the assumption of responsibility for the welfare of the world
Biodiversity is, broadly, variety in life and species. It is not a single, measurable quality. One form of biodiversity is species richness, the number of species per area; we can speak of species richness in a place, or we can standardize it to a unit of area. Species richness ignores the number of individuals; a species with two individuals and one with a thousand individuals count the same. Thus, population size is also an important indicator of biodiversity. This is in part because a large population increases genetic diversity, the total of variation in available genes. Genetic diversity is good for the survival of an individual species, because it increases that species’ adaptability; if a particular adverse environmental condition or disease should arise, a genetic anomaly held by a few members could be the key to surviving it. Genetic diversity is also increased, of course, by the sheer number of species.
Biodiversity is particularly high in the tropics, for two reasons: the tropics receive disproportionate energy and disproportionate rainfall. These are good conditions for abundant life, which tends towards significant variation. The Mediterranean biome is also a significant contributor to global biodiversity, because the various Mediterranean ecosystems are isolated from each other and environmentally quite different from their neighbors.
A species is endemic to a place if it is only found in that place. ‘Endemic’ should not be confused with ‘native’; a species is native to a place if it evolved in that place.
Small, isolated landmasses give rise to a high number of endemic species (a phenomenon known as island endemism). Because the landmasses are isolated, ecological niches tend not to be filled by migration; rather, the variety of niches provide multiple evolutionary opportunities and generally no competition to fill them. The Galapagos finches that inspired Charles Darwin are a clear example of this.
Biodiversity is often greater in mountainous areas, because of the diversity of environments. One reason is orogenic (mountain-caused) precipitation and the rain shadow effect. A cloud (which is water suspended in the air), blown in from the ocean and against a mountain, will be forced up, where the lower pressure will cause the water to drop out of the air, falling disproportionately on the seaward side of the mountain. The inland side of the mountain will be comparatively dry (existing in the so-called rain shadow). Elevation works like latitude in creating climatic zones, so that moving up a mountain we encounter different microclimates. By affecting drainage, slope also helps to determine microclimate. Finally, mountainous areas often include multiple parallel valleys separated by the mountain ridges. Migration over the ridge may be limited by the physical height of the ridge; also, species that could otherwise migrate over time, by expansion of habitat, may be unable to exist in the varying microclimates of different elevations. This means that two valleys right next to each other are effectively isolated from each other, so that even if the valley floors contain identical ecological niches, each might evolve different species to fill them.
Non-native or invasive species pose a threat to existing ecosystems because they upset the equilibrium and displace native species. Invasive species that fill similar niches to a native species can often outcompete that native species, because the invasive species may have no natural predators. Some invasive species are predators or parasites to which native species have no evolutionary resistance. While invasive species have the potential to reduce global biodiversity by leading towards the extinction of endemic species, most cases of invasive species actually increase local biodiversity in the short to medium term, since the invasive is in fact a new species and a new genetic component of the ecosystem. Biodiversity is a complicated issue, and we cannot just summarize it as “more biodiversity is better”. Species richness, for example, could easily be increased by human intervention, and populating a previously-barren place with multiple new species might well be aesthetically pleasing; but it would be a divergence from the existing natural state.
We can use individual species to define an ecosystem, especially in the case of dominant plant life, but an ecosystem is always a system of multiple, interdependent species. There are, however, a couple of reasons why conservation often focuses on individual species. One reason is scientific. One species may be identified as an indicator species, which is deemed especially sensitive to changes in the ecosystem equilibrium. The health of that species’ population is taken as an indication of the health of the ecosystem as a whole. A famous example of this is the spotted owl, a bird of the western coastal and central highland regions of North America. The decline of spotted owls, particularly the northern spotted owl, has been used to justify curbs on logging, under the US Endangered Species Act. Pushback against these measures questioned whether one species was worth the loss of jobs, but failed to understand that the spotted owl was merely indicating the deterioration of the old growth forests as a whole.
A second reason for focusing on individual species is political. Certain charismatic megafauna, large animals with particular appeal to the public, have been highlighted as a means of engaging the public in broader conservation efforts. Examples include elephants, giant pandas, tigers, various whales, and polar bears, which have featured heavily in campaigns to protect habitat, tighten laws, and raise money. This isn’t dishonest, merely strategic; the featured animals are indeed threatened, and focusing on them drives conservation efforts that benefit these species but also others.
The Columbian Exchange, the great transfer of people, other species, diseases, technologies, and ideas between the Old and New Worlds begun with the voyages of Christopher Columbus (Cristoffa Corombo / Χρ̃οFERENS « K‛r’oferens »), has profoundly affected the ecologies of both hemispheres. It was relatively easy for species to migrate within the Old and New Worlds, but migrating between them was largely blocked. This cannot be attributed to the ocean, since the landmasses are nearly connected now, and have been actually connected during glacial periods (when water is locked up in glaciers, thus lowering sea level). Rather, it was the climate that blocked migration. The polar climate is inhospitable to all but a few adapted species. For species (including humans) to make the migration, they would essentially need to adapt their way north into the polar zone and then adapt their way back south. This is generally unlikely.
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