Human and ecosystem health converge around biological diversity issues. Cultivated and wild plants as food and medicine make essential contributions to human health, which in turn provides rationales for conservation. While wild and cultivated plant diversity reasonably facilitates dietary diversity and positive health outcomes, the challenges of demonstrating this relationship limit its impact in concept, policy, and practice. We present a rationale for testing the dietary contribution of biological diversity to improved eye health as a case study based on existing phytochemical, pharmacological, and clinical knowledge. We consider the empirical evidence needed to substantiate, interpret, and apply this relationship at a population and ecosystem level within a unified research framework. Epidemiological data strongly support the prevention of childhood vitamin A deficiency blindness, cataract, and age-related macular degeneration by fruit and vegetable consumption. Phytonutrients, including the carotenoids lutein and zeaxanthin, protect the eye from oxidative stress and harmful light exposure. Laboratory, community, and population level research should prioritize food composition of dietary plants from both agriculture and the wild. Intervention studies, focus groups, and transmission of knowledge of local species and varieties within communities will further interpretation of epidemiological data. Population-based studies combining clinical data and measures of access and consumption of biological diversity are key to demonstrating the important relationships among biodiversity, dietary diversity, and health outcomes.