Lec 11-12 Flashcards
Types of interactions between species
Mutualism: +/+ Commensalism: +/0 Predation: +/- Amensalism: -/0 Neutralism: (0/0) --No interaction b/w species, so not really even an interaction
What do organisms compete for?
Resources:
- Food, water, light (among plants)
- Space
Mates:
- Apparent in animals
- Also in plants, with pollen grains “racing” down the style to the ovary
- Competition within species
Levels of Competition
Intraspecific competition
Interspecific competition
Intraspecific Competition
Competition between individuals of the SAME species
Leads to density-dependent logistic growth curve seen in study of population ecology
Interspecific Competition
Competition between individuals of 2 different species
Exploitation (indirect) competition
Occurs when one competitor uses resources and in so doing reduces the resource availability to the other competitor
-The most common form of competition in all organisms
Interference (direct) competition
Occurs when two competitors physically challenge or harm each other to obtain a resource
- Reasonably common in animals
- Also seen in plants, when one plant releases toxins to harm the competing plant: ALLELOPATHY
Demonstrating competition with Field Experiments: Barnacles
Large acorn barnacles (Semibalanus) and small acorn barnacles (Chthalamus) are both found within intertidal zones
Large acorn barnacles are found throughout the lower and middle intertidal
Small acorn barnacles are found only in the upper intertidal
How can it be shown that this is the result of competition? Do an experiment!
Removal experiment: Barnacles
When large acorn barnacles are experimentally removed, the small acorn barnacles colonize all of the intertidal zone (upper, middle, and lower)
When small acorn barnacles are experimentally removed, the large acorn barnacles remain out of the upper intertidal
Shows:
- Large acorn barnacles are superior competitors, keeping small acorn barnacles out of the lower and middle intertidal
- Large acorn barnacles are susceptible to desiccation, which prevents them from surviving in the upper intertidal
Demonstrating competition with Field Experiments: Paramecium
G.F. Gause experimented with 3 species of Paramecium (unicellular protozoans)
P. aurelia and P. caudatum feed on bacteria
P. bursaria feeds on yeast (unicellular fungus, eukaryotic)
When placed SEPARATE from each other: grow logistically
P. aurelia ad P. caudatum placed together: Both feed on bacteria, P. aurelia is the stronger competitor
P. caudatum and P. bursaria placed together: Coexist; compete for space only, have separate food sources
Competitive Exclusion Principle
Gause established competitive exclusion principle in 1934
Two species cannot coexist in a community if they use the exact same resources in the exact same way
–One species will persist, while the other will be outcompeted and driven to local extinction
Ecological Niche
Niche = When, where, and how a species makes its living
- -All of the environmental factors necessary for a species’ existence (its survival, growth, and reproduction)
- -A species’ role in its environment
- -An n-dimensional hypervolume
- –Abstract, intangible concept
Resource Partitioning
Redefine COMPETITIVE EXCLUSION PRINCIPLE:
-Two species CANNOT occupy the exact same niche within a community
Very similar species can coexist by way of resource partitioning:
-Use the same resources but in slightly different ways to avoid complete competitive overlap
AKA niche differentiation
Modeling Competition
Begin with the logistic equation:
dN/dt = rN (1 - N/K) => dN/dt = rN ((K-N)/K)
Lotka-Voltare Competition Equations:
Species 1: dN1/dt = r1N1 ((K1-N1-aN2)/K1)
Species 2: dN2/dt = r2N2 ((K2-N2-BN1)/K2)
a and B = competition coefficients
a indicates the effect of an individual of species 2 on the individual of species 1
When a=1
If a competitor of species 2 decreases the survival, growth, and reproduction of species 1 by the same amount as another individual of species 1 would, then a=1
Intraspecific and interspecific competition have the SAME effect in this case
When a=2
If a competitor of species 2 decreases the survival, growth, and reproduction of species 1 twice as much as another individual of species 1 would, then a=2
Interspecific competition is then STRONGER than intraspecific competition
Interpretation of B is _________ to interpretation of a
identical
If a and B are both GREATER than 1, the two species ______________-
Could NOT likely coexist
Equilibrium solutions
Set the Lotka-Volterra equations equal to zero to find the population size (N1 or N2) at which the species is in equilibrium (not increasing or decreasing)
Species 1: N1 = K1 - aN2
Species 2: N2 = K2 - BN1
Equilibrium solutions can be demonstrated graphically as ____________________
Zero-growth isoclines
Isoclines show equilibrium sizes for one species in combination with different numbers of individuals of other species
Isoclines can also be used to determine the condition under which each species will increase or decrease in size
X-axis in Competition model
Species 1
Y-axis in Competition Model
Species 2
There are four possible outcomes when the 2 isoclines are plotted together
1) If the species 1 isocline lies entirely above the species 2 isocline, species 1 is the stronger competitor - it will always outcompete species 2 and reach its own carrying capacity
2) The exact opposite will occur if the species 2 isocline lies entirely above the species 1 isocline - both of these outcomes represent competitive exclusion
3) If the isoclines cross and interspecific competition is stronger than intraspecific competition, one species will again outcompete the other, but which persists and which is outcompeted depends on the starting population sizes
4) If the isoclines cross and intraspecific competition is stronger than interspecific competition, both species do better competing against the other species than against their own species - the two species will coexist at a stable equilibrium point
Does Competition Only Occur Between Closely Related Species?
NO
Guilds consist of different (potentially distantly related) species that acquire nutrition in the same way
Competition will occur between members of the same guild
Example: Many rodents and ants belong to the granivore (seed-eating) guild, so these very different species compete for seeds
Amensalism
-/0
Really a special form of competition (-/-)
–Asymmetric or one-sided competition
All competition lies somewhere between completely equal and completely one-sided
Types of Predators
1) True Predators
2) Grazers (herbivores)
3) Parasites
True Predators
Consume many prey items throughout their lives
Almost always kill their prey
Examples: lions, insectivorous birds, spiders, baleen whales (eat krill) (and toothed whales)
Grazers (Herbivores)
Consume only parts of many prey items throughout their lives
Don’t usually kill their prey
Examples: Cows, sheep, aphids, leeches, mosquitoes
ALL herbivores are grazers but not all grazers are herbivores
Parasites
Consume only parts of just one or a few prey items throughout their lives
Don’t usually kill their prey
Examples: tapeworms, malaria-causing Plasmodium, Lyme disease-causing spirochete bacteria
Similar to grazers BUT restricted to ONE or FEW prey items (hosts)
Trends in Predators and Herbivory
Foragers:
- Some true predators forage throughout their habitat in search of prey
- -Examples: wolves, sharks, hawks
Sit-and-wait predators:
- Others are sit-and-wait predators, remaining in one place and attacking prey that move within striking distance
- -Examples: Eels, anemones, spiders
Most true predators are __________________
Polyphagous
- They are generalists that feed on many prey species
- They have a tendency to switch their diet to whichever prey species is most abundant
Most herbivores are ____________
Oligophagous
-They are specialists that feed on just a few plant prey species
Leaves are the most common plant structures consumed (as opposed to stems, roots, fruits, seeds, or sap)
-Leaves are the most abundant plant structures and have a high nitrogen content
Adaptations of Animal Prey
1) Physical defenses include large size (elephants), rapid or agile movement (gazelles), and body armor (snails and pangolins)
2) Chemical defenses include toxins along with warning coloration to advertise the toxin
3) Other forms of coloration:
- -Crypsis - camouflage by resembling background (hide from predator)
- -Mimicry - resembling another species that is fierce or toxic
4) Behavioral adaptations, such as foraging less in the open or keeping lookouts for predators
Predators and prey exert strong ___________ on each other
Selection pressures
For any __________ mechanism exhibited by a prey species, there is usually a predator species with an ____________ mechanism to counter it
Defensive; offensive
Most snakes can swallow prey that are ____________________-
Larger than their heads
Cheetahs are capable of _________________ to catch rapid and agile prey
Quick bursts of speed
Strong _________ (sight, smell, touch, echolocation in bats and dolphins, electroreception in sharks) to detect cryptic prey
senses
Predators also use _______________ (stonefish)
crypsis and toxins
Adaptations of Platn Prey
1) Structural defenses
- -Spines (modified leaves) and thorns (modified branches), tough leaves, sharp margins (edges), trichoscleroids (like ninja star
2) Chemical defenses
- -Secondary compounds that are distasteful or toxic to the herbivores or attract the predators of the herbivores
- –Secondary compound: Unnecessary directly for survival, aid greatly in survival
3) Masting
- -Producing hugs numbers of seeds in some years and hardly any in other years
- -This enable plants to avoid herbivores temporally and then overwhelm them with their numbers
- –Example: Semelparous species
4) Compensation
- -Plants are able to compensate for and therefore tolerate herbivory
- -Removal of plant tissue by herbivores stimulates growth responses, enabling plants to produce new tissues to replace lost tissues
- –Oxin: Hormone preventing outward growth; herbivores eat oxin off with buds, allowing more growth
Adaptations of Herbivores
herbivore adaptations generally involve the evolution of mechanisms to tolerate the plant defenses
Spines can be consumed or secondary compounds can be digested by certain herbivores
This provides an abundant food source to the tolerant species
Modeling Predation
The Lotka-Volterra predator-prey models:
Prey model: dN/dt = rN - aNP
Predator Model: dP/dt = faNP - dP
The PREY MODEL
dN/dt = rN - aNP
N = number of prey P = number of predators r = prey population growth rate a = capture efficiency of predators
When P = 0, the prey population simply grows according to its growth rate r
With predators present (P>0), the rate of prey capture depends on how frequently predators and prey encounter each other (NP) and the efficiency of capture (a)
The overall number of prey killed is aNP
The PREDATOR MODEL
dP/dt = faNP - dP
N = number of prey P = number of predators d = death rate of predators a = capture efficiency of predators f = efficiency of predators at turning prey into offspring (turn energy/nutrients obtained from prey into offspring)
When N = 0, the predator population simply decreases at its death rate, d
With prey present (N>0), individuals are added to the predator population according to the number of prey killed (aNP) and the efficiency by which prey are converted to predator offspring (f)
faNP represents the number of predator offspring born
Equilibrium solution and zero-growth isoclines can be used to determine what happens to predator and prey populations over time
Prey: P = r/a
Predator: N = d/fa
There is a certain number of predators that will keep the prey population constant
The prey population will decrease if P>r/a
The prey population will increase if P
There is a certain number of prey that will keep the predator population constant
The predator population will DECREASE if N < d/fa
The predator population will INCREASE if N > d/fa
Combining both isoclines reveals that predator and prey populations _______ over time
Cycle
Predator-Prey Cycles in Nature
Snowshoe hare (prey) and Canadian lynx (predator) have been tracked in Hudson Bay (northeastern Canada) since the mid-1800s
