Ecological theory recognizes the importance of the variety of species
for maintaining the functioning of ecosystems and their derived
services. In this context, adaptive changes in functional traits plays a
crucial role. We assert that when studying the effects of shifts in
biodiversity levels using mathematical models, they must be
scale--invariant, i.e., should be sensitive to the variety of species
traits but not to raw species numbers. We present a testing procedure
for verifying scale--invariance of eco--evolutionary network models
expressed as ODEs. Furthermore, we applied our test to several
influential models used for evaluating biodiversity effects on ecosystem
functioning. In most of the surveyed studies the equations used failed
our test. This raises doubts about the validity of previous results and
calls for revisiting the theory derived from these studies. Our results
foster the creation of artifact--free models, a necessary step towards
building a more robust theory of ecosystem functioning.