UBC Bio 111 - Intro to Biology

1. 4.1 Biotic Interactions
Learning Outcomes
1. Identify different ways in which organisms interact and the outcomes of those interactions.
2. Distinguish among different types of predators.
Readings: Chapter 14: 366-367, 370-378
Reminders: Reading quiz for Module 4 is due TODAY (before start of class)
The learning centre has additional activities you have never seen to reinforce the key concepts for each
week. At least 5 visits/over the semester will earn you 0.5% added onto your final grade.
Although abiotic factors are important for determining where a species lives, biotic factors also have an important
effect. Biotic factors are all the influences an organism faces from other living organisms such as what food it eats, what
species preys on it, as well as what species competes with it within the same habitat.
Under the forest canopy light can be limiting. These two species of ferns
have spread their fronds to maximize the interception of light. Because they
are growing closely together their fronds will overlap.
Competing for light
Below the surface of the soil, the root systems of plants seek out nutrients
and water. Plant root forms range from fibrous and shallow to individual
deep tap roots. The root systems from many individuals will intermingle.
Competing for H20 & nutrients
Hyenas and vultures both search the savannas for carrion (dead animals).
Both species will feed on the carrion.
Competing for food
Blue mussels and barnacles inhabit the intertidal zone of marine shores.
Both need to attach to the crowded rock surface.
Competing for space
Competition: Two species require the same resource that is in limited supply (Inter specific competition).
They may actively interfere with each other or one may exploit the resource before the other competitor can
access the resource.
Competitive exclusion: The elimination of one competitor from the habitat by the successful acquisition of
resources by the other competitor.

2. Garden snail (Pacific sideband snail) grazes vegetation using a file-like
radula.
Herbivory
Leaf miners are the larvae of insects that eat the chlorophyll rich inner
layer of leaves.
Herbivory (Consumers only eat producers)
Leeches attach to chordates, such as fish and non-hairy animals. They
attach with a circular set of jaws to pierce the skin and draw out the blood.
Parasitism obtains nutrients from an other organism by harming but not
usually by killing them
- Form of predation
Sea gulls effectively hunt for food by searching the shallow waters and
attacking live animals such as this sea star.
Predation
(Carnivory)
Predation: The process of collecting, harvesting or hunting another organism (plants, algae, fungi, or
animals) for consumption.
A hummingbird feeds on nectar from a thistle flower. In the process the
head of the bird is covered with pollen. The hummingbird is critical in the
reproduction of the plant.
Mutualism
Lichens are formed from a sandwich of a blue green bacteria or algae
between two layers of fungus. Both organisms are dependent on the
other for survival.
Symbiotic form of Mutualism b/c of close contact

3. Corals are animals that are encased in hard calcium carbonate shells.
They consume other small organisms that they can catch with tentacles
but also have algae embedded in their tissues that supply an extra source
of energy through photosynthesis.
Symbiotic form of Mutualism b/c of close contact
Mychorrizal fungi are associated with the roots of plants. They provide an
extensive network of hyphae (thread-like filaments) that absorb water and
nutrients from a large area that they share in exchange for sugars from
the photosynthetic plant.
Very important association for plants in many soils
(Similar stuff: rhizobium – bacteria. Capture N2 gas convert it into NO3 or
NH4. These are usuable by plants)
Symbiotic form of Mutualism b/c of close contact
Mutualism: Individuals of two different species provide a benefit to each other.
Symbiotic associations: Two mutualistic species are in intimate contact with each other, often not being
able to survive without the other.
Small fish like the damselfish often hide within
crevices of coral to avoid predation by larger fish.
The coral obtains no benefit from this interaction.
Commensalism Individuals of one species benefit while there is no effect on the other species.
Table of interactions – refer to your text to summarize the types of interactions
Type of interaction Effect on Species 1 Effect on Species 2
+ +
Mutualism
+ -
-
Commensalism +
Your Turn
Identify the type of interactions illustrated (by pictures in class):
1. Squirrels eat mushrooms
2.
3.
4.

4. 5.
6.
Please be prepared to answer questions about this case study in Monday’s class. You will be asked some of the
questions given below as well as some related questions.
Case Study: Temperature and competition determine habitat locations for three fish species living in
mountain streams
These fish species have been observed to live at different locations along Rocky Mt. streams.
Salvelinus fontinalis Salmo trutta Semotilus atromaculatus
brook trout brown trout creek chub
high elevation middle elevations low elevations
A study was performed by Taniguchi et al. 1998 which indicated that competitive abilities between the fish changed with
temperature. An artificial large oval stream tank was used to test competition between the fish at different
temperatures. Researchers could observe fish by watching them from the centre of the oval; also the tank mimicked a
real stream by having sections with different depths and current speeds.
Draw in layout of the tank as if looking at it from above.
Competitive abilities were measured by number of aggressive acts (chasing and nipping other fish) and by the ability to
successfully obtain food (floating invertebrates) compared to the other species. Besides the laboratory experiments,
experimental field studies were also performed to study distribution of the three species in nature in streams of
different temperature.
The following figures indicate how many stream sites at a specific temperature contained the different species of fish.
Brook trout alone
QUESTION: 1. Describe the temperature ranges
Brook and brown trout of tolerance for
together each individual species based on this data.

5. Within the artificial stream for one replicate at a specific temperature one individual fish of each species was placed. All
fish were similar sizes and had been previously acclimated to the temperature being tested. No brook trout were tested
above 24oC because they did not survive acclimation above this temperature. For each temperature a number of
replicates were performed. During each replicate the number of food items consumed by each fish was recorded (top
graph) and the number of aggressive acts (bottom graph) was also recorded.
QUESTIONS: 2. When you compare brook trout to brown trout are
there any significant differences between the two species for how
many food items they consume at different temperatures?
3. Describe food consumption by creek chum at different
temperatures. When is it significantly less than the trout species?
When is it not significantly different? Is it ever significantly higher
than either of the trout species?
The researchers wanted to know whether a particular species was consuming less food at a particular temperature due
to the presence of a competitor or solely due to the particular temperature. After all 3 species had spent time together

6. at a particular temperature the fish which appeared to be the strongest competitor was removed. Then it was observed
whether the remaining fish showed any increase in the number of food items consumed.
QUESTIONS: 5. If food consumption increased after removal of the strong competitor what do you conclude was causing
a fish of a particular species to eat less when the competitor was present?
6. Conversely, if food consumption did not increase after the removal of the competitor then what do you conclude?
In the fig(a) for brook trout you can assume that the brown trout were removed and in the
fig(b) for brown trout you can assume brook trout were removed.
QUESTIONS: 7. Is brook trout inhibited by brown trout? Is there any
influence of temperature on the results?
8. From 3-24oC was brown trout inhibited by brook trout? What can you
conclude about the feeding behavior of brown trout at 26oC?
9. Do creek chub eat more when competitors are removed?
In nature, brook trout are found in the highest elevation Rocky Mt. streams which tend to be the coldest. In this study
they were found in the coldest temperature streams also.
10. Was there any significant difference in the amount of food they ate or in their competitive abilities at the lowest
temperatures (3oC) tested compared to brown trout?
11. Why might they be found alone in the coldest (and highest) streams in nature then?
12. **Does temperature have an effect on competitive behaviour? Describe any significant differences in food
consumption and aggressive behaviour below 20oC.
13. Did the creek chub ever have a competitive advantage over brook trout, if so at what temperature?
14. Did the creek chub ever have a competitive advantage over brown trout, if so at what temperature?

7. 15. ***What is your overall conclusion or conclusions from this study? Does temperature mediate competitive
interactions between these species (and possibly effect patterns of species distribution in streams)?
Trout dominate below 22 – 25 degrees. Because they are likely better adopted to the temperature allowing them to be
better at obtaining food and have more aggressive behavior
Trout experience thermal stress above this temp, whereas creek chub do not, and thus they will dominate.
16. What other factors may have an effect on where within a stream a species lives?
Reference cited:
Taniguchi, Y., Rahel, F.J., Novinger, D.C., and Gerow, K.G. 1998. Temperature mediation of competitive interactions among three fish
species that replace each other along longitudinal stream gradients. Canadian Journal of Fish and Aquatic Sciences. 55: 1894-1901.