Here is another great study that I
am sure many of us remember from our youth. Hold on dearly to these memories
for I can attest to the negative effects of being a graduate student. Already
the proportion of gray hair atop my head has increased dramatically, and the
absolute number of said hairs would also have increased tenfold I am sure if
not for the additional increase in my rate of hair loss. So, I repeat, hold
onto these memories of your youth for they are all you have now that life is
heading down hill so quickly, and the stresses of life are increasing “exponentially”
(I totally pay attention to your math lessons Helen.)
Okay, let us begin with my summary,
or one can head to the end of the paper for a very nice summary by the author
Joseph H. Connell, who is still alive and very old. 92 years old to be exact.
Connell is an American ecologist, but, since his study on how interspecific
competition regulates the distribution of the barnacle Chthamalus stellatus was conducted on the shores of Scotland, I personally
think he should be referred to as Sir Joseph Connell because the United Kingdom
is the land of knights and chivalry, and everyone knows that academics are the
epitome of chivalry and honor. However, I am not the queen and therefore cannot
bestow such an honor so I will call him Connell.
Connell starts off his paper with
an observation on the shores of Scotland. He noticed that the adults of Chthamalus and the adults of Balanus balanoides are separated into
two distinct zones with the smaller Mediterranean Chthamalus distributed mainly above the tide level, and the arctic Balanus distributed at and below the
tide level. Connell also noted that while he observed young Chthamalus below the tide level
intermixed with the Balanus, he saw
very few adult Chthamalus below the
tide level. Something must be at play that is separating these two species of
barnacle on the majestic coast of Scotland, and Connell was set out to discover
what this was or die trying.
And bravely we soldier on into the
rather detailed, and dry, methods section. So, who had to look up what a spring
tide and neap tide are? Don’t lie, Christmas is just around the corner, and if
we all get coal then London will be polluted again and all the typical moths will be eaten. I know I had
to look up what these two kinds of tide are, but we can discuss the specifics
in class; in short these two kinds of tide are tied (pun intended) to the
relative positions of the sun and moon, and were used in conjunction with the
mean tide level as demarcations for the distributions of the two barnacle
species in question. I don’t think it is very effective to go through piece by
piece what exactly Connell did, so I will try my best to be concise. Basically,
Connell had several areas of different depths where he mapped the locations of
barnacles for several years. At each area there was a control portion that he
did not remove the presumed dominant species, Balanus, and an experimental portion where he did remove Balanus. In order to test for
competition at depths where Chthamalus did
not occur he performed translocations of stone with barnacles already attached.
Connell also tested for the effect of a predatory snail on barnacle
distribution at several testing areas by using a mesh cage to prevent predation.
(It’s
a Scottish fold, the primary predator and snuggler in Scotland)
Enough with that methods jazz, onto
the results, which is the real meat of any paper. Connell Briefly discusses
physical factors that play into the distributions of Chthamalus and Balanus.
The main point to take away from this section is that Chthamalus appears to be capable of surviving at a higher level
than Balanus due to a greater
tolerance to heat and desiccation. This explains the upper limit of the
distribution of Balanus; they simply
can’t survive being dried out and baked as well as Chthamalus. In addition to this Connell also pointed out that the
data suggests that Chthamalus is
capable of surviving at deeper depths than it is currently found at, meaning
that there are other factors contributing to the lower limit of Chthamalus.
Now we move on to the crux of the
paper, competition for space between the two species. Figures 2 and 3 sums up
this paper quite well actually. Each graph represents a study area with the
horizontal axis being time in months, and the vertical axis being total number
of Chthamalus in the study area. The
dotted line on the graph is the number of Chthamalus
in the study area where Balanus
was removed, and the solid line is the number of Chthamalus in the study area with Balanus present. A majority of the graphs show the same trend; Chthamalus survives longer when it is
not living, however short that life might be, sympatrically with Balanus. Connell provides several tables
of ancillary data in regards to how Balanus
either directly kills or removes Chthamalus
when they are competing for space, and other details about mortality rates.
Figure 4 is not from Connell’s study, but shows that survival of Chthamalus is negatively correlated to
the growth rate of Balanus.
Connell, again briefly, discusses
the effects of predation by the snail Thais.
What is significant in this section is that predation by this snail does not
explain the distributions of these two barnacle species, but, interestingly, it
does lessen competition. Connell proposes that the mechanism by which Thais reduces competition is by
preferentially preying upon larger barnacles, which happen to usually be Balanus. This would obviously reduce
competition between Balanus and Chthamalus because it lowers the
population of Balanus.
I remember reading about this study
as an undergrad, and it was just a given that Balanus is a better competitor than Chthamalus. It was never explained exactly why or how Balanus out competed Chthamalus. Lucky for us Connell does
just that in his discussion section with his part on “The Causes of zonation.”
Connell ends his section on competition for space with a quote from Elton and
Miller (1954) defining interspecific competition as “in which one species
affects the population of another by a process of interference, i.e., by
reducing the reproductive efficiency or increasing the mortality of its
competitor.” In this final section Connell explains how Balanus does just this to Chthamalus.
And for anyone teaching a 203 or 204 lab, you can use this as an example of a
redundant sentence since I said basically the exact same thing before the
quotation. According to Connell, Balanus
directly affects the fitness of Chthamalus
by increasing mortality of adults through physically smothering or removing
adults from an area, but also indirectly affects fitness by deforming and
reducing the size of adult Chthamalus,
which in turn decreases the amount of larvae produced thus reducing
reproductive efficiency. Balanus is
able to do this because of two traits; One, Balanus
produces more larvae resulting in a larger population density relative to Chthamalus; and two, Balanus has a faster growth rate than Chthamalus. These two traits allow to Balanus to out compete Chthamalus when the two species exist
sympatrically.
The last section is Connell’s
wonderful summary that you could read and skip mine, but hopefully this has
been somewhat entertaining and informative, now for some questions.
What exactly makes this a
foundation paper and why is it included? (Everyone has to ask this question so
cut me some slack)
How does this paper deal with
interspecies interactions in comparison with Kettlewell and Park? What is the
ultimate conclusion to the interactions in each of these systems?
Is Billy Connolly Scottish, or over
the top Scottish?
I thought that the experimental
design was robust, but I am sure people can find holes and flaws, what might
these be?
It was mentioned both in the
introduction to this section and Connell’s introduction to this paper that part
of the beauty of this system is that the two competing species are sessile, how
might one accurately test for completion in a more complicated and mobile
system?
Do I
actually know how to use a semi-colon, or was I randomly throwing them in there
for brownie points?
This entire
section for experiments in the lab and field is animal centric when a majority
of what we have read up till now has been looking at plants, why do you think
that is?
How could
Mel Gibson go from making Braveheart to The Beaver?
This paper did a nice job of combining observational work with field perturbations. The author elucidates the effect of competition by testing the effect of predation, abiotic factors, and competitive interactions between the two species of barnacle. Interestingly they find that only the competitive interaction has a significant effect on the minority species. This was a nice way of putting an observation to the test and using the scientific method to draw conclusions about how the natural world works. It is also notable (and a nice addition to the paper) that the authors describe how the dynamics of this relation shift in other geographic areas. I thought it was interesting that young of the minority species migrated to the deeper waters of the majority never to return. It seems like this puts them at a steep disadvantage. I love the review questions (and the pictures!)
ReplyDeleteThis is a good paper on competitive exclusion of barnacles. I do recall something about this study, either in undergrad or high school, but to be honest it never really lingered. Good observations, aligned with a good experiment, I liked how the predatory snail appeared to lessen the competition between these two genera. I enjoyed the post, pictures and questions.
ReplyDeleteThis one I do not remember hearing about in high school or college, but I do remember going on a field trip the intertidal zone and counting/measuring stuff, so maybe I was supposed to have learned about this then. Oh to be an innocent, naive undergrad again...
ReplyDeleteI believe this paper is the first we've read that has looked at range limits. It's a nice demonstration of how they can be set by either biotic or abiotic factors. Connell also shows how competition can be mediated by abiotic factors - the strength of Balanus competition with Chthamalus weakens in higher zones more subject to desiccation. I think the strength of this paper relative to others we've read in this section is that it is a) a field experiment conducted under more natural rather than artificial conditions, and b) it involves direst observation of the mechanisms of competition (i.e. crowding, gouging etc.). The experimental design seems relatively strong, but sample sizes are small. There are also could be more controls e.g. sometimes exclosure experiments will include an exclosure that still lets the consumer in to test the effect of the exclosure alone. And might the moving and bolting of rocks affect the barnacles? I don't think these are fatal flaws, but things to consider and that could be done to make the design more robust.
One question I had was about the types of competition. The intro (p. 722) mentions that originally there were two types (interference and exploitation), but that since there are now at least six. Perhaps we can cover what these are in class?
Like Dunbar was saying, this paper has some cool stuff that is centered around the how biotic and abiotic factors are affecting a population. This fleshes out the niche and really shows some good things about the realized vs fundamental niche. The competitive nature of the barnacles seems to be a great study system, especially since their zones are so well defined. Something interesting that could be studied further (and may have already been) is to test this hypothesis more by trying to develop a model that would predict what happens with climate change. Since we can predict the effects on the ocean, we can think about how the distribution of the barnacles might have changes now that the sea temperatures have changed. It would also be interesting to see if the distribution changes over time since I assume the conditions in the ocean change over time, but that wouldn't be an experimental manipulation and I guess would not be appropriate for this book section.
ReplyDeleteIn contrast to Park and Huffaker, Connell does not really emphasize availability of food for the species being studied, although he writes a lot about competition for space. I'd like to see what happens at various tide levels with presence/absence of Balanus, if the areas were flooded with a surplus of plankton.
ReplyDeleteI thought Schuyler's semi-colon usage was outstanding; however, someone needs to provide some intraspecific competition for at least some kind of dampening effect on his sense of humor.
The image of Connel chasing rabbits for his graduate work reminded me of many students who choose to study very evasive animals, and how hard that can be at times. That is why he decided to move onto the barnacle, a much easier animal to catch and study. Here is a little article from Connel himself, giving us some background on his exp. as well as some advise at the end to future grad students.:
ReplyDeletehttp://garfield.library.upenn.edu/classics1981/A1981LP44800001.pdf
I really liked that his experiment was not in a lab. I think because of his chosen organism he was able to do this. Many others cannot with larger more mobile organisms, so there will always have to be some kind of lab design and extrapolation of these ideas from smaller less mobile organisms.
Also, it would be neat to see a long-term evolution study of Chthamalus under successively drier conditions with neutral drift as the null hypothesis, to see what traits of Chthamalus are adaptive for different tidal zones. And then it would be interesting to do the same long term-evolution studies with presence/absence of Balanus to see how the interspecific competition affects these putative adaptations.
ReplyDeleteI remember reading this paper in both high school and undergrad; oh, those were the days!
ReplyDeleteTo me, this paper seems to be foundational because of the inclusion of something Dunbar mentioned above, the talk of range limits. Clearly each species has its ideal habitat range. This study took that to the next level and not only determined why the barnacles had the range they did, but what happens when they are growing in the same area and why one species does better than the other. Reproduction rate and size clearly play a part in determining the competitive mechanisms, but the abiotic conditions come into play as well. The experimental design for this study seems sound to me considering it was done in the field, but I'm sure there are ideas about how it could be improved. This study really stands out to me as one that, while it was important at the time, has only increased in notoriety as the field of Ecology has developed.
I feel like semicolons only win you points if they are followed by pithy comments; and are almost always negated by the use of the Oxford comma.
ReplyDeleteAs most have already mentioned, this paper is a nice addition to the book because of its incorporation of factors that influence the distribution. I like that not only did Connell study how the two barnacles impacted each other, but how other factors might also be causing this segregation of barnacles.
ReplyDelete