Contributed Paper Assessing Citizen Contributions to Butterfly Monitoring in Two Large Cities K. C. MATTESON, ∗ ‡ D. J. TARON,† AND E. S. MINOR ∗ ∗ Biological Sciences (M/C 066), University of Illinois at Chicago, 3346 SES, 845 W. Taylor Street, Chicago, IL 60607, U.S.A. †Chicago Academy of Sciences/Peggy Notebaert Nature Museum, 2430 N. Cannon Drive, Chicago, IL 60614, U.S.A. Abstract: Citizen science may be especially effective in urban landscapes due to the large pool of potential volunteers. However, there have been few evaluations of the contributions of citizen scientists to knowledge of biological communities in and around cities. To assess the effectiveness of citizen scientists’ monitoring of species in urban areas, we compared butterfly data collected over 10 years in Chicago, Illinois (U.S.A.), and New York City, New York (U.S.A.). The dates, locations, and methods of data collection in Chicago were standardized, whereas data from New York were collected at any location at any time. For each city, we evaluated whether the number of observers, observation days (days on which observations were reported), and sampling locations were associated with the reported proportion of the estimated regional pool of butterfly species. We also compared the number of volunteers, duration of volunteer involvement, and consistency of sampling efforts at individual locations within each city over time. From 2001 to 2010, there were 73 volunteers in Chicago and 89 in New York. During this period, volunteers observed 86% and 89% of the estimated number of butterfly species present in Chicago and New York, respectively. Volunteers in New York reported a greater proportion of the estimated pool of butterfly species per year. In addition, more species were observed per volunteer and observation day in New York, largely due to the unrestricted sampling season in New York. Chicago volunteers were active for more years and monitored individual locations more consistently over time than volunteers in New York. Differences in monitoring protocol—especially length of sampling season and selection protocol for monitoring locations—influenced the relationship between species accrual and sampling effort, which suggests these factors are important in volunteer-based species-monitoring programs. Keywords: citizen monitors, citizen sensors, Lepidoptera, sampling effort, species accumulation, urban biodi- versity, volunteer efficiency Evaluaci´ on de Contribuciones de Ciudadanos al Monitoreo de Mariposas en Dos Ciudades Grandes Resumen: La ciencia ciudadana puede ser especialmente efectiva en paisajes urbanos debido a la gran disponibilidad de voluntarios potenciales. Sin embargo, existen pocas evaluaciones de las contribuciones de los cient´ıficos ciudadanos al conocimiento de las comunidades biol´ ogicas en y alrededor de las ciudades. Para evaluar la efectividad del monitoreo de especies por cient´ıficos ciudadanos en ´ areas urbanas, comparamos los datos de mariposas recolectados a lo largo de 10 a˜ nos en Chicago, Illinois (E.U.A.) y Nueva York, Nueva York (E.U.A.). Las fechas, localidades y m´etodos de recolecci´ on de datos en Chicago fueron estandarizadas, mientras que los datos de Nueva York fueron recolectados en cualquier localidad en cualquier tiempo. Para cada ciudad evaluamos si el n´ umero de observadores, d´ıas de observaci´ on (d´ıas en que se registraron las observaciones) y localidades de muestreo se asociaban con la proporci´ on reportada de las especies de mariposas regionales. Tambi´en comparamos el n´ umero de voluntarios, la duraci´ on de la participaci´ on de voluntarios y la consistencia de los esfuerzos de muestreo en localidades individuales en cada ciudad. De 2001 a 2010 hubo 73 voluntarios en Chicago y 89 en Nueva York. Durante ese per´ıodo, los voluntarios observaron 86% y 89% del n´ umero estimado de especies de mariposas presentes en Chicago y Nueva York, respectivamente. Voluntarios en Nueva York reportaron una mayor proporci´ on de la riqueza estimada de especies de mariposas ‡Current address: Department of Zoology, Miami University, Oxford, OH 45056, U.S.A., email kevmatteson@gmail.com Paper submitted June 9, 2011; revised manuscript accepted October 27, 2011. 557 Conservation Biology, Volume 26, No. 3, 557–564 C 2012 Society for Conservation Biology DOI: 10.1111/j.1523-1739.2012.01825.x