Decline in Distribution and Abundance: Urban Hedgehogs under Pressure

Abstract

Increasing urbanization and densification are two of the largest global threats to biodiversity. However, certain species thrive in urban spaces. Hedgehogs Erinaceus europaeus have been found in higher densities in green areas of settlements as compared to rural spaces. With recent studies pointing to dramatically declining hedgehog numbers in rural areas, we pose the question: how do hedgehogs fare in urban spaces, and do these spaces act as refuges? In this study, recent (2016-2018) and past (1992) hedgehog abundance and distribution were compared across the city of Zurich, Switzerland using citizen science methods, including: footprint tunnels, capture-mark recapture, and incidental sightings. Our analyses revealed consistent negative trends: Overall hedgehog distribution decreased by 17.6% ± 4.7%, whereas abundance declined by 40.6% (mean abundance 32 vs. 19 hedgehogs/km², in past and recent time, respectively), with one study plot even showing a 91% decline in this period (78 vs. 7 hedgehogs/km², respectively). We discuss possible causes of this rapid decline: increased urban densification, reduction of insect biomass, and pesticide use, as well as the role of increasing populations of badgers (a hedgehog predator) and parasites or diseases. Our results suggest that hedgehogs are now under increasing pressure not only in rural but also in urban areas, their former refuges.

Keywords: Erinaceus europaeus; citizen science; population reduction; population trend; settlement area; urban densification.

Figure A1

Footprint tunnel with removable plate in front, showing hedgehog footprints on them. Two paper sheets and two brushes of ink are applied to it. A spoon of bait is placed in the brown plate. © Thomas Massie/stadtwildtiere.ch.

Figure A2

Linear regression between the abundance measures from the capture-mark-recapture (CMR) study (whiskers indicate confidence intervals of the CMR estimates) as the independent variable and the hedgehog levels from the footprint tunnel study (ranging from 1–10) in the respective area as the dependent variable. Regression line in black with confidence (dashed line) and prediction interval (dotted line).

Figure A3

Estimation of the decrease in median distribution area (in percent (± 95% confidence interval), revealed through subsampling. With increasing size of the subsample, the estimate becomes more accurate, on the other side the degrees of freedom become more restricted (because overall sample size = 987 for 1992 and 1096 for 2016–2018). Our estimate resulted by averaging the values for subsample sizes 500 through 900.

Figure 1

City of Zurich and study areas that were assessed in the course of 25 years (past (1992) vs. recent (2016/2017)). The outline delineates the municipal border of the city of Zurich with forest area (green) and water bodies (blue). Study areas are divided into 1 km² patches (light blue) and study plots (red: Altstetten (A), Enge (E), Schwamendingen (S), Wipkingen (W); in 1992, only Wipkingen was investigated; for details, see text).

Figure 2

Distribution map of hedgehogs Erinaceus europaeus in the city of Zurich, Switzerland in the past study 25 years ago (1992, circles) and in the recent study (2016–2018, points). The outline delineates the municipal border of the city of Zurich with forest area (green) and water bodies (blue). Distribution circles represent the ¼ km² survey grid. The distribution of hedgehogs declined by −17.6% (± 4.7%) in the 25 years between the studies.

Figure 3

Changes in hedgehog abundance estimates between the past (1992) and recent (2016–2018) study (% change in abundance/km²). The outline delineates the municipal border of the city of Zurich with forest area (green) and water bodies (blue). Squares signify the study area divided into km²: declining hedgehog abundance (red), increasing hedgehog abundance (green).

Figure 4

Proportion of km-squares per survey period (past in light blue; recent in dark blue) with low, medium, and high hedgehog abundance per km².