Designing and conducting an experiment around something as complex as ecology might sound overwhelming. So I decided to share with you a serie of article to explain the different steps to conduct an experiment in marine ecology with a concrete example (i.e. one of my own experiment). Last time, I told you about what to do before conducting an experiment. If you haven’t already read it, check this article first!
#2: Conducting your experiment to prove or disprove the hypothesis
#3: Collecting samples after an experiment
#2 Conducting your experiment to prove or disprove the hypothesis
2.1. Identify your independent and dependent variables
A controlled scientific experiment is set up to test whether a variable has a direct effect on another. A variable is something that may vary such as number of individuals, body size or life expectancy.
The independent variable is commonly known as the cause, while the dependent variable is the effect.
For instance, in the statement A causes B, A is the independent variable and B is the dependent.
INDEPENDENT variable = CAUSE
DEPENDENT variable = EFFECT
A controlled experiment can only manipulate one variable at a time. If more than one variable is manipulated, it is impossible to say for certain which caused the end result and the experiment is invalidated. That is why you need to plan to have controls and to measure all variables possible while the experiment is running!
In our case study
Independent and dependent variables
In this study:
Independent variables: presence or absence of non-native worms and fish
Dependent variables: bacteria, big and small invertebrates quantity and diversity (number of species that we can find at the end)
In other words: does the presence of non-native worms cause differences in communities (of big and small invertebrates and of bacteria)? And does the presence of predators cause differences in communities (of big and small invertebrates and of bacteria)?
In this study, we make sure that all environmental variables (sediment characteristics, temperature, salinity,…) are the same for all aquariums.
2.2. Setting up your experiment
To set up an experiment in marine ecology, you need to build the system (in our case study, we have aquariums with a flow-through system) and to collect organisms you want to include.
We have started by putting the same amount of sediment as well as the same amount and sizes of invertebrates (clams, mosquito larvaes, worms and snails) in each aquarium, to recreate natural conditions as much as possible. This allowed us to have exactly the same conditions at the starting point.
Then, in order to see the impact of the worms, we needed to add the two different non-native worms in amounts reflecting what we can find in Åland Islands. The different aquariums therefore present either (1) no added worms (control), (2) 40 added worms of Marenzelleria spp., or (3) 40 added worms of Boccardiella ligerica.
Finally, to understand the implications for predators, a fish, the sand goby (Pomatoschistus minutus), has been added in half of the aquariums. This is one of the most common small predators in the Baltic Sea.
SPECIES PRESENT IN OUR AQUARIA
This small fish is one of the most common small predator in the Baltic Sea!
The worm Marenzelleria spp. is one of the most successful non-native species in the Baltic Sea. First observed there in 1985, they quickly colonized the entire sea, occupying a dominant position in the ecosystem.
The non-native worm Boccardiella ligerica was first found in southwestern Finland in 1963. They are small and live in a tube constructed of mucus and sediment.
Most common invertebrates in the Baltic Sea
Mosquito larvaes (Chironomidae)
Snails (Hydrobia sp.)
2.3. Checking your experiment
It is important to check up your experiment while it is running to make sure that all variables are the same in all aquariums., except for the one we are interested in.
In our case study
Checking an aquarium experiment
While an experiment is running, it is important to check if everything is fine by measuring conditions in the aquariums (such as temperature and salinity). You want all the aquariums to have the same conditions to be able to compare them at the end.
[q multiple_choice=”true”] What is an independent variable?
[c*] a cause
[f] Right! The independent variable is commonly known as the cause!
[c] an effect
[f] False, try again 🙂
[q multiple_choice=”true”] What is a dependent variable?
[c] a cause
[f] False, try again 🙂
[c*] an effect
[f] Right! If the independent variable is commonly known as the cause, the dependent variable is the effect. For instance, in the statement A causes B, A is the independent variable and B is the dependent.
[q multiple_choice=”true”] An essential thing to think about while planning an experiment?
[c*] Randomizing your treatments
[f] Randomizing is ESSENTIAL while planning an experiment. If you don’t randomize your treatment you are not gonna be able to conclude anything at the end of your experiment. Think about it before, when you are planning!
[c] Where you are gonna leave for vacations after ending your experiment
[f] This can be a good idea to plan a holiday because it can be quite an intensive work to set up an experiment but maybe try another answer 🙂
Which species is the most successful non-native worm in the Baltic Sea?
[c] Boccardiella ligerica
[f] The worm Boccardiella ligerica is also a non-native species in the Baltic Sea but is less successful in the Baltic Sea than Marenzelleria spp.
[c*] Marenzelleria spp.
[f] GOOD JOB!! The worm Marenzelleria spp. is one of the most successful non-native species in the Baltic Sea. First observed in the Baltic Sea in 1985, they quickly colonized the entire sea, occupying a dominant position in the communities.
[q] Which predator species was added in our aquariums?
[c*] Show me the answer
[f] The sand goby (Pomatoschistus minutus) which is one of the most common small predator in the Baltic Sea!
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