The continued conversion and development of forest land pose a serious threat to the ecosystem services derived from forested landscapes. We argue that developing an understanding of the full range of consequences from forest conversion requires understanding the effects of such conversion on both components of ecosystem services: products and processes. However, there are unavoidable challenges involved in quantifying the threats from forest conversion and their related costs to human well-being. First, most attempts to quantify the costs of forest conversion on ecosystem services will necessarily rely on specific ecological science that is often emerging, changing, or simply nonexistent. For example, the role that many species play in ecosystem processes is poorly understood. Second, given the interconnected nature of ecosystem products and processes, any attempt to quantify the effects of forest conversion must grapple with jointness in production. For example, the cost of losing a species from forest conversion must account for that species' role as both (1) a product that directly contributes to human well-being, and (2) as a component in an ecosystem process. Finally, the ecology and the human dimensions of ecosystems are highly specific to spatial-temporal circumstances. Consequently, the effects of forest conversion in one spatial-temporal context are likely to be quite different than effects elsewhere.