Lab 12
ECOLOGICAL INTERACTIONS
December 1-5
Thought questions:

What is more important for speciation: intraspecific or interspecific interactions? Explain.

What types of interactions are possible between two species? What kind of evidence can
be used to verify the type of interaction that is occurring?

Is it useful to define ecological interactions in a pair-wise fashion? Why or why not?

Biologists are extremely interested in competitive interactions. Why?

Interspecifically, are competitive or mutualistic interactions more common? Why?

Are ecological interactions a form of biodiversity? Should they be conserved?

Can you give a few examples of ecological interactions that are economically important?
Objectives of this week’s lab
(1) To survey several major types of two-species ecological interactions, including
mutualisms, commensalisms, predator-prey interactions, and parasitism.
(2) To distinguish mutualistic and commensal interactions conceptually, and to explore
experimental and technical challenges associated with gathering evidence to make this
distinction.
(3) To examine morphological features that are adaptations to a parasitic lifestyle.
(4) To consider how ecological interactions can be relevant economically, agriculturally,
and medically.
Readings: Campbell et al. [5
th
edition, pages 1109-1121],
Lab 12-1
6th edition pages 1176-1186.
LAB OVERVIEW
ECOLOGICAL INTERACTIONS
The Big Picture:
Ecologists often distinguish between abiotic and biotic factors in the environment, and
they also divide biotic interactions into two categories: (1) interactions within species
and (2) interactions among individuals of different species. Our focus today is a special
case: two-species interactions between pairs of species. Such seemingly basic
interactions can be amazingly diverse, complex, and challenging to study.
BEFORE CLASS
 Prepare for today’s quiz by reading your lab
handout and textbook and by attending recitation.
 Observe/sketch examples of mutualisms and
commensalisms; complete a worksheet.
IN CLASS THIS WEEK
 Observe/sketch examples of predator-prey and
parasite-host interactions; complete a worksheet
Worksheets are due at the end of your lab period this week!!
Lab 12-2
Introduction to Ecological Interactions
Ecology is the study of how organisms affect, and are affected by, biotic (living) and abiotic (nonliving)
factors. This week we focus on biotic interactions, which can be divided into (1) interactions among
individuals of the same species, and (2) interactions among organisms of different species.
Here are a few thought-provoking examples of interactions within species, often called intraspecific
interactions:
 Animal parents sometimes cannibalize their offspring rather than providing them with food.
Under what ecological circumstances would such behavior be advantageous for rates of
population growth?
 In colonial animals such as termites, a single “queen” is the only reproductive individual in the
colony. Non-reproductive “workers” supplying food and “soldiers” fight nest invaders. What
makes up for this loss of reproduction? Does only the colony benefit? Is there a benefit to the
individual worker or soldier?
 Competition among individuals has been convincingly demonstrated for many plant species and
for certain species of animals. What types of animals are commonly used for competition
experiments, and why?
What about our focus this week: interactions among different species, or interspecific interactions?
Clearly, they can be as diverse as life itself. In a temperate forest, for example, one species such as the
gypsy moth may eat leaves from several tree species. But it is by no means the only herbivorous insect on
a given tree species. In that same forest, generalist bird species such as woodpeckers may eat many
different insect species. But a more specialized bird like a bay-breasted warblers tends to eats just one
species and also to forage almost exclusively near the trunks of coniferous trees.
To simplify the multi-species complexity, ecologists often shift their focus away from species, assigning
species to trophic levels: trees are producers, many insects are primary consumers or herbivores, and
insectivorous birds are secondary consumers. By focusing on trophic levels, it is possible to draw certain
general conclusions. For example, forests remain green despite the presence of herbivores. This suggests
that herbivores are limited by predators rather than by food supply. Another approach is to assign species
into guilds. A guild is a group of species with similar requirements for resources and similar strategies for
acquiring those resources. For example, woodpeckers, warblers and nuthatches belong to an
“insectivorous bird” guild. Maples and oaks and elms are all in the “deciduous tree” guild.
Alternately, it is possible to focus on two-species interactions, which are special cases. This week’s
recitation will discuss the two-species relationships summarized in the grid on the next page. In recitation
we will also consider why it is difficult to find examples of intraspecific competition.
The experimental part of today’s lab will focus on mutualisms, specifically how to distinguishing
mutualisms from commensalisms.
We will also observe examples of two types of victim-exploiter relationships: predator-prey and
parasite-host interactions.
Lab 12-3
Effect of Species A on Species B
Effect of
species B on
species A
none
+
–
none
No interaction
Commensalism
Amensalism
+
Commensalism
Mutualism
Victim-exploiter
–
Amensalism
Victim-exploiter
Competition
In this table, two meanings can be assigned to the “+” and “–” signs. First, they are shorthand for how the
population densities of interacting populations change over time as compared to how population densities
change when the interaction is not occurring. Second, they can be used to model the interaction as either a
positive feedback loop or a negative feedback loop.
Mutualism is a “+/+” interaction. In mutualism, the population density of Species A is higher in the
presence of Species B than when it occurs alone, and vice versa.* Mutualisms include familiar
relationships such as plant-pollinator interactions; for example, each species of yucca plant is pollinated
by a single species of yucca moth. Both species are “winners” — if the interaction is occurring, both show
increasing population density over time. The “win/win” nature of mutualisms can be modeled as positive
feedback loops, something we’ll cover in recitation.
Competition is a “–/–” interaction. If an interaction is competitive, the population density of Species A is
lower in the presence of Species B than when it occurs alone, and vice versa.* A textbook example is the
relationship between the barnacle species Balanus balanoides and Chthamalus stellatus, which compete
so strongly that they occupy distinct zones within rocky intertidal habitats (Figure 53.2 in Campbell 6th
edition; Figure 53.13 in Campbell 5th edition). If a competitive interaction is occurring, it is often possible
to identify one species as the “winner” and the other as a “loser” — that is, one species increases its
population density over time while the other decreases. As you’ll learn in lecture, this type of “win/lose”
relationship can also be modeled as a positive feedback loop.
Victim-exploiter relationships are “+/–” interactions. Think of Species A as an exploiter (e.g., a
predator or a parasite), and species B as a victim (e.g., a prey or a host). The population density of
exploiter Species A is higher in the presence of victim Species B compared to the absence of species B.
But the population density of victim Species B is lower in the presence of exploiter Species A compared
to the absence of Species A. An example is the mite Varroa jacobsoni, which can only lives as a parasite
of honeybees, Apis mellifera. The mites live on the bodies of bees, sucking their blood; this shortens adult
bee’s lifespans and reduces growth and survival of larval bees. If a victim-exploiter interaction is
occurring, population densities of both species tend to oscillate in time, and victim-exploiter interactions
can be modeled as negative feedback loops.
Commensalism and amensalism are examples of a non-reciprocal interaction. Today’s experiment will show how
to discern mutualism from commensalism. Amensalisms are rarely discussed in ecology.
*
Lab 12-4
Lab 12, Part 1: Biodiversity of Mutualisms and Commensalisms
You will observe four additional two-species interactions that are either mutualisms or commensalisms.
While exposing you to an interesting type of biodiversity, this is an opportunity to consider the
conceptual and technical challenges involved in studying ecological interactions.
There are four interactions to observe: (1) lichens; (2) Azolla ferns and cyanobacteria in the genus
Anabaena; (3) Rhizobium bacteria that infect and live inside the roots of leguminous plants; and
(4) Trichonympha, a genus of protozoan that lives inside the guts of “wood-eating” termites.
Lichens
Lichens are close associations between two species in partnership, a fungal partner and a photosynthetic
partner.* Biologists recognize ‘species’ of lichens with Latin binomials because the partners live in such
close association that they result, essentially, in the formation of a new organism. The partners often have
a completely different physiology and morphology when they grow independently, compared to when
they grow in partnership.
There is great diversity in the gross morphology of lichens, and several are on display. You should also
examine a prepared slide of a lichen and note the fungal and algal cells in close proximity.
On Worksheet 1, there is space to sketch what you observe under the microscope. Further details about
lichens are available in your textbook. (6th edition, pp. 627-628; 5th ed. pp. 584-585)
Azolla-Anabaena in rice fields
Globally, rice is the most important grain crop. Where rice is cultivated continuously, in flooded fields or
paddies, Azolla ferns are often found floating in the water, providing the benefit of nitrogen fertilization
because the ferns almost always live in associating with a nitrogen-fixing cyanobacteria in the genus
Anabaena. Nitrogen is an essential nutrient — it is needed to make amino acids and proteins — and plant
growth is often limited because not enough is available in a usable form without the help of the nitrogenfixing bacteria.
Take a lobe of one of the upper leaves of an Azolla plant and place it in a drop of water on a microscope
slide. Add a coverslip and gently apply enough pressure with your thumb or pencil eraser to rupture the
leaf. You should be able to see filaments of Anabaena amongst the crushed leaf tissue. If you are not
convinced, or not sure what you are looking at, place a drop of a pure Anabaena culture onto a slide and
cover it with a coverslip. Observe the slide under the microscope.
The cells that you are examining form filaments of well-defined shape and generally predictable size.
Although the cells lack a nucleus and membrane bound organelles (they are prokaryotic cells), they are
nonetheless capable of performing sophisticated metabolic processes and coordinating their activity with
a filament.
Look closely at the filaments of Anabaena and locate cells that are round and thick-walled. These
specialized cells, called heterocycts, are the sites of nitrogen fixation. On Worksheet 1 there is space to
make a labeled sketch of an Anabaena filament with heterocysts. (See your text, figure 27.11 in the 6th
edition; figure 27.9b in the 5th edition.)
*
Usually the photosynthetic partners are green algae, but sometimes they are cyanobacteria.
Lab 12-5
Nitrogen-fixing Rhizobium bacteria
Another economically important plant-microbe interactions is between Rhizobium and leguminous plants
including important crops such as soybeans or alfalfa. This topic is discussed in detail in your textbook
(6th ed. pp. 776-778; 5th ed. pp. 722-724)
Familiarize yourself with the gross morphology of root nodules using the plant specimens provided.
Does the root run through the center or are the nodules to the side or ends of the root? You should also
study the prepared slide of the cross-section of cells within the nodule. Note that the nodule is filled with
masses of stained bacterial cells.
Worksheet 1 provides you with space to sketch root nodules.
Trichonympha and termites
So-called wood-eating termites, pests that are exterminated from
wood-frame homes, lacks the enzyme for digesting cellulose, a
major energy-storing carbohydrate molecule found in wood tissue.
Protozoans in the genus Trichonympha can produce an enzyme for
digesting cellulose, and they are often found dwelling in the
intestines of termites.
To observe Trichonympha, place a termite on a glass slide and
hold it in place by gently grasping its neck with forceps while you
use a scalpel to cut through the abdominal wall in the region of the
last two segments. By gently pulling on this posterior region with
a dissecting needle, the intestine can be withdrawn from the
abdominal cavity.
Agitate the intestine in a drop of 0.6% saline. Then cover it with a coverslip, and observe under the
microscope. Careful adjustment of the light source will enable you to see the characteristic shapes of this
protozoan as well as other species of protozoans, nematodes, and spirochetes.
On Worksheet 1, make a rough sketch of Trichonympha.
Lab 12-6
Lab 12, Part 2: Biodiversity of Victim-Exploiter Interactions
Interactions such as predator-prey and parasite-host interactions are similar, and are sometimes referred to
with the generalized label “victim-exploiter” relationship.
Like mutualisms and commensalisms, predator-prey and parasite-host interactions are a form of
biodiversity. Today we will examine an example of predation amongst microscopic protozoans.
Worksheet 2 provides you with a place to sketch and make notes about your observations.
Predation
Many protozoans prey upon other small organisms — bacteria, algae, or other protozoa. In this exercise,
you will examine the predation of one protozoan on another.
1. Prepare and examine separate wet mounts of the two protozoans — Stentor and Chilomonas — by
adding a drop of the cultures to a slide and gently lowering a coverslip (may not even be necessary).
Be sure that you can recognize the differences between the two species.
Note: To dislodge Stentor in order to sample it, agitate the contents of the jar with a pipette.
Leave the jar near a lamp (75 watts) for five minutes, and Stentor will move away from the light
(exhibiting negative phototaxis). Now you can easily pipette the protozoans from their
concentrated gathering on the opposite site of the jar.
2. WAIT to make a wet mount of the two organisms together. It is important to make your observations
immediately after preparing the slide. (Why?) Isolate a few Stentor on a microscope slide and then
add a drop from the Chilomonas culture (making sure not to mix up the pipettors). Gently lower a
coverslip (again, it may not be necessary).
Observe carefully, using appropriate magnification(s) with your compound light microscope. Sketch and
describe what you see on Worksheet 2. Which organism is the predator? Which is the prey?
Parasitism
In a relationship of parasitism, one species, the parasite does not kill and consume its host. Rather, it lives
on or in its host and consume some of its tissue. When the host is readily available, individual parasitic
organisms benefit and the parasite population’s density increases. Although the host is not killed by the
parasite immediately, it is usually harmed in some way that reduces the population density of the host
population.
There are parasites among bacteria, protozoans, fungi and “worms” from several animal phyla, especially
Phylum Platyhelminthes and Phylum Nematoda. Viruses are obligate parasites. Some parasites live on,
rather than inside of, their hosts. Today we will examine several parasites that live inside of humans and
other animals, including examples of both blood parasites and gut parasites.
1. Blood parasites
The genus Plasmodium causes malaria in human beings. This parasitic protozoan has no means of
locomotion, and a complex life cycle that depends on two hosts: mosquitoes and humans.
Infected mosquitoes that bite humans inject asexual cells that travel through the bloodstream to the liver
and eventually infect red blood cells. Examine a slide of a blood smear from a human infected with
Lab 12-7
Plasmodium and note the parasites among the red blood cells. Plasmodium can achieve very high
population densities in the bloodstream because it undergoes asexual (mitotic) reproduction there. This
proliferation results in the symptoms of malaria, and can be fatal.
Before the infected human dies, the Plasmodium may also undergo sexual (meiotic) reproduction. This
results in gametes forming in the bloodstream. Gametes may be ingested by mosquitoes that bite an
infected human. The final phase in the life cycle, fusion of gametes, is completed after a mosquito bites
an infected human and the gametes fuse in the mosquito gut.
By what method(s) has malaria usually been controlled? What are the costs and benefits of various
control strategies?
2. Gut parasites
Various types of “worms” — multicellular invertebrate animals — infect humans the human digestive
tract. Worms in the Phylum Platyhelminthes are likely to have complex life cycles that depend on
interaction with one or more hosts. They are also likely to be modified morphologically to suit to their
parasitic way of life. Taenia, like most parasites, shows a reduction or loss of some systems and
corresponding increase in reproductive capabilities.
Examine Taenia, a tapeworm, from the prepared slide and preserved specimen. Tapeworms are covered
with a specialized epidermis that protects them from the host’s digestive juices while also permitting
absorption of nutrients. They have no mouth or digestive cavity and absorb food through the body wall.
They do have a small knob-like head, the scolex, with muscular suckers and a circle of hooks on the
elevated tip. A short neck joins the scolex to the body, which consists of up to 1000 sections or
proglottids. New proglottids are continually formed by budding in the neck and are forced posteriorly.
Each proglottid contains muscles, excretory canals, nerve cords and a complete set of male and female
reproductive organs. Self-fertilization in one or separate proglottids, or cross-fertilization between two
worms in the same host are all possible. As fertile proglottids mature, they are cast loose and shed in the
host feces. If fertile eggs from such proglottids are ingested by an intermediate host, e.g. a pig or cow, the
egg coverings are digested off and six-hooked larvae are released. The larvae bore into blood vessels and
are carried by blood or lymph to muscle tissues where they encyst. Tapeworms from pigs or cows are the
ones that humans can get from eating undercooked meat. You don’t actually eat the worm itself; instead
you consume a dormant, encysted stage of the tapeworm’s life cycle. When raw or improperly cooked
pork or beef containing these cysts is eaten, the larvae develop a scolex, attach to the second host’s
intestine and grow into new tapeworms.
Trichinella are roundworms in the Phylum Nematoda that cause the disease trichinosis. Examine
preserved, infected meat or a slide of infected meat and locate the cysts, which contain the juvenile
worms. When humans eat infected meat, the juvenile worms are released into the digestive tract and
mature to reproduce sexually, again producing juvenile worms that infect human muscle. A human with
trichinosis has muscular aches and pains that can lead to death if respiratory muscles fail. Trichinosis can
be prevented by thoroughly cooking all pork and pork products.
3. External Parasites
Many parasites live on their hosts, rather than inside their guts or in their blood. Think of examples of
external parasites that affect humans, and note them at the bottom of Worksheet 2.
Lab 12-8
Name_____________________________
Lab time/day/instructor____________________
Biology 2003 Biodiversity Lab, due at the end of lab today
Fall 2003
Lab 12, Worksheet 1: Biodiversity of mutualisms and commensalisms
Make your sketches below and answer the accompanying question on a separate, attached page .
Examine the prepared slide of a cross-section through a
lichen and sketch the layering of algal and fungal cells.
Sketch the gross anatomy of root
nodules along the roots of an
uprooted plant.
After observing Anabaena filaments in pure culture and
growing insideAzolla ferns, make a labeled sketch of
filament with heterocysts.
Sketch what you observed when
examining a prepared slide showing
cross-section through root nodule.
Make a rough sketch of
Trichonympha found in the gut of
termites.
Written assignment: For two of the examples above, design an experiment that could be used to distinguish whether
the interaction is mutualistic or commensal. Be sure to discuss both the types of data that would be required to
demonstrate either mutualism or commensalism and the technical feasibility of your proposed experiment. Answer on a
separate page with your name on it and attach to this worksheet (ONE page maximum!).
Lab 12-9
Name_____________________________
Lab time/day/instructor____________________
Biology 2003 Biodiversity Lab, due at the end of lab today
Lab 12, Worksheet 1: Biodiversity of mutualisms and commensalisms
Written Assignment from previous page.
Lab 12-10
Fall 2003
Name ______________________________ Section day/time/instructor _________________________
Biology 2003
Fall 2003
Lab 12, Worksheet 2: Biodiversity of victim-exploiter interatctions.
Due at the END OF LAB TODAY. Make sketches and answer the questions.
Predation
Stentor
Chilomonas
Describe your observations when the two organisms are together. Which is the prey? Which is the predator?
Blood Parasites
Plasmodium infecting human blood cells
Gut parasites
Taenia (label: proglottids, neck, scolex, hooks)
Trichinella cysts
External parasites
Give one or more examples of parasites that live on humans, rather than inside their blood or digestive tract.
Lab 12-11