Manipulating Hosts:
Worms ∆ Pillbug Behavior
Acanthocephalan infects pillbug
makes pillbugs seek light
Starling eats now-exposed pillbug,
and parasites grow in their gut.
Worm eggs pass out
to be eaten by other pillbugs.
Do Acanthocephalans manipulate hosts
to increase their chance of spreading?
YES!!!
Pillbugs experimentally infected were
eaten at a higher rate.
Manipulating Hosts:
Pucinia Fungus ∆ Plant Morphology
Makes mustards form pseudoflowers; they attract pollinators.
Pollinators bring fungal spermatia to pseudoflowers,
“pollinating” the fungus.
Changing Outcome
of Competition
In flour beetles, T. castaneum
normally beats T. confusum.
When protozoans infect
T. castaneum, it loses.
T. confusum isn’t affected
Grazers
& Plants
Opuntia conquered Australia.
Far denser than in NA, where Cactoblastis moth.
Cactoblastis introduced to Australia as biocontrol.
Now Opuntia only has small, sporadic eruptions.
Parasite, Predator, & Prey:
Mange in Swedish Foxes
Mange reduced fox popn by >70%
Did this help the prey of foxes?
Hares
Grouse
Deer fawns
Predator, Prey, & Plant:
Lynx, Hares, and Willows
Lynx eat snowshoe hares
Predators alone don’t explain
cycles.
Records Hudson Bay Co.
show cycles.
Lag time
Snowshoe hares eat willows
Predator, Prey, & Plant:
Lynx, Hares, and Willows
Popn cycles also influenced by
grazing pressure on willows:
hares overgraze them
they take time to recover
they make toxins to inhibit further overgrazing
other predators:
Arctic foxes eat lynx
parasites?
Thus, cycles can be of complex structure and causation
Let’s look at them more.
Lotka/Volterra Mathematical Models
(1920’s) Of Population Dynamics
Based on cycles in nature.
Shows predator’s numerical
response to ⇑ prey: dNp/dt
Graphs have lag time.
Combined graph of the two
popns is circular.
Lab Experiments
Try To Mimic Nature
Most studies end in extinction: Gause
Paramecium & yeast worked for 20 days.
Paramecium and Didinium.
When prey go extinct, predators do too.
He added refuge, allowing
prey to wait out predators.
sediment
Immigration by repeated restocking
of predator and prey finally worked.
Refuges Reduce
Capture Rate Of Prey
p = capture rate
Not = in every environment
lower w/ refuges.
Refuges can be where
prey physically hide
air
In burrows, crevices, water - land -
predators don’t like it:
Cactus persists at high el.
where it’s too cold for moths
too high, cold, dry, hot
beyond physiology
Refuge by Predator Saturation
Overwhelm predator’s ability to consume all prey
by massive, synchronized reproduction.
Functional response levels out.
Additional prey ≠ more eaten.
Refuge by Predator Saturation
Cicada emergence cycle
13 year in South; 17 year in North
Results in high densities
Up to 4x106 per hectare
The highest known biomass of a terrestrial animal
Does this decrease the proportion of cicadas eaten?
Refuge by Predator Saturation
Masting in many tree species.
Pines
Williams et al. (1993)
trapped emerging nymphs
E.g., 1,000
caught falling dead adults
Oaks
E.g., 900
Wildebeests on African
savannas flood their
predators with calves.
also caught falling wings
E.g., 100
This leads into another
strategy … Safety in Numbers
Any individual’s chance of being
eaten declines.
Allows group defense.
Refuge in Size
Large size works too.
Remember size-selective
predation?
Get BIGGER!