Download • Predators “know” which prey are most beneficial and will switch to

Optimal Foraging Theory
• Predators should optimize energetic gains by balancing
the costs/benefits of capturing prey.
• Costs
– Search time
– Handling time
– Digestion
• Benefits
– Calories assimilated
Predator strategies
• Ambush predator – use stealth and crisps,
wait for prey to come to you.
• Active hunter – use speed and strength to
capture prey.
• Grazer – may or may not kill prey, different
degrees of selectivity.
• Parasite – do not directly kill prey, allow it
to provide continuous supply of food.
Optimal Foraging, Predator Choice and Search Image
• Predators “know” which prey are most beneficial and
will switch to more beneficial prey. Should switch as
prey become scarce (optimizing cost/benefit)
• Search image – mental picture of prey predators
are search for.
Giving up density
• Giving Up Density (GUD) – density of prey at
which a predator will abandon a prey type.
– Prey are depleted – cost/benefit of pursuing
prey no longer beneficial
– Competing predator more efficient –
cost/benefit of prey no longer beneficial
– Predator response – switch or find new patch
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Predator effects on communities
• Eliminate weak and
sick
• “Switch” to most
abundant species,
preventing one from
dominating
– Biodiversity
implications:
• Prevents competitive
exclusion, increasing
diversity
• More energy = more
trophic levels = more
predators = more diversity
Prey Responses - Mimicry
• Mimicry – look like something toxic, dangerous or inedible
– Batesian – harmless species looks like dangerous one
Prey Responses
• Crypsis – increase
predator search time
• Escape
• Avoidance – move to
an area w/out
predators
• Both increase search
times
Prey Responses
• Armor, physical defenses – increase handling time
• Swamping – prey group together/produce more
young than predators can consume. Safety in
numbers.
– Mullerian – two dangerous species look similar
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Functional Response
Predator-Prey oscillations
• Predator and prey populations often tightly
linked.
– Prey (hare) and predator (lynx) population
sizes are clearly linked…why?
– What controls dynamic?
– Two hypotheses
Number Eaten
• Predator/prey functional response – describes
predation rate at different prey densities.
– Note – assume # predators constant, number
eaten ~ benefit.
Prey Density
Top-down control
• Top-down control – consumption at higher tropic levels
controls population sizes
– lynx prey on hare, reduces hare population
– Fewer hare → support fewer lynx, reduce lynx
– Fewer lynx → hare recover and increase
– More hare → support more lynx
– cycle continues
Fox
Bottom-up control
• Bottom-up control – productivity in lower trophic levels
controls population sizes
– Hare population increases → resources (primary
producer biomass) limited → Hare population declines
– Hare population declines → fewer lynx supported, primary
producers recover → hare population recovers
Fox
rabbit
rabbit
Grass
Grass
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Which is it?
Modern revision to “the world is green”
• “The world is green” (Hairston et al. 1960)
• Primary produces are not limited by resources
• Herbivores limited by carnivores, not primary
production
• Primary produces are climate or nutrient
controlled on the local scale.
• Different climates and nutrient environments will
favor selection for different traits.
• Problems
– Many parts of the world are not green
– Much primary production does not represent a
quality meal
Trophic Cascades
Food Web Interactions
• Trophic Cascade - reduction or removal at one
trophic level has the opposite affect at
alternating lower levels.
Hawk
Snake
Bird
+
Grasshopper
Grasses
-
+
Hawk
-
Snake
+
Bird
-
Grasshopper
Grasses
• Communities more complex than 1 predator, 1 prey
or linear food chains.
• Removal of one species can have ripple effect
through community.
+
-
4
• Species who’s importance to the ecosystem is
disproportionate to its abundance (biomass)
• Traditionally keystone predators – species that
controls abundant species, prevents competitive
exclusion.
• Modern view - does not have to be predator (eg.
figs).
Keystone Species
Change in Ecosystem Function if Removed
Keystone Species
Keystone
species – rare,
but vital to
ecosystem
Abundant
species And vital role in
ecosystem
Moderate
abundance moderate role in
ecosystem
Rare species not a vital role in
ecosystem
Species Biomass
Parasitism
• Similar to predation, predator usually does not kill host
(prey). Parasite dependent on host for nutrition.
– Microparasites – bacteria, viruses etc.
– Macroparasites – flea, tick, mosquito, tapeworm etc.
– Biotropic – parasite can only live as long as host
lives
– Necrotrophic – parasite may kill host, feed on corpse
– Parasitoids – parasite always kill host
Other species interactions
• Parasitism and Predation (+/-)
• Mutualism (+/+) – both species
benefit from relationship
– Intestinal fauna aid in
digestion
– Leaf cutter ants
• Plant (-)
• Ant (+)
• Fungi (+)
• Comensalism (+/0) - one
species benefits, the other not
affected
– Shark-ramora
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Humans as Predators
• Overexploitation –
• Switching -
Humans as Predators
• Most desirable fish – large body
– Other Competitor (K-selected)
traits associated
• Slow maturation
• Long life span
• Few, large young (low fecundity)
Endangered Megafauna
Of the worlds 20
largest freshwater
fishes, 17 (18) are
endangered due in
part to overharvest.
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Review of some basic definitions
Community Properties
• community - all populations of organisms that
live together & potentially interact in particular
area at particular time
• ecosystem – collection of communities in an
area & their biotic and abiotic interactions with
the environment
Example of Resistance and Resilience
• Diversity - # of species, relative abundances (S and H`)
• Productivity – Primary productivity and energy transfer
through ecosystem
• Trophic structure – number of trophic levels, food web
structure
• Species composition
– What species occur in a region and why?
• Patterns of colonization and local extinction
– What species persist or disappear from a community?
• Resistance and Resilience
– Is the community resistant to change (do we observe
the same species through time)?
– Is the community resilient (if disturbed, does the
community return to pre-disturbance conditions)?
Example of Resistance and Resilience
16
14
Chickasawhay River
12
Diversity
Leaf River
10
8
6
Pascagoula River
Chickasawhay
Leaf
Pascagoula
4
2
Pre Storm
Fall 2005
'2006
Time Period
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Species Pools and Environmental Filters
Ecological Succession
• Communities tend to transition from one set of
species to another
• Generally, r selected species replaced by K
selected species
Disturbance
• Natural process that removes biota,
generally produces a patch of bare
habitat.
• Measure in frequency and intensity
• Aquatic and terrestrial
Ecological Succession
• Primary succession
– Start with empty habitat, Pioneer community
colonizes
• Secondary succession
– Start with pioneer community, is replaced by
later successional community
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Ecological Succession
• Succession involves changing
plant, animal and microbial
communities.
Ecological Succession - Climax Community
• Fredrick Clements – all communities naturally
progress towards a stable climax
• Made the analogy that succession was similar to
the growth and development of an organism
• Community composition is not random but
predetermined
Ecological Succession and Facilitation
• Succession changes both the biotic and abiotic environment
• Facilitation – presence of one species facilitates the presence
of others (provide a resource other species require)
• Bare land to forest change
– Erosion
– Nutrient export
– Temperature
– Shade
–…
Gaia Hypothesis
• Gaia Hypothesis - Earth is a superorganism
– Species are analogous to cell types
– Communities (analogous to tissues) will change
due to disturbance, but ultimately return to original
state.
– Popular with the environmental movement…no
scientific basis.
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Ecological Succession – Individualistic hypothesis
• Henry Gleason – individualistic hypothesis.
– There is no stable climax. Random events and
properties of individual species determine final
successional state.
– Determined by
• Local climate
• Population interactions (competition, colonization ability)
• Disturbance frequency
Krakatoa – a natural experiment
Post Disturbance Community Composition
• Climax community
– Only early
succesional species
suitable
– Community progress
predictably, reaches
climax
• Individualistic
hypothesis
– First species are the
best dispersers, best
adapted for postdisturbance
communty
– Later successional
species are poorer
dispersers, but better
competitors
Dominance vs. Founder Controlled Community
• Founder controlled community - Early
community determined by dispersal ability
– R-selected
• All life destroyed on
island by 1880
volcanic activity.
• Change in community
structure observed
over 100+ years since
– K-selected
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Dominance vs. Founder Controlled Community
• Dominance Controlled Community – Over
time, poorer dispersing species (better
competitors) arrive and dominate.
– R-selected
– K-selected
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