Invasive Alien Species: The Nature of the Problem

Harold A. Mooney

The increasing human population is altering the natural resources on which our societies depend to an ever-greater extent. Many of these changes are purposeful and to the benefit of society. Others, although purposeful, have inadvertent negative impacts on the goods and services that natural resources deliver to society. In order to manage these resources in a sustainable manner we must understand the interactions and trade-offs between resource alteration and the natural, generally renewable processes on which we depend. This book addresses one particular driver of resource alteration: alien species invasions. In aggregate, these invasions are global in extent and are having consequences that are generally unappreciated but quite threatening to many human activities.

The vast numbers of species that populate the earth provide innumerable goods and services that society values. Equally important for society are the services that natural systems provide free of charge (Daily et al. 1997). On the other hand, invasive alien species can represent "ecosystem bads and disservices" (as characterized by Madhav Gadgil, 2000: 16) to systems on which society depends. In this introduction I concentrate not only on how invasive aliens alter ecosystem properties but also more directly on their effects on goods and services valued by society. This information provides a backdrop to the chapters that follow, which focus on what to do about this pervasive problem.

For comprehensive overviews of the problem there are a number of recent summaries (Vitousek et al. 1997; Lonsdale 1999; Parker et al. 1999; Williamson 1999; Mack et al. 2000; D'Antonio et al., 2004), edited volumes (Sandlund, Schei, and Viken 1999; Mooney and Hobbs 2000), and popular books (Bright 1998; Devine 1998).

What Do We Know in General?

The reviews just listed give us some general conclusions about the status and impacts of alien species that can be summarized as follows:

• There has been a massive global mixing of biota.

• This mixing has been both purposeful and accidental.

• There has been both biotic enrichment and impoverishment in any given area (species view).

• A small fraction of alien species have become invasive.

• Invasive alien species come from all taxonomic groups.

We know with less precision the kinds of habitats in which invasive alien species are most successful, the traits of successful invaders, and the mechanisms of habitat degradation caused by invaders. We do know that invasive alien species have altered evolutionary trajectories, can disrupt community and ecosystem processes, are causing large economic losses, and threaten human health and welfare.

Character of the New Biotic World

It is easy to demonstrate that the nature of the biological world is very different from what it was before the age of exploration. The natural ecosystems that evolved in isolation on the various continents and large islands, constrained by biogeographic barriers such as oceans, have become functionally connected through the capacity of humans to transport biological material long distances in a short amount of time. The consequences of this biotic exchange are staggering when you tally up what the biotic world looks like now in comparison with the recent past. In Hawaii, a prime example of the onslaught of alien species, there are 3,500 more species of flowering plants and insects than there were before the age of exploration. Of the flowering plants, there are more alien species than endemics (Eldredge and Miller 1997). In California, more than 1,000 established alien plant species have been added to the approximately 6,300 natives (contrast this to the 64 plant taxa that are threatened or extinct) (Hobbs and Mooney 1998).

Looking across the world, Hawaii is at the extremes of biotic introductions, as are many other islands, and California probably is somewhere in the middle. However, a survey of the number of alien plants in various parts of the world shows that established alien species are everywhere. For plants alone, the following numbers of established alien species have been noted in the large continental areas of the Russian Arctic, 104; Europe, 721; tropical Africa, 536; southern Africa, 824; Canada, 940; continental United States, 2,100; Chile, 678; and Australia, 1,952. For islands, New Zealand has 1,623, the British Isles 945, and the Canary Islands 680. These numbers indicate the extent of the changes in biotic systems that have occurred. Similar numbers, at least proportionately to natural abundance, can be seen for other taxonomic groups (Vitousek et al. 1997).

How Fast Has All of This Happened?

The exchange rate of biological material across biogeographic barriers that have separated continents for millions of years has been extremely low until very recently. Similarly, climate has been fairly constant in recently millennia. However, both climate change, as driven by the changing composition of the atmosphere, and the large-scale intercontinental movement of biological material have greatly accelerated in recent times. To get a sense of the comparative rates of change in atmospheric composition over the past 200 years and in biotic composition caused by biotic introductions, one can examine the detailed records of the changing CO2 concentration of the atmosphere. In 1800 it was 280 ppm, and in 1990 it had increased to 354 ppm, an increase of 26 percent (Boden et al. 1994). In contrast, to use one well-documented case, the numbers of established alien arthropods in the United States grew from 50 in 1800 to 2,000 in 1990 (Sailer 1983; U.S. Congress 1993: 1263), a forty-fold increase! This is not to say that the potential global environmental...