Fundamentals of Conservation Biology. Malcolm L. Hunter, Jr.
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of ecosystems, most species and their genetic diversity can be protected as well (Hunter 1991; Beier et al. 2015a). This idea is often described using a metaphor of coarse filters and fine filters first proposed by The Nature Conservancy (1982) (Fig. 4.8). The coarse‐filter approach to conserving biodiversity is appealing because it is efficient and provides broad protection. It is efficient because, compared with the number of species in the world, there are relatively few different types of ecosystems and thus protecting a representative array of these is comparatively straightforward. It is broad because it will protect, to some degree, little‐known species such as invertebrates and fungi, even undescribed species, plus their genetic diversity. The Nature Conservancy (1982) originally estimated that 85–90% of species could be protected this way, although this seems optimistic based on the few empirical tests that have been undertaken (e.g. MacNally et al. 2002; Grantham et al. 2010).
Figure 4.8 The strategic value of ecosystems is illustrated by the coarse‐filter–fine‐filter approach to conserving biodiversity. Protecting a representative array of ecosystems constitutes the coarse filter and may protect most species. However, a few species will fall through the pores of a coarse filter because of their specialized habitat requirements or because they are overexploited. These species will require individual management, the fine‐filter approach. In this example, a coral reef ecosystem with all of its constituent species is protected by the coarse‐filter approach, but fine‐filter management is still required for the hawksbill turtle and spiny lobster.
Importantly, the coarse‐filter approach can be an effective strategy regardless of whether ecosystems are tightly connected systems or loose assemblages of species (Fig. 4.3). It is only necessary that the distribution of species and their habitats have some degree of concordance so that a complete array of ecosystems will harbor a reasonably complete array of species (Hunter et al. 1988; Rodrigues and Brooks 2007). We will return to this point and the coarse‐filter approach in general in Chapter 11, “Protecting Ecosystems.” Finally, it is notable that some ecosystems are analogous to flagship species; that is, they elicit public concern about conservation writ large. Tropical rain forests and coral reefs are perhaps the best examples of this phenomenon, but others, such as traditional agricultural ecosystems with high cultural value, are emerging as a new emblem for biodiversity conservation (Chapter 14, “Conservation near People”).
Uniqueness Values
The process of ecosystem classification clouds the issue of ecosystem uniqueness. If we define many different types of ecosystems, each type of ecosystem will not be very different from similar types. Alternatively, if we make gross distinctions (e.g. all coniferous forests are one type of ecosystem), then each type of ecosystem will clearly be unique. Some types of ecosystems may seem unique under any classification, for example, caves and hot springs, but there is a danger of confusing uniqueness and rarity. In short, different ecosystems may have different uniqueness values, but these will be difficult to evaluate until the classification schemes currently being developed are widely accepted. Nevertheless, some types of ecosystems, such as those found on remote islands and dominated by endemic species, are widely acknowledged as unique (Fig. 4.9 depicts one example).
Figure 4.9 The forests of Socotra, a small remote island, could be said to constitute a unique type of ecosystem because they are dominated by endemic species, notably the dragon’s blood tree pictured here.
(Ovchinnikova Irina/Shutterstock)
Ecosystem Diversity and Species Diversity
The coarse‐filter–fine‐filter metaphor (see Fig. 4.8) captures the strategic value of protecting ecosystems as a vehicle for maintaining species diversity, but the relationship between ecosystem‐level conservation and species‐level conservation is more complex than this. Some of this complexity is captured in two related questions that have long intrigued ecologists. First, are species‐rich ecosystems more stable than species‐poor ecosystems? Second, why do some ecosystems have more species than other ecosystems?
Diversity and Stability
Conservation biologists have long been concerned that species extinctions could have dire consequences for the stability of entire ecosystems. This idea is captured in a well‐known metaphor suggested by Anne and Paul Ehrlich (1981). Imagine you were flying in a plane, looked out the window, and saw a rivet fall out of the wing. You might not worry too much because there are thousands of rivets in a plane, and the loss of one rivet would not make it fall apart and crash. In fact, many rivets could probably fall out before the situation became dangerous, but, eventually, if enough rivets fell out, the plane would crash.
By analogy, an ecosystem could survive the loss of some species, but if enough species were lost, the ecosystem would be severely degraded. Of course, all the parts of a plane are not of equal importance, and, as explained in Chapter 3’s discussion of keystone, controller, and dominant species, not all species are of equal importance in an ecosystem. Thus it is possible that the loss of even a single important species could start a cascade of extinctions that might dramatically change an entire ecosystem. A good illustration of this occurred after fur hunters eliminated sea otters from some Pacific kelp bed ecosystems: the kelp beds were practically obliterated too, because, in the absence of sea otter predation, sea urchin populations exploded and consumed most of the kelp and other macroalgae (Estes et al. 1989). The likelihood of such calamities is related to the synergistic systems versus loose assemblage debate we discussed earlier (see Fig. 4.3); obviously, significant degradation is more likely if ecosystems are highly synergistic systems and the species lost are those that exert disproportionate control.
Three mechanisms for higher diversity increasing ecosystem stability have been proposed by Chapin et al. (1997). First, if there are more species in an ecosystem, then its food web will be more complex, with greater redundancy among species in terms of their ecological niche or role. In other words, in a rich system if a species is lost, there is a good chance that other species will take over its function as prey, predator, producer, decomposer, or whatever. Second, diverse ecosystems may be less likely to be invaded by new species, notably exotics that would disrupt the ecosystem’s structure and function. Third, in a species‐rich ecosystem, diseases may be less likely to spread because most species will be relatively less abundant, thus hampering transmission among individuals, and some disease organisms may be diluted among multiple host species. Variations on these themes have been proposed, such as species‐rich ecosystems being more productive because they use a site’s resources more efficiently, or species‐rich plant communities being less vulnerable to herbivory because of the dilution effect.
Research to illuminate these ideas has been accumulating, albeit slowly because of an incomplete understanding of what constitutes stability. For example, diverse grasslands are more resistant to being changed by a drought than species‐poor grasslands, but they are not more resilient in terms of recovering quickly after a drought (Isbell et al. 2015). Overall, support