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Using Unmanned Aerial Vehicles (UAVs) To Estimate Densities of Two Threatened Cactus Species in the Pinacate Biosphere Reserve, Sonora, Mexico

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There are around 1400 species of cacti worldwide, and nearly half — 669 species — are found in Mexico, with 518 (77%) being endemic. This project focused on changes in cactus populations in the Mexican Sonora Desert between 2006 and 2019. The densities of two threatened cactus species, the saguaro (Carnegiea gigantea) and the teddy bear cholla (Cylindropuntia bigelovii), were recorded in the Pinacate Biosphere Reserve. Cacti were censused by analyzing photos from an Unmanned Aerial Vehicle (“drone”). Raw images were imported into ArcGIS Pro and stitched together to create a high-resolution map of all individuals in the ~100 km2 study area. We compared data from these images to densities recorded from on-the-ground surveys in 2006. Population declines were detected for both cacti (C. bigelovii populations decreased by 37%, and C. gigantea by 61%) between 2006 and 2019. To address concerns about the change in methodology between censuses, we used historical census data for C. bigelovii that were available for the years 2004, 2014, and 2018 from a small (~0.001 km2) study site nearby called Cactus Junction. A decline in C. bigelovii populations at Cactus Junction (~77% decrease between 2004 and 2018) suggests that the changes noted are not strictly a function of methodology. Precipitation data were analyzed as a possible explanation for these declines; however, my results showed no significant correlation between rainfall and cactus density. Additional surveys across a broader geographic region and comparing methodologies may provide insights into the cause of the decline in cactus.

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Using Unmanned Aerial Vehicles (UAVs) To Estimate Densities of Two Threatened Cactus Species in the Pinacate Biosphere Reserve, Sonora, Mexico

There are around 1400 species of cacti worldwide, and nearly half — 669 species — are found in Mexico, with 518 (77%) being endemic. This project focused on changes in cactus populations in the Mexican Sonora Desert between 2006 and 2019. The densities of two threatened cactus species, the saguaro (Carnegiea gigantea) and the teddy bear cholla (Cylindropuntia bigelovii), were recorded in the Pinacate Biosphere Reserve. Cacti were censused by analyzing photos from an Unmanned Aerial Vehicle (“drone”). Raw images were imported into ArcGIS Pro and stitched together to create a high-resolution map of all individuals in the ~100 km2 study area. We compared data from these images to densities recorded from on-the-ground surveys in 2006. Population declines were detected for both cacti (C. bigelovii populations decreased by 37%, and C. gigantea by 61%) between 2006 and 2019. To address concerns about the change in methodology between censuses, we used historical census data for C. bigelovii that were available for the years 2004, 2014, and 2018 from a small (~0.001 km2) study site nearby called Cactus Junction. A decline in C. bigelovii populations at Cactus Junction (~77% decrease between 2004 and 2018) suggests that the changes noted are not strictly a function of methodology. Precipitation data were analyzed as a possible explanation for these declines; however, my results showed no significant correlation between rainfall and cactus density. Additional surveys across a broader geographic region and comparing methodologies may provide insights into the cause of the decline in cactus.