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Predation of young tortoises by ravens: the effect of habitat structure on tortoise detectability and abundance

  • Instituto de Investigación en Recursos Cinegéticos, IREC (CSIC-UCLM-JCCM)

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The predation of young tortoise is considered a major cause of mortality for many tortoise species. The predation by common ravens has been identified as being responsible for significant decreases in tortoise populations. Mediterranean spur-thighed tortoise hatchlings and juveniles in Maamora forest (Morocco) were studied in order to describe the size/age class predation of common ravens on young tortoises and infer the drivers of predation risk and population abundance. The results showed a high level of predation on young tortoises (<75 mm carapace length) attributed to ravens in areas with low vegetation cover, representing 100% of the cases of mortality (n = 147), but it was moderate in covered areas (n = 19), representing 12–27%. The population structure of living juveniles differed significantly between covered and uncovered areas, thus suggesting that raven predation might modify juvenile population structure. Finally, N-mixture models showed a positive relationship between (i) bare cover and tortoise detectability that is only evidenced when the plot is far from a perch and (ii) population abundance and shrub species-richness, being higher in uncovered areas. Our results improve the knowledge on predation and survival on this critical stage in life, which is crucial for the conservation of the Mediterranean spur-thighed tortoise.
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SCIENTIFIC REPORTS | (2020) 10:1874 |
Predation of young tortoises
by ravens: the eect of habitat
structure on tortoise detectability
and abundance
Amalia Segura
1*, José Jimenez1 & Pelayo Acevedo
The predation of young tortoise is considered a major cause of mortality for many tortoise species.
The predation by common ravens has been identied as being responsible for signicant decreases in
tortoise populations. Mediterranean spur-thighed tortoise hatchlings and juveniles in Maamora forest
(Morocco) were studied in order to describe the size/age class predation of common ravens on young
tortoises and infer the drivers of predation risk and population abundance. The results showed a high
level of predation on young tortoises (<75 mm carapace length) attributed to ravens in areas with low
vegetation cover, representing 100% of the cases of mortality (n = 147), but it was moderate in covered
areas (n = 19), representing 12–27%. The population structure of living juveniles diered signicantly
between covered and uncovered areas, thus suggesting that raven predation might modify juvenile
population structure. Finally, N-mixture models showed a positive relationship between (i) bare cover
and tortoise detectability that is only evidenced when the plot is far from a perch and (ii) population
abundance and shrub species-richness, being higher in uncovered areas. Our results improve the
knowledge on predation and survival on this critical stage in life, which is crucial for the conservation of
the Mediterranean spur-thighed tortoise.
Tortoise populations are characterised by high adult survival and low recruitment rates, which probably inuence
their demographic dynamics1. e lack of reliable information on recruitment complicates evaluations of the real
importance of young individuals as regards population demography2. In this respect, threats that limit recruit-
ment bouts may have an important regulatory eect on population dynamics3, especially when these threats
continue over long periods of time (e.g.4).
Hatchlings and juveniles are more vulnerable than adults owing to their smaller size and higher susceptibility
to environmental conditions, such as the temperature or rain, which aect their physiology, but also to vegetation
cover, which inuences thermoregulation, the availability of food and the risk of predation3,5,6. Indeed, the low
temperature and high rains of winter2,7 and the high temperature of summer8,9 are considered some of the most
common causes of mortality in hatchlings. e predation of young tortoises –their shell is so or not suciently
solid to protect them from predators– by mammals7,10,11 and birds1214 is also considered a major cause of mortal-
ity in many tortoise species, which was particularly studied in North America7,10,12,15,16.
e common raven Corvus corax is considered to be one of the most relevant predators of tortoises and
is responsible for 70–91% of the mortality of desert tortoises Gopherus agassizii4,13,17. Both single individuals
and breeding pairs have been identied as responsible for signicant decreases in tortoise populations, also in
addition to modifying population structure by aecting juvenile size/age classes.12,18,19 Indeed, its high numbers
and opportunistic feeding habits have severe impacts on its target prey, whose population size may be reduced
without aecting the condition of the predator, which will switch to another prey when this resource becomes
scarce (e.g.4). But the prey species has mechanisms by which to reduce predation. Vegetation cover has been
documented as a key determinant as regards reducing young tortoises’ detectability, since it facilitates their cam-
ouage20. Crypsis has anti-predatory benets, and refuge areas, such as those areas dominated by vegetation
cover, may reduce the risk of predation. It has also been documented that ravens’ predation on tortoises diers
1Instituto de Investigación en Recursos Cinegéticos, IREC (CSIC-UCLM-JCCM). Ronda de Toledo, 12, 13071, Ciudad
Real, Spain. 2Escuela Técnica Superior de Ingenieros Agrónomos (UCLM), Ronda de Calatrava, 7, 13071, Ciudad Real,
Spain. *email:
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according to their spatial distribution of single individuals or breeding pairs. Tortoise predation is greater in
adjacent human developments, which attract large numbers of single ravens, and in adjacent successful nests
of breeding pairs throughout human developed and undeveloped areas4. Even the success of juvenile tortoise
releases is compromised in the head-starting programs included in conservation strategies owing to the fact that
certain ravens are attached to the predation of certain tortoise size classes17,21. It could, therefore, be hypothesized
that the risk of raven predation on young tortoises might be high in areas with high dense tortoise populations
and mediated by the overlaid eect of raven presence and vegetation cover.
Maamora forest, an anthropogenic cork oak forest located in northern Morocco, is considered to be close to
the optimum niche –the core range– of the Mediterranean spur-thighed tortoise’s distribution Testudo graeca22
and one of the areas with the highest density populations documented to date23. However, the common raven
has, over the last 40 years, increased its numbers and distribution worldwide, and this cork oak forest is no
exception24. is increase in predators may be a threat to the Mediterranean spur-thighed tortoise, and especially
to highly susceptible young individuals. Concretely densities of 0.8 breeding pairs km2 had been observed in
the lustrum (A. Segura unpublished data). In this context, our specic objectives were to: (i) describe predation
and size classes’ preference of common ravens on young tortoises, and (ii) infer the main drivers of predation
risk and abundance. ese goals are relevant as regards improving knowledge on the predation and survival of
the Mediterranean spur-thighed tortoise at this critical stage in its life and, therefore, the conservation of this
threatened species.
Raven predation on hatchlings and juveniles. Twenty-two occurrences of ravens, not including groups
(7, 3, 6 and 6, in A, B, C and D, respectively), were detected on our four study sites (Table1). ree raven nests
with active breeding pairs were found and two breeding pairs had between 1 and 2 chicks (Table1).
One hundred and sixty-six dead young tortoises (<100 mm Carapace Length; hereaer CL) (11, 8, 55 and
92, in A, B, C and D, respectively) were detected during the surveys. Most of the dead juvenile tortoises found
(88%) were located in the uncovered areas (areas C and D), where the main CL size category of dead tortoises
corresponded to between 41 and 70 mm (Fig.1). e cause of mortality in those areas was associated with preda-
tion by the common raven (100%, n = 147), where only < 75 mm CL tortoise carcasses were found with signs of
raven predation under perch and nest trees. e ravens ate the hatchling and juvenile tortoises by pulling o their
head and limbs (6%) or pecking holes through the carapace (60%) or plastron (34%). Indeed, 74 and 15 juvenile
tortoises were predated by two active breeding pairs of ravens in spring 2018 (in areas D and C, respectively).
Nevertheless, unknown causes of mortality dominated in the covered areas, and only 12–27% were related to
raven predation. e threshold size above which young tortoises are safe from raven predation in both covered
and uncovered areas was 75 mm CL. Dead young tortoises associated with raven predation diered signicantly
between covered and uncovered areas (A vs. C: X2 = 4.07, p < 0.05, n = 66; A vs. D: X2 = 4.25, p < 0.05, n = 103; B
vs. C: X2 = 5.05, p < 0.05, n = 63; B vs. D: X2 = 5.19, p < 0.05, n = 100), but did not dier between either covered
(A vs. B: X2 = 0.08, p = 0.77, n = 19) or uncovered areas (C vs. D: X2 = 0, p > 0.99, n = 147).
Live hatchlings and juveniles. One hundred and sixty-three live young tortoises (39, 30, 65, 39 in A, B, C
and D, respectively) were found during the surveys. Figure1 shows the size-class distribution on the four study
sites. e juvenile structure of live tortoises did not dier between the covered areas (A vs. B; X2 = 7.68, p = 0.26,
n = 69) but diered signicantly between the uncovered areas (C vs. D; X2 = 21.19, p < 0.05, n = 104) and also
Bare ground cover 0 0 ± 0.2 17.9 ± 17.4 21.4 ± 13.8
Shrub cover 59.7 ± 21.1 52.0 ± 14.8 45.0 ± 23.0 28.2 ± 26.1
Shrub height 62.1 ± 17.7 71.1 ± 30.6 31.9 ± 27.3 19.8 ± 17.6
Shrub richness 2.0 ± 0.8 1.5 ± 0.7 0.8 ± 0.4 0.7 ± 0.7
Predation risk
Perch distance 203.6 ± 35.4 510.8 ± 120.8 72.5 ± 36.3 89.2 ± 35.3
Nest distance 999.0 ± 84.5 1076.7 ± 131.1 184.9 ± 89.2 144.7 ± 37.7
Raven occurrence
Single individuals 5 2 4 5
Breeding pairs 2 1 2 1
Groups 1(28) 1(5) 0 1(17)
Number of live tortoises 39 30 65 39
Number of dead tortoises 11 8 55 92
Raven predated tortoises 3 1 55 92
Table 1. Bare ground cover (%), shrub cover (%), shrub height (cm) and shrub species richness; raven perch
and nest distance (m) in the four study sites, A, B, C and D (mean ± SD); maximum raven occurrence in the
Mediterranean spur-thighed tortoise survey distributed by single individuals, breeding pairs and groups of
>2 individuals (number of individuals); live (<100 mm carapace length, CL), dead (<100 mm CL) and raven
predated tortoises (<75 mm CL) (in all of them, the number of individuals).
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between the covered and the uncovered areas (C vs. A: X2 = 16.06, p < 0.05, n = 104; C vs. B: X2 = 16.06, p < 0.05,
n = 95; D vs. A: X2 = 36.89, p < 0.05, n = 78; D vs. B: X2 = 36.77, p < 0.05, n = 69). e main dierences concerned
the CL size category of 81–100 mm, which represented 41% versus 16% in the covered and uncovered areas,
Tortoise detectability and abundance: risk of predation by ravens. According to previous results
regarding dead animals found on our study sites, only those animals < 75 mm CL were susceptible to predation
in Maamora forest. Predation risk analysis was, therefore, restricted to these size classes: 140 detections (A: 27
tortoises in 11 occupied grids; B: 21 tortoises in 10 occupied grids; C: 57 tortoises in 13 occupied grids, and D: 35
tortoises in 7 occupied grids). It varied from a maximum of 3 to 5–11 tortoises per grid in covered and uncovered
areas, respectively (Table1).
In the N-mixture model, we used a negative binomial model (Table2). e stepwise procedure carried out
to select predictors explaining detectability and abundance processes is summarized in Table2. e bootstrap
p-values for the nal model based on the SSE, Freeman-Tukey, and Chi-square statistics were 0.03, 0.00 and 0.01,
respectively. e value of ĉ (ratio of observed/expected) was 1.59.
e nal model included the interaction between bare ground cover and the distance to the raven perch in
the detection process (Fig.2), and the site and the number of shrub species in the abundance process (Fig.3,
Table3). e detectability increased with the percentage of bare ground, but this eect was apparent only when
the distance to a raven perch was greater. e abundance was higher in uncovered areas and in grids with high
shrub species richness.
Evidence of common raven predation on hatchlings and juveniles. is study reveals the predation
of common ravens on juveniles of Mediterranean spur-thighed tortoises in certain areas of Maamora forest. Both,
single individuals and breeding ravens were observed killing, carrying away and consuming juvenile tortoises,
their preference being for class sizes 40–70 mm CL. e selection by ravens of certain size classes of tortoises over
others has been found in other Testudinidae populations too18,19. Despite some tortoises being paint-marked to
identify them as recaptures, no evidence of any increased risk of predation was observed for the recognisable
fraction of the population (but see25); only in C uncovered area was one individual found dead, representing 2%
of the marked individuals.
Bearing in mind that raven predation might vary between years and among individuals, and the fact that our
study comprised only one year, our results still suggest that the predation of juvenile tortoise in the study area
was higher when compared with that of populations of Mediterranean spur-thighed tortoises in southern Spain8.
Indeed in those areas raven predation did not aect tortoise populations at all. Nevertheless, some similarities
Figure 1. Distribution of carapace length (mm; CL) in the four study areas: covered (A,B) and uncovered
(C,D). Data represent the frequencies of young tortoises: live shown in black, dead by raven predation in light
grey and dead by any other reason in dark grey.
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with other Testudinidae populations aected by raven predation were found12,16,26, although the ravens involved
in predation in our study appeared to have a slightly lower size threshold above which juveniles are safe from
predation (75 mm CL) than reported for other Testudinidae populations (85 and 100 mm CL; 27, 21 respectively).
is might be associated with the length of time shell hardening takes in Mediterranean spur-thighed tortoises,
which has been documented to limit the probability of predation by ravens (e.g.27), and this merits further studies.
Nevertheless, when comparisons of raven predation on tortoises were restricted to covered versus uncovered
areas, in the former such occurrences were all anecdotal while high mortality rates were rigorously recorded in
the latter. Certainly, we were surprised by the high amount of predation by one pair of breeding ravens, which
predated 74 tortoises of <75 mm CL in a single breeding season. Indeed, raven predation might be modifying
juvenile tortoise population structure through altering recruitment (e.g.4). Further studies are required to dis-
entangle the role played by predation in tortoise population structure within the Maamora forest.
Model specication nPars AIC ΔAICc < 2 AICwt cltvWt
(1) Latent abundance distribution
Negative binomial 3 541.51 0.00 1.00 1.00
Zero Inated 3 584.01 42.50 0.00 1.00
Poisson 2 620.03 78.52 0.00 1.00
(2) Covariates of abundance (γ) and detection (p)
γ (Shrub richness + Site) p (Bare ground cover*Perch distance) 10 527.82 0.00 0.98 0.98
γ (Site) p (Bare ground cover*Perch distance) 9 536.24 8.42 0.01 0.99
γ (.) p (Bare ground cover*Perch distance) 6 538.68 10.86 0.00 1.00
γ (.) p (.) 3 541.51 13.69 0.00 1.00
Table 2. Model selection of Mediterranean spur-thighed tortoise hatchlings and juveniles (<75 mm carapace
length; see text for details): (1) Latent abundance distribution and (2) covariates of abundance and detection.
Covariates considered: shrub richness; site, bare ground cover (%) and raven perch distance (m). Model
selection based on Akaike’s Information Criterion (AIC), number of parameters (nPars), the dierence in AICc
from the best tted models (ΔAICc < 2), model weights (AICwt), and cumulative model weights (cltvWt).
D=350 D=400 D=450
D=150 D=200 D=250
D=20 D=50 D=100
0.00.2 0.40.6 0.00.2 0.40.6 0.00.2 0.
Bare ground rate
Detection probability
Figure 2. Interaction between two continuous covariates: bare ground cover rate and distance to the nearest
raven perch (D, in meters) over the probability of detection.
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Predation risk by ravens, population size and structure of young tortoises. In this study, tortoise
detectability –associated with predation risk– was mediated by the interaction between predator presence and
bare or low cover areas; with tortoise detectability increasing with greater areas of bare ground, mainly in loca-
tions far from perch trees (e.g.18,28). is might suggest that ravens could be modulating the behavioral response
of young tortoises, e.g. they will reduce their activity in areas near perch trees in order to be less detectable by the
ravens (e.g.29). In this respect, it is reasonable to assume that the survivorship of juveniles in areas with higher
predation risk might be lower and, therefore, a lower juvenile population size will characterise populations in
such areas. Conversely, we found a higher abundance of young tortoises in uncovered areas, where they suered
higher predation, but also in areas where there was high diversity of shrub species in both covered and uncovered
areas (e.g.30). It is thus plausible to assume that many other factors involving juvenile physiological costs (e.g.29)
or even female reproduction traits –the number of clutches, clutch size and recruitment success–3, might explain
part of the variation found in the size of juvenile populations in covered and uncovered areas3. However, in higher
risk predation areas, it might also be expected that juvenile population structure would be modied and show
dierences in size/age classes12,18,19. Indeed, the higher percentage of longer/older juveniles (76–100 mm) –which
are not considered susceptible to predation– found in covered areas compared to in uncovered ones evidenced a
threat to juvenile survivorship in the lower size/age classes in those areas where hatchlings are more detectable,
e.g. uncovered areas, and whose eects, among others, might be mediated by vegetation cover (e.g.30).
Finally, hatchlings are challenging to study since they are rarely encountered in the eld31,32 and, as such, very
low capture rates and practically nonexistent recapture rates of Mediterranean spur-thighed tortoise hatchlings
have been reported in southern Spain33,34. Further studies are required to discover the role played by predation in
050 100 150 200 250 300
Number of species
Figure 3. Relationship between sampling sites (A, B, C and D; le graph) and the richness of shrub species
(number of species; right graph) with the abundance of Mediterranean spur-thighed tortoise hatchlings and
juveniles (<75 mm carapace length).
Process Var iable Estimate Standard
Error ZP (>|Z|)
Intercept 1.025 0.49 2.094 ***
Shrub richness 0.709 0.222 3.196 **
Site B 0.419 0.619 0.677 ns
Site C 2.697 0.564 4.78 ***
Site D 2.102 0.586 3.588 ***
Intercept 2.416 0.454 5.32 ***
Bare ground cover 0.636 0.516 1.23 ns
Perch distance 1.9 0.588 3.23 **
Bare ground cover* Perch
distance 1.961 0.78 2.52 **
Table 3. Summary and statistical parameters of the nal N-mixture model parameterized to estimate
Mediterranean spur-thighed tortoise hatchlings and juveniles (<75 mm carapace length) as regards both
detectability and abundance (signicance codes: ns no signicative; ** < 0.01 and *** < 0.001).
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the recruitment process, which will have later impacts on the size and structure of tortoise populations. is will
also help to disentangling the possible physiological costs associated with predation risk.
Material and Methods
Study area. e study was conducted in areas of low elevation (72–185 m a.s.l.) and sandy soil in Maamora
forest (northwest Morocco; 34° 02 54.19 N, 6° 27 19.24 W). e climate is Mediterranean, with hot and dry
summers, and the annual range of average rainfall is from 300 to 500 mm. Maamora forest is dominated by cork
oak trees, Quercus suber, with scattered endemic wild pear, Pyrus mamorensis, wild olive Olea europaea, green
olive Phyllirea latifolia and mastic Pistacia lentiscus, and a sparse understory represented by bush and shrub
species, such as Mediterranean broom Genista linifolia, Cytisus arboreus, Stauracanthus genistoides, dwarf palm
Chamaerops humilis, French lavender Lavandula stoechas, sage-leaved rockrose Cistus salviifolius, Halimium hal-
imifolium and ymelaea lythroides35.
e study specically took place on four sites that were close together (separated by 3 km), but always ensuring
that the tortoise populations’ territories were separated by a sucient distance for them to be considered as inde-
pendent populations34,36; Fig.4. Despite the fact that all the sampling sites were located on private land on which
there has been no pet trade (>10 years protected) and that the undergrowth is well represented when compared
with other sites in unprotected Maamora, the study sites diered as regards their vegetation cover. Two of them
were characterized by high cover and the diversity of their shrub and herbs (hereaer denominated as covered
areas, A and B), while the other two were characterized by a lower cover and diversity of shrubs and a high per-
centage of bare ground (hereaer denominated as uncovered areas, C and D; see Table1 for further details). is
experimental design allowed us to test for dierences in tortoise detectability, and, therefore, infer predation risk,
in relation to vegetation cover.
Mediterranean spur-thighed tortoise and common ravens. e study sites were surveyed during
the 2018-breeding period, comprising the end of February until the end of May, covering an area of 15 ha in
each of the covered areas and of 18 ha in each of the uncovered ones. Each of the four sites was intensively sur-
veyed in order to detect young tortoises. is was done for four (uncovered areas) or ve (covered areas) days
by four trained people. e survey consisted of searching the entire territory for the occurrence of individual
tortoises, but focusing on the detection of hatchlings and juveniles (<100 mm CL). e tortoises were recorded
from 12 h until 16 h on foot and in adequate weather conditions (sunny days with temperatures of between 20
and 24 °C). Each recorded individual was georeferenced using a GPS and the CL was measured using a vernier
Figure 4. Location of the study area in Morocco and tortoise populations studied (sites A, B, C and D) is
shown. Grey circles size is proportional to the maximum abundance of live young tortoises (<75 mm CL)
observed per 1ha-grid (1, 2–3, 4–5, 6–11 ranges). Small grey squares mark grids where no tortoises were found.
Black stars represent the raven perch or nest locations.
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SCIENTIFIC REPORTS | (2020) 10:1874 |
calliper (accuracy ± 1 mm). Size classes were used to characterize the tortoises’ population structure in covered
and uncovered areas24. e Chi-square test was used to assess any dierences between covered and uncovered
areas in terms of hatchling and juvenile size-classes structure, considering measurements of 30–40, 41–50, 51–60,
61–70, 71–80, 81–90 and 91–100 mm. e tortoises’ carapaces were lightly marked with non-toxic paint so as
to be able to identify any recaptured individuals. In addition, two additional days per area at the beginning and
the end of the raven breeding season were carried out looking for any dead animals, which were collected and
measured and, when possible, their cause of death was determined on the basis of a visual examination of the
remains of the animal and the area in which it was found. ese data were then used to determine the relevance of
predation by ravens and to characterize the size-classes of the tortoises that are predated by this species.
During the tortoise survey, the location of common raven perches and nests were registered with a GPS. For
this purpose, we also surveyed a buer area of 1.5 km around the sampling areas, which is slightly greater than the
average distance between the ravens’ territories4. e number of single individuals and breeding pairs of ravens
was also recorded, along with their breeding status and their recruitment success (chicks or edglings).
Modelling tortoise detectability and abundance. e recorded data were referred to 1-ha grids, which
were our territorial unit for analytical purposes. e number of young tortoises susceptible to predation (accord-
ing to our data < 70 mm CL, see below; but see21,37) was, therefore, quantied (our response variable) for each
grid and survey. In addition, each grid was characterized during the survey in relation to vegetation cover, and
specically to the shrub cover (%), shrub height (cm) and species richness, and the bare ground cover (%). In
order to incorporate the eect of the common raven into species detectability, the mean distance to the nearest
raven perch and nest (m; two variables) in each grid was also calculated.
We used N-mixture models to model the detectability and abundance of the hatchlings and juveniles that
were, according to the count data, susceptible to predation, while we accounted for imperfect detection38 using
the unmarked package39 in R40. Our assumption is that the detectability of young tortoises during the surveys
can be considered a proxy of young individuals’ detectability by the ravens. Hatchlings and juveniles are well
camouaged and dicult to follow in their environment. eir detectability is very low8,10,16,41, since it is linked
to the central hours of the day when animals are active. We, therefore, assume that detectability by observers is
a proxy of detectability by ravens in order to be able to explore the eect of dierent factors on predation risk.
is does not mean that the observer is able to detect the same number of young tortoises, but that the higher the
detectability for the observer, the higher detectability for the ravens.
We used Akaike’s Information Criterion corrected for small sample sizes (AICc)41 to select the most appro-
priate error distribution by comparing the performance of Poisson, zero-inated Poisson and Negative Binomial
models. e detection and abundance processes were then modelled sequentially. A forward stepwise procedure
was used to identify the most relevant predictors explaining the detection process. e same procedure was sub-
sequently followed in order to identify the predictors explaining the abundance process. AICc was also used to
compare models in the stepwise procedures by following the ΔAICc < 2 rules42. N-mixture models require an
integer value that species the upper bound used in the integration (K). In our study, this upper bound was set at
K = 120, which is suciently large for it not to have an eect on the model results. We used the parametric boot-
strap approach (1000 samples) in unmarked to attain p-values from the sums of squares, along with Chi-square
and Freeman-Tukey t statistics as a measure of the goodness of t of the nal model.
Ethic statements. Sampling of Mediterranean spur-thighed tortoises in Maamora forest was conducted
under the authorization and following the protocols approved by Le Haut-Commissariat aux Eaux et Forêts et
à la Lutte Contre la Désertication of Morocco (High Commission for Waters and Forests and the Fight against
Desertification). The field surveys were done in accordance with the guidelines and regulations. There was
non-invasive sampling.
Data availability
e datasets generated during and/or analyzed during the current study are available from the corresponding
author on reasonable request.
Received: 22 October 2019; Accepted: 22 January 2020;
Published: xx xx xxxx
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We are very grateful to Oscar Rodríguez, Hassan Belhajjamia, Absallam Belhajjamia and Bouhali Kaddouri for
their eld assistance. We truly appreciate their commitment to the surveys and their ability to detect hatchlings.
We would like to thank HCEFLCD services for their guidance. We would also like to thank Greg Trollip and
Jacob Mwanzia for their support and interest in wild species conservation. PA is supported by MINECO-UCLM
through a “Ramón y Cajal” contract (RYC-2012–11970).
Author contributions
A.S. and P.A. conceived the initial ideas, A.S. did the eld surveys, A.S., J.J. and P.A. analysed the data and led the
writing. All the authors discussed the ndings.
Competing interests
e authors declare no competing interests.
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SCIENTIFIC REPORTS | (2020) 10:1874 |
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... Despite, during a long period of persecution, ravens almost becoming extinct in the US and central Europe in the late nineteenth/early twentieth century [38], raven populations have increased dramatically over the past several years throughout the US, Europe and North Africa [39] due to the growing human activity footprint and its associated anthropogenic food subsidies, as well as the species having been afforded protection (EU bird directive in Europe and federal laws in the US). However, recent increases in raven populations have threatened some vulnerable species, including Desert tortoises Gopherus agasizzii, Spur-thighed tortoises, Sandhill crane Antigone canadensis, Marbled Murrelet Brachyrampus marmoratus, Snowy plover Charadrius nivosus and Least Terns Sternulla antillarum [29,40,41]. Currently, management techniques, such as lethal removal, behavioral modification and habitat modification, have been employed to protect threatened and endangered species from raven predation in certain states in the US [21] but in the Maamora forest, no control measures have as yet been implemented. ...
... km 2 . Because breeding pairs spend 90% of their time within 400 m of their nest [42], the area within a 400 m radius of each of nest site was intensively surveyed in order to detect dead tortoises with signs compatible with predation by Common ravens (recent holes in the carapace or plastron; see [29]). The surveys to detect dead tortoises were carried out on two days each month throughout the raven breeding season in both 2019 and 2020 (for further details see [29]). ...
... Because breeding pairs spend 90% of their time within 400 m of their nest [42], the area within a 400 m radius of each of nest site was intensively surveyed in order to detect dead tortoises with signs compatible with predation by Common ravens (recent holes in the carapace or plastron; see [29]). The surveys to detect dead tortoises were carried out on two days each month throughout the raven breeding season in both 2019 and 2020 (for further details see [29]). Each dead individual was georeferenced using a GPS and the carapace length (CL; mm) was measured using a vernier caliper (accuracy ±1 mm). ...
Full-text available
Bird nest selection in forests can be influenced by the composition of key structural elements and resources. This has important consequences in terms of species population dynamics since it can determine reproduction success. Here, we assessed Common raven nest-site selection and reproductive success, and how these might be determined by foraging behavior and habitat structure. A previously documented breeding raven population that exerts high predation pressure on young Spur-thighed tortoises (Testudo graeca) in a Mediterranean forest was monitored. Generalized linear mixed models were performed to determine the singularities of the trees with nests and the drivers of reproductive success of breeding pairs of ravens. The results showed a high density of breeding pairs in the study area (0.8 pairs/km2), which selected taller trees in areas with higher bare ground cover and a high density of tortoises for nesting. Nests were spatially aggregated; breeding pairs occupied smaller territories and intraspecific competition seemed relaxed, reflecting the abundance of food resources. Most breeding pairs occasionally predated on young tortoises. Tortoises seem to play a part in raven reproductive success in our study area, which might be associated with the availability/catchability of young tortoises. The study illustrates that Spur-thighed tortoise distribution and abundance plays a role in the breeding behavior of ravens and is mediated by habitat structure. Understanding the drivers of nest-site selection and the breeding behavior of ravens is pivotal to implementing appropriate habitat management and conservation strategies across their distribution range, particularly in areas where ravens potentially affect threatened species.
... The size composition of the Karoo dwarf tortoise population was similar to a population of speckled dwarf tortoises that had declined by 66% (Loehr 2017: Figure 1). Declines in chelonian populations due to insufficient recruitment and aging adults appear common (Epperson and Heise 2003, Smith et al. 2013, Van Dyke et al. 2019, Segura et al. 2020, Price et al. 2021, and we consider lack of recruitment a survival threat for Karoo dwarf tortoises. ...
... Both species are tortoise predators (Hockey et al. 2005, Fincham andLambrechts 2014), pied crows especially in arid regions (Durà i Franch 2017). Corvids in general affect tortoise demographics (Boarman 2003, Berry et al. 2013, Segura et al. 2020) and were a likely cause of decline in a speckled dwarf tortoise population (Loehr 2017). We found 3 dead adult Karoo dwarf tortoises with shattered shells next to large boulders on which they were probably dropped by birds (Hockey et al. 2005, Branch 2008). ...
Full-text available
Inconspicuous, secretive, or sparsely distributed species receive relatively little research attention, potentially leading to uncertainty about their status and lack of efforts to conserve them. Karoo dwarf tortoises (Chersobius boulengeri) are endemic to South Africa, spend most of the time in retreats at remote arid locations, and are seldom seen. We conducted a 3‐year (2018–2020) mark‐recapture study to investigate the size and structure of the only Karoo dwarf tortoise population currently known to exist. The population in the 16‐ha core of our study site consisted primarily of adult males and females, at a density of 3.3 individuals/ha. Many individuals had severely worn shells and appeared old. Small individuals (straight carapace length <65 mm) represented just 8% of the population and were mostly recent hatchlings. Overall, tortoises had high estimated survival rates (0.77–0.95; lower 95% confidence limit for the smallest tortoises was 0.16), despite a 15‐month drought. The lack of small individuals may reflect low levels of recruitment and population decline. Predation by corvids was an obvious threat to all size classes. We estimated that the local population across the 250‐ha study area was 800–900 males and females, and recommend precautionary conservation measures focused on reducing human‐subsidized avian predation. In South Africa, the only Karoo dwarf tortoise population currently known to exist comprised mostly adults, many of which appeared old, whereas juveniles were almost absent. To improve recruitment into adult stages and counter projected population decline, removal of potential nesting sites, food and water sources that are attractions for predatory corvids is recommended.
... However, in spite of the interest in this field, very few experimental studies have provided sound results for the offspring size and fitness relationship (hereafter "offspring size/fitness") and survival in vertebrates [8,9,19,20]. The low detectability of breeders or newborns and juveniles [21,22] and the limited long-term studies play a part in this sense [15]. Studying recruitment patterns and how they are affected by environmental conditions is thus essential to determine demographic traits, to model and predict local distributions and to identify causal factors affecting long-term population dynamics [23]. ...
... Overall survival from emergence to overwintering in this study was higher in hatchlings from old females (56% when extrapolating fortnight survival) than their counterparts in Southern Spain (39% [31]). In our study, raven predation is a plausible threat for the hatchling population, as occurred in other populations of Maamora forest [22] and in other Testudinidae populations, where it is known to cause high mortality [72]. However, the effect of traffic and trampling by large ungulates found in our study, and as occurred in northern populations of T. graeca [31], is a minor cause of mortality. ...
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Long-lived species are particularly interesting for investigation of trade-offs that shape reproductive allocation and the effective contribution to the next generations. Life history theory predicts that these species will buffer environmental stochasticity via changes in the reproductive investment, while maintaining high adult survival rates. The spur-thighed tortoise was selected as a case study in order to investigate the relationship between the linked maternal characteristics (size and age) and related traits in their hatchlings. We tracked naturally emerging hatchlings from young and old females under semi-natural conditions to test variations in hatchling numbers, body mass, size and survival over two years. We used linear mixed-effect models to analyze variations in hatchling body mass and size, and a mark–release–recapture framework to model their survival. Our study illustrates that old females of long-lived species have greater offspring numbers, greater survival and smaller size when compared with those of young females. The interannual variability evidenced the reduced offspring number and survival in the lower autumn rainfall and spring mean temperature year. Our results highlight the role of maternal age and climatic conditions in the population dynamics and the need for long-term studies of reproduction traits for designating adequate conservation strategies.
... limited to juveniles 18 . Strikingly, golden eagles (Aquila chrysaetos), the paradigm of generalist avian predators 19 , are able to successfully feed on adult terrestrial tortoises. ...
Full-text available
A reduction in adult survival in long-living species may compromise population growth rates. The spur-thighed tortoise (Testudo graeca) is a long-lived reptile that is threatened by habitat loss and fragmentation. Golden eagles (Aquila chrysaetos), whose breeding habitats overlap that of tortoises, may predate them by dropping them onto rocks and breaking their carapaces. In SE Spain, the number of golden eagles has increased in the last decades and the abundance of their main prey (i.e., rabbits Oryctolagus cuniculus) has decreased. Our aims were to 1) describe the role of tortoises in golden eagles’ diet, and 2) estimate the predation impact of golden eagles on tortoises in eagles’ territories and in the regional tortoise population. We collected regurgitated pellets and prey remains under eagle nests and roosts, and obtained information on tortoise abundance and population structure and rabbit abundance. We found that tortoises were an alternative prey to rabbits, so that eagles shifted to the former where the latter were scarce. The average predation rate on tortoises was very low at the two studied scales. However, eagles showed a marked selection for adult female tortoises, which led the tortoise sex ratio to be biased towards males in those eagle territories with higher tortoise predation. Whether this may compromise the spur-thighed tortoise long-term population viability locally deserves further attention.
... Young tortoises are especially vulnerable to raven mortality as their relatively soft, developing shells are susceptible to beak punctures and they lack the developed musculature to keep their limbs tucked, protecting areas of softer flesh (Boarman 2002(Boarman , 2003. In fact, raven depredation has been shown to account for the majority (up to 100%) of young desert tortoise predation mortalities in some areas (Nagy et al. 2015a, b;Segura et al. 2020). Data collected between 2013 and 2019 by the California Desert Common Raven Monitoring and Management Program indicated that most raven-depredated tortoise carcasses are within the size class of 0-9 years old (i.e., midline plastron length <124 mm), consistent with Medica et al. (2012). ...
Full-text available
Some avian species have developed the capacity to leverage resource subsidies associated with human manipulated landscapes to increase population densities in habitats with naturally low carrying capacities. Elevated corvid densities and new territory establishment have led to an unsustainable increase in depredation pressure on sympatric native wildlife prey populations as well as in crop damage. Yet, subsidized predator removal programs aimed at reducing densities are likely most effective longer-term when conducted in tandem with subsidy control, habitat management, and robust assessment monitoring programs. We developed decision support software that leverages stage structured Lefkovitch population matrices to compare and identify treatment strategies that reduce subsidized avian predator densities most efficiently, in terms of limiting both cost and take levels. The StallPOPd (Version 4; available at software enables managers to enter the area of their management stratum and the demographic properties (vital rates) of target bird population(s) of interest to evaluate strategies to decrease or curtail further population growth. Strategies explicitly include the reduction in fertility (i.e., eggs hatched) and/or the culling of hatchlings, non-breeders and/or breeders, but implicitly comprise reduction in survival or reproduction through subsidy denial. We illustrate the utilities of the software with examples using common ravens (Corvus corax; ravens) in the Mojave Desert of California, USA. Unfortunately, the survival and reproduction effects of each unit of a particular subsidy in that system have remained elusive, though this is the priority of current research. Because the software leverages a life history representation that is known to characterize hundreds of wildlife species in addition to ravens, the work expands the suite of tools available to wildlife managers and agricultural industry specialists to abate bird damage and impacts on sensitive wildlife in habitats with persistent human subsidies.
... The posterior estimates of the TFD and AWT models presented herein are directly incorporable for designing and implementing new occupancy surveys within cliff habitat of the Appalachian region and eastern United States. The effectiveness of the time-to-detection method for estimating detectability and abundance makes this framework conducive to raven monitoring not only for targeted species conservation but also for management and control in ecosystems where ravens depredate species of concern, such as the threatened desert tortoise (Gopherus agassizii, Cooper) in the Mojave Desert, USA (Kristan & Boarman, 2003;Shields et al., 2019); spur-thighed tortoise (Testudo graeca L.) in Morocco (Segura et al., 2020); and greater sage-grouse (Centrocercus urophasianus, Bonaparte) in the western United States (O'Neil et al., 2018). To utilize the time-to-detection framework for survey planning in other geographic areas, studies similar to ours that evaluate detection patterns should be repeated to derive accurate location-specific detectability curves and recommendations of required survey effort. ...
Full-text available
Common ravens (Corvus corax) inhabited much of the eastern United States prior to European colonization but were nearly extirpated by the mid‐1900s. Although remnant raven populations have since begun recolonizing portions of their historic range in the eastern United States, the extent of recovery remains largely unknown because of the species' elusive behavior. To aid development of targeted monitoring programs for this rare and cryptic species, we investigated factors that may influence detectability of ravens in natural cliff habitat during the nesting season in the Central Appalachian Mountains. Using a time‐to‐detection framework, we performed surveys at cliff sites with positive raven occupancy and recorded time to first detection (TFD) and confirmed cliff occupancy (i.e., occupied detection) to estimate detection probability curves as a function of survey time. We further compared multiscale habitat features of occupied and unoccupied cliff sites to characterize regional nesting habitat. Mean TFD at occupied cliffs was 14 ± 2 (SE) min. The TFD and subsequent detection probability increased with warming temperatures, likely as a result of heightened activity at nests as the reproductive season progressed. Shorter distances from observation point to the surveyed cliff resulted in longer TFD, indicating that ravens are likely sensitive to disturbance at nest sites. Analysis of cliff‐nesting habitat data identified raven selection of cliffs with large, exposed faces positioned on predominant west‐facing aspects. Mean detection probability calculated from detectability curves was ≥0.8 after the first 30 min. Using a probability‐based model and empirical detection probabilities, we estimated that raven absence from a cliff site during the nesting season may be inferred with p = 0.99 after three independent 30‐min surveys. Although ravens have been considered elusive and rare in the eastern United States, we find that their detectability at occupied cliff sites during the nesting season is relatively high and that observation of distributional changes may be easier for this species than for other, more cryptic birds.
... The presence of (old) adults and hatchlings, but entire lack of juvenile stages, implies that the population is not stable, i.e. deceased adults may not be replaced by maturing juveniles. We do not know the cause of the lack of juveniles, but predation by corvids is a likely explanation (Kristan & Boarman 2003;Nagy et al. 2015;Loehr 2017;Segura et al. 2020). During every sampling period, white-necked ravens (Corvus albicollis) were patrolling the site daily, and pied crows (Corvus albus) were present occasionally. ...
In arid regions with summer rainfall, herbivorous reptiles are able to acquire water and fresh food in the presence of high environmental temperatures that can promote ectotherm activity. However, extremely high temperatures and below average rainfall may also limit foraging opportunities due to risks of overheating and predation while gathering scarce food. Karoo Dwarf Tortoises (Chersobius boulengeri) inhabit an arid region in South Africa where most rains fall around austral summer (October-May). We used focal-animal observations and instantaneous recording to assess their behavioral patterns. Despite relatively high rainfall and available plant growth, Karoo Dwarf Tortoises spent approximately 80-90% of their time in retreats. Whereas activity (behavior outside retreats) in the spring was unrelated to time of the day, possibly due to moderate ambient temperatures, activity in the summer was restricted to the afternoon and evening, when tortoises walked and scanned for food and retreats, and fed only 11 min/d on average. In summer, body temperature of tortoises within retreats was positively associated with retreat temperatures, but tortoises appeared to thermoregulate using bodily postures and possibly other means. We suggest that Karoo Dwarf Tortoises mitigate predation risks by maintaining a low level of activity and thermoregulating within retreats. The short feeding time of Karoo Dwarf Tortoises compared to other tortoise taxa may result in slow growth and reproductive rates, which might in turn affect population resilience and conservation needs of this endangered species.
... Freshwater turtles rely on both the aquatic and terrestrial habitats of wetlands to complete their life cycles (Burke et al. 2000). While many species are either apex predators or high in the food chain in the aquatic environment, once in the terrestrial environment, turtles are highly vulnerable to predation (Segura et al. 2020;Spencer and Thompson 2003;Tucker and Janzen 1999). Freshwater turtle life histories are characterised by long generation times, high adult survivorship, late age maturation, low annual reproductive effort, as well as low and variable recruitment (Congdon et al. 1993;Iverson 1991;Spencer 2018). ...
Snake-Turtle interactions have been rarely documented. We recorded a hatchling Chelodina oblonga within the stomach contents of a Western tiger snake (Notechis scutatus occidentalis). This is the first recorded observation of an interaction between snakes and hatchling freshwater turtles in Western Australia. Field based palpation failed to detect the hatchling, suggesting that without dissection, turtle hatchling predation by snakes more generally could be higher than commonly reported. Snake predation of hatchlings could be placing additional pressure on threatened populations of freshwater turtles in Australia, warranting further investigation.
... ha) is likely due to mobility restrictions created by a small body size, which is only 6% of an adult's mass. Additionally, a small home range size could be a survival strategy for hatchling and juvenile lizards because large movements away from vegetative cover increase risk of predation for reptiles (Pietrek et al. 2009, Segura et al. 2020, and hatchling horned lizards are less protected than adults from predators by their body armor and occipital horns (Ballinger 1974, Sherbrooke 2003. Alternatively, hatchlings could have smaller home ranges because of more specific microhabitat preferences in the hatchling life stage than later life stages. ...
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Habitat fragmentation has negative consequences on threatened and endangered species by creating isolated populations. The Texas horned lizard (Phrynosoma cornutum) is experiencing population declines and localized extirpations throughout its range and has been classified as a species of greatest conservation need in Oklahoma, USA. Younger age classes have been poorly studied but may be vital to the stability of remaining populations. To address gaps in knowledge concerning subadult (hatchling and juvenile) morphometrics, survivorship, and home range sizes, we studied 2 cohorts of subadults, for 2 years each, covering their hatching and juvenile years (2016–2019). We used a combination of radio‐telemetry and novel harmonic radar methodology to study a closed population of Texas horned lizards in 15 ha of native grassland at Tinker Air Force Base, Oklahoma. Population abundance for adults and juveniles was estimated as 56.5 ± 5.5 lizards and density as 7.96 lizards/ha. Our lowest estimates of survival indicated an average survival probability for the hatchling life stage of 0.285 (95% CI = 0.15–0.44), which is lower than for adults on the site. Average home range size increased from hatchling to adult life stages. Our results will have an immediate effect on the planning and assessment of ongoing headstart and management programs for Texas horned lizards. © 2021 The Authors. The Journal of Wildlife Management published by Wiley Periodicals LLC on behalf of The Wildlife Society. We estimated Texas horned lizard hatchling survivorship using direct, field‐based monitoring methodology, and report ontogenetic increases in morphometrics and home range area from hatchling to adult life stages. Our results can be used in the planning and assessment of future headstart and management programs for this threatened lizard.
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Ecología y dinámica poblacional de las tortugas moras de Doñana. 1987. Universidad de Sevilla. España.
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The use of color markings (e.g., non-toxic paint, dye, or pens) is frequently employed by herpetologists to track individuals within a population, but effects of these markings on study animals are generally unknown. Markings could affect survival rates, although this can be difficult to determine through mark-and-recapture methods. With clay models, however, we can quantify interactions with predators and measure predation risk associated with color markings. We used 126 clay models of Western Fence Lizards (Sceloporus occidentalis) to examine how marking individuals with colored paint influences predation risk, and to determine whether conspicuous colors enhance risk more than inconspicuous colors. We compared avian attacks on unmarked models to attacks on two treatments: models marked with white nail polish or purple polish. We modeled how these treatments are viewed under an avian visual system and found that white markings exhibited greater achromatic contrast against the clay while purple markings exhibited greater chromatic contrast. Models marked with purple paint received significantly fewer attacks than the control, while the attack rate for the white treatment was similar to that of the control. These results show that purple markings could have positively affected survival rates of marked animals. Conducting experiments on the effects of artificial markings might help researchers minimize negative impacts on their study animals. These studies also suggest that markings may have unintended effects by skewing predation risk, thereby potentially influencing research outcomes.
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Common ravens (Corvus corax) are a major predator on the threatened desert tortoise (Gopherus [=Xerobates] agassizii). Large numbers of juvenile tortoise shells have been found beneath known raven nests and perches; many shells that show evidence consistent with raven predation have been found sporadically throughout the range of the tortoise; significant proportional decreases in juvenile size/age class distributions have been identified; and people have observed ravens killing, carrying, and consuming juveniles. In 1988 the U. S. Bureau of Land Management initiated a process to evaluate, design, and implement a program to reduce raven predation on desert tortoises. A pilot program was temporarily halted by a law suit filed by the Humane Society of the United States, and a draft long-term plan and Draft Environmental Impact Statement were subsequently issued and are now being modified. Several complex issues have arisen in attempting to design and implement control of ravens including: pitting one native species against another, making management decisions in light of data of varying scientific validity and depth, targeting individuals versus populations, and managing a predation problem over a broad geographic range. Addressing each of the concerns is highly problematic and the solutions are not always satisfying.
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Recruitment is integral to population persistence, therefore characterizing this process is essential for evaluating recovery actions for species in decline. We gathered all data available and used Bayesian analyses to quantify annual recruitment of Mojave Desert (Gopherus agassizii) and Sonoran Desert (G. morafkai) tortoises as the product of four components: proportion of females that reproduced, number of eggs produced per reproducing female, hatching success, and hatchling survival. For Mojave Desert Tortoises, the estimated proportion of females that reproduced (0.81 [95% Confidence Interval: 0.52–0.99]) and number of eggs produced per year (6.90 [5.51–8.16]) were higher than for Sonoran Desert Tortoises (0.52 [0.07–0.94] and 5.17 [3.05–7.60], respectively). For Mojave Desert Tortoises, hatching success averaged 0.61 (0.25–0.90). Data on hatching success for Sonoran Desert Tortoises and hatchling survival for both species were sparse, therefore we represented these components with a range of plausible values. When we combined components, average recruitment for Mojave Desert Tortoises ranged from 0.51 females/female/y assuming that hatchling survival was 0.30 to 1.18 females/female/y with hatchling survival assumed to be 0.70. For Sonoran Desert Tortoises, average recruitment ranged from 0.25 to 0.57 females/female/y for the same values of hatchling survival. Differences in recruitment between species likely reflect the evolution of different life-history strategies for tortoises in Mojave and Sonoran Deserts, perhaps in response to variation in precipitation regimes. To better inform conservation and recovery of desert tortoises, more information is needed for all recruitment components, but especially for hatching success and hatchling survival.
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Camouflage via animal coloration and patterning is a broadly important antipredator strategy. Behavioral decision making is an influential facet of many camouflage strategies; fitness benefits often are not realized unless an organism selects suitable backgrounds. Controlled experimental studies of behavioral strategies in selection of backgrounds conferring camouflage, however, are rarely paired with observations of wild populations. In order to investigate how substrate composition influenced habitat preference and selection by juvenile desert tortoises (Gopherus agassizii), we completed a manipulative experiment in captivity and an observational study in the wild. In our captive experiment, we found that tortoises spent a greater portion of their time near rocks. We similarly found that wild tortoises preferentially placed themselves in areas with equivalent or larger-sized rocks. Additionally, juvenile tortoises were found to be less detectable on rock substrate by observers than they were on substrate-lacking rocks. We hypothesize that rocks improve juvenile tortoise camouflage and thus that tortoises select for habitat containing rock substrate, in part, due to a survival advantage conferred by such use. The desert tortoise is a threatened species, and the present study provides a model for examining the intersection between behavior and conservation, with implications for how suitable habitat is defined and measured in species conservation programs.
Collection for the pet trade has been considered one of the major threats to the Mediterranean spur-thighed tortoise Testudo graeca , since it modulates the size and structure of the species’ populations and, therefore, their demography. Maamora forest is one of the most suitable habitats for this species. The proximity of the forest to Rabat indicated the possibility of these tortoise populations being particularly sensitive to over-collecting. Population demography was studied in four populations, in protected and unprotected areas in Maamora forest. The results showed significant differences as regards population size and structure between protected and unprotected areas. They specifically highlighted: i) higher density (23-17 indiv · ha ⁻¹ ) balanced populations in the protected areas, in which young adults were predominant, ii) a higher body condition in the protected areas, especially the females, and iii) a low density (5.5 ind · ha ⁻¹ ) more unbalance population in the unprotected areas, in which older females and younger males were predominant. In addition, a survey carried out by interviewing local adults ( ) showed that most people owned tortoises (61%, ), mainly juveniles (65%, ). The respondents stated that their tortoises had been captured in the forest (68%, ). Maamora forest is home to one of the highest density populations of Mediterranean spur-thighed tortoises documented to date, and its conservation is essential if this species is to be maintained. Our social survey suggested that one of the challenges is to change the social perception of the Mediterranean spur-thighed tortoise as a pet and highlight its intrinsic ecological value.
1.Preventing biodiversity loss in fragmented agricultural landscapes is a global problem. The persistence of biodiversity within remnant vegetation can be influenced by an animal's ability to move through the farmland matrix between habitat patches. Yet, many of the mechanisms driving species occurrence within these landscapes are poorly understood, particularly for reptiles. 2.We used scented and unscented plasticine lizard models and wildlife cameras to (1) estimate predation risk of reptiles in four farmland types (crop field, pasture paddock, restoration tree planting and areas with applied woody mulch) relative to the patch edge and remnant vegetation, and (2) examine how predation risk was influenced by temporal change in the matrix (crop harvesting). 3.Birds (55.1%), mammals (41.1%), reptiles (3.4%) and invertebrates (0.5%) attacked models, of which 87% were native species. Mammalian predators were 60.2% more likely to attack scented models then unscented models. Bird predators were not influenced by scent. 4.We found predator attacks on models were highest at edges (49%, irrespective of adjacent farmland type, with a reduced risk within farmland (29%) and remnant patches (33%) (P<0.01). Both mammal and bird predators contributed to high numbers of predation attempts at edges. 5.Removal of crops did not increase predation attempts in crop fields or other farmland types, although predation attempts were significantly lower along the crop transect after harvesting, compared to the woody debris transect. However, numbers of predation attempts were higher in edge habitats, particularly prior to harvesting. 6.Synthesis and applications. Reptiles are at risk of predation by birds and mammals in both remnant patches and the farmland matrix, particularly in edge habitat. Our results demonstrate that edge habitats are potentially riskier for lizards than the farmland. Vulnerability to predation may be increased by a lack of shelter within edge habitats such as by increasing visibility of reptiles to predators. Therefore, to benefit reptiles, land managers could provide shelter (rocks, logs and grasses), particularly between remnants and linear plantings which could improve landscape connectivity. This article is protected by copyright. All rights reserved.
In spite of growing reliance on translocations in wildlife conservation, translocation efficacy remains inconsistent. One factor that can contribute to failed translocations is releasing animals into poor quality or otherwise inadequate habitat. 2.Here we used a targeted approach to test the relationship of habitat features to post-translocation dispersal and survival of juvenile Mojave desert tortoises Gopherus agassizii. 3.We selected three habitat characteristics—rodent burrows, substrate texture (prevalence and size of rocks), and washes (ephemeral river beds)–that are tied to desert tortoise ecology. At the point of release, we documented rodent burrow abundance, substrate texture, and wash presence and analysed their relationship to maximum dispersal. We also documented relative use by each individual for each habitat characteristic and analysed their relationships with survival and fatal encounters with a predator in the first year after release. 4.In general, the presence of refugia or other areas that enabled animals to avoid detection, such as burrows and substrate, decreased overall mortality as well as predator-mediated mortality. The presence of washes and substrate that enhanced the tortoises’ ability to avoid detection also associated with reduced dispersal away from the release site. These results indicate an important role for all three measured habitat characteristics in driving dispersal, survival, or fatal encounters with a predator in the first year after translocation. 5.Synthesis and applications. Resource managers using translocations as a conservation tool should prioritize acquiring data linking habitat to fitness. In particular, for species that depend on avoiding detection, refuges such as burrows and habitat that improved concealment had notable ability to improve survival and dispersal. Our study on juvenile Mojave desert tortoises showed that refuge availability or the distributions of habitat appropriate for concealment are important considerations for identifying translocation sites for species highly dependent on crypsis, camouflage, or other forms of habitat matching. This article is protected by copyright. All rights reserved.