Lecture 10: Predation EEES 3050

The Nature of Predation Types of predators  predators: eat other living organisms  taxonmic and functional classifications true predators Herbivores or grazers parasites Parasitoids Cannibals

Classifications true predators:  kill living prey,  Need more prey/lifetime (lions, tigers and bears) Herbivores or grazers:  small amounts of living prey  Need less prey/lifetime (ungulants, leeches, mosquitos)

Classifications parasites:  small amounts of living prey  1 or few prey/lifetime (aphids, flukes, worms) parasitoids:  kill living prey  1 per lifetime  developmental stage (wasps, nematodes) Cannibalism  Eating others of same species  Sexual cannibalism (praying mantis)  Size-Structured (found in ~90% of aquatic species)

Predation as a process Effect distribution Influence the structure of a community  Top down effects Predation as selective force  Predator-prey coevolution  Red-queen hypothesis.

Models of predation Discrete generations:  Need to understand, but won’t explain in class.  Based on previous models with addition of predator term. Continuous  Lotka-Voltera – again modified previous equations Found to be too simplistic  Rosenzweig – MacArthur model.

What happens to prey?

What happens to predators?

You are responsible for – from book. Lab studies of predation  Are there ever systems where a prey and predator establish an equilibrium in the lab?  What about oscillations. Field studies of predation  Do predators actually impact prey populations?  What are the four responses of predators to an increase in prey populations as predicted by Holling?

You are responsible for – from book. Optimal foraging theory.  What are the primary factors in determining what prey items a predator eats, i.e. how do you determine the profitability of a prey type?  What is the difference between a specialist and generalist predator? And how does this relate to optimal foraging models

Predation Example: Dingoes and Kangaroos

Examine Article Does dingo predation control the densities of kangaroos and emus?  By G. Caughley, G. C. Grigg, J. Caughley and G. J. E. Hill

Structure of research… Observations Hypotheses  Developed from existing theory Experiment Results Discussion

What are the 3 observations? Read the 1 st four paragraphs:  Stop at the first figure:

3 paradoxical observations Red Kangaroo density in 1975 in New South Wales is vastly greater than it was in In South Australia the number of kangaroos appears to have remained constant. Densities of kangaroos much lower in South Australia and Queensland compared to New South Wales  Local opinion was that kangaroos just avoided the fence.  Densities of dingoes is opposite of kangaroos.

What are some of your hypotheses or questions? Develop questions/hypotheses:

What were the questions developed by authors?  Read rest of introduction.

Read Methods:  Don’t worry too much about Analysis section. How did they test their questions?

Is this a “true” experiment? They did not manipulate anything.

Results 1 st New South Wales – South Australia

Results: NSW vs. SA

What is variance? In the analysis… the States account for 77% of the variance in the data.  What does this mean? Results: NSW vs. SA

Results Read Queensland-South Australia

Results: QLD vs. SA No significant difference.

Results Read New South Wales – Queensland (far west)

Results: NSW vs QLD (far west)

Results Read New South Wales – Queensland (central west)

Results: NSW vs QLD (central west)

General Results

Revisit questions:

Discussion Read Discussion to middle of page 10

Discussion points: The finding that the density differences between States hold 40 km from the border rules out explanation of the contrast by any local fence effect.  The difference has something to do with environmental conditions specific to the States What about shooting of kangaroos or water sources?

leaves only two hypotheses that we consider credible:  that the density contrasts reflect differing vegetations  differing rates of predation by dingoes. Read rest of pg 10 and 1 st paragraph of 11. Looked at changes in vegetation with satellite imagery.  What did they find? Discussion points:

What is going on in the mid west? Read rest of discussion. Direct interpretation:  The primitive densities of red kangaroos is kept low by dingo predation.

This study has documented a phenomenon but has not revealed a cause. We have presented and discarded several hypotheses to account for the reported contrasts in density, and are left with only one-the distribution pattern reflects different levels of predation by dingoes.

Ties to theory! A predator usually cannot force the prey to very low density because its own numbers will decrease reciprocally as its food supply declines.  Remember Rosenzwieg-MacArthur models. That tight dynamic is uncoupled when the prey comprises more than one species.

How can multiple prey result in low density of one prey?  if the threatened prey is taken at a higher rate than alternative prey its numbers will be lowered disproportionately.  if its intrinsic rate of increase is significantly lower than that of alternative prey We suspect that the second mechanism is in play when dingoes prey upon kangaroos in the presence of rabbits Ties to theory!

Final thoughts on article: Sometimes ecology feels like quantifying the obvious.  Doesn’t mean it is easy. On the other hand, sometimes the “obvious” isn’t quite so clear.  At the time current expectations were that predators could not impose such a large effect. Still could be other mechanisms – no one has thought of them yet.

Evolution of predator-prey systems Predation can be a strong selective process. How have predators adapted to be more efficient? How have prey adapted to stay alive?

Predator adaptations: Examples Prey detection  Owls: not only eye sight.  Smell  Sound Capturing prey  Sit and wait predators  Behavior Grasshopper mice and darkling beetles  Cooperative groups.

Prey adaptations: Examples Avoiding detections:  White-tailed ptarmigan Avoiding capture  “eye-spots” on moths  Safety in numbers Warning coloration

Safety in numbers How often do sparrows look up when feeding? Number of sparrows Number of head-cocks/Ind