Local homogeneity is a common assumption in ecology, which can lead to the aggregation of populations without regard for their unique needs and characteristics. Here we discuss whether this assumption is justified, and we hypothesize an approach to formulate an informed decision, by discerning the source of variation between plasticity and adaptation. We test this approach on plants, performing a common garden experiment to guide our choice discerning between the two at a local scale (~60 Km). Using functional trait measurements for three species, Quercus ilex, Pistacia lentiscus and Cistus salviifolius, hailing from three sites chosen along a local aridity gradient, we were able to reject the local homogeneity assumption for Q. ilex and C. salviifolius at this scale due to the adaptive divergence observed among neighbouring populations. We conclude that gathering empirical evidence can provide with solid bases to decide whether to adopt the local homogeneity assumption.