Download Resource Partitioning in Ants (Lab Write

Resource Partitioning in Ants
(Lab Write-Up = 20 Points)
Introduction
As we have discussed in class, each organism has a niche that can be described using both
biotic and abiotic factors which impact the organism. If we look at different organisms, they often
will have different niches because each species utilizes these biotic and abiotic factors in different
ways. One way we can assess these differences is to calculate the amount of niche overlap, or the
degree to which two (or more) species share a given niche factor (such as food types). As we’ll see
later in the semester, as overlap increases the importance of competition increases as well. This
means that we can use niche overlap values to assess the likelihood that two species will compete
with each other…at least within the niche dimensions we measure!
Ants are excellent organisms in which to examine resource partitioning for several reasons.
They are both abundant and speciose in most habitats across the U.S.; most areas are likely to
contain at least 6 coexisting species. Temperate species are also primarily scavengers, feeding on
whatever food is available.
General Methods
In this lab we will work in the grassy and wooded areas just west of the TTU baseball/
softball field. We’ll have a grid of Petri dishes laid out that will contain one of four food types: (A)
cat food; (B) cat food mixed with artificial maple syrup; (C) grape nuts cereal mixed with vegetable
oil; and (D) sliced green grapes. This provides some food sources that are mainly proteins (i.e., cat
food), some that are mainly carbohydrates (grapes), and some that are a mixture of these two. We
will have 16 dishes in the open, grassy habitat and 8 in the shaded, woody habitat. There will be
equal numbers of each food type in each habitat.
We will visit the sites during lab time and then once outside of class. During each visit you
will need to make note of the following information:
(1) number of individuals of each ant “species” at each plate;
(2) number of individuals of other animals (bees, beetles, snails, etc.) at each plate;
(3) an estimate of the amount of food left;
(4) weather conditions;
(5) any interesting behavioral observations (fighting, predation events, etc.).
We will define the ant “species” very generally, using size and color of the ants. We’ll determine
this once we get to the site, but in the past we’ve had at least large red ants, large black ants, small
red ants, and small black ants.
Data Analysis
Using the counts for the different ant “species,” you’ll calculate several overlap values. For each,
the higher the number the greater the niches overlap, and thus the greater the chance that those two
species will compete:
(A) spatial overlap (SO) is given by the formula:
,
where a = # dishes containing species a, b = # dishes containing species b, and c = # of dishes
containing both species a and species b. SO values can range from zero (no overlap) to one
(complete, or 100%, overlap).
(B) food overlap (FO) is given by the formula:
,
where pif = the proportion of food type f used by species i, and pjf = the proportion of food type f
used by species j. Note that the double lines means to take the absolute value; that is, just ignore
any negative signs you get. FO values also range from zero (no overlap) to one (complete overlap).
(C) combined niche overlap (NO) is given by multiplying these two previous values together:
NO = SO * FO. As with FO and SO, NO can range from zero (no overlap in space or food use)
to one (complete overlap of food and space use).
We will gather the data as a class; once that is done, I will send you an example calculation for each
of these as a guide. You will need to do each overlap calculation for each pair of ant
“species;” this is very tedious, but tells us a lot about how these ants compete with each other! For
example, if we have large red ants (LRA), small red ants (SRA), large black ants (LBA), and small
black ants (SBA), you will need to do the following comparisons: (1) LRA vs. SRA; (2) LRA vs.
LBA; (3) LRA vs. SBA; (4) SRA vs. LBA; (5) SRA vs. SBA; and (6) LBA vs. SBA. You do not
need to compute overlap values for any non-ant species!!
Lab Write-Up
This lab will be in the short report format (Results and Discussion) like the physiological ecology
lab. You should include the raw class data (#s of individuals at each plate, # of species at each
plate, weather conditions) and the calculations for the three overlap values for each pair of ant
species. In the Discussion, you’ll want to write about the implications of these results, namely what
they say about the likelihood of competition among each pair of ants. You should also consider the
following in your Discussion:
(1) Does there seem to be any time differences? Do certain ants appear at specific times? Are
some ants more numerous at certain times of the day? How does the amount or quality of
food remaining affect overlap or abundance of ants?
(2) Is there a difference between ant species or numbers found in the open grassy area and the
shaded woody area?
(3) What other animals are found at the traps? Does this differ among food types or time of
day? Do ant abundances seem to be affected by the presence of any of these other species?
(4) If the weather varies, does this seem to affect ant abundance or the types of species you see
at the traps?
Note that there are no statistical tests for this lab! You’ll have some concrete data to back up your
conclusions using the overlap values, but for the rest you’ll have to make informed opinions. Be
sure to include any interesting observations in the Discussion section, as appropriate.