Ecosystem indicators: factors affecting the choice of predator performance indices for use in monitoring programmes

The ability to use upper-trophic level species as ecosystem indicators is determined by the ability to relate changes in indices of their performance to changes at lower trophic levels. Using indices of predator performance from four species of krill-eating predator together with independent ship-based acoustic estimates of krill abundance from South Georgia the relationship between a range of indices of predator performance and krill abundance was examined. There was a distinct relationships between the variability of indices and the biological processes that they measured; body mass parameters had the lowest variability (CV< 10%) whereas parameters measuring breeding success showed the greatest variability with CVs > 50 %. Predator parameters that reflected processes occurring during the summer showed the closest relationship with krill abundance, especially those for species with foraging ranges similar to spatial scales at which krill surveys were undertaken. Population size parameters showed no functional response relationship with annual krill abundance estimates. Combining the summer parameters into a single combined index provided a better fit with the krill data than any of the individual parameters.