Total score: 88/100 – nice work
Title [[4/4 – you’ve got all the important bits, but the word “benthic” reads a little
strangely]]
An Analysis of the Species-Species and Species-Habitat Associations in a Benthic
Kelp Forest Community at Hopkins Marine Station
Ian Moffitt
Clarity [[13/14 – your sentence structures are clear and your grammar is good, but
you can work on connecting sentences and improving flow so that ideas build more
naturally from one sentence to the next.]]
Introduction [[17/20 – nice job with the broad ecological context and the specific
questions. You could do a bit better explaining the novelty of the study and why
Hopkins in particular is suitable for this kind of study, but you’ve got all the basic
pieces for a good intro]]
Ecologists have long been trying to understand how diversity is maintained
in natural ecosystems. Some of the hypotheses that attempt to explain how diversity
is maintained are the lottery model and the intermediate disturbance hypothesis.
The lottery model accounts for diversity by assuming that openings are filled at
random by recruits from a large pool of potential colonists (Morin 1999). The
intermediate disturbance hypothesis suggests that the highest diversity is
maintained at intermediate scales of disturbance (Connell 1978). However, one of
the most interesting and controversial hypotheses is niche partitioning. This
hypothesis states that diversity can occur by partitioning resources among different
species to avoid competitive interactions. [[this is a good intro to the broad
ecological concepts, but you probably need to explain why it’s difficult to maintain
diversity]]
Ecologists have been studying niche partitioning by looking at specieshabitat, and species-species associations. For example, Hallacher and Roberts
(1985) looked at species-species associations of two species of rockfish (Sebastes
carnatus and Sebastes chrysomelas) while James Watanabe (1984) looked at specieshabitat associations of different species of Tegula spp. (gastropods) to study niche
partitioning. Generally, ecologists have focused on highly diverse ecosystems such
as tropical rainforests in terrestrial ecosystems (Connell 1978), as well as coral reefs
and kelp forests in marine ecosystems (Hallacher and Roberts 1985, Watanabe
1984, Connell 1978). [[nice examples]]
The study of how diversity is maintained in marine ecosystems is
underrepresented. This is due to the fact that working in marine environments can
be very challenging in comparison with terrestrial studies. However, understanding
how resource partitioning influences community structure and function in a marine
ecosystem is essential for the management of resources, such as food and
recreation, associated with such important ecosystems. Furthermore due to climate
changes, kelp forests are subject to further alterations that we cannot fully
comprehend. In order to better conserve these areas of high diversity, one of the
factors we need to understand is how diversity is maintained through resource
partitioning. [[nice “so what”]]
Kelp forests are one of the most highly diverse and productive ecosystems in
the world. They can be found in cold, eutrophic waters associated with western
boundary currents (Coleman 2011). In central California, kelp forests are
characterized by giant kelp (Macrocystis pyrifera), which acts as biogenic habitat for
a variety of invertebrates, fish, and mammals (Carr 1994, Watanabe 1978, Estes
1998). M. pyrifera kelp forests are found on different types of substrate and relief.
Although M. pyrifera can only colonize regions with hard substrate (NOAA 2007),
substrate type can vary within a kelp forest. Such substrate types can include: sand,
cobble, boulder, and rock. Relief can also range from flat to high. Previous studies
have suggested that different types of substrate and relief can accommodate
different species. For example Watanabe (1984) showed that different species of
Tegula spp. occupy different substrate and relief types in the kelp forest ecosystem.
These variable habitat characteristics make the kelp forest an ideal place to study
resource partitioning. [good description of why kelp forests are a good study
system, but I’d like you to connect this directly back to Hopkins]]
The aim of this study is to explore species-habitat and species-species
associations in a diverse kelp forest ecosystem to better understand how resource
partitioning influences benthic community structure. We aim to answer these
specific questions: First, are there species-habitat associations in the kelp forest?
Second, do these species-habitat associations vary in strength? Third, are there
species-species associations in the kelp forest? Fourth, do these species-species
associations vary in strength? Fifth, which of these associations
(physical/biological) are more important with respect to maintenance of diversity.
[[good intro of the specific questions]]
Methods [[16/18 – nice!]]
We conducted an observational field study by using SCUBA in a healthy kelp
forest ecosystem off the coast of California. We tested for the effects of habitatspecies and species-species associations to better understand resource partitioning
in a kelp forest ecosystem. [[a few more details for general approach]]
System (Figure 6)
The sampling was conducted using SCUBA off a permanent transect cable in
the sub-tidal kelp forest at Hopkins Marine Station (36º36’N, 121º54’W) of Stanford
University. The sampling depth ranged from seven to ten meters. This research
station has been a marine reserve since 1985 (Jones 1985). With minimal
anthropogenic effects due to the sites protected status, it is ideal for studying
naturally occurring habitat and species associations. There are an abundance of
sessile and mobile invertebrate species in the area as well as a multitude of varying
substrate and relief types. Large granitic outcrops in shallow water give rise to
moderate and high relief bedrock surrounded by sand flats in deeper water
(Watanabe 1984). This diversity of substrate and relief types, allows us to examine
the effects physical habitat characteristics have on resource partitioning. The
abundance of invertebrate species allows us to examine the effects of biological
associations on resource partitioning. [[good]]
Data collection
Data collection was performed using two basic underwater observational
techniques, uniform point contact (UPC) and Swath (described below)
Uniform point contact:
UPC transects were performed by reeling out (30 m) meter tapes on and off
shore (270,90 degrees respectively) of the permanent transect cable. The substrate
and primary placeholder were recorded at every half meter (30 m total) as
whatever was directly under the meter tape. The relief was recorded by
determining the maximum height change in a .5/.5 meter window of the uniform
data point. This was done to estimate the percent cover of, various
substrates(Figure 1), relief types(Figure 1), and invertebrates(Figure 7). The
transects were conducted by ten buddy pairs using SCUBA along the permanent
transect cable from 90-135 meters. Each buddy team collected 120 data points for a
total of 1,200.
Four types of substrate were differentiated as sand, cobble (<10 cm), boulder
(10cm-1m), and bedrock (>1m). Four classifications of relief were differentiated as
flat (0-10 cm), slight (10cm-1m), moderate (1-2m), and high (>2m). The primary
placeholders that were counted are referred to in Figure 7.
Swath:
The swath transects were performed by reeling out transect tapes (30 m) on
and off shore (270, 90 degrees respectively) of the permanent transect cable. This
was done by ten buddy pairs using SCUBA from 90-135 meters. Eight species of
invertebrates/algae(Figure 1) were individually counted in a 2x30 meter area of
each transect. Each 30 meter transect was broken up into 5x2 meter increments
with each buddy sampling one meter on either side of a 5 meter section. This was
done to count the total number of invertebrates within each transect. The total area
covered was 1200 square meters. [[you could use fewer details here and
incorporate it into the general approach section, saving the details for the end of the
methods section ]]
Study Design
Species-habitat associations
To see if there are species-habitat associations in the kelp forest at Hopkins
Marine Station we combined densities of swath invertebrates with previously
obtained UPC physical habitat data. We performed an analysis of variance with this
data using a 95% confidence interval. UPC data collection (see data collection
section) allows us to estimate the total percent cover of various substrate and relief
types. Swath data collection (see data collection section) allows us to quantify the
total number of invertebrates and algae in a given area. Association values were
measured as the deviation from expected values if species were distributed
randomly on various habitats by chance. A positive association indicated that a
species was found more often than expected by chance, while a negative value
indicated that a species was found less often then by chance. Species habitat
associations were determined by combining UPC substrate and relief data with
swath invertebrate data. This was done by calculating a dissimilarity matrix based
on Euclidean values for the environmental variables (UPC species, substrate, relief)
as well as a dissimilarity matrix based on Bray-Curtis values for the swath species.
Correlations between the two matrices were tested for using a weighted spearman
rank correlation. [[nice, this was what I wanted you to do]]
Species-habitat association strengths
Association values were measured as the deviation from expected values if
species were distributed on various habitats by chance. A positive association
indicated that a species was found more often than expected by chance, while a
negative association indicated that a species was found less often then by chance. A
strong association was determined to be any value >0.1 or <-0.1 while a weak
association was any value between these. Strong associations are important because
they show a trend that is most likely not due to chance that may play a role in niche
partitioning. However, some strong associations may only appear to be strong
because of a small sample size. Weak associations are still important with respect to
niche partitioning, because they allow us to see trends whose effects may have been
masked by a large sample size (see discussion).
Species-species associations
To see if there were species-species associations in the kelp forest at Hopkins
Marine Station, we combined densities of swath invertebrates with the densities of
UPC invertebrates. We performed an analysis of variance with this data using a 95%
confidence interval. Association values were measured as the deviation from
expected values if species were distributed on various biogenic habitats by chance.
A positive association indicated that a species was found more often than expected
by chance, while a negative value indicated that a species was found less often then
by chance. Species-species associations were determined using the statistical
methods described in the species-habitat associations section (above).
Species-species association strengths
Association values were measured as the deviation from expected values if
species were distributed on various biogenic habitats by chance. A positive
association indicated that a species was found more often than expected by chance,
while a negative value indicated that a species was found less often then by chance.
A strong association was determined to be any value >0.1 or <-0.1 while a weak
association was any value between these. Strong associations are important because
they show a trend that is most likely not due to chance that may play a role in niche
partitioning. However, some trends can be amplified by a small sample size. Weak
associations are still important with respect to niche partitioning, because they
allow us to see trends whose effects may have been masked by a large sample size
(see discussion)
Importance of physical and biological factors
The relative importance of physical and biological factors was measured as
the percent of variance explained in swath species abundances.
Results [[13/16 – good job with general results and directly addressing the
questions. The figures need some formatting work because they ended up on
different pages, but the captions look good]]
The results of our study show that there are species-habitat and speciesspecies associations in the kelp forest ecosystem of Hopkins Marine Station. These
associations are positive, negative, and variable in strength. In addition, physical
factors account for more species diversity than the biological factors. [[nice general
results]]
Species-habitat associations (Figure 1)
We found that there are species-habitat associations in the kelp forest at
Hopkins Marine Station (T=10.01, P<0.000001). There were eight species that
exhibited associations with different substrate and relief types. Species associations
are clearly not randomly distributed across the various habitat types. [[good job
directly answering the questions with your results]]
There were a total of eleven positive associations with various substrates
and twelve positive associations with various relief types across all eight species.
For example: Cystoseira osmundacea, M. pyrifera, Styela monteryensis, and Lithopoma
gibberosa all exhibited positive associations with bedrock; while Balanus nubilus, S.
monteryensis, Cryptochiton stelleri, and L. gibberosa all exhibited positive
associations with high relief.
There were also a total of nineteen negative associations with various
substrates, and a total of thirteen negative associations with various reliefs. For
example, every species except Patiria miniata exhibited negative associations with
sand and flat relief. [[counting up the number of associations isn’t that helpful to
me]]
Species-habitat association strengths (Figure 1)
We found that the species-habitat associations (described above) were
variable in strength. There were eight strongly positive associations with various
substrates as well as seven strongly positive associations with various relief types.
For example: C. osmundacea, M. pyrifera, and S. montereyensis, each exhibited
strongly positive associations with bedrock while C. osmundacea, M. pyrifera, and S.
montereyensis each exhibited strongly positive associations with slight relief.
There were also nine strongly negative associations with various substrates
as well as eight strongly negative associations with various relief types. For
example, all of the species except M. pyrifera and P. miniata exhibited strongly
negative associations with sand while C. osmundacea, L. gibberosa, and P. miniata
each exhibited strongly negative associations with moderate relief.
Species-species associations
We found that there are species-species associations in the kelp forest at
Hopkins Marine Station (T=4.40,p=0.000011). There were eight swath species that
each exhibited associations with thirty-three different UPC species. Swath Species
associations are clearly not randomly distributed among the UPC species.
There were a total of one hundred-twenty two positive associations among
the eight swath species and the thirty-three UPC species. For example: P. miniata
and Pisaster giganteus both exhibited positive associations with Bryozoans (Figure
4). There were also one hundred-forty one negative associations among these same
species combinations. For example: L. gibberossa is negatively associated with
Diopatra ornate (Figure 5) .
Species-species association strengths
We found that species-species associations were variable in strength. There
were fifty-five strongly positive associations between the eight swath species and
the thirty-three UPC species. For example, M. pyrifera and C. osmundacea both had
strongly positive association with each other (Figure 3). There were also forty
strongly negative associations between these same species. For example, P. miniata
and P. giganteus both exhibit strongly negative associations with Corynactis
californica (Figure 4).
Importance of physical and biological factors
Physical habitat characteristics(substrate/relief) were found to account for
76% of the variance in swath species diversity. Biological features were found to
account for 24% of the variance in swath species diversity.
Discussion [[19/22 – nice job! A general discussion paragraph that sets it all up
would do wonders. I especially like the way your conclusion links back to the big
questions (as it should)]]
For the five questions addressed by this observational field study (see
introduction) we hypothesize, first, that there will be species habitat associations.
Second, these species-habitat associations should vary in strength. Third, there
should be species-species associations. Fourth, these species-species associations
should vary in strength. Fifth, we hypothesize that the physical habitat
characteristics will prove to be the most important factor affecting species diversity.
[[don’t need to repeat the questions all here, try to start with a general discussion
paragraph that links your findings back to the big ecological questions from the
intro and sets us up to understand the hypothesis-specific discussions to follow]]
Species-habitat associations (Figure 1)
There are many examples of species-habitat associations occurring in natural
ecosystems. For example, Phillips et al (2003) studied the affects of soil on various
Amazonian tree species and found that soil composition played an important role in
the distribution of some species. Accordingly, our study shows that species-habitat
associations are clearly occurring (see Results). For example, S. montereyensis and L.
gibberosa both exhibited positive associations with bedrock and high relief. When
ecologically similar species are found to co-occur on the same type of habitat, it is
often indicative of competition currently taking place. Although both these species
are invertebreates, they are also ecologically distinct in that S. montereyensis is a
suspension feeder while L. gibberosa is an herbivore. These differences in diet could
be a reason that these species are able to co-occur on the same time of habitat. In
this case, resource partitioning didn’t lead to habitat diversification, but niche
diversification is still a possible explanation for the observed habitat correlations of
both species. In this circumstance, the “niche” is diet. [[why are you comparing these
two species?]]
Species-habitat association strengths (Figure 1)
A clear example of niche diversification occurring through habitat
partitioning can be found by analyzing the varying strengths of habitat associations
between B. nubilus and S. montereyensis. B. nubilus exhibited a strongly positive
association with boulders while S. montereyensis only exhibited a weakly positive
association with boulders. Furthermore, S. montereyensis exhibited a strongly
positive interaction with bedrock while B. nubilus exhibited a weakly negative
association with bedrock. These results show that these two suspension feeders
have adapted to different substrate types to avoid competing for nutrient resources,
as predicted by the theory of niche partitioning. This is further supported by the
relief correlations between both species, which are also variable in strength. For
example, B. nubilus exhibits strongly positive associations with both moderate and
high relief while S. montereyensis exhibits weakly positive associations with both
these relief types. Furthermore, S. montereyensis exhibits a strongly positive
association with slight relief while B. nubilus exhibits a strongly negative association
with this relief type. The varying strengths of substrate and relief data for both
species suggest niche partitioning through habitat differentiation as the mechanism
by which these two species are able to coexist in the kelp forest ecosystem.
Species-species associations
There are many examples of species-species associations occurring in natural
ecosystems. For example, Herbold (1983) studied several species of fish that
occurred in a similar region of a stream in Indiana. He concluded that niche
diversification was the most plausible model to explain these fishes ecological
distributions, as they spatially co-occurred but did not exhibit competitive
interactions. In our study, species-species associations were not as important with
respect to variance in swath species abundance when compared to the specieshabitat associations (see results). We did identify some species-species associations.
However, it is very likely that many of these associations are a result of the
underlying habitat associations that form the basis for the species-species
associations.[[good]] For example, P. miniata and P. giganteus both exhibited
negative associations with C. californica (Figure 4). This could be due to avoidance
of the colonial anemone due to its stinging nematocysts. However, another
explanation can be found by looking at the substrate preferences of C. californica
(Figure 8). Since C. californica associates negatively with sand substrate, it would
not be expected to associate with P. miniata that exhibits a positive association for
sand. Furthermore, C. californica associates positively with bedrock while P.
giganteus associates negatively with bedrock. Therefore, the fact that these species
do no associate with each other may be a consequence of the underlying habitat
diversification. It is impossible to say if these organisms segregated to different
habitats because of physical or biological associations. However, negative speciesspecies associations are still occurring.
Species-species association strengths
Looking at association strengths, we were able to identify some interesting
species-species associations. For example, C. osmundacea exhibits a strongly
positive association with coralline algae (Figure 3). This could be because C.
osmundacea preferentially recruits to coralline algae (Reed and Foster 1984).
Furthermore, we found that M. pyrifera exhibits strongly positive associations with
C. osmundacea and vice versa (Figure 3). This does not make sense from the
standpoint of niche partitioning because these two species utilize the same
resources and should therefore not be found in the same habitat. However, I have
observed these algae in close proximity to one another many times, and perhaps
niche diversification does not apply as well to these algae species as an explanation
for how they can co-exist. It seems that C. osmundacea could recruit
opportunistically after the M. pyrifera canopy is wiped out during a large storm.
However this is entirely speculative and was not considered during this study.
Importance of physical and biological factors
Physical factors clearly account for more of the variance among swath species (see
results). However, truly understanding the source of niche partitioning is a difficult
task. For example, in the case of C. californica, P. giganteus, and P. miniata, it is
impossible to say if the niche diversification was due to habitat or species
characteristics.
Potential sources of error
Error could have accumulated in the UPC data if anyone consistently
identified data points with species that they recognized as opposed to the species
that was actually the direct primary placeholder. Error could have accumulated in
the swath if anyone sampled an organism in the wrong section of the swath. For
example, if someone counted an organism at 5.1 meters when it was actually at 4.9
meters.
Conclusion
From this observational field study in a kelp forest, we can conclude that speciesspecies and habitat-species associations of varying strengths contribute to resource
partitioning that may lead to niche diversification. Furthermore, we can conclude
that physical characteristics (substrate and relief) contribute more to swath species
variance than UPC species. Many of these association make sense, as in the case of B.
nubilus and S. montereyensis, but others do not make immediate sense, as in the case
of M. pyrifera and C. osmundacea. It is important to understand how species
associate with various habitats and other species because it allows us to see how
these species are sharing resources to maintain diversity. By understanding how
diversity is maintained, we are better able to conserve and manage resources
provided by this kelp forest ecosystem. Although resource partitioning is clearly a
factor in maintaining diversity in the kelp forest ecosystem, it is not the only factor,
as shown in the case of M. pyrifera and C. osmundacea (see species-species
association strengths). More work is needed, to understand how other factors affect
the maintenance of diversity in the kelp forest ecosystem.
Citations [[6/6]]
Carr, C.H 1994. Effects of Macroalgal Dynamics on Recruitment of a Temperate Reef
Fish. Ecology 75(5):1320-1333
Coleman, M. A., Roughan, M., Macdonald, H. S., Connell, S. D., Gillanders, B. M.,
Kelaher, B. P. and Steinberg, P. D. (2011), Variation in the strength of
continental boundary currents determines continent-wide connectivity in
kelp. Journal of Ecology, 99: 1026–1032.
Connell, J.H., 1978. Diversity in Tropical Rain Forests and Coral Reefs. Science.
199(4335): 1302-1310
Estes, J.A., Tinker, M.T., Williams, T.M., Doak, D.F. (1999) Killer Whale Predation on
Sea Otters Linking Oceanic and Nearshore Ecosystems. Science, 282:473-475
Foster, M.S., Reed, D.C., 1984. The Effects of Canopy Shading on Algal Recruitment
and Growth in a Giant Kelp Forest. Ecology. 65(3): 937-947
Hallacher L, D.A. Roberts. 1985. Differential utilization of space and food by the inshore
rockfishes (Scorpaenidae: Sebastes) of Carmel Bay, California. Environmental
Biology of Fishes. 12.2: 91.
Herbold, B. (1983) Structure of an Indiana Stream Fish Association: Choosing an
Appropriate Model. The American Naturalist. 124(4): 561-572
Jones, B. 1985. The Hopkins Marine Life Refuge: Information and Background.
<http://www.stanford.edu/group/dbr/HMLRhistory.htm>
Morin, P.J., 1999. Community Ecology. 321
NOAA. 2007. Ecosystems: Kelp Forests
< http://sanctuaries.noaa.gov/about/ecosystems/kelpdesc.html>
Watanabe, J. 1984. The Influence of Recruitment, Competition, and Benthic Predation on
Spatial Distributions of Three Species of Kelp Forest Gastropods (Trochidae:
Tegula). Ecology. 65.3: 920-936.
Figures
Figure 1
This figure shows the associations between
substrate (starting from left: sand, cobble, boulder,
bedrock) and relief(starting from right: high,
moderate, slight, flat) with respect to the
associated species(shown at top of graph).
Figure 2
This figure shows the associations between various UPC species (listed on
X axis) and their associated swath species (listed above graph)
Figure 3
This figure shows the associations between various UPC species
(listed on X axis) and their associated swath species (listed above
graph)
Figure 4
This figure shows the associations between various UPC species
(listed on X axis) and their associated swath species (listed above
graph)
Figure 5
This figure shows the associations between various UPC species (listed
on X axis) and their associated swath species (listed above graph)
Figure 6
Map of Hopkins Marine Station in Pacific Grove California.
Figure 7
Inanimate
Red Algae
Coralline
Brown Algae
Tube Worms
Bare rock
Branched
Crustose
Cystoseira
Solitary
osmundacia
tubeworm
Dictyoneurum
Diopatra ornata
Bare sand
Leafy
Articulated
californicum
Shell debris
Bushy
------------------
Egregia
Phragmatopoma
menziesii
californica
Sediment
Lacy
------------------
Desmarestia
Dodecaceria spp
Dead
Encrusting
------------------
Macrocystis
--------------------
holdfast
red
----------------
Turf
holdfast
------------------
Laminariales
--------------------
holdfast
-----------------
------------------
------------------
Dictyotales
--------------------
spp.
Snails
Cnidarians
Tunicates
Other
Serpulorbis
Corynactis
Colonial tunicate
Scallop
squamigerus
californica
Petalochonchus
Cup coral
Solitary tunicate
Embedded
monteryensis
cucumber
-----------------------
Other anemone
------------------------- Barnacle
-----------------------
Hydroids
------------------------- Bryozoan
-----------------------
Stylaster
------------------------- Sponge
californica
-----------------------
-------------------------- ------------------------- Mussel
List of all UPC species counted
Figure 8
This figure shows all of the strong associations between
substrate and UPC species. Corynactis californica(CORCAL) is
the only species referred to in this table. It is located 3rd from
the top, in the middle.