Evaluation of the use of drones to monitor a diverse crocodylian assemblage in West Africa

Abstract

Context West African crocodylian populations are declining and in need of conservation action. Surveys and other monitoring methods are critical components of crocodile conservation programs; however, surveys are often hindered by logistical, financial and detectability constraints. Increasingly used in wildlife monitoring programs, drones can enhance monitoring and conservation efficacy. Aims This study aimed to determine a standard drone crocodylian survey protocol and evaluate the drones as a tool to survey the diverse crocodylian assemblage of West Africa. Methods We surveyed crocodile populations in Benin, Côte d’Ivoire, and Niger in 2017 and 2018, by using the DJI Phantom 4 Pro drone and via traditional diurnal and nocturnal spotlight surveys. We used a series of test flights to first evaluate the impact of drones on crocodylian behaviour and determine standard flight parameters that optimise detectability. We then, consecutively, implemented the three survey methods at 23 sites to compare the efficacy of drones against traditional crocodylian survey methods. Key results Crocodylus suchus can be closely approached (>10 m altitude) and consumer-grade drones do not elicit flight responses in West African large mammals and birds at altitudes of >40–60 m. Altitude and other flight parameters did not affect detectability, because high-resolution photos allowed accurate counting. Observer experience, field conditions (e.g. wind, sun reflection), and site characteristics (e.g. vegetation, homogeneity) all significantly affected detectability. Drone-based crocodylian surveys should be implemented from 40 m altitude in the first third of the day. Comparing survey methods, drones performed better than did traditional diurnal surveys but worse than standard nocturnal spotlight counts. The latter not only detected more individuals, but also a greater size-class diversity. However, drone surveys provide advantages over traditional methods, including precise size estimation, less disturbance, and the ability to cover greater and more remote areas. Drone survey photos allow for repeatable and quantifiable habitat assessments, detection of encroachment and other illegal activities, and leave a permanent record. Conclusions Overall, drones offer a valuable and cost-effective alternative for surveying crocodylian populations with compelling secondary benefits, although they may not be suitable in all cases and for all species. Implications We propose a standardised and optimised protocol for drone-based crocodylian surveys that could be used for sustainable conservation programs of crocodylians in West Africa and globally.

Towards the fully automated monitoring of ecological communities

“…These outstanding perspectives brought by the fully automated, high resolution and multidimensional monitoring of ecological communities should not eclipse the potential negative effects of these technologies on wildlife. For example, unmanned and self‐navigating devices such as drones can affect animal physiology (Ditmer et al, 2015 ) and behaviour (Bennitt et al, 2019 ; Bevan et al, 2018 ; Mulero‐Pázmány et al, 2017 ; Schroeder et al, 2020 ), although these disturbances may be less detrimental than those caused by traditional survey methods, with less impact per unit of data (Aubert et al, 2022 ; Christiansen et al, 2016 ; Gallego & Sarasola, 2021 ) and some species becoming rapidly habituated to the presence of unmanned vehicles (Ditmer et al, 2019 ). Nevertheless, it is timely to (i) better quantify these impacts to avoid the generation of biased and unstandardised data; and (ii) aim to minimise these impacts to prevent animal stress.…”

Existing and emerging uses of drones in restoration ecology

“…Many other studies have used drones to successfully count wildlife, from waterbirds to crocodiles (Aubert et al, 2021; Marchowski, 2021). Counting wildlife is an important facet of restoration ecology––for example, it can be part of measuring the composition, functionality and complexity of ecosystems prior to, during and following a restoration intervention.…”

“…Drones can also be less disturbing to wildlife (if species‐specific protocols are followed) (Aubert et al, 2021; Gallego & Sarasola, 2021) and cover larger spatial extents than traditional field surveys. If protocols are not followed, drones can be detrimental to wildlife.…”

Foreword to the Special Issue on ‘The rapidly expanding role of drones as a tool for wildlife research’

“…Featuring studies from both aquatic and terrestrial ecosystems, this special issue of Wildlife Research highlights the environmental and taxonomic reach of drone research today for observing wildlife and, as a corollary, the myriad ways in which drones are helping overcome limitations of and complement more traditional sampling approaches. For example, Aubert et al (2022) reported a pioneering drone survey of a West African crocodilian assemblage. They found that although they were less effective than nocturnal visual (on-the-ground) surveys, drone surveys were better at detecting crocodilians than were diurnal visual surveys, in large part because their aerial perspective overcomes on-the-ground visual obstructions caused by plants and other forms of habitat complexity.…”

“…Moreover, drones alleviated many of the considerable logistical constraints imposed by both traditional techniques. This marked efficiency advantage is critical in the system studied by Aubert et al (2022), and many others, where focal taxa are simultaneously imperilled and difficult to monitor. Sudholz et al (2022) pushed a different research boundary, showing that drone surveys are an effective means of monitoring invasive species, in their case Rusa deer (Rusa timorensis) in Queensland, Australia, particularly when paired with automated detection via machine learning.…”