Predator facilitation or interference: a game of vipers and owls
Keren Embar, Ashael Raveh, Ishai Hoffmann and Burt P. Kotler
Ben Gurion University of the Negev, Blaustein Institutes for Desert Research, Mitrani
Department of Desert Ecology, Sde Boker Campus, Midreseht Ben-Gurion 84990
Israel.
Corresponding author: Keren Embar
Ben Gurion University, Institute for Desert Research, Mitrani Department of Desert
Ecology, Midreseht Ben-Gurion 84990 Israel. Phone: 08-659-6785 Fax: 08-659-6772
E-mail: embar@bgu.ac.il
Online Resource 1 – Prediction Tree
In a community of predators, one predator may sometimes hinder or facilitate
another’s hunting success. Predator facilitation and interference can occur among the
predators of desert rodents. Consider vipers and owls hunting gerbils in a mosaic of
bush and open microhabitats. Figure 1 describes the possible interactions between
owls and vipers as mediated by gerbil behaviour. In this figure, let Δ represent the
difference in foraging time that gerbils allocate between bush and open patches,
calculated as (foraging time allocated to the bush microhabitat – foraging time
allocated to the open microhabitat). The bigger the difference, the more the gerbils
prefer the bush.
Without predators, gerbils prefer foraging in bushes where seeds tend to
accumulate and where they are less exposed to any undetected predator. In response
to predators, gerbils lower activity. When owls loom nearby, gerbils treat the open as
more dangerous and show even sharper bias towards the bush (Kotler et al. 1991). In
contrast, when vipers are present, gerbils bias their activity to the open microhabitat
(Kotler et al. 1993). Thus, owls make the open more dangerous (e.g., Kotler et al.
1988, 1991), and snakes make the bush so (Brown 1989, Kotler et al. 1992, 1993,
Bouskila 1995). When both owls and vipers are present, gerbils lower the risk of owl
predation by preferring the bush, but are nonetheless forced more into the open
because of the vipers. Thus through their effect on gerbil microhabitat use, the vipers
facilitate the owls (Kotler et al. 1992) (fig. 1, upper arc).
Patch use
-
Δ
+
Vipers
Predator facilitation
Owls
-
Activity
-
Competition
Fig. 1. Possible interactions between owls and vipers. Predator facilitation or
competition are mediated by the risk management behavior of the gerbils. Under risk
of predation, gerbils may choose to reduced their overall foraging activity or change
their patch use to forage more in safer habitats, with the later here represented as Δ,
the difference in foraging time allocated between bush and open patches. Another
possible interaction, not shown here, may be a direct confrontation between the two
predators (unlikely, in our model).
Vipers have their own foraging decisions to make, and they may choose not to
remain in the bushes (Kotler et al. 1993). If the vipers remain in the bushes, they make
bushes more dangerous and so drive gerbils to reduce their patch use in the bushes
and increase their patch use in the open. This facilitates owl predation, so owls should
allocate more time to foraging where the vipers are present. If vipers instead forage in
the open, competing with the owls in the same microhabitat, they will kill some
gerbils in the open and drive gerbils to increase bush patch use- out of reach of both
owls and snakes. This would cause the owls to experience competition through direct
resource depression through gerbil mortality from viper hunting success and indirect
resource depression by reducing gerbil activity in the open microhabitat (fig. 1, lower
arc). In such a case the owls, should allocate time away from where the vipers forage.
If the vipers forage in both the open and the bush patches equally, they will suffer
competition from the owls in the open microhabitat, and the gerbils will reduce their
overall activity and patch use in both patches. Again the owls will experience
resource depression, though not as strongly as when both predators compete solely in
the same microhabitat. In these ways, the presence of vipers may affect the hunting
behavior of the owls. Furthermore, both owls and snakes cause gerbils to reduce
foraging time (Kotler et al. 1991, 1992) and increase vigilance (e.g., Embar et al.
2011), leading to behavioral resource depression and fewer capture opportunities. If
the effect of the predators on resource depression is greater than their effect on
microhabitat use, then the predators will overall be competitors.
We summarize these predictions in Table 1.
Risk of predation
Gerbil Patch use
Gerbil activity
Outcome
No predator
Bush > Open
High
Background
foraging
Owl
Bush > Open
Medium
Predator
avoidance
Viper
Bush < Open
Medium
Predator
avoidance
Owl + Viper:
If Owl > viper
Bush > Open
Medium
Predator
facilitation
Owl + Viper:
If Viper > owl
Bush = Open
Low
Predator indirect
Interference
Predator direct
Interference:
Owl + Viper:
If they kill each
other
If Owl kills Viper: Medium,
Bush > Open
irrelevant
If Viper kills Owl: Medium,
Bush < Open
irrelevant
Predator foraging
behavior depends
on their own risk
of predation
Table 1. Outcomes of the possible interactions between owls and vipers, and their
common gerbil prey. Based on previous finding we assume that (1) gerbil consider
foraging in bush habitats as safer than open habitats, (2) vipers forage predominantly
in bush habitats and (3) owls forage predominantly in open habitats.
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