2. Food Web Evaluation

This version of the Puget Sound Science Update provides an initial evaluation of food web indicators, but is not intended to be comprehensive. Highlights include the evaluation of individual species or species complexes as food web indicators due to their key functional roles (e.g., forage fish, jellyfish), and the identification of existing data sources for assessing food web structure and function at Washington State agencies and via satellite.

Species abundance and biomass data can be used to paint a synthetic picture of community composition, especially when viewed collectively with respect to particular Domains and in relation to species’ trophic and functional roles. Even in isolation, insight into the status and trends of keystone species (i.e., those that have a disproportionate influence on food web structure relative to their abundance), highly connected species (i.e., those that are consumers of and consumed by many other species), minimally connected species, and those species representing a large proportion of the biomass in Puget Sound can be useful for interpreting the structural configuration of the food web (Soulé et al. 2005). In addition, species abundance and biomass data can be summarized into index values that describe the three different types of diversity defined in Section 3.2.3.3 (species, trophic, and response diversity). Dietary composition data, especially for higher trophic level predators such as marine mammals and birds, offer an alternative inroad to understanding community composition in Puget Sound and are available for a limited subset of species. Ongoing monitoring programs led by the Washington Departments of Ecology, Fish and Wildlife, and Natural Resources, among others, provide a rich source of information on community composition in Puget Sound. The challenge is to sort through these data to extract meaningful summary descriptions.

Proxies for primary productivity such as chlorophyll a concentration and phytoplankton biomass (in the Marine Domain) and leaf area index (in the Terrestrial Domain) are the most widely available indicators for energy and material flows in Puget Sound. Remote-sensing data are a valuable source of this information, though other, labor-intensive approaches are available for obtaining spatially explicit and finely resolved understanding of primary productivity as well (e.g., plankton tows, forest inventories, etc.). Alternatives to remote-sensing data are especially important in the Marine Domain, where it is difficult to obtain reliable estimates of primary productivity in nearshore areas at small spatial scales. More detailed data collection or modeling efforts (e.g., Ecopath with Ecosim) are needed to estimate the magnitude of secondary production and pathways of energy flows throughout the food web. Biogeochemical approaches for measuring cycling rates are well developed, especially with respect to inorganic nutrients, but such data are not widely available and can be quite expensive to obtain. Making up for this deficiency will require detailed, broad-scale studies of how different species interact with the physical and chemical oceanography of Puget Sound to affect processes such as nitrogen fixation, carbon sequestration, and microbial decomposition.

There were nineteen Food Web indicators identified and of these, we have evaluated fifteen. The degree to which food web indicators satisfy our evaluation criteria is highly variable, and about half of them did not perform well against any of the criteria. The majority of evaluated indicators were from the Marine Domain, and no indicators have yet been evaluated for Freshwater Food Webs. The current status of indicator evaluations for the Food Webs Goal is summarized below.

Eleven indicators of Marine Food Web community composition and two indicators of Marine Food Web Energy and Material Flows were evaluated (Table 11 and Table 12). The status and trends of benthic and pelagic fish communities species, marine shorebird diets, and jellyfish abundance performed best against the primary considerations for indicators of community composition. Of these indicators, however, only marine shorebird diets also met a majority of the Data and Other Considerations criteria. The general deficiency of quantitative data suggests the potential utility of several indicators while highlighting the need to begin data collection and monitoring. Most of the community composition indicators that did not perform well against the Primary Considerations also were deficient under the Data Considerations criteria. One of the biggest challenges for developing Marine Food Web indicators will be to increase their specificity prior to evaluation; several indicators, like the marine biodiversity index, shellfish, and benthic macroinvertebrates, were considered too vague to evaluate properly.

Phytoplankton biomass and chlorophyll a concentration provide similar information about primary productivity in the Puget Sound Marine Food Web. Both indicators performed well against the Primary Considerations for indicators of energy and material flows. However, chlorophyll a concentration met more of the Data and Other Considerations. Due to this indicator’s reliance on remotely sensed data, however, it is unlikely to provide information about energy and material flows on spatial scales smaller than the PSP Action Areas. We suggest the evaluation of additional indicators of energy and material flows in the future.

Table 11. Summary of Marine Food Webs – Community Composition indicator evaluations. The numerical value that appears under each of the considerations represents the number of evaluation criteria supported by peer-reviewed literature. For example, Macro benthic inverts has peer-reviewed literature supporting 0 out of 5 Primary Considerations criteria. Details can be found in the accompanying spreadsheets.

Table 12. Summary of Marine Food Webs – Energy and Material Flow indicator evaluations. The numerical value that appears under each of the considerations represents the number of evaluation criteria supported by peer-reviewed literature. For example, Chlorophyll a has peer-reviewed literature supporting 3 out of 5 Primary Considerations criteria. Details can be found in the accompanying spreadsheets.

Three indicators of Freshwater Food Web community composition were identified (Table 13), but unfortunately none were evaluated for this version of the PSSU. No indicators of Freshwater Food Web energy and material flows appear on the list of candidates suggested by O’Neill et al. (2008). Indicators of this Focal Component clearly deserve greater attention in future evaluation processes.

Table 13. Summary of Freshwater Food Webs – Community Composition indicator evaluations.

O’Neill et al. (2008) identified one indicator of Terrestrial Food Web community composition (Table 14), the terrestrial biodiversity index. Unfortunately, because it is still in development, this indicator did not meet many of the evaluation criteria under the Primary, Data, and Other Considerations. No indicators of Terrestrial Food Web energy and material flows were proposed by O’Neill et al. (2008) and none were evaluated. As with Freshwater Food Webs, indicators of Terrestrial Food Webs clearly deserve greater attention in future evaluation processes.

Table 14. Summary of Terrestrial Food Webs – Community Composition indicator evaluations. The numerical value that appears under each of the considerations represents the number of evaluation criteria supported by peer-reviewed literature. For example, Terrestrial biodiversity index has peer-reviewed literature supporting 2 out of 5 Primary Considerations criteria.

Two related indicators of Interface Food Web community composition were identified by O’Neill et al. (2008) (Table 15): forage fish and herring status and trends. Both indicators performed well against the Primary Considerations, though many of the Data and Other Considerations were not met. No indicators of Interface Food Web energy and material flows were proposed by O’Neill et al. (2008) and none were evaluated. In general, new, additional indicators of this Focal Component should be evaluated in the future.

Table 15. Summary of Interface Food Webs – Community Composition indicator evaluations. The numerical value that appears under each of the considerations represents the number of evaluation criteria supported by peer-reviewed literature. For example, Forage fish status & trends has peer-reviewed literature supporting 4 out of 5 Primary Considerations criteria. Details can be found in the accompanying spreadsheets.