Homework 2 – SMALL MAMMALS’ HABITAT CHOICE:
Generate Hypotheses/Predictions
(To be completed during lab)
Name: _KEY________________________
In a survey of the published literature, we have found background information about the
mammals and successional fields. Table 1 provides a summary of some important traits that
distinguish the three main groups of small mammals in the successional fields.
Table 1. Trophic level, dominant food items, and agility of three majors types of mammals in
abandoned fields at Phillips Tract.
Mammal type
Shrews
Voles
Mice
Trophic Level
Diet
Carnivore insects/worms/small mammals
Herbivore
plants
Omnivore
plants, insects
Agility
Low
Low
High
Note how the types of mammals differ in trophic level, diet and agility (ease and speed of
movement in vegetation).
We also know that the two successional fields differ in amount of and density of vegetation
(plant biomass) and arthropod abundance.
Table 2. Amount of plant biomass, density of vegetation, and abundance of arthropods in
successional fields of two ages.
1-year field
5-year field
Amount of biomass (g)
high
low
Number of plant stems/m2
high
low
Number of arthropods
low
high
Note how the fields differ in amount of potential food for mammals, as well as density of
vegetation in which they must move and can hide from predators.
We are interested in possible factors that determine the distribution of small mammals in these
two successional fields (1-year and 5-year). Is each type of small mammal equally abundant in
both successional fields? Or does each type have a preferred field? If so, what factor(s)
determine their habitat choice?
Develop two formal hypotheses/predictions that address this question:
What are two factors that determine habitat choice of small mammals in successional
fields? (1-year vs. 5-year)
Use the Tables above for mammal traits and successional fields to develop your speculation.
Factor 1 __ ease of escape from predators
Factor 2 __ food availability/preference
Hypothesis/Prediction 1
Incorporate your first factor into a complete sentence that answers the question posed.
Convert your sentence to a formal hypothesis:
If ease of escape from predators determines habitat choice of small mammals,
Now develop a prediction of the hypothesis. Consider what is a logical consequence of that
hypothesis? If the hypothesis is correct, what would you predict about the distribution of the
type of mammal in the two fields? (HINT: You may need to make a separate prediction for each
mammal type).
then 1) shrew and vole density will be greater in the successional plot with greater plant
biomass (or stem density);
2) mice density will be equal in the two successional plots.
1) Is the experimental design contained in the hypothesis or the prediction? I.e.,where are
located your independent variable (something with a natural categorization) and
dependent variable (operationally, what you will measure) in prediction
2) Circle your independent variable (successional plots; underline your dependent variable
(shrew, mice, and vole density)
3) Is the independent variable continuous or categorical? categorical
4) How many treatments are in your experimental design? Two (yr 1 and yr 5)
Hypothesis/Prediction 2
If food availability/preference influences habitat choice,
Then 1) shrew abundance will be greater with greater arthropod abundance (or biomass),
2) vole abundance will be greater with greater plant biomass, and
3) mice abundance will be more independent of arthropod abundance (or biomass) and
plant biomass (This omnivore can switch food choices).
What is the unit of measurement for your dependent (response) variable? One small mammal
What represents one sampling unit? One successional field
To do a statistical test to decide if your prediction is correct, it is desirable to get replicates of
your sampling unit. Ideally you would gather data in multiple fields of each age. These repeated
samples are called replicates.
How many replicates of the sampling unit of each treatment would you plan to collect? Five??