The Ultimate Classic Paper Analysis
Andrew Chao
Bio 112
11/15/05
By the mid twentieth century, substantial laboratory evidence existed which
illustrated the role of interspecific competition in shaping community structure. However,
direct evidence in nature has been small, shedding little light on the subject and making
uncertain the actual role of interspecific competition in nature. In his 1961 paper titled,
“The influence of interspecific competition and other factors on the distribution of the
barnacle Cthalamus stellatus,” Joseph H. Connell attempts to address not only the role of
interspecific competition on animal community structure in nature, but also whether or
not competition shapes the distribution of animals in nature. His findings have served as
a basis upon which continual research has built in order to form a more comprehensive
picture of the effects of interspecific competition in the environment.
Let us take the moment to briefly describe the concept of interspecific
competition, as it was perceived in the early days of its study. Original research was done
by observing two species of diatoms in a laboratory setting. A limited vital resource
(silicate) was in the media containing these diatoms, and the effects of competition with
respect to this resource. The eventual result was one species being able to survive at
lower levels of silicate while the other could not, thus resulting in one species remaining
in the experiment. However, these results cannot be applied to the natural world due to
the simple design of the experiment; this is but one interaction among countless others in
nature.
In his investigations, Connell observed the relationships of the barnacles
Cthalamus stellatus and balanus to their environment in a rocky intertidal habitat.
Specifically, he initially noted that the adults of both species occupy two distinct
horizontal zones with small areas of overlap (between younger individuals between the
two species). Connell’s hypothesis for this observation was that stellatus (inhabiting the
upper zone) was eliminated by the balanus in the lower level, most likely from
competition for a number of potential resources. A series of experiments were done with
varying the presence of one species in relation to the other in order to detail the effects of
their presence, in addition to that of physical factors in their environment.
From his balanus exclusion experiments, Connell believes he was able to
demonstrate that it was not intraspecific competition that was resulting in most of
Cthalamus mortality. By demonstrating the ability of Cthalamus to survive well (without
contact with balanus) at different heights, Connell was also able to discount the role of
physical forces constraining the distribution of Cthalamus barnacles. Therefore,
considering the significant reduction in Cthalamus on plots with balanus, we can infer
that it is indeed interspecific competition that is contributing to high barnacle mortality
outside of the high zone. This is confirmed also via direct observation of balanus
undercutting and covering (growing over) Cthalamus over time when the two populations
were placed in the same plot. Taking into account that balanus grows at a faster rate than
Cthalamus, but also does not survive at higher levels (being less tolerant of increased
stress than Cthalamus), strong evidence is put forth by Connell in support of the role of
interspecific competition in species interaction and distribution in nature.
The importance of Connell’s work on interspecific competition may seem less
significant today as the findings of his research are now basic study in ecology.
Nevertheless, the role of interspecific competition plays in nature was not firmly
established by this one paper. Research continued in the years following Connell’s work
that attempt to prove and dismiss the importance of this, in addition to expanding upon it.
A paper release in 1966 by Thomas Paine was one of the challenges put forth to
Connell’s conclusions about competition. To him and other critics, interspecific
competition, despite Connell’s efforts, does not sufficiently explain the outcome that is
observed in nature; too many important factors (in the view of the author) are neglected.
In nature, organisms are highly unlikely to interact with just one other species; rather,
they are more likely to be members of a complex food, preying on lower trophic
levels/making their food, or being preyed upon.
Here, Paine seems to show great interest in undercutting the role of interspecific
competition by stating that “if predation operates to reduce competitive stresses, evidence
for this reduction can be drawn by comparing the extent of niche diversification as a
function of trophic level” (Paine et al.). Paine presents some evidence to this, showing
that the high trophic predator heliaster preys all gastropods in its environment; the
gastropods in turn have highly specialized niches in which they each have a different prey
which constitute their primary nutrition. As such, Paine points out, that it is not unlikely
for other factors in nature to make the effects of competition insignificant, which would
thus diminish Connell’s case.
While debate between the true role of competition continued for a number of
years, the subject of competition was further studied in a number of ways. Nelson
Hairston was one among many investigators who built upon the research done by Connell
with his field experiments on salamanders (1980). Hairston states that the widely
accepted view of evolution is that it works by means of niche diversification, which in
turn influences community structure as suggested by Paine. However, here Hairston
chooses to elaborate upon an alternative pathway of evolution, with selection favoring
improvement in competitive ability.
To demonstrate this, Hairston used salamander species P. glutinosus and P.
jordani in the Appalachian Mountains as his specimens for study. The distributions of
glutinosus and jordani vary throughout the region, and Hairston posited that if
competitive ability is evolutionarily derived, the intensity of competition will increase as
distance between competitors decreases. This was accomplished by taking jordani from
areas of large separation between the two species and transplanting them into a close
proximity with a glutinosus population used to close competition with a jordani
population (which was removed).
The results of Hairston’s experiment was that the long-distance separation jordani
species was out-competed by an apparently evolutionary adapted glutinosus population.
The reverse showed true as well, with close-competition jordani thriving when paired
with long-distance glutinosus. The findings of Hairston strongly support the rarely
discussed pathway of evolution via competition; additionally, his findings also supports
Connell’s view on the significance of interspecific competition in nature.
Also in 1980, Arthur Dunham conducted more experimentation to test
interspecific competition. For his investigation here, Dunham examined competition
between insectivorous iguanid lizards S. merriami and U. ornatus, both of whom are
higher trophic level predators in their system (which can allow for testing of Paine’s
position). An exclusion experiment with controls was done, showing that even for higher
trophic levels, without a competitor in the same habitat, the “foraging success, growth
rates, body masses, prehibernation lipid levels, and population densities” were higher in
U. ornatus populations with their competitor excluded (Dunham et al.). However, only
one of several experiments could show any increases at all for S. merriami only plots,
which show that results can be variable depending on species and environment. So here
we see a compromise of sorts between Connell and Paine, where the significance of
competition is highly variable, present in some cases and absent in others.
With finer points of the whole interspecific competition controversy being
debated with a few other papers, other investigators in the field began looking at
mechanics behind competition rather than evidence for it. Scott Wilson, in a 1986 study,
hypothesized that a species’ ability to compete was dependent along its distribution along
a vital environmental gradient, and that this also was related to the incidence of a
particular species (in a particular part of the habitat).
Wilson’s findings were predictable: competitive ability was the highest at higher
ends of a critical gradient, and decreased correspondingly as species move down the
gradient. That is not to say, however, that because of increased nutrients or resources a
species’ competitive ability increased; rather, because these resources are highly
desirable, the most competitive species are able to maintain control of the most profitable
regions, leaving the lower levels of the gradient to less competitive species and shaping
the overall shape of the community of organisms in the shoreline habitat. While this
result may have been clear intuitively, it’s just one more piece in a greater puzzle as
scientists continue to develop a greater sense in how ecological systems work, and the
role competition plays in them.
As a greater acceptance and understanding of interspecific competition has been
achieved, more and more new fields and applications of this understanding (and others in
the field of ecology) are being sought and utilized. In a recent study by Cari-Ann
Hickerson et al (2005), a scenario of increased contact between an invasive European
centipede species and a native species existing in the interior of the forest. They found
that the native species was able to outcompete the invasive species at any point in the
habitat, but surprisingly was able to prey upon the exotic centipedes as well. These
interesting results have some significance in the realm of resistance to invasive species,
and suggest the more capabilities a native species has to employ against an exotic (in this
case, the ability to exploit more and even eat the competitor) the greater the chance of a
native community staying intact.
Another recent study by Hickerson (2004) analyzes another interesting aspect
behind competition: behavioral adjustments. In this study of salamander-salamander
competitive and salamander-centipede interactions, competition for space varies
significantly among closely related species and between salamanders and centipedes. It
was posited decades prior that there is an intrinsic level of competitiveness that a species
will display based on it’s evolutionary development; yet here, we find that salamanders
respond more aggressively and violently to centipedes intruding on their territories than
they would other salamanders (even of a different species). Therefore, we must conclude
that behavior and awareness of species certainly plays a role in the competitiveness
exhibited by individuals when they are confronted with varying opponents.
The field of studying interspecific competition is still expanding even today, yet
as more research is conducted we find that we must go back and address old issues
thought long since established. As Brewser et al. show us in a low-tide environment,
gradients may not be the prime factor behind distribution of competitive ability in
habitats. With nitrogen additions to the soil in the habitat, the hypothesis would be that
competitive spartina plants would push their borders outward and intrude upon the
normally nitrogen-poor regions occupied by juncus plants (with nitrogen facilitating the
expansion). Yet, while some invasion did occur, it was found that despite increase the
resource for which competition was high over, competitiveness was not lost in the lownitrogen tolerant species juncus. Furthermore, in the last year of the study, juncus invaded
the spartina boundaries, which was the opposite result expected. More must be at work
here than simply nutrient use efficiency, and Brewser is unable to offer an explanation for
what else accounts for it.
Interspecific competition theory has come a long way since it was first developed.
While it is a concept that is easy to visualize and imagine, proving its existence and
relevance in the real world has been no easy task. With debates swinging popular
opinion in different directions, eventually enough light has been shed on the subject that
tentative compromises on theory were developed to satisfy the growing collection of
evidence concerning this phenomena. As such, we can now begin to integrate this
concept with others that have been developed in ecology, helping humans gain a better
understanding of how the world around us functions and why.
Yet nonetheless, work still remains to be done in this field. As some research has
shown, not all that we know of competition is certain; revision of past knowledge will
almost certainly occur. And applications of interspecific competition have not been as
widespread in realms such as the microbial world and in very extreme environments,
where the peculiarities of the habitat may have strange ramifications on our traditional
understanding of how organisms interact. In doing this, a greater unification of ecological
concepts and theories can be achieved by tying all levels of our world together.
Bibliography
1. Connell., J.H. 1961. The influence of interspecific competition and other factors on
the distribution of the barnacle Cthalamus stellatus. Ecology 42:710-723
2. Paine, RT. Food web complexity and species diversity. The American Naturalist
100(910): 65-75 1966
3. Hairston, NG. Evolution under interspecific: field experiments on terrestrial
salamanders. Evolution 34(3): 409-420 1980
4. Wilson, SD. Species competitive ability and position along a natural stress/disturbance
gradient. Ecology 67(5): 1236-1242 1986
5. Hickerson, CM. Edge effects and intraguild predation in native and introduced
centipedes: evidence from the field and laboratory microcosms. Oecogia 146(1): 110-119
2005
6. Hickerson, CM. Behavioral interactions between salamanders and centipedes:
competition in divergent taxa. Behavioral Ecology 15(4): 679-686 2004
7. Brewser, JS. Nitrogen addition does not reduce belowground competition in a salt
marsh clonal plant community in Mississippi (USA). Plant Ecology 168(1): 93-106 2003