Population ecology mainly involves calculations of population size and contents. Thus, firstly get acquainted with theory and then do some practice.
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Population Ecology practice problems
Answers to Population Ecology practice problems
Big thanks to http://www-pvhs.stjohns.k12.fl.us/teachers/lyonss/S07678DCE.55/Population%20Ecology%20Practice%20Problems%20%28ABrokaw2012-13%29.pdf