Role of invasive species in biodiversity
It is the variety of life on Earth, in all its forms and all its interactions. If that sounds bewilderingly broad, that's because it is. Biodiversity is the most complex feature of our planet and it is the most vital. "Without biodiversity, there is no future for humanity," says Prof David Macdonald, at Oxford University. The term was coined in 1985 -- a contraction of "biological diversity" -- but the huge global biodiversity losses now becoming apparent represent a crisis equaling -- or quite possibly surpassing -- climate change.
More formally, biodiversity is comprised of several levels, starting with genes, then individual species, then communities of creatures and finally entire ecosystems, such as forests or coral reefs, where life interplays with the physical environment. These myriad interactions have made Earth habitable for billions of years. A more philosophical way of viewing biodiversity is this: it represents the knowledge learned by evolving species over millions of years about how to survive through the vastly varying environmental conditions Earth has experienced. Seen like that, experts warn, humanity is currently "burning the library of life".
The diversity of life on our planet is critical for maintaining the basic planetary life support systems we rely on every day. Ecosystem services or the resources nature provides us free of charge, like drinking water, crop pollination, nutrient cycling and climate regulation, all rely on biodiversity. For instance, the diversity of insect and avian pollinators is crucial to global agricultural productivity; ensuring plants produce harvestable crops for human use.
The Earth's staggering biodiversity is also responsible for more tangible human goods. In many parts of the world, plants are the main source of medicine used for primary health care, linking the survival of plant diversity with human well-being. Additionally, many of our most important pharmaceutical drugs come from compounds discovered only in specific plants or organisms, meaning future drug discoveries may well depend on the survival of species that have yet to be studied for their medicinal properties.
Barriers such as large rivers, seas, oceans, mountains and deserts encourage diversity by enabling independent evolution on either side of the barrier, via the process of allopatric speciation. The term invasive species is applied to species that breach the natural barriers that would normally keep them constrained. Without barriers, such species occupy new territory, often supplanting native species by occupying their niches, or by using resources that would normally sustain native species.
The number of species invasions has been on the rise at least since the beginning of the 1900s. Species are increasingly being moved by humans (on purpose and accidentally). In some cases the invaders are causing drastic changes and damage to their new habitats (e.g.: zebra mussels and the emerald ash borer in the Great Lakes region and the lion fish along the North American Atlantic coast). Some evidence suggests that invasive species are competitive in their new habitats because they are subject to less pathogen disturbance. Others report confounding evidence that occasionally suggest that species-rich communities harbor many native and exotic species simultaneously while some say that diverse ecosystems are more resilient and resist invasive plants and animals. An important question is, "do invasive species cause extinctions?" Many studies cite effects of invasive species on natives, but not extinctions. Invasive species seem to increase local (I e : alpha diversity) diversity, which decreases turnover of diversity (I e: beta diversity).
Overall gamma diversity may be lowered because species are going extinct because of other causes, but even some of the most insidious invaders (e g : Dutch elm disease, emerald ash borer, chestnut blight in North America) have not caused their host species to become extinct. Extirpation, population decline and homogenization of regional biodiversity are much more common. Human activities have frequently been the cause of invasive species circumventing their barriers, by introducing them for food and other purposes. Human activities therefore allow species to migrate to new areas (and thus become invasive) occurred on time scales much shorter than historically have been required for a species to extend its range.
Not all introduced species are invasive, nor all invasive species deliberately introduced. In cases such as the zebra mussel, invasion of US waterways was unintentional. In other cases, such as mongooses in Hawaii, the introduction is deliberate but ineffective (nocturnal rats were not vulnerable to the diurnal mongoose). In other cases, such as oil palms in Indonesia and Malaysia, the introduction produces substantial economic benefits, but the benefits are accompanied by costly unintended consequences.
Finally, an introduced species may unintentionally injure a species that depends on the species it replaces. In Belgium, Prunus spinosa from Eastern Europe leafs much sooner than its West European counterparts, disrupting the feeding habits of the Thecla betulae butterfly (which feeds on the leaves). Introducing new species often leaves endemic and other local species unable to compete with the exotic species and unable to survive. The exotic organisms may be predators, parasites, or may simply outcompete indigenous species for nutrients, water and light.
At present, several countries have already imported so many exotic species, particularly agricultural and ornamental plants, that their own indigenous fauna/flora may be outnumbered. For example, the introduction of kudzu from Southeast Asia to Canada and the United States has threatened biodiversity in certain areas.
The preservation of the number, types, and relative abundance of resident species can enhance invasion resistance in a wide range of natural and semi-natural ecosystems (medium certainty). Although areas of high species richness (such as biodiversity hot spots) are more susceptible to invasion than species-poor areas, within a given habitat the preservation of its natural species pool appears to increase its resistance to invasions by non-native species. This is also supported by evidence from several marine ecosystems, where decreases in the richness of native taxa were correlated with increased survival and percent cover of invading species.
Biplob Kumar is an environmentalist and author