NO, 3652 ISOPODA AND TANAIDACEA—MILLER 35 
Bermuda and transport to the Antillean region, probably by boats— 
seems less plausible. A crucial point, which can be determined only by 
more collections, is how well and widely is it established in other parts 
of the Antillean region. 
EsTABLISHMENT OF INTRODUCED SPECIES.—The above-cited ex- 
amples leave little doubt that isopods and related crustaceans can and 
have been dispersed considerable distances by various means and that 
the transported species have become established in new localities. As 
Hedgpeth (1957) states, establishment requires some minimum popu- 
lation, as well as ecologically favorable conditions. 
Enough immigrants of both sexes or a sufficient number of females 
carrying young in the brood pouch must be introduced to sustain a 
colony. How many individuals would be required is problematical, but 
probably more than a single pair or a single ovigerous female would be 
needed. Predators such as fish easily might wipe out the immigrants or 
reduce their numbers below the minimum sustaining level. The aliens 
might not be able to compete with or outbreed native species occupy- 
ing the same ecological niche. On the other hand, absence of predators 
or a selective advantage of some sort over competitors would favor 
establishment of the introduced species. In any event, chance would 
play an important role in establishment. 
One established, introduced species may radiate into different, 
perhaps unoccupied, ecological niches in the new region. Segregated, 
populations may undergo speciation as a consequence of reproductive 
isolation, mutation, genetic drift, and natural selection. Many of the 
taxonomic difficulties encountered in zoogeographical studies may be 
attributed to differentiation of local, isolated races, perhaps incipient 
species. 
Ecological conditions in the new environment must be favorable 
not only for survival but also for reproduction of the introduced species. 
As Hutchins (1947) points out, within the survival limits for any 
ecological factor, there is a somewhat narrower range of conditions 
over which reproduction and repopulation can be completed. For 
temperature, he shows that maximal and minimal values for survival 
and/or reproduction are the critical parameters limiting the north- 
south distribution of many species. That temperature is an important 
limiting factor for isopods is evident from the latitudinal zonation 
shown by many species and even higher taxa. 
Likewise, extremes (more so than averages) of other factors such as 
salinity may be limiting. Whether or not these conditions operate 
directly on the survival and reproduction of the animals themselves, 
or indirectly through their influence on other organisms on which 
