Читать книгу Fundamentals of Conservation Biology - Malcolm L. Hunter Jr. - Страница 33

Extinction usually refers to the disappearance of a species from the Earth, but the term is also routinely used, with modifiers, to describe the disappearance of a species from a smaller area. For example, when a species disappears from a small area, this is called a local extinction, even though the area may later be recolonized by immigrants, e.g. when beavers return to a valley from which they had disappeared. On a somewhat larger scale one can refer to regional extinction. For example, during the last century the bald eagle went extinct in many regions of the United States while remaining abundant in Alaska. Although conservation biologists are most concerned about global extinctions, smaller‐scale extinctions are also of some concern because they may foreshadow extinctions on a larger scale and because they may represent a loss of genetic diversity. Another key term is endemic, which refers to species found only in a defined geographic area; thus, koalas are endemic to Australia. If a species is found only in a very limited area (e.g. inhabiting only a single small island or lake), it is sometimes called a local endemic. For example, there is a snail known only from the base of a single waterfall in Chittenango, New York – the Chittenango ovate amber snail.

The risks of extinction at different spatial scales are a key consideration when deciding which endangered species are a higher priority. The larger the scale at which an extinction is likely to occur, the more important it is to try to prevent it. For example, the Iberian lynx, a species confined to southern Spain, is a higher priority for Spanish conservationists than the Eurasian lynx, which has a huge range that just reaches northern Spain (Fig. 2.2).

Figure 2.2 Conservationists do not consider all species to merit equal attention. For example, Spanish conservationists place a higher priority on protecting the Iberian lynx, a species now endemic to southern Spain that faces global extinction, than on the Eurasian lynx, a species threatened with regional extinction from the Pyrenees Mountains along the Spanish–French border but still relatively secure in other parts of Europe and Asia.

(Apple2499/Shutterstock)

The ecologist Robert Whittaker (1960) devised a simple system for classifying the scales at which species diversity occurs: alpha, beta, gamma (A, B, C in Greek). Alpha diversity is the diversity that exists within an ecosystem. In Fig. 2.3 two hypothetical lizard species, spotted lizards and long‐tailed lizards, illustrate alpha diversity by coexisting in the same forest, living at different heights within the forest. A third species, banded lizards, illustrates beta diversity (among ecosystems diversity) by occurring in a nearby 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.