Finally, my favorite topic of the semester . . . parasitism!

1. • Parasites, like predators, cause harm to their hosts for their own benefits (to varying degrees); but unlike predators tend to be closely associated with only one host, and many parasites may infection a single host.
   • Infection is the state of a host being colonized by a parasite, disease is the expression of symptoms or loss of fitness by the host
   • We dichotomize parasites as micro- or macro-parasites . . .  though this dichotomy is imperfect.
   • Parasites may be entirely dependent on a single host (direct lifestyle) or dependent on free-living stages, multiple hosts, or vectors
   • Parasite’s need to use host as both habitat and resource creates evolutionary tension

As we’ve seen — predation can be either disruptive (e.g. Huffaker’s experiment without dispersal) or stabilizing (e.g. limit cycles, or damped oscillations) from the perspective of the predator and its prey species.  Today we’ll discuss what some of the indirect effects of predation may be on non-target species.

1. • Predation can produce similar community effects as disturbance — generating gaps that can allow persistence
   • Effect of predation on diversity may depend on resource availability
   • Specialist predators can permit persistence if the effect is biased towards dominant competitor
   • Apparent competition can occur in species that don’t specifically interact, but share a predator
   • Keystone Predators are those whose presence or absence can cause a large shift in community composition
   • The complexity of community interactions can generate pattens that are difficult to interpret as due to bottom-up or top-down control

Case Study: Deer on the Kaibab Plateau

Burk1973

Caughley_Ecology_1970

Here is a great video, and story, about Bob Paine and the origin of the Keystone concept in ecology:

http://www.hakaimagazine.com/video/how-ecosystems-got-keystone

And a story by Ed Yong about Bob Paine’s legacy in the field:

http://www.nature.com/news/scientific-families-dynasty-1.12205

For homework you’ll be asked to read three papers (linked below) and answer questions about them.  The instructions are in the attached document — please submit the assignment to the Homework 3 dropbox on the CANVAS site by 5PM on 1 November.

Assignment 3

Hairston et al 1960

Murdoch 1966

Holdo et al 2009

Predator-prey dynamics drive cycles, which keep ecological systems continually in flux through time.  These dynamics can result in complex patterns in space and time — and affect the selective pressures on both predators and prey:

1. • The canonical Lotka-Volterra model generates stable limit cycles, but there are lots of other cyclic behaviors that can be generated when you add real-life ecology to that simple math
   • Disturbance can maintain cycles even in presence of intraspecific competition
   • Cycles necessarily go through numerical bottlenecks — low numbers are susceptible to demographic stochasticity and local extinction
   • Metapopulation dynamics can rescue unstable cycles
   • Predation can drive strong selection in both predator and prey species — the positive feedback loop can drive co-evolution (i.e. the Red Queen hypothesis)
   • Evolution can be directional — e.g. cheetahs and gazelles

Today we’ll look at the population dynamics of coupled predator-prey systems:

1. • Predator-prey interactions generate stable, linked cycles of both predator and prey
   • The L-V model of predator prey dynamics assumes that prey are only limited by predation and that predators are dependent on prey as food source
   • In these cycles the predator population always lags the prey population by 1/4 phase
   • Intraspecific competition among prey generates damped oscillations that lead to stable population sizes of both predator and prey

There is a new paper just out by Dr. Rosie Woodruffe about about African wild dogs that illustrates several of the phenomena we have discussed in class. Rather than summarize them myself, I’ll let Dr. Woodruffe do so in tweet form:

Today I published a paper – not headline-grabbing or especially policy-relevant – but I'm proud of it because it solves a problem I've wanted to understand for years: how African wild dog populations work pic.twitter.com/GHJbZg3yve

— Rosie Woodroffe (@RosieWoodroffe) October 21, 2019

This thread outlines the story very well, and links to the new paper at the end. It also has great pictures of African wild dogs, which are terrifically cute.  Consider this a required course reading that is fair game for exam questions. Also, consider checking out Dr. Woodruffe’s research, she’s an absolute rock star in the fields of wildlife parasitology and wildlife population dynamics.

Last time we discussed how prey respond to consumption, and how consumers respond to prey abundance.  Today, we address how consumers “choose” what and how much to eat.  If nothing else, you’ll never look at a buffet table the same way again after this lecture:

• Consumers will rarely consume prey in direct proportion to abundance for all prey density
• Consumption often saturates at some prey density because of inherent limits of the handling time for consumption
• Consumption may be limited at low prey densities because of refugia or prey switching
  foragers should rank prey items based on the ratio of energy gain to handling time
• a current food item should be added based on the cost due to search time
• Diet breadth should be determined by foraging strategy (searchers vs. handlers) or resource availability (high quality vs. low quality environments)
• The duration that foragers should stay in resource patches depends on both the resource quality and the traveling time between patches

Here are some short movies to illustrate two key concepts in optimal foraging:

The first shows the calculations for the rule about whether foragers should add items to their diet. This addresses the question “if there’s a food item in front of me, should I eat it?” (), which is central to the question of why some consumers are specialists and some are generalists:

Optimal diet breadth

The second shows the rules for how long a forager should stay in a patch before moving on to the next patch. This addresses the question as to whether foragers should “clean their plate” or move on to the next patch when resources become rare.  Note, this is the same calculus that is used in resource extraction fields to work out the optimal level of extraction: e.g. should we get every last drop of oil out of the ground, even though it gets harder and harder, or just move on to the next well.

Optimal patch leaving time

So far, “resources” have been static, passive things in our discussions.  Starting today, we let the meek rise up and consider what happens when the consumed have their own population behavioral and population dynamics that respond to consumption.

1. • Consumers can be classified as either true predators, grazers, parasites, or parasitoids
   • Consumers alter prey species numbers by increasing mortality
   • Consumers drive prey species life history
   • Predator consumption responds to prey availability
   • Functional response is the relationship  the per capita predation rate and the prey population size
   • Consumers will rarely consume prey in direct proportion to abundance for all prey density
   • Consumption often saturates at some prey density because of inherent limits of the handling time for consumption
   • Consumption may be limited at low prey densities because of refugia or prey switching

For those of you nostalgic about the past — it was a snow day a few years ago for this lecture, so I made this make up video . . .

Consumer-Resource Dynamics Introduction