The Power of Citizen Science to Quantify Ecological Risks in Cities

Urbanization is an extreme form of habitat modification that can alter ecological relationships among organisms, but these can be hard to study because much of the urban landscape is inaccessible private property. We show that citizen science can be a powerful tool to overcome this challenge. We used photo-vouchered observations submitted to the citizen science platform iNaturalist to assess predation and parasitism across urbanization gradients in a secretive yet widespread species, the Southern Alligator Lizard (Elgaria multicarinata), in Southern California, USA. From photographs, we quantified predation risk by assessing tail injuries and quantified parasitism rates by counting tick loads on lizards. We estimated urbanization intensity by determining percent impervious surface around each lizard observation. We found that tail injuries increased with age of the lizard and with urbanization, suggesting that urban areas are riskier habitats, likely because of elevated populations of predators such as outdoor cats. Conversely, parasitism decreased with urbanization likely due to a loss of mammalian hosts and anti-tick medications used on companion animals. Moreover, our citizen science approach allowed us to generate a large dataset on a secretive species extremely rapidly and at an immense spatial scale that facilitated quantitative measures of urbanization (e.g. percent impervious surface cover) as opposed to qualitative measures (e.g. urban vs rural). This study demonstrates that citizen science is allowing researchers to answer ecological questions that otherwise would go unanswered.