Overall score 88/100
Title [[3/4]]
An Observational Analysis of Fish Species Relative Abundance and Fish Species Diversity as a Function of Depth and habitat characteristics in a Sub-Tidal Kelp Forest at Hopkins Marine Reserve
Ian Moffitt
Clarity [[13/14 – your writing style is fairly clear, but you should work on tightening your writing]]
INTRODUCTION [[18/20 – a little long, but you have all the key elements. In the future work on tightening your writing and removing the fat]]
Ecologists have long been trying to identify how diversityis maintained in natural ecosystems, as well as why relative species abundance differs with respect to various habitats. Species diversity is sometimes called species accumulation, which consists of species evenness and species richness. Species evenness describes the likelihood of seeing a subset of the total species in the sampled area on any given transect. The more evenly distributed the species types, the more likely it is too see all the species present in the area on a given transect. Species richness is merely a measure of how many species there are in a given area, regardless of how evenly distributed they are.
There are many models that attempt to explain how diversity is maintained in natural ecosystems. Two of these include, the intermediate disturbance and niche diversification hypotheses. The intermediate disturbance hypothesis suggests that the highest amounts of diversity occur at sites of intermediate disturbance(Connell 1978). This is because areas of intermediate disturbance maximize the survivorship of species as well as keep spaces open for new species to recruit. The niche diversification hypothesis suggests that diversity can occur through resource partitioning to avoid competition. Competitive interactions result in a loss of fitness of both interacting species, ultimately decreasing species diversity (Connell 1978). One of the most commonly acknowledgedfactors for maintenance of species diversity and differing relative species abundances is habitat complexity (Gratwicke and Speight 2004). The complexity of a habitat is often based on three characterizing features: substrate type, relief type, and percent of biological cover. [[good context, but I’m not quite sure why the intermediate disturbance hypothesis comes in here.]]
Habitat complexity is an important factor influencing species diversity and relative species abundance in a variety of ecosystems. Numerous studies show that as habitat complexity increases, so too does species diversity and relative species abundance. This may be due to the availability of more niches for colonization(Mac Arthur et. al. 1966). For example, August (1973) found that more mammal species in a Venezuelan grassland were associated with complex habitats than less complex habitats. Furthermore, Almany (2004) showed that fish abundances are strongly affected by habitat complexity in coral reef ecosystems. The complexity of an ecosystem clearly plays a role in the maintenance of diversity as well as the relative species abundances in a variety of diverse ecosystems.
Kelp forests are one of the most complex and productive ecosystems in the world (Edwards 2003).They can be found in cold, eutrophic waters associated with western boundary currents (Coleman 2011). In central California, kelp forests are characterized by Macrocystispyrifera, which acts as biogenic habitat for a variety of invertebrates, fish, and mammals (Carr 1994, Watanabe 1984, Estes 1998).M. pyrifera can only grow on hard substrate and needs a lot of light and nutrients to thrive (NOAA 2007). M. pyriferatherefore adds additional habitat complexity to an already complex habitat of rock and bedrock substrate of varying relief type.
The kelp forest ecosystem at Hopkins Marine Reserve (36º36’N, 121º54’W) of Stanford University is pristine. It has been a designated marine protected area since 1984 (Jones 1985) and therefore experiences minimal anthropogenic influences. An interesting feature of this particular site is that physical complexity (substrate and relief) decreases with increasing depth (UCSC KFE 2012)(Figures 1 and 2). This decrease in physical habitat complexitywith increasing depth also corresponds to a decrease in biological habitat complexity with increasing depth(UCSC KFE, 2012) (Figure 3). This is interesting in that it allows us to formulate predictions on species diversity and relative abundance based on zones (deep/shallow).
The variance in habitat complexity with depth at Hopkins Marine Reserve, compels us to ask questions about how relative species abundances and species accumulation are affected. We would like to know, first, if relative species abundances vary as a function of zone (shallow/deep). Second, we would like to know if species accumulation varies as a function of zone..
Based on the hypothesis that increased physical and biological habitat complexity will correspond to higher species diversity and greater relative species abundances, we have formulated the following predictions for a subset of the fish assemblage found at Hopkins Marine Reserve. First, we predict that there will be more relative fish species abundance in the shallow regions [this is awkward]] of the site because of the increased complexity of habitat found in this zone. Second, we predict that there will be more fish species diversity in the shallow regions of the site because of the increased complexity of habitat found in this zone. To our knowledge, this is the first study of its kind to address the accumulation and relative abundance of fishes in the kelp forest ecosystem at Hopkins Marine Reservebased on different depth zones that also takes into account physical and biological habitat characteristics.
METHODS [[17/18 – good job!]]
We conducted an observational field study using SCUBA in a healthy kelp forest ecosystem off the coast of California at Hopkins Marine Reserve. We looked at how various substrate, relief, and biological features varied at different depth zones. Various fish species (Figure 5) were also counted and associated with different depth zones. With this data, we calculated fish species abundances per unit area and fish species accumulation at two different depth zones. [[good general approach]]
System
The sampling was conducted using SCUBA off the deep and shallow sides (270 degrees and 90 degrees respectively) of a permanent transect cable in the sub-tidal kelp forest at Hopkins Marine Station (36º36’N, 121º54’W) of Stanford University (Figure 7). The sampling depth ranged from seven meters on the shallow side to ten meters on the deep side. 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 is an abundance of algal cover and fish species in the area as well 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 type, allows us to examine the affects of physical habitat characteristics on relative fish species abundances at different depth zones. The abundance of fish and algal species allows us to examine the affects of biological habitat on various fish species at different depth zones.
Study Design
Fish species relative abundance as a function of zone
To test if species relative abundances vary as a function of zone, we looked at fish species densities in shallow and deep depth zones. We did this by performing fish transects on April 24, 2012(see data collection). These transects gave raw fish species density data. The fish densities in different depth zones were then compared. A chi square analysis was used to determine if there was a significant difference between species relative abundances in the shallow and deep. We graphed the abundance per unit volume to determine if the higher abundances for a given species occurred in the shallow or deep zones (Figure 4 and 5).
Fish species accumulation as a function of zone
To test if species accumulation varies as a function of zone, we looked at fish species density in a given 5 m segment of a 30 m transect. Fish densities were acquired by performing fish transects (see data collection). We then combined and resampled random segments of all the transects performed to determine species accumulation. Two species accumulation curves were graphed according to this re-sampling theory (Figure 6). The initial slope of the curve indicates species evenness. The steeper the initial slope, the more likely and observer is to see all of the species present in the sampled area. The asymptote of the curve represents species richness. If one asymptote is higher than another, it is indicative of more species richness. Species diversity is a measure of species richness and evenness.
Data Collection
Data collection was performed using three basic underwater observational techniques, uniform point contact (UPC), algal swath, and fish transects (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 what type of substrate and relief types were present the most at different depth zones. 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. Two species of algae(Cystoseiraosmundacea and Macrocystispyrifera) 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 amounts of C. osmundaceaand M. pyrifera within each transect. The total area covered was 1200 square meters.
Fish transects:
The fish transects were performed in one day on April 24, 2012. They were performed by ten buddy pairs using SCUBA from 90-135 meters before the UPC and Swath transects to avoid attracting or scaring away various fish species. For this same reason, transect tapes were run out to 30 m along the bottom while fish were being counted in 5 m increments. 19 fish species were counted in a 2m wide by 2m high by 2m long “box” with the center of the transect tape running directly through the middle. This was done on and off shore of the permanent transect cable (270, 90 degrees respectively) to calculate various fish densities. These densities were used to look at varying species abundances and accumulation at different depth zones (onshore= shallow, offshore= deep). Lights were used to look into crevices.
RESULTS [[14/16]]
Our results show that fish species relative abundance varies as a function of zone. Furthermore, we found that fish species accumulation varies as a function of zone as well. Most species relative abundances were greater in the deep zone. The most species evenness and richness was found to occur in the deep zone as well. [[good]]
Fish species relative abundances as a function of zone
Fish species densities were found to vary significantly between the shallow and deep zones (T= 66.041898 , P= <0.000001). More fish species were found in the deep zone than the shallow zone (Figure 4). Fifteen of the eighteen species of fish counted in this study were found to occur more often in the deep zone (Figure 4).
Our results clearly show that there are differences in relative species abundances as a function of depth zone. Although there was more relative abundance of various species in the deep zone, some species exhibited higher relative abundance in the shallow zone as well. About 35% of all fish found in the deep zone were composed of pile perch (Figure 5). Gopher rockfish were found to occur only in the deep zone while black and yellow rockfish were found to occur predominantly in the shallow zone (Figure 4).
Fish species accumulation as a function of zone
The slope of the species accumulation curve was steeper for species in the deep zone (Figure 6). This shows that species in the deep zone occur more evenly, thus contributing to a greater observed species diversity in this zone. The asymptote of the shallow zone was lower than that of the deep zone (Figure 6), suggesting that more species richness occurs in the deep zone as well. The higher species evenness and richness present in the deep zone shows that the deep zone is overall more diverse. However, the deep zone curve does not appear to approach an asymptote. This suggests that there are more species in the deep zone that were not sampled, further increasing the apparent fish diversity of this zone.
DISCUSSION [[17/22 – your mechanisms are weak as you mainly just talk about how you don’t believe the results]]
Many studies have shown that fish species relative abundance increases with increasing habitat complexity in a kelp forest. For example, Schmitt and Holbrook (1990) have shown that the relative abundance of the black surfperch increased dramatically with the addition of more M. pyrifera. Many studies have also shown that fish species diversity increases with increasing habitat complexity in a kelp forest. For example, Graham (2004) showed that the complex habitat formed by M. pyrifera can cause dramatic increases in species diversity. The large number of studies that have found that complex habitats positively correlate to more species relative abundance and diversity, caused us to predict that more relative species abundance and species accumulation would be found in the shallow zone. We predicted this because the shallow zone was found to consist of more complex habitat as determined in a previous study (UCSC KFE 2012) (Figures 1, 2, and 3).
Fish species relative abundance as a function of zone
There are many examples of fish species relative abundance varying as a function of zone. For example, Hallacher and Roberts (1985) found that Sebasteschrysomelas(black and yellow rockfish) are found primarily in shallow zones, while Sebastescarnatus (gopher rockfish) are found primarily in deep zones. They reasoned that this was because S. chrysomelas had competitively excluded S. carnatus from the more nutrient rich shallow waters. Indeed, our results show that S.chrysomelas has much morea higher relative abundance in the shallow zone at Hopkins Marine Reserve, while S. carnatus has much morea higher relative abundance in the deep zones at this same site (Figure 4). Based on our prior understanding of kelp forest ecosystem dynamics as influenced by Hallacher and Roberts (1985), these results make theoretical sense. Furthermore, these results make sense specifically for our study site, as the shallow zone has much more physical and biological habitat complexity suggesting that resource partitioning formed the asymmetric bathymetric distribution (Figures 1,2, and 3).
It should be noted that fish transects are highly prone to error. For instance, it is very hard to visualize a 2 by 2 by 2 meter box to estimate fish density while underwater. Furthermore, visibility is often variable, with the shallow zone often being more murky than the deep zone. This could have made our dataless precise. For example, pile perch aggregate in large schools in the mid-water column(U.S Fish and Wildlife Service 1983). Since our transects were conducted on the bottom and the shallow zone often experiences less visibility, it would make sense that far less pile perch were seen in the shallow zone than the deep zone. Furthermore, the seemingly huge relative abundance of pile perch in the deep zone may have been due to a large school swimming through all of the transects and thus being counted on each transect.
Fish species accumulation as a function of zone
Many studies have shown that increases in habitat complexity can lead to areas of higher diversity in kelp forest ecosystems. For example,Johnson (2007) found that habitat complexity in the kelp forest ecosystem could account for higher recruitment of blue rockfish thus leading to more species diversity in the area. Previous studies have shown that physical and biological habitat complexity at Hopkins Marine Reserve is greatest in the shallow zone (UCSC KFE 2012). However, our species accumulation analysis, shows that species diversity is highest in the deep zone (Figure 6).
It seems to us highly unlikely that species diversity is actually greater in the deep zone at Hopkins. Our results may have been influenced by poor visibility as visibility in the shallow zone is often worse than visibility in the deep zone. Our results may have also been influenced by sampling error, as fish transects are often difficult to perform, especially in poor visibility conditions.