An Underlying Question in the Study of Ecology Is How Diverse Assemblages of Species Are

Overall score: 85/100

Title [[3/4 – you can’t attribute causality, so should be talking about relationships or associations, not effects]]

The Effects of Habitat Complexity on Relative Abundances and Species Accumulation of fishes in the kelp forest at Hopkins Marine Station

By: Kyle M. Swann

Abstract

An underlying question in the study of ecology is how diverse assemblages of species are maintained. The ‘habitat heterogeneity hypothesis’ predicts that highly complex habitats will exhibit greater individual abundances and increased species diversity due to greater amounts of available resources. Results from a past study show a distinction of two zones within the reef at Hopkins Marine Station: a shallow and a deep zone, which vary in complexity. We conducted an observational study in the kelp forest at Hopkins reef to answer the following questions: 1) Do fish species differ in relative abundance as a function of zone (shallow, deep)?; 2) Does fish species accumulation differ as a function of zone (shallow, deep)? We found that fish species assemblages do differ as a function of zone at Hopkins reef. However, these results do not support our predictions, or those of the ‘habitat heterogeneity hypothesis.’ Examples of support and contradiction of current literature in our results suggests the need for more robust survey designs.

Clarity [[13/14 – you have a nice clear writing style]]

Introduction [[18/20 – nice job]]

An underlying question in the study of ecology is how diverse assemblages of species are maintained. One hypothesis used to explain this maintenance is the ‘habitat heterogeneity hypothesis’ (HHH) (MacArthur & Wilson 1967). The HHH predicts that highly complex habitats will exhibit greater individual abundances and increased species diversity when compared to less complex habitats (Tews et al. 2004). This is because the HHH assumes that structurally complex habitats will provide more ecological niches and resources than structurally simple habitats, allowing for a greater diversity of species and higher abundances of organisms (Tews et al. 2004). Previous studies have made observations in accord with the predictions of this hypothesis.

Hansen & Coleman (1998) studied the effect of forest litter complexity on the species diversity and composition of oribatid mites. They found that mite species richness was greater in treatments with mixed litters of 3 or 7 litter fall species when compared to treatments with one litter fall species. Similar results were observed in a study conducted by Friedlander & Parrish (1998). They assessed the effects of multiple habitat variables across different reef types on fish assemblages in Hanalei Bay, Kauai. Species diversity and individual abundances of reef fishes were found to be greatest on areas of the reef with high habitat complexity and heterogeneity. Like these studies, the majority of research of habitat complexity effects on species assemblages has focused on terrestrial forest and coral reef ecosystems. Few examinations of these interactions have been conducted in kelp forest systems.

Kelp forests are one of the most diverse ecosystems in the world. These habitats provide resources for species from at least 10 different phyla (Stenech et al. 2002). The foundation species of these forests comprise the large brown seaweeds of the Orders Laminariales and Fucales (Dayton 1985). These kelps provide a multitude of three-dimensional habitat for invertebrates, fish, and marine mammals. For example, Macrocystis pyrifera is the primary forest building species in central California. The holdfast of this species can provide habitat for scores of invertebrates, while the stipes and blades that arise from the holdfast are utilized by fish, other invertebrates, and marine mammals (Carr 1983; Estes et al. 1998; Watanabe 1984). The brown algaes that create kelp forests require a hard surface for establishment. This results in the formation of forests on substrates of varying complexity (Dayton 1985). Thus, the diversity in species and habitat present in kelp forests makes them an excellent system to study the effects of habitat complexity on species diversity and abundances. Of particular interest is a kelp forest in the Monterey Bay at Hopkins Marine Station.

A previous study conducted in the kelp forest at Hopkins Reef (36 36’ N, 121 54’ W) quantified differences in substrate characteristics and biological attributes (KFE 2012; Fig. 1, 2 & 3). These results show a distinction of two zones within the reef: a shallow and a deep zone. When compared to the deep zone, the shallow zone is characterized by greater amounts of large substrate, high relief, and three-dimensional structure provided by the brown seaweeds Cystoseira osmundacea and M. pyrifera. We conducted an observational study in the kelp forest at Hopkins reef to answer the following questions: 1) Do fish species differ in relative abundance as a function of zone (shallow, deep)?; 2) Does fish species accumulation differ as a function of zone (shallow, deep)? Based on the HHH, we predict that the more structurally complex shallow zone will have greater species abundances and species accumulation than that of the deep zone. To our knowledge, this is the first study of its kind to be conducted in the kelp forest at Hopkins Marine Station.

Materials and Methods [[15/18]]

General Approach

To test our hypotheses we conducted comparative observational surveys in the field. The surveying method used was benthic band transects. This method was chosen as it provides the most efficient means to characterize fish species composition. Through comparisons of habitat data from previous studies (Fig. 1, 2 & 3) with fish species composition data collected here, we are able to identify the effects of habitat complexity on species abundances and accumulation.

Study System

This study was conducted at Hopkins Marine Station of Stanford University in Pacific Grove, California (36 36’ N, 121 54’ W) on April 24, 2012. A large M. pyrifera kelp forest on granitic outcrops surrounded by shell-fragmented substrate characterizes the Hopkins reef (Watanabe 1984). The variability in substrate and relief type at this site allow for a large diversity in algal species, from the canopy forming M. pyrifera to understorey reds such as Rhodymenia spp. and Chondracanthus spp. Likewise, this variability in biogenic habitat maintains a large diversity in the animal species utilizing these resources. Thus, Hopkins reef is an ideal site for studying the effects of habitat complexity on species abundances and accumulation. The station’s status as a no-take marine reserve is also appealing, as it should limit the influence of direct anthropogenic forces.

Survey Design

Benthic band transects were conducted by buddy pairs perpendicular to the permanent transect cable at 5 m increments between the 90 m and 135 m marks. At each 5 m increment along the cable two transects were completed, one at a 90 heading and the other at a heading of 270. The depth range of all surveys was within 7 – 13 m. Every buddy pair was assigned to a meter mark between 90 m and 135 m along the permanent cable. Upon securing the transect tape to the permanent cable we began a survey on one of the designated headings, making counts as we reeled out the tape.

Fish abundances were recorded in 5 m segments during the 30 m survey. Each diver counted all fish species 2 meters above the transect tape, which lay along the benthos, and 1 m on either side of the tape. To assure that fish were not counted twice, a mental snapshot method was developed. Divers would take a mental snapshot of the water column to 2 m in front of them, only counting midwater fish that occurred in this 2 x 2 x 2 m box at that moment. Divers would then begin counting all benthic fish to the end of this 2 m mental snapshot zone and repeat until the survey was completed. Once completed, a second survey was conducted on a reciprocal heading using the above methods. Averages of each buddy’s counts were taken after all surveys were complete.

Prediction 1: Relative abundances of fish species in the shallow zone will be greater than that of the deep zone.

A graph of species densities per transect per zone will be created. A chi square analysis of the raw counts of this graph will allow us to determine if there is variance in the relative abundances of species between the deep and shallow zones.

Prediciton 2: Fish species accumulation of the shallow zone will be greater than that of the deep zone.

A species accumulation curve will be calculated. This curve will depict the number of species expected to be observed during transects as a function of the total number of 5 m segments sampled. For example, the graph will allow us to compare how many species we would expect to see in the shallow and deep zones if we sampled 5 segments or 35 segments. Two key features of the graph allow for interpretation of each zone’s diversity. The slope of the graph defines the evenness of the population. A steep slope represents more evenly distributed species than that of a shallow slope. The asymptote depicts the total number of species present in the population. If no asymptote were present, we would expect there to be more species in the population that have not been sampled. [[this should really all be past tense]]

Results [[13/16]]

General Results

The deep zone exhibited greater relative abundances of species, as well as greater numbers of species than did the shallow zone (Fig 4 & 5; Table 1).

Prediction 1: Relative abundances of fish species in the shallow zone will be greater than that of the deep zone.

These results refute our prediction that relative abundances of fish species in the shallow zone would be greater than those exhibited in the deep zone (Fig. 4; Table 1). While overall relative abundances of fish species were greater in the deep zone, specific examples contradict or support this result. Kelp rockfish, juvenile rockfish, and gopher rockfish were all found at greater relative abundances in the deep zone, while black-and-yellow rockfish showed greater relative abundances in the shallow zone.

Prediction 2: Fish species accumulation of the shallow zone will be greater than that of the deep zone.

These results refute our prediction that fish species accumulation of the shallow zone would be greater than that seen in the deep zone (Fig. 5). When 35 segments were sampled, total accumulated species were 14 and 18 for the shallow and deep zone respectively (Fig. 5). The steeper slope of the deep zone represents a more evenly distributed population from that of the shallow zone (Fig. 5). However, unlike the shallow zone, the deep zone curve does not reach an asymptote (Fig. 5). This indicates the possibility of unsampled species present within the deep zone population.

Discussion [[17/22 – excellent context, but you should do more than just come up with reasons why your results are not that robust. It’s also possible that they’re correct, so you should propose some mechanisms for that]]

General Discussion

The results of this study do not support the predictions of the HHH. Unlike results from previous studies (Friedlander & Parrish 1998; Hansen & Coleman 1998), areas characterized by less complex habitat (deep zone) exhibited greater relative abundances of species than did areas of more complex habitat (shallow zone). Patterns of abundances of some species (kelp rockfish and juvenile rockfish) contradict current literature (Carr 1983, 1989 & 1991; Holbrook et al. 1990). However, other species abundance patterns (black-and-yellow and gopher rockfishes) resemble results in current literature (Hallacher & Roberts 1985). The disparity between results of this and other studies is likely due to human sampling error and limitation in survey design.

Prediction 1: Relative abundances of fish species in the shallow zone will be greater than that of the deep zone.

Species whose abundance patterns contradict current literature are the kelp rockfish and juvenile rockfish species complex of kelp, gopher, and black-and-yellow rockfishes (KGBs). Past studies have shown associations of these fish species with the giant kelp M. pyrifera (Carr 1983, 1989 & 1991; Holbrook et al. 1990;). Hallacher & Roberts (1985) observed feeding behavior of kelp rockfish in which individuals would ambush invertebrate and fish prey a short distance from the kelp thallus. Carr (1989) observed strong affinities in juvenile rockfish to aggregate near M. pyrifera individuals, possibly for shelter. Our study suggests patterns of abundance that are not consistent with these authors’ findings.

Individuals of these species exhibited greater relative abundances in the deep zone at Hopkins reef (Fig. 4). KFE (2012) observed this deep zone to have fewer M. pyrifera individuals than that of the shallow zone (Fig. 3). Thus, our results of greater relative abundances of kelp rockfish and KGB individuals in areas with less M. pyrifera individuals is not consistent with the associational patterns and abundances observed in previous studies (Carr 1983, 1989 & 1991; Holbrook et al. 1990). However, the patterns of abundance in black-and-yellow and gopher rockfishes in our study similar to observations made by Hallacher & Roberts (1985). Hallacher & Roberts (1985) found that black-and-yellow rockfish were more prevalent in shallow waters, while gopher rockfish had higher abundances in deeper waters. Likewise, our results show higher abundances of black-and-yellow rockfish in the shallow zone and higher abundances of gopher rockfish in the deep zone (Fig. 4). However, in our study these species exhibited greater overlap from observed by Hallacher & Roberts (1985). The inconsistencies between previous studies results and ours are likely due to error in sampling and design.

The day this study was conducted was the first day in which all dive buddy groups completed benthic band surveys using this methodology. Disparities in the way divers counted fish may have led to skewed results. For example, some divers only counted fish to the left or right of the tape, thus only sampling a 2 x 1 x 30 m box. However, other divers sampled fish in a 2 x 2 x 30 m box. Because each dive buddy pair’s counts were averaged, this could have led to an under representation of some species. Another discrepancy may have resulted in miscommunication of the mental snapshot method between buddy pairs. For example, some divers may have taken mental snapshots more frequently than every two meters. This could result in an overestimation of some species. Better standardization of methods will be essential to future studies.

Prediction 2: Fish species accumulation of the shallow zone will be greater than that of the deep zone.

Our results of species accumulation are also inconsistent with past studies (Friedlander & Parrish 1998; Hansen & Coleman 1998). Figure 5 shows the less complex habitat of the deep zone exhibiting a greater number of species than the more complex shallow zone. Further, the lack of an asymptote in the deep zone curve suggests that this population was not adequately sampled (Fig. 5). In order to adequately sample the fish population at Hopkins reef it seems that more replicates are needed. However, the shallow zone curve does exhibit a slight asymptote (Fig. 5). Thus, more replicates may increase the difference in species accumulation of the deep and shallow zones. While these results may also be due to human sampling error, it is possible that aspects of biology that have been overlooked in our surveys may have an influence on distributional patterns (Hallacher & Roberts 1985; Holbrook et al. 1997).

Conclusion

In order to properly assess the abundances of fish species extensive replication through space and time is necessary. For example, Hallacher & Roberts (1985) studied rockfish community assemblage under differing physical oceanic conditions. They found that the distribution and feeding behavior of rockfish species differed between upwelling and non-upwelling periods. Similarly, Holbrook et al. (1997) observed a shift in the distribution of fish species following a warming event in 1975. These studies highlight the importance of replication when attempting to accurately assess ecosystem interactions. While the results of this study refute the HHH, other author’s results suggest its importance. It is clear that various mechanisms act to maintain the diversity within kelp forests. Future authors are urged to consider multiple biotic and abiotic factors, and to design surveys and experiments that will account for these variables when investigating the maintenance of diversity in kelp forest ecosystems..

References [[6/6]]

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