 Lauri Oksanen argued that the top trophic level in a food chain increases the abundance of producers in food chains with an odd number of trophic levels (such as in Hairston, Smith and Slobodkin's three trophic level model), but decreases the abundance of the producers in food chains with an even number of trophic levels.Additionally, he argued that the number of trophic levels in a food chain increases as the productivity of the ecosystem increases.Although the existence of trophic cascades is not controversial, ecologists have long debated how ubiquitous they are.