By Achim Steiner, UN Under-Secretary General and Executive Director, UN Environment Programme (UNEP)
22 May 2009 - International Biological Diversity Day 2009 spotlights perhaps one of the least know threats to biodiversity and economies-alien invasive species.
Some governments such as New Zealand are facing up to the challenge with tough customs controls on foreign plants and animals.
Others such as South Africa have well-funded eradication programmes. But far too many countries have failed to grasp the threat or are far too casual in their response.
This is a mistake of profound environmental and economic proportions. By some estimates alien invasive species may be costing the global economy $1.4 trillion or more while representing a further challenge to the poverty-related UN Millennium Development Goals.
So where are they coming from? In HG Wells' celebrated sci-fi saga, "The War of the Worlds" aliens invaded in space ships to wreak havoc and mayhem.
In the real world they are spread from one Continent to another via the global agricultural, horticultural and pet trades or by hitchhiking lifts in ballast water and on ship's hulls.
Free from natural predators and checks and balances, alien invasive spaces can explode in numbers in their new homes ousting native species, clogging waterways and power station intakes, bringing novel infections including viruses and bacteria, poisoning soils and damaging farmland.
Take water hyacinth. A native of the Amazon basin, it was brought to Continents like Africa to decorate ornamental ponds with its attractive violet flowers.
But there is nothing attractive about its impacts on Lake Victoria where it is thought to have arrived in around 1990 down the Kigera River from Rwanda and Burundi.
Hyacinth can explode into a floating blanket, affecting shipping, reducing fish catches, hampering electricity generation and human health.
Annual costs to the Ugandan economy alone may be $112 million. The plant has now invaded more than 50 countries world-wide.
In sub-Saharan Africa, the invasive witchweed is responsible for annual maize losses amounting to US$7 billion: overall losses to aliens may amount to over $12 billion in respect to Africa's eight principle crops.
The challenge is both a developed and developing economy one and the more scientists look at the issue the more concerned they become.
In the United States researchers believe they now know why a weed from Europe - garlic mustard - is damaging native hardwoods. The alien produces a poison which kills native fungi which the trees need to grow.
The scale is perhaps only now unfolding. One of the most comprehensive Continent-wide assessments to date has just been completed.
The Delivering Alien Species Inventories for Europe or DAISIE says there are now 11,000 invaders in Europe of which 15 per cent cause economic damage and threaten native flora and fauna.
Globalization and international trade will, when the economy recovers, increase the chances of new aliens to travel from one part of the world to another.
Meanwhile climate change is likely to favour some alien species currently constrained by local temperatures.
Scientists have termed them 'sleepers'-foreign agents who become embedded in a community to be activated some years later. Introduced rainbow trout into the UK is a case in point.
In War of the Worlds the Martians are defeated by an Earthly infection - perhaps a bout of flu - to which they have no resistance. Real world aliens are often made of sterner stuff.
Improved international cooperation through the UNEP-linked Convention on Biological Diversity is needed and stepped up support for the Global Invasive Species Programme.
Important too to boost the capacity of the responsible national customs and quarantine agencies, especially in developing countries and to accelerate via the UN's International Maritime Organization.
Preventing alien species entering a new country is going to be demonstrably cheaper than the cure of trying to eradicate a well-entrenched species.
Alien invasive species have for too long been given a free ride - raising awareness among policy-makers and the public and accelerating a comprehensive response is long overdue.
The DAISIE network of scientists is clear. Inaction "is becoming increasingly disastrous for Europe's biodiversity, health and economy". They could be speaking for the whole world.