Abundance of wild prey modulates consumption of supplementary food
in the Iberian lynx
José V. López-Bao *, Alejandro Rodríguez, Francisco Palomares
Department of Conservation Biology, Estación Biológica de Doñana (CSIC), Seville, Spain
a b s t r a c t
Keywords:
Adaptive management
Behavioural dependency
Food supplementation
Lynx pardinus
Prey abundance
Trophic ecology
Food supplementation is increasingly used as a conservation tool. However, little is known about how
much supplemental food is used by target populations or the degree to which the abundance of natural
food affects the utilization of supplemental food. Long-term supplementation programmes could cause
individuals to rely almost exclusively upon supplemental food and, consequently, lose some skills needed
to forage efficiently on natural food. This may result in reduced fitness upon discontinuation of supplemental food. The Iberian lynx (Lynx pardinus) preys almost exclusively upon European wild rabbits (Oryctolagus cuniculus), and some populations are thought to be food limited. We quantified the contribution of
supplemented domestic rabbits, whose guard hairs could be distinguished from hairs of wild rabbits, to
the diet of the Iberian lynx. We also examined whether the consumption of domestic rabbits varied with
the availability of wild rabbits, and with the duration of exposure to supplemental food. Domestic rabbits
made up over 50% of the diet. Consumption of domestic rabbits decreased non-linearly as the relative
abundance of wild rabbits increased; however, this pattern was true only above a threshold density of
one wild rabbit km—1. Below this threshold, supplementation was apparently strictly necessary to retain
Iberian lynx. The consumption of domestic rabbits did not increase with the length of the supplementation period. Lynx continued consuming wild rabbits proportionally to their abundance, suggesting lynx
did not become dependent upon supplemental food. Understanding how the abundance of natural food
modulates consumption of supplemental food may help to adjust supplementation schedules to food
availability and to the needs of the target populations.
1. Introduction
Providing supplemental food to wildlife is widely used as an
experimental approach in the study of food limitation and its ecological consequences as well as a tool in the management of wild
populations (Boutin, 1990; Putman and Staines, 2004; Jones and
Reynolds, 2008; Robb et al., 2008). In a conservation context, supplementation is used to increase overall fitness by increasing productivity, survival, or both (Elliot et al., 2001; Schoech et al., 2008;
López-Bao et al., 2010). However, unwanted side effects can arise,
including increased obesity (Powlesland and Lloyd, 1994), disease
and pathogen transmission (Miller et al., 2003; Jones and Reynolds,
2008), increased predation risk (Turner et al., 2008), and behavioural alterations (Reese and Kadlec, 1984; Boutin, 1990; Václav
et al., 2003).
Since supplementary feeding is usually designed as a transitory measure to be employed only while levels of natural food
* Corresponding author. Address: Estación Biológica de Doñana, CSIC, c/Américo
Vespucio s/n, Isla de la Cartuja, 41092 Seville, Spain. Fax: +34 954 621 125.
E-mail address: jvlb@ebd.csic.es (J.V. López-Bao).
:
remain depressed (López-Bao et al., 2008), a key goal should be
to minimize the likelihood that a target population develops
dependence upon supplemental food, implying the alteration or
loss of foraging abilities. Under constant supply, animals may become increasingly reliant upon supplemental food (i.e. irreversible dependence; Brittingham and Temple, 1992; Jones and
Reynolds, 2008). In these instances, reducing the supply, whether
gradually or precipitously may have negative effects. On the
other hand, supplementation programs guided by the principles
of adaptive management (Walters, 1986; Lee, 1999; Armstrong
et al., 2007) dictate that the amount of supplemental food provided should be adjusted to the abundance of natural food. Few
studies have assessed the contribution of supplemental food to
the diet of target populations (Brittingham and Temple, 1992;
Partridge et al., 2001; Fleischer et al., 2003; Margalida et al.,
2009), but even fewer have assessed whether fluctuations in
the abundance of natural food influence consumption of supplemental food (Sauter et al., 2006).
The Iberian lynx (Lynx pardinus) is a specialist predator feeding
almost exclusively upon European wild rabbits (Oryctolagus cuniculus; Delibes et al., 2000). Rabbit scarcity has been identified as a
major factor underlying the decline of lynx populations throughout
its range (Rodríguez and Delibes, 2002). During the last 20 years,
rabbit abundance has been extremely low at many sites within
the Doñana area (Moreno et al., 2007), an area that supports one
of the world’s two remaining Iberian lynx populations (Ferreras
et al., 2010). In 2002, food supplementation was implemented as
an emergency measure for lynx conservation and has proven
useful for lynx retention under food-limited conditions (LópezBao et al., 2008, 2009). However, long-term food supplementation
at a given locale, which by definition reduces the spatio-temporal
variability that normally accompanies resource availability
(López-Bao et al., 2008), could cause supplemented individuals to
become dependent upon this resource. It is also conceivable that
long-term supplementation could result in the loss of the skills
that are needed to forage on wild rabbits. Were this the case,
behaviourally dependent lynx may starve as a consequence of
drastic variations or long disruptions in supplementation schedules, or when they leave supplemented areas.
In this study, we quantify the relative contributions of natural
and supplemental food in the diet of the Iberian lynx. Specifically,
we examined: (i) the importance of supplemental food in two lynx
sub-populations with markedly different levels of natural prey
abundance, (ii) whether consumption of supplemental food was
influenced by temporal fluctuations in natural prey abundance,
and (iii) whether consumption of supplemental food increased
with exposure to food supplementation, as a sign of behavioural
dependency.
2. Materials and methods
2.1. Study area and the supplementary feeding programme
We conducted our study in Doñana National Park, SW Spain
(37°100 N, 6°230 W; Fig. 1). Lynx diet was studied in two well-defined lynx sub-populations called Vera (VE; 80 km2) and Coto del
Rey (CR; 25 km2; Fig. 1), around 10 km apart, that have traditionally played the role of sources within the Doñana meta-population
(Gaona et al., 1998). Sub-populations were isolated by unsuitable
habitat (marsh; Fig. 1) and exchange of individuals occurred only
rarely through natal dispersal (Ferreras, 2001). VE and CR differed
markedly in rabbit abundance and lynx density. Over the last decade, maximum lynx density in VE (5 ind 100 km—2) was lower than
in CR (24 ind 100 km—2). Likewise, relative rabbit abundance in autumn, the low rabbit abundance period (Palomares, 2001), in VE
was markedly lower (average number of rabbits per kilometer of
transect ±SD = 0.07 ± 0.05 rabbits km—1; n transects = 7) than in CR
(average number of rabbits per kilometer of transect
±SD = 4.08 ± 2.33 rabbits km—1; n transects = 6; Equipo de Seguimiento de Procesos Naturales – Estación Biológica de Doñana).
We provided live domestic rabbits kept in 4 x 4 m enclosures
with walls 1.3 m high, hereby referred to as feeding stations. Supplementation was continuous and feeding stations were checked
every other day to guarantee a regular and predictable source of
food. In VE, food supplementation began in 2002, whereas it was
implemented in CR beginning in 2006. Further details about the
supplementation procedure are given by López-Bao et al. (2008).
The provision of domestic rabbits was approved by the Regional
Government of Andalusia through the implementation of the conservation project LIFE-02NAT/8609.
Iberian lynx easily entered feeding stations, regularly used the
supplemental food, and consumed almost all of the supplemental
food in some frequently visited stations (López-Bao et al., 2008).
In fact, during the study period, all animals identified living in VE
and CR sub-populations used the supplemental food regularly
(López-Bao et al., 2009).
2.2. Collection and analysis of lynx faeces
Lynx diet was studied by means of faecal analysis. Between January 2005 and April 2007 we collected lynx faecal samples every
3 months. We searched for faeces along paths where Iberian lynx
deposit most of their faeces (Robinson and Delibes, 1988).
Shape, size, colour, and smell are, in combination, diagnostic
attributes of Iberian lynx scats. Some red fox (Vulpes vulpes) faeces
could be wrongly attributed to lynx, but this occurs with very low
frequency. DNA analyses showed that 99.7% of faeces (n = 369)
were correctly assigned to lynx in the field (authors, unpublished
data).
We were able to collect faecal samples in all breeding territories. Each breeding territory holds one adult female, one adult male
and one or more juveniles (Ferreras et al., 1997; López-Bao et al.,
2009). During the study period, VE contained two breeding territories and CR contained three breeding territories (Fig. 1). Preliminary results of a parallel study, in which lynx DNA was extracted
from faecal samples collected with a similar sampling scheme,
showed that faeces were produced by all individuals living in the
sampled area (Rodríguez et al., 2009).
Faecal samples were broken, cleaned, and prey remains classified and identified to the species level whenever possible (Debrot
et al., 1982; Brown, 2003). We determined whether consumed rabbits were wild or domestic on the basis of differences in the colour
pattern of guard hairs found in faeces. Guard hairs of domestic rabbits are known to be a single colour: white, black or brown (50%,
13% and 37%, respectively; n = 3560 supplemented rabbits). In contrast, guard hairs of wild rabbits exhibit a characteristic tricoloured
band at the tip (black at the base, tawny in the middle, and black at
the apex). We used a 10x binocular lens to distinguish between
the two types of rabbits.
2.3. Availability of wild rabbits
Rabbit abundance shows temporal fluctuations along the annual cycle with a low rabbit abundance period in autumn and
a high rabbit abundance period in spring (Palomares, 2001;
Palomares et al., 2001). We estimated relative rabbit abundance
at each of the two study sites by counting rabbits from a vehicle
along fixed transects (>10 km, one fixed transect per area:
14.7 km in VE and 16.2 km in CR) 1 h before dusk (when rabbit
activity is least influenced by seasonal and environmental factors;
Villafuerte et al., 1993) on three consecutive days each month
(Moreno et al., 2007). Transects were conducted along paths of
similar characteristics, guaranteeing a consistent wide and stable
sampling area: a 10-m belt on each side of the transects. We used
the highest count for the 3 days to calculate a monthly index of relative abundance (index of abundance; henceforth, IA) for months
during which faeces were collected. The IA was expressed as the
total number of rabbits counted per kilometer of transect. This
method to estimate relative rabbit abundance and their temporal
fluctuations has been frequently used in the Doñana area (Moreno
et al., 2007).
2.4. Data analyses
We calculated the frequency of occurrence of domestic rabbits
in the lynx diet. For each faecal sample we recorded the occurrence
of wild rabbits and the occurrence of domestic rabbits, and expressed them as two binary variables. We used generalized linear
models with binomial error and logit-link to analyse the influence
of the area and relative abundance of wild rabbits on the occurrence of each type of rabbit in the lynx diet. We included the time
elapsed (months) between the date when supplemental food was
first provided in each area and the date when each faecal sample
Fig. 1. Distribution of the two lynx sub-populations studied in the Doñana area, VE and CR (80 km2 and 25 km2 respectively), SW Spain. Doñana National Park is highlighted
in dark grey (dunes and scrubland) and clear grey (marsh). Spatial location of all the breeding territories existing in both sub-populations (two in VE and three in CR) during
the study period are shown as fixed kernel home ranges using 90% of positions of resident adult females (black polygons). Black points denote the spatial location of all faeces
collected in this study.
was collected as a covariate in the models to test for variations in
consumption of supplemental food over time. All statistical analyses were performed using the R statistical software V.2.8.0 (R
Development Core Team, 2008).
3. Results
A total of 323 faeces samples were collected, 158 in VE and 165
in CR. Overall, only 1.5% of faeces contained prey items other than
rabbits: red-legged partridge (Alectoris rufa), red deer (Cervus elaphus) and ducks (Anas spp.). Out of 323 faeces, 212 (65%) contained
only hair of domestic rabbits, 93 (29%) had only hair of wild rabbits, and 18 (6%) had hair of both types of rabbits. The proportion
of faeces containing hair of domestic rabbits accounted for >50% in
both areas, but its proportion in VE (83%) was significantly higher
(v2 = 21.19, df = 1, P < 0.001) than its proportion in CR (59%). Hair
of wild rabbits appeared in 27% and 41% of scats in VE and CR,
respectively.
Overall, the use of supplemental food was influenced by the relative abundance of wild rabbits. In spite of this, the consumption of
domestic rabbits in VE was relatively constant over time (mean
monthly proportion in the diet = 0.86, range = 0.72–1.00), irrespective of the numbers of wild rabbits which did not statistically
change over the course of the study (v2 = 1.53, df = 1, P = 0.216;
Fig. 2) and remained below the 1 rabbit km—1 (range 0–0.91 rabbits
km—1). However, in CR, IA varied in the range of 1.5–15.0 rabbits
km—1, and the proportion of domestic rabbits in the lynx
diet also varied widely around a mean of 0.56 (range = 0.25–
0.83). The relationship between consumption of domestic rabbits
and relative abundance of wild rabbits was negative and had a
steep slope (Fig. 2). Abundance of wild rabbits began to negatively
influence the consumption of supplemental food above a threshold
of one rabbit km—1, and this effect was significant (v2 = 5.55, df = 1,
P = 0.019). Our model predicted an equal consumption of wild and
domestic rabbits when the relative abundance of wild rabbits was
about seven rabbits km—1 (Fig. 2). Consequently, the frequency of
occurrence of wild rabbits in the lynx diet was only modulated
n.s
Frequency of occurrence
1.0
0.8
0.6
n.s
0.4
0.2
0.0
0.0
0.2
0.4
0.6
0.8
1.0
Relative abundance of wild rabbits (rabbits km-1)
Frequency of occurrence
1.0
P < 0.05
0.8
0.6
0.4
P < 0.05
0.2
0.0
2
4
6
8
10
12
14
16
Relative abundance of wild rabbits (rabbits km-1)
Fig. 2. Predicted frequency of occurrence of wild (dashed line) and supplemented
rabbits (solid line) in lynx faeces against the relative abundance of wild rabbits in
VE (upper panel) and CR (lower panel). Note that patterns with a significant slope
are shown. ns = non significant.
by their density in CR (v2 = 4.50, df = 1, P = 0.034 for CR and
v2 = 0.01, df = 1, P = 0.971 for VE; Fig. 2).
The length of the period that lynx were exposed to food supplementation had no significant effect on the proportion of domestic
rabbits in the lynx diet (v2 = 0.01, df = 1, P = 0.968 for CR;
v2 = 2.59, df = 1, P = 0.107 for VE).
4. Discussion
The fact that supplemental rabbits were an important resource
for the Iberian lynx in both areas may be explained by two lines of
reasoning. Firstly, a high level of consumption of domestic rabbits
may reflect the severe scarcity of wild rabbits at many sites in the
Doñana area, particularly in VE (Moreno et al., 2007), combined
with the strict trophic specialization of the Iberian lynx (Delibes
et al., 2000). Secondly, all else being equal, predators take the most
vulnerable prey (Quinn and Cresswell, 2004). Therefore, high consumption levels of supplemental food may arise in part due to the
capture of domestic rabbits in small enclosures requiring lower
foraging costs compared to seeking wild rabbits at very low densities. This might be especially true in VE.
As previously suggested by López-Bao et al. (2008), we found
that lynx continued to feed on wild rabbits, even in VE where the
supplemented rabbits were an ecological necessity. This suggests
that lynx may prefer to feed on natural food where available. The
hypothesis that lynx prefer wild rabbits was also supported by
the fact that as the abundance of wild rabbits increased, lynx consumed a greater proportion of wild rabbits (see Fig. 2). This possible preference for wild rabbits, even when the alternative prey is so
similar, could be associated with a lack in some important nutritional component in the supplied food or with unknown costs of
visiting feeding stations (e.g. competitive interactions with other
lynx; López-Bao et al. (2009). In fact, we tend to believe that without food supplementation the lynx may have abandoned the VE
area.
Although domestic rabbits made up a considerable fraction of
the lynx diet, in CR consumption of supplemental food greatly varied over time. The role that supplemental food played in the lynx
diet was apparently modulated by the abundance of wild prey.
The consumption of domestic rabbits decreased with increasing
abundance of wild rabbits. However, our results suggest that this
prey shift only occurs above a threshold value of wild rabbit abundance. Below this threshold, lynx seem to be compelled to survive
upon supplementary food.
Feeding stations were operative between 2 and 5 years (LópezBao et al., 2008, 2009). After such a long exposure to supplementary food Iberian lynx continued to prey upon wild rabbits and
we found no sign that they progressively reduced the intake of natural food over time. Lynx seemed to be able to discriminate between natural and artificial food (Sauter et al., 2006), with prey
choice being determined by wild rabbit abundance. From this we
infer that, overall, lynx apparently did not lose the skills necessary
to forage efficiently on wild rabbits. We conclude that the hypothesis that food supplementation promotes high levels of behavioural dependency in the Iberian lynx was not supported.
There have been many efforts to increase wild rabbit densities
in Doñana through habitat management and rabbit translocations
to aid conservation of the Iberian lynx (Moreno and Villafuerte,
1995; LIFE, 2002; Delibes-Mateos et al., 2009). To date, most of
these attempts have been relatively unsuccessful, with results
being only satisfactory on a small scale (Moreno et al., 2004). We
recommend the use of supplementary feeding during periods
where the relative density of wild rabbits within lynx territories
does not reach at least a value of IA = 1 rabbit km—1. As wild rabbit
density increases above this threshold value, supplementation may
still be necessary but schedules may be adjusted to rabbit abundance and the energetic demands of lynx (López-Bao et al., 2008,
2009) following an adaptive management approach (Walters,
1986; Lee, 1999; Armstrong et al., 2007). To adapt supplementation to wild rabbit abundance efficient methods to monitor rabbit
abundance and its variations in the short and mid-term is needed,
as well as at different spatial scales.
Acknowledgements
This research was funded by the Spanish Ministry of Education
and Science (project CGL2004-00346/BOS), the Spanish Ministry of
the Environment under the National Parks research programme
(Grant-17/2005), and BP-Oil Spain. The Consejería de Medio Ambiente (Junta de Andalucía, JA) partially financed the supplementary
feeding programme under the project LIFE-02NAT/8609. Land-Rover España S.A. kindly lended the vehicles for this work. JVLB was
supported by a FPU fellow (Ministry of Education).
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