Vigie-Chiro
Bat population trends
The prime objective of Vigie-Chiro is to compute population trends for a maximum of bats in France. These trends are calculated based on recordings collected between 2006 and 2023 on 195 car transects, 138 walk transects and 1822 fix points.
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The car and walk transects have been monitored during a mean of 4 years. Fix points, from the most recent protocol, have been monitored during a mean of 3 years.
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Methods:
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The data are modelled with models called Generalised Linear Mixed Models (GLMM) with a negative binomial family. The variable of interest is the number of bat passes per section/point/night.
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Three types of temporal modelling of the trend were performed: log-linear on the period 2006-2023 to assess long-term trends, og-linear on the period 2014-2023 to assess short-term trends, and an estimation year by year.
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The models take into account the day of the year, the type of protocol, the type of recording (direct or time expansion), and the timing of the road transect as fix variables. They also account for the site and the recording material used as random variables.
We also controlled the capacity of the models to separate these effects from the population trend using the VIF (Variance Inflation Factor). The model is said to be robust only if the VIF of the trend parameter is inferior to 2. It is robust for Nyctalus leisleri, N. noctula, Pipistrellus pipistrellus, P. kuhlii, P. nathusii and Eptesicus serotinus. Other species for which monitoring increased thanks to the fixed points protocol still display a high VIF because their trend highly depends on the very last years (see at the end of this page) and the temporal trends can still be confused with some meteorological variables.
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A possible degradation of the material is also controlled by the measurement of the bat pass length of Pipistrelles, after having removed all recordings suspected to be deficient. We noted a decrease in bat pass length of -0.39% (confidence interval (CI) +/- 0.19%). Over the whole period, the loss in bat pass length is of -6.65% (CI +/- 3.16%). Therefore, declines do not seem to be attributed to a possible bias in the loss of sensitivity of the material.
Results of​ March 2024. Numbers in brackets are the lower and upper confidence intervals.
Barbastella barbastellus
Linear trend 2008 – 2023 : + 41,1 % (+ 14 % ; + 74,7 %) p-value < 0,01
Linear trend 2014 – 2023 : + 35,4 % (+ 13,3 % ; 61,9 %) p-value < 0,001
Between 2008 and 2023, the Barbastelle bat population increased significantly by 41.1%. This positive trend also continues over the more recent period from 2014 to 2023, with a significant increase of 35.4%. These results highlight a significant increase in the numbers of this species over time.
© Ludovic Jouve
Eptesicus serotinus
Linear trend 2006 – 2023 : + 13,7 % (- 3,4 % ; + 33,8 %) Not significant
Linear trend 2014 – 2023 : + 39,1 % (+ 21,1 % ; + 59,9 %) p-value < 0,001
Analysis between 2006 and 2023 reveals an upward trend of 13.7% for the Serotine bat, although this is not significant. In contrast, over the more recent period from 2014 to 2023, a significant increase of 39.1% was observed, suggesting significant recent growth in the numbers of this species.
© Ludovic Jouve
Hypsugo savii
Linear trend 2014 – 2023 : - 21,6 % (- 42,5 % ; + 6,8 %) Not significant
Between 2014 and 2023, the calculated trend indicates a decrease of 21,6% of the population size, although this decrease is not statistically significant. These resultats suggest a possible decline of the species along the years, but it is impossible to conclude with certainty.
© Charlotte Roemer
Miniopterus schreibersii
Linear trend 2014 – 2023 : - 32,9 % (- 52,8 % ; - 4,5 %) p-value < 0,05
Over the period from 2014 to 2023, a significant decline of 32.9% has been observed in the numbers of the Bent-winged bat.This indicates a significant decline in the numbers of this species over this period.
© Blandine Carré
Nyctalus leisleri
Linear trend 2006 – 2023 : + 21,3 % (+ 1,6 % ; + 44,8 %) p-value < 0,05
Linear trend 2014 – 2023 : + 76,3 % (+ 51,1 % ; + 105,9 %) p-value < 0,005
Between 2006 et 2023, the population of Leisler's bats increased significantly of 21.3 %. Again, during the more recent period (2014 to 2023), a strong and significant increase of 76.3 % was observed. This indicates a remarkable increase of the population of this species though time, especially during recent years.
© Charlotte Roemer
Nyctalus noctula
Linear trend 2006 – 2023 : - 52,5 % (- 62,8 % ; - 39,4 %) p-value < 0,001
Linear trend 2014 – 2023 : + 6,9 % (-12,2 % ; + 30,3 %) No significant
Between 2006 et 2023, lthe population of common Noctule bats decreased significantly of 52,5 %. However, during the more recent period (2014 to 2023), although a slight increase of 6,9 % was observed. This was not significant, indicating little change during this period.
© Ludovic Jouve
Myotis daubentonii
Linear trend 2006 – 2023 : - 29,2 % (-45,5 % ; - 7,9 %) p-value < 0,05
Linear trend 2014 – 2023 : - 31,7 % (-45,5% ; -14,4 %) p-value < 0,001
Between 2006 et 2023, the population of the Daubenton's bat decreased significantly of 29,2 %. Again, during the more recent period (2014 to 2023), a significant decrease of 31,7 % was observed. These results suggest a continuous decrease of the ppulation size of the Daubenton's bat over time.
© Ludovic Jouve
Myotis emarginatus
Linear trend 2014 – 2023 : + 65,1 % (+ 29,1 % ; + 111,2 %) p-value < 0,001
From 2014 to 2023, a significant increase of 65,1 % is observed for the notch-eared bat, which indicates a remarkable increase of the population size of this species.
© Laurent Arthur
Myotis mystacinus/brandt
Linear trend 2014 – 2023 : - 21,1 % (- 40,5 % ; + 4,5 %) Not significant
The analysis between 2014 and 2023 reveals a negative trend of 21,1 % for the group whiskered bat/brandt's bat, although thus decrease is not statistically significant. These results suggest a possible decline of this group over the last years, but it is not possible to confirm this
© Gilles San Martin
Myotis nattereri/Myotis crypticus
Linear trend 2014 – 2023 : + 79,9 % (+ 49,1 % ; + 11,7 %) p-value < 0,001
From 2014 to 2023, a significant increase of 79.9 % is observed for this group, which indicates a remarkable increase of its population size .
© Manuel Ruedi
Pipistrellus nathusius
Linear trend 2006 – 2023 : - 29,9 % (- 45,8 % ; - 9,2 %) p-value < 0,005
Linear trend 2014 – 2023 : - 20 % (- 37,5 % ; - 0,5 %) p-value < 0,01
Between 2006 et 2023, the population of the Nathusius bat decreased significantly of 29,9 %. Again, during the more recent period (2014 to 2023), a significant decrease of 20 % was observed. These results suggest that the population size of the Nathusius bat is continuously decreasing over time.
© Céline Le Barz
Pipistrellus kuhlii
Linear trend 2006 – 2023 : - 7,7 % (- 18,9 % ; + 5,1 %) not significant
Linear trend 2014 – 2023 : + 0,1 % (- 11 % ; + 12,7 %) not significant
Between 2006 et 2023, the population of the Kuhl's Pipistrelle is not significant, athough the decrease if of 7.7 %. Again, during the more recent period (2014 to 2023), a slight increase of 0.1 % is observed, but it is not significant. These results indicate a relative stability of the effectives of this species over time.
© Laurent Arthur
Pipistrellus pipistrellus
Linear trend 2006 – 2023 : - 23,6 % (- 30,3 % ; - 16,2 %) p-value < 0,001
Linear trend 2014 – 2023 : + 1,5 % (- 7,2 % ; + 11 %) not significant
Between 2006 and 2023, the population of the common Pipistrelle decreased of 23.6 %. However, during the more recent period (2014 to 2023), a slight increase of 1.5 % is observed, but it is not significant, which suggests, that there was no change during this period.
© Charlotte Roemer
Plecotus austriacus
​Linear trend 2014 – 2023 : + 86,8 % (+ 45,5 % ; + 138,2 %) p-value < 0,001
The analysis between 2014 and 2023 reveals a positive and significant trend of 86,8 % for the grey long-eared bat, showing a notable increase for the population size of this species.
© Ludovic Jouve
Plectorus auritus
Linear trend 2014 – 2023 : - 19,3 % (- 44,1 % ; + 16,4 %) p-value < 0,2 not significant
Between 2014 and 2023, a decrease of 19,3 % is observed for the brown long-eared bat, although this decrease is not statistically significant. These results suggest a possible decline for this species over the last years, although it is not possible to confirm.
© Charlotte Roemer
Pipistrellus pygmaeus
Linear trend 2014 – 2023 : - 2,4 % (- 27,4 % ; + 31,2 %) not significant
Between 2014 and 2023, a non-significant decrease of 2.4 % is observed for the soprano pipistrelle, suggesting a relative stability of its population size over this period.
© Ludovic Jouve
Tadarida teniotis
Linear trend 2014 – 2023 : + 160,1 % (+ 72,8 % ; + 291,7 %) p-value < 0,001
Over the period 2014 to 2023, the population of the European free-tailed bat showed a significant increase of 160,1 %, showing an increase of the population size of this species.
© Blandine Carré