Population size and density of green mambas were estimated by capture-mark-recapture procedures in a moist rainforest area of southeastern Nigeria (Eket, Akwa-Ibom State). The surface of the area used for the capture-mark-recapture procedure was 100 ha. The capture-mark-recapture study lasted 109 days, from early June to late September 1997. Snakes were individually marked by ventral scale-clipping, and painted with a white number on the back for further observations at distance without necessity of recaptures. We considered as “recaptures” (second sample) all of the later sightings of marked (i.e. dorsally painted) individuals, when at least 10 days from the date of the first capture had passed.
Population density was calculated by applying the Lincoln-Petersen index, with its relative 90% confidence limits (Caughley, 1977; Seber, 1982). For population size estimates, only adult specimens were considered. The Lincoln-Petersen index was used under the assumption that our mamba population was nearly without emi/immigrants. In fact, the study area is surrounded in part by a large river (Kwa-Ibo River) and in part by much deforested areas, that are hardly crossed by green mambas.
The effects of the macro-environmental parameters on the presence/absence of D. jamesoni in the study region were assessed by using a logistic regression model (forward stepwise conditional procedure) for discrete values (Hosmer and Lemeshow, 1989). For this analysis, we used data of presence/absence of the green mamba in 52 study sites which were accurately surveyed during our environmental works (Anonymous, 1998). The surface of each site was approximately 50 ha, and was separated from the closest surveyed site by at least 10 km of linear distance. Eight macro-environmental parameters were identified during these large-scale environmental studies (see Anonymous, 1998), and we checked for the eventual presence of any of them in each of the 52 sites surveyed. The designed macro-environmental parameters were as follows:
(1) primary dry forest (PDF); (2) secondary dry forest (SDF); (3) shrubland (SHL); (4) primary swamp-forest (PSF); (5) secondary swamp-forest (SSF); (6) mangrove (MGR); (7) farmland and plantation (FPL); (8) large water body (main river tract or wide lake, PWB).
The place-names, the geographic coordinates, and the macro-environmental parameters of each of the 52 study sites are omitted in this paper to save space, but are fully presented in Luiselli and Politano (1998).
In the logistic regression model, the study areas were the cases (total n = 52), the presence/absence of D. jamesoni was the dependent variable, and the environmental parameters were the covariates (total n = 8).
All data were statistically processed by means of a SPSS (version 4.5, for Windows) personal computer package, with all tests being two-tailed and with alpha set at 5%.