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34 Cards in this Set

  • Front
  • Back
What are the effects of predation?
- reduction in abundance of prey
- indirect effects on habitat use
- foraging
- cascading interactions and prey adaptations to reduce risk
How does a predator respond to an increase in prey density?
- Prey density: functional response
---> The consumption of prey by predators changes with prey density.

Ex. Rainbow trout and damselfly (invert) overlap over food resource (mayfly)
-- trout feeds by whatever convenient method, opportunistic, but also typically drifts
-- fish in general, visual predators, detect prey via motion
-- damselfly is a stalker/hunter, does not use vision, tactile, feels prey
---> Poor Model though because fish prefer invertebrates away due to low E expenditure and higher gain from invertebrates
How to reduce prey: different methodologies
01: Consumption (global)

02: Alter Behaviour (Local)
-- ex. scaring away; no change in numbers, but in distribution

03: Foraging times

04: Usage of substrate types
Prey Density: Functional Response

What are the consequences in terms of prey regulation?

Type 1
- # kill/time increases in unit proportionality
---> should be constant
- linear response: Prey INCR, feeding rate INCR

(-) No prey regulation: disregards satiation, proportion stays constant
(-) Poor handle time on prey
---> Can predator keep moving that fast?
(-) Not realistic, rarely seen in nature

- #killed/time vs. prey density LINEAR
- proportion killed vs. prey density: constant, horizontal
Prey Density: Functional Response

What are the consequences in terms of prey regulation?

Type 2

- #killed/time vs. prey density: LOGISTIC GROWTH
- proportion killed vs. prey density:

- Takes into account predator satiation / handling time:
---> At every increment of prey density the predator is killing less of the proportion of prey
---> More realistic: hungry, eat fast, stop when full
---> Get to point where there are so many prey but cannot handle quick enough
---> "Can't shove it in your mouth fast enough"

(-) No prey regulation
---> For every increment prey population, less effect

-- Can find this in real rivers where the bottom is smooth (sand, silt, bedrock)
-- Environment lacks heterogeneity
Prey Density: Functional Response

What are the consequences in terms of prey regulation?

Type 3

- # killed/ time vs. prey density: LOGISTIC CURVE
-->Lag: search time (takes time to figure out what to eat, find, and consume it), learning curve
--> Incr: kill lots once knows were prey is and how to get it
--> Plateau: predator satiation or handling time, gets full/ reaches max feeding efficiency

- proportion killed vs. prey density: BELL SHAPES (INCR then DECR)
--> INCR: can regulate prey population (other models lack this)
--> DECR: lose ability to regulate

- Takes into account predator satiation and search time:

01: Predator effect is increasing: can regulate the prey pop
02: Predator effect is decreasing: cannot stabilize the dynamics of the prey
Type 2 vs. Type 3 Functional Response

Why do we see Type 2 in the lab and Type 3 in nature?
- Habitat heterogeneity: predator effect only when refuges are full
- Lab: don't have to search, no place for prey to hide, everything is right in front of predator
- Also, small spatial scale
- Thus, this is an artifact of expt. apparatus, but can remedy by addition of rocks.
Prey Size: Size Selectivity of Predators

Expected vs. Observed

Why monotonic increase in size selectivity for fish?
01: Size Selection -- Bell Curve
---Increases to a certain point, then decreases because size is too big, cannot handle/gape limitation/capture efficiency

- Macroinvertebrate is a model match
- Fish however are a linear increase


- General organisms eat big, or as big as they can handle for high energy return and decr. searching time for smallers orgs (waste of time to college and find)
- Inverts avoid if prey gets too big because they cannot handle, may be too hard to process, or they may be attacked

- Nothing is too big for a fish to eat (macroinvert wise)
- Hardly any bugs escape the ability to be consumed by a fish
- Everything is small enough (can also be too small, not enough E return)
Predator Size: What determines the set range (size of food)?
GAPE: inhale items, sometimes crush food (no chewing)

RAKERS on the GILLS: distance btwn gill rakers set amount of food consumed by fish -- determines size of prey; item is literally caged then swallowed

Predator size determines:

01: Diet breadth (area between gape limitation and gill raker spacing)
02: For fish, diet breadth increases with fish size
Prey Vulnerability; What adaptations do prey have to avoid being eaten?
Prey can alter encounter rates and enhance ability to survive

01: Reduce activity
-- RED encounter rates, RED resource acquisition

02: Avoid top of substrate: prey are easily seen on top, contrast easier to pick up, all food is also on top (esp. if benthic invert)
--RED encounter rate, RED resource acquisition

03: Nocturnal Activity: can still obtain resources up top, not as easily seen, but give up half a day's worth of resources by hiding (fish visual predators, can hide in dark)
- Only helpful for visual preds and not inverts (tactile)
-- RED encounter rates, RED resource acquisition

04: Morphological Defense (armor)
- Energy costly, and only efficient against invertebrates
- For fish, no effect, armor = incr in size = yummy

05: Escape Responses:
- Drift: great when at night against visual preds and also tactile, because they can just leap off substrates and still have no visibility to fish because of the dark

- Posturing: shape makes it appear larger than it really is
---> looks harder to swallow (works only for inverts, not fish)
---> incr perceived size
---> (-) no feeding, decr fitness and burns E

-- Remember defensive adaptations to one predator may be useless against another predator type
Predator Effects: Why are variable results the rule?

Naive vs. Experienced Communities

Expt: Rattlesnake Creek
- Naive prey communities (never seen fish before) -- tested what happens when predators are added to streams that lack predators.

01: In sumer, stream is in isolated pools
02: Some pools contain trout, others do not

01: Add trout to troutless pools
02: Remove trout from pools with trout

01: Trout effects STRONGER when trout were ADDED (+)
-- Bugs skirted to outer sides and a decrease in bottom biomass

02: Trout effects were WEAKER when trout were REMOVED (-)
-- Hardly any effects
Predator Effects: Naive Communities, Rattlesnake Creek

Why was there a difference in the strength of effect?
Time Scale Problems:

01: Trout addition has rapid effects (consumption)
--Consumption is immediate
02: Trout removal has slow effects (colonization)
-- Colonization slow due to drift (conditions: little flow, drift primary source of colonization)
-- See quicker results during winter or spring when there is an increase in flow

Predators added to naive communities often have large effects
What happens when predators are introduced where they already are? (straight addition)
Weak or undetectable effects are common. WHY?

01: Consumption (global effect)
-- Already are adapted and are therefore not vulnerable

02: Behavioural Modification (local)
-- Behaviour can be short term acclimation response

- Benthic invets have larger effect vs. drift feeders in experiencec communities
--> Lots of ways to get away form fish vs. inverts
--> Studies usually sample bottom vs. water columns (fish are found in the pools and water column while inverts are at the bottom)
--> Riffles too turbid for fish to pull out food resources
Exceptions: fish feeding at bottom that specialize in benthic substrates like catfish have big impacts
Multiple Predator Impacts: What happens when multiple predators are present?

Ex. Soluk and Richardson: Channels with and without trout
Interaction modifications:

Something being done is either:

01: Facilitation (helping)
02: Inhibition (stopping)

another organism in doing what it's doing.

- Add stonefly predator, which decreases mayflies
- Trout grow faster in the presence of stoneflies
- What is the mechanism??
---exchange rate: rate difference between what flows in and what flows
---both also have the same prey (mayflies)
---tested in cages: no flow, big impact because nothing flows in to replace mayflies, thus they all get consumed
Multiple Predator Impacts: So Cal Streams (Even)

Treatments and Predictions
01: Added trout and odonates (dragon larvae) in isolation
-- Trout: want to eat mayflies, not dragonflies
-- Make fish too small to eat the dragonflies (gets rid of artifacts)
02: Added trout and odonates together

Mayflies are NAIVE


01: Predators will decrease density of mayfly prey (by eating or altering
02: Multiple predators will have a greater impact than predators in isolation

(-) Artificial channels: lose some heterogeneity
-- Prey are still drifting, (+) control situation but (-) take away from reality
Multiple Predator Impacts: So Cal Streams (Even)

01: Predators DECREASE prey DENSITY

-- Adding both O and T separately and at same time decr. density (anomale**)

02: Predators INCREASE prey DRIFT

-- Escape
-- Emigration incr but doubles when add both

03: Predators DECREASE prey EXPOSURE

-- All decr exposure to same degree
-- Measure of how to find prey: find refuge on tiles, count how many on top of substrate vs. underneath

- No difference in # of prey consumed by multiple predators and predators in isolation.
01: Predators DECREASE prey DENSITY

-- Adding both O and T separately and at same time decr. density

02: Predators INCREASE prey DRIFT

-- Escape
-- Emigration incr but doubles when add both

03: Predators DECREASE prey EXPOSURE

-- All decr exposure to same degree
-- Measure of how to find prey: find refuge on tiles, count how many on top of substrate vs. underneath

Why is there no difference in # of prey consumed by multiple predators and predators in isolation?

Hypothesis: Both O+T compete for prey
Test: Who has more mayflies in them?
---> cut open guts

Findings: T eat more with O around. Why do O eat less with T?
---> Odonates HIDE from trout. Trout try eat odonates, but can't swallow them...just pop in and out. However, they do try --> harassment
---> O larvae scare mayflies, thus mayflies as a rxn to immediate threat incr in drift but get eaten by fish


01: Trout decrease foraging rate of odonates (inhibition)
02: Odonates increase foraging rate of trout (facilitation)
---> These both have the same effect overall
---> Averaging: one compensates for the other; get a net effect
Predator Effects: Exchange Rates
Immigration and emigration of prey can influence the impact of predators
Test: cages with prey and +/- predators

- Predator Impact (PI): Impact on Consumption/ Behaviour of Prey


PI = -ln (Np / No)
Np = prey density in cages WITH predators
No = prey density in cages WITHOUT predators
---> If PI = 0, there is NO predator effect
---> If PI increases, predator consumption/be alteration increases
How much the predator effect change as flow rate increases through the cages?****
Before flow: likely to see impact

Increasing flow, increases drift, which causes "swamping"
---> Cannot measure in a local space

River = conveyer belt of food, can come too fast for predator to handle
Exchange Rates: General Model

What does it do?
- Predicts how exchange across cage boundaries affects predator impact

Immigration (I) --> Density Population (Ne) --> Emigration (E)

System can be modeled as:

dN/dt = I-E

Must also take into account consumption
Exchange Rates: General Model Equation and Implications
dN / dt = I-E
E = emigration
E= mNi
m = per capita emigration rate
Ni = prey population density INTERIOR of cage
I = Immigration
I = iNe
i = per capita immigration rate
Ne = prey population density exterior of cage

- If immigration rate is equal to emigration rate:
---Assumer i = m = c where c = exchange rate (the rate at which an organism moves across a boundary)

Then: dN/dt = I-E = iNe - mNi

Since i = m - c then dN/dt = c(Ne-Ni)
Exchange Rates: General Model -- Explain newly derived equation that takes into account attack rate
dN/dt = c(Ne - Ni)

The predator effects (Ni), prey population density interior of the cage.

(-) Loss term: aNiP = # prey consumed
a = attack rate
Ni = pop density of prey inside cage (interior)
P = # predators

Complete interaction modeled as:

dN/dt = c(Ne-Ni) - aNiP

-- incr a, incr PI
-- constant cycling of new prey that you don't see effects
-- incr P, incr PI bc incr aNiP (consumption and attack)
General Model Exchange Rates:

dN/dt = c(Ne-Ni) - aNiP

Explain dynamics between cages as well as Predictions and Conclusions
At equilibrium:
- For predator free cages: Ni* = Ne
- For predator cages: Ni* = cNe / c(c+aP)

Equivalently: ratio of interior to exterior prey densities

--density of prey is being replenished, which is both a global and local function
-- still an impact, just not measurable
-- the river in a conveyer belt of food, ex. I Love Lucy and the chocolates :)
-- more global than local effect
-- Whenever conveyer belt speeds up, decreases PI

Conclusions: Predator effects on prey density are larger in habitats with reduced prey exchange
Do predator effects change as exchange rate increases?

Expeirmental Evidence: Sierra Nevada Streams (Even)

Treatments and Predictions
Treatments: Artificial Stream channels
01: Added trout and stoneflies
02: Across a gradient of prey (beatif mayfly) measured exchange rates in channels

-- had nets that catch bugs: could collect and redistribute in x quantity to the different channels

01: Predators will decrease density of mayfly prey
02: An increase in exchange rates (c) will decrease predator impacts (PI)
Do predator effects change as exchange rate increases?

Expeirmental Evidence: Sierra Nevada Streams (Even)

Results and Conclusions

Drift rate (y) vs. Benthic density (x): LINEAR response (pos)

PI (y) vs. Prey Immigration (x): BELL SHAPED CURVE
-- beginning of curve is due to no drift: the prey hiding, thus they are not exposed to predtors and have refuge space
-- the gradual incline signifies space is filled but not at a very high density yet
-- graph then peaks: see the highest PI because the benthic density fills up the refuge space and must colonise the top, which is easier to feed off of, but also there is no swamping so it is still measurable
-- the decline is explained by replacement --> swamping: there are too many
Trophic Cascade:
The effects of upper trophic levels cascade via direct and indirect effects through lower trophic levels
- Top down effect, modifying higher trophic level cascades down
- Seen in simple food chains:
more likely to show cascading effects, otherwise don't really see in the real world
Direct vs Indirect Effect
Direct: I eat you
Indirect: I ate something that AFFECTS you
Trophic Cascade; Experimental Evidence

Ex. Oklahoma Streams (Powers and Matthews)

(simple systems)

Observations: two kinds of pools

01: Bass (predator)
-- no stoneroller minnows: either eaten or escape (too high of metabolic rates to freeze)
-- (+) algae increased

02: No predator
-- (+) stoneroller minnows (herbivores -- algavore)
-- (-) no algae
Trophic Cascade; Experimental Evidence

Ex. Oklahoma Streams (Powers and Matthews)

Treatments and Results
01 : Removed bass, divided pools in half, added stonerollers
02: Added tethered bass to stoneroller pools

01: stoneroller INCR, algae DECR
02: stoneroller DECR, algae INCR
03: bass INCR, stoneroller DECR, algae INCR

-- stoneroller moved to periphery, emigrated or were consumed, algae increased
-- as the bass were tied to leashes, algae was abundant wherever it could reach (DIRECT result, minnow to scared to come close)
-- minnows fed along radius and did not flee pool
Trophic Cascade Experimental Evidence

Cascading effects of the lead litter decay in Utah streams
01: Cages with and without invertebrate predators (stonefly, eats detrivores, which eats CPOM)

--INCR in preds, DECR in shredders, INCR in leaf pack weight
---> decay rate is SLOWER

-- DECR in preds, INCR in chredders, DECR in lead pack weight
---> FASTER rate of decay

- invertebrate predators reduced shredders causing a decrease in lead decomposition rates
Trophic Cascade Experimental Evidence

Eastern Sierra Streams (Cooper 1994)

01: INCR in preds, NO EFFECT on baetid grazers, INCR in periphyton

-- baetids go from top of rocks to bottom
-- does not alter density, but behaviour

02: DECR in preds, NO EFFECT on baetids, DECR in periphyton

If there's no effect of trout on baetis density, how is there a periphyton effect?

-- INCR in preds, DECR baetids on ROCKS, INCR periphyton
-- preiphyton is released from predation
-- decr. exposure at top of rocks
-- baetids go from top of rocks to bottom
-- does not alter density, but behaviour
Trophic Cascade: Complex Food Webs

Northern California Streams (Powers)
Observations: Baetis is dominant food of roach and trout in the Eel river

Predictions: Removal of fish much decrease periphyton by increasing Baetis


Cages excluded and included large fish

Results: DECR in fish, NO EFFECT on baetid grazer, INCR in algae. WHY?

- Did not take into account a fourth trophic level:
--> DECR fish, INCR predatory inverts, DECR chironomid (midge), INCR in algae

01: Fish do not eat chironomids because of retreats (less E return)
02: Baetis does not control algae
03: Chironomids control algae
-- weave algae, not consumer (not a grazer)
-- uses as protection from predators, can effectively snap at prey source from algae
-- mayfly does not control algae, midge does; midge effect >baetis
--weaving: less SA in plant for sun, detrimental
Trophic Cascade: Effect of Disturbance

Expt: Northern California Streams (Power)
Observations: During the drought years an increase or decrease in fish has no cascading effects on algae. WHY?

01: DECR in fish, NO EFFECT dicosmoecus, DECR algae
02: INCR in fish, NO CHANGE dicosmoecus, DECR in algae

Conclusions: no cascade because caddisfly is armored an inedible to predators (cannot be eaten and thus reduces algae). Physical conditions are also different, which may rid or add trophic levels.

- large caddisfly, only in rivers of low flow, cannot hold on for that long; poor colonizer
- very efficient scraper
- hard shell, thus fish cannot eat / resistant to predation
- dominant for algal resources
Breaking a trophic cascade. What is the term?
Decouping a trophic cascade