The power of community science to quantify ecological interactions in cities.

Studying animals in urban environments is especially challenging because much of the area is private property not easily accessible to professional scientists. In addition, collecting data on animals that are cryptic, secretive, or rare is also challenging due to the time and resources needed to amass an adequate dataset. Here, we show that community science can be a powerful tool to overcome these challenges. We used observations submitted to the community science platform iNaturalist to assess predation and parasitism across urbanization gradients in a secretive, 'hard-to-study' species, the Southern Alligator Lizard (Elgaria multicarinata). From photographs, we quantified predation risk by assessing tail injuries and quantified parasitism by counting tick loads on lizards. We found that tail injuries increased with age and with urbanization, suggesting that urban areas are risky habitats. Conversely, parasitism decreased with urbanization likely due to a loss of hosts and anti-tick medications used on human companion animals. This community science approach generated a large dataset on a secretive species 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). We therefore demonstrate that community science can help resolve ecological questions that otherwise would be difficult to address.