Patterns and Possible Explanations of Spatial Variation in the Community Structure of Kelp Forests
Talley Sagot
October 6, 2011
Abstract
One approach that ecologists use to better understand the processes that influence the structure and dynamics ofecosystems, is to compare how communities vary spatially. Differences in spatial variability of distribution and abundance can enable scientists to identify relationships andimpacts of trophic cascades, as well as biotic and abiotic factors. By looking at different sites we may be able to distinguish components that structure kelp forest ecosystems.
Community structure of kelp forests rely heavily on species composition, distribution and abundance. We are interested in quantifying the relative abundance of species present to understand if there are differences in community structurebetweentwo relatively close kelp forest locations. And, if so, do those differences vary with taxa?
We quantitatively assessed the distribution and abundance in species composition. Hopkins Marine Station and Point Lobos Natural Reserve are located within miles of one another on the central coast of California. The kelp communities at Hopkins and Pt. Lobos are different. Algae and invertebrates species composition vary between the kelp communities. However, species composition of fish did not vary between sites. We think these differences in species composition are driven byphysical factors, such as location with regards to the wave exposure gradient and biological factors, such as species like M. pyrifera that create the foundation for a unique community structure (Hughes, 2010).
Introduction
One approach that ecologists use to better understand the processes that influence the community structure and function is to compare how communities vary spatially (Ling et al, 2009).The study of the distribution and abundance of species at different locations can enable scientists to identify,the mechanisms influencing structure and function of these communities (Estes, 1998). Studying communities at two different places with different physical (i.e. temperature, nutrients, light) and biological (i.e. predation, competition)characteristicscan help us identify which species, or group of species,is more important in structuring the community. Esteset al. (1998) were able to estimate differences in community structure around the Aleutians Islands. They found that even at very close locations the species composition was very different. For example, kelp beds with otters had a lot of algae; in contrast, kelp beds without otters had very little algae. Therefore, studying distribution and abundance of species at different locations can be very usefulto better understandecosystem function and structure response to sources of physical and biological variation.
Kelp forests are one of the worlds' most productive and species rich ecosystems and are helpful in describing species distribution and abundance (Mann, 1973). They provide ecosystem services such as replenishment of fish stocks by fuelling primary production, increase local species diversity, kelp harvesting and recreational practices for human use (Graham, 2007).They are distributed along a latitudinal gradient and respond to different physical and biological characteristics (Graham et al., 1997). Therefore, it is very important to study the distribution and abundance of species in the kelp forest.
Hopkins Marine Station and Pt. Lobos are two very close kelp forest systems with different physical characteristics, such as wave exposure.Hopkins Marine Station is very protected from high levels of wave action relative to Pt. Lobos, due to its’ geographical placement and the direction of the wave action gradient along the California coast(Figure 1, map). Due to proximity, easy access and species diversity and abundance, these are great places tocompare how community composition, distribution and abundance of species in the kelp forest, respond to different physical and biological sources of variation.
The aim of this study is to quantitatively assess the differences between species composition and abundance at Hopkins Marine Station and Whalers Cove, Pt. Lobos, by counting target species present along a series of 30m transects. By looking at different sites we may be able to identify the components that structure kelp forest ecosystems. Thus, studying species composition and abundance with two physically different kelp forest sites, Hopkins and Lobos, I can assess species distribution and abundance. I want to know if there is a difference in species composition between sites, and if so do they vary in taxa? Is there variability in the differences due to a day affect, if so, do they vary in taxa? Is there variability in the differences due to an interaction of day and site effect, and if so, does that vary with taxa?
Methods
To assess the differences between the kelp forests at Hopkins and Pt. Lobos, we quantitatively surveyed transects by counting 13 species of invertebrates, 9 species of fish, 7 species of kelp and one genus of surf grasson replicate 30m long by 2m wide transects. We did this on October 11 and 13, 2011, at Hopkins Marine Station(latitude: 36.6207929 and longitude: -121.9041211) and Whalers Cove, Point Lobos (Latitude: 36.5199616 - Longitude: -121.938566) in Monterey, Ca (Figure 1, map).
We did our quantitative sampling on SCUBA along an offshore 80m North South transect line at both sites. On both days and at both locations, 7 buddy pairs took 4 quantitative swaths of a separate meter sitealong the transect line. The first thirty meter transect, leg one (out) was a fish count and two (in) was an algae and invertebrate count and the same was done for the second thirty meter transect onshore. At Hopkins Reef the first two swaths were done at a 90degree heading off the deep side of the black cable and the last two swaths were done onshore at a 270degree heading off the shallow side of the cable transect. At Pt. Lobos the data was collected at a 90degree heading off two separate meter marks ranging 5 meters away (i.e. 20-25m) on the transect line. On each leg of the procedure target species were counted, number of M. pyrifera stipes about a meter off the bottom were counted and fish counts were taken from a 1m wide and 2m high volume. In our survey each transect swath was one replication.
In order to estimate differences in species composition between Hopkins and Point Lobos, each buddy pair counted two transects of individual target species present each day. We used Permanova, did a Bray Curtis similarity plot and statistically analyzed species distribution and abundance with Excelto find a difference in species composition between Hopkins and Point Lobos. We analyzed the differences of all target species to see if species composition varied by taxa between Hopkins and Pt Lobos. The target algal species were: Cystoseira osmundacea, Chondracanthus corymbifera, Dictyoneurum californicum, Macrocystis pyrifera, Dictyonueropsis reticulata, Eisenia Arborea and Pterogophera californica.The target invertebrate species were: Patiria miniata, Pycnopodia helianthoides, Pisaster giganteus, Pisaster brevispinus, Urticina lofotensis, Urticina piscivora, Pachycerianthus fimbriatus, Ballanophyllia elegans, Tethya aurantia, Calliostoma ligatum, Loxorhyncus grandis, Heliotis rufescens, Strongylocentrotus franciscanus.The target fish species were: Oxylebius pictus, Sebastes atrovirens, Sebastes mystinus, Sebastes chrysomelas, Sebastes carnatus, Hexagrammos decagrammus, Embiotoca jacksoni, Embiotoca lateralis, Damalichthys vacca.
In order to examine differences in species composition between days, each buddy pair surveyed two transects of individual target algal species present, at both locations on two days. This was to account for the variability a day affect would have between samples.We used Permanova, did a Bray Curtis similarity plot and statistically analyzed each day species distribution and abundance to find a difference in species composition between days. We used Excel and Permanova when we analyzedalgae, fish and invertebrates to see if the statistical difference in species composition between days varied by taxa.
In order to examine differences of the interaction with both sampling day and sample site, we used a Bray Curtis similarity plot and statistical analysis with Permanova. This was to account for the variability associated with site and sampling day interaction effect. We used Excel and Permanova when we analyzed algae, fish and invertebrates to see if the statistical difference in species composition between the interaction of sample day and sample site varied by taxa.
We looked at differences in species composition between location and if it varied by taxa. We looked at differences in species composition between sample days and if it varied by taxa. We looked at differences in species composition between the interaction of location with sample day and if it varied by taxa. In our survey we collected 33 replications; each transect swath counted as one replication to control for unexplained variability. We used a power index to assess good survey design including sufficient replication for maximum statistical power, sufficient days a site is sampled, and a look at the ease with which species may be adequately sampled. We chose a p-value below .05 to determine statistical significance.
Results Bold this
General comment here: state the result and reference it. Don’t talk about the P-value unless you have some specific reason to. There is a statistically significant difference in all species composition between Hopkins and Pt. Lobos (Figure 2). The Permanovas showed a p-value associated with the site comparison as .001(Table 1.1). We found that the species composition did vary by taxa. The statistical differences in algal and invertebrate species composition had a p-value of .001 associated with the site comparison (Table 1.2, 1.4). The specific species that primarily contributed to the differences were: B. elegans, P. californica, C. osmundacea, C. corymbifera, P.miniata(Table 2).The source of variation with algae and invertebrates was one hundred percent explained by site(Chart 1). However, there was not a statistically significant difference in species composition of fish, with an associated p-value of 0.3(Table 1, Fish).
There is no statistically significant difference in species composition between days over all species (Table 1, All Species). However, there is a difference in species composition between days that varies with taxa. There is a statistically significant difference in species composition with fish between days which has an associated p-value of .04 careful….,there was an interaction which means that you cannot conclude a day effect without dissecting it by site. (Table 1.4).The specific species that contributed predominantly to the differences were: E. lateralis, S. mystinus, H. decagrammus, S. atrovirens (Table 2, Fish-Comparison of Days). The source of variation with fish was roughly explained by an 80% variance in sample day and 20% variance site (Chart 1).Not Chart- FIGURE!
The results of the interaction effect of sampling day and sampling site were statistically insignificant for overall species composition. The interaction effect on species composition does vary by taxa, though. In regards to algae or invertebrates, there was no significant difference due to an interaction effect, but there is a statistically significant difference in species composition with fish which has an associated p-value of .01 (Table 1.4). Once again, the source of variation with fish was roughly explained by an 80% variance in sample day and 20% variance site (Chart 1).
In our survey each transect swath counted as one replication to control for unexplained variability by making sure we had sufficient replications. The power index suggests that we collected an adequate amount of replication in regards to all species compositionReally? To me it looks like Algae were adequately sampled but not Fish or Invertebrates. (Chart 2). Species of algae, such as;M. pyrifera, C. osmundacea, P. californica have the highest power index (Chart 3). Species of invertebrates, such as; B. elegans, P.miniata, T. aurantia and U. piscivora have the highest power index (Chart 5).All species of fish had a high power index, except S. chrysomelas(Chart 4). The analysis of variance shows that number of stipes per transect varies as a function of site with an associated p-value of .000008,but not sample day or interaction between site and sample day (Table 3, Stipes per Transect).
Discussion
We found a significant difference in species composition between Hopkins and Point Lobos. This difference doesn’t include fish. The variation between locations was mainly due to invertebrates and algae; species abundance of algae are 81% dissimilar between sites. Species that contributed to this difference were mainly P. californica, C. osmundacea, C. corymbifera, D. reticula, D.californicum, E. arborea, M.pyrifera. Thealgal community at Hopkins is mainly M. pyrifera, D. californicum, D. reticulata, and C. corymbifera. The algal community at Pt. Lobos is primarily P. californica and E. arborea. The distribution and abundance of invertebrates at Hopkins were B. elegans, T. aurantia, and P. fimbriatus. The abundance of these invertebrates at Pt. Lobos was far less.
This is sounding like a regurgitation of the results. In the discussion, you want to talk about reasons for the differences as younbring them up.
The difference between days and interaction of both site and day effect, really only affected the fish samples. The first day dives were done in relatively calm waters while the second day dives had to deal with a considerable amount of swell. This may have scared the fish away or our divers flying through the benthos might have scared them away or into hiding. Most of the variance was due to day and a little due to site. The power index for fish looks as if more replications are needed for stronger correlations (Chart 2). Generally, avoid referencing figures in the discussion…only in the results
There is variation between species that require more replications.Replications worked better to accurately quantify some species more than others. For example, B. elegans could have been more accurately quantified using random quadrat placement for measuring densities in kelp beds rather than transects due to its distribution and abundance. We probably did more than the optimal number of transects that could be sampled for algae; 20 would’ve probably been fine (Chart2). The optimal number of transects done for fish and invertebrates should be more than 35 to maximize statistical power. The number of sample days is adequate since most of the species power index was above 2.
Our findings support our hypothesis and predictions.The dense canopies at Hopkins Marine Station are mainly formed from the species of giant kelp, Macrocystis pyrifera, which provide food and shelter to a plethora of organisms.Since Pt. Lobos varies in community structure due to algal differences it would suggest that the species associated with M. pyrifera would be less abundant (Hughes, 2010). Hopkins is relatively protected from strong wave exposure from Pacific Ocean currents and wind patterns, while Pt. Lobos is much more exposed to the open ocean and therefore more influenced by wave motion (Figure 1, map). Comparing ecosystems we know sites differ as an effect of exposure to wave motion. Pt. Lobos is exposed to higher levels of wave action which leads to tearing and breaking of portions of kelp plants. This wave exposure gradient allows giant kelp to flourish at Hopkins. Pt. Lobos is exposed to a higher wave motion gradient which does not allow for such large kelp to do as well. The smaller kelp such as P.californica and E. arborea are much better adapted to absorb the impact of higher wave motion (Graham et al., 1997). The abiotic factors of physical disturbance to the area, coupled with the biotic factors of the local kelp species present, contribute greatly in characterizing each unique kelp forest system (Hughes, 2010).
At the end, it is good to step back and mention some of the big picture. Why do we care about variation in communities across exposure gradients for example. What application do thes results have? Future research?
References
Dayton, D.K. 1985. The ecology of kelp communities. Ann. Rev. Ecol. Syst. 16: 215-245.
Estes, J. a. (1998). Killer Whale Predation on Sea Otters Linking Oceanic and Nearshore Ecosystems. Science, 282(5388), 473-476. doi:10.1126/science.282.5388.473
Graham, MH, Harrold, C, Lisin, S, Light, K, Watanabe, JM, Foster, MS. 1997. Population dynamics of giantkelpMacrocystis pyrifera along a wave exposure gradient. MARINE ECOLOGY-PROGRESS SERIES:148, 1-3, 269-279.
Graham, M H, Vasquez, J A, & Buschmann, A H. (2007). Global ecology of the giant kelp Macrocystis: From ecotypes to ecosystems. Oceanography and marine biology, 45, 39-88.
Harrold, C., Watanabe, J. and S. Lisin. 1988. Spatial variation in the structure of kelp forest communities along a wave exposure gradient. P.S.Z.N.I. Marine Ecology 9(2):131-156.
Hughes, B. B. (2010). Variable effects of a kelp foundation species on rocky intertidal diversity and species interactions in central California. Journal of Experimental Marine Biology and Ecology, 393(1-2), 90-99. Elsevier B.V. doi:10.1016/j.jembe.2010.07.003
Ling, S. D., Johnson, C. R., Frusher, S. D., & Ridgway, K. R. (2009). Overfishing reduces resilience of kelp beds to climate-driven catastrophic phase shift. Proceedings of the National Academy of Sciences of the United States of America, 106(52), 22341-5. doi:10.1073/pnas.0907529106
Mann, K.H. 1973. Seaweeds: their productivity and strategy for growth. Science 182: 975-981.
Results (25)
__3__/4 Figure legends Accurate
__2__/4 Figure Legends well composed (complete and concise)
__3__/5 Results organized according to questions
__3__/4 Graphs presented in a logical order, case made for the order
__3__/4 Grammar, sentence structure and spelling
__3__/4 Clarity and conciseness of writing
Discussion (25)
____/11 How well did they answer the questions they present in the Intro?
1)__2__/2 Discuss the results from the specific to the general.
2)__3__/3 Answered question about how sites differ in community composition?
3)___2_/3 Assessed sources of variation and made a strong case for whether we are able to detect differences among sites.
4)__2__/3 Presented sound logic as to why certain taxa or species were sampled well.
__2__/2 Grammar and Spelling
__3__/3 General Thoughtfulness
__1__/2 Clarity and conciseness
__3__/5 Organization of discussion
__1__/2 Context and Bigger Picture
General Notes:
See the notes in the figures file. Call them figures, not charts. Be concise and more descriptive in the legends. Think standalone.
In the results state your result and reference it.
Good thoughts on the topic, but the discussion can use better organization.
Subheadings would have been useful in the results