The Galapagos Islands are home to a variety of flora and fauna, whose actions are magnified by the partial isolation of the islands. Lytechinus semituberculatus is a green sea urchin that is very common in large numbers located in the intertidal zones and shallow coastal waters with low wave action around the Galapagos. The urchins belonging to this species are considered browsers, nibbling on plants and debris with five, sharp, chisel-like teeth. Another species of urchin Eucidaris thouarsii, is located in similar habitats and often found intermixed with the green urchins. These urchins graze on encrusting algae and are considered predators on both pocillipora and pavona corals.
There are three prominent algaes located in the intertidal and shallow water zone in the area of the Galapagos in which the survey was performed. Ulva lactuca is a type of green algae that is two cell layers thick. It is not limited in length or width and will continue to grow if cut or torn due to grazing or wave action. It is found in the intertidal zone at depths up to 20 meters. Padina pavonica is a type of brown algae that grows on rocky or hard substrates also to a depth of 20 meters. The plants can grow up to 15 centimeters and are often calcified with calcium carbonate giving them their distinctive fan shape. Turf algae grow on the hard substrates and are often guarded by damselfish that maintain “algal lawns”. The turf at the sample site was brown in color and usually around 3-5 centimeters long.
All of these species interact with each other in Loberia Grande, a lagoon that is partly sheltered from the wave action of the ocean. The lava rock in the lagoon is covered with a combination of algae and urchins. According to Glynn et al. (1979), grazing by sea urchins can alter the distribution, relative abundance, and species composition of marine plants which may also indirectly affect animal populations. After noticing the large numbers of green sea urchins, I wondered if the high density had an impact on the ecosystem. The goal of the study was to determine whether the density of urchins had an impact on the abundance and type of algae. I hypothesized that a higher density of green urchins would correspond with a lower percentage of algae coverage due to the grazing pressure of the green urchins. I formed the same null hypothesis for both green urchins and slate pencil urchins. One of the goals of this experiment was to determine what kind of algae the urchins eat. This would be determined by the type of algae that has the lowest percent cover when numerous amounts of urchins are present. I also predicted that the slate pencil urchins would have no correlation with the abundance or type of algae because it does not prey on the types of algae present in Loberia Grande.
Null Hypothesis A: There is no relationship between density of green sea urchins and algal percent cover
Null Hypothesis B: There is no relationship between density of slate pencil urchins and algal percent cover.
Methods: Samplings were taken on February 17th and 23rd, 2011 at Loberia Grande on the island of Isabela in the Galapagos. The times of the survey were not relevant to the outcome of the experiment. A 0.25 m2 quadrat was used to estimate percent cover of the substrate along with the number and species of urchins in each quadrat. The quadrats were placed relatively randomly over the shallow, rocky regions of the lagoon. Some quadrats were purposefully placed to collect data for a certain amount of urchins in one area to gather a wider spread of data. This included intentionally collecting data from locations with no urchins present inside the area of the quadrat. A total of 40 quadrats were analyzed.
Results: A range of 0-13 green urchins and 0-5 slate pencil urchins were counted in the quadrats. Three different types of algae were most prominent in the sampled quadrats; ulva, padina, and turf. Other substrates found were cyanobacteria, sand, and rock.
Figure 1. Substrate composition and percentages of the 40 quadrats sampled during the study.
When the percentages for all types of algae were totaled, it was apparent that quadrats which contained more green urchins had less total percentage of algae. The R2 value of 0.6388 and the p value of less than 0.0001 show that this correlation is statistically significant.
Figure 2. The correlation between the number of green sea urchins and total percentage of algal cover in the 40 quadrats sampled. All three major types and “other” algae percentages were added together. The R2 value was 0.6388 and the p value was less than 0.0001.
The number of slate pencil urchins showed almost no correlation to the total percent of algae in each quadrat. In fact, the quadrat that contained 5 slate pencil urchins also had 95% algae cover. The trendline for the data also showed no steady correlation and produced a R2 value of 0.0006 and a p value of 0.883496, both of which indicated no statistical correlation.
Figure 3. The correlation between the number of pencil urchins and total percentage of algal cover in the 40 quadrats sampled. The R2 value was 0.0006 and the p value was 0.883496.
When comparing the number of green urchins to the percentage of different algal species, there is an inverse relationship with algal cover. As the amount of green urchins increases, the amount of algae decreases. This stands true for all 3 types of algae. The ulva and padina had downward trends, but the R2 values were 0.0771 and 0.0235 respectively, leading to no significant correlations. The relationship between the number of green urchins and the percentage of turf was much more pronounced and yielded an R2 value of 0.4566 and a p value of less than 0.0001 leading to statistical significance.
Figure 4. The relationship between the density of green urchins and the percentage of turf in the quadrat yielded an R2 value of 0.4566 and a p value of less than 0.0001.
Graphs and R2 values were calculated for each type of urchin-algae relationship, but only the green urchin vs. turf relationship was statistically significant.
Discussion: Null Hypothesis A stating that the density of green urchins had no correlation with algal cover percent cover was disproved. There is a relationship between the density and algal cover shown by the R2 value and p value. Both of these statistically lead to the conclusion that higher densities of green urchins will lead to lower percentages of algal cover. Since green urchins are known to feed on algae in the intertidal and shallow rocky zones, it is logical to assume that the lower algal cover is due to the grazing pressure of the green urchin.
Statistics also shows that the green urchins are eating turf more than any other type of algae. All algae show a decline in percent cover as the density of green urchins increases, but only the values for turf show that the correlation is statistically significant. This shows that the urchins may be grazing on all the types of algae, but turf is their preferred food source and they eat more of it than the other types of algae. This also shows that if there is a higher density of urchins in one area, the chances of turf growing in that same area are diminished. This could be a reason why the damselfish diligently remove urchins from their algal lawns and the area surrounding the algal lawns. Since damselfish like to maintain an intermediate amount of grazing, they are likely to remove urchins because urchins decimate the algae instead of maintaining it.
Null Hypothesis B was not able to be disproved. The slate pencil urchin density did not show any relationship to the percentage of algal cover. Comparing the density of the urchins to both total percent algae and individual species of algae yielded no significant correlation data. This is an expected result when incorporated with the information that this species of urchin does not eat algae, but mostly feeds on coralline algae and corals. The study by Glynn et al. (1979) reported that the slate pencil urchin had a significant (P<.05) inverse correlation with live coral cover which is similar to the correlation found in my own study between green urchins and algae.
Glynn et al. (1979) determined that removal of algae by sea urchins provides space for coral settlements on reefs. If green urchins are removing a significant amount of algae from a reef system, hard corals can settle which provides food for the slate pencil urchins. Although much of the coral was eliminated on Isabela by the 1982-1983 El Niño, it is still present in smaller populations. If it were completely eliminated, then there would be no slate pencil urchins present because they would have no food to sustain their populations. My study shows that the green urchins are effective in removing considerable amounts of algae from the reef. This information combined with the study by Glynn et al.(1979) shows a cycle of algae, corals, and urchins. The grazing by green urchins removes algae leaving room for corals to grow. The corals are eaten by the slate pencil urchins leaving room for algae to grow once again. When this cycle is in effect and in equilibrium across a reef system then populations of both green and slate pencil urchins will be sustained as they are in Loberia Grande.
What do you think? I think that Peter Glynn is my not so new hero. I'm so lucky that I got to learn from him- the Galapagos are his turf when it comes to scientific studies. Truly. Look up any published scientific papers about the Galapagos and I guarantee his name will pop up on at least 50% of them and he is probably referenced in the other half. Along with Peter Glynn I'd like to thank the Academy and also my sister Meredith for helping me through her amazing scientific librarian ninja genius powers. Basically she downloaded PDFs and emailed them to me since there's not enough bandwith? here and the connection times out before PDFs can be downloaded of of UM's server, but somehow it doesn't time out when downloading from email. Make sense to you? It doesn't make sense to me either. Anyways thank you Merry and go check out her Etsy business. It's pretty awesome and there is a link on the right side of my blog. Over there-----> You might have to scroll down a little.
The parents arrive on Saturday afternoon and we are going on a cruise (my first!) from Sunday to Wednesday. I have mixed feelings about this that I will explain in another post, but I am not sure if I will have internet on the boat. It makes sense that I won't, right? But I promise tales of adventure from the hotel in Santa Cruz towards the end of next week and maybe some pictures from my rental computer that my parents are bringing me. Is that enough to get you to stick around? Oh man am I going to need coffee in the morning!