In populations with size-structured dominance hierarchies, such as basilisk lizards, intraspecific competition may force smaller individuals into less preferred habitats where they experience greater predation. The size of the basilisk population at Corcovado National Park was nearly halved from 1998 to 1999, which created an opportunity to test a hypothesis that relates microhabitat use, sizestructured dominance hierarchies, and population dynamics. We surveyed the basilisk and avian predator populations along a river (Rio Claro) and two smaller streams. Our data, combined with data from two previous censuses, were used to estimate the age structures of each population and test whether size classes differed in their recovery from the population decline. Basilisks in all three populations used sheltered microhabitats more than expected based on availability, with the strongest preference occurring in the Rio Claro population, which also had the highest number of avian predators. We found a greater proportion of smaller individuals along the Rio Claro, but similar population structures along Streams 1 and 2, compared to 1999 surveys. This indicates that avian predation pressure exerts a stronger influence on the Rio Claro population. All populations declined from 1999 to 2000, and sensitivity analyses indicate that these populations will continue to decline unless the proportion of large individuals increases dramatically in the next few years.