January 27, 2014
Dear Sally,
Hello! It sure was good to hear from you. How are things in 7th grade? I am happy to have completed
my first semester of eighth grade. In your last letter, you mentioned your friend with whom you had
been discussing ecology. In my science class, we just finished an ecology unit and I have really learned a
lot about it. I was troubled when you stated in your letter that abiotic and biotic factors have no impact
on populations in ecosystems. With all due respect, I am sorry to say that I think you and your friend
have it all wrong. First of all, abiotic and biotic factors are limiting factors. They can directly affect some
or all of the populations in an ecosystem. Secondly, one of the fundamental principles of ecology is that
abiotic and biotic components of ecosystems are interconnected. This would mean that these factors
rely on or influence one another. Lastly, your thoughts about biotic and abiotic factors not having any
impact on populations would mean that populations can exist without interacting with or being
influenced by the ecosystem. All right, I know I just rocked your world. Don’t be a hater. Just stay with
me and maybe I can provide you with the evidence that will help to change your way of thinking. After
all, I don’t want you to finish 7th grade with your mind clouded with misinformation!
To begin, abiotic factors and biotic factors are limiting factors. The definition for limiting factor is simply
any abiotic or biotic factor than restricts the size of a population. The definition of limiting factor alone
speaks to the fact that abiotic and biotic factors affect populations. In order to be able to restrict a
population you have to be able to have an impact on it. In class we looked at a lot of different
relationships and examples that bear this out. For instance, for one of my class assignments I had to
graph actual data for wolves and moose at Isle Royal National Park in Michigan. It is the longest
ecological study of its kind and has lasted over five decades. Wolves and moose would be considered
biotic factors since they are both living organisms. The graph I created from the data showed some
interesting interactions between these two species. First of all, when the moose population peaked, it
wasn’t long before the wolf population also peaked. The wolves were a predator of the moose. When
the moose population reached a low, soon after the wolf population also would decline. This provides
proof that biotic factors can influence one another. The prey controlled the predator population while
at the same time; the predator controlled the number of prey. Please realize that I am simplifying what
actually happened in Isle Royale over time, but hopefully you get the idea. This example shows how
biotic factors impact one another, but you are probably wondering about abiotic factors as well. As it
turns out, in the same ecosystem, ecologists documented a severe decline in moose population at a
time when wolf populations were very low. This event varied from the usual predator/prey pattern in
the graph. Based on a little more research, I found out that the population crash was due in large part
to the weather; an abiotic factor! That’s right, a severe winter reduced winter forage and many of these
big mammals starved to death. It is pretty hard to deny the facts, huh Sally? Abiotic and biotic factors
are limiting factors. They are all about impacting populations! Still a skeptic, read on.
Obviously this idea that abiotic and biotic factors can impact a population is much more than an idea. It
is a fundamental principle of ecology. I thought about the definition of ecosystem. It is a system formed
by the interaction of organisms with their environment. That says to me that it is a system formed by
the interactions between living and non-living things.