Dr. Mooney entered the University of California at Berkley as a political science major, but was forced to halt his studies for economic reasons and accepted a job on a freighter traveling down the west coast of the Americas. While transiting the Panama Canal, he read in a magazine about being a collector for the United States Department of Agriculture's Plant Exploration unit, which lead to a major change in his career path. Dr. Mooney, who had taken a strong interest in plants through his activities in the mountains of California, was extremely attracted to an occupation in which he could both study plants and go on adventurous travels. So he transferred to the University of California's Santa Barbara campus, which had a plant ecology program.
In 1957, he researched the physiological processes of Arctic-Alpine plants over a vast natural range extending from Alaska to the Rocky Mountains. He studied photosynthesis and respiration of the plants using an infrared gas analyzer and equipment that he helped to design and was able to demonstrate the physiological basis for ecotypic differentiation by comparing the plants that were raised in controlled environments with naturally occurring plants. He showed that plants adapted their physiological processes to their local environments.
After he obtained his doctorate in 1960, he embarked on research into convergent evolution that showed that different plant species develop the same physiological characteristics in response to the same severe environments. He earned acclaim for demonstrating that similarities between different species were not limited to form, which had already been demonstrated, but also extended to function. He accomplished this by comparing the ecology and physiological characteristics of plants in the drought-limited Mediterranean climates in the geographically disparate California and Chilean coastal regions and Mediterranean Basin.
In the 1970s, he took a broader approach to examine not only carbon gain but carbon use by plants in an area of California ranging from the desert to the White Mountains and applied a cost-benefit approach to clarify how carbon resources are allocated to different sites in plants for photosynthesis, or various other functions. He had a significant impact on later studies into plant physiological ecology and advanced research into carbon gain and use in plants by showing in a detailed cost analysis how plants obtain carbohydrates and nitrogen, and how they distribute and store them to obtain the greatest effect with the lowest expenditure of energy.
Through these studies, Dr. Mooney showed how plant species and groups of species respond to their environments, thereby contributing to the theoretical framework of plant physiological ecology, and developed research methodologies for assessing how plants interact with their biotic environments. To date he has authored over 400 scientific books, papers and articles.
In the latter half of the 1980s, he pursued research into the effect of the invasion of different plant species on naturally occurring species under the auspices of the Scientific Committee on Problems of the Environment (SCOPE), setting up the first global evaluation of invasive plant species. He regarded the acceleration of problems related to invasive species due to increased international commerce with grave concern, recognized the need for joint research between naturalists and social scientists, and launched the Global Invasive Species Program with many international institutions as partners. Through such programs, he has brought awareness to the topic of the impact of human activities upon ecosystems through species introductions.
Dr. Mooney has played an international leadership role in recent years, especially with problems related to biodiversity and global warming. In addition, he has been active in building up worldwide communities and networks of ecologists and scientists in other disciplines and arranging international conferences on the environment. He played a central role in the International Geosphere-Biosphere Program (IGBP), building up an international organization of scientists and having an influential part in setting the guidelines for the formulation of environmental policies. He has advanced numerous international research programs as Secretary General and Vice-President of the International Council for Science (ICSU). Furthermore, he is working to solicit the interest of the general public in many scientific topics through the media and other channels. As president of the Ecological Society of America he helped launched the publication of a new journal called Ecological Applications that is intended to make use of ecology as a useful tool for management, and worked to promote the designation of the International Biodiversity Observation Year.
Dr. Mooney has demonstrated the importance of ecological studies in the research of changes in the global environment and helped to build the foundation for the field of global ecology. Now universities around the world are establishing global ecology research departments. He continues to work toward the development of new environmental sciences that will be required for the continued existence of humankind.