many ecologists would say that a young oak forest and an adjacent old oak forest are the same ecosystem even though a fair number of their species would be different, as would some key ecological processes such as decomposition and water cycling. Separating two adjacent ecosystems is particularly difficult when the edge between them, often called an ecotone, is a gradual transition zone. For example, on the side of a mountain, ecosystems change continuously in response to the climate gradient that parallels elevation, and it is quite arbitrary to draw lines among them.
Figure 4.1 Deciding where one ecosystem begins and another ends is a complex task because the web of ecological interactions does not have clean breaks. In this example, distinguishing 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 |
|
|
