Download Non-consumptive Predator Effects

Non-consumptive
Predator Effects
Risk Effects and the Ecology of
Fear
ESRM 450
Wildlife Ecology and
Conservation
The Role of Predators in Ecosystems
•  Predators eat their prey
–  In so doing, can influence prey population dynamics, prey
distribution, and community organization
–  But is that all predators do?
Painted dogs
(Lycaon pictus)
Predation is Costly
“During any given day, an animal may fail to obtain a
meal and go hungry, or it may fail to obtain matings
and thus realize no reproductive success, but in the
long term, the day’s shortcomings may have minimal
influence on lifetime fitness. Few failures, however, are
as unforgiving as the failure to avoid a predator: being
killed greatly decreases future fitness”
- Lima and Dill (1990) Can J Zool*
*Modern classic: cited 2946 times (Web of Science)!
So “Risk Management” Is
Expected
•  From an evolutionary perspective, we expect most
animals to make adjustments that mitigate possibility of
predation
–  Predation is unforgiving, no room for mistakes
•  Risk management
–  No eating involved
–  Can be physiological,
morphological, or…
•  Behavioral
–  Antipredator behavior
–  Today’s focus
Didn’t make the proper adjustment
Defining Predation risk
•  When we discuss risk management, we need to define
what is being managed
•  Predation risk: an animal’s likelihood of predator-induced
mortality
–  Two components: chances of encounter * chances of escape
–  Meaning: risk isn’t always highest where/when predators are most
numerous
–  Risk is highest where product of components is greatest
Lima and Dill (1990) Can J Zool
The Ecology of Fear
•  Implications of risk management often called the
“Ecology of Fear”
–  Because, for many species, predation risk may inspire fear…
–  …but this notion is hard to demonstrate and anthropomorphic
–  However, idea of ‘fear’ works well as a metaphor for the way
predators influence prey non-consumptively
•  In truth, species need not be able to experience fear to
respond to predation risk
•  Instead, only have to have evolved tendency to manifest
adaptive trait changes when exposed to cues associated
with heightened likelihood of mortality
–  i.e., risk effects are products of natural selection; lack of
response to risk is maladaptive
Kinds of Antipredator Behavior
•  Antipredator behavior can take many forms, all of which
share something in common
•  Namely, they all carry a price
-  i.e., investment in defense usually requires that something of
value is given up, such as energy reserves, resources or
reproductive opportunities
-  Why? Anti-predator investment is energetically expensive and/or
incompatible with efficient foraging, searching for mates
-  Otherwise, prey would be maximally defended all the time
-  Let’s consider some common anti-predator adjustments
Lima and Dill (1990) Can J Zool
Kinds of Antipredator Behavior
•  Major means of risk management: alteration of space
use
–  i.e., habitat shifts
Predator
*
Hugie and Dill (1994) J Fish Biol
Spatial Shifts
•  Across landscape, patches
–  offer different foraging payoffs (benefits) and
–  feature different levels of predation risk (cost)
•  So, benefits gained from foraging in any patch must be
discounted by the probability of being eaten while
acquiring resources
–  i.e., predation risk is a foraging cost1
•  Spatially - food and predation risk often positively
correlated
–  Why? Predators “game” against prey; best policy often to choose
areas where resources for prey are abundant2
–  Meaning: plentiful (resource-rich) patches are often of lower quality
(in terms of fitness) than they seem
1Brown and Kotler (2004) Ecol Lett
2Hugie
and Dill (1994) J Fish Biol
Spatial Shifts: Negotiating the
Food-Safety Tradeoff
•  When resource-rich patches are also most dangerous
–  Safety-conscious prey use resource-poor patches to degree
greater than expected based on expected energy gain (or
another fitness currency)
–  i.e., prey individuals will trade food for safety
Predator
Safe (but fewer resources)
Dangerous
Exploiting The Fear Metaphor
Per capita net rate of
energy gain
Back to IFD Figure – No Risk
Good patch
Poor patch
Density at which consumers start
using poor patch
Number of competitors
Per capita net rate of
energy gain
Back to IFD Figure – With Risk
Food that is given up to be safe
Good patch (same resources, but
now more dangerous)
Poor patch
(safer)
*
New ‘switching’
density
*
Old ‘switching’
density
Number of competitors
Kinds of Antipredator Behavior
•  Another means of mitigating danger: enhanced vigilance
–  i.e., spending more time watching out for predators
–  Costly because, in many cases, individuals cannot forage,
or forage less efficiently, when surveying for predators
–  Result: Where predators are a threat, prey foraging is
reduced
e.g., Guppies (Poecilia reticulata) are less
vulnerable to predators when in ‘head-up’ position,
but can’t forage
Krause and Godin (1996) Anim Behav
Costly Antipredator Behavior An Example
•  Elk (Cervus elaphus) under risk of wolf (Canis lupus)
predation in YNP
–  One defensive tactic: increased vigilance
Winnie and Creel (2007) Animal Behaviour
Vigilance
•  Female elk increase
vigilance, at the expense
of grazing, when wolves
are nearby
•  Vigilance and grazing
times of male elk not
affected by wolf proximity
•  Why this sex difference?
Grazing
Winnie and Creel (2007) Animal Behaviour
Are Some Species Immune to
Risk?
•  A qualified yes
–  “True” apex predators
–  Perhaps some “megafaunal herbivores” (e.g., elephants, rhinos)
•  But most are not
–  Even many that experience negligible predator-induced
mortality!
Are Some Species Immune to
Risk?
•  African buffalo (Syncerus caffer) in Kruger national Park,
S. Africa
–  Eaten by lions but, because of high survival and effective defenses, long
considered immune to risk
•  Yet, in some cases, these mega-herbivores appear to
avoid prime foraging areas in exchange for safety from
lions
Are Some Species Immune to
Risk?
Plant quality
NVDI = Normalized Difference Vegetation Index
Lion risk
uniform
Lion risk correlates
with NDVI
•  African buffalo accept
low-quality food where
high-quality foraging
patches feature high
lion predation risk
(basalt)
Winnie et al. (2008) Ecology 89: 1457-1468
Humans and Predation Risk
•  Gordon Orians (emeritus, UW Biology)
–  Compared levels of refuge symbolism (e.g., cabin, cave, open area) in
paintings of sunsets and sunrises
–  Prediction: more refuges in sunset paintings; why?
Humans and Predation Risk
•  Gordon Orians (emeritus, UW Biology)
–  Compared levels of refuge symbolism (e.g., cabin, cave, open area) in
paintings of sunsets and sunrises
–  Prediction: more refuges in sunset paintings; why?
•  35 sunsets, 11 sunrises
•  66% of sunset paintings offered built refuge, 9% for
sunrises
Is Cognitive Complexity a
Prerequisite for Risk-Sensitivity?
•  No
–  Responses to predation risk manifested by many invertebrates
•  e.g., zooplankton exhibit daily movement patterns known
as diel vertical migration (DVM)
–  Deep strata used during day, shallow at night
•  Best explanation: response to predation risk
–  Use of deep strata facilitates avoidance of visually orienting fish
predators that hunt near the surface
Hays (2003) Hydrobiologia
Risk Effects
“Risk effects are the non-consumptive effects of
predators on prey, namely the lost foraging
opportunities and lower levels of growth and
reproduction experienced by prey investing in
antipredator behavior ”
Risk Effects Impact Prey
Populations
•  Even in the absence of heavy mortality, risk effects can
depress prey population size
•  How?
–  Carrying capacity reduced because prey animals do not exploit all
available resources (acquisition of some resources too dangerous)
Without Predators
Fully Exploited
With Predators
Exploited
Eschewed
Risk Effects Impact Prey
Populations
•  Example: Elk in Yellowstone
–  Since reintroduction of wolves (1995-1996), northern Yellowstone elk
population has declined by 43% (14,500 to 8,300); ~ double the predicted
decline based on wolf-induced mortality
–  Wolf-free populations elsewhere in MT have increased
–  Why? Reduced calf recruitment stemming from foraging adjustments
–  i.e., females are giving up food in prime but dangerous foraging locations
and suffering the nutritional consequences
Creel et al. (2005) Ecology
Creel and Christianson (2008)
TREE
Why Study Risk Effects?
•  To assess full impact of predators on prey
–  Consumption plus Intimidation
–  If we focus only on consumption, we risk underestimating or
completely missing predator effects
•  To gain insight regarding possible non-consumptive
impacts of humans on wildlife
–  Seemingly benign activities may have negative consequences if
they induce costly antipredator behavior
•  So that we can predict impacts of predator removal,
restoration
–  Prey responses presage population, ecosystem changes