WWF and IUCN: 

Effects: immense, insidious, irreversible

IUCN, the World Conservation Union, states that the impacts of alien invasive species are immense, insidious, and usually irreversible. They may be as damaging to native species and ecosystems on a global scale as the loss and degradation of habitats.

Hundreds of extinctions have been caused by alien invasives. The ecological cost is the irretrievable loss of native species and ecosystems.

WWF

The worldwide movement of non-native organisms damages the environment and endangers native species, undermining decades of conservation efforts. Because federally regulated international trade is the source of most invaders, we must reverse some U.S. policies and improve others.

What you can do to help

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the UN and the Global Biodiversity Assessment:

"In continental terrestrial ecosystems, the most important mechanism [by which the underlying causal factors result in the reduction and loss of populations, the extinction of species and the transformation and degradation of ecological communities] is the loss, fragmentation and degradation of habitat. On islands, species introductions and habitat loss have been equally important mechanisms. In oceans, overharvesting and pollution are the most important factors. All of these mechanisms have been major influences on species and populations in freshwater ecosystems." 

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The Australian Conservation Foundation has a good site for kids titled "Feral Peril."

http://www.acfonline.org.au/default.asp?section_id=164

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Global invasive species program:

http://www.gisp.org/

a personal experience, and why bells don't work:

http://www.matrifocus.com/LAM02/earth.htm

Global invasive species database:

http://www.issg.org/database/species/search.asp?st=100ss&fr=1&sts=sss

"Invasive species have been recognised globally as a major threat to biodiversity (the collected wealth of the world’s species of plants, animals and other organisms) as well as to agriculture and other human interests."

United Nations convention on biodiversity:

http://www.biodiv.org/convention/articles.asp

Global biodiversity assessment:

http://www.dhushara.com/book/globio/ass.htm

An underlying causes of the loss and degradation of biodiversity is failure of people to consider the long-term consequences of their actions, often due to a basic lack of knowledge.

The specific mechanisms by which the underlying causal factors result in the reduction and loss of populations, the extinction of species and the transformation and degradation of ecological communities include the introduction of non native species.

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earthwatch:

http://earthwatch.unep.net/emergingissues/biodiversity/invasivespecies.php

Invasive species constitute a global-scale problem affecting inter alia health, agricultural potential, and biodiversity

The control of invasive species can be difficult, if not impossible, making prevention particularly important. The invasion of alien species continues to cause biodiversity loss and can have negative economic impacts. Despite the fact that a considerable amount of information now exists on invasive species, that information is not easily accessible.

Eradication, control, mitigation of their impacts combined with legislation and guidelines at national, regional and international levels are some of the ways that the Convention on Biological Diversity (CBD) is addressing the issue of Invasive species. Paragraph h of Article 8 (In-situ conservation) of the Convention notes that "Each Contracting Party shall, as far as possible and as appropriate, ….prevent the introduction of, control or eradicate those alien species which threaten ecosystems, habitats or species;…". The Subsidiary Body on Scientific, Technical and Technological Advice (SBSTTA) of CBD identified alien species that threaten ecosystems as a priority issue

According to the UN global biodiversity assessment, a peer reviewed scientific publication, introduced species contribute just as much as habitat destruction to biodiversity loss in many areas. This is a huge problem that is often underestimated or deliberately played down.

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UN Convention on biological diversity:

http://www.cbd.int/invasive/default.shtml

Alien species that become invasive are considered to be a main direct driver of biodiversity loss across the globe. In addition, alien species have been estimated to cost our economies hundreds of billions of dollars each year.

http://www.cbd.int/invasive/problem.shtml

Increasing travel, trade, and tourism associated with globalization and expansion of the human population have facilitated intentional and unintentional movement of species beyond natural biogeographical barriers, and many of these alien species have become invasive. Invasive alien species (IA are considered to be one of the main direct drivers of biodiversity loss at the global level^{1, 2}. It is clear that IAS can produce substantial environmental and economic damage, and their negative effects are exacerbated by climate change, pollution, habitat loss and human-induced disturbance. Increasing domination by a few invasive species increases global homogenization of biodiversity, reducing local diversity and distinctiveness.

IAS can change the community structure and species composition of native ecosystems directly by out-competing indigenous species for resources. IAS may also have important indirect effects through changes in nutrient cycling, ecosystem function and ecological relationships between native species. IAS can also cause cascading effects with other organisms when one species affects another via intermediate species, a shared natural enemy or a shared resource. These chain reactions can be difficult to identify and predict. Furthermore, aggregate effects of multiple invasive species can have large and complex impacts in an ecosystem.

Invasive species may also alter the evolutionary pathway of native species by competitive exclusion, niche displacement, hybridization predation, and ultimately extinction. IAS themselves may also evolve due to interactions with native species and with their new environment.

IAS can directly affect human health. Infectious diseases are often IAS imported by travellers or vectored by exotic species of birds, rodents and insects. IAS also have indirect health effects on humans as a result of the use of pesticides and herbicides, which infiltrate water and soil.

http://www.actionbioscience.org/biodiversity/simberloff.html

http://www.dhushara.com/book/globio/ass.htm

In continental terrestrial ecosystems, the most important mechanism is the loss, fragmentation and degradation of habitat. On islands, species introductions and habitat loss have been equally important

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