Download Foraging efficiency of Akodon azarae under different plant cover and

J Ethol (2009) 27:447–452
DOI 10.1007/s10164-008-0140-x
ARTICLE
Foraging efficiency of Akodon azarae under different plant cover
and resource levels
Jimena Fraschina Æ Carol Knight Æ Marı́a Busch
Received: 11 January 2008 / Accepted: 26 November 2008 / Published online: 17 January 2009
Ó Japan Ethological Society and Springer 2009
Abstract The goal of this work was to determine how the
foraging behaviour of Akodon azarae changes with
predation risk and food availability in cropfield borders
of Buenos Aires, Argentina. Our hypotheses were that
A. azarae has a greater foraging efficiency in safe areas
than in risky ones and that the foraging behaviour of
A. azarae also depends on the level of resources. We
measured giving-up densities (GUDs) and food consumption twice a year in artificial foraging patches (bottles with
known amounts of millet seed) in covered and open areas
and with two different levels of seed abundance. In both
periods, GUDs were lower in the covered areas than in the
open ones independently of food level. Consumption
increased with food level in covered areas but not in open
areas. Based on these results, we conclude that A. azarae
appears to maximize its consumption depending on predation risk.
Keywords Constraints Foraging efficiency Giving-up density Predation risk Rodent
J. Fraschina (&) M. Busch
CONICET, Avenida Rivadavia 1917,
CP C1033AAJ Ciudad de Buenos Aires, Argentina
e-mail: [email protected]
J. Fraschina C. Knight M. Busch
Departamento de Ecologı́a, Genética y Evolución,
Facultad de Ciencias Exactas y Naturales,
Universidad de Buenos Aires,
Av. Intendente Cantilo s/n Ciudad Universitaria,
Pabellón 2, 48 Piso, lab. 63, CP 1428 Núñez,
Buenos Aires, Argentina
Introduction
Foraging behaviour involves various steps that include the
choice of where to seek food, which food patches to exploit
and for how long, and what to eat (Caccia et al. 2006).
Herbivores face a trade-off between maximizing food
intake and reducing the time of exposure to predators
(Brown 1988), but this trade-off is also affected by overall
resource availability (Holt and Kotler 1987) and population
densities (Schnurr et al. 2004).
Several prey species modify their use of a feeding patch
in order to balance predation risk and energy intake
(Holmes 1984; Brown 1988; Brown et al. 1988; Kotler
1992; Hughes and Ward 1993). These modifications
include changes in food intake due to variations in foraging
time, predation risk at the food patch, harvest rate and
vigilance (Brown et al. 1988; Kotler et al. 1992, 2002).
Predation risk has been found to alter microhabitat use and
foraging behaviour of small mammals, and many rodents
exhibit strong preference for highly vegetated habitats that
provide refuge from predators (Kotler et al. 1991; Jacob
and Brown 2000). In agrarian systems, the success of
control campaigns for M. musculus has been found to be
influenced by the effects of shelter on bait uptake (Jacob
et al. 2003).
Traditional optimal foraging models predict that a
forager will continue to exploit a patch until rewards
decline to its average over all patches (including both the
benefits and the costs of foraging in other patches and of
travelling between them). Brown (1988) extended the
marginal value theorem to the effect of predation risk on
patch use. His model predicts that a forager will stop
depleting a patch when the benefits of feeding rate no
longer exceed the sum of energetic, predation and missed
opportunity costs of foraging. Schoener (1971)
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448
considered two main strategies: maximization of food
intake or minimization of predation risk. The effect of
predation risk and resource availability on foraging
activity may be assessed by the giving-up densities
(GUD) when known amounts of resources are provided
(Brown 1988). Animals that minimize predation risk do
not increase consumption with greater availability
because they quit the patch when some minimal energetic
requirement is satisfied (GUD will be higher in richer
habitats because with higher availability, more resources
are left without being consumed). However, animals
maximizing intake will increase their total consumption
in richer habitats, and GUD will not differ between rich
and poor habitats.
Although feeding patches may be visited by many
individuals, the GUD value reflects the feeding decision of
a single animal, the last one to visit the patch (Brown 1988;
Kotler 1997; Morris 1997; Mohr et al. 2003). On the other
hand, GUDs can be compared between related situations
despite different densities when the same number of individuals have access to the different types of foraging
patches (Ziv et al. 1995).
Akodon azarae is a crepuscular nocturnal cricetide
rodent (adult average weight 25 g) that inhabits Pampean
agrarian ecosystems. In these systems, rodents are found
in crop fields, pastures and longitudinal habitats, such as
railway and crop field borders and river banks (Bonaventura et al. 1988; Ellis et al. 1997; Busch et al. 2001).
Edge habitats are less disturbed than agricultural fields,
maintaining high plant cover throughout the year, thereby
providing good habitat conditions for small rodent species
(Hodara and Busch 2006). A. azarae shows a strong
habitat selection for less disturbed habitats with high
vegetal cover (Bonaventura et al. 1988; Busch et al. 2001;
Hodara et al. 2001), but it increases its use of fields when
crops are mature and when crops and weeds are well
developed (Bilenca and Kravetz 1998). It is an omnivorous opportunistic species. Apart from the reproductive
period for females (Bilenca and Kravetz 1995), its habitat
selection is not associated with food levels but seems to
be aimed at minimizing predation risk (Busch et al.
2001).
Most potential predators of rodents in the area are birds,
while terrestrial predators are scarce (Bellocq 1988; Hodara and Busch 2006). In the study area, owls capture their
prey preferentially along crop field borders, but predation is
greater for other rodent species than for A. azarae, probably because of the competitive dominance of the latter
species, which has access to safer sites (Bellocq 1987,
1988).
Because plant cover influences habitat selection of
A. azarae at both macrohabitat and microhabitat scales
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J Ethol (2009) 27:447–452
(Busch et al. 2001), we hypothesize that foraging behaviour of this species will be influenced by predation risk.
These effects can be measured by the animal’s use of
artificial food patches. As such, the aim of our study was to
assess the foraging behaviour of A. azarae under different
cover conditions and different abundances of feeding
resources. The hypotheses were: (1) A. azarae has a greater
foraging efficiency in safe sites than in risky ones and,
consequently, GUDs will be lower in covered areas than
in open ones; (2) A. azarae is a forager that minimizes
predation risk subject to an energetic constraint. As a
consequence, the GUD will increase when food is
augmented.
Study area
We studied the foraging efficiency of A. azarae in crop
field borders of the Exaltación de la Cruz Department
(34°190 S and 59°140 W), Buenos Aires province, Argentina.
The study was conducted in the autumn (nonreproductive
period with a high population density) and spring (reproductive period, with a low population density) of 2004.
The study area is located in the Pampean region
(Cabrera 1953). The climate is sub-humid temperate with
a mean annual temperature of 16°C and an annual precipitation of about 1000 mm. The original vegetation has
been replaced by crop fields and pastures, while a spontaneous and particular flora has developed along the road
and crop field borders (Soriano et al. 1991). The borders
are dominated by plant species such as Stipa neesiana,
S. papposa, Paspalum dilatatum, Bromus unioloides, the
forbs Solidago chilensis and Senecio grisebachii and the
thistles Carduus acanthoides, Cirsium vulgare, Cynara
cardunculus (Bonaventura and Cagnoni 1995; Bilenca and
Kravetz 1998). The most frequent crops include wheat,
maize, soybean and sunflower. Seasonal changes in plant
phenology and in the stage of development of crops cause
seasonal qualitative and quantitative variations in resources—both in cropfields and borders (Busch et al. 1997;
Hodara and Busch 2006). Resources for rodents are both
less abundant and less available in the winter than in the
other seasons, with variations largely due to changes in
plant cover, plant species composition and availability
of invertebrate prey (Busch et al. 2001). Borders have
abundant plant cover throughout the year, while the plant
cover of cropfields varies with the stage of the crops,
from low cover after ploughing and sowing to high cover
when crops are mature. These fluctuations in habitat
structure, resources and A. azarae populations are reflected in changes in individual resource requirements and
predatory risks.
J Ethol (2009) 27:447–452
449
In order to select sites for assessing foraging efficiency, we
assessed the presence of A. azarae in six crop field borders
(located at distances ranging between 200 and 500 m from
each other) by placing 15 trapping stations spaced at 10-m
intervals in each border. There were two Sherman traps at
each trapping station, which were baited with a peanut
butter, bovine fat and oat mixture. Traps were checked
every morning for three consecutive days. The individuals
captured were identified to the species level. Because
GUDs are species-specific (Ziv et al. 1995), we removed
all rodents which were not A. azarae. We recorded sex,
breeding condition, trap location and date of capture for all
individuals.
system, where seeds are mixed in the litter. This design was
selected after preliminary experiments with 5, 7 and 10 g
of seeds and using litter as the substrate. The bottles protected the seeds from precipitation and from other animals
(see Morris 1997 for a similar approach to rodents in
Canada), but they did not provide a refuge for rodents
because they are transparent, and although most predators
cannot fit through the opening, they would be able to break
or move the bottles and thereby gain access to the rodents
inside.
Bottles with seeds were provided 4 days before the
GUD experiment to allow rodents to locate them. The
GUDs were then measured for three consecutive nights.
Each morning we collected the contents of each bottle,
which were later sieved in the laboratory to recover and
weigh the remaining seeds. Bottles were refilled each day
with the same original amount of seeds and substrate.
Measurement of GUDs
Data analysis
The effects of cover and resource availability on the foraging behaviour of A. azarae were assessed by measuring
GUDs at two cover conditions and two levels of resource
availability. The GUD study was conducted 1 week after
the completion of a trapping round in the same borders
where the rodent trapping had been carried out. To ensure
that consumption was due to the target species, we placed
foraging stations at sites located\5 m away from A. azarae
sites of capture (according to the range of movements of
A. azarae; Cittadino et al. 1998). We used 44 foraging
stations in the autumn and 36 in the spring. At each foraging station, one bottle was placed in a covered patch
while another was placed 2 m away in the centre of an area
of about 1 m2 where the plant cover had been artificially
removed (open area). In half of the foraging stations we
provided 5 g of millet seed (6 ml), and in the other half, we
provided 10 g (12 ml). Foraging bottles were 500 ml
plastic bottles with a single 3 to 4 cm opening that was
covered by an adhesive tape to prevent access by ants and
other insects. Foraging was assessed by the consumption
of unhusked millet seeds because previous experiments
of food addition confirmed that although A. azarae is
omnivorous, it also exploits rich seed patches when
available (Cittadino et al. 1994). We mixed the millet seeds
with an artificial substrate in order to cause a diminishing
return; we did not use the natural substrate of the study area
because this would have made it difficult to recover the
remaining seeds and, therefore, to assess consumption. The
artificial substrate consisted of small pieces of rubber
(diameter 0.5 cm, thickness 0.01 cm) that resembled pieces
of plant remains that are frequent in the natural habitat. The
volume of artificial substrate used (35 ml) was determined
according to the natural conditions of seed density in our
For the statistical analysis of foraging efficiency, we only
considered data of foraging stations where consumption by
rodents was confirmed by the presence of droppings and/or
hairs in at least one bottle of a pair (covered or open) to
avoid over estimation of GUDs. We considered visited
each foraging station with signs at least one time of the
3 days period, and for the estimation of GUDs and consumption we considered the average over the 3 days. We
studied foraging stations located at different sites within
the crop field borders at each month.
We conducted a mixed design ANOVA (Winner 1962)
to analyze the effect on GUDs of one within subject factor
(cover, two levels) and two between subject factors (the
amount of seeds and the time period, two levels each). In
all cases we used the average value of GUDs over the
3 days. When we found a significant interaction between
factors, we assessed the effect of each factor at the two
levels of the other factors (Zar 1996). We also conducted
an ANOVA to compare seed consumption according to
cover and resource availability because changes in food
consumption between richer and poor patches may have
been produced with or without changes in GUDs.
Materials and methods
Rodent sampling
Results
Akodon azarae represented 89% of the captures (81 individuals) in the autumn and 88% (27 individuals) in the
spring. Prior to carrying out the experiments, we removed
eight Oligoryzomys flavescens, one Calomys musculinus
and one Cavia aperea in the autumn and two Oligoryzomys
flavescens and two Calomys musculinus in the spring. The
low abundance of other species in the area and the short
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J Ethol (2009) 27:447–452
time between rodent sampling and GUD measurements as
well as the competitive dominance of A. azarae (Busch and
Kravetz 1992; Cittadino et al. 1994) made us confident that
only the target species consumed seeds. In some cases,
insects were found stuck to the adhesive band that surrounded the opening of the seed bottles. We did not
observe any birds eating at the bottles, nor did we find
feathers or droppings.
We found signs of rodents feeding at the bottles of 40 of
the 44 (90%) foraging stations in the autumn, 21 at stations
with 5 g and 19 at stations with 10 g. In the spring, we
found signs of rodents feeding at 26 of the 36 (72%) foraging stations, 15 at sites with 5 g and 11 at sites with 10 g.
In order to obtain a balanced design in the analysis, we
used 19 sites in the autumn and 11 sites in spring at each
offer level (randomly deleting sites with 5 g).
Giving-up densities and food consumption
The GUDs were lower (1) in the autumn than in the spring,
(2) in covered areas than in open areas and (3) in areas with
5 g seeds compared to those with 10 g seeds (Fig. 1a, b).
There was a significant effect of month, resource availability and cover on GUDs, and a significant interaction
between month and cover and between resource availability and cover (Table 1). According to the analysis of
direct effects, cover had a significant effect in both months
a
GUDs Autumn
10
Grams
8
6
4
Table 1 Analysis of variance results for the effect of cover (within
subject factor), resource availability and time period on Giving-up
densities
Effect
df
Month
1
Offer
1
1
56
0.07
5.52
0.01
Month 9 offer
Error
MS
F
P
88.53
16.03
0.0002
385.77
69.86
0.0000
0.91
Cover
1
76.54
31.22
0.0000
Cover 9 month
1
13.66
5.57
0.022
Cover 9 offer
1
10.23
4.17
0.046
Cover 9 month 9 offer
1
0.82
0.34
0.564
56
2.45
Error
(F1,56 = 43.07; P = 0.0001 for autumn; F1,56 = 4.11;
P = 0.0474 for spring). The month effect was also significant at the two levels of cover (F1,56 = 35.02;
P = 0.0001 for covered areas; F1,56 = 6.65; P = 0.0125
for open areas). Resource availability had a significant
effect on GUDs at the two levels of cover (F1,56 = 57.10;
P = 0.0001 for covered and F1,56 = 116.21; P = 0.00001
for open areas).
We observed more consistent results for seed consumption than for GUDs, with the exception that resource
availability had a significant effect on consumption only in
covered areas (F1,56 = 23.14; P = 0.00001) and not in
uncovered ones (F1,56 = 2.52; P = 0.1182). The proportion of seeds consumed was greater in sites with a lower
initial abundance of seeds (5 g) with respect to sites with
10 g seeds, at both seasons and in open and covered sites.
The differences in proportions of seed consumed according
to initial abundance were greater in autumn than in spring,
but were similar in open and covered sites (Fig. 2).
2
Discussion
0
5 g Cov
b
5 g Open
10 g Cov
10 g open
GUDs Spring
10
Grams
8
6
4
2
0
5 g Cov
5 g Open
10 g Cov
10g open
Fig. 1 Mean giving-up densities [GUDs; mean weight of seeds
remaining (g) after a foraging night by Akodon azarae] ± standard
errors according to resource availability and cover. a In autumn, b in
spring. Cov covered sites, Open open sites
123
Although both covered and open patches were used to
some degree, our results confirmed that A. azarae avoids
open areas and has a greater foraging efficiency in covered
areas than in open ones, suggesting that this species’
behaviour is influenced by predation risk. The effects of
augmented food on GUDs in covered and open areas
suggest that there was no fixed response to quit a patch
after a specific level of consumption had been obtained,
suggesting that this species can change its foraging strategy
according to predation risks. In another South American
rodent, Octodon degus, Vasquez et al. (2006) found that
foraging efficiency can change according to the information about patch quality acquired during foraging. In
A. azarae, however, we did not find a change in consumption
in the richer habitats during the 3-day experimental period.
J Ethol (2009) 27:447–452
1.0
451
Proportion of seeds consumed
0.8
0.6
0.4
0.2
0.0
5
Cov Autumn
10
Open Autumn
Initial abundance
of seeds (grams)
Cov Spring
Open Spring
Fig. 2 Proportion of seeds consumed by A. azarae according to the
initial abundance of seeds, in cover and open sites, and in autumn and
spring. Cov covered sites, Open open sites
The absence of an increase in consumption in richer
habitats may be a good evidence of the existence of a
landscape-scale effect (Morris 1997). Our design, with
richer patches occurring as pairs together at a station, may
have created a richer environment, confounding the effects
of the two scales: patch and environment (Morgan et al.
1997).
Differences in GUDs between months may reflect
changes in energetic requirements, missed opportunity
costs (according to food availability), foraging costs and/or
predation risks that affect the balance between the benefits
and costs of foraging in a given patch (Brown 1988). The
higher food consumption in the autumn than in the spring is
possibly attributable to an increase in energetic requirements due to the lower temperatures registered in the
autumn. These may have caused an increase in thermoregulatory costs and a decrease in missed opportunity costs
due to lower per capita resource availability with increasing population density. Based on to the differences in
reproductive conditions (reproductive season in spring;
inactive in autumn), we expected a higher consumption in
spring. As this was not the case, reproductive costs do not
appear to be important in determining the foraging strategy
of A. azarae. Another alternative explanation is that the
measure of GUDs has been affected by night brightness
(Kotler et al. 1993; Kotler 1997). During the days of
sampling, night brightness was lower in the autumn
(between 68.6 and 93.8%) than in the spring (between 92.9
and 99.7%), and rodents may have experienced a lower risk
of predation in the first month due to the sky conditions.
An alternative explanation for the observed variation in
GUDs between months is that population density acts
directly and not through a decrease in missed opportunity
costs. Brown (1988), Kotler (1997) and Morris (1997)
consider that GUDs are not affected by population density
if the same individuals have access to the different types of
patches. This condition is met when we compare GUDs
between covered and uncovered sites within each month,
but it is not met when we compare GUDs between months
because rodent density was found to be higher in the
autumn than in the spring. Lower values of GUDs and a
higher proportion of foraging stations with consumption in
the autumn are consistent with a higher population density
during the autumn when compared with spring.
Differences between months may have also been due to
a day effect because the short 3-day period studied in each
month may have confounded moon phases and seasonal
effects.
The effect of predation risk on foraging behaviour of
A. azarae may be of increasing importance due to changes
in land management in the study area, where there is a
progressive loss of nondisturbed areas and vegetal cover is
reduced by the use of herbicides, which affect not only
crop weeds, but also the vegetation of the borders. These
changes in land management can affect A. azarae populations by reducing available food, increasing the risk of
predation and reducing the foraging efficiency.
Acknowledgments We thank Gerardo Cueto and Beatriz Gonzalez
for their statistical advice and Joel S. Brown for his useful comments
on early versions of this manuscript. We also thank D. Gaynor people
for their assistance during field work. We greatly acknowledge
commentaries on a previous version of the manuscript by R. Cavia,
M.S. Fernández, I.E. Gómez Villafañe, D. Bilenca and V. León. This
work was funded by University of Buenos Aires and CONICET
grants.
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