"Indirect and Manifold Effects . . .," These are the words used by Dr. J. Alan Pounds as he described the biological response to climate change on a tropical mountain ecosystem (see Nature Vol 398; April 1999). His recent presentation at the monthly USGCRP Seminar Series on the extinction of the Golden Toad and the link to a rise in the orographic cloud bank in Costa Rica is an excellent example of the "indirect and manifold effects" that must be considered in assessing the consequences of climate change and variability on ecosystems.

While we tend to focus on the direct effects of climate change because establishing a stress-response relationship is at the heart of an assessment, much of the effects on ecosystems will be indirect and manifold; i.e., of many kinds and multiples. It is essential that as we assess the consequences of climate change on ecosystems that we explicitly build the notion that the effects are "indirect and manifold" into the design of the research, the collection of data, and the formulation of assessments. An example of the indirect and manifold effects involves non-native invasive species.

Climate change may result in a longer growing season, which may give some invading weedy plants time to flower and set seed where previously they could only spread asexually. This newfound ability to flower could have profound implications for not only the invader but for those organisms associated with the invading species. Certain invading weeds like cheatgrass (Bromus tectorum), which dominates vast areas of the American West, appear to adapt more quickly to rising levels of carbon dioxide in the atmosphere than native species.

We know that ecosystems already face a wide range of stresses: e.g., habitat destruction and fragmentation; loss of species; invading non-native species; changes in the nutrient cycles; and xenobiotic chemicals. Assessments need to be conducted that not only incorporate information on global environmental change, other stresses, ecosystem functioning and biodiversity, but that also include social and economic factors and responses. Perhaps the biggest challenge in conducting assessments on ecosystems is in answering the "So what?" question.

Most people agree on the value of protecting human lives and improving conditions of human health. Human health assessments deal with effects already familiar to the public, such as cancer rates and birth defects, and the debate rarely is about the importance of these effects. Ecological assessors, however, are faced with assessing effects (or endpoints) driven by ecological values that are poorly understood by the public. Despite the "So what?" problem, the practice of ecological assessments has become inculcated into the operations of regulatory agencies, the regulated sector and non-governmental interest groups.

This evolution of the practice and the science of assessments has led to a better understanding of how cause-effect relationships of stressors and receptors are analyzed and quantified. It requires that our assessments be structured to account for the "indirect and manifold" effects in a way that minimizes the debate about the scientific plausibility of the assessment while focusing more on what to do about the consequences.