Predators can trigger defensive traits in their prey through a variety of mechanisms. We studied whether predators stimulate inducible defenses of sea urchins in temperate kelp forests in the Northern Channel Islands of Southern California. We hypothesized that inside of Marine Protected Areas (MPAs), where fishing is completely restricted or limited, urchins would be better defended than in fished areas due to the recovery of predators in the MPAs. We collected urchins from four management zones that varied in predator and resource abundance and measured variation in three defensive traits: load resistance, spine length, and test thickness. While we did not find strong differences in load resistance or test thickness between management zones, we did document significant variation in spine length. However, this variation was attributed to resource abundance but not predator abundance, suggesting that resource availability may interact with urchin behavior to influence this trait. Urchins develop longer spines when resource abundance is minimal because they must be exposed while searching for food, placing them at higher risk of predation even when predator abundance is low. Urchins also showed energetic trade-offs between distinct defensive traits and between defensive traits and reproduction, further suggesting that different habitat types impact how urchins allocate resources towards their morphology. Lastly, we found strong variation in gonadosomatic index and spine lengths between sampling zones directly adjacent to one another, demonstrating the plasticity and lack of mobility of purple urchins. Our findings suggest that the morphological responses of purple urchins to changing environments reflect the plasticity of these animals, and therefore, the challenges present in kelp forest community returning to natural states without direct human-management techniques.