Walker is a plant ecologist interested in the mechanisms that drive primary plant succession. Primary succession can be defined as directional changes in plant communities after severe disturbances that leave no soil layers intact. Ecologists have been studying succession for about 100 years but are still in disagreement about what causes it, and even if it exists! Attempts at general theories that will explain successional change are useful exercises but are not readily applicable to specific examples of succession. His approach to this dilemma is to examine many examples in different systems, using similar methods, and then to look for similarities or differences between ecosystems.