This measure is based on muskrat aerial surveys in the Peace-Athabasca Delta (PAD), done in collaboration with local indigenous groups. Muskrats are a keystone species. They play a pivotal role in the food web of the PAD and as such are a good indicator of what is going on at higher trophic levels. They have a predictable response to change, which can be reliably and easily measured. Muskrats also eat a range of plant species, alter vegetation composition of wetlands, and create a patchwork of closed and open habitats. Culturally, they play a fundamental role in the diet, materials, medicine, and traditional values, culture and economy of First Nations and Métis of the PAD. The monitoring question must consider two elements of the muskrat population: abundance and cyclicity. Muskrat houses are counted by observers from a fixed wing plane every other year in the spring. Transects are 1 km wide and 20 km long and the plane flies at an altitude of 200m at 120 knots.