Download Marine Ecology 2008, Lecture 5 july 10 final pred-parasite

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.
- 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 predators
keep prey populations below K then we have 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 ratein 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 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
Used experiments to find predators that
controlled prey species that fed on
commercially important citrus crops. He
concluded that Gause’s experiments were
too simple to adequately reflect nature.
He studied 2 species of mites (predator and prey) on oranges. Prey
fed on oranges and predators fed on prey. Arenas in the lab had
oranges placed in rectangular trays and densities of predators/prey
were manipulated. He also increased habitat complexity by adding
rubber balls and vasoline barriers.
oranges only: predators ate prey and eventually starved to death.
oranges, balls and vasoline: 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 morphs 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 verses 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 particular prey item as a valuable food source. The
Predator then focuses on and begins to exploit 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 reproductive increase in the population)
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 Predator
Predators can influence prey populations dramatically
(can drive them to extinction). You might conclude that
there is some optimal level of predation intensity that will
serve to maximize the # of predators without driving the
prey populations to extinction.
The idea was suggested that predators might
“manage” prey populations and that this might explain
why predators and prey coexist.
Problem: individuals would have to cooperate with each other
(no penalty for cheaters in the short term)
Some evidence: predators can be prudent without
“altruistic behavior” because exploitation of the prey is
determined by the ability of predators to capture prey.
Which individuals are 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, Washington)
Observations
Juveniles barnacles
Adults
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)
- 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 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
potential prey species, one of which is a dominant
competitor. If we introduce a predator system and it
prefers the dominant competitor it can reduce the
population 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 the most
important consumer
Experimental Design
8x2m
Plots in
intertidal
Control
Piaster 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 mussel population 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 microfauna
(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 – rarely used it himself
Others picked up on it (particularly conservation literature)
(conserving keystones to maintain diversity)
Problems:
- identifying them
- context dependent
- may overlook other important species
Criticized by Hurlburt (1997)
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
Pisater more important at exposed sites
Other sites: diffuse predation
- 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 compound)
Effects of predation on morphology,
distribution and abundance
4. Morphological modifications
– inference from
observation
c. crypsis – organisms match the color and surroundings of
their background. Believed this morphology shaped by
predatory pressure over time.
Artificial camouflage
Decorator crabs
Crabs put algae on
their backs, which
increases their survival
In areas with Dictyota
algae, 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: Fix 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.
Size Class:
1
2
3
Low Abundance
(available)
2
4
8
Low Abundance
(eaten)
30%
65%
5%
High Abundance
(available)
10
20
40
High Abundance
(eaten)
60%
35%
5%
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 polychaete worms on mudflats 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
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)
• 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)
• Parasite benefits, the host loses
• Effects on host
– Reduced feeding efficiency
– Depletion of food reserves
– Reduced reproduction
– Lowering of disease resistance
Isopods
Acanthocephalans
In intestine
Behaviour and parasitology?
Behaviour is ‘the way in which individual
organisms interact with other components of
their environment, including prey, competitors,
predators, potential mates and parasites.
1. Parasites exploit natural
patterns of host behaviour to
maximise transmission
The types of host behaviour that can be exploited by parasites is
variable, but usually involves feeding / foraging.
This is especially important for parasites with an indirect life-cycle
Example 1: The Gasterosteus -Schistocephalus system
Stickleback
Free-swimming
coracidium
Copepod sp.
Example 2: A tale of two fishes…
Atlantic halibut
Hippoglossus hippoglossus
• Diurnal forager
• Rests during night (sand)
• Infected by:
Entobdella hippoglossus
Common sole
Solea solea
• Nocturnal forager
• Rests during day (sand)
• Infected by:
Entobdella 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
behavioural 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 antiparasite 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 important: BUT is energetically expensive,
Negative effects regarding growth, fat storage and reproduction
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 as available as
they appear to us
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 mass
• 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 cont.
• 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 (which
reduce the competitiveness of some plants) 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 less than 30% of
NAPP is consumed
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
ectoparasite
– 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
• Relationship seems to be mutualism (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.
Food chains to food webs
• Multiple predators and
multiple prey
• Same energy loss as
move to higher trophic
levels
Parasitism (cont)
• Parasite benefits, the host looses
• Effects on host
– Reduced feeding efficiency
– Depletion of food reserves
– Reduced reproduction
– Lowering of disease resistance
Parasitism con’t
• 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
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
Paine 1966- early
evidence of important
top-down effects
•
Effect of Pisaster on intertidal
assemblage
•
15 species coexist in intertidal
•
Food web: Pisaster the most
important consumer
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)