an undisturbed forest would have just those species associated with a late‐successional stage (i.e. the disturbed forest would exhibit beta diversity among the different patches). Of course, many ecosystems have heterogeneous environments with or without the patchiness of disturbances, and this is also an important source of niches for additional species. For example, an ecosystem that has an array of substrates ranging from clay to boulders will support more species than one that is covered by only clay. Similarly, the vertical dimension of forests and aquatic ecosystems is a form of spatial heterogeneity that adds opportunities for many species.
One simple explanation for why species richness varies among ecosystems is their size. Not surprisingly, more species can fit into a large ecosystem than a small one. There are many reasons for this, which we will discuss in Chapter 8 in the section on fragmentation. That discussion will also cover isolation, another factor that limits species richness by curtailing colonization, especially on islands. Time may also be a factor. Notably, the species richness of the tropics may be partly related to having long periods available for coevolution to generate new life‐forms without being bulldozed by glaciers, as has happened repeatedly at higher latitudes.
Finally, we need to recognize that species richness probably operates in a positive‐feedback loop, a “snowballing effect” in more colloquial language, to further increase the diversity of species‐rich ecosystems. Compare two ecosystems, one with 50 species of plants and the other with 200. The latter is likely to support a much wider spectrum of herbivores, pollinators, parasites, pathogens, and so on (Haddad et al. 2009; Lin et al. 2015). In other words, species beget species.
To summarize, the primary driver of species richness is the physical environment, especially how big, warm, and wet it is and how much it varies in space and time because of disturbances and other factors. Secondarily, the dominant species in the system (plants in terrestrial systems and a mixture of plants, algae, corals, and more in aquatic systems) shape diversity by enhancing spatial heterogeneity and providing the basis of a food web. Ultimately, every species may play some role, if only as food for its suite of predators, parasites, and pathogens.
An Important Postscript
This focus on the relative species richness of different ecosystems returns us to our earlier discussion about mismeasuring biodiversity by overemphasizing species richness (Chapter 2). Becoming fixated on species richness can lead conservation managers astray. For example, although maintaining the stability of ecosystems is an important argument for avoiding the loss of species, the converse of this argument does not hold: we should not seek to increase the stability of ecosystems by artificially augmenting the number of species, for example by planting new tree species in a forest or adding fish species to a stream network. Similarly, although sustaining species‐rich ecosystems like tropical forests may be a somewhat higher priority than sustaining species‐poor ecosystems, overemphasizing species richness to the exclusion of species‐poor ecosystems would be very short‐sighted. Recall the discussion about salt marshes, home to a narrow range of species but a very important type of ecosystem because of their productivity (see Fig. 4.5). Finally, because each type of ecosystem harbors a unique suite of species, the coarse‐filter approach requires protecting a complete array of ecosystems, even those that may have relatively few species (Fig. 4.10). In particular, many islands support a precious biota of endemic species, but are not very diverse overall; the Galápagos Islands may be the best example of this.
Figure 4.10 The extreme climatic conditions of a high‐latitude or high‐elevation ecosystem (tundra and alpine ecosystems) are the main reason why they support far fewer species than the coral reef depicted in Fig. 4.8. Such ecosystems still merit conservation because of their unique biota and other attributes.
(National Park Service/Public domain)
Ecosystems and Landscapes
The mosaic of ecosystems we see from a plane is not just a random array. There are patterns to the spatial configurations of ecosystems. Lakes are drained by rivers and bordered by marshes, woodlots are patches embedded in a matrix of agricultural ecosystems, clearcuts are patches in a matrix of forests, and so on. Human‐dominated landscapes in particular have a regularity of pattern and a sharp‐edged character not found elsewhere. Ecologists call these mosaics of ecosystems landscapes, and a subdiscipline called landscape ecology has developed (Forman 1995 ; Turner and Gardiner 2015) (Fig. 4.11). For example, landscape ecologists are interested in ecosystems that occur as long, narrow strips such as rivers and their associated riparian (shore) ecosystems because these ecosystems may serve as corridors that facilitate organisms moving among ecosystems. Also of interest to landscape ecologists are the edges between ecosystems. The interface between a forest and a field is one example: it will be avoided by some species and preferred by other species (Hunter and Schmiegelow 2011).
Figure 4.11 Ecologists refer to a mosaic of interacting ecosystems as a landscape. How many different types of ecosystem can you recognize in this fine‐scale landscape on the coast of Maine, USA?
(Drew Tarvin/Flickr/CC BY 2.0)
Conservation biologists are interested in landscape phenomena for a number of reasons that we will examine further in subsequent chapters. Two brief examples will suffice here. First, many endangered species are large animals that have large home ranges – tigers, wolves, elephants, etc. – that encompass many ecosystems. If we wish to maintain habitat for these species, we must maintain entire landscapes that provide for all their needs. Second, human activities have left many natural ecosystems as islands, isolated in a “sea” of human‐altered ecosystems, and conservation biologists are concerned with what happens along the edges of these small, residual patches. Are they being degraded by factors that originate externally such as exotic species, pesticides, and changes in local climate?
These and similar issues have led conservation biologists to advocate maintaining biodiversity from a landscape perspective (Groves and Game 2016). This is a way of saying that it is not sufficient to protect a representative array of ecosystems. We must also ensure that these ecosystems are arranged spatially so as to maintain the natural relationships among them. In short, to conserve biological diversity we should maintain natural, functioning landscapes composed of many interacting ecosystems.
CASE STUDY 4.1 Mangrove Swamps
Tropical shores are not all white‐sand beaches lined by coconut palms. In many places the land–sea transition is marked by dense stands of trees and shrubs that form mangrove swamps or mangal (Fig. 4.12). The seaward edge of a mangrove swamp is usually sharply delineated, but moving inland it often grades into other types of swamps as the elevation rises and the water becomes less saline. This gradation is one reason why the term “mangrove”
