to trace the origins of particular invasive species. An exam-

ple is the demonstration that the Cuban anole (Anolis sagrei)

in Florida must have undergone multiple introductions be-

cause many locations in Florida have a greater diversity of

mtDNA haplotypes than does any one location in Cuba

[38]. The same study was able to show that invasions by

this lizard of Hawaii and Taiwan must have arrived from

Florida rather than Cuba. This particular study did not

demonstrate that the multiple origins had consequences for

the invasion. However, for reed canary grass (Phalaris arun­

dinacea) in North America [40] and the multicolored lady

beetle (Harmonia axyridis) in North America and Europe

[47], similar genetic analyses show that, as noted above,

hybridization between individuals introduced separately

from different regions can produce more invasive geno-

types. Hybridization between a native and an introduced

oomycete is responsible for alder blight (Phytophthora alni),

a new pathogen that is killing alders (Alnus spp.) through-

out Europe [113].

In general, the plethora of genetic studies on invaders has

detected far more multiple introductions than had been sus-

pected as well as frequent hybridizations between popula-

tions introduced from different regions. These studies also

revealed that hybridizations between introduced and related

native species occur more often than previously assumed

based on simple morphological analyses. The frequency of

multiple introductions at least partly resolves the “paradox of

invasion genetics.” That is, it has long been noted that al-

though very small populations are frequently presumed to be

endangered by genetic deterioration, engendered by genetic

drift and inbreeding-induced genetic depression [2,28],

many strikingly successful invasions have originated from

very small propagules, which greatly reduced genetic varia-

tion by virtue of the “bottleneck effect” [75]. However, we

now know that some introduced populations, such as the

Florida populations of the Cuban anole, have greater genetic

variation than any one native population, thereby hindering

the expected genetic deterioration [66].

Many introduced populations have evolved morpholog-

ically in their new homes. A remarkable example is the Old

World fruit fly Drosophila subobscura, introduced into west-

ern North and South America. Old World populations have

a pronounced latitudinal cline in wing length whereas in

North American populations no wing length cline was de-

tected ten years after introduction of the species; but after

20 years a cline had evolved that largely converged with the

Old World cline [35]. However, different sections of the

wing are responsible for the cline in North American vs. Old

World populations. Thus, the evolution of geographic varia-

tion in wing length was predictable, but expression of the

genes by which the cline was achieved depended on other

factors. Introduced South American populations also rapid-

ly evolved a cline of increased wing length with increased

latitude, but a different section of the wing is responsible for

the cline in South American than in either North American

or Old World fruit flies. Furthermore, many traits other

than morphology have evolved in introduced populations,

including changes in life history, physiology, and behavior

[16]. Perhaps best known to the public are the many cases

in which insects have evolved resistance to insecticides, ei-

ther physiological changes to tolerate or detoxify the chemi-

cal or behavioral changes to avoid it [67,108]. Native spe-

cies sometimes also evolve very quickly in response to

invasions [91]. For instance, after introduction of the preda-

tory green crab (Carcinus maenas) to the Atlantic coast of

North America, the dog whelk (Nucella lapillus), a native

prey species, evolved thicker shells [100].

The explosion of research publications on invasions has

led to a proliferation of formal meta-analyses of that litera-

ture—as the method became known outside the field of

medicine [11]—particularly regarding the first two ques-

tions of the SCOPE agenda: what determines the invasive-

ness of species and the invasibility of sites or habitats (e.g.,

[43,44,102]). However, it seems unlikely that such efforts

will advance our understanding of invasions substantially for

two reasons. First, particular invasions are highly idiosyn-

cratic such that a fundamental requirement of meta-analysis

is violated: the different studies can by no means truly be

viewed as replicate tests of the same hypothesis. Second, this

same idiosyncrasy implies that an effect size in an analysis in

one case will have limited predictive value for an invasion by

the same species or type of species in another. This is most

clearly shown by the fact that a single species can be highly

invasive at one site and either fail utterly or have minor im-

pact at another [114]. What is needed most to advance our

understanding of invasions is not the study of effect sizes but

of actual effects, on the ground and in a multitude of cases

[74]. Unfortunately, this sort of research is largely in the tra-

dition of detailed natural history at the community level,

which has fallen from academic favor precisely because com-

munity dynamics are too variable and idiosyncratic [41]. Yet,

even though community studies are highly idiographic, they

are precisely what is needed if we are to understand and suc-

cessfully address many environmental and conservation is-