The negative impact of heavy rains on the abundance
of a Mediterranean population of European rabbits
By F. PALOMARES
Department of Applied Biology, Estacioon Bioloogica de Donana, CSIC, Sevilla, Spain
Abstract
European rabbits, Oryctolagus cuniculus, are the basic prey of several endangered predators and an
important hunting bag in the area of its origin in southwestern Europe. Conversely, they are considered an undesirable species in other areas where they were introduced. Therefore, there is great
interest in understanding the factors that influence population dynamics and abundance of this
species. I studied the effects of relatively common heavy rains on inter-annual variations in rabbit
density in four habitats of Donana National Park and surrounding areas (southwestern Spain) during several years when rainfall was either lower or higher than average. I estimated spring and autumn rabbit densities by line transect sampling between autumn 1993 and autumn 1998, and
counted rabbit warrens and entrances in two of the habitats (one with and the other without scrubland vegetation) in 1995, 1996 and 1997. Rabbit density significantly decreased in all habitats
during the rainy years, densities being on average 5.3 and 4.6 times lower for spring and autumn
censuses, respectively. Both number of warrens and entrances significantly decreased after two
consecutive years of heavy rain in both habitat types, although in the scrubland habitats some recovery was observed during the second consecutive year of heavy rains. The area where warrens
were apparently free of the effects of rains was only between 2.7 and 3.8o in the open habitat
and 21.5o in the scrubland. At least for the open habitat, no clear relationship was observed between the height above sea level and whether warrens were affected by rain or not. The results indicate that heavy rains may be an important factor decreasing rabbit density, at least in flat areas,
by negatively acting on warrens during the breeding period.
Key words: Oryctolagus cuniculus, flooding, warrens, interannual density
Introduction
The European rabbit, Oryctolagus cuniculus, is a species of extraordinary but contrary concern in both the area of its origin
and places where it was introduced (TH0MPS0N and KING 1994). In the areas of origin
in southwestern Europe (BRANc0 et al.
2000), rabbits are an important hunting
bag and the staple prey of many endangered predators such as Iberian lynx, Lynx
pardinus, or imperial eagle, Aquila adalberti
(DELIBES and HIRALD0 1981; R0GERS et al.
1994), but in many other parts of its distribution, they are considered an undesirable
species due to economic losses in agriculture or to damage of conservation actions
for endangered species (e. g. K0LB 1994).
Therefore, population dynamics of rabbits
have received considerable attention by
both field and theoretical biologists (e. g.
MYERS and PARKER 1974; PEcH et al. 1992;
SMITH and TR0UT 1994; GIBB and FITZGERALD 1998).
It has been well documented that rabbit populations undergo high intra-annual oscillations in abundance as a direct consequence
of seasonality in reproduction (DELIBES
and CALDER0N 1979), triggered by the availability of food (WALLAGE-DREES 1983; VILLAFUERTE et al. 1997). In Mediterranean environments of Europe, the breeding period
can extend from mid autumn to the end of
spring (R0GERS et al. 1994). Myxomatosis,
which is usually more virulent during summer and autumn in Europe, and viral hemorrhagic disease (RHD) have also a significant effect on intra-annual variations in
rabbit numbers (R0GERS et al. 1994; VILLAFUERTE et al. 1994; MARcHANDEAU et al.
1998; MUTZE et al. 1998). In the same way,
rabbit abundance also varies widely among
years (e. g. MYERS and PARKER 1975; R0GERS
et al. 1994; PAL0MARES et al. 2001), although
not in a predictable way as in snowshoe
hares, Lepus americanus, or small mammal
populations in northern ecosystems (KREBS
1996). Between-year oscillations in rabbit
abundance have been related to variations
in the incidence of myxomatosis or RHD
(S0RIGUER and R0GERS 1981; VILLAFUERTE
et al. 1995), food availability (VILLAFUERTE
et al. 1997; GIBB and FITZGERALD 1998),
drought periods (MYERS and PARKER 1975),
or predator abundance (PEcH et al. 1992;
BANKS 2000).
Here, I studied the effect of heavy rains on
rabbit density on an inter-annual basis in
an area of the Iberian Peninsula, where the
European rabbit has its evolutionary origin
(BRANc0 et al. 2000).
Material and methods
Study area
The study was carried out in the north of Donaana
National Park and surroundings, in an area called
Coto del Rey (south-western Spain; 37°9’ N,
6°26’ W) between November 1993 and October
1998. The Donaana National Park is a flat, sandy
area containing three main biotopes: scrubland,
dunes, and marsh (VALVERDE 1958). The climate
is Mediterranean sub-humid, with an annual rainy
period usually starting in autumn and ending in
spring, and a dry period when rain is very rarely
lasting 3—5 months around summer. Annual rainfall is between 500 and 600 mm, although during
the study period rainfall was lower than average
during the first 2years (annual totals of 413 and
253 mm, respectively; annual rainfall measured
between dry seasons), followed by 3 rainy years
(mainly the first 2years) when annual totals were
1 032, 885 and 629 mm, respectively. During these
years, several times monthly rainfall exceeded
200 mm (Fig. 1 a). In addition, the number of days
with heavy rains (here considered as days with
rainfall 25 mm) was much more frequent from
winter 1995/96 onward than previously (Fig. 1 b).
The low rainfall years of the study period were a
continuation of a longer period that started in
1990/91 with rainfall close to or lower than average (480 mm for 1990/91, 494 mm for 1991/92,
and 374 mm for 1992/93).
Coto del Rey is characterized by a mixture of relatively well-conserved areas of Mediterranean
scrubland and ash stands where Pistacia lentiscus
shrubs predominate, plantations of mainly Pinus
pinea, and cleared areas for cattle grazing with
isolated trees (Quercus suber and Olea europaea);
marsh extends to the south of the study area.
Ground heights in the study area range between
1—6.4 m a. s. l. More information on the study area
can be found in PAL0MARES et al. (2001).
The effect of the rain on rabbits was studied in
4 areas/vegetation types of Coto del Rey: Mediterranean scrubland, ash stands inside the National Park, ash stands outside the National Park,
and pastureland. The first three areas were scrubland habitats, while the last was an open habitat
(PAL0MARES et al. 2001).
Rabbit density
Rabbit density was estimated twice per year between autumn 1993 and autumn 1998 in the
4 areas/vegetation types described. Rabbits were
counted in October—November and June each
year (a priori times of the lowest and highest rab-
Fig. 1. Monthly rainfall (a) and number of days with rainfall higher than 25 mm (b) between December 1992 and
October 1998 in a meteorological station situated within the Donana National Park to 12 km from the study area.
bit density; S0RIGUER 1981). Rabbit density in
each area was estimated by line transect sampling
and the program TRANSECT (BURNHAM et al.
1980; BUcKLAND et al. 1993). Transects ranged
from 1 100 m to 1 870 m per habitat and were
slowly (ca. 1.5—2.5 km/h) surveyed on foot at dusk
(15—20 min before sunset to 25—30 min after sunset). In total, there were 72days of valid censuses,
during which 110 580 m were walked and 3 176
rabbits were counted. More details on the
methodology can be found in PAL0MARES et al.
(2001). Results are given as mean rabbit density
and 95% confidence intervals of the estimated
density.
Significant differences in rabbit densities between
censuses carried out in drought (autumn 1993—autumn 1995) and rainy (spring 1996—autumn 1998)
study periods were tested by general lineal mod-
els using the GLM procedure in SAS (SAS INSTITUTE INc. 1990), and a Type III sums of squares.
In addition to general weather conditions, season
and habitat were also included in the model to
control for the possible effect of these variables
on results of the test.
Warren surveys
Warren surveys were carried out in the most representative and distinct habitats of the study
area (Mediterranean scrubland and pastureland)
between April and October of 1995 (the end of
inter-years drought period) and at summers 1996
and 1997 (both years with winters of heavy rains;
see Fig. 1). Areas surveyed in each habitat were
the same 342ha each year for the pastureland
and 126 ha for 1995 and the same 23 ha (included
within 126 ha of 1995) for both 1996 and 1997 at
the Mediterranean scrubland. Warren surveys
consisted of recording the size (i. e. number of entrances) and position of every warren within these
areas in U. T. M. coordinates using a Global Positioning System (GPS; model Garmin 75). Used
and unused entrances were counted for each warren, but not collapsed ones. Used entrances were
these with clear signals of use such as clean paths,
absence of cobweb or presence of rabbit tracks,
whereas unused ones lacked these signals. In the
scrubland habitats of the study area, rabbits
mainly use warrens during the breeding period
(PAL0MARES 2001). For warren mapping, teams of
3—5 persons slowly walked small sections of the
area inspecting for warrens everywhere. Differences in the total number of warrens and entrances recorded in 1995 (before heavy rains)
and 1996 and 1997 (after one and two years, respectively, of heavy rains) were tested by Chisquare tests considering the data from each habitat type independently.
In addition to these warren surveys, the immediate effect of rain on warrens was field-assessed
on 2—3 February 1996 in the pastureland habitat,
and on 12—13 January 1997 in both pastureland
and Mediterranean scrubland habitats. In these
surveys, I searched for warrens and considered
that warrens were not affected by rain if they
had at least one entrance apparently free of the
effect of rain (i. e. the visible part of the entrance
was not flooded or blocked by sand). Obviously,
this approach was conservative because it is reasonable to think that in many cases warrens could
actually be flooded although water could not be
seen. These surveys were carried out 2—3 days
after heavy rains in 1996 (22 l/m2/day), and 5—
6 days after heavy rains in 1997 (41 l/m2/day).
Results
Rabbit density
Rabbit density oscillated between 55 and
less than one individual per hectare in the
Mediterranean scrubland during June 1994
and pastureland during both October of
1997 and 1998, respectively (Fig. 2). After
the start of the rainy period in November
1995, rabbit density was lower in all habitats (Fig. 2). Density of rabbits was on
average 5.3 times lower (range = 2.6—9.0
for the different habitats) in the rainy years
than in the drought years for the June censuses, and 4.6 times lower (range = 2.4—8.3)
compared with the October censuses. Differences between censuses carried out during drought and rainy periods were highly
significant (F = 74.03, d. f. = 1, p < 0.001),
once the effect of season and habitat was
controlled.
Effect of rain on warrens
Warren densities in summer 1995 were 7.0
and 1.5 per hectare for the habitats Mediterranean scrubland and pastureland, respectively (Fig. 3). Density of total entrances were 38.1 and 18.2per hectare for
the same habitats (Fig. 3). Both the number
of warrens and entrances decreased in both
habitats after the first year of heavy rains
(Fig. 3), although significant differences between 1995 and 1996 for both habitat types
combined were only detected for the
number of entrances (x2 = 17.7, d. f. = 1,
p < 0.001). During the second year of heavy
rains, the number of warrens and entrances
continued to decrease in the pastureland,
but not in the Mediterranean scrubland
where some recovery was observed
(Fig. 3). This recovery was also observed in
the direct rabbit censuses, which mirrored
quite well what happened with warrens
and entrances in both habitat types (Figs. 2
and 3). The overall effect of rain on warrens
after the two years of heavy rains was significant both for warrens (x2 = 28.5, d. f. = 1,
p < 0.001) and entrances (x2 = 161.0, d. f. = 1,
p < 0.001).
Fig. 2. Spring and autumn rabbit density in four habitats of the Donana National Park or surroundings between
November 1993 and October 1998. Cape bars indicate the 95o confidence intervals around the mean estimates.
NP = national park; NNP not national park.
The winter surveys of the study area during
1996 and 1997 showed that not more than
3.8% in 1996 and 2.7% in 1997 of pastureland, and in 1997 21.5% of Mediterranean
scrubland were at least partially free of the
effect of the rain on warrens. In the pastureland no clear relation was observed between warrens with at least one entrance
free of the effect of rain and the height
above sea level. These warrens were located in 8 different sites that coincided
quite well in both years and were between
2and 6 m a. s. l. Many other warrens located at the same height were affected by
rains. This probably depended more on the
topography of the ground in each site
Fig. 3. Summer warren and entrance densities in two habitats (Mediterranean scrubland and pastureland) of the
Donana National Park for 1995, 1996 and 1997.
where warrens were built than on the elevation above sea level. Most entrances were
either flooded, or blocked by the ground
falling off on the top of the entrance, or
blocked by the sand carried down by water
during the rain (Fig. 4). Conversely, the results were different in the Mediterranean
scrubland as most warrens were not fully affected by rain (all except one) and situated
higher than 4.6 m a. s. l.
Discussion
Factors determining the variations in rabbit
density among habitats in the study area
have already been discussed elsewhere (PAL0MARES et al. 1996; 2001; PAL0MARES and
DELIBES 1997), therefore I will focus here
on between-years variations and the role of
heavy rains.
Fig. 4. EEamples of rainfall effects on rabbit warrens in the pastureland habitat. (a) flooded warren; (b) blocked
warren by falling off the ground; (c) blocked warren by sand carried down by rain; and (d) detail of entrance
blocked by sand carried down by water where it is possible to see the track left by running water. Arrows indicate
warren entrances.
The data presented suggest that an excess
of rain, concentrated in a few months and
even days, negatively affected rabbit populations of the study area. The effect was simultaneous and significant in different habitats, although rabbits in open habitats
suffered the highest negative impact. Heavy
rains acted directly, by flooding and collapsing most warrens, at the time of the year
when rabbit reproduction is common in the
study area (DELIBES and CALDER0 N 1979).
Flooding and collapsing of warrens most
likely directly killed most young rabbits
that were unable to escape. C0PS0N et al.
(1981), PARER et al. (1987) and GIBB (1993)
reported young rabbit deaths in warrens
due to wet weather. In addition, most rain
in the study area was concentrated at the
end of autumn and beginning of winter,
the time of first litters (DELIBES and CALDER0 N 1979; CALZADA 2000). Young rabbits
born at these times of the year are more
healthy and experience a higher probability
to survive than young rabbits born later
(RIcHARDS0N and W00D 1982; WEBB 1993).
Furthermore, rabbits born at these times
became reproductively active during the
same breeding season. Therefore, the negative effect of rain on rabbit population dynamics is likely to be profound, as it hit the
more potentially productive fraction of
young rabbits.
Other factors such as the RHD could
have caused the declined observed in the
rabbit populations of the study area.
Although this hypothesis cannot be totally
discarded, it seems unlikely since in Dona ana RHD arrived in 1989 (VILLAFUERTE et
al. 1994), and during both dry and wet
years along the study, a few rabbits were
found dead of RHD in the study area
(C. G0RTAZAR, pers. comm.; P. MATIN, pers.
comm.).
Even though rabbit populations in both
open and scrubland habitats suffered from
the negative effect of heavy rains, the effect
was lower in the scrubland. This was probably because warrens were less affected in
scrubland than in open habitats. There was
even a slight recovery of warrens in the
scrubland habitat during the second year of
heavy rains. In scrubland habitats, most of
the warrens are built between roots of tall
shrubs (PAL0MARES 2001), which can therefore help to prevent them from collapsing
and being fully blocked by water-deposited
sand as was very frequent in the pastureland. The recovery in Mediterranean scrubland could be produced by rabbits selecting
new and safer places to build the warrens
in the second year of heavy rains. However,
in the pastureland, rabbits must be limited
in their building capabilities because they
are more exposed to predation while excavating warrens than in the Mediterranean
scrubland (VILLAFUERTE and M0REN0
1997). In fact, in the pastureland, warren
distribution seems to mainly be the consequence of human management of the area
carried out in 1970 when scrubland vegetation was removed and warrens destroyed
except in a few places (PAL0MARES et al.
1996). Since then the spread of rabbits over
the full area has been limited.
In this study, I show that inter-annual variations in rabbit abundance may also be
caused by heavy rains, which had a particularly negative effect in altered-open habitats. However, the continuous decline observed in rabbit populations during the
last decades in the Mediterranean ecosys-
tems where they originated (R0GERS et al.
1994) may have been caused by other factors. On this larger temporal scale, competition with larger herbivores (S0RIGUER
1983), mesopredator release due to decline
in numbers of top predators (PAL0MARES et
al. 1995), loss of traditional land management practices (M0REN0 and VILLAFUERTE
1995), arrival of myxomatosis first (R0GERS
et al. 1994), and later the viral hemorrhagic
disease (VILLAFUERTE et al. 1995; G0RTAZAR 1997), have been proposed as potential
factors explaining the decline. Probably all
these factors have acted jointly on rabbit
populations to produce their present status.
Results of this study can have important
management implications for rabbits. In
addition to an important hunting bag (R0GERS et al. 1994), rabbits are also the prey
of species of great conservation value such
as the Iberian lynx and the Spanish imperial eagle. Therefore, rabbit recovery plans
are being undertaken in many places of
the Iberian Peninsula (e. g. see G0RTAZAR
et al. 2000), including the Dona ana region.
Amongst the management activities undertaken, building of artificial warrens is common (G0RTAZAR et al. 2000; SANZ-ZUASTI
and PABL0S 2000). This study indicates that
in flat or sandy areas, warrens should be
built in places with low probability of
being affected by heavy rains, perhaps simulating the levees that rabbits frequently
use to built their warrens in the Camargue
(R0GERS 1981).
Acknowledgements
The research was supported by DGICYT and
DGES (projects PB90-1018, PB94-0480 and
PB97-1163), Ministerio de Medio Ambiente and
Consejer’ia de Medio Ambiente de la Junta de
Andaluc’ia. E. REVILLA, J. CALZADA and several
students helped during warren counts. M. DELIBES, E. REVILLA, A. R0DRIGUEZ and S. M0REN0
made useful suggestions on previous drafts of the
manuscript.
Zusammenfassung
Der negative EinfluB von starken Niederschlaagen auf die Abundanz einer mediterranen
Population europaaischer Wildkaninchen
Europa” ische Wildkaninchen, Oryctolagus cuniculus, sind die wichtigste Beute verschiedener gefa” hrdeter Predatoren und, in ihren Herkunftsgebieten in Su” dwesteuropa, eine wichtige Jagdbeute.
Demgegenu” ber werden europa” ische Kaninchen in Gebieten wo sie eingefu” hrt wurden als unerwu” nschte Art betrachtet. Daher ist es wichtig, die Faktoren zu verstehen, welche die Populationsdynamik und Abundanz dieser Art bestimmen. In dieser Studie wurde der EinfluR der relativ ha” ufigen
starken Niederschla” ge auf die ja” hrlichen Vera” nderungen der Kaninchendichte in vier Habitaten des
Donana National Park und den angrenzenden Gebieten in Su” dwestspanien untersucht. Datenanalysen wurden fu” r mehrere niederschlagsreiche und niederschlagsarme Jahre durchgefu” hrt. Kaninchendichten wurden zwischen 1993 und 1998 im Fru” hling und im Herbst mit der Linientransektmethode
bestimmt. Kaninchenbaue und Einga” nge wurden 1995, 1996 und 1997 in zwei der vier Habitate
(eines mit und eines ohne Buschvegetation) geza” hlt. Die Kaninchendichte verringerte sich in allen
Habitaten wa”hrend niederschlagsreicher Jahre signifikant und war im Fru” hjahr bzw. im Herbst um
einen Faktor 5,3 und 4,6 geringer. Sowohl die ahl der Kaninchenbaue als auch die der Einga” nge
verringerten sich in beiden Habitattypen signifikant nach zwei aufeinander folgenden niederschlagsreichen Jahren, obwohl im Habitat mit Buschvegetation ein leichter Anstieg im zweiten
Jahr beobachtet wurde. Die Kaninchenbaue, die offenbar keinem EinfluR von Niederschla” gen unterlagen, lagen nur zu 2,7o und 3,8o in offenen Habitaten und zu 21,5o in Buschhabitaten. umindest fu” r das offene Habitat bestand keine klare Beziehung zwischen der Ho” he u” ber dem Meeresspiegel und dem EinfluR von Regen auf Kaninchenbaue. Diese Ergebnisse zeigen, daR starke
Niederschla” ge, zumindest in Ebenen, die Kaninchendichte negativ beeinflussen ko” nnen, indem sie
die Kaninchenbaue wa”hrend der Tra” chtigkeit der Weibchen scha” digen.
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Author’s address:
FRANcISc0 PAL0MARES, Department of Applied
Biology, Estacio’ n Biolo’ gica de Dona ana, CSIC,
Avda. Mar’ia Luisa s/n, 41013, Sevilla, Spain
(e-mail: ffpaloma@ebd.csic.es)