Breakout Session 3 -- Carrying Capacity

What is carrying capacity?

Discussion Leaders: Dan Costa and Jim Schumacher
Participants: Karl Banse, George Boehlert, Robert Haney, George Hunt, Patricia Livingston, William Pearcy, Daniel Ware.

Working Definition

Carrying capacity is a measure of the biomass of a given population that can be supported by the ecosystem. The carrying capacity changes over time with the abundance of predators and resources (food and habitat). Resources are a function of the productivity of the prey populations and competition. Changes in the physical and biotic environment affect the distributions and productivity of all populations involved.

The group agreed that carrying capacity exists, however they did not believe that it was worthwhile to attempt to measure it as an absolute value. Further, it was not instructive to attempt to provide a rigorous definition that would facilitate its direct measurement. Rather, they assumed that when a species approaches carrying capacity, density dependence will start to affect the important population parameters. Developing a set of parameters that can be used to assess the relative changes in the status of a population would be a useful exercise (Table 3). These parameters will vary with the scale of the system and the relative importance of abiotic (forcing & temperature) effects. To this end, it was informative to describe the various levels of scale that were relevant to physical forcing and ecosystem interactions.

Strictly speaking, carrying capacity refers to population parameters of a single species. However, it is useful to apply this concept to species groups. As a first approximation the function systems of the Subarctic Pacific into the following tentative grouping.

Size spectrum theory

A potentially valuable conceptual framework to examine these interactions may come from the "size spectrum theory". This idea relates rates of productivity to the size class of the various organisms in the ecosystem. There are a number of important insights that are gained from this approach. Specifically, this approach could address how the ecological efficiency of a community changes as the productivity of the primary producers remains constant, while their relative body size shifts.

Fundamental to the study of species interactions is the need to develop models that can relate the various trophic levels and provide some integration. For this purpose it may be more instructive to select important indicator species for each trophic level that can be used to track these relationships described in Table 3. Ideally these species should have retrospective data sets available and sub–populations that exist within different regimes for comparative purposes.

Time Scale of Carrying Capacity Indices

Most of the indices presented in Table 3 can be measured using existing methodologies. Some data sets exist for various components in specific environments. Ideally, examination of carrying capacity would require simultaneous collection of these parameters over appropriate time periods. The importance of long time series to measure these parameters was emphasized in discussions. However, there are fiscal and logistical limitations. At the present time it is likely that GLOBEC would plan on time-series of 5-7 years. This is appropriate for measurements of quasi biennial oscillations (QBO) and annual fluctuations. However, short time series will not allow examination of interdecadal processes, regime shifts and lower frequency change. However, if a 5-7 yr time series was acquired now, then a follow-on time series of 5-7 yr. could be used to examine these longer term processes. To resolve ENSO effects, we require a 15-35 year time series because the data record should be 3-5 times the period of the cycle of interest.

Determinants of Community Structure

A final aspect of the discussions was to address the issues of the importance of bottom up versus top down control of marine ecosystems.

The most powerful method of examining the factors that control community structure have been "removal" or "exclusion" experiments. One possibility is to examine human fisheries as a removal experiment. However, there are always the confounding variables of regime shifts and other physical features such as temperature. It would be interesting to examine fisheries removal within a given regime. As this may not be possible, the best approach to this problem would be to use models to examine ecosystem and community structure. A critical component of such models would include predation rates of both natural marine predators and humans.

Why questions on carrying capacity should be addressed

There are several reasons why the time is right to initiate studies to examine the impact of climate change on living resources. These include: 1) the existence of bio-physical models (see breakout session 4), 2) technology available to study on appropriate time and space scales (see breakout session 5), 3) PICES provides a comparative approach that would not be possible in a research effort launched by a single nation in a single region of North Pacific, and 4) measurements of ecosystem changes must be started as soon as possible to provide the necessary data for future generations of studies.