Conservation in a changing world: assessing the conservation status of an agriculturally adapted species, the blue crane

Craig, Christie Anne

Conservation in a changing world: assessing the conservation status of an agriculturally adapted species, the blue crane

Thesis / Dissertation

2024

Publisher

University of Cape Town

Department

Department of Biological Sciences

Faculty

Faculty of Science

Abstract

With rapid human development and global change, conservation scientists need to stay abreast of changes to threatened species' demographics. The Blue Crane Anthropoides paradiseus has adapted to land-use change in some parts of its range. By the 1980s, numbers of cranes in their natural core range in the eastern grasslands and Karoo of South Africa had declined dramatically due to agricultural development and poisoning, leading to the species being listed as Vulnerable. These impacts were partly offset by the species colonising a novel ecosystem, the Western Cape wheatlands. Currently approximately half of all Blue Cranes are found in these wheatlands. However, there are concerns that the Western Cape population may be decreasing, given the many threats in intensive agricultural landscapes. This thesis assesses the conservation status of the Blue Crane, with particular focus on the Western Cape wheatlands. In Chapter 1 I frame the topic, review the literature, and describe the study areas. In Chapter 2, I use aerial survey data to estimate the Blue Crane population at 51000 (range 34000–68000) cranes, which is double the last conservative estimate in 2002. To assess population trends for Blue Crane over the last 30 years, I analyse two citizen science datasets: Southern African Bird Atlas Projects (SABAP) and Coordinated Avifaunal Road-counts (CAR). These datasets indicate that Blue Crane numbers have increased over the long term but CAR data show that the population trend changed from positive to negative in 2010. Since 2011, summer crane counts have declined by 19% (95% CI –31% to –5.2%), driven primarily by numbers in the Overberg (southern Cape wheatlands) declining by 22% from 2011–2019 (–37% to –1.9%). In Chapter 3 I focus on one of the key threats to Blue Cranes, collision with powerlines. For two years I conducted quarterly surveys for dead birds along powerlines in the Karoo, and annual surveys in two areas in the Western Cape wheatlands (Overberg and Swartland), where agricultural activity prevented quarterly surveys. The Blue Crane was the most frequently recorded species in all three regions, with collision rates of 0.18 (95% CI 0.13–0.24) cranes/km/year in the Karoo, 0.08 (0.02–0.17) in the Swartland and 0.05 (0.02– 0.08) in the Overberg. Estimates of scavenger, observer and crippling bias indicate that we may underestimate collision rate by factors of 6, 7 and 8 times respectively. I developed the first predictive model based on rigorous survey data to identify key collision risk factors for Blue Cranes. Using a binomial Generalized Linear Mixed Model, I found proximity to seasonal water to be the strongest predictor of Blue Crane collision on powerlines; collision probability reduced to nearly zero for powerlines > 2.5 km from a seasonal water body. This finding could guide placement of new powerlines, and to guide marking of existing powerlines close to water bodies. 7 In Chapter 4, I explore Blue Crane movements and survival from GPS-GSM and Iridium satellite trackers fitted between 2016 and 2021, on 31 adults, 3 juveniles and 25 Blue Crane fledglings. Using autocorrelated kernel density home range analysis, I demonstrate that Western Cape Blue Crane home ranges were on average 1655 km2 (SD 2757). Mean daily distance travelled did not differ between regions, Swartland: 9.5 km (SD 11.7), Overberg: 7.5 km (SD 9.8), but non-breeding cranes moved significantly farther per day, 9.5 km (SD 13.3) than breeding cranes, 5.8 km (SD 5.4). As a result, non-breeding cranes intersect powerlines twice as often as breeding cranes. This may contribute to the lower immature survival rate than adult Blue Cranes. Using a Known Fate Mark Recapture Model, I estimate survival for non-breeding birds to be 0.85 (95% CI 0.61–0.95, n = 13), and 0.95 (0.71–0.99, n = 12) for breeding birds. In Chapter 5, I summarise my findings and use these insights to assess the Blue Crane on the IUCN Red List and the Green Status of Species. In the Red List assessment, I draw from a population viability analysis and the CAR data in Chapter 2 and project that the Blue Crane population (currently 21860 mature individuals) will decline by 53% (95% CI –19% to –72%) over three generations, narrowly meeting the criteria for Endangered. However, with the lower end of uncertainty indicating that a Near Threatened listing is appropriate, I recommend that Blue Crane be listed as Vulnerable. I conclude that the Western Cape wheatlands may pose an ecological trap for the species, and that declines are likely being driven by poor breeding productivity and recruitment (Appendix 1). Drawing from the literature, and interviews with farmers (Appendix 2), I contextualise Blue Crane conservation in this landscape and make recommendations for the conservation of this flagship species. This study highlights some of the risks in land-sharing conservation approaches, and the challenges of conservation in man-made ecosystems. In this context, it is important for conservation science to be interdisciplinary and crosscutting in its approach.

Keywords

conservation science

Reference:

Collections

PhD / Doctoral

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