Abstract This paper develops a novel tolerance range approach that allows for the quantitative assessment of ecosystems with only a minimum amount of information. The quantitative assessment is achieved through the determination of tolerance range scores and indices that indicate the vulnerability of species. For the purposes of demonstrating the tolerance range approach an ecosystem assessment is performed on Te Waihora/Lake Ellesmere, a large shallow lake found in the Canterbury region of New Zealand. From the analysis of tolerance range scores and indices it was found that brown trout and lake-margin vegetation are the most vulnerable species of value to further degradation. This information implies that management actions should prioritize towards preserving these species to maintain all valued species along sustainable pathways.