A fundamental problem in evolutionary ecology research is to explain how different species coexist in natural ecosystems. This question is directly related with species trophic competition. However, competition theory, based on the classical logistic Lotka-Volterra equations, leads to erroneous conclusions about species coexistence. The reason for this is incorrectly interpreted interspecific interactions, expressed in the form of the competition coefficients. Here I use the logistic Lotka-Volterra type competition equations derived from the so called resource competition models to obtain the necessary conditions for species coexistence. These models show that only species with identical competitive abilities may coexist. Due to such relations between competing species ecosystems biodiversity decreases in the course of evolution.