The study group has always been the center of the Community, the glue that held these different characters together. But "Competitive Ecology" presents the group in a new light where these personalities actually clash. Whether the inclusion of Todd was the main reason for the group blowing up is up for debate, but it was very funny to see all of them go at it.
The Chang plot had nothing to do with the rest of the episode, but it filled up time and was pretty clever, making him into one of those fictional detectives who make random jumps in logic to find the conclusion. Even though there wasn't much sense to the plot, I enjoyed it, especially when he was playing the saxophone.