Which method of classification requires that each taxonomic group be monophyletic

Cladistics

Explanation:

Cladistics is the most scientific method used to classify organisms that are categorized into distinct groups or 'clades'. The cladistic approach is derived from the studies made by the entomologist Emil Hans Willi Hennig (1913-1976). This methodology is based on the identification of the most recent common ancestor (MCRA), where organisms are classified based on shared traits (i.e., how similar they are). By using this method it is possible to draw a cladogram, i.e, an evolutionary tree that exhibits the ancestral relationships among organisms.

as resources become scarce because of population growth, the growth decreases after few years due to lack of resources and food.