In the Philippines, lowland farmers are migrating to the uplands in increasing numbers, placing themselves in a situation of potential competition with upland tribal populations for natural resources in the area. How do the tribal populations whose subsistence strategies are affected by this situation adjust? This question bears directly on the tribal populations' subsistence styles, especially their utilization and management of increasingly scarce resources. In addition, how does this lead to ethnic differentiation, or the dissolution of ethnic markers? This concerns the process of maintaining or eliminating ethnic boundaries that may help groups preserve control over, or increase access to, various resources. These are questions that interest Philippine anthropologists today. In this chapter, I will compare two Philippine Negrito groups, the Ata and the Batak, who live in different social and physical environments. Both groups were originally nomadic forest foragers, but they have evolved very different subsistence styles in recent years. Taking into account their existing environments, the differences in their subsistence styles in adjusting to migration of lowlanders to the uplands will be explained, and the theoretical implications of the findings for ethnic diversity examined.