Internal structure and patterns of contraction in the geographic
range of the Iberian lynx
Alejandro Rodrı́guez and Miguel Delibes
We use reports obtained in a field survey to characterize the internal structure, and
to reconstruct the strong contraction in the geographic range of the Iberian lynx
Lynx pardinus during a 35-yr period. Lynx were distributed in one large central
population surrounded by smaller peripheral ones. Abundance was autocorrelated,
attained high values only in a few scattered sites mostly within the central population,
and increased from west to east along major mountain chains. Abundance and
occupancy were positively related. The strength of range contraction was similar in
both large and small populations. We were able to date two peaks of local extinction,
which were accompanied by many events of population fragmentation. We also
identified five areas where local extinctions aggregated in space. Lynx relative
abundance appeared to be site-specific, and the probability of local extinction
decreased with increasing abundance. We suggest that sublethal deterministic factors
operating with similar intensity all over the range, rather than in progression from a
focal point, best explain the observed trajectory of contraction. Two factors that meet
such characteristics may have had an outstanding role in lynx decline. One of them
is myxomatosis, an introduced viral disease that decimated rabbits Oryctolagus
cuniculus, the staple prey of the lynx. The other is related to the changes in land use
prompted by extensive human emigration from the countryside 40 yr ago. Other
stochastic or superimposed deterministic factors may have accounted for regional
aggregation of local extinctions. The identification of these factors is very important
to reverse the lasting decline of this endangered felid.
Rodrı́guez, A. (alrodri@ebd.csic.es) and M. Delibes, Dept of Applied Biology, Estación
Biolólogica de Doñana, CSIC Avda. Marı́a Luisa s /n, E -41013 Sevilla, Spain.
Detecting and monitoring populations of rare endangered species is difficult. This is one of the reasons why
the decline and extinction of many species pass unnoticed (May et al. 1995). In the absence of direct data on
population trends, the study of changes in the limits of
geographic ranges can be used to document declines, as
well as to draw inferences about their causal processes
(Rosen 1988, Myers and Giller 1988, Gaston 1990,
Caughley and Gunn 1996, Brown and Lomolino 1998,
Shaffer et al. 1998). A range contraction may be viewed
as the sum of many local extinctions. Therefore, when
the aggregation in space or time of local extinctions is
correlated with the action of a given putative factor,
this factor can be proposed as an agent of decline.
Two types of extinction forces can be distinguished
according to their strength and initial impact on populations. On the one hand, enduring intense disturbance
can drastically modify environmental conditions, bringing local populations to a rapid extinction and impeding their recovery. An example of this, for forest
species, would be the lasting transformation of forest
into artificially maintained agricultural land. On the
other hand, the extinction force may not necessarily
result in a longstanding extinction, either because a
severe disturbance may be exerted only temporarily
allowing future recolonization, or a less acute disturbance may merely reduce population size. The response
of a forest species to a large forest fire or a moderate
decline in prey would illustrate these two effects respectively. Each type of extinction force would produce a
distinctive trajectory of range contraction (Gaston
1994, Bohning Gaese and Bauer 1996, Channell and
Lomolino 2000a). Strong permanent disturbances
would wipe out both weak and healthy populations.
Conversely less severe and/or temporal disturbances
would produce immediate but perhaps reversible extinctions only in the weakest local populations. In the
context of the range dynamics of single species, the
prediction of a positive relationship between persistence
and population density relies on the assumption that all
populations are subjected to the synchronous action of
the extinction factor. This disturbance should also be
exerted with uniform intensity over the entire geographic range (Channell and Lomolino 2000a), e.g. a
global rise in temperature due to climatic change.
The two types of extinction force differ in nature too.
Leaving aside natural catastrophic events, nowadays
overwhelming disturbances are almost invariably associated with human intervention (Diamond 1989). Several authors have reported that such anthropogenically
caused extinctions often spread contagiously over space
and sequentially over time (Towns and Daugherty
1994, Lomolino and Channell 1995, Channell and Lomolino 2000a). In contrast, the proximate cause of
reversible declines and temporal extinctions is usually
environmental, irrespective of whether the ultimate inducing factor is natural or anthropogenic (Mehlman
1997, Cade and Woods 1997).
To study the relationship between population density
and resistance to the forces of extinction, details of the
distribution and abundance of species (i.e. the internal
structure of the geographic range, Brown et al. 1996)
need to be known. Models based on extensive empirical
evidence indicate that local population density reflects
environmental suitability, which is autocorrelated and
varies gradually in space (Brown 1984, Gaston 1990,
Brown et al. 1995). Most species show a concentric
decreasing density towards the periphery of their range.
Thus peripheral populations, presumably smaller and
more fluctuating in numbers than central ones, are
expected to be at a higher risk of extinction (Pimm et
al. 1988) unless maintained by immigration (Hanski
1999). However, environmental suitability does not decrease towards the range boundaries in all species (e.g.
Blackburn et al. 1999). Abundance may be relatively
high in several separate areas where the combinations
of the niche components or dimensions are most favourable, producing multimodal patterns of abundance
(Brown 1984, Lawton 1995). Further, environmental
conditions may become abruptly, rather than gradually,
unfavourable across space. Therefore, in the absence of
factors causing unavoidable extinction, the internal
structure of the range will strongly influence the resulting pattern of contraction.
The range of the endangered Iberian lynx (Lynx
pardinus Temminck) has suffered a strong contraction
during the second half of the 20th century (Rodrı́guez
and Delibes 1990). In this paper we reconstruct the
trajectory of this range contraction and discuss possible
explanatory factors. We describe the internal structure
of the lynx range and identify aggregations of local
extinctions in space and time which could be linked to
intensive action of extinction forces. Finally, we examine the relationship between the contraction trajectory
and the distribution of abundance across the range.
Specifically we analyse whether the pattern of range
collapse reflects the distribution of lynx density and
whether there is any sign of contagion in the contraction trajectory.
The Iberian lynx range
Historical distribution can be defined as the area a
species occupied at the time of its maximum expansion.
The precise limits of the historical distribution of the
Iberian lynx are not well known. It is believed that this
species is the evolutionary result of a trend towards
body size reduction which was initiated by its European
Pleistocene ancestors to escape competition (Werdelin
1981). A further stage of this trend would have resulted
after the immigration of Lynx lynx L. from Asia, which
may have replaced the Iberian lynx in parts of Europe
other than Iberia (Kurtén and Granqvist 1987). If this
is true, perhaps there was not much distributional
overlap between the current forms of L. lynx and L.
pardinus. Fossils of the Iberian lynx’ close ancestor L.
pardinus spaeleus Boule have been found both in the
Iberian peninsula and north of the Pyrenees, as far as
southern Germany (Kurtén 1968, Kurtén and Granqvist 1987, Garcı́a et al. 1997, Garcı́a and Arsuaga
1998). On this basis we roughly guess the boundaries of
its historical distribution (Fig. 1). More recently, the
location of subfossils from archaeological studies indicates that the lynx was already restricted to the Iberian
peninsula (Lauk 1976, Morales pers. comm.), although
widespread over most of it. The naturalists of the 19th
century still reported lynx occurrence in many localities
of northern and eastern Iberia (Rodrı́guez and Delibes
1990) as well as near the Atlantic coast (Almaça 1992).
Interestingly, lynx density appeared to be quite high in
some of these sites only a few decades before local
populations went extinct (e.g. mountains around
Madrid, Graells 1897). By 1950 the Iberian lynx was
confined to the southwest of the Iberian peninsula,
although some uncertain, relict populations were still
reported in the north and east (Valverde 1963, Garzó n
1978, Delibes 1979). Around 1990 the species persisted
in ten highly fragmented populations in the southwest
(Rodrı́guez and Delibes 1990, 1992, Castro and Palma
1996). Today some of these populations may have
disappeared (Delibes et al. 2000).
Methods
Lynx survey
We first defined the area which comprised the estimated
lynx range at ca 1950. To delimit this area we a)
compiled reports from the literature, private archives
and museums, b) published a request for reports of lynx
in hunting and nature journals, which yielded 57 communications, and c) sent a postal enquiry, between
November 1987 and May 1988, to 3816 forest guards,
hunters, naturalists, and taxidermists living in the
southern two-thirds of peninsular Spain (Rodrı́guez
and Delibes 1990), which produced 1310 answers. Most
reports obtained during this preliminary phase were
dated after 1940 and restricted to the four southern
mountain ranges in the Iberian peninsula plus the flat
coastal area of Doñ ana (Fig. 1). The resulting framework area roughly occupied 74 000 km2.
The next step was to determine whether lynx occurred in sampling plots later than 1950 and, if ever
present, when observations stopped. To build a data
base of Iberian lynx reports (location plus date of
sightings and deaths) we designed a field survey based
on interviews. This method has proved valuable to
estimate distribution and density of elusive carnivores
over large areas (Blanco et al. 1992, Gros et al. 1996,
Gros 1998). After excluding extensive farmland gaps
from the framework area field work coverage was
restricted to 45 950 km2. We searched actively for firsthand reports provided by gamekeepers, landowners,
and herdsmen who lived permanently in the coun-
Fig. 1. The Iberian lynx
geographic range. Top: light
grey shows the estimated
boundaries of the historical
range. The rectangle delimits
the area containing the
Spanish fraction of the
estimated lynx range in 1950.
Bottom: altitude increases as
the shading becomes lighter.
Names and approximate limits
of major mountain chains are
given. Principal rivers are also
shown. Hatched: Doñ ana
coastal plain.
tryside and were not contacted previously. We also
personally interviewed people who reported lynx
records by mail. Respondents were asked to date and
describe, in as much detail as possible, all reports they
could remember, either recent or old. Unless supported
by proof, the acceptance of reports obtained or checked
during the field work was dependent upon a favourable
assessment of respondent reliability. Following the interview, respondents were subjectively classified as reliable or unreliable according to their field experience,
knowledge, and tendency to exaggerate facts. We only
noted reports from respondents classified as reliable
when date and location were accurately remembered.
Around 2500 people were contacted in the field, although only 1483 interviews were used after discarding
unreliable respondents.
We assumed that, all else being equal, the probability
of obtaining lynx reports was positively related to
relative lynx abundance, given a fixed searching effort.
Dominant big game hunting practices implied that the
distribution of potential observers in the field, mainly
gamekeepers and hunters, was similar over most of the
surveyed region (one keeper 10 – 30 km − 2, one hunting
day per year and per 5 km2). Overall mean density of
interviews was 3.2 100 km − 2 (range 3.0 – 3.7). Average
sampling effort was enough to identify relict populations during the first half of the century (Rodrı́guez and
Delibes 1990) as well as populations living at very low
densities in more recent times (Rodrı́guez and Delibes
1992).
In all interviews we specifically asked whether lynx
had always been there or had colonised the area. The
answers were fully consistent across the lynx range. We
only recorded reports where lynx were known to ‘‘always’’ be present in the area. There was no single case
of lynx reported as a newly established species, either in
the present or in the past, on the basis of local tradition. Further, in areas where no reliable reports were
found, over 70% of people had never heard of lynx in
their vicinity, and some could not even identify a lynx
picture. In these areas, lynx may have been absent for a
much longer period. This clear pattern strongly suggests that the lynx range contracted steadily during the
second half of the 20th century. We thus made this
continuous contraction a central assumption of the
treatment of the database. Consequently, distribution
maps estimated for a given year were constructed only
with the reports dated from that year on. A further
assumption was that lynx occurred in a locality until
the date of the most recent report, either continuously
or discontinuously in time (local turnover), and was
extinct thereafter. Although our method does not allow
unequivocal demonstration that lynx were absent in a
particular cell after the last known report, this second
assumption relies on our large sampling effort, our
ability to detect populations at very low densities (Rodrı́guez and Delibes 1992), and more recent data sup-
porting these local extinctions ten years after the end of
our study (Rodrı́guez et al. 2000, Palomares pers.
comm.).
Maps of distribution and abundance
The database contained 2240 reports. We represented
lynx distribution on a 10 km UTM projection grid. One
cell in the grid provides sufficient space, on average, for
13 adult lynx (Ferreras et al. 1997). This spatial resolution will then indicate the extinction of local populations rather than changes in the size and occupancy of
individual territories. With the whole database we produced a reference map that we consider an estimate of
lynx distribution in 1950 (Fig. 2a). Seven similar distribution maps were constructed at five-year intervals.
Each one contained all reports dated during and after
the title year. We chose a temporal scale of five years
because it allows lynx detection in cells where the
species lives at low density so reports are not generated
every year, but is also a short enough period to record
rapid changes in lynx distribution. To compare relative
abundance over time with independent data, another
series of maps was prepared using only the reports
dated in each group of five consecutive years.
In each map, a population was defined as a continuum of occupied adjacent cells, connected either by
sides or corners. Local population refers to each single
occupied cell, whether isolated from others or not. The
number of reports per cell (raw or in categories) was
used as an estimate of local abundance (Fig. 2). When
comparing two maps, local extinction was defined as
the shift of a cell from occupied to empty. Population
size refers to the number of occupied cells. We used the
term ‘‘area loss’’ to denote the number of local extinctions in a population or a natural region during a given
period. The rate of area loss indicates the number of
local extinctions divided by the size of the population in
an early stage of lynx decline. Area loss, as applied to
the whole study area, was termed ‘‘range loss’’. Fragmentation means loss of continuity between occupied
cells in the grid. When fragmentation occurred, we
recorded the number and size of new populations and
assigned new identities to small fragments, while the
largest resulting fragment kept the identity of the former population. Regarding temporal scales, long-term
and short-term respectively denote comparisons between the first and last map of the series (35 yr) and
pairs of consecutive maps (5 yr).
Analyses
We digitized all reports, prepared maps and made
further calculations using Idrisi32, a Geographic Information System (Eastman 1999). We examined the
spatial patterns of distribution and abundance only on
the first and last stages of range contraction (reference
and 1985 maps, respectively). We determined the range
centre as the average position of the occupied cells on a
presence/absence reference map. To see whether the
pattern of density was concentric across the range, we
measured abundance along linear transects both
through the range centre and along mountain ranges.
We also measured abundance and frequency of occurrence in concentric rings 10 km wide. Spatial autocorrelation of abundance was measured with Moran’s I
I=
n
S0
Wij(xi − x̄)(xj − x̄)
i
j
,
(xi − x̄)2
i
where Wij is the distance between cells i and j, xi is the
class of abundance in the cell i, x̄ is the average of xi
over the n occupied cells, and S0 = iiij Wij (i j). The
significance of this index was tested as the probability
of drawing the observed distribution from all possible
spatial arrangements of the same set of values (Upton
and Fingleton 1985).
We compared rates as well as the spatial aggregation
of local extinctions between central and peripheral
parts of the lynx range and within large populations. In
a raster map, and for a focal cell with a given value, the
index CVN is the number of cells with different values
in any defined neighbourhood (Murphy 1985). We used
the index CVN to count the number of cells that
persisted in the immediate neighbourhood of a given
extinct cell (range 0 – 8). Low values of CVN indicate an
aggregation of local extinctions. We subsequently examined whether the values of CVN were autocorrelated. We compared the frequencies of local extinction
in east-west and north-south halves of the lynx range,
as well as within its largest population, to look for
evidence of a contagious contraction. We finally used
stepwise logistic regression to determine the relative
effects of local density and position within the range on
the probability of local extinction.
Results
Internal structure of the lynx range
Fig. 2. Maps of distribution and abundance of the Iberian
lynx at four stages of the range contraction. The colour of
each cell indicates lynx relative abundance in terms of number
of reports per cell dated in the title year or afterwards until
1988. The arrow in the reference map points to the range
centre. For geographical reference note that the map outline
matches the rectangle in Fig. 1.
Pattern of distribution
The minimum rectangle containing the Spanish portion
of the lynx range in 1950 measured 510 × 440 km =
224 400 km 2 (Figs 1 and 2). Since the species also
occurred in Portugal (Castro and Palma 1996), this is
an underestimate of its extent of occurrence (sensu
Gaston 1991), as was the lynx area of occupancy (Gaston 1991) which measured 406 cells in the reference
map, only 18% of the extent of occurrence. Henceforth
we shall refer to the Spanish portion of the area of
occupancy as lynx range (over 95% of the total area of
occupancy; Rodrı́guez and Delibes 1992, Castro and
Palma 1996).
Lynx distribution was spatially discontinuous. In the
1950 reference map, the species occurred in 32 populations (Fig. 2). One large population (269 cells) placed in
a central position was surrounded by smaller satellite
populations, whose average size was 4.4 cells (Table 1).
The ‘‘one large and many satellite populations’’ structure remained until 1985 when the central population
split into two fragments whose combined size was only
123 cells. The total number of populations was similar
in all maps (Table 1).
Patterns of abundance
In most cells relative abundance was extremely low, i.e.
just one report. In the reference map only 57 cells (14%)
had > 10 reports, and this was reduced to only ten cells
(4%) in the 1985 map. The spatial distribution of lynx
abundance in the reference map was non-random and
the dispersion of reports over space clearly heterogeneous (Fig. 3). This same pattern persisted after the
strong contraction of the lynx range (Fig. 3), which
may be due in part to non-independence between the
data used to construct the first and last maps, the latter
being a nested subset of the former. Nevertheless, we
obtained a similar curve from independent lynx reports
dated between 1955 and 1984 (significant departure
from a Poisson distribution: x2 = 822, DF = 328, p <
0.001). Since about half of the cells occupied in the
reference range were empty at the end of the observations (Table 1), we divided the study area into two
halves (western and eastern) to test whether the same
pattern appears in subsamples of this size drawn from
the range. The sigmoidal shape of ranked abundances
was found again in the eastern half, which contained
several hot spots, but not in the western half of the two
maps analysed.
1) Centre vs periphery
The geographical centre of the lynx range in the refer-
ence map was located near the western border of the
central population (Fig. 2a). Plots of abundance along
linear transects through the centre (Fig. 4) showed that
1) sites with the highest abundance were displaced
between five and ten units from the centre, 2) there was
no clear decreasing trend of abundance toward the
boundaries, and 3) spatial variation in abundance was
discrete rather than gradual. In the 1985 map, the
smaller number of occupied cells rendered similar plots
uninformative. However, the asymmetrical and fragmented lynx distribution hides a decreasing trend in
abundance along radial transects. This trend appeared
by averaging both occurrence and abundance in concentric rings (Fig. 5). The abrupt drop of mean abundance at 13 distance units from the range centre
matched the northern and southern boundaries of the
central population. In fact, the mean number of reports
per cell in the central population was 7.0 in the reference map, whereas the highest mean recorded in satellite populations was 4.5. Respective values in the 1985
map were 4.9 and 3.9. The mean ( ± SD) number of
reports per cell in satellite populations averaged 1.7 ±
1.0 in the reference map and 1.7 ± 0.8 in the 1985 map.
In both maps the mean in the central population fell
outside the 99% confidence interval for the mean in
satellites.
Within each mountain range there was a superimposed spatial pattern of abundance. In mountain chains
close to the centre of the lynx range, i.e. Toledo Mountains and Sierra Morena (Fig. 1), abundance clearly
increased from west to east in the reference map (Table
2). This trend persisted more weakly in the 1985 map.
Similar patterns were weak or absent in other mountain
ranges (Table 2).
2) Autocorrelation
Abundance was autocorrelated. In the reference map
the value of Moran’s I for the spatial distribution of the
number of reports per cell was 0.411 (Z = 13.16, p <
0.001), whereas in the 1985 map Moran’s I was 0.434
(Z = 8.91, p < 0.001). Cells belonging to hot spots were
Table 1. Iberian lynx range size in Spain, as estimated by the number of occupied 10×10 km cells, and number and size of lynx
populations between 1955 and 1985, in snapshots at 5-yr intervals.
Map
Range size
Number of populations
Population size
Central
1950
1955
1960
1965
1970
1975
1980
1985
406
391
385
365
356
325
289
223
32
32
33
34
32
29
36
36
269
257
253
241
236
219
169
62+61
Satellites
Mean
SE
Maximum
4.42
4.16
4.13
3.76
3.87
3.79
3.33
2.94
1.17
1.12
1.10
1.02
1.09
1.03
0.74
0.45
33
33
33
33
33
30
25
12
Fig. 3. The frequency
distribution of ranked relative
abundance at the initial and
final stages of the Iberian lynx
range contraction (circles:
1950, squares: 1985). Lines
indicate the expected values
from a Poisson distribution
with the same mean (solid:
1950, dotted: 1985). Observed
distributions differ significantly
from the Poisson curves. These
plots compare with those of
Brown et al. (1995) for
common species in a variety of
taxa.
defined as those that fell within the upper 5th percentile
of the ranked distribution of the number of reports per
cell ( > 20 reports in the reference map, > 10 reports in
the 1985 map). There were several hot spots instead of
a single central area of high lynx density. In the reference
map there were nine aggregations of cells containing
> 20 reports (Fig. 2a): one in western Central Range
(Granadilla), two in Toledo Mountains (Bullaque and
Urda), four in eastern Sierra Morena (Arenoso, Andú jar,
Despeñ aperros and Guadalmena), one in central Sierra
Morena (Bembézar), and one in the flat coastal plain of
Doñ ana. All but Granadilla and Doñ ana were within the
large central population. The mean size of hot spots was
2.5 cells (range 1 – 8) in the reference map which decreased to 1.7 cells (range 1 – 3) in the 1985 map, although
the difference was not statistically significant (MannWhitney U = 21, p = 0.699). The total number of cells
belonging to hot spots, however, dropped from 20 in the
reference map to 10 in the 1985 map.
3) Abundance -occupancy relationship
Average local abundance was higher in populations
which extended over larger areas than smaller ones. For
populations with three or more cells, the mean number
of reports per cell increased with the log-transformed
number of cells occupied (reports dated between 1955
and 1984, r = 0.823, DF = 8, p = 0.003; reports dated
between 1985 and 1988, r = 0.662, DF = 16, p = 0.006).
Further, at least in the reference map, lynx distribution
in the periphery was more fragmented than around the
more densely populated range centre. Frequency of
occurrence and mean number of reports per cell at a
given distance from the centre were coupled in the
reference map (rs = 0.493, p = 0.010; Fig. 5). Outer peaks
of abundance resulted in this correlation disappearing in
the 1985 map (rs = 0.313, p = 0.128; Fig. 5).
Range contraction
Changes in distribution
1) Range loss
At the end of the 35-yr period the Iberian lynx had
disappeared from 183 cells, i.e. 45% of the initial range
size (Table 1, Fig. 2). Area loss occurred over the entire
range: 26 populations (81%) in the reference map lost
between 30 and 100% of their cells. The relative strength
of area loss was similar in both large and small populations (Fig. 6). The arcsin transformed proportion of lost
cells in the long-term was not correlated with initial
population size (r = − 0.13, n = 32, p = 0.487). After
correcting for temporal changes in the rate of area loss
(see below), the residual proportion of area loss in the
short-term was also not correlated with population size
(r = − 0.015, n = 170, p > 0.5).
Significant changes in the slope of range loss over time
revealed two critical peaks of extinction occurring between 1970 and 1975 and between 1980 and 1985 (Fig.
6). Initially the short-term rate of area loss was quite
steady until 1970 at under 0.05, then suddenly increased
in 1975 (x2c = 12.24, DF = 1, p < 0.001) and again in 1985
(x2c = 14.44, DF = 1, p < 0.001), when it was four times
higher than in the early periods.
2) Fragmentation
Range loss was accompanied by population fragmentation (Fig. 2). Twenty-four new populations were generated by the splitting of existing ones. New fragments were
generally small, with a mean size ( ± SE) of 2.8 ±
0.5 cells. As a result, through the study period there was
a significant increase in the proportion of populations
with five or less cells (rs = 0.928, n = 8, p < 0.001). The
contribution of these small populations to the total
range size continuously increased with time from 10%
in the reference map to 31% in 1985 (rs = 0.976, n = 8,
p < 0.001; Fig. 7), indicating an acceleration of the
fragmentation process. This happened in spite of the
fact that many small populations went extinct during
the same period (Rodrı́guez 1997).
The rate of population fragmentation changed with
time. Most fragmentation events (72%) took place after
1975. The two extinction crises revealed by trends in
range loss were also fragmentation crises. These were
the only transitions between successive maps when
there was a significant increase in the division of populations (Fig. 7).
Changes in relative abundance
Relative abundance in each cell showed a remarkable
consistency over time (Fig. 2). The number of reports
per cell was strongly correlated between different periods (Table 3). Similarly, the number of reports per cell
in the period 1955 – 1984 was positively correlated with
the number of reports per cell after 1985 (log transformation, r = 0.591, DF = 160, p < 0.001). Since abundance was autocorrelated, the degrees of freedom may
be inflated. Nevertheless, this correlation remained significant with 10 DF. Nine out of 10 hot-spot cells in
1985 also belonged to hot spots in the reference map.
Seven hot spots (78%; all but Arenoso and Bembézar)
persisted after 35 yr.
Spatial aggregation of local extinctions
Local extinctions over the 35-yr period were autocorrelated (Moran’s I = 0.168, Z = 5.133, p < 0.001). Given
that the normality assumption in the distribution of the
Moran’s coefficient requires a sample size > 20 (Upton
and Fingleton 1985) we restricted our analyses to the
two populations which attained this size in the reference map: the central population and the Gata popula-
Fig. 4. Lynx abundance along
linear transects through the
range centre in the reference
map. The number of reports
per cell in increasing classes of
abundance are 1, 2, 3 – 5,
6 – 10, 11 – 20, 21 – 50, and
> 50. The arrow indicates the
location of the range centre.
tion in western Central Range. In the latter, extinctions
were not autocorrelated (Moran’s I = 0.006, Z = 0.335,
p < 0.369), but they were in the former (Moran’s I =
0.155, Z = 4.260, p < 0.001).
To identify gaps in the distribution produced by the
spatial aggregation of local extinctions we looked at the
values of CVN which were also autocorrelated
(Moran’s I = 0.844, Z = 9.854, p < 0.001). Groups of
cells that became empty with
3 surrounding additional extinctions occurred mostly in the western half of
the central population (91%, n = 47). There were three
groups (Fig. 8): western Sierra Morena (Sevilla
province; 6 cells), central Sierra Morena (Có rdoba
province; 18 cells) and western Toledo Mountains (19
cells). Groups of local extinctions with two additional
extinction events in their neighbourhood were more
abundant in the eastern half of the central population
(69%, n = 36). Their aggregation was less evident than
in the former case, yet two areas were identified: the
northern edge of eastern Sierra Morena (9 cells) and
western Toledo Mountains (NW Ciudad Real province,
9 cells).
Trajectories of range contraction
The frequency of local extinction in the western half of
the range was higher than in the eastern half (51 vs
38%, x2 = 6.89, p = 0.009). This cannot be attributed to
an increased disaggregation and vulnerability of western populations because we found the same pattern in
the large central population (56 vs 31%, x2 = 15.33,
p < 0.001). This trend could indicate a contagion, a
directional front of extinction spreading from the western end of the lynx range. Conversely, the frequency of
local extinction did not decrease significantly with distance to the western boundary in seven N-S oriented
strips 70 km wide (rs = − 0.500, p = 0.253). There was
no other indication for a contagiously spreading extinction across the range (Fig. 2).
Fig. 5. The proportion of cells
occupied (thick lines) and the
average number of reports per
cell in concentric rings 10 km
wide around the centre of the
lynx range.
We found no indication that the prediction of a
concentric pattern of contraction was true. The longterm rate of area loss in the central population (42%)
was lower but not significantly different (xc2 = 2.03,
DF = 1, p = 0.155) from that in satellites (50%). Moreover, plots of short-term rates of range loss over time
produced almost parallel curves for central and satellite
populations (x2c < 1.86 in all comparisons of area loss
between consecutive 5-yr periods; DF = 1; p > 0.173;
Fig. 6). These results suggest, or at least cannot ignore,
a similar strength in extinction forces over the entire
lynx range.
Interestingly, stepwise logistic regression of local extinction frequencies in the long-term revealed that relative density in each cell had a significant negative effect
on extinction probability, whereas three measures of
cell position within the range did not affect it (Table 4).
Discussion
Internal structure of the lynx range
Brown et al. (1995) showed the prevalence of the
clumped pattern of abundance among common species.
They argued that rarer species would have lower absolute variation in abundance among sites. Similar to
other specialist species with a high trophic level (Terborgh 1974), the Iberian lynx cannot be considered a
common species. Among the possible forms of rarity,
lynx could be described as constantly sparse across
most of their range (Rabinowitz et al. 1986). Further,
average local abundance and range size are positively
correlated in closely related species (Brown 1984, Gaston 1990, Gaston et al. 1997). The estimated historical
range of Iberian lynx is much smaller than the current
ranges of other Lynx species (0.5 vs 5 – 13 × 106 km2;
see Fig. 1 and Nowell and Jackson 1996), so rarity in
terms of average low densities should be expected. Yet
the spatial distribution of lynx abundance departs from
random as in more widespread species, on the basis of
both recent and older reports. Differences in the distribution of abundance between common species and this
kind of rare species may reside solely in the steep slope
of its initial exponential decline (Fig. 3). We propose
that rare species may also have peaks of high relative
abundance, but in fewer, smaller and perhaps more
scattered sites.
Lynx abundance was autocorrelated, as probably
were combinations of environmental variables determining the lynx niche (Brown 1984, Brown et al. 1995).
Table 2. Spearman rank correlations between class of lynx
abundance and cell position along transects through major
Iberian mountain chains in Spain. For east-west oriented
chains the lowest and the highest values of cell position were
located at the western and eastern ends of the transects,
respectively. In the Betic mountains lynx populations were
aligned north-south, and the lowest value of location was at
the southern end of the transect. The number of reports per
cell in increasing classes of abundance was 1, 2, 3–5, 6–10,
11–20, 21–50, and > 50. Abundance was measured only in
occupied cells. Spacing between adjacent transects was at least
50 km.
Transect
E-W oriented
Central Range
Toledo Mountains
Sierra San Pedro
+Toledo Mountains
Sierra Morena
N-S oriented
Betic Mountains
Map
rs
n
p
Reference
1985
Reference
1985
Reference
1985
Reference
1985
−0.452
−0.046
0.551
0.413
0.655
0.714
0.530
0.372
20
14
24
16
18
13
38
29
0.045
0.876
0.005
0.112
0.003
0.006
<0.001
0.047
Reference
1985
−0.208
−0.289
15
5
0.457
0.638
Fig. 6. Area loss undergone by
the Iberian lynx range expressed
as the number of cells occupied
at 5-yr intervals in the whole
range (circles), the area occupied
by the large central population
in the reference map (squares)
and peripheral areas (triangles).
Bars show the short-term
percent of range loss.
Hot spots were restricted to a few cells, indicating that
the most favourable combinations of environmental
conditions rarely occur. The identity of cells enclosing
many reports was consistent between the first and final
maps. This suggests that favourable conditions were a
permanent site-specific property at our time-scale of
observation. The same can be inferred from the strong
within-site correlation of lynx abundance across maps
built with time-independent datasets. Concordance of
abundance at the same localities over time has also
been found in bracken insects (Lawton and Gaston
1989), soil arthropods (Bengtsson 1994) and birds
(Blackburn et al. 1998).
The two exceptions to the normal probability density
distribution of abundance that Brown (1984) pointed
out occurred in the Iberian lynx range. Firstly, density
changed abruptly rather than gradually over space.
Along transects density often dropped to zero reflecting
a discontinuous distribution which in 1950 coincided
already with the location of Iberian mountains (compare Figs 1 and 2). This is not surprising as the Iberian
lynx positively selects Mediterranean scrubland and
avoids open land (Palomares et al. 1991, 2000, Rodrı́guez 1997). Scrubland is largely restricted to rough
terrain of low quality for agriculture following the early
extensive removal of Mediterranean forest in the lowlands of the Tagus and Guadalquivir river valleys (Rodrı́guez
and
Delibes
1992).
However,
both
discontinuous distribution within mountain ranges and
lynx absence in other mountainous areas of similar
ecological conditions (e.g. the large Betic Mountains
since 1930, Rodrı́guez and Delibes 1990) suggest that
scrubland was not the only important factor determining the location of remnant populations. The fragmented distribution of peripheral populations had its
roots in previous stages of the Iberian lynx decline, and
is therefore difficult to interpret without deeper investigation of history at lower spatial scales. Similarly, we
found a positive relationship between distribution and
abundance already in the reference map but, as with
other species, it is hard to discern between several
alternative explanations (Hanski et al. 1993, Gaston
1994, Gaston et al. 2000).
Secondly, the pattern of abundance was multimodal
with most hot spots placed in the eastern half of the
central population, partly reflecting the increasing trend
of abundance eastwards. Within each mountain range
there is a typical descending gradient of precipitation
from west to east (Font 1983) which determines corresponding gradual differences in ecosystem productivity,
composition of vegetation communities, and land uses
(Peinado and Martı́nez 1985, Moreira and Ferná ndez
Palacios 1995). For instance, in western Sierra Morena
forest patches with little understorey dominate the
landscape and livestocking is the prevalent land use,
while in eastern Sierra Morena scrubland and big game
are more common. More importantly, the best habitat
for rabbits Oryctolagus cuniculus L. occurs in the eastern thirds of Sierra Morena, Toledo Mountains and
Central Range where their highest densities have been
reported (Blanco and Villafuerte 1993). Rabbit distribution is scattered and density very low in the agricultural
land of intermediate
valleys where Mediterranean
scrubland is virtually absent. The distribution of rabbit
density could largely explain the geographical variation
in lynx density. This makes sense since the Iberian lynx
is a stenophagous species whose diet is composed almost exclusively of subadult and adult rabbits wherever
it has been studied (Doñ ana: Delibes 1980, Beltrá n and
Delibes 1991, Palomares et al. 2001; central population:
Aymerich 1982). The correlation between rabbit density
and lynx density, already demonstrated at the local
level (Palomares 2001), has to be further tested with
regional studies. Other factors thought to have influenced lynx distribution, such as hunting or habitat
removal (Rodrı́guez and Delibes 1992), do not show, at
first glance, any increasing pattern eastwards.
Crises of local extinction
Two widespread phenomena, namely myxomatosis and
land use changes, have been proposed so far as major
causes of lynx decline in recent times (Delibes 1979,
Rodrı́guez and Delibes 1992). We were able to identify
two temporal peaks and five spatial clumps of local
extinctions. If the range contraction were primarily
prompted by the action of these hypothesised extinction
factors significant changes in them should have
matched the temporal peaks of local extinction in the
periods 1970 – 75 and 1980 – 85.
Myxomatosis, a viral disease of rabbits, entered the
Iberian peninsula from the north in 1953 and its
catastrophic effects on rabbit populations were evident
virtually everywhere over the next few years (Fenner
and Ross 1994, Rogers et al. 1994). To our knowledge,
there are no quantitative data on the effects of myxomatosis on rabbit numbers in Spain. As a reference, in
Britain, the initial outbreak killed over 99% of rabbits
and populations remained low for years (Fenner and
Ross 1994). Despite some recovery, population levels
14 yr after the arrival of the disease were only 20% of
pre-disease levels (Lloyd 1981). In Australia, rabbit
density in 1970 ranged between 0.2 and 6.5% of reference densities in 1950 before the introduction of myxomatosis (Myers 1970). If the rabbit population crash
due to myxomatosis had an inmediate effect on lynx
populations, then a lynx extinction crisis should have
been observed in 1960 or 1965, but not as late as we
found it. Our results suggest that lynx could survive the
initial sudden shortage of rabbits in the majority of
places but probably at the cost of reducing its density
everywhere. A possible mechanism could be that reproduction almost stopped, while a fraction of the adults
was able to survive rabbit scarcity for years in a similar
way to Canadian lynx populations (Lynx canadensis
Kerr) coping with strong fluctuations in their prey
(Brand et al. 1976). Thus, lynx populations would have
become more vulnerable to subsequent fluctuations of
rabbit density or to other factors operating more
recently.
The second general phenomenon can be loosely
defined as the ecological repercussions of the massive
emigration of people from the countryside that took
place in Spain after 1960. Namely loss of diversity in
the Mediterranean scrubland at the landscape level
(Ferná ndez Alés et al. 1992), extensive replacement of
native vegetation by conifer and eucalyptus plantations,
and changes in land uses as a consequence of the rise of
the big game business (Ló pez Ontiveros 1991). In contrast to myxomatosis, these changes were gradual and
their relative magnitude varied between regions. They
have probably affected both rabbits and lynx (Delibes
et al. 2000), but so many (poorly identified) factors are
involved that specific predictions about the time lag of
lynx response cannot be easily derived at our scale of
analysis.
The spatial correlation of local extinctions in five
areas may have been the result of local deterministic
factors acting with particular intensity. It is most interesting identifying these factors by seeking correlations
between the spatiotemporal variation of their intensity
and the dates and places of local extinctions. Specific
studies at the regional level will again be needed to
answer these questions.
Fig. 7. The contribution of lynx
populations of different size
classes to the geographic range
in maps at 5-yr intervals.
Comparisons of the frequencies
between consecutive periods
were not significant except in
two cases. Between 1970 and
1975, 2 – 5 cell fragments
increased its contribution to
the total range size at the
expense of populations of
6 – 20 cells (x2 = 10.4, DF = 4,
p = 0.034). Between 1980 and
1985 the large central
population generated many
2 – 5 cell populations while
splitting up (x2 = 9.8, DF = 3,
p = 0.020).
Table 3. Spearman rank correlation coefficients of the number of reports per cell between pairs of 5-yr periods containing at
least 200 reports. N (within brackets) denotes the number of cells containing reports in the two periods compared. *p<0.01,
**p<0.001.
Period
1970–74
1975–79
1980–84
1985–89
1965–69
0.546**
(48)
0.562**
(35)
0.707**
(148)
0.360*
(47)
0.453**
(116)
0.398**
(89)
0.429**
(54)
0.455**
(117)
0.471**
(92)
0.490**
(118)
1970–74
1975–79
1980–84
Trajectories and mechanisms of contraction
Lynx range collapse probably started thousands of
years ago. It has been much slower than that of many
mammals in Australia, New Zealand and North America after the arrival of Europeans (Short and Smith
1994, Towns and Daugherty 1994, Lomolino and
Channell 1995, Smith and Quin 1996, Channell and
Lomolino 2000a, b). Waves of human colonisation,
especially those of advanced civilizations, entered the
Iberian peninsula from the eastern and southern coast
(Martı́n 1998, Blondel and Aronson 1999), whereas
lynx range contraction apparently started in the north.
In spite of this, some relict populations persisted in
northern Iberia until the first half of the 20th century
(Delibes 1979, Rodrı́guez and Delibes 1990). Early settlers generated cultural landscapes (Pons 1981) which
added spatial heterogeneity (Le Houérou 1981, Godron
et al. 1981, Blondel and Aronson 1999) and benefited
species linked to successional stages of Mediterranean
forest such as the European rabbit (Flux 1994). Therefore, humans may have initially benefited lynx by generating mosaics that improved habitat and prey
(Delibes et al. 2000). It is then doubtful that the successive arrival of cultures triggered the lynx decline. Moreover, during the 35-yr period we examine, the
contraction did not start at one or more places along
the range boundary or at any other identifiable focal
point. Fragmentation of large populations occurred
everywhere and there was no evident extinction front.
Nor did extinction follow the likely contagious and fast
spread of rabbit myxomatosis from north to south.
This is not to say that deterministic factors have not
played a role in lynx decline. Stochasticity alone would
have predicted a much larger proportion of area loss in
small isolated populations than in large ones. However,
both large and small populations were hit with similar
intensity. There is indication that human activities,
directly and indirectly, have been involved, including
hunting, habitat removal, changes in land use, and
introduction of diseases affecting lynx prey (Rodrı́guez
and Delibes 1990, Ferreras et al. 1992, Garcı́a Perea
2000, Delibes et al. 2000).
In agreement with a uniform distribution of extinction forces, area loss was similar in different parts of
the range and independent of initial population size.
Moreover, the relative density within individual cells
was similar over time, and cells with high relative
density had a higher probability of persistence. This
suggests that the response of local populations to the
extinction forces was proportional to local density:
weak local populations could not bear the disturbance
and went extinct whereas dense local populations persisted, albeit most likely at lower levels of abundance.
Early reductions in local density may signal further
local extinctions (the ‘‘double jeopardy’’, Lawton 1995)
as we observed in the final disintegration of the central
population. Concerning the strength of extinction
forces, the simplest explanation of the density-dependent extinction probability is that they may have operated with almost uniform intensity over the entire
range. We show elsewhere that the probability of population extinction was a function of population size and
isolation which points out the importance of (regional)
metapopulation dynamics for persistence (Rodrı́guez
1997). Differentially higher extinction rates of small
populations would explain why the contribution of
Fig. 8. Values of the index CVN (number of neighbour extinct
cells) for cells of the large central population in the reference
map (see Fig. 2a) that became extinct after 35 yr. Light:
CVN = 1, medium: CVN = 2, dark: CVN 3.
Table 4. Stepwise logistic regression analysis of the effects of lynx density and cell position within the range on the long-term
frequency of local extinction. Relative density was expressed as the number of reports per cell. Position was indicated by three
variables: 1) central vs satellite populations as defined in the reference map, 2) inner vs outer 50% area of a circle around the
range centre, and 3) actual distance of each cell to the range centre. DF = 1 for all explanatory variables.
Variables
STEP 1
Density
Position:
1. Central/satellite
2. Inner/outer
3. Distance
Coefficient
SE
G
p
−0.23
0.04
78.7
<0.001
0.32
0.02
0.01
0.21
0.22
0.02
2.3
0.1
0.3
0.129
0.752
0.584
one-cell populations to the total range size was nearly
constant in all maps, despite many populations of that
size being generated through the 35-yr period. Independent stochastic dynamics of isolated populations may
also explain why in 1985 secondary peaks of abundance
appeared in regions with low average occupancy: density can remain high as long as the local environmental
conditions allow an isolated population to grow without immigration.
In summary, we conclude that the lynx range contracted towards the areas of maximum abundance and
greatest environmental suitability mainly as a result of
deterministic factors which exerted a similar pressure
over the entire range. We found signs that the extinction forces were not consistently lethal to all populations whatever their density, and that they operated
over the whole range instead of in progression from a
starting point. Local populations persisted or went
extinct depending on their density. Similar results were
obtained by Mehlman (1997) who observed the range
contraction of three bird species produced by the action
of a uniformly distributed abiotic factor.
Humans have coexisted with the Iberian lynx for
millennia. The most likely causal factor in the decline of
the lynx are humans and their activities as they show a
nearly uniform distribution across the lynx range. Technological improvement, economic or demographic
trends, or changes in social habits may have provoked
changes in land uses, hunting intensity, and prey or
habitat availability. In Spain, during the period we
consider, these culturally driven pressures have been
widely distributed and may have been exerted evenly
over the lynx range. The introduction of myxomatosis
has likely played an outstanding role in making lynx
populations vulnerable. Its relative regional importance, however, may have varied depending on the
strength of other factors, which should be examined at
lower spatial scales. Identifying these factors and neutralizing their effects is of paramount importance to
reverse the lasting decline of the Iberian lynx. This is a
task that should be incorporated into plans and strategies for the conservation of this endangered species.
Acknowledgements – We are grateful to J. N. Guzmá n and J.
C. del Olmo for their invaluable help during the field work, to
H. Andrén, R. Channell, and F. Palomares for useful com-
% explained deviance
14
ments on the manuscript, and to L. Swankie for additional
editorial advice. The lynx survey was funded through an
agreement between Instituto Nacional para la Conservació n de
la Naturaleza (ICONA) and Consejo Superior de Investigaciones Cientı́ficas (CSIC). During several phases of the preparation of this work A.R. was supported by CSIC.
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