Mini Ecosystem inside a Jar
Lab Report
By Student X, B2
Question/Purpose:
Does the amount of water in the jar/ecosystem affect the organisms living inside? (And if so,
what kind of effect does it have on them?)
Hypothesis:
If we only add half the amount of water we put in the control jar to the experimental jar, then the
ecosystem will be less healthy (for example, the plants, fish, and snails might die earlier) because
organisms need water to survive. Also, since the water is what they’re living in, more water
means more room to live.
Materials:
Two 1-liter glass jars with lids
One guppy per jar
Two snails per jar
Aquatic plants (equal amount in both jars)
200 mL of gravel total (1 inch layer on bottom of each jar)
Water – almost up to the top of the jar for the control group, and about halfway full for the
experimental
Variables:
Constants:
Guppy x1
Snail x2
Aquatic plants
Gravel
Manipulated:
Water (specifically, the amount of water – half full in experimental, full in control)
Method/Procedure:
1) Set up the control jar by filling an empty jar with water and adding a layer of gravel on the
bottom, one guppy fish, and two snails. “Plant” aquatic vegetation by putting the plant’s roots
into the gravel.
2) Label the jar “Control”.
3) Set up the experimental jar by adding everything exactly in Step 1, except only put in half the
water.
4) Label this jar “Experimental”.
5) Cap both jars and put them on the windowsill (to get sunlight) right next to each other.
6) Do NOT feed the fish!
7) Wait for about two to three weeks, periodically making observations about what is happening
inside the jars.
Observations:
9/22
Control:
- water in jar filled almost to the top; is completely clear, nothing in it but a few leaves from
the plants
o water is clean
- guppy is orange and white, about 3 cm long with a “fan” tail; swims around a lot
- layer of gravel on bottom of jar is clean
- snails have dark black/brown shells and don’t move, just stay on the floor of the jar
o snails don’t move that much
- plants are green with a tiny bit of brown around the edges of the leaves
- plants are already uprooted from the gravel; roots are dangling upside down in the water
9/24
Control:
- fish still swimming around
o fish is alive
- slightly larger portion of the plants’ leaves are browning
o not enough nutrients/sunlight?
o leaves dying
- more pieces of plant on the gravel/floating in the water
- snails stuck to side of jar
o are alive and actually did move
Experimental:
- fish is orange, a tiny bit longer/bigger than control jar fish
- plant’s leaves also pretty green
o in good condition
- top of plant is above surface of the water
- everything else is the same as the control jar (fish is swimming around, snails are on wall, etc.)
10/6
Control:
- fish still swimming around
o no change, still alive
- snails lying on jar floor like they were when first put in
o came down from wall, maybe dead?
o maybe just resting or eating/decomposing the leaves on the gravel
- clumps of brown leaves on jar bottom
o more plant matter dying
- water is slightly green but only slightly
Experimental:
- droplets of water on sides of jar not covered with water (at the top)
o condensation, water evaporating
o water level might decrease over time
- fish nowhere to be found
- several gray, dusty-looking chunks of something are scattered very far apart from each other
on the gravel
o fish died (and started decomposing, which is why the chunks are gray); chunks are parts
of its skeleton/corpse
o seems as if fish ‘exploded’ because of widely scattered remains
-
plants also turning brown
10/10
Control:
- fish still swimming around
o still alive
- both snail shells (still laying on gravel) changed color, dulled and went from dark/black to
yellowish brown
o look empty, snails probably died
- water remains relatively clear and not murky
- a lot of brown leaves floating in water/on gravel
o plants are dying?
o still not enough nutrients?
Experimental:
- gray chunks are mostly gone, just bundles of gray dust remaining
o decomposed further
- more condensation
- snail seem dead because aren’t moving/changing location
- everything else is the same as 10/6
10/14
Control:
- fish still swimming
o alive
- snails still seem dead and are completely motionless
-
blanket of dead leaves on gravel
water has leaves in it and bits of dusty-looking stuff, but otherwise clean
Experimental:
- gray chunks pretty much gone
- snails still haven’t shown signs of movement
- thin coating of muddy-green algae on wall of jar
- dead leaves, similar to control jar
- water is cloudier and turning greenish brown
o leaves and algae are making it greener
o bacteria growing in the water, causing it to become filthy?
10/22
Control:
- fish still swimming around
o alive!
- snails are the same
- plant is the same; looks alive, but browning
Experimental:
- everything is the same, although the water appears a lot dirtier/foggier and more green
o bacteria multiplying?
o more algae getting mixed into water
- algae layer on jar wall seems thicker
Sketches & Diagrams:
9/22
Control:
9/24
Control:
Experimental:
10/6
Control:
Experimental:
10/10
Control:
Experimental:
10/14:
Control:
Experimental:
10/22:
Control:
Experimental:
Analysis:
Our two jars started out pretty much the same; the fish were both healthy and swimming around,
and the snails showed signs of movement by changing their locations and clinging to the jar wall.
The water in both jars was pretty clear at first, and the plants had a couple dying/brown leaves,
but they were otherwise healthy. After about a week, a few changes started happening. The
control jar snails came down from the wall and were found lying on the gravel (10/6). I think that
they were dead by then, because they stayed in the same position for the rest of the experiment.
Also, by the time we next observed the control jar, the snail shells in there had gone from dark
brown to yellow, and they looked empty. Because I observed that once they died, the pile of dead
leaves at the bottom of the jar built up faster, I believe that they break down dead plant material
and so play the role of decomposer in our jar ecosystem.
The plants in both jars started dying very early on (9/24), with their leaves turning brown and
falling onto the gravel/floating in the water. There was no significant difference between the
amounts of dead leaves in the control group versus the experimental group. From the fact that the
health of the aquatic plants deteriorated fairly quickly, I can infer that living in a jar is not the best
for a plant. I think that they didn’t have enough nutrients, since we put gravel in the jar, not soil
(and there aren’t any nutrients in gravel). This shows that more rich and fertile soil is a necessary
abiotic factor for the specific species of aquatic plant we put in our jars.
The inferences and observations above don’t indicate that less water means organisms will die
earlier; however, I can infer that my hypothesis is still more-or-less true, because the fish in the
control jar outlived the experimental one by more than a week. I’m not sure why, but I can infer
that the cause was related to the cleanness of the water – I noticed that the water in the
experimental group was considerably dirtier than the water in the control group. I deduced that
this was due to the fact that the pollutants in the control group (like fish waste, algae, etc.) were
mixed in with twice the amount of water, so it was diluted, but the experimental group only had
half the water, so it would have been twice as filthy.
Mucky water means that the fish was probably harmed by the waste in the water, or even
poisoned by it. I think this shows that it isn’t the water itself that radically affects the organisms,
it’s the space provided by the water (because the fish, plants, and snails live in the water).
Conclusion + Future/Application:
In conclusion, I learned that the amount of water in an ecosystem probably does affect the
organisms living there, as the water in the experimental jar was much dirtier than the control jar’s,
and foul water means poor living conditions. Furthermore, the fish in the control jar lived longer
than the one in the experimental. Actually, it was the decrease in the space provided by the water
that really affected the organisms. This tied in with our lesson on limiting factors for population,
because space was one of the factors, and it evidently limited the population of fish in our
experimental group. Therefore, my hypothesis was correct.
In the future, it might be useful to try this experiment again, since we only did it once so there’s
no guarantee that we managed to isolate the variable. Something else could have been affecting
the organisms in the experimental jar other than the water; for example, maybe the fish we put in
the experimental jar was sick (and the control jar fish wasn’t), so it died earlier. Also, if we did
the experiment again, I would do more research on the amount of water’s role in an ecosystem,
because I don’t think I have enough background knowledge of that to make better inferences on
exactly why less water caused the ecosystem to be less healthy.
I learned a lot about how to use the scientific method to conduct an experiment effectively, and I
now more thoroughly understand the concept of isolating just one variable so you can tell just
what was affecting the results of your experiment. Before this lab, I never realized that you could
make an ecosystem in a jar with only three organisms (fish, snails, and plants), but now I know
that these things, combined with the abiotic factors that every ecosystem needs (water, sunlight,
etc.) do make one.