CH 55 SPECIES INTERACTIONS
2. Predation, Herbivory, and Parasitism
3. Mutualism and commensalism
- Intraspecific interactions take place among members /populations of same species.
- Interspecific interactions : how individuals of one species interact with individuals or populations of other species that
live in the same locality.
- Competition: interaction that affects both species negatively (-/-)
- Sometimes interaction is one sided, where it’s detrimental to one species but not to the other.
o Amensalism (-/0)
o Predation, herbivory, parasitism all have positive effect on one species.
- Mutualism is an interaction in which both species benefit (+/+)
- Commensalism benefits one species and leaves the other unaffected (+/0)
- Neutralism, when 2 species occur together but don’t interact in any measurable way (0/0)
o Ex. Rabbit and fox (-/+) predation
o Rabbit and Deer (-/-) for grass competition
o Rabbit and Plants (+/-) herbivory
o Rabbit and Disease organisms (-/+) parasitism
Several Different Types of Competition Occur in Nature
- Exploitation competition: organisms compete indirectly through the consumption of a limited resource
o Ex. Fly maggots compete in a mouse carcass
- Interference competition: individuals interact directly with one another by physical force or intimidation
- Competition isn’t always equal, so you remove one species temporarily and examine the effect on the other species.
Connell’s Classic Experiments with Barnacle Species Showed that One species can Competitively Exclude another in a Natural
- Studied Chthamalus stellatus and Semibalanus balanoides , 2 species of barnacles that dominate the Scottish coastline
and has a well defined niche or physical distribution.
- Chthamalus occurred in upper intertidal zone and Semibalanus restricted to lower intertidal zone.\
- Connell wanted to find how Chthamalus fares without competition from Semibalanus.
- Took rocks at high-tide level where Chthamalus grew and moved them to Semibalanus zone to be colonized.
- Took rocks out, remove Semibalanus and returned rocks to lower intertidal zone – mortality of Chthamalus was high.
- Also found that Semibalanus was not as resistant to dessication/drying up as Chthamalus and thus could not survive in
the upper intertidal zone
The Outcome of Competition Can vary with Changes in the Biotic and Abiotic Environments
- Park examined competition between 2 flour beetles, Tribolium castaneum and Tribolium confusum, under a variety of
biotic and abiotic conditions
- Presence of a parasite killed T. castaneum, but without it, T. castaneum usually won.
- Competitive environment influenced by climate, T. confusum did better in dry conditions and T. castaneum, moist.
- Results of competition vary as a function of parasites, temperature, and moisture.
- Similar species can coexist if they occupy different niches or occupy similar niches but undergo physical changes in
form and character. Field Studies Show Competition Occurs Frequently in Nature
- Competition was found in 55% of 215 species (Connell)
- More likely when 2 species utilize a resource
- Plants had high degree of competition because they’re rooted to the ground and cannot easily escape or competing for
same set of limiting nutrients.
Species May Coexist if They Don’t Occupy Identical Niches
- Gause studied Paramecium Aurelia, P. bursaria and P. caudatum, all which fed on bacteria and yeast.
- The bacteria occupied more in the oxygen-rich upper part of culture tube and yeast in the oxygen-poor lower part.
- Population growth was calculated as a combination of numbers of individuals per mL of solution x unit volume.
1. Grown separately: all 3 had logistic growth pattern
2. P. caudatum and P. Aurelia, P. caudatum went extinct because P. Aurelia grew at faster rate so was better at
converting food into offspring.
3. P. caudatum and P. bursaria: neither went extinct because P. bursaria used the yeast on bottom.
- Competitive exclusion hypothesis: complete competitors can’t coexist.
- Resource partitioning : differentiation of niches both in space and time that enables similar species to coexist in a
o Ex. Passerines (birds) had similar lifestyles but segregated based on habitat.
o All fed on insects but some would feed exclusively in grasslands, others in forests…
o Bigger species took on different sized food.
o Feed in different winter ranges.
Morphological Differences May Allow Species to Coexist
- Allopatry – different geographical species may be nearly identical
- Sympatry: same geographical area – competition may cause meaningful differences
- Character displacement: tendency for 2 species to diverge structurally.
o Schluter: studied stickleback fish – sympatric species exhibit character displacement in traits related to
o Benthic species feed on organism in sediments few, short gill rakers and wide gape
o limnetic species feed on planktonic species in the open water zone. many long gill rakers and narrow
o Predatory fish may contribute to morphological character displacement character displacement in
55.2 Predation, Herbivory, and Parasitism
- Herbivory usually involves nonlethal predation on plants, while predation generally results in the death of the prey.
- Parasitism, like herbivory, is typically nonlethal and differs from predation in that the adult parasite lives and reproduces in the
Animals have evo