Non-consumptive Predator Effects Risk Effects and the Ecology of Fear. ESRM 450 Wildlife Ecology and Conservation

Transcription

1 Non-consumptive Predator Effects Risk Effects and the Ecology of Fear ESRM 450 Wildlife Ecology and Conservation

2 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)

3 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)!

4 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

5 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

6 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

7 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

8 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

9 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 cost 1 Spatially - food and predation risk often positively correlated Why? Predators game against prey; best policy often to choose areas where resources for prey are abundant 2 Meaning: plentiful (resource-rich) patches are often of lower quality (in terms of fitness) than they seem 1 Brown and Kotler (2004) Ecol Lett 2 Hugie and Dill (1994) J Fish Biol

10 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

11 Exploiting The Fear Metaphor

12 Back to IFD Figure No Risk Per capita net rate of energy gain Poor patch Good patch Density at which consumers start using poor patch Number of competitors

13 Back to IFD Figure With Risk Per capita net rate of energy gain Poor patch (safer) New switching * density Food that is given up to be safe Old switching * density Good patch (same resources, but now more dangerous) Number of competitors

14 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

15 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

16 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

17 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!

18 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

19 Are Some Species Immune to Risk? NVDI = Normalized Difference Vegetation Index Plant quality 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:

20 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?

21 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

22 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

23 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

24 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 With Predators Fully Exploited Exploited Eschewed

25 Risk Effects Impact Prey Populations Example: Elk in Yellowstone Since reintroduction of wolves ( ), 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

26 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