A tolerance range approach for the investigation of values provided by Te Waihora/Lake Ellesmere.

Abstract

This report develops and applies a tolerance range approach for investigating the set of values provided by the shallow lake ecosystem, Te Waihora/Lake Ellesmere. A tolerance range refers to the extent to which a selected species (representing an indicator of a value) can continue to produce and reproduce in the long-term for a particular environmental condition critical to its survival. The set of values provided by Te Waihora/Lake Ellesmere that are investigated in this report include: the Lake Margin Vegetation Value, the Birdlife Value, the Agricultural Livestock Value, the Native Fishery Value, the Ngāi Tahu Value and the Trout Fishery Value. Indicator species, which are a few selected species that represent a value, were determined by experts for each of the values, so that it becomes possible to consider all values without having to account for the myriad of possible species found on the lake. Critical environmental conditions or controlling factors that determine the survivability of indicator species for each of the values were also established and weighted by experts. Finally, tolerance ranges for each indicator species were elicited by experts for each controlling factor. From this information tolerance range indices were calculated on a zero-to-one scale for each indicator species in relation to each controlling factor. The calculated tolerance range indices established the degree of tolerance for each indicator species for that particular controlling factor considered. These tolerance range indices were then multiplied by the weights given to controlling factors in order to establish weighted indices. From these weighted indices, which were then summed, it was established that the Lake Margin Vegetation Value, the Agricultural Livestock Value and the Trout Fishery Value are the most vulnerable to further losses. Accordingly, this tolerance range approach was able to investigate all values considered, yet provide systematically and quantitatively developed insights using only a limited amount of critical information for determining the present vulnerability of values provided. The determination of the present vulnerability of values provided by Te Waihora/Lake Ellesmere is important in understanding where lake management is required for all values to be preserved along sustainable pathways.