Download Marine Ecology 2009, final Lecture 5 pred

Predation
Predation
– one species feeds on another  enhances
fitness of predator but reduces fitness of prey
(+/– interaction)
Charles Elton (1942)
British ecologist who studied mammalian
population data. He concluded that
oscillations were common and suggested
that predators regulate prey populations.
Cycles caused by predator –prey
interactions
prey
predators
Lotka – Volterra
Predation model
Mathematicians who tried to model predator-prey interactions
wondered: .
- to what extent do predators cause these cyclic fluctuations
(are they ½ responsible? Completely responsible?)
- are other factors important?
- do predators keep prey populations below K? (If so then no
reason to believe completion is important because resources
would not be limiting)
- if predators are so efficient, why don’t the prey populations
go extinct?
Predator-Prey Interaction Model
Changes in prey population
dN
 r1 N  k1 PN
dt
N = # of prey
r1 = intrinsic rate of increase in prey populations
k1 = constant that measures the ability of the prey to
escape predators (0-1)
P = # of predators
r1N =density independent growth
- k1 PN = reflects the negative effect of predators on prey
Predator-Prey Interaction Model
Changes in predator population
dN
 r2 P + k 2 NP
dt
r2 = death rate in predator population
k2 = constant that measures the ability of the predators
to capture prey (0-1)
P = # of predators
- r2P = negative of density independent growth
(drag on predator populations
Assumption: no density dependent effects (no carrying
capacity, no competition) only predation
Predator-Prey Interaction Model
This model did produce an oscillation between prey and
predator populations (thus, it appeared to reflect
natural situations). However, more complicated models
showed that they were math. unstable.
Gause : 1st test of model – observed two species of
protozoans (prey and predator) grown on an oat
medium. Predator always totally consumed the
prey, then starved to death.
Predator-Prey Interaction Model
Gause : By adding sediment to the oat medium
(habitat complexity) for hiding places for the
prey (paramecium). Predators starved to death,
then the prey populations increased dramatically.
Gause concluded that the cycles
seen in nature are the result of
constant migration, because he
could not get coexistence in his
experiments.
Carl B. Huffaker (1950’s)
Insect ecologist
Experimented with predators and their prey
species that fed on commercially important
citrus crops. He concluded that Gause’s
experiments were too simple to reflect nature.
Studied predator and prey mite spp. on oranges. Prey fed
on oranges and predators fed on prey. Lab arenas had
oranges in rectangular trays and densities of
predators/prey were manipulated. Also increased habitat
complexity by adding rubber balls and vasoline barriers.
oranges only: predators ate prey and then starved to
death.
oranges, balls and vasoline: complexity allowed coexistence
C. S. Holling (1960’s)
Conducted studies on the components of
predatory interactions (acts among
individual organisms)
Worked with vertebrates and invertebrates
(entomologist, concerned with the outbreaks
of insects in forests which denuded trees.)
Functional response – relationship between prey
density and the rate at which an individual
predator consumes prey
Numerical response- increase in predator numbers
with increases in prey abundance
Components of Functional Responses
(for individual predators)
1. Rate of successful search
a. ratio of the speed of predator to prey
b. size of the field of reaction of the predator
(distance at which the predator can perceive the
prey)
c. success rate of capture (does a predator get a prey
every time he encounters it)
Why are predators size-selective?
Encounter frequency: Encounter of large prey is higher than
small prey
Reaction distance —how close to
the fish does a prey item have to
be for the fish to see it and react
to (eat) it?
Pumpkinseed
Lepomis gibbosus
Confer and Blades 1975 (L&O)
Reaction distance translates to overall volume
searched, which influences vulnerability of the prey
Reaction distance = radius of
sphere
Longer radius = higher encounter
rate
It is not just size that matters, it is overall visibility
Zaret 1972
Two types of Ceriodaphnia
Big eye
Small eye
Fish always took the
big-eye form.
Artificially made small-eye morph more visible by feeding
them india ink. Predation rate increased
Components of Functional responses
(for individual predators)
2. Time available for hunting verses other
activities
other activities necessary for an organism to carry on
to reproduce is going to influence the amount of time
available to hunt
a. avoiding other predators
b. time looking for a mate
c. patrolling a territory
Components of Functional Responses
(for individual predators)
3.
Time spent handling prey
amount to time it takes to capture a prey after
recognizing a potential food source
a. pursuit of prey
b. subduction of prey
c. eating prey
d. digesting prey
4.
Hunger level – function of the size of the gut of
the predator and the time spent in digestion and
assimilation (rapid versus slow metabolsim)
Type I: Functional Response
Linear increase; same assumptions as the Lotka-Votera
growth models
No examples of Type I functional response curve observed
in natural systems
Type II: Functional Response
Leveling off of # of prey eaten even though the # of prey
increases (satiation of predators or time spent hunting prey)
Holling found Type II curve with invertebrate predators
Type III: Functional Response
Lag period: even as density of prey increases the # of prey
eaten doesn’t increase dramatically (thought to result from the
formation of search image by predators)
These were all the results of laboratory experiments that
Holling conducted. People later found a few examples of
Type II and more examples of Type III.
Search image – when prey are rare there is no value in
hunting for them. Only when the prey population
increases above some threshold level does the predator
form a search image and begin to recognize that prey
item as a valuable food source. The predator then
focuses on and exploits that food source heavily.
Numerical response – when there is a large
increase in prey density, the predators present can
become satiated as prey densities increase and the
rate of prey eaten is not going to increase for each
individual predators.
However, if predators are added to the population
increased exploitation of the prey can occur (due to
immigration not reproduction)
Switching
If predators exploit prey populations heavily and
drive prey populations down, eventually prey
densities will decline below some threshold value
and predators will switch to another prey item.
If switching occurs then more than one prey
species can coexist (many studies have found
switching to take place).
Murdoch (1960’s)
Conducted the 1st switching experiments –
examined gastropods that feed on mussels and
barnacles and found that switching took place.
-best candidate for switch occurred when the
predator exhibited a weak preference for prey
species
Prudent Predators
Predators can drive prey pops. to extinction. But
there is some optimal level of predation intensity that
will maximize the # of predators without driving the
prey extinct. It has been suggested that predators
might “manage” prey populations and that this might
explain why predators and prey usually coexist.
Problem: individuals must cooperate with each other.
But why not cheat?
Evidence: predators can be prudent without
“altruistic behavior” because exploitation of prey is
determined by the ability of predators to capture
them. And which individuals are usually removed
from prey populations?
Back to the Rocky Intertidal
• Early work by Connell (1970)
• Conducted a 9 year study of barnacles and
predatory whelks (San Juan Island, WA)
Observations
Juveniles barnacles
Adult
barnacles
Predatory Whelks
Model
Model: Lower limit of adults caused by
predation
H1: Excluding predators in low areas leads to
presence of adults
Experiment: predator exclusion cages on a pier
piling
Results
Midshore level: excluding whelks resulted in an
adult population
Lowshore: small whelks got into cages
Conclusion
•Predation controls lower limit of barnacle population
•Contrast with Connell (1961), where
competition controlled lower limit of Chthamalus
•Reason for difference?
- predation reduced density below which
competition could occur
- space not limiting
Effects of predation on morphology,
distribution and abundance
1. Change in size structure of prey population
(if predator prefers the largest individuals in a prey
population) or there are shifts in the relative abundance of
prey species (such that smaller species become quite
abundant and the larger prey species becomes rare).
Lakes in North America
When fish introduced
there were huge changes
- predators preferred the
larger zooplankton
- small zooplankton
became dominant
- large phytoplankton
become abundant
Brooks and Dodson 1965 (over 1350 citations)
Effects of predation on morphology,
distribution and abundance
2. Decreases in overall diversity – if predators are
very efficient at removing prey, they drive
populations to extinction which reduces diversity
3. Increase in diversity – in simple systems with few
prey species, one of which is a dominant
competitor. If a predator prefers the dominant
competitor it can reduce the number of the
dominant competitor, allowing the inferior
competitors to exist.
All three of these can occur in “ecological time” = one
to a few generations
Paine 1966
• Effect of Pisaster on
intertidal assemblage
• 15 species coexist in
intertidal
• Food web: Pisaster starfish
the dominant consumer
Experimental Design
8x2m
Plots in
intertidal
Control
Pisaster
removal
Monitored changes over one year
Results
Control plot: no change
Removal plot: 80% barnacles (3 months)
Mussels starting to dominate (1 yr)
Species diversity decreased 15 to 8 spp.
Predicted mussels would dominate available space
Conclusions
Pisaster interrupts successional process
After removal, superior competitor dominates
Produced the concept of the
“keystone predator”
Limitations: no replication; did not examine smaller fauna
(can be very diverse)
Keystone species
paradigm
Pisaster become known as a
“keystone species”
Paine (1966) cited 850 times 1970 –
1979
Defined as “a single native species,
high in the food web … which greatly
modifies the species composition and
physical appearance of the system”
(Paine 1969)
Is the keystone a useful concept?
Paine intended the term as metaphor – he rarely used it
Others picked up on it -particularly conservation biologists
(conserving keystones to maintain diversity)
Problems:
- identifying them
- can be context dependent
- may overlook other important species
Criticized by Hurlburt (1997) and others
Menge (1994)
Effects of Pisaster under different conditions
Transplanted mussel clumps
Hypothesis: Pisaster will consume mussels at all
locations where it is present
Experimental Design
Boiler Bay
Sheltered
Pred
No pred
Strawberry Hill
Exposed
Pred
Turf Bare
Replicates (n=5)
No pred
Sheltered
Pred
No pred
Exposed
Pred
No pred
Results
Pisaster more important at exposed sites
Other sites: diffuse predation, with strong effect
shared among species
“Keystone” effect context dependent
Why? – low productivity
Take home message
Predation can regulate assemblage structure
- Directly: influences prey distribution
- Indirectly: can mediate competition
Keystone concept – beware of generality
Trophic Cascade in Kelp Forests
• When the keystone sea otter is removed, sea
urchins overgraze kelp and destroy the kelp forest
community.
Figure 5.15b
Effects of predation on morphology,
distribution and abundance
4. Morphological modifications – inference from
observation
a. protective devices (spines on sea urchins; strong
shells)
Effects of predation on morphology,
distribution and abundance
4. Morphological modifications – inference from
observation
b. mimicry – organisms that resemble unpalatable
species (usually because they contain toxic
compounds)
Effects of predation on morphology,
distribution and abundance
4. Morphological modifications – inference from
observation
c. crypsis – organisms match the color and shading
of their habitats. Believed this morphology shaped
by predatory pressure over time.
Artificial camouflage
Decorator crabs put algae
on their backs, which
increases their survival
In areas with Dictyota algae,
crabs use this species for
decoration, but rarely food
Optimal Foraging
- types of feeding behaviors that would maximize food
intake rate.
Important because increased food intake results in
larger and healthier organisms with more energy for
growth and reproductive output. (maximize fitness)
Is taking the largest prey item the best strategy?
Elner and Hughes (1978)
Used predatory green crabs (Carcinus maenas) and
mussels. Several different sizes of mussels were
offered (small, medium and large). Feeding trials
looked at the amount of energy/unit time.
Manipulation: Fixed the proportion of different
sized mussels but varied the overall abundance (# of
each size class in a given area)
Observation: Proportion of each size class eaten
under different abundances.
Conclusion:
The crabs are foraging in a size-selective manner
AND they get more selective at higher abundances
However, they still sample unprofitable size
classes
Goss-Custard (1977)
Studied size selection of worms eaten by redshanks (Tringa
totanus). Redshanks selected prey of 7mm more than any
other size class even though prey of 8mm were more
common. Thus, worms were eaten in proportion to their net
energy benefit --not in relation to their abundance.
Also….
• When large #’s of worms available birds were selective
• When small #’s of worms were available all were consumed
(take what you can get)
2. Optimal foraging —take the prey that provides the
greatest energy return for cost of capture/handing.
Werner and Hall (1974) Ecology
With abundant prey, bigger is better
80
% in diet
60
Expected from
encounter rates
Observed in Diet
40
20
0
Small (< 1.5 mm)
Medium
Large (>2.5 mm)
Fed fish choice of three sizes of Daphnia magna
Why may organisms not follow
predictions of Optimal Foraging Theory
1. The idea that organisms maximize energy uptake
is an assumption – other factors may be involved
(predator avoidance). An organism may not
maximize energy consumption because of the
need to minimize predation risk.
2. Organisms may not be able to detect all available
prey.
3. Caloric value may not account for all needed
resources (essential vitamins).
Optimal Foraging needs to be thought of as a
concept not a theory.
Inducible versus Constitutive
defenses
A bryozoan makes
spines when placed in
contact with a predatory
nudibranch.
A hydrozoan, Hydractinia,
produces defense stolons
armed with nematocysts
when in contact with
another colony.
Inducible Defense: The conical (right) and bent
(left) forms of the acorn barnacle Chthamalus
anisopoma. The animal develops the bent form if
predatory snails are present.
Mytilus edulis (Blue mussel)
• Thicker shells
• Leonard et al (1999)
• Smith & Jennings
(2000)
• Larger adductor muscle
• Reimer & Tedengren
(1996)
• Increased gonad ratios
• Reimer (1999)
• Increased byssus volume
• Cote (1995)
Predation: Indirect Effects
• Non-lethal effects
– Injury by browsing predators
– Trait-mediated indirect effects (TMII)
• Risk averse foraging
• More shelter dwelling in the presence of predators
– Trophic cascades
Predation: Indirect Effects
• Non-lethal effects
– Injury by browsing predators
– Trait-mediated indirect effects (TMII)
• Risk averse foraging
• More shelter dwelling in the presence of predators
– Trophic cascades
Emergent Multiple Predator Effects
(MPEs)
• Types of interactions among predators (Soluk and
Collins, 1988):
– Neutral: predators do not affect one another’s
rates of prey consumption
– Negative (interference): combined prey
consumption less than neutral values  MPE
– Positive (facilitation): combined prey
consumption greater than neutral values 
MPE
Parasitism
• A two species interaction in which one species
(parasite) lives in or on a second species (host)
for a significant period of time and obtains its
nourishment from it.
Parasitism (cont’d)
• Viruses, bacteria and fungi
• Protozoa, arthropods, helminths
(nematodes, cestodes, trematodes, and
acanthocephala)
• Parasites are ubiquitous and should
probably be considered in every
ecological study (but aren’t)
Parasitism cont’d
• Parasite classifications
– Ectoparasites which live attached to or
embedded in the external body surface (gills,
body walls etc) of an organism
– Endoparasites lie within the body, and may
occupy circulatory vessels or internal organs
Parasitism (cont’d)
• Parasite benefits, the host loses
• Effects on host
– Reduced feeding efficiency
– Depletion of food reserves
– Reduced reproduction
– Lowering of disease resistance
Isopods
Isopods
Fish Lice (Branchiurans)
Acanthocephalans
In fish intestine
Behavior and parasitology
1. Parasites exploit natural
patterns of host behavior to
maximise transmission
The types of host behavior that can be exploited by parasites is
variable, but usually involves feeding / foraging.
This is especially important for parasites with an indirect lifecycle
The Gasterosteus -Schistocephalus system
Stickleback
Free-swimming
coracidium
Copepod sp.
Ex 2: A tale of two fishes…
Atlantic halibut
Hippoglossus hippoglossus
• Diurnal forager
• Rests during night (sand)
• Infected by the worm:
Entobdella hippoglossus
Common sole
Solea solea
• Nocturnal forager
• Rests during day (sand)
• Infected by: E. soleae
Parasites are closely related, but cannot successfully
infect the ‘wrong’ host
Sole
Halibut
E.
Hippoglossus
hatching
E. Soleae
hatching
Parasites lay sticky eggs that adhere to sand
particles, near their potential hosts
Eggs of E. hippoglossus and E. soleae exhibit
opposite hatching periodicity, which match host
activity patterns
• We know that host behaviour can influence
parasite infections
• Therefore…..variation in host behaviour
patterns can create variation in parasite infection
levels
2. Hosts can evolve behavioral
resistance as a response to
infection threat
Behavioural resistance ‘the first line of defence’
Behavioral Defense
Prevention better than cure. Least energetically
demanding defense.
Structural Defense
Skin of Red Sea cling fish can produce enough
anti-parasite mucus to cover its entire body in a
few minutes.
Crinotoxic fishes (sedentary) have epidermal toxins that
protect against parasites.
Immunological Defense
Immune defense is impt, BUT energetically expensive,
with negative effects on growth, repro and maintenance.
Herbivory
• Herbivory is a special case of predation
– herbivory differs from predation in that the
prey cannot move (but remember that much
herbivory in the ocean is really predation)
The importance of herbivory to
nearshore ecosystems
• It is the first step in the transfer of energy in
nearshore food webs
• It provides a major trophic link for the cycling of
nutrients within these food webs
• It often affects the productivity and structure of
plant communities
Plant Community Shifts due to
Herbivory
• Increases prevalence of species with:
–
–
–
–
Low nutritive value (low nitrogen)
Chemical Defenses (secondary compounds)
Structural Defenses (calcareous skeletons)
Shifts in functional groups (from erect fast
growers to prostrate slow growers)
– mutualisms between grazers and host plants
Why is the world green?
• Why don’t herbivores consume more of
the plants that are available to them?
– Maybe herbivores aren’t food limited
(predators control herbivore density)
– alternatively the plants are not really as
available as they appear to us, either because
they have chemical or morphological
defenses (spines) or they are nutritionally
inadequate
Herbivory: Secondary
compounds
• Secondary compounds are not part of
primary metabolic pathways so they
must be synthesized at some cost to the
plant
• The primary function of these
compounds is controversial:
– some view them as waste products of plant
metabolism that are only coincidentally toxic
– others suggests they are so costly as to have
evolved specifically to thwart herbivores
Herbivory: Secondary
compounds cont.
• Types of chemical defenses:
– quantitative
• examples include tannins and resins
which occupy as much as 60% of the
plants leaf dry weight
• these compounds are thought to deter
specialized herbivores
– qualitative
• comprise < 2% of a plants leaf dry mass
• examples include alkaloids and phenols
• deter generalist herbivores
Herbivory:Secondary compounds (2)
• The amount of energy invested in a plant depends
on the vulnerability of the tissue
– growing shoots and leaves are more heavily
defended than old leaves
– usually compounds are concentrated near the
surface of the plant
• Because these compounds are expensive to produce
some plants have the ability to turn the production
of these compounds on and off (induced defenses) in
as little as 12 hours after a bout of herbivory
– only studied for a few marine species
Secondary compound tradeoffs
• Reduced competitive capability for the
defended plant
• grazer may seek out defended plants to
sequester these compounds for their own
defense
• some grazers are well equipped to defeat
chemical defenses
The impact of herbivores on
plant communities
• How much plant production is taken:
– in macroalgae on reefs and phytoplankton in
some estuaries virtually all of the Net
Aboveground Primary Production (NAPP) is
consumed by herbivores
– in seagrass and mangroves usually less than
30% of NAPP is consumed (but more in
past).
Population Interactions
• Competition (--) when both species suffer from
an association
• Predation (+-) when one benefits and one suffers
• Commensalism (+0) when one species benefits
from another
• Amensalism (-0) when one species negatively
affects another
• Mutualism (++) when both species benefit from
another
Mutualisms
• Both species benefit
• One species provides nutrition while the other
provides either protection or cleaning services
• Examples include:
– Clownfish-anemone
– Giant clams/corals-zooxanthellae
– Goby-shrimp
– Decorator crabs-sponges, tunicates and
anemones
– Deep sea worms-sulphur metabolizing
bacteria
Mutualisms
• Classifications of mutualisms
– Obligatory
• At least of the species can not live in the
absence of the other
– Facultative
• Each species can survive singly but
quality of life is much less
Mutualisms
• How did mutualisms evolve?
– Most think they develop as a result of some
intense negative interaction (parasitism or
predation)
– Organisms had two options
• Escape the interaction
• Adapt to the interaction
Mutualisms
• Benefits of mutualisms
– Each species grows, survives, or reproduces
at a higher rates in the presence of the other
Mutualisms
• Mutualisms less important when resources are
plentiful
• They are more common in stressful
environments
• Benefits maybe density-dependent
Cleaner Stations
• An experiment conducted
with cleaner fishes and
larger predators
– Cleaners feed on
ectoparasites
– In some cases parasites
within the mouth
• When cleaners are removed
parasite infestation increase
within a very few hours
• Cleaner stations are sites of
very high species richness
Another example
gobies and snapping shrimp
Another example
Sea anemone - protection
against predators
Clown fish highly evolved
to survive cnidarian
nematocysts
Mucus - thicker & lacks
sialic acid groups which
trigger nematocyst
discharge.
Algal-Invertebrate Mutualisms
• Found in protozoans, sponges, cnidarians, asciidians,
flatworms, and mollusks.
• Alga generally lose motility during symbioses and
may lose cell walls
• Animal hosts may change behavior – e.g. Cassiopeia,
Convoluta, Tridacna
• Relationships seeming to be mutualisms (trading
food for food, protection, oxygen, carbon dioxide)
•Vertical or horizontal transmission possible
Algal-Invertebrate Symbioses,
cont.
Population Interactions
• Competition (--) when both species suffer from
an association
• Predation (+-) when one benefits and one suffers
• Commensalism (+0) when one species benefits
from another
• Amensalism (-0) when one species negatively
affects another
• Mutualism (++) when both species benefit from
another
Commensalism
Facultative
commensal
e.g. – barnacles
• The Remora fish (Echeniedea) has its dorsal fin
modified as a sucker-like attachment organ. It
attaches to the sides of larger fish and turtles
using them as transport hosts but in addition,
obtains food fragments dropped from the host.
Trophic amensalism
• Amensalism is the opposite of mutualism, and
occurs when one organism alters the
environment such that another type of
organism cannot live there.
• Deposit feeding organisms cause bioturbation,
making suspension feeding more difficult or
impossible. This is amensalism among trophic
levels, or Trophic amensalism.
Ghost shrimp burrows
Prevent clams from living because the loose sediment
around burrows clogs their feeding apparatus
Population Interactions
• Competition (--) when both species suffer from
an association
• Predation (+-) when one benefits and one suffers
• Commensalism (+0) when one species benefits
from another
• Amensalism (-0) when one species negatively
affects another
• Mutualism (++) when both species benefit from
another