in turn eat kelp. When otters are removed, urchin populations grow and reduce the kelp forest creating urchin barrens. This reduces the diversity of the ecosystem as a whole and can detrimental effects on all of the other organisms. In other words, such ecosystems are not controlled by productivity of the kelp, but rather, a top predator. One can see the inverse effect that top-down control has in this example; when the population of otters decreased, the population of the urchins increased. Bottom-up control in ecosystems refers to ecosystems in which the nutrient supply, productivity, and type of primary producers (plants and phytoplankton) control the ecosystem structure. If there are not enough resources or producers in the ecosystem, there is not enough energy left for the rest of the animals in the food chain because of biomagnification and the ecological efficiency. An example would be how plankton populations are controlled by the availability of nutrients. Plankton populations tend to be higher and more complex in areas where upwelling brings nutrients to the surface. There are many different examples of these concepts. It is common for populations to be influenced by both types of control, and there is still debates going on as to which type of control affects food webs in certain ecosystems. Philosophy and ethics Top-down reasoning in ethics is when the reasoner starts from abstract universalisable principles and then reasons down them to particular situations. Bottom-up reasoning occurs when the reasoner starts from intuitive particular situational judgements and then reasons up to principles. Reflective equilibrium occurs when there is interaction between top-down and bottom-up reasoning until both are in harmony. That is to say, when universalisable abstract principles are reflectively found to be in equilibrium with particular intuitive judgements. The process occurs when cognitive dissonance occurs when reasoners try to resolve top-down with bottom-up reasoning, and adjust one or the other, until they are satisfied they have found the best combinations of principles and situational judgements. See also The Cathedral and the Bazaar Pseudocode References cited https://philpapers.org/rec/COHTNO Citations and notes Further reading Corpeño, E (2021). "The Top-Down Approach to Problem Solving: How to Stop Struggling in Class and Start Learning". . Goldstein, E.B. (2010). Sensation and Perception. USA: Wadsworth. Galotti, K. (2008). Cognitive Psychology: In and out of the laboratory. USA: Wadsworth. Dubois, Hans F.W. 2002. Harmonization of the European vaccination policy and the role TQM and reengineering