Reliability of Trend Lines Obtained from Variable Counts

Standard errors (SE) of trend lines arising from highly variable counts of animals are derived analytically given the variability of the counts, the number of years of trend monitoring, the number of replicate counts each year, and certain assumptions. Examples of the analytical solution's use and graphs of expected trend line variability are presented. When counts are highly variable, performing multiple counts each year is shown to be the only way to achieve precision of a population trend estimate within a short (100%. Goddard's (1967) 18 replicate counts of black rhinoceros (Diceros bicornis) have a CV of 42%. A CV ₅0% is not uncommon with counts of rare species, such as grizzly bears; e. g. , Reynolds (1974). Assuming lognormality, the a2 required for equation (3) is: 0. 8