field. Finally, if you imagine spotted, long‐tailed, and banded lizards living on one island, and a fourth species, speckled lizards, living a thousand kilometers away on another island, this would represent gamma diversity, or geographic‐scale diversity, that is, the total number of lizard species among all the ecosystems in question.
Figure 2.3 The distribution of four hypothetical lizard species showing alpha diversity (within an ecosystem, a plus b), beta diversity (among ecosystems, a/b plus c), and gamma diversity (geographic scale, a–c plus d). See text.
We can use this hypothetical example to show how a narrow‐scale perspective on maintaining biodiversity can lead would‐be supporters of biodiversity astray. Some people might look at Fig. 2.3 and think, “There are more lizard species in forests, so let’s plant trees in the field.” By doing so they might increase the alpha diversity of the field from one lizard to two (from banded lizards to spotted and long‐tailed lizards), but they might also decrease the beta diversity of the island from three species to two because banded lizards would no longer have any suitable habitat. Similarly, they might think, “Let’s bring some of the speckled lizards from the other island to our forest and have four species here.” However, the speckled lizards might outcompete and replace one of the local lizards or introduce a disease. The whole archipelago could end up with only three, two, or one lizard species instead of four and thus decreased gamma diversity.
The idea of spatial scale is so fundamental to maintaining biodiversity that a mnemonic phrase is worth remembering: “Scale is the tail that w‐a‐g‐s biodiversity” (w, within ecosystem diversity; a, among ecosystem diversity; g‐s, geographic‐scale diversity).
Diversity components usually vary dramatically from one scale to another, but not always. Take the extreme case of the native flowering plants of Antarctica. They include just two species – a grass, Deschampsia antarctica, and a cushion‐forming plant, Colobanthus quintensis – that usually co‐occur at the same sites. This is a very rare case where alpha and gamma diversity are the same. Or think about Madagascar with its high alpha diversity (many species at a given site), diverse ecosystems (high species turnover among ecosystems or beta diversity), and very high levels of endemism. In short, alpha and beta diversity and Madagascar’s unique contribution to gamma diversity make it a global priority for conservation.
Some readers may think that some intuitively obvious ideas are being belabored here, but these ideas are frequently overlooked in the real world of natural resource management. For example, foresters who manage large tracts of contiguous forest often claim that they can increase the biodiversity of their forest by logging moderate‐sized patches in their forest (Hunter and Schmiegelow 2011; Mayor et al. 2012). This claim is usually true; cutting a few patches in a mature forest typically increases species richness by increasing beta diversity because it will provide new habitat for many early successional species, while most of the species associated with a mature forest ecosystem will persist in the remaining uncut forest. On the other hand, building logging roads fragments forests and may facilitate access for excessive hunting and invasive species. These threats (to which we will return in later chapters) will probably diminish populations of some uncommon species and thus decrease evenness. Local extinction of some sensitive species is possible. Would this be a good tradeoff for increasing the beta diversity of a forested landscape? In sum, whenever we manipulate diversity at a local scale, we should consider the consequences at a larger scale and not rely on simple measurements of local biodiversity to judge the outcome. Case study 2.1 illustrates this issue well.
CASE STUDY 2.1 Clear Lake
In northwestern California lies Clear Lake, a large body of water (17,760 ha) that is shallow, warm, and productive; thus it supports a great abundance of fish. Originally, Clear Lake was home to 14 native kinds of fish, at least three of which were endemic to the lake: the Clear Lake splittail, Clear Lake hitch, and Clear Lake tule perch (Moyle 1976a ; Thompson et al. 2013) (Fig. 2.4). However, beginning in the late 1800s and continuing through the 1980s, people tried to increase the fish diversity of the lake by importing exotic species, primarily sport fish sought by anglers. Stocking began with carp and two species of catfishes and continued at irregular intervals, primarily with members of the Centrarchidae family (sunfishes and basses) native to the eastern United States. One species introduced in 1967, the Mississippi silversides, soon became the most abundant species in the lake. In the face of this competition and some other issues, all of the native species have declined dramatically, and only six native species may remain common in the lake. Worse still, two of the native species that have disappeared from the lake (the Clear Lake splittail and the thicktail chub) are globally extinct. The net scorecard: misguided attempts to enrich the fish fauna of Clear Lake increased the number of fish species there from 14 to 30 by adding 26 exotic species, but these introductions have decimated the lake’s native fish fauna, eliminating two elements of biodiversity from the entire planet and reducing gamma diversity. Most conservation biologists would argue that this was not a good trade. Recently some conservation biologists have disagreed about the relative importance of alpha versus gamma diversity (or, as more generally expressed, local versus global species richness), primarily because they believe that ecosystem function is more important than the nativeness of the biota (Primack et al. 2018).
Figure 2.4 Clear Lake in northern California used to be inhabited by 14 native species of fish until fisheries managers began introducing new fish species, 26 in all. These introductions decimated the native fish populations, but still produced a net increase in alpha diversity of 16 species. This increase came at the expense of gamma diversity at a global scale because two of the original species, the Clear Lake splittail and the thicktail chub, are now globally extinct.
Biodiversity Verbs
People change, manipulate, and manage the world, and consequently affect biodiversity, often negatively. Conservationists promote positive actions and use a variety of verbs to describe these activities. The verb maintain is dominant in this book because a major goal of conservation biology is to keep all the elements of biodiversity on Earth, despite human‐induced threats. In this section we will evaluate some alternative verbs that are often encountered in the conservation biology literature. This may seem like a pedantic exercise, but some verbs carry implications that are not always consistent with the goal of maintaining biodiversity. For example, to maximize biodiversity implies manipulations such as increasing the alpha diversity of an ecosystem, even importing non‐native species, without considering the consequences for biodiversity at a larger scale. Manipulating the lizard populations in Fig. 2.3 was a good example of this. To increase or to enhance biodiversity may imply the same shortsightedness, unless we are referring to an ecosystem in which biodiversity has been diminished by previous human activity and the goal is to return it to its previous state. If this is the case, it is probably best to refer to restoring biodiversity. Protecting biodiversity is similar to maintaining biodiversity but
