One way of determining if snakes are present is to look at the populations of the prey. A decline over time could indicate snake predation. Long term studies are needed for this so that normal population fluctuations will not be mistaken for snake predation. The 1998 dry season, exacerbated by the El Nino, is a good example as many lizard species experienced a drop in population levels only to rebound after the rains came.

Species chosen - Anoles and Shrews

Anoles (Anolis carolinensis): Introduced to Guam in the 1950's for insect control. First documented in the CNMI (on Saipan) in the1970's. Data from Guam show that, when it is present, this lizard is a preferred food item, especially for the juvenile snakes. Probably due to the fact that it sleeps in the leaaves of trees and bushes at night making it easy prey for a nocturnal arboreal predator.

Shrews (Suncus murinus): Also introduced to Guam in the late1950's, albeit accidentally and since then habe make to Saipan and Tinian. When present they are readily eaten by snakes. Shrews also seem to have an effect on skink (ground lizard) populations by preying on these lizards. Skink densities on Saipan are much less than other shrew free islands. Interestingly, skink densities on Guam are much higher than on Saipan. The snakes eliminated the shrews from Guam possibly causing an increase in skink populations, which the snakes now prey on.

The monitoring method for both species is Mark-Recapture. A plot is marked off in the jungle and the animals are captured, marked and resleased. This is done for upto 12 capture events. The number of animals that are recaptured (have been marked previously) compared to those that are new captures (have not been marked) can beused to calculate a population estimate.