Biological invasion happens when an organism, any sort of organism, arrives somewhere beyond its previous range. Nowadays, most invasions come from human actions, deliberate or accidental. But natural invasions happen too, from minor changes of range to major invasions across continents. One reason for studying invasions is that many invasive species have become serious pests. The cumulative losses in the USA from some non-indigenous species were estimated at almost $100 billion by 1991. It has to be noted that most invaders fail and have small effects, but that the cumulative effect of those that succeed has been and will continue to be large. Another important fact is that invasions have been an important component of the evolutionary process throughout geological history. Researchers are therefore not only interested in what makes a successful invader, but also in the ecological as well as the genetic impacts of invasions.
When dealing with invasions one should realise the fact that they can be natural or due to human actions as mentioned above. This will be illustrated by some examples. The Great American Interchange, the progressive exchange of mammalian faunas, and their subsequent evolution, between North and South America, is an example of a major natural invasion. It was the result of tectonic movements that brought these continents close together and formed the Central American bridge between them, a process starting 10 million years ago in the Miocene, and possibly still underway. In the 19th century the pattern of colonisation and trade meant that introductions were predominantly from Europe. European settlers often set about recreating a European agricultural landscape, and started to introduce European species. Nowadays, the flow of commerce is much more widely spread, and faster, and so invasions travel in all directions. One example of this is the flow of marine organisms in ballast in ships.
The Scientific Committee on Problems of the Environment (SCOPE) ran for about ten years from 1982 and it tried to investigate the ecology of biological invasions. SCOPE showed that it is unusual for the course of a particular invasion to be predictable and so an approach was launched in which the study of invasions became statistical characterising the probability of outcomes for classes of invasions. A basic rule called the tens rule was introduced in 1989 to estimate how frequently invasive species establish and how frequently they become pests: 10% of feral (introduced) invaders, invasive species living outside captivity in any sense, become established, and 10% of those established become pests. This rule is very rough and pests are defined by human perception rather than by ecological effects. Studies have shown that tens rule applies to a variety of British groups as well as outside Britain, but there are also a number of cases in which the tens rule, or parts of it, clearly do not hold. This means that the tens rule does seem to be a good general guideline when looking at invasions, but that it needs to be interpreted with care.
Many statistical studies have been performed to investigate the properties of successful invaders for example by Griffith et al. The following factors have been suggested as predictors of invasion success: propagule pressure, suitability of habitat, previous success in other invasions, intrinsic rate of natural increase, reproductive and genetic characteristics, abundance and range in native habitat, taxonomic isolation, climatic matching, vacant niche and ancestral habitat.
Increasing the number of propagules increases the chances of a species establishing because:
- few individuals introduced may disperse and not meet again i.e. no reproduction
- with more individuals there is a greater chance of finding a suitable habitat, or avoiding difficulties from weather, or of not bringing in parasites or pathogens.
- general predators or herbivores can easily destroy small initial populations.
An example for propagule pressure is the common starling, a western Palaearctic species which appears to have increased in numbers in Western Europe in the 19th and 20th century. There has been a general decline in Europe since about 1960. the common starling has been introduced into North America, South Africa, Australia, New Zealand and various other islands, but not always successfully. It has been argues that unsuccessful introductions were due to the size of the inoculum which was critical. Starlings in America have only one third of the species of parasite found in European birds, but there do not seem to be advantages of this. It might be the case that the birds who were successfully established might have been more free of parasites than others. There are several data sets showing that increasing the number released has a steadily increasing effect on the probability of establishment. Some studies have shown both the importance of propagule pressure and that the pressure comes from human introductions in most cases. In nature reserves both in South Africa and in North America there is a striking positive linear relationship between the number of introduced species found and the logarithm of the annual number of visitors. Previous success in other invasions certainly is a good indicator of whether an invader will succeed in a new place, but this does not seem true for related species and it has to be considered.
A population introduced into a new suitable habitat will increase exponentially. The population will settle down to increase at a rate r when it has also settled down into a stable age distribution. r is the maximum rate of increase under certain specified conditions. It seems natural to many ecologists to assume that a population that reproduces faster than another will be a better invader. The actual evidence that establishment depends on r is thin and contradictory. What is certain is that some successful invaders have rather low rates of increase e.g. Homo sapiens has a maximum rate of increase of only 3% per year. Nevertheless, when trying to predict which of several related species is the most likely to be a successful invader, and particularly is the most likely to become a pest, r may be relevant.
There is also an assumption that successful invaders have definable genetic characters such as being inbreeders or asexual, polyploid, heterozygous, etc. It has been found that successful invaders may have any one of a large number of suites of characteristics. In fact, some successful invaders are without sexual reproduction (plant examples) and these suggest that genetic variation is irrelevant to invasion success. It has to be noted that some invasions seem to have gone along with a progressive loss of alleles, presumably from founder effects. As far as predicting success of invasion in general, genetics for the moment has little to offer, but under standing may be useful in some cases.
There may be a relationship between invasion success and the abundance of a species in its native range, or the size of that range, or both. A positive relation of range size and invasion success in passerines in Hawaii has been found. Crawley found for both range and abundance that insect species that are widespread in their native lands are significantly more likely to become established than species with local or patchy distributions. In general there seems to be a weak relationship between abundance and range. It seems that abundance is the important indicator of a good invader, but that range is simply a correlated variable. This has to be considered with care though as evidence from biological control has shown that abundance can be a bad predictor due to the role of specific enemies. A relatively rare species, rare due to enemies, may perform excellently as a control agent once freed of those enemies.