ECOLOGY -- INTERACTIONS UNIT
Chapter 13: Competition
Species interactions: competition, mutualism, predation/parasitism, commensalism
Intra-/interspecific; interference, resource (exploitative) competition
Damsel (and other) fish example
Pine forest root trenching experiment
Intraspecific Competition -- the logistic growth curve has the assumption with it that intraspecific
competition for limited resources largely determines K.
Sorghastrum nutans (competing for N in soil); Medicago sativa (Alfalfa)
Two important results can occur as plants are grown in increasing density:
1. Low density -- plants of same species may be larger than when grown under higher
2. Self-thinning -- exhibited by many plants; the idea being that as a patch of one species
grows, more and more of the biomass is in fewer and fewer large individuals. WHY?
Homopterans -- aphids, planthoppers, leafhoppers. All share a feeding habit almost unique
in the insect world -- can tap into flowing "sap" with a very flexible "beak"
At higher densities, these insects experience reduced survivorship, slower growth and
development, resulting from reduced protein, water content and photosynthetic capability
of host plants.
Isopods (Grosholz) -- supplementing food APPEARED to have little effect on survivorship
(though Grosholz used what turned out to be rather low densities of isopods), but higher
density decreased survivorship, apparently because of cannibalism. Is there a possible
paradox here? Should Grosholz have done further experiments to clarify food limitation?
Niches -- a niche consists of all the necessary factors to allow a species to subsist in a particular
place and time. Another way of looking at the word niche would be that the niche represents the
role the species plays in the ecosystem -- all it's interactions, including what it eats and, in turn,
acting as food for other predators. One important result of this definition is that the niche does
not exist without the species. Although you may see the discussion of empty "niches" in various
ecosystems, technically this is strictly an incorrect usage of the term as there IS no niche without
the species, as indicated, and there may be no single species that can "fill" the "niche" that is
being considered empty -- instead, IF species "fill" the niche, it could be many species that divide
up the "empty niche".
Fundamental vs. Realized Niche -- realized niche breadth necessarily narrower than fundamental
The Competitive Exclusion Principle -- two species with exactly the same resource requirements
cannot occupy the same niche in the same place, and should result in local extinction of one of the
two species. However, we also know that instead of extinction that resource partitioning can result
from character displacement (see below; remember MacArthur's warbler study in Chapter 1?)
the warbler example mentioned above
the classic: Galapagos (Darwin's) Finches -- Beak width and seed size. Different species
feed on different sized seeds based on beak width and length. During drought times (remember
from Chapter 11), smaller & softer seeds eaten first, leaving larger & harder seeds. So drought
may impact the smaller species more severely, and within species, it may be the larger, stronger
individuals that survive through the drought. So, food and water availability largely shape the
niche of the finches (of course, many, many organisms are largely influenced by food/water)
Interspecific Competition -- Mathematical Models. (an extension of the Lotka/Volterra models)
This involves modifying the logistic growth curve to include competitive effects.
dN1/dt = rmax1 N1 ((K1 - N1- α12N2)/K1) This represents growth of species 1 with affects
on numbers from competition with species 2.
dN2/dt = rmax2 N2 ((K2 - N2- α21N1)/K2) This represents growth of species 2 with affects
on numbers from competition with species 1.
α12 & α21 are competition coefficients, where the first represents competitive effects of species
2 on species 1, and the second represents competitive effects of species 1 on species 2.
If α12 > 1, then competitive effects of species 2 on 1 are greater than intraspecific effects.
If α12 < 1, then competitive effects of species 2 on 1 are less than intraspecific effects.
Zero population growth will occur for each species when:
N1 = K1 + α12N2 (for species 1) and N2 = K2 + α21N1 (for species 2)
Hopefully, it is obvious that with the added competition from another species, the ACTUAL
value for K COULD go down for both species. Also, needless to say, if you added other
competitive species, the equation becomes more complex, but K COULD "decline" for
EACH species involved. However, depending on competitive capabilities of the species,
one could exclude the other, or they could partition the resources and coexist (see pg. 290)
Paramecium species: aurelia and caudatum
Grown separately, both species quickly reached a K based on intraspecific competition,
and K was higher for aurelia on both half (HS) and full strength (FS) growth media.
Grown together, aurelia outcompetes caudatum, w/ complete exclusion on HS in 16 days.
Tribolium (flour) beetles: confusum and castaneum
Grown separately, the species both did approximately equally as well.
Grown together, castaneum outcompetes confusum when conditions were warm and
humid, but confusum outcompetes castaneum when conditions were cool and drier.
Under intermediate conditions, the species first establishing a pop with greater numbers
typically wins out -- the priority effect.
Clearly, competition can have a very strong influence on shaping niches
the lab. What
about in nature?
Competition and Niches
Natural examples: PLANTS
Bedstraw (Galium) species on acidic (saxatile "wins") vs. basic (sylvestre "wins") soils
Both establish healthy populations on a variety of soils when grown separately
Natural examples: ANIMALS
Barnacles: Chthamalus and Balanus (mentioned before in Chap 9, pg. 203)
Competition critically important in middle intertidal, but desiccation-resistance becomes
more important in upper intertidal, and predation becomes more important lower.
Desert Rodents: Dipodomys (Kangaroo rats; large granivores); Perognathus (Pocket
mice; small granivores); and Onychomys (insectivorous rodents)
very ambitious study in SE AZ with large plots and removal strategies; we'll discuss
this example at some length (make sure to read pages 291 and 292 carefully, and
understand figures on page 293 and 294).
Character Displacement -- will expand further on our previous discussion of Geospiza
finches and then other organisms. A definitive example of character displacement must meet
several criteria (page 293, 295; figures on page 294).
Chapter 14: Exploitative Interactions -- Predation, Herbivory,
Predators, parasites and pathogens influence distribution/abundance/density (pop. structure) of
host/prey pops (see chapter 9). Herbivores clearly do the same for the plants they feed upon.
Trophic levels in the ecosystem:
Plants (the producers), herbivores (first order consumers; depending on plant consumed,
may or may not eat entire plant), predators (second and higher level consumers; DO kill
food), parasites (feed on, but do not usually kill, host), parasitoids (larva consumes host,
often from inside; functionally equivalent to predators), pathogens (induce disease; for the
most part, functionally equivalent to . . . ?). All consumers are exploitative, i.e., make their
living at the expense of others.
Exploitation and Abundance -- Predator and prey abundance intimately tied together
Already talked about several examples (like the Canadian Lynx, Snowshoe Hare, Dwarf
Willow example -- see below); more examples presented here in Chapter 14:
1. Bats and birds and arthropod densities on plants (in Panama).
2. Cactoblastis cactorum and Opuntia stricta in Australia; the moth was introduced
from South America as a biocontrol agent. Larval feeding introduces both fungal and
bacterial pathogens into the Opuntia. Extremely effective -- reduced pops from 12,000 per
hectare to 27, and from area of infestation of 24 million hectares to a few thousand. This same
moth, however, has turned out to be a potential threat to North American Opuntia, as the
moth is now spreading out from Florida north and westward.
Similarly, rabbits overran parts of Australia as well early in the last century, but were largely
brought under control by introduction of the rabbit papilloma virus.
Both rabbits and Opuntia are now in low abundance in Australia, where much lower density
allows them to escape the moth/virus for a while (probably similar to where they naturally occur).
3. Foxes, Hares and Mange Mites (Sarcoptes scabiei) in Sweden; as mange spread across
Sweden, reduction of fox populations resulted in increased hare populations, as well as other
prey species of the fox (grouse, deer fawns).
Dynamics -- Virtually all interactions are dynamic; as indicated previously, parasites may reduce
host populations which may increase populations of prey (what would happen to a predator
whose prey was affected by a parasite/pathogen?). However, very few of these interactions
result in some sort of continuously stable population levels for all species involved; instead, the
populations are temporally dynamic.
Example: The classic example of the Canadian Lynx/Snowshoe Hare/Dwarf Willow (and
other shrubs) shows how abundances of each species are tied to each other, and may change
through time. An added interactive effect we had not discussed before is that the shoots of the
Dwarf Willow (and others) that the Snowshoe hares browse on increase their terpenoids and
phenolic resins in response to high rates of feeding, making the plants less tolerable to hares,
effects which can persist for up to two years.
On the predator side of the story, there are other predators which feed on Snowshoe Hares,
such as foxes, coyotes, goshawks, owls, and mink/weasel. It appears as though these predators
experience some population cycling as well. Coyote diet may be 2/3 Snowshoe hares at peak
hare density, suggesting a predatory functional and numerical response to hare density. Both
coyote and lynx numbers increase 6 - 7 fold during high hare densities, though the functional
responses were a bit different. Both killed more than they could eat at the highest hare densities,
with coyotes in particular caching for later, though coyotes kill the most during hare increases
(and apparently would kill more if they could as their predatory response showed no sign of
levelling off), while lynx killed the more when hare pops were declining already (probably
attacking already weakened individuals). Experimental manipulation of food abundance and
predator abundance led to expected results (see Fig. 14.10).
Mathematical Modelling of population cycles:
(more Lotka/Volterra "stuff")
dNh/dt = rhNh - pNhNp where "h" refers to host & "p" refers to predators/parasites/pathogens
"p" is the RATE of predation/parasitism, which means that "pNhNp"
is the rate at which exploiters destroy hosts in the whole population.
Similarly for the predator/parasite/pathogen population numbers:
dNp/dt = cpNhNp - dpNp where "c" is a conversion factor representing rate at which exploiters
convert hosts to offspring; "d" represents death rate
SEE Figs. 14.11 & 14.12. These equations tie numbers of hosts to numbers of exploiters --
indeed, as Nh increases, so, too, does exploitation rate (pNhNp), which, in turn, declines Nh and,
in turn, Np. So, increasing host and exploiter population numbers in this model lead to significant
decrease in numbers later -- a cycle that is repeated. Though this model has some obvious utility,
remember again, however, that NO system in nature is so simple as to be ONE prey item with
just ONE predator or parasite/pathogen.
Laboratory attempts to produce cyclic fluctuations in exploiter/host populations have proved
extremely difficult, though Utida was able to show remarkable cyclic fluctuations between prey
bean weevils (Callosobruchus chinensis) and it's parasitoid wasp (Heterospilus prosopidis) for
up to 112 generations in very small, simple, environmentally controlled, petri dish habitats.
Refuges -- to persist in the face of exploitation, prey/hosts need refuges
1. Didinium nasutum (predators) and their prey Paramecium aurelia (see Fig. 14.19)
Gause was able to produce oscillations, but only if there was a refuge for the Paramecium, and
an immigration source (lab cultures) for the Didinium.
2. Huffaker and the ballooning/crawling herbivorous mite (Eotetranychus sexmaculatus) and its
crawling predatory mite (Typhlodromus occidentalis). The ballooning capability of the herbivore
allowed it to get to at least temporary refuges on other fruits ahead of the predator, and resulted in
oscillations of predator-prey populations (see Fig. 14.20)
Spatial Refuges -- typically what we think of in terms of refuges; Opuntia has not been driven
extinct in Australia as a result of the introduction of the moth Cactoblastis cactorum. Why? It
has spatial refuges where, at least momentarily, it has escaped the moth.
Numerical Refuges -- "safety in numbers". Hosts/prey can overwhelm exploiters, resulting in
predator satiation. Schools of fish, flocks of birds, etc.
Examples: 1. The thirteen and seventeen year periodical Cicadas
2. Masting by plants (synchronous seed/fruit production), often after disturbance (eg., fire)
Size as a refuge -- Elephants
Though young may fall prey to predators, many species reach an ultimate adult size that puts them
outside the range of prey that can be handled by the predator.
This has led to "appear big" behaviors -- cat defensive posture
Complex Interactions -- There are far more interactions than numbers of species. WHY?
Pathogens/Parasites that alter Host Behavior
Starling (Sturnus vulgaris), Isopod (Armadillidium vulgare), and a parasite (a spiny-
headed worm [acanthocephalan], Plagiorhynchus cylindraceus). The parasite
changes the phototactic behavior (- to +) in the isopod. WHY? (See Fig. 14.23).
Fungal parasites of moth caterpillars. Near maturation of the fungus inside the host, the
caterpillar crawls to top of a plant, becomes stiff and dies. WHY?
Mustard (Arabis sp.) host and a rust fungus (Puccinia monoica). Puccinia alters life
history of infected individual, causing plant to form a pseudoflower, with sweet sticky
hyphae that attract "pollinators" to move spermatia from one fungus to another. Flies
are the most common and effective rust fertilizers, but bees and butterflies also can.
Infected plants may be killed, though some survive . . . however, those that do never
flower, so fitness of infected plants is zero.
Obviously, it will be difficult to untangle the different interactions and their individual
effects on organisms; some experiments, however, show clear alterations in numbers
and success due to competitive and exploitative interactions.
Examples: Tribolium castaneum/confusum
Besides being affected by humidity/temperature, a parasite (Adelina tribollii) also
influences the competitive balance. Both species of Tribolium a bit cannibalistic on
eggs, but T. castaneum strongly prefers T. confusum eggs when present (explains a
LOT about the competition experiments). The Adelina parasite strongly affects
castaneum but not confusum, largely shifting the competition in favor of confusum.
Chapter 15: Mutualism -- Interactions where both benefit
OBVIOUS examples of mutualism: You and your E. coli; pollinators and flowers; fruits and
seed dispersers; many plant (cellulose) eaters and their digestive microorganisms
Facultative vs. Obligate
Margulis (and others) and the endosymbiont theory (now "proven" by DNA analysis)
Plant Mutualisms -- (some already mentioned above)
Plants and Mycorrhizal Fungi -- a 400 million± year relationship [a facultative mutualism]
Some plants grow root "chambers" for the fungi, other types form network around roots.
These fungi help plants w/ water extraction; nitrogen and phosphorus/copper/zinc sequestration;
access to more phosphorus allows plants to grow more roots, which allow them to claim more
water. The fungi, in turn, get carbohydrates produced by the plant.
Conflict of interest? The balance sheet is not always nice and equal. Different mycorrhizal
species and even different strains within the same species may not "treat" their hosts the same, and
may parasitize the host plant. Fungi may be more likely to act parasitically in more fertile soils,
as plants release less soluble carbs in root exudates in more fertile soils, which apparently has
selected for more aggressively carbohydrate sequestering mycorrhizae. In Andropogon (bunch-
grass), both shoot growth and inflorescence production are maximized when grown with associated
mycorrhizae from unfertilized soils rather than fertilized.
Bullhorn Acacias and ant (Pseudomyrmex) symbiotes [an obligate mutualism for ant]
Hollow thorns for ant abodes. The ants are fast, aggressive, agile climbers, that vigorously attack
growing vines/epiphytes or other animals (caterpillars) that grow/eat the tree, which is already well
protected by nasty spines! Additionally, the Acacias provide foliar nectaries and Beltian bodies
to "keep" ants fed and on the host tree. Acacias without ants both grow slower (leaves, shoots,
thorns, etc.) and show higher infestation (by other insects) and lower propagation.
Conflict of interest? The ants may be at odds with pollinators, both in competition for nectar and
driving them away. However, inflorescences offer scant nectar, and have an ant repellant chemical.
Aspen Sunflowers (Helianthella) and Formica (and other) ants in the temperate zone
Ants are attracted to this Rocky Mtn. sunflower because of extrafloral nectaries, with nectar
rich in sucrose and numerous amino acids, sufficient to keep the ants attention. During times when
the flowers are budding and just opening, ants rigorously patrol the flowers and help deter potential
bud/seed predators, but once flowers are open, the rays form a "shield" between the nectaries and
the open face of the flower, so the ants do not generally interfere with pollinators.
There are several species of ants involved here, and NONE have an obligate relationship with
Helianthella. The reason probably has to do with, at least in some populations, late frosts which
kill the flowers (including the nectaries; see Fig. 15.15) -- if ants were obligate, the ants would die
at that point. This event actually helps the sunflower, as it kills any and all seed predators in that
year. Clearly, the TEMPERATE climate plays a significant role in the nature of this mutualism.
Coral Mutualisms -- a mutualism between the coral and algae (zooxanthellae)
Coral reefs have exceptional diversity, and among the highest productivity of any ecosystem, even
though surrounded by the nutrient poor ocean. How?
The mutualism: dinoflagellates that are photoautotropic, and therefore productive. They live within
coral tissues, and receive nutrients from the coral animal, while the coral receives organics from the
zooxanthellae produce during PS. Corals induce release of nutrients from zooxanthellae when
needed by coral with chemicals that change zooxanthellae cell memb permeability, whereby they
release large amounts of fixed carbon. The corals also keep growth of the zooxanthellae at 1/10 to
1/100 that of individuals cultured separately, because of higher levels of carbon fixation. Doesn't
sound like a mutualism so far, but . . .
The zooxanthellae appear to get access to higher levels of nutrients, especially nitrogen (in
ammonia waste). Corals with zooxanthellae partners, don't APPEAR to release ammonia, as it is
metabolized by the zooxanthellae.
Crab/Shrimp mutualism with coral:
The coral gain protection, and the crab/shrimp mutualists get food (extra fat bodies produced)
Other examples of mutualisms: Orchids and orchid bees (obligate, see
video), Yucca and Yucca Moths
(obligate), Mullerian mimicry (facultative), the honeyguide and humans (see the "Applications" section
of chapter 15).
Evolution of Mutualism -- in general, we would
expect mutualism to evolve and persist in a population
when and where mutualistic individuals have a higher fitness than non-mutualistic individuals. This
will happen when the benefits to the two organisms exceeds the costs of the relationship.