Much financial activity is zero-sum. While providing transactional and diversification services to others, participants also prey upon each other. High-ability predators trade opportunistically with less-able prey. In our dynamic model these features amplify real shocks. The presence of more low-ability traders reduces expected losses to high-ability traders, leading to equilibria with high levels of financial activity and employment. Shocks to profits can motivate exit by low-ability traders, rendering those of intermediate skill more vulnerable. Thus, our relatively simple model generates boom-bust dynamics suggestive of Wall Street. (JEL G00, G20, E44)