Evidence for a Southern California
Subtidal Trophic Cascade

 
Natalie Bollinger

Patrick Robinson

 

Introduction:

            Marine reserves have recently been a hotly debated topic in marine conservation.  The questions of which species and habitats to protect are of utmost importance in preserving both species interactions and community-level dynamics.  These questions are of particular importance when considering members of a trophic cascade.  Removal or loss of members of a trophic cascade will have both direct and indirect effects on other species in the community:  their importance in the community is disproportionately large.  An example of the diversity of these interactions can be seen in the trophic cascades in the Kelp forests of the west coast (fig. 1).  The presence of species making up higher trophic levels will often result in increased species diversity.  It is, therefore, extremely important to identify trophic cascades and the associated interactions to increase the likelihood of conservation success.

 

Figure 1:  Potential direct (solid lines) and indirect (dashed lines) effects in the dominant trophic cascade of temperate Kelp forests.


 

Pattern:

            In the kelp forests of the California Channel Islands, Sheephead (Semicossyphus pulcher), Lobster (Panilurus interruptus), Urchins (Centrostephanus coronatus), and Giant Kelp (Macrocystis pyrifera) are abundant species of varying trophic levels.  Within the marine reserve (Catalina Marine Science Center Marine Life Refuge), it has been observed that the density of Sheephead greater than 30cm (legal fishing size) is positively correlated with total Urchin density (fig. 2).  The effect of Lobster density on the regulation of Urchin density is not known for this system; however, Tegner and Levin (1983) show that Lobsters can have a significant negative effect on the density of different species of Urchins (Strongylocentrotus spp.).  Lastly, it was found that total Urchin density is negatively correlated to total Kelp density (fig. 3).  Thus, the observed relationships indicate the potential for a trophic cascade to be acting in this system.

 

Figure 2:  Comparison of the density of large Sheephead to total Urchin density within the Catalina Marine Science Center Marine Life Refuge.


 

Figure 3:  Comparison of total Kelp density to total Urchin density within the Catalina Marine Science Center Marine Life Refuge.



Goal:

            The goal of this project is to determine if the predator-urchin-kelp system acts as a trophic cascade (fig. 4).  Specifically, we will determine if predators, namely Sheephead and Lobster, regulate the urchin density and whether urchins regulate the kelp density; this implies that Sheephead and Lobster densities indirectly regulate kelp density.  Also, we will determine the relative importance of sheephead and lobster in the trophic cascade.  Lastly, we hope explain the positive correlation between Sheephead and Urchin densities by providing evidence that Urchins may find predation escape in size.

 

Figure 4:  Potential direct (solid lines) and indirect (dashed lines) effects in the hypothesized trophic cascade.

Background:

            In the past, several key studies have shown that a single apex predator can have disproportionate effects on the community in which it lives.  Paine (1966) described such an effect in the rocky intertidal zone.  The Ochre sea star was found to be the top predator in this system.  Through predation, the sea star increased the species richness via indirect effects.  The keystone predation of mussels by sea stars was found to disrupt the competitive exclusion of barnacles from the system.  Therefore, the presence of sea stars played a disproportionately large role in determining the structure of the community.

Estes and Duggins (1995) found a similar pattern in the subtidal kelp forest.  Sea Otters in Alaska demonstrate this effect via a trophic cascade.  In this system, Otters were hunted nearly to extinction.  Their subsequent population recovery was patchy:  some of the Aleutian Islands quickly repopulated while others remain Otter-free.  These two regions can be compared with respect to the relative abundance of the different species present.  On islands with a healthy population of Otters, Urchin density was kept low via predation.  Consequently, the low Urchin density resulted in a lowered predation intensity on the Kelp and, therefore, higher Kelp density.  On islands free of otters, the opposite pattern was observed.  Urchins were released from predation and had higher densities, causing an increase in predation intensity on Kelp.  Otters are, therefore, having a disproportionate effect on the kelp forest community due to the cascading effects the predation has on lower trophic levels.

            Based on the work of Cowen (1983) and Watanabe and Harrold (1991), a similar pattern was observed between Sheephead-Urchins and Urchins-Kelp, respectively.  Experimental results showed that Sheephead removal resulted in increased urchin density. Also, Urchins were found to decrease the abundance of Kelp through destructive grazing.  These two parts work together to show the potential for the existence of a trophic cascade in this system.  Similarly, Lobsters were found to have disproportionate effects on their environment via predation (Breen and Mann 1976).  Lobster predation was found to regulate Urchin density.  Likewise, Urchins were found to regulate Kelp density, thus completing the trophic cascade.   It should be noted that the Urchin and Lobster species in the aforementioned  studies are different than those described in our pattern, despite fulfilling similar ecological roles.  In this study, we hope to find evidence of these patterns, namely evidence of a trophic cascade, by conducting experiments on the similar species found in Southern California waters.

 

Hypotheses:

            The goal of this project is to determine if the predator-Urchin-Kelp system in Southern California acts as a trophic cascade, with the predators having a disproportionate effect on the community.  To determine whether Urchin density is regulated by predation, three specific hypotheses have been created.  First, the exclusion of lobster will result in higher urchin density. Second, the exclusion of sheephead will result in higher urchin density.  Third, the exclusion of predators (i.e. both Sheephead and Lobster) will result in higher density and increased variance in the size-frequency distribution of urchins.  This third test will hopefully explain the unexpected result of a positive correlation between Urchin density and Sheephead density (fig. 2) by showing a size refuge from Sheephead predation for larger Urchins.  To determine whether Kelp density is regulated by grazing, a single specific hypothesis is offered; namely, the exclusion of urchins will result in higher kelp density. The above links will build support for the presence of a trophic cascade acting to shape the community structure in the Southern California Kelp forest ecosystem.

 

Species descriptions:

Sheephead:

                        The California Sheephead is a large protogynous hermaphroditic diurnal demersal labrid.  The sex change occurs at approximately 30cm total length; thus, the individuals we are concerned with in this study are primarily male.  Sheephead are abundant in Kelp forest habitat from the Baja peninsula to Southern California and can grow to over three feet in length.  They maintain relatively small home-ranges (personal observation), thus potentially having a large impact on their immediate environment.  They have a diverse diet consisting of:  Crustaceans, Mollusks, Worms, and Echinoderms (including most species of Urchins) (Cowen 1983).  While Urchins (specifically C. coronatus) are not a large proportion of the Sheephead diet, Sheephead are thought to have a strong impact on Urchin behavior and, therefore, density (Nelson and Vance 1979).

Lobster:

                        The California Spiny Lobster ranges from Baja to Central California (www.pbs.org).  This hard-bodied crustacean can reach two feet in length.  They are nocturnal predators that feed primarily on Worms, Snails, Mussels and Urchins.  Studies have shown that Lobster predation has the potential to regulate Urchin population density (Tegner and Levin 1983).  This study focused on Strongylocentrotus spp.; the impact of Lobster predation on C. coronatus has not been observed.

Urchin:

                        The Crowned Sea Urchin ranges from the Galapagos to California.  They can reach sizes in excess of 15cm test diameter.  Vance and Schmitt (1979) show the range of the Urchin diet including Kelps (the dominant food source), Algae, and Bryozoans.  Their spines are significantly longer than the other common Urchin species in Southern California:  Purple Urchins and Red Urchins.  Crowned Sea Urchins are nocturnal and are central-place foragers (Nelson and Vance 1979).  The life history of these Urchins also includes a planktonic larval stage, with subsequent benthic recruitment.

Kelp:

                        Giant Kelp is a subtidal brown algae that occurs on rocky reef habitat.  Dense forests can grow rapidly within a single season, providing valuable habitat for an array of commensal companion species.  Live Kelp, in addition to drift Kelp, is an important food source for many species.  Kelp disperses via a planktonic stage as a spore before settling to the benthos.

Site Description:

        This experiment will take place in the Catalina Marine Science Center Marine Life Refuge located near the Isthmus of Catalina Island, California.  The interactions will be studied in the subtidal (20-50ft depth) over rocky reef substrate.  A dense Kelp forest occurs in this area, making it an ideal location for our experiments.  Due to its northeast orientation and the protection by a close island, the region is relatively sheltered from wave exposure.

Methods:

            To determine whether Urchin density is regulated by predation, three exclusion experiments will be conducted.

             To test whether the exclusion of Lobster will result in higher Urchin density, four experimental plots and four control plots will be created.  All plots will be 3m x 3m in size and located randomly within the reserve between 20 and 50 feet in depth.  The experimental plots will be completely bordered by a solid plastic edging one foot in height, thus excluding Lobster while not excluding Sheephead.  All Lobsters will be removed from the experimental plots in the beginning of the experiment.  For the first two weeks of the experiment, nightly and daily observations will be made to confirm the exclusion of Lobster and the presence of Sheephead, respectively.  Control plots will be bordered by intermittent plastic edging of the same height as the experimental plots.  On each side of the control plots, four one-foot gaps will be created, allowing the free passage of Lobster.  For the first two weeks of the experiment, nightly observations will be made to confirm the passage of Lobster through these plots.  Weekly observations of Urchin density will be made for one year.  Statistical comparison of experimental versus control plots will then be conducted to test the specific hypothesis that the exclusion of Lobster will result in higher Urchin density.  Expected results are shown in figure 5.

 

Figure 5:  expected results of Lobster exclusion experiment.


                        To test whether the exclusion of Sheephead will result in higher Urchin density, four experimental plots and four control plots will be created.  All plots will be 3m x 3m in size and located randomly within the reserve between 20 and 50 feet in depth.  The experimental plots will be covered by a taut mesh (2 inches x 2 inches) elevated one foot above the bottom by metal poles, thus excluding Sheephead while not excluding Lobster.  For the first two weeks of the experiment, daily and nightly observations will be made to confirm the exclusion of Sheephead and presence of Lobster, respectively.  Control plots will consist of only the support poles.  For the first two weeks of the experiment, daily observations will be made to confirm the passage of Sheephead through these plots.  Weekly observations of Urchin density will be made for one year.  Statistical comparison of experimental versus control plots will then be conducted to test the specific hypothesis that exclusion of Sheephead will result in higher Urchin density.  Expected results are shown in figure 6.


Figure 6:  expected results of the Sheephead exclusion experiment.

                        To test whether the exclusion of predators (i.e. both Sheephead and Lobster) will result in higher density and increased variance in the size-frequency distribution of urchins, four experimental plots and four control plots will be created.  All plots will be 3m x 3m in size and located randomly within the reserve between 20 and 50 feet in depth.  The experimental plots will be cleared of all Lobster, Sheephead and Urchins, then completely caged with a taut mesh (2 inches x 2 inches) supported by one foot poles at each corner, thus excluding both Lobster and Sheephead while allowing Urchin larvae free passage.  The control plots will be cleared of Urchins and consist of only the support poles.  For the first two weeks of the experiment, daily observations will be made to confirm the exclusion of Sheephead and Lobster in these plots.  Monthly observations of Urchin density and test size will be made for five years.  Statistical comparison of experimental versus control plots will then be conducted to test the specific hypothesis that exclusion of predators (i.e. both Sheephead and Lobster) will result in higher density and increased variance in the size-frequency distribution of urchins.  Expected results are shown in figures 7 and 8.

 

Figure 7:  expected result of predator (Sheephead and Lobster) exclusion experiment.

 

Figure 8:  expected result of Urchin size-distribution experiment.


     To determine whether Kelp density is regulated by grazing, one exclusion experiment will be conducted.

     To test whether the exclusion of Urchins will result in higher Kelp density, four experimental plots and four control plots will be created.  All plots will be 3m x 3m in size and located randomly within the reserve between 20 and 50 feet in depth.  The experimental plots will be cleared of all Urchins and Kelp, then fenced with fine mesh (1cm x 1cm).  The mesh will be attached to the 3m x 3m square on the bottom and extend 10 feet off of the bottom, supported by “fun noodles.”  This will exclude Urchins while not excluding most other species.  For the first two weeks of the experiment, nightly observations will be made to confirm the exclusion of Urchins.  The control plots will be cleared of both Kelp and Urchins and will be intermittently fenced by a fine mesh (1cm x 1cm).  On each side of the control plots, two two-foot gaps will be created, allowing the free passage of Urchins.  Weekly measurements of Kelp density will be made for one year.  Statistical comparison of experimental versus control plots will then be conducted to test the specific hypothesis that the exclusion of Urchins will result in higher Kelp density.  Expected results are shown in figure 9.

 

Figure 9:  expected results of Urchin exclusion

References:

Breen, P.A. and K.H. Mann 1976. Changing lobster abundance and the destruction of kelp beds by sea urchins. Mar. Biol. 34: 137-142.

Cowen, R. K. 1983. The Effect of sheephead (Semicossyphus pulcher) predation on red sea urchin (Strongylocentrotus franciscanus) populations: an experimental analysis. Oecolgia 58: 249-255.

Estes, J. A. and D. O. Duggins 1995. Sea otters and kelp forests in Alaska: generality and variation in a community ecological paradigm. Ecological Monographs 65 (1): 75-100. Paine, R.T. 1996.  Food web complexity and species diversity.  The American Naturalist 100:  368-378.

Nelson, B. V. and R.R. Vance 1979. Diel foraging patterns of the sea urchin Centrostephanus coronatus as a predator avoidance strategy. Mar. Biol. 51: 251-258.

Tegner, M. J. and L. A. Levin 1983. Spiny lobsters and sea urchins: analysis of a predator-prey interaction. J. Exp. Mar. Biol. Ecol. 73: 125-150.

Vance, R. R. and R. J. Schmitt 1979. The effect of predator-avoidance behaviour of the sea urchin Centrostephanus coronatus, on the breadth of its diet. Oecologia 44: 21-25.

Watanabe, J. M. and Harrold, C. 1991. Destructive grazing by sea urchins Strongylocentrotus spp. in a central California kelp forest: potential roles of recruitment, depth, and predation. Mar. Ecol. Progr. Ser. 71: 125-141.

http://www.pbs.org/oceanrealm/seadwellers/darknessdwellers/spinylobster.html