HUMAN IMPACTS ON OTHER SPECIES
As we have seen, humans and our activities have had immense impacts on the rest of the biological world. In geological history, there have been five extinction events that biologists consider “mass extinctions.” fte Permian-Triassic extinction event and the Cretaceous-Paleogene extinction event that included the non-avian dinosaurs are probably the two most well-known. fte recent Holocene (or Anthropocene) has seen extinction levels that many biologists consider to be the 6th mass extinction. fte Holocene extinction rate has been estimated at 140,000 species per year, with most of those being undocumented.
ftese extinctions include such recent and well known ones as the flightless dodo of the island of Mauritius, the thylacine of Tasmania, Australia, and New Guinea, and the passenger pigeon of North America. fte dodo was driven to extinction in the late 17th century, apparently through a combination of habitat loss, mortality caused by introduced animals that plundered dodo nests, and human hunting. fte thylacine (Fig. 8.10), also known as the Tasmanian wolf or Tasmanian tiger, was officially considered extinct in 1936, when the last captive specimen died at the Beaumaris Zoo in Hobart, Tasmania. fte passenger pigeon was probably the most abundant bird in North America, with numbers estimated at up to 5 billion at the time of European arrival. fte bird was driven to extinction by hunting and loss of habitat; the last individual died at the Cincinnati Zoo in 1914.
Figure 8.10: Thylacines in Washington DC, USA, 1906. (Wikipedia ‘Thylacine;’ Attribution: Baker; E.J. Keller)
For every high-profile extinction such as these, countless species have disappeared before they were even known to science. For most of human history, our primitive hunting abilities and small numbers probably made the impacts of our hunting on other species negligible. But as we have seen, by the Pleistocene, human hunting capabilities had advanced to the point that it probably played a role in the Pleistocene megafaunal extinctions. Modern human hunting on an individual basis is regulated in many countries, and this can allow sustainable harvesting of wild game. However, on a large-scale, commercial level, human predation on other species of animals has reached a level that, according to a recent study by Chris Darimont and colleagues, now makes H. sapiens a “super predator,” with the ability to cause mortality rates far in excess of those caused by non-human predators. For example, humans kill adult reproductive members of prey species at rates up to 14 times higher than do other predators; terrestrial carnivores and fishes are exploited especially intensely. ftis level of exploitation reflects both the hyper-efficiency of many human harvesting technologies as well as the demands of feeding the large and highly consumptive human population. Of course, these human hunting pressures affect not only the species being harvested, but other species in the food web that depend on them.
Since the beginnings of agriculture, H. sapiens have transformed the Earth’s landscape. ftis transformation has undoubtedly been the most important source of anthropogenic extinctions. Over the last few centuries, this transformation has occurred in an accelerating fashion, due both to our increasing population and our increased technological abilities to alter the landscape. Columbus’ 1492 voyage to the New World marked the beginning of a large-scale exchange of plants, animals, people, and technology between the Old and New Worlds, often referred to as the Columbian Exchange. One result of this exchange was the growth of Europe as an economic power; this would in time lead to colonialism and large- scale establishment of exploitative plantation economies in which primary products such as crops, timber, and extracted minerals would be produced using cheap (slave) labor in the colonies and then exported to the home country where the materials would be processed and re-exported at a large profit.
From a land-use standpoint in the colonies, this system would result in large areas of forest being cut and large areas of land being converted to agriculture with, of course, an accompanying loss of native biodiversity. Since then, of course, humankinds’ ability to alter the landscape has dramatically increased in geographic scope and impact. In many parts of the world, such as the Midwestern U.S.A. where I live, one would be hard-pressed to find even a few hectares of contiguous land that does not show significant human impacts. ftis degradation and fragmentation of landscapes has resulted in small islands of suitable habitat in an ocean of agriculture and urbanization. As we saw in Chapter 6, smaller, more isolated oceanic islands have lower species richness; this pattern applies to terrestrial habitat islands as well.
As mentioned previously, the Columbian Exchange involved large-scale movement of species between the Old World and New World, so introductions of exotic species are not a new phenomenon. In many cases, these were economically important crops or domestic animals, and the ecological damage associated with them was to a large extent a function of land-use patterns in their new homes. fte terms exotic and invasive are sometimes used interchangeably, but there is an important difference between the two. An exotic species, also known as an introduced, alien, or non-native species, is one that has arrived at a location outside its native geographic range, usually through human transport. Exotic species often, but not always, become invasive species. An invasive species is a (usually) exotic species that tends to proliferate and spread, causing ecological, economic, or even health-related harm.
Invasive species are often “r-strategists” rather than “K-strategists.” In ecology, these two concepts relate to selection pressures for different life history traits. Species that are r-strategists can quickly colonize new locations. ftey tend to be relatively small-bodied and short-lived, but have rapid reproduction and growth. Offspring receive little or no parental care and have low survival rates, but because so many are produced, large enough numbers still survive. ftese species can disperse and spread rapidly. ftey are generally poor competitors, and depend on exploitation of newly-available environments with less crowded ecological niches to perpetuate themselves. Examples of r-strategists include ruderal or “weedy” plant species such as dandelions and many grasses, many insects and other invertebrates, and, among the mammals, many rodent species. K-strategists show the opposite traits (long-lived, slow reproduction but lots of parental care, good competitors, etc.), and include many tree species, orchids, and mammals such as primates and elephants. It is important to keep in mind that the r- vs. K-strategist concept represents a continuum, and that both types of traits can sometimes be found in the same species.
ftere are also other factors that can play a role in a species becoming invasive. If natural enemies or competitors kept the exotic species in check in its native environment, the absence of those species in the new environment can allow ecological release of the exotic species populations, resulting in an invasive species. Likewise, an exotic species can become invasive if an important limiting resource is more abundant in the new environment. ftere is also evidence that many invasive species are capable of undergoing rapid evolution, allowing them to quickly adapt to new environments. Islands and disturbed environments appear to be particularly susceptible to invasive species, probably because of lack of effective natural enemies and competitors, and greater availability of open ecological niches.
ftere is abundant evidence that invasive species can drive down numbers of other species and thus contribute to loss of biodiversity. On the western Pacific island of Guam, the brown tree snake was accidently introduced from the Indonesian region sometime in the late 1940s-early 1950s. Since then, this snake has created havoc, causing the extinction of 12 native bird species on Guam. Because of the greater abundance of food resources than in its native environment, brown tree snakes have reached densities of 100 individuals per ha on Guam. Plants can also be invasive exotics. Old man’s beard is a climbing vine that was introduced as an ornamental plant in New Zealand in the early 1900s (Fig. 8.11). fte seeds germinate most readily on disturbed ground. Once established, vines can cover trees and form a dense canopy that prevents sunlight from reaching the ground. ftis adversely affects native vegetation already present, as well as inhibiting germination of new individuals. In the Tierra del Fuego region of extreme southern Argentina and Chile, introduced beavers are radically changing the landscape and adversely affecting native species through their tree-felling and dam-building habits. ftese beavers are descendants of ones evidently imported from Canada by the Argentinian government in the 1940s in an attempt to establish a commercial fur trade. fte beavers are now the focus of what may be the largest eradication project ever attempted, but it now appears that control, rather than eradication, is a more realistic goal.
Figure 8.11: Old man’s beard (Clematis vitalba) smothering a New Zealand endemic, Cordyline australis, on the Port Hills in Christchurch. (Wikipedia ‘Old man’s beard in New Zealand;’ Attribution: Alan Liefting)