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John M. Hranitz et al. 
diversity could be caused by genetic bottlenecks, most likely due to 
founder’s effects, such as only small immigrants colonizing the is- 
land (Carlquist 1974) as might occur simply by chance or because 
smaller body size confers an energetic or cryptic advantage during 
dispersal. This hypothesis can also explain why larger island than 
mainland conspecifics are observed in other instances. 
An inverse relationship between size and abundance of toads 
on the mainland and the island could occur if the growth of toads 
on the island was stunted while “ecological release” (Soule 1966) 
permitted the species to be abundant in response to the depauperate 
herpetofauna of Assateague Island (Lee 1972). Size frequency distri- 
butions of toads at each location in both years suggest that this is 
true. Factors that can cause stunting on the island include intraspe- 
cific competition, which may be the predominant biotic factor con- 
trolling population size considering the depauperate herpetofauna of 
the island, and physiological stress caused by fluctuating environ- 
mental factors (Parsons 1990). The more important factors likely 
include salinity of impoundments, availability of water on the is- 
land, and quality of habitats at each location. Instraspecific compe- 
tition and physiological stress are consistent with the food supply 
hypothesis (Carlquist 1974) because each involves a limiting envi- 
ronmental factor that either favors or produces individuals with small 
body size. 
The island and mainland demes could have different age struc- 
tures, with mainland toads being older and larger, and island toads 
being younger and smaller. The lack of sexual dimorphism and the 
overall increase in mean body size in each deme from 1988 to 
1989 is similar to growth of young cohorts of B. calamita (Boomsma 
and Arntzen 1985). These results suggest that both demes are com- 
posed of young toads, although the demes might not be the same 
age or have the same proportion of toads from different cohorts. 
This concept is not consistent with any of the ideas of Carlquist 
(1974) but is consistent with an explanation for body size variation 
in Thamnophis sirtalis and Nerodia sipedon in the Lake Erie area 
(King 1989). 
Studies reporting differences between island and mainland con- 
specifics often attribute differences between populations to unique 
features of islands (Carlquist 1974, Gibbons et al. 1979). In some 
cases, differences between island and mainland populations could, in 
fact, be artifacts of sampling two distinct populations, an opportu- 
nity that does not always present itself in continuous or adjacent 
mainland populations. Studies by Oldham (1985) and King (1989) 
