Foresters have always bemoaned the continued loss of forests generated by agricultural expansion. Briefly, the tide seemed to turn. Following the invasion of the coconut leaf beetle (B. longissima) into the continental Asian region, some agriculturists attributed the uncontrolled movement of ornamental palms from the Indo–Malayan Archipelago as the likely cause. While this may be true, a quick scan of the various invasives suggests both the agricultural and forestry sectors are equally responsible for spreading invasive species. One classic case is the introduction of the cane toad (Bufo marinus) to control cane beetles in Australia. However, the toad shifted its attention to native forest areas, and wreaked havoc on the ecosystem. Among tree species, out of the 1 121 species used worldwide, 443 introduced species are reported to be invasive. The story of invasives is dotted with disastrous introductions from both forestry and agriculture. This being the case, and in order to reduce these incidences, there is merit in learning how invasive species work: what are their attributes, and the characteristics of invaded communities?
Our concern for invasive species stems primarily from their impact on the economy. Some countries like the United States claim that invasive species cost their economy US$137 billion annually. Disease organisms have likewise resulted in a loss of US$41 billion. A single mammal like the rabbit is costing Australia US$400 million annually. A startling number of diseases are moving around the globe – these include HIV/AIDS, Avian flu, Nipah virus, and the bubonic/pneumonic plague. The invasions are increasing because of increased trade and movement of people.
Invaders come from all the major taxa, from viruses to higher plants and mammals. Generally, the invaders are not a problem at the source. But in their new environment, they end up transforming the structure and species composition of the ecosystems by repressing or excluding native species. This may even lead to a cascading effect on the ecosystem as well: for example, when an invasive insect replaces a native insect which is an important pollinator. The drop in the number of pollinators can result in lowered fruit production of certain trees, which can then lead to a decline in the vertebrate dispersal agents. Increasing global domination by a few invaders is leading to relatively homogeneous conditions worldwide.
Enough is known about invaders to draw some key predictors. In the case of plants, species with high invasive potential usually have a large native geographical range, are small in genome size, are associated with disturbed habitats, have small seed mass, have a short juvenile period and produce large seed crops at frequent intervals; even better if they are soil-stored. Vegetative propagation definitely gives the invader an advantage. The habitat conditions in the new location are usually similar as the point of origin. Species with specialized needs, such as very specific pollinators and dispersal agents are less likely to invade. Some other factors include the invader’s ability to out-compete the native species for resources through better foraging techniques, its lower maintenance needs and fewer natural enemies.
Some ecosystems are more vulnerable to invasions than others. Oceanic islands, habitats that are periodically disturbed and areas that have high human encroachment are more vulnerable to invasive species. In reality, only a fraction of the species introduced to a new location become invasive. Invaders exhibit considerable similarities in species attributes such as size, life span, fruit type and dispersal habits. This explains why Cecropia sp. of South America are invading sites in Africa where normally the Musanga cecropoides, an ecological equivalent, are found. Another interesting perception is that habitats with high species diversity are more resistant to invasions. However there are too many discrepancies for this to become a rule. Thus it can be concluded that in locations where conditions favour high diversity, there are more niches to occupy likewise for the invaders. Overall, much of the invasive ecology can be elucidated in the theoretical framework of community ecology. According to the niche theory, how a species responds to resources, the physical environment and natural enemies determines the success of a species in a specific environment. This approach provides an adequate basis for understanding why and when an exotic species turns into an invasive one.
In fact, an invader faces many barriers before it can be classified as an invasive species. It must first move to the new range – the geographical move. Once this is accomplished, it needs to establish itself in the new environment, reproduce and disperse effectively. Even if it establishes itself in disturbed habitats, it cannot qualify as an invasive species technically. This only occurs when it crosses the final barrier, i.e. establishes itself in the natural habitat. We have considerable understanding of how invasive species move. We also have at our disposal quarantine and other control systems to minimize the invasion of alien species. But their implementation remains imperfect. This is where regional collaboration, sharing of information and networking can be of tremendous assistance in controlling the invasion of alien species. In addition, excessive alteration of the natural habitat greatly predisposes the site to invasives. It will be a challenge to control human activities so invasive species can be halted or slowed down. The history of alien species invasions reveals, more often than not, that they appear to be thwarting our efforts. It often seems as if we are merely slowing the rate of invasions. However, when invasive species have truly affected the bank balance or life, human success has been considerable. This means it is possible to control invasive species from spreading. We need not live in a homogeneous world.
* National Forest Programme Advisor, FAO Regional Office for Asia and the Pacific, Bangkok, Thailand; e-mail: Simmathiri.Appanah@fao.org