Good point. OTOH, perhaps the most fruitful sites were already monitored in 1997 and the new sites were peripheral and not expected to see as many butterflies.

Good point. OTOH, perhaps the most fruitful sites were already monitored in 1997 and the new sites were peripheral and not expected to see as many butterflies. Also, the graph doesn't define "Total Monarch Abundance". That number might be an extrapolation of butterflies scaled by the number of sites, not the literal number of butterflies counted.