Stratifying the reserve into three strata based on these forest types and sampling separately in each stratum will yield two advantages. The ability to make habitat specific density estimates may be important for guiding management or protection measures. Second, if density really does differ by forest type then samples within each forest type should cluster more tightly around their respective stratum means than they would around a pooled mean for all forest types. Thus, making separate density estimates for each stratum should produce a more precise estimate of elephant abundance for the entire reserve than making a single estimate for all forest types. Any environmental factor can be used to define sampling stratum, including general spatial location, habitat type, or proximity to points of high human activity.
Stratification can also be temporal.
For instance, samples collected during the dry season might form one stratum with samples from the wet season constituting a second stratum. Sometimes during the course of a study you come to suspect that a certain environmental factor that you had not recognized or been aware of beforehand has an influence on density. However, you should make separate density estimates for each stratum only if an approriate statistical test shows that the samples from that stratum differ significantly from other strata. A test of significance is necessary because small samples taken from exactly the same underlying density can differ substantially. Only stratify if a statistical test indicates that the difference between strata is so large that it could not be due simply to sampling error.