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Numerical responses and population decline of an avian predator dependent on cyclic prey
Stockholm University, Faculty of Science, Department of Zoology, Animal Ecology.ORCID iD: 0000-0001-5124-2534
Göteborgs universitet.
Stockholm University, Faculty of Science, Department of Zoology, Animal Ecology.ORCID iD: 0000-0001-5535-9086
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Specialist predators per definition show numerical responses to changes in food supply. Numerical responses are broadly divided into a reproductive response, where reproductive output increases with increased food supply, and an aggregative response caused by breeding suppression and movements. Numerical responses are crucial for understanding predator-prey relations, and also for appropriate management of predator populations. Declining populations of keystone herbivores (voles and lemmings) have been described as a widespread pattern in Europe. Negative effects of dampened small mammal cycles on numerical responses, and thereby population dynamics, have been predicted but so far demonstrated for relatively few specialist predators. We therefore monitored relationships between a common sub-arctic avian predator, the rough-legged buzzard Buteo lagopus, and small rodents in NW Sweden for 19 years (1970-1978 and 2001-2010, 369 observed breeding attempts). Rough-legged buzzards were food-limited and exhibited aggregative and reproductive responses to current rodent abundance in both study periods, but with a weaker coupling in recent years. Density of breeding pairs in rodent peak years was 32-50 % lower in the 2000s than in the 1970s. Further, reproductive output was lower in the 2000s, possibly preventing a population increase. Mean clutch size decreased with 0.77 eggs/clutch (from 4.53 to 3.73, an 18 % reduction), and mean number of fledglings per breeding attempt decreased with 1.08 juveniles/pair (from 3.88 to 2.80, a decrease of 28%). Hatching success and brood survival did not change between 1970s and 2000s, which suggests that reproductive output is constrained by clutch size, rather than by nestling mortality. The observed changes in reproductive parameters support a long-term change in food supply at the onset of breeding as the causal factor. Our study demonstrates the link between predator-prey theory and the declining population-paradigm of conservation biology, illustrating how estimation of numerical responses can be used to predict the outcome of perturbations to predator-prey systems.

Keyword [en]
Buteo lagopus, rough-legged, rodent cycles, population status
National Category
Ecology
Research subject
Animal Ecology
Identifiers
URN: urn:nbn:se:su:diva-102725OAI: oai:DiVA.org:su-102725DiVA: diva2:712950
Available from: 2014-04-17 Created: 2014-04-17 Last updated: 2014-10-28
In thesis
1. Predator responses to non-stationary rodent cycles
Open this publication in new window or tab >>Predator responses to non-stationary rodent cycles
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Regular fluctuations in population size, cycles, are common in small mammals and have important effects on predator populations and life histories. In this thesis, I identify long-term patterns and processes in two specialist predators, the arctic fox Vulpes lagopus and the rough-legged buzzard Buteo lagopus, in relation to their prey (lemmings and voles) and in the case of the arctic fox also to a dominant competitor, the red fox Vulpes vulpes. The results demonstrate that the specialist predators as expected were limited by food supply, which was revealed by combining long-term monitoring with a pseudo-experimental approach. But dampening of cycles has led to long-term declines in the predator populations. Cycles in red and arctic foxes disappeared in the 1970s and 1980s. A return to cyclic dynamics was found locally in the red fox in the boreal zone, but monitoring in the mountain region identified a widespread return of cycles in both lemmings and voles in the early 2000s. This increase in natural food was sufficient to halt the decline in the arctic fox population, but a large-scale field experiment revealed that only supplementary fed subpopulations increased in size. Competition with the red fox further had a negative impact on the arctic fox population. A theoretical model suggested that this asymmetric competition is context dependent and most severe if red fox numbers are independent of rodent density due to access to alternative food. The number of breeding rough-legged buzzards was determined primarily by rodent abundance, but has been nearly halved since the 1970s and was decoupled from rodents in the most recent years. Reproductive output is currently also lower due to smaller clutch sizes. The functional responses of the rough-legged buzzard were complex and differed between lemmings and voles. Rodent cycles are clearly essential for maintaining biodiversity, but spatiotemporal patterns and effects on ecological communities are increasingly variable.

Place, publisher, year, edition, pages
Stockholm: Department of Zoology, Stockholm University, 2014. 32 p.
Keyword
Buteo lagopus, Vulpes lagopus, Lemmus, Myodes, population cycles, predation, numerical response, functional response, arctic, lemmings, voles, fox, monitoring, raptors
National Category
Ecology
Research subject
Animal Ecology
Identifiers
urn:nbn:se:su:diva-102728 (URN)978-91-7447-922-5 (ISBN)
Public defence
2014-05-28, Nordenskiöldsalen, Geovetenskapens hus, Svante Arrhenius väg 12, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defence the following paper was unpublished and had a status as follows: Paper 2: Manuscript.

Available from: 2014-05-06 Created: 2014-04-17 Last updated: 2014-10-28Bibliographically approved

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