Global patterns in ecological indicators of marine food webs: a modelling approach

Abstract

Background: Ecological attributes estimated from food web models have the potential to be indicators of good environmental status given their capabilities to describe redundancy, food web changes, and sensitivity to fishing. They can be used as a baseline to show how they might be modified in the future with human impacts such as climate change, acidification, eutrophication, or overfishing.

Methodology: In this study ecological network analysis indicators of 105 marine food web models were tested for variation with traits such as ecosystem type, latitude, ocean basin, depth, size, time period, and exploitation state, whilst also considering structural properties of the models such as number of linkages, number of living functional groups or total number of functional groups as covariate factors.

Principal findings: Eight indicators were robust to model construction: relative ascendency; relative overhead; redundancy; total systems throughput (TST); primary production/TST; consumption/TST; export/TST; and total biomass of the community. Large-scale differences were seen in the ecosystems of the Atlantic and Pacific Oceans, with the Western Atlantic being more complex with an increased ability to mitigate impacts, while the Eastern Atlantic showed lower internal complexity. In addition, the Eastern Pacific was less organised than the Eastern Atlantic although both of these systems had increased primary production as eastern boundary current systems. Differences by ecosystem type highlighted coral reefs as having the largest energy flow and total biomass per unit of surface, while lagoons, estuaries, and bays had lower transfer efficiencies and higher recycling. These differences prevailed over time, although some traits changed with fishing intensity. Keystone groups were mainly higher trophic level species with mostly top-down effects, while structural/dominant groups were mainly lower trophic level groups (benthic primary producers such as seagrass and macroalgae, and invertebrates). Keystone groups were prevalent in estuarine or small/shallow systems, and in systems with reduced fishing pressure. Changes to the abundance of key functional groups might have significant implications for the functioning of ecosystems and should be avoided through management.

Conclusion/significance: Our results provide additional understanding of patterns of structural and functional indicators in different ecosystems. Ecosystem traits such as type, size, depth, and location need to be accounted for when setting reference levels as these affect absolute values of ecological indicators. Therefore, establishing absolute reference values for ecosystem indicators may not be suitable to the ecosystem-based, precautionary approach. Reference levels for ecosystem indicators should be developed for individual ecosystems or ecosystems with the same typologies (similar location, ecosystem type, etc.) and not benchmarked against all other ecosystems.

Figure 1. Food web diagram of the Venice lagoon with 27 nodes or funtional groups.

Colors of flows depict different fishing target (artisanal fisheries in blue, and clam fishery in red) and non-target species (for clam harvesting, in green). Modified from Pranovi et al. .

Figure 2. Distribution of food web model used in this study.

Models are divided into Eastern Pacific (black), Western Atlantic (red), Eastern Atlantic (blue), North and Baltic Seas (orange), Mediterranean Sea (green), Indian Ocean (purple) and Eastern Pacific (pink). Each model is numbered on the graph according to its number in Table S1, where details and references to each model area are given.

Figure 3. Boxplot of significant differences of food web ecological indicators by ecosystem type.

The smallest observation (minimum), lower quartile, median, upper quartile, largest observation (maximum) and outliers are indicated. The boxes are drawn with widths proportional to the square-roots of the number of observations in each class. TST  =  total systems throughput (t.km⁻².year⁻¹), TB community  =  total biomass of the community (t.km⁻²), and mean EE  =  mean ecotrophic efficiency (proportion).

Figure 4. Boxplot of significant differences of food web ecological indicators by depth class.

The smallest observation (sample minimum), lower quartile, median, upper quartile, largest observation (sample maximum) and outliers are indicated. The boxes are drawn with widths proportional to the square-roots of the number of observations in each class. FD/TST  =  flow to detritus/total systems throughput (proportion), R/TST  =  respiration/total systems throughput (proportion), TEm  =  mean transfer efficiency (%) and mean EE  =  mean ecotrophic efficiency (proportion).

Figure 5. Boxplot of significant differences of food web ecological by size class.

The smallest observation (sample minimum), lower quartile, median, upper quartile, largest observation (sample maximum) and outliers are indicated. The boxes are drawn with widths proportional to the square-roots of the number of observations in each class. Mean EE  =  mean ecotrophic efficiency (proportion), Ex/TST  =  export/total systems throughput (proportion) and FCI  =  Finn cycling index.

Figure 6. Boxplot of significant differences of food web ecological by ocean basin.

The smallest observation (sample minimum), lower quartile, median, upper quartile, largest observation (sample maximum) and outliers are indicated. The boxes are drawn with widths proportional to the square-roots of the number of observations in each class. R/TST  =  respiration/total systems throughput, Ex/TST  =  export/total systems throughput (proportion), mean TE  =  mean transfer efficiency, FD/TST  =  flow to detritus/total systems throughput (proportion), MTLco  =  mean trophic level of the community, Q/TST  =  consumption/total systems throughput (proportion), SOI  =  systems omnivory index, IFO  =  Internal flow overhead (%), A/C  =  relative ascendency (%), O/C  =  relative overhead (%).

Figure 7. Key ecological roles of functional groups of marine food web models.

a) Keystone (KS ≥0; black circles) and dominant groups (KD ≥0.7; grey circles), respectively, in terms of absolute overall effect () for each food web. Open dots represent non key functional groups. b) The 105 trophic groups franking first in terms of absolute overall effect within each food web model (Fig. 2, Table S1). The figure shows the trophic level (TL) vs. the fraction of top-down effect (td%). Groups identified as keystones are represented in black symbols and dominant groups are reported in grey symbols, respectively, whereas open circles represent non key functional groups. Groups are highlighted for both keystones and dominant: birds (star within square), marine mammals (triangles), sharks and rays (squares), top-predators (romboid), primary producers (crossed squares), other groups (circles). Large squares with error bars identify mean+/−SD for all keystones and dominants identified in the 105 models.

Figure 8. Proportion of key functional groups by food web ecological traits and by exploitation.

Graphs report proportion of Keystone (KS ≥0) and dominant groups (KD ≥0.7) by a) ecosystem size, b) depth, c) ecosystem type, d) ocean basin and e) fishing category. Only traits showing significantly different patterns (on the basis of total Chi-squared) are reported (***, ** respectively p<0.01, p<0.05). Main contribution to Chi-squared are highlighted by asterisk (see also Table 4).

Figure 9. Boxplot of significant differences of food web ecological traits by exploitation and of fishing indicators.

The smallest observation (sample minimum), lower quartile, median, upper quartile, largest observation (sample maximum) and outliers are indicated. The boxes are drawn with widths proportional to the square-roots of the number of observations in each class. PP/P  =  primary production/total production (proportion), meanEE  =  mean ecotrophic efficiency (proportion), TBco  =  total biomass of the community (t.km⁻²), Mean TLc  =  mean trophic level of the catch, L_index  =  loss in production index, and P_sust  =  probability of being sustainably fished.