On the geography of activity: Productivity but not temperature constrains discovery rates by ectotherm consumers

Consumer activity—the rate that individuals move through and discover items in their environment—can constrain population interactions and ecosystem services. We introduce a model that assumes consumer activity is co-limited by the abundance and velocity of consumers, which in turn are constrained by two global drivers: net primary productivity (NPP) and environmental temperature, respectively. We test it with data from a recent study showing how arthropod activity decreases with latitude and elevation. The maximum discovery rates (discoveries per day) of these ectotherms increased linearly with NPP and accounted for the observed latitudinal gradient in activity. The mean temperature of the warmest month in contrast had no consistent effect on activity. An ecosystem's NPP, which provides carbon to build individuals and sugars to fuel them, can thus be an important constraint on the activity of its ectotherm consumers.