Wheat fields as an ecological trap for reptiles in a semiarid agroecosystem

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Wheat fields as an ecological trap for reptiles in a semiarid
agroecosystem
Guy Rotem
a,c,
⇑
, Yaron Ziv
a
, Itamar Giladi
a,b
, Amos Bouskila
c,b
a
Spatial Ecology Lab, Department of Life Sciences, Ben-Gurion University of the Negev, Beer-Sheva, Israel
b
Mitrani Department of Desert Ecology, Jacob Blaustein Institutes for Desert Research, Ben-Gurion University of the Negev, Sede Boqer Campus, Israel
c
Behavioral Ecology Lab, Ben-Gurion University of the Negev, Beer-Sheva, Israel
article info
Article history:
Received 27 January 2013
Received in revised form 8 August 2013
Accepted 18 August 2013
Keywords:
Agroecosystem
Dispersal
Ecological trap
Fragmentation
Habitat selection
Trachylepis vittata
abstract
Intensive agricultural activity over large areas on earth, which is necessary to meet the increasing
demand of a growing human population, may lead to biodiversity loss. This loss may be mitigated by
keeping natural and semi-natural patches within agricultural fields to allow the maintenance of biolog-
ical diversity (‘Wildlife Friendly Agriculture’). We conducted our study in an agroecosystem comprised of
small isolated patches nested within agricultural fields. We trapped reptiles in 13 sampling sites, each of
which included arrays of pitfall traps in a natural patch, in the adjacent wheat field and at the patch-field
edge. We conducted six trapping sessions in the spring – four times before, once immediately after and
once a week after the wheat harvest. Prior to the harvest, we found an intensive movement of Trachylepis
vittata, the most common reptile in our study, from the semi-natural patches into the fields, but negligi-
ble movement in the opposite direction. This pre-harvest directional movement corresponded with
higher abundance of prey (i.e., arthropods) in the wheat field compared to the natural patches in early
spring. The individuals that moved into the fields were adults of better body condition than those remain-
ing in the patch, suggesting that the motivation for movement was habitat preference by individuals with
high prospective fitness rather than the exclusion of subordinates. The population of T. vittata in the
wheat fields and movement across habitats dropped to zero during and after the harvest. Our results pro-
vide strong evidence that the agricultural fields serve as an ecological trap to organisms inhabiting
nearby natural habitats. We suggest that plans for Wildlife-Friendly Agriculture for biodiversity conser-
vation should consider also potential negative effects, such as the ecological trap effect.
Ó2013 Elsevier Ltd. All rights reserved.
1. Introduction
A rapidly growing global human population coupled with an in-
crease in per-capita consumption challenge modern agriculture to
increase productivity in order to meet the increasing demand. This
challenge is being tackled by both an expansion of farming area
and an intensification of agricultural practices. The vast terrestrial
areas affected by agriculture (about 80% globally; MEA, 2005), agri-
cultural intensification, and the cultivation of monocultures are all
expected to cause biodiversity loss (FAO, 2007; Green et al., 2005).
One recent approach to alleviate the negative effects of agriculture
on biodiversity is ‘Wildlife Friendly Agriculture’, which apparently
promotes a balance between food production and conservation by,
among others, leaving natural habitat patches within a heteroge-
neous agricultural landscape (Green et al., 2005). Accordingly,
preservation of natural or semi-natural patches within the agricul-
tural matrix is considered an effective and relatively cheap way to
preserve biodiversity (Aarssen and Schamp, 2002; Benton et al.,
2003; Duelli and Obrist, 2003). In addition to biodiversity conser-
vation, this approach may be beneficial also for farmers because
of the positive ecosystem services that natural habitats provide
for agriculture (Rosenzweig, 2003a,b; Tscharntke et al., 2005;
Bommarco et al., 2013).
However, the proximity of natural habitat patches to agricul-
tural matrix may also affect animal behavior, in general, and hab-
itat selection, in particular (Tscharntke et al., 2012). The selection
of habitats in which to shelter, feed and reproduce can dramati-
cally impact organism fitness. Consequently, most animals have
evolved abilities to sense reliable cues regarding habitat quality
and to move to a better habitat whenever possible (Abramsky
et al., 1985; Pulliam, 1988).
However, the ability to reliably assess habitat quality is often
compromised in human-made environments (Kristan, 2003;
Battin, 2004). Cultivation-related fluctuation in habitat quality
0006-3207/$ - see front matter Ó2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.biocon.2013.08.028
⇑
Corresponding author. Address: Department of Life Sciences, Ben-Gurion
University of the Negev, P.O.B. 653, Beer-Sheva 84105, Israel. Tel.: +972 8 6461350,
mobile: +972 52 3354485; fax: +972 8 6479221.
E-mail addresses: rottemguy@gmail.com (G. Rotem), yziv@bgu.ac.il (Y. Ziv),
itushgi@bgu.ac.il (I. Giladi), bouskila@bgu.ac.il (A. Bouskila).
Biological Conservation 167 (2013) 349–353
Contents lists available at ScienceDirect
Biological Conservation
journal homepage: www.elsevier.com/locate/biocon

may attract individuals at certain times and be detrimental at
other times (Best, 1986; Bollinger et al., 1990). The case where
an organism prefers low-quality habitats over other available bet-
ter habitats is called an ‘ecological trap’ (Dwernych and Boag,
1972; Donovan and Thompson, 2001; Hawlena et al., 2010), which
might be considered a special case of source-sink dynamics (Pul-
liam, 1988; Battin, 2004). Such ecological traps may have far-
reaching consequences for the populations in both the low and
the high quality habitats. Robertson and Hutto, (2006) offer three
criteria that define an ‘ecological trap’: ‘‘(1) individuals should have
exhibited a preference for one habitat over another;(2) a reasonable
surrogate measure of individual fitness should have differed among
habitats; and (3) the fitness outcome for individuals settling in the pre-
ferred habitat must have been lower than the fitness attained in other
available habitat’’.
Our study area, the Beit-Nir agroecosystem, is located at the
northern part of Southern Judea Lowlands (SJL), central Israel
(31°30
0
52
00
N34°52
0
36
00
E), approximately 50 km southwest of Jeru-
salem (Fig. 1a). Thousands of years of human inhabitance (Ben-Yo-
sef, 1980) and recent intensive agricultural practice formed a
landscape consisting of natural habitat patches at different degrees
of isolation, surrounded by agricultural fields (mainly wheat) vine-
yards and olive groves. The presence of semi-natural patches with-
in this agricultural landscape can potentially host a high diversity
of reptiles. However, these patches are positioned within wheat
fields, a habitat with potentially highly fluctuating quality due to
seasonal cultivation.
Using reptiles, we examine the main hypothesis that the agri-
cultural system serves as an ecological trap, as defined by Robert-
son and Hutto (2006), where many individuals move to and
permanently occupy the agricultural fields, eliminated by the agri-
cultural machinery before or during the reproduction season. We
contrast this hypothesis with an alternative one, stating that the
agricultural system is used for a daily foraging ground by individ-
uals that mainly occupy the adjacent natural habitats.
Our model species Trachylepis vittata [Scincidae] is common
along the eastern Mediterranean basin and in North Africa (Van
der Winden et al., 1995). It is frequently found under stones in
the early morning until the ambient temperature rises above
14 °C. This species also uses rocks as shelters to escape rain and
other extreme weather (Clark and Clark, 1973). It measures
225 mm from snout to tail and feeds on arthropods (Schleich
et al., 1996). Females give birth to live offspring between July
and August (Disi et al., 2001, p. 226).
2. Methods
2.1. Study design and survey protocol
We surveyed reptiles in 13 sampling sites, each including a nat-
ural patch, an adjacent wheat field and the patch-field edge
(Fig. 1b). At each site we installed 40 traps, positioned in two ar-
rays, each comprised of 20 one-liter dry pitfall traps. The traps
were arranged in two parallel lines at distances of 10 m and
15 m on either side of the patch-field edge (Fig. 1b). Additionally,
on the patch-field edge we used a polypropylene multiwall sheet
to build a 100 m-long and 40 cm-high fence (Fig. 1b) that directs
all reptiles’ movement between the natural patch and the agricul-
tural field to passageways located every 20 m along the fence
(Fisher et al., 2008). At those passageways we placed two one-liter
dry pitfall traps, one at each side (total of 10 one-liter dry pitfall
traps along each fence). These sampling methods enabled us to
simultaneously asses the community structure and monitor the
physical condition of reptiles in the natural patch, in the field,
while crossing from the natural patch to field and while crossing
in the opposite direction (Jenkins and McGarigal, 2003).
We trapped reptiles during six sessions throughout the spring
(March to June) – four times before the wheat harvest, immedi-
ately after the harvest and one week later. In each session, traps
were left open for 72 h. Trapped animals were measured (i.e.
weight, snout-to-vent-length, tail length) and identified to species
(and sex when possible; see results). Individuals’ physical condi-
tion was assessed by an index of body condition (IC; Andrews
and Wright, 1994). Initially we intended on using individual
marking to follow the reptiles’ movement. However, as marking
of individuals during the four first sampling sessions resulted in
no recapture at all, this method was not used further on. We re-
leased all captured individuals back to the habitat where they were
captured (in the natural patch or agricultural field) or to the habitat
they were aiming for (in the patch-field edge). We averaged all the
observations from each combination of ‘habitat’ ‘session’ ‘site’
prior to any statistical analysis and used these summarized data as
our replicates, thus avoiding any pseudo-replication.
Incidentally, the pitfall traps also collected arthropods that
were later identified in the lab to their order level. Previous studies
have found a positive correlation between insect abundance and
reptile abundance (Rocha et al., 2008). As all the studied reptile
species were predators, having insects as a dominant component
of their diet, we assumed that arthropod abundance could serve
as a good indicator for habitat quality.
3. Results
Throughout the study, we trapped 352 reptiles, belonging to 9
species. Most of the trapped individuals (271) belonged to our
model species, T. vittata. The vast majority (244) of the 271 individ-
uals of T. vittata, throughout the season and in all habitat types
were adults, 16 were sub-adults (mainly in the pre-harvest ses-
sions only, and in all habitat types) and only 11 were juveniles,
all of which were captured in the natural patch habitat in the
post-harvest session. Although it was sometimes possible to deter-
mine the sex of trapped individuals, in most cases it could not be
reliably done. Therefore, our analysis was not stratified by sex or
by age.
We found a significant effect of both sampling time and habi-
tat (repeated-measures ANOVA, F
(5, 240)
= 10.43, p< 0.001, and
F
(3, 48)
= 72.46, p< 0.001, respectively) as well as their interaction
(F
(15, 240)
= 9.0643, p< 0.0001) on T. vittata’s abundance (Fig. 2).
T. vittata abundance (Fig. 2) in natural patches remained rela-
tively constant throughout the entire study period. In contrast,
the number of T. vittata found in the wheat field varied. Early in
the season only a few individuals occurred within the field habitat,
but their number increased throughout the spring until the har-
vest. After the wheat harvest, not a single individual was found
within the field habitat. The reptiles’ movement across habitats
was unidirectional with an intensive movement from the natural
patches into the wheat fields in early spring (38 individuals ob-
served). Only two individuals attempted crossing in the opposite
direction throughout the entire season. The very low densities of
other reptile species precluded us from conducting meaningful
analyses at the species level. Nevertheless, the general patterns
for all the rest of the reptile community combined was similar to
the results found for T. vittata. The number of reptiles (excluding
T. vittata) captured per trapping array per session remained con-
stant in the natural patch habitat throughout the season (0.69
and 0.77 for pre-harvest and post-harvest, respectively). It dropped
sharply in the field habitat (from 0.25 individuals in the pre-har-
vest to 0 in the post-harvest). Prior to the harvest, twice as many
individuals crossed from the patch to the field than in the opposite
350 G. Rotem et al. / Biological Conservation 167 (2013) 349–353

direction (0.15 and 0.08 individuals per trapping array per session,
respectively) and no movement was observed in either direction
after the harvest.
Habitat type significantly affected body condition of T. vittata
(Fig. 3; one-way ANOVA, F
(2, 61)
= 33.7, p< 0.05) and we found that
individuals in the natural patches were in poorer physical condi-
tion than those captured in the field or those striving to cross from
the natural habitat to the agricultural field (p= 0.0001, Tukey’s
honestly significant difference post hoc test).
All individuals in the field and those crossing from the natural
patch to the field were adults, whereas juveniles and newborn
were found in the natural patches only.
In early spring arthropod abundance within the wheat fields
was significantly higher compared to that in the natural patches
(in early spring: t-test, t
17
= 3.791, p= 0.001, SD = 77.173; after
the harvest arthropod abundance within the wheat fields was
not significantly different than that in the natural patches: t-test,
t
17
= 1.912, p= 0.07, SD = 72.115).
4. Discussion
Robertson and Hutto (2006) criteria for the existence of an eco-
logical trap include preference for one habitat over another and
lower fitness (measured directly or using a surrogate) in the pre-
ferred relative to the other habitat (see Introduction). The rela-
tively higher insect abundance in the field in early spring may
explain the extreme asymmetrical movement of individuals of
the insectivorous skink, T. vittata, from the natural patches to the
field. Furthermore, the superior body condition of adults crossing
to, or already in the field, relative to those remaining in the natural
patches, clearly indicates that the movement to the habitat with
high food abundance is mainly executed by the individuals in a
better condition, with high potential for reproduction. Meylan
et al. (2002) showed that when movement between patches (i.e.,
dispersal) incurs a high energetic cost, only individuals of better
body condition make an attempt for such movement. Whether or
not this mechanism drove our results, the superior body condi-
Fig. 1. Map of the research area and the study site. (a) White polygons represent natural and semi-natural patches surrounded by agricultural fields, mainly wheat. A diagram
of a trapping array and (b) showing the fence (black line) and the trapping array in each habitat and along the separating fence. A picture of the patch-field edge and the
separating fence is given in c.
Fig. 2. Mean number of Trachylepis vittata individuals per trapping array that was
captured in natural patches, wheat fields and while crossing between these habitats
at different times along the wheat growing season. T. vittata was captured in four
occasions prior to the wheat harvest (Pre-H1–H4 corresponding to end of February,
end of March, Mid April and end of May), immediately after the harvest (Harvest)
and one week later (post-H).
G. Rotem et al. / Biological Conservation 167 (2013) 349–353 351

tioned individuals clearly showed preference to the agricultural
field.
Combined our observations suggest that in early spring, individ-
uals behave according to the expectation of ideal density-depen-
dent habitat selection (Fretwell and Lucas, 1969), i.e., moving to
a higher quality habitat to increase fitness. However, this seem-
ingly optimal habitat selection eventually led individuals to be
trapped in a very poor habitat following the wheat harvest. We
have not found even a single live T. vittata in the field following
the harvest activity. Furthermore, we have not found evidence of
movement of any individual from the field into the patch during
or after the harvest, nor immediately prior to the harvest, despite
a buildup of substantial population in the field at this time
(Fig. 2). Some of the reptiles in the field were presumably killed
by the agricultural machinery; others, exposed to predators like
Corvus monedula,Falco tinnunculus or Circaetus gallicus that accom-
panied the harvest activity, were likely consumed. We indeed
counted more than 20 individuals of each of these predatory birds
following the harvester, apparently collecting prey uncovered by
the harvester (personal observations). This phenomenal scene is
typical to harvesting activity throughout SJL and Israel, and, as
far as we know, also throughout the world.
Much of the results that we obtained, at least early in the sea-
son, could have been generated by a daily movement of individuals
that reside and reproduce in the natural patch and conduct daily
foraging forays into the fields where they benefit from the high
arthropod abundance. If that was the case, we would expect that
at least some individuals that arrived during a trapping session will
be captured on their return to the natural patch. The almost com-
plete lack of such movement, especially in the last session prior to
the harvest, when substantial population was found in the field,
clearly rejects the daily pattern hypothesis. Furthermore, through-
out the spring, the T. vittata population within the agricultural field
grew, which could indicate that individuals that moved from the
natural patches remained in the field and have not used it just
for diurnal foraging. Finally, if daily foraging individuals benefited
from high quality food in the fields, one could expect a negative
correlation between the physical state of individuals in natural
patches and the distance from the high quality field habitat. Using
auxiliary data, where traps were located in different distances from
the patch-field edge (Rotem, 2012), we found no such correlation
(Linear regression, F
1,34
= 1.10, p= 0.30, R
2
= 0.03).
Clearly, our results and observations indicate a large difference
between the fitness provided by the two habitats – while repro-
duction of T. vittata occurs in the natural patch (as evident by the
observation of a few newborns late in the season after the harvest),
the fitness in the fields equals zero (not even a single live T. vittata,
adult or juvenile, in the field following the harvest activity). Fol-
lowing the criteria set by Robertson and Hutto, (2006), we affirm
that the agricultural fields serve as an ecological trap for T. vittata
– better-conditioned individuals show preference for the field, as
indicated by their directionality of movement; the field offers high-
er resource quantity indicating a potential difference in prospec-
tive fitness; and the preferred habitat has eventually a lower
fitness. Although the data enabled us to conduct detailed analysis
for only one common species, the similar patterns observed for
the rest of the community suggest that the implications of the re-
sults may be pertinent for many species, including rare species for
which data is always hard to obtain.
Organisms make decisions regarding their future success based
on currently available information. Most of these decisions are
based on the long process of evolutionary promotion of optimal
habitat selection (Schlaepfer et al., 2002). However, adaptations
for optimal habitat selection that have been shaped by long-term
evolutionary processes may be out of context in cases of anthropo-
genic intervention with the natural environment (Hawlena et al.,
2010). Such intervention, which is usually much faster than almost
any evolutionary process, leads to situations in which organisms
select habitats according to their ‘‘evolutionary knowledge’’
(Battin, 2004), leading, on occasions, to ecological traps (e.g.,
Hawlena et al., 2010). In our case, the wheat field serves as an eco-
logical trap by attracting individuals of better physical condition in
the population to migrate to the seemingly better habitat. These
individuals, of high prospective fitness, find themselves in a very
poor habitat after the harvest, leading to no fitness at all.
The passage of individuals in better physical condition from the
natural patches into the wheat fields, where their fitness is very
low, may further decrease both population size and the quality of
the natural patches’ populations (Schlaepfer et al., 2002). Small
fragmented populations are exposed to inbreeding depression
and genetic drift, which further decrease the population’s genetic
diversity and weaken its ability to cope with both short-term sto-
chasticity (e.g., drought period) and long-term environmental
change (Porlier et al., 2009). The effects of genetic isolation and
the negative qualitative and quantitative effects of ecological traps
on isolated populations in natural patches may be additive, or even
synergistic, increasing the probability of extinction for those popu-
lations. The asymmetric, almost unidirectional, movement across
habitats and the functioning of the wheat fields as an ecological
trap pose a risk, particularly to small patches that share a long bor-
der with the fields relative to their patch area. In such small
patches, the loss of individuals that do not reproduce in the patch
may be detrimental, due to the reduction in the number of individ-
uals available to replace natural mortality in the patch and due to
potentially gradual loss in the quality of the remaining individuals
in the patch.
We believe that the phenomenon of an ecological trap in agro-
ecosystems is not unique to our study area or to our study species,
but may represent an example of a broad phenomenon, probably
found in agricultural areas in many places worldwide (see Bollin-
ger et al., 1990; Shochat et al., 2005). Consequently, we think that
possible risks of ecological traps should be incorporated in the
‘Wildlife Friendly Agriculture’ approach (see Introduction) that is
currently proposed to promote conservation.
Acknowledgements
This study was funded by Nekudat Hen foundation. This re-
search was also partially supported by a grant from the Israel Sci-
ence Foundation (ISF grant 751/09) to Y.Z. We thank our assistants,
Fig. 3. Index of body condition (IC index) of individuals in the fields, natural
patches and along the fence separating patch and field. The analysis is based on
early-spring trapped adults with intact tail.
352 G. Rotem et al. / Biological Conservation 167 (2013) 349–353

especially Gal Aviad, for their help in the field and the farmers of
Kibutz Bet-Nir for their cooperation.
This study was conducted under Israel National Parks Authority
permit number 2011/38096.
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