Landscape effects on predator activity and nest predation in grey partridges Perdix perdixDoctoral thesis
Date of Examination:2023-03-07
Date of issue:2023-04-14
Advisor:Prof. Dr. Matthias Waltert
Referee:Prof. Dr. Matthias Waltert
Referee:Prof. Dr. Christoph Leuschner
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EnglishEurope's agricultural landscapes used to be home to a wide variety of plants and wildlife that have adapted to these open, cultivated landscapes. Following the onset of large-scale agricultural intensification in the mid-20th century, there was a dramatic decline in biodiversity on agricultural land, affecting all major taxa, including birds, mammals, insects, and plants. Among other causes, many farmland birds are threatened by high predation rates, which are more difficult to counter than, for example, by simply providing replacement habitats. This motivated my doctoral thesis on the relationship between landscape composition and structure and predation risk for grey partridges Perdix perdix. In the agricultural landscape south and east of Göttingen in central Germany, I collected and analysed camera trap data on predator activity over three summer seasons between 2019 and 2021 and analysed grey partridge nesting success data from 2009 to 2017. The results and new insights of these studies are presented in this dissertation. Chapter 1 provides an overview of the current biodiversity crisis in agricultural landscapes with a focus on farmland birds, the threat posed by high predation rates for ground-nesting birds like grey partridges and my thesis objectives. Farmland birds in Europe are currently facing numerous threats, and ground-nesting birds in particular are at risk from high predation rates. Apart from rising predator densities, changes in land use and landscape structure are the main contributors to an increase in predation risk. The aim of this PhD project was therefore to better understand the effects of landscape structure and composition on predation risk in different vegetation elements, and to find out whether it is possible to reduce predation pressure on grey partridges through landscape management. Chapter 2 investigates predator activity in different vegetation types in the agricultural landscape and how it is influenced by the surrounding landscape. My co-authors and I used camera traps to study predator activity in five different vegetation types, i.e., flower blocks, field margins, hedgerows, winter cereal and winter rapeseed fields. Overall, we deployed 240 cameras over two summer seasons in 2019 and 2020 and analysed the data with negative binomial generalized linear models (GLMMs). Additionally, we looked at the differences in predator activity at the edge and inside of flower blocks with 48 cameras in August 2020. We found that red foxes Vulpes vulpes were the most prominent predators, and that vegetation type was the most important factor influencing fox activity: it was highest in hedgerows, followed by winter oilseed rape and lowest in winter cereal fields. Compared to other extensive vegetation structures, i.e., hedgerows and field margins, fox activity was low in flower blocks. In addition, predator activity within the flower blocks was about 9 times lower than at the edge. With regard to the landscape, we found that fox activity increased with woodland area and decreased with increasing structural richness and distance to settlements. Overall, our results confirm the importance of red foxes as a predators and the high value of flower strips as relatively safe nesting sites for ground-nesting birds. Open, small scale agricultural landscapes with a high structural richness and broad extensive vegetation elements seem to be ideal for ground-nesting farmland birds. Chapter 3 focuses on how small-scale landscape structure and composition affect predator activity in flower blocks. My co-authors and I studied the effects of the surrounding landscape at different scales (100 m, 200 m, and 500m) on predator activity by deploying 132 camera traps in flower blocks over three summers from 2019 – 2021 and analysing the data with negative binomial GLMMs. In particular, we investigated the effects of habitat elements that are highly attractive to predators on neighbouring nesting habitats: Either increasing predation risk due to a higher density of predators (hypothesis A) or reducing predation risk by distracting predators and diverting them away from the nesting habitats (hypothesis B). We identified the length of linear structures in the environment as the most important parameter for predator activity in flower blocks. The more linear structures there are, the lower the predator activity, presumably because predator activity is diluted with increasing structural richness, i.e., distributed over more structures. Additionally, we found that in line with hypothesis B, predator activity decreased with increasing proximity to woody structures, suggesting that these structures can successfully distract predators from flower blocks. In line with hypothesis A, predator activity increased with increasing extensive vegetation area and settlement area, indicating that these may attract predators and increase predator densities. Overall, this study has shown that careful consideration of where we place conservation measures in the landscape can help reduce predation pressure. And most importantly, it has shown that richly structured landscapes are of great importance in reducing predation pressure on ground-nesting birds. Chapter 4 investigates the effects of nest site and surrounding landscape on nest predation in grey partridges. In this study, my co-authors and I looked at the problem of predation from the perspective of grey partridges. We investigated the fate of 56 grey partridge nests identified in a large telemetry study of grey partridges between 2009 and 2017 near Göttingen and used Bayesian logistic regressions to analyse the relationship between nest predation and its environment. Predation was by far the main reason for nest losses. The distance between the nest and the nearest edge was the most important factor for nest predation, the greater the distance, the lower the probability of predation. Regarding landscape, predation probability tended to decrease with increasing area of extensive vegetation and increasing habitat diversity, while it tended to increase with distance from settlements. Altogether, these results strongly suggest that the width of nesting habitats is the most important factor for nest safety – the wider the better. Diverse landscapes and a high proportion of extensive vegetation elements can help to reduce predation pressure, as they provide many opportunities for microhabitat selection and predator avoidance by grey partridges and can potentially dilute predator activity. Chapter 5 summarises the main findings from the studies shown earlier and how they relate to other research on predation and conservation of farmland birds. I argue that a) landscape-based implementation of conservation measures, i.e., careful consideration of how and where to place conservation measures, can help reduce predation pressure on ground-nesting birds and can be an alternative or complement to other methods such as lethal predator control and b) sufficiently wide extensive vegetation elements, such as flower blocks, can be highly recommended and should form the backbone of grey partridge conservation programs. Furthermore, I discuss possible shortcomings of my dissertation project, such as the fact that I was only able to conduct a small number of years of field research due to time constraints. Finally, I give recommendations for future research.
Keywords: ground-nesting farmland birds; Perdix perdix; grey partridge; predation risk; camera traps; farmland; conservation; landscape effects; nest predation; predator management; flower blocks