Thomas W. Schoener and David A. Spiller

 This passage is adapted from Thomas W. Schoener and 
David A. Spiller, "Trophic Cascades on Islands." ©2010 by 
Island Press
In the 1970s, one of us visited more than 500 
Bahamian islands to survey distributions of 
vertebrates, with special emphasis on lizards and 
birds. A key objective was to determine the threshold 
island area on which vertebrate populations could just 
survive. We were astonished to find lizards, 
particularly Anolis sagrei, on some tiny islands, a 
discovery that multiplied by at least two orders of 
magnitude the list of Bahamian islands surmised or 
known to have resident populations of vertebrates. 
We realized we had to check many quite small islands 
to determine such thresholds, and in the course of 
that endeavor we came upon a large number of 
islands without lizards. This led to a second, even 
more exciting discovery: Such islands sometimes had 
extraordinarily high densities of spiders, the 
omnipresent webbing giving them the appearance of 
the proverbial grandmother's attic.
In 1981, we had to investigate this phenomenon 
systematically for the many small islands in the 
central Bahamas near the relatively large island of 
Staniel Cay, a major stopover in our earlier survey. 
Our first study found that spiders were about an 
order of magnitude denser on no-lizard than lizard 
islands (adjusted for the positive and negative 
correlations with area and distance from large 
landmasses, respectively). A second observational 
study in 1982 examined numbers of spider species, 
finding that no-lizard islands had 1.5-2 times the 
number of species as had lizard islands (again 
adjusted for area and distance, and for the maximum 
height attained by the vegetation on the island, which 
correlated positively with number of spider species). 
This result was quite different from Paine's (1966) 
famous one in the rocky intertidal, in which diversity 
increased increasing predation, and it presaged other 
such results for terrestrial arthropods in our system 
and in others also.
Such comparative data pointed to a strong 
negative effect of lizards on spiders, but as is true of 
all comparative studies, the observations did not 
suffice to eliminate alternative hypotheses about why
islands with and without lizards might differ. A more 
reliable investigation would be experimental, and 
toward that end we staked out nine approximately 83-
square-meter plots on Staniel Cay in 1985. Three of 
the plots were unenclosed, and the others had wood-
framed fences made of hardware cloth topped with 
smooth plastic to impede lizard locomotion in and 
out. Three of the enclosed plots were randomly 
chosen to maintain lizards at natural densities, 
whereas the other three had lizards removed. Thus we 
had three treatments: The two types of enclosed plots 
tested the lizard effect, and the unenclosed plots were 
a cage control, to be compared with the enclosed 
lizard plots. The 18-month experiment showed that 
lizard removal enclosures had spider densities three 
times higher than those control enclosures and the 
unenclosed (also having lizard) plots. Numbers of 
spider species were higher without lizards as well, in 
parallel to the comparative studies. Numbers and 
biomasses of insects caught in the sticky traps were 
also higher in lizard removal enclosures; therefore, an 
increase in spiders did not completely compensate for 
the absence of lizards. There was some effect of the 
enclosures: Sticky traps in the enclosed plots caught 
about 20 percent fewer arthropod individuals than 
those in open plots.
 What was the mechanism of the (now firmly 
established) lizard effect on spiders? The obvious one 
is predation. However, a second is competition for 
food: Spiders consume prey large in relation to their 
own size, so lizards and spiders might overlap in prey 
size well beyond their relative body sizes alone.

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