lecture 4 review 1) liebig’s law of the minimum says the growth of a population of organisms will...
DESCRIPTION
Lecture 4 Review 4) The two types of interspecific competition are interference and exploitation 5) Character displacement-through time two closely related species tend to become distinct morphologically and therefore use different portions of limiting resources 6) Connell’s experiments on barnaclesTRANSCRIPT
Lecture 4 Review 1) Liebigs law of the minimum says the growth
of a population of organisms will increase until the supply of a
critical resource becomes limiting 2) Results of intraspecific
competition are reduced body size or growth and reduced fitness 3)
The competitive exclusion principle says when two species compete
for identical, limited resources, one will eventually eliminate the
other Lecture 4 Review 4) The two types of interspecific
competition are interference and exploitation 5) Character
displacement-through time two closely related species tend to
become distinct morphologically and therefore use different
portions of limiting resources 6) Connells experiments on barnacles
Lecture 4 Review 7) In competitive release the niche of the
competitively-inferior species expands in the absence of the
competitively-superior species 8) In character displacement two
competing species diverge in a trait that reduces the strength of
interspecific competition Review Questions Define or
compare/contrast: intra- vs. interspecific competition; the
competitive exclusion principle; Gause; exploitative vs.
interference competition; niche breadth Explain how character
displacement can take place over evolutionary time Describe
Connells classic experiment Explain competitive release Predation
Predation one species feeds on another enhances
fitness of predator but reduces fitness of prey (+/ interaction) 7
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 modeled predator-prey interactions wondered: -
to what extent do predators cause these cyclic fluctuations
(arethey responsible? Completely responsible?) - 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 dont the prey populations go
extinct? Predator-Prey Interaction Model
Changes in prey population 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 dt r P k NP = - + 2 r2 = death
rate in predator population k2 = constant that measures the ability
of the predators tocapture 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 (1950s) Insect ecologist Experimented with
predators and their prey species that fed on commercially important
citrus crops.He concluded that Gauses 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 (1960s) 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 (for individual predators)
Components of Functional Responses (forindividual predators) Rate
of successful search a. ratio of the speed of predator to prey b.
size of the field of reaction of the predator (distance atwhich 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) Two types of
Ceriodaphnia
It is not just size that matters, it is overall visibility Zaret
1972 Two types of Ceriodaphnia Big eyeSmall eye Fish always took
the big-eye form. Artificially made small-eye morph more visible by
feeding them india ink.Predation rate increased (for individual
predators)
Components of Functional responses (for individual predators) 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 (for
individual predators)
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
doesnt 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 preyitem
as a valuable food source.The predator thenfocuses 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 (1960s) 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? Effects of
predation on morphology, distribution and abundance
Change in size structure of prey population (if predator prefers
the largest individuals in a prey population) Brooks and Dodson
1965 (over 1350 citations) Lakes in North America When fish
introduced there were huge changes - predators preferred the larger
zooplankton small zooplankton becamedominant large phytoplankton
become abundant Effects of predation on morphology, distribution
and abundance
Decreases in overall diversity if predators are very efficient at
removing prey, they drive populations to extinction which reduces
diversity Increase in diversity in simple systems with fewprey
species, one of which is a dominant competitor.If a predator
prefers the dominant competitor it can reduce the number of the
dominant competitors, 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 8 x 2 m 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 Produced
the concept of the
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 Results Take home
message
Pisaster more important at exposed sites Other sites: diffuse
predation, with strong effect shared among species Keystone effect
context dependent Why? low productivity Predation can regulate
assemblage structure - Directly: influences prey distribution -
Indirectly: can mediate competition Keystone concept beware of
generality Take home message Effects of predation on morphology,
distribution and abundance
Morphological modifications inference from observation a.
protective devices (spines on sea urchins; strong shells) Effects
of predation on morphology, distribution and abundance
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
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 Important because
increased food intake results in
- 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? With abundant prey, bigger is better
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 Fed fish choice of three sizes
of Daphnia magna 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 measured the
amount of energy gained/unit time. Manipulation: Fixed the
proportion of different sized mussels but varied the overall
abundance (# of each size class in a given area) Observed:
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
ate prey of 7mm more than any other size 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) Why
may organisms not follow predictions of Optimal Foraging
Theory
The idea that organisms maximize energy uptake is an assumption
other factors may be involved (e.g., 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. Threat of predation leads
to:
Mytilus edulis (Blue mussel) Threat of predation leads to: 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) Mytilus
edulis also undergoes inducible changes when exposed to predator
effluent however it is unlikely that you will ever see it pictured
in a textbook. The mussel on the left was exposed to water-borne
cues from the sea star Asterias for 3 months and the one on the
right was a control.Now any differences you perceive between these
mussel shell are probably not the result of phenotypic plasticity
However in the presence of predators blue mussels are known to: 50
Predation: Indirect Effects
Non-lethal effects Injury by browsing predators Trait-mediated
indirect interactive effects (TMII) Risk averse foraging More
shelter dwelling in the presence of predators Can produce larger
effects than consumption does 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 Can produce more dramatic effects than
actual predation does Trophic cascades 54 Dugongs can modify the
structure of seagrass beds through their foraging
Tiger sharks cause dugongs to change habitats, which canaffect
seagrass communities 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 Can produce more dramatic effects than
actual predation does Trophic cascades 56 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 Emergent
Multiple Predator Effects (MPEs)
Types of interactions among predators (Soluk and Collins, 1988):
Neutral: predators do not affect one anothers 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 (contd) 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 arent) Parasitism contd
Parasite classifications
Ectoparasites- 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 (contd) Parasite benefits, the host loses Effects
on host
Reduced feeding efficiency Depletion of food reserves Reduced
reproduction Lowering of disease resistance Isopods Isopods 64 Fish
Lice (Branchiurans)
65 Acanthocephalans In fish intestine Behavior and
parasitology
67 Parasites exploit natural patterns of host behavior to maximise
transmission
68 This is especially important for parasites with an indirect
life-cycle
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 life-cycle The
Gasterosteus -Schistocephalus system Stickleback Free-swimming
coracidium Copepod sp. 69 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 70 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 71 We know that host behaviour
can influence parasite infections
Therefore..variation in host behaviour patterns can create
variation in parasite infection levels Hosts can evolve behavioral
resistance as a response to infection threat
73 Behavioral resistance the first line of defence
Behavioral Defense Prevention better than cure.Least energetically
demanding defense. But there are also.. Structural Defenses 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
Defenses Immune defense is impt, BUT energetically expensive, with
negative effects on growth, repro and maintenance. 74 Herbivory
Herbivory is a special case of predation
herbivory differs from predation in that the prey cannot move
(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 dont herbivores
consume more of the plants that are available to them? Maybe
herbivores arent 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 generalistherbivores
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). Bucktooth Video 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 Cleaners in Hawaii
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 Back to the Rocky Intertidal
Early work by Connell (1970) Conducted a 9 year study of barnacles
and predatorywhelks (San Juan Island, WA) 103 Observations
Juveniles barnacles Adult barnacles Predatory Whelks 104 Model
Results 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 105 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 106
Menge (1994) Effects of Pisaster under different conditions
Transplanted mussel clumps Hypothesis: Pisaster will consume
mussels at all locations where it is present 107 Experimental
Design Boiler Bay Strawberry Hill Sheltered Exposed
PredNo pred PredNo pred PredNo pred PredNo pred Turf Bare
Replicates (n=5) 108 Reaction distance translates to overall volume
searched, which influences vulnerability of the prey
Reaction distance = radius of sphere Longer radius = higher
encounter rate