Eugene Cittadino <> John Haught <> Stuart Pimm <> Larry Rasmussen <> Elspeth Whitney

Gary Belovsky

Rates of change of Natural Ecosystems: The Impact of Humans
Abstract

     Ecologists have become more interested in how ecological systems change. An element of any change is the rate at which it occurs. Ecological changes can have different rates, which can span an immense temporal scale: from millennia to longer for evolution to produce new species; centuries to exhibit the succession of the species comprising a community; decades to exhibit climatic variation; a year to exhibit seasonality; a day to exhibit physiological and behavioral rhythms. Therefore, from a practical perspective, environmental policy and management must not only recognize that change occurs in ecological systems with or without human activity, but must also be concerned with how rapidly ecological systems respond to these changes.
     When changes in ecological systems have been recognized, people often assume that they occur incrementally and continuously over time until a new steady state is attained. However, there is growing evidence that steady states may not be common and changes can occur very abruptly and quickly. One way this occurs is when environmental impacts accumulate over time until the ecological system's ability to overcome these changes is exceeded, and dramatic and sudden changes are produced. Another way this occurs is when an ecological system exhibits alternative states depending on initial conditions (e.g., the relative abundances of the species initiating the system). Either of these abrupt changes poses problems for environmental policy and management because environments appear to withstand impacts for long periods until sudden and dramatic changes occur.
     People's perceptions that environments are constant or gradually changing may be due to our collective cultural memory, which has conditioned our expectations. Changes in ecological systems that could be attributed to human activity or environmental change required people to associate human activity or environmental change with ecological change. Developing these associations with limited records and data processing capabilities that have existed through most of human history was easiest for incremental and gradual change. For example, it would be easy for early agriculturalists or pastoralists to associate declines in land productivity with extended use or reduced rainfall. With these associations, appropriate mitigation actions could be taken (rotational use, storage of products, migration, etc.). Consequently, these types of associations conditioned our perceptions.
     However, when environmental changes or human activities need to accumulate before ecological systems exhibit a response, or ecological systems shift between alternative states, the ability to make the appropriate associations taxes human capabilities without long-term records and sophisticated data processing capabilities. With less sophisticated data and means of interpretation, dramatic and sudden changes may be interpreted as catastrophes beyond human comprehension, such as the whimsical beneficence and retribution of deities. Scientific advances based on long-term weather data and computer analyses have begun to elucidate all over the world how periodic climatic shifts can abruptly change ecological systems. This includes periodic environmental changes in the Middle East that influenced the Judeo-Christian tradition, which is the dominant ethical/moral foundation for Western Society.
     Interpreting how and when dramatic and sudden changes occur is a challenge facing ecologists and environmental policymakers/managers. Human activities have been accumulating for millennia and longer, while our intense dependence on ecological systems for products and services may no longer permit the luxury of system failures. However, we may now be paying for accumulated human activity, which only now is exceeding ecological thresholds. What are the ethical and moral implications of these ecological changes emerging from accumulated human activity, especially in the Judeo-Christian perspective?
     A good example of sudden and rapid environmental changes that have moral and ethical implications are current threats to biodiversity and the products and services they provide. Throughout the 3.5 billion year history of our planet, an average species persisted for approximately 7 million years before extinction. In the last 10,000 years, this species persistence time has declined rapidly until current average persistence may be only a few centuries or millennia. The same threat applies to species assemblages called ecosystems. Thresholds and alternative states with accumulated human activity tell us that the loss of a few individuals in a population or species in an ecosystem can produce sudden ecological collapses such as extinction and ecosystem failure. This is why people may be asking how dramatic increases in species extinction and ecosystem losses can occur, when we now are often curtailing our environmental exploitation (e.g., currently fewer US hectares are logged annually than 100 years ago).