A pilot study to estimate the population size of endangered Galápagos marine iguanas using drones

Abstract

Background Large-scale species monitoring remains a significant conservation challenge. Given the ongoing biodiversity crisis, the need for reliable and efficient methods has never been greater. Drone-based techniques have much to offer in this regard: they allow access to otherwise unreachable areas and enable the rapid collection of non-invasive field data. Herein, we describe the development of a drone-based method for the estimation of population size in Galápagos marine iguanas, Amblyrhynchus cristatus. As a large-bodied lizard that occurs in open coastal terrain, this endemic species is an ideal candidate for drone surveys. Almost all Amblyrhynchus subspecies are Endangered or Critically Endangered according to the IUCN yet since several colonies are inaccessible by foot, ground- based methods are unable to address the critical need for better census data. In order to establish a drone-based approach to estimate population size of marine iguanas, we surveyed in January 2021 four colonies on three focal islands (San Cristobal, Santa Fe and Espanola) using three techniques: simple counts (the standard method currently used by conservation managers), capture mark-resight (CMR), and drone-based counts. The surveys were performed within a 4-day window under similar ambient conditions. We then compared the approaches in terms of feasibility, outcome and effort. Results The highest population-size estimates were obtained using CMR, and drone-based counts were on average 14% closer to CMR estimates—and 17–35% higher—than those obtained by simple counts. In terms of field-time, drone-surveys can be faster than simple counts, but image analyses were highly time consuming. Conclusion Though CMR likely produces superior estimates, it cannot be performed in most cases due to lack of access and knowledge regarding colonies. Drone-based surveys outperformed ground-based simple counts in terms of outcome and this approach is therefore suitable for use across the range of the species. Moreover, the aerial approach is currently the only credible solution for accessing and surveying marine iguanas at highly remote colonies. The application of citizen science and other aids such as machine learning will alleviate the issue regarding time needed to analyze the images.

A bird’s-eye view: Evaluating drone imagery for the detection and monitoring of endangered and invasive day gecko species

“…However, the use of drone imagery was highly complementary to the visual counts, since we detected additional individuals on Pandanus leaves in the canopy. We assume that the benefit of drone-based monitoring might become clearer in less accessible areas such as cliffs and shrublands and may outperform visual counts (Monks, Wills, and Knox, 2022; Varela-Jaramillo et al, 2023). Phelsuma inexpectata is distributed along the coastline, inhabiting steep slopes and cliffs.…”

“…We showed that species-level identification was reliable within 5 m distance from the geckos. However, this approach may not be suitable for individual-level identification with the current resolution of standard mid-range drone cameras and may only be possible for larger species (e.g., photo-identification of Galàpagos marine iguanas; Varela-Jaramillo et al, 2023). Further improvement of mid-range drone camera lenses in the future might allow for higher resolution imagery and thus, individual identification.…”

“…Novel methodologies such as computer assisted slow-speed road cruising (Jones et al, 2022) or camera-trapping (e.g., Roesch, Hansen, and Cole, 2021; Deso, Crouzet, and Bonnet, 2022) have already proven efficient. Drones have recently been used for the monitoring of endangered species (Landeo-Yauri et al, 2020; Varela-Jaramillo et al, 2023), including cryptic reptiles (Monks, Wills, and Knox, 2022), and have improved the detection of invasive reptile species living in the tree canopy (Aota et al, 2021).…”

A bird's‐eye view: Evaluating drone imagery for the detection and monitoring of endangered and invasive day geckos

“…In our case, we were not able to perform individual-level identification for these species with the current resolution of our drone camera. However, individual-level identification was already possible with standard mid-range drone camera lenses for larger species (e.g., photo-identification of Galàpagos marine iguanas; Varela-Jaramillo et al, 2023) and even for jeweled geckos, which are about the size of our smaller study species (Monk et al, 2022). This offers the possibility to integrate drone-based counts in studies that take into account imperfect detection aimed at The habitat use of P. inexpectata is dynamic, with more frequent use of the canopy during winter (Choeur et al, 2023).…”

“…Drones have already been used to estimate species abundance, monitor individual behaviour and animal disturbance related to its presentations (Schad & Fischer, 2022). They were recently used for the monitoring of endangered species (Landeo‐Yauri et al, 2020; Varela‐Jaramillo et al, 2023), including cryptic reptiles (Monks et al, 2022), and have improved the detection of invasive reptile species living in the tree canopy (Aota et al, 2021).…”