Ecosystem services are not always easy to quantify for a number of reasons. One reason is that information about a particular service, or the natural capital that generates it, may be imperfect or even completely lacking. For example, the benefits of aquatic ecosystems in developing countries may be unaccounted for because no research has been conducted on the scope and status of these systems. In developed nations, markets may distort the value of ecosystem goods and services, inflating or deflating them due to factors such as the influence of media attention or politics. An example is the recent, rapid colonization of inland North American waters by the zebra mussel (Dreis-sena polymorpha). This invasive species is generally regarded as a pest that devalues water bodies through removal of plankton that fuels the food web and hence affects fisheries. Although true, increased water clarity from zebra mussel filtering is highly valued as well and can quite likely be seen by examining changes in waterfront property values over time.
Another problem is that ecosystem goods and services are often 'multifunctional,' and involved in more than one process. Thus, it has been difficult to parse the multiple values generated by the same component (e.g., marsh vegetation is important as structure, as habitat, and as food or building material). One approach has been to calculate the 'total ecosystem value,' which sums all the values; another is to estimate the major value, or the values of the most clearly distinguishable values. Clearly, these different approaches will yield different estimates with different ability to capture the full value.
Other issues are those of scale and uncertainty, even when ecosystems are fairly well researched. Different ecosystems may contribute one type of service at local scales (or short-time horizons), but contribute, either collectively or individually, different services at larger (or longer) scales. Uncertainty arises not only from spatiotemporal variability, but also from the system's degree of resilience, that is, its responses to stochastic events (e.g., oil spills or hurricanes). Some aquatic ecosystems, such as lakes, can be 'pushed' from one stable state (e.g., oligotrophic) to another (e.g., eutrophic) through pollution with excess nutrients that ultimately build up in sediments, and are released for many years after the pollution load has diminished or ceased. The specifics of lake morphometry, residence time, climate, and trophic structure all play a role in lake resiliency and maintenance of trophic state.
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