ICE 3: The Ideal Free Distribution

ICE 3: The Ideal Free Distribution

The ideal free distribution makes five assumptions:

1       1. Adopting “idea free” tactics is adoptive.

2       2. All individuals participating in the “game” have equal competitive ability.

3       3. There are no barriers to movement among patches.

4       4. There are no costs to moving around patches.

5       5. More competition in a patch means fewer resources for an individual.

From these assumptions, I can make two predictions.  The first states that the number of competitors in a patch is proportional to patch quality.  The second is that all competitors experience equal fitness gains regardless of which patch they occupy.

There are situations in which the assumptions made by ideal free distribution could be false.  If the first prediction made is false, the new situation would be the number of competitors in a patch is NOT proportional to patch quality.  This could occur in seasonal drought and would challenge assumptions 2, 4, and 5.  From assumption 2, I can now assume that individuals have unequal competitive ability.  Many individuals would have to compete for the limited and depleting resources.  Without proper hydration, individuals in the patch would grow weak from fatigue resulting in a decrease in competitive ability.  We can assume that as the drought ends, resources will become more available.  Only the strongest that survived the drought would be able to gain quick access to these resources, while the weak would need assistance or adapt quickly as stated by assumption 1.  Competition will decrease as patch resources increase or the number of individuals decreases due to death.  Conversely, few individuals in a rich patch, one that was not heavily affected by the drought, would not have to compete at all for resources, but as resources are depleted, competition will increase.  This also goes along with falsifying assumption 5.  Less competition means more resources.  Equilibrium could be reached when taking into account assumption 3 (no barriers), but ideal free distribution states that there are no costs to moving (assumption 4).  If patches are not proportional to competition, then moving from a currently occupied patch may be costly for the individual.    Without any knowledge of how the drought affected adjacent patches, an individual may migrate to a patch of poorer quality or one of higher competition. But, as I know, the higher the cost, the higher the potential reward.  An individual could move to a patch with no competition and high resources and establish itself before equilibrium is reached and the drought ends.

If the second prediction is false, then the new prediction is that competitors will not have equal fitness regardless of which patch they occupy.  This situation then makes assumptions 1, 3, and 4 false.  A decrease in fitness could be due to an inability to adapt to the environment.  This idea contradicts assumption 1 which states that these tactics are adoptive.  Without the ability to adapt, an individual might experience direct effects to high competition and few resources gained.  In essence, the fitness would decrease, because these individuals are unlikely to survive on a short term scale.  These individuals have a higher chance of survival if they Ire to move betIen patches, but if you are unable to adapt, the cost of moving increases, falsifying assumption 4.  If an individual is unable to adapt at a current location, the probability that it would be successful in another is low.  Adding that to the allocation of energy needed to move, the individual is unlikely to flourish.  Adding a barrier (assumption 3) presents another problem besides not being able to move betIen patches.  This could disrupt the ratio of males to females.  Without migration, the occupants of a patch would remain static.  Competition within a patch would increase, and feIr individuals would gain reproductive success.  All in all, I would observe a sharp decline in fitness of the competitors.

I can generate hypothesis by challenging the assumptions made by ideal free distribution.

Hypothesis 1:
                Assumptions Challenged: (1) Adopting “ideal free” tactics is adaptive, and (4) there are no costs to moving among patches.

There is a cost to moving if an individual is unable to adapt.  I would predict that there is a high cost to individuals who are unable to adapt.  They must base their decision to move on their successes in the current patch.  Conversely, individuals who are able to adapt have a high chance of doing well in either patch, and have no reason to move.  Under these circumstances where competitive and adaptive individuals stay at the current patch, the focal individual (unable to adapt) stands a higher chance of survival by moving.  However, if many individuals are moving to the current patch, assuming again that competitors and adapters stay, then the focal individual has a higher chance of staying at the current patch where it will be competing with individuals who are also unable to adapt.  Any movement between patches by competitive and adaptive individuals decreases the chances that the focal individual will survive.

Hypothesis 2:

        Assumptions Challenged: (2) All individuals participating in the “game” have equal competitive ability and (3) there are no barriers to movement among patches and (5) more competitors in a patch means fewer resources for the individual.

Individuals whose movement between patches is restricted by a barrier will experience higher competition.  I will be conducting an experiment based on this hypothesis.  This hypothesis can be easily viewed from a habitat fragmentation perspective.  Due to anthropogenic effects, many habitats (forests, reef, etc.) have been divided.  A prime example is deforestation in which the loss of trees separates once connected patches.  Many of these patches could be saved if connected by a corridor: a path that has not lost many of its ecosystem resources.  However, it is not always easy or even feasible for animals of one patch migrating to another.  Let us say that two individuals of the same species engage in aggressive behaviors.  Losers might experience injury during a competition, and moving to another location would be beneficial.  But if there is a barrier restricting movement, then individuals will have to face the same competitors, increasing the likelihood of injury and decreasing competitive ability.  Conversely, winners will likely continue to win and gain resources.  In this way, they do not have to compete as much (or at all) as other individuals within the path.

From this, I will conduct an experiment.  I will control for variables made in prediction one.  The number of individuals in a patch will be proportional to the resources available.   This can easily be seen in nature with patch fragmentation due to deforestation or other anthropogenic effects on habitat.  The patches will be separated by patch fragmentation and connected by a small corridor located at the center of each patch. 

For this experiment, I will be using ordinary field mice taken from fragmented habitat.  To eliminate parental effects, I will use the F₂ generation for study.  Only males will be used.  This will eliminate fitness from prediction 2 so that the focus of the experiment relies on competition between individuals.  Below you will see four diagrams denoting each manipulation.  One square is representative of the resources allotted to one individual.  A corridor connects two patches, one with a high number of competitors (18) and one with a low number of competitors (6).  By this standard we can see that patch one is three times as large as patch two.  A camera will be situated in such a way that each treatment can be seen and any movement within the corridor can be recorded.  All mice will be of the same size (within 2 g of each other), the same muscle density, and the same age.  I will also record the time spent eating for each mouse and the time spent engaging other mice in aggressive behavior for resources.  The mice will be watched for eight thirty-minute increments: two in the morning, two in the afternoon, two in the evening, and two at midnight.  This will be repeated for a full workweek.  Each test will be run three times, after which the data will be analyzed.

 

All individuals in treatment one have equal opportunity to access resources.  Their competitive ability will begin at the same rate as denoted earlier.  I would expect some mice to experience a decrease in competition if they are unable to cross the corridor to the other patch.  This would be a result of continued competition among the same individuals, and after time a hierarchy should establish.  Any individuals who sustain injury will be noted and each mouse will be analyzed for health after the experiment.  My expectation for Treatment One can be seen in Graph 1.

  

 

 

In Treatment 2, competitors in patch one will be manipulated for competitive ability. The black X represents an individual that will be manipulated for each run.  Note: the superscript denotes how many individuals will be manipulated for each run of the treatment.  The mice will be chosen at random, but for aesthetics, I have placed the X in the middle.  During the first run, one mouse will be manipulated.  During the second, two, and during the third, three mice will be manipulated.  Manipulation will include grinding teeth down to a dull edge and clipping the tail.  This will decrease their competitive ability as proposed by assumption 2.  From this, I can see how assumptions 3 and 5 are affected.  Do the barriers restrict their movement?  Do they lose or gain resources by staying in the current patch or migrating to the other?

 

From this, I would expect the mouse in the first run to attempt to cross the corridor.  He will experience high competition and would fare much better in patch 2, as there are fewer individuals to compete with.  If he is unable to cross, I would expect him to gain fewer resources and as a result suffer health consequences.  During the second run, I would expect at least on individual to cross.  If the second were to watch the first, it might notice that, if it does not return, it is experiencing higher resource availability in the second patch.  This would prompt the second individual to attempt to cross the corridor.  If both manipulated individuals arrive at patch 2, the will experience higher competition with each other.  They are more likely to interact within a patch containing fewer individuals.  However, they will not experience a decrease in competition with individuals of higher competitive ability because of the control of prediction 1.  The resources within the patch are only able to support 9, not 11 individuals.  They still have an equal chance of competing with a competitive individual.  I would expect the same in run 3 as I did in run 2.  The only difference is that after time, it might be more beneficial for at least one manipulated individual to return to patch 1.  He would more likely be able to sneak resources from competitive individuals than have to compete for them with his fellow manipulated individuals.  My predictions for Treatment 2 can be seen in Graph 2.

 

 

 

 

 

Treatment 3 will have the same manipulations as Treatment 2, except the individuals’ manipulated will begin in Patch 2.  I expect run 1 to stay the same as competition between fewer individuals is more beneficial.  It will look similar to that of Graph 1.  In the second run, I would expect at least one individual to venture across the corridor to patch 1.  This individual will not have to compete with the other manipulated individual and might be able to sneak resources.  In the third run, I expect something similar to runs 2 and 3 of Treatment 2.  That is, when one individual crosses, the other might see that he doesn’t return and assume success.  Once the third follows the second (the first has remained in patch 2), he might be wary of the high competition and try to venture back to patch 2.  The expected outcomes can be seen in Graph 3.

 

Treatment 4 combines Treatments 2 and 3 in that both patches have manipulated individuals.  The number of individuals manipulated in each patch is equivalent and increases with each run.  This makes the number of manipulated individuals in each patch disproportionate; however, I believe it is a fair treatment as it will be comparable to the two previous treatments.  If we combine the predictions from Treatments 2 and 3, I will see one individual cross from patch one to patch two.  During the second run, I will see two individuals cross from patch one to patch two and one individual cross from patch two to patch one.  In the third run, I will see three individuals cross from patch one to patch two then one cross back to patch one, and two individuals cross from patch two to patch one and one cross back to patch two.  These predictions run with the assumptions made in Treatments 2 and 3.  These predictions can be seen in Graph 4.

The graphs above only predict the movement of manipulated individuals.  It would be incorrect of me to assume that no competitive individuals also cross; however, this experiment was designed to see the effects of decreasing individual competitive ability to see how they respond.  To accurately watch manipulated individuals, I will place a large blue dot on their back.  It would also be incorrect to assume that individuals move in such an orderly fashion, but in order to accurately see my predictions, I made the graphs to portray movement.  It could reasonably occur at any point along the time axis; although, I do not think more than one individual will occupy the corridor due to size constraints of the corridor.  Individual assessments before and after the experiment will be conducted to gauge the competitive abilities of each rat, manipulated and competitive.

From this experiment on habitat fragmentation, we can conclude that assumption five is correct.  If there are more competitors in a patch, then there are fewer resources available to an individual.  This compounds while taking into account assumption 2.  As resources decline for the less competitive, the more divided individuals are in their competitive ability.  The weak (manipulated) will experience a decrease in ability and access fewer resources while the strong (competitive) experience the reverse.  I would also expect as the number of manipulated individuals increases, they would want to spread across the two patches so that they do not have to compete with other manipulated individuals.  In this way they can adapt sneaky tactics to gain resources.  While this was not initially tested, this supports assumption 1 (tactics are adaptable).

  

Habitat fragmentation offers another challenge to ideal free distribution: the presence of barriers (assumption 3).  This may greatly affect an individuals’ ability to compete.  Unless they can gain access to the corridor and attempt success in the other patch, they will have to continue to compete with individuals occupying the same patch.  This further divides those who are able to gain resources and those who are outcompeted.  This also presents some cost to the individual (assumption 4).  The mice have no knowledge of the resources available and the number of competitors in a patch connected by a corridor.  Because I control for prediction one, the number of individuals is proportional to the resources available, competition is equal within each patch.  This provides a huge cost for the individual crossing as they will likely gain no more resources in another patch, even if the number of competitors is less.

 

From this experiment on ideal free distribution, I conclude that habitat fragmentation proves assumptions 1 and 5 true.  Adopting “ideal free” tactics is adaptive, and the more competitors in a patch mean fewer resources for the individual.  This also proves that assumptions 2, 3, and 4 are false.  Individuals participating in the “game” do NOT have equal competitive ability, there ARE barriers to movement among patches, and there ARE costs to moving among patches.