Kenneth S. Norris Rancho Marino Reserve

01 Jan, 04 AM - 06 Jan, 04 AM

The ecological effect of reduced body size on rocky intertidal gastropods. Goal of Research Rocky intertidal invertebrates have provided for a major source of nutrition for humans since prehistoric times. Exploitation of these resources is far more intense and wide-ranging today compared to subsistence collecting in the past. Both pre-historic and modern humans preferentially select the larger individuals of a population for consumption. The effect of this size selective harvesting has been a significant decrease in maximum body size of rocky intertidal gastropods in the last century from southern California (Roy et al, 2003). These decreases are not strictly limited to gastropods collected directly for food. Therefore, human impacts are causing an overall size decline in an entire assemblage of molluscs. The widespread decline in maximum body size of rocky intertidal gastropods in California is a critical problem that must be understood in order to effectively manage these precious ecosystems. The goal of my research is to investigate how this reduction in body size is influencing the ecology of a highly exploited gastropod and its impact on the ecology and community structure of the rocky intertidal of California. Body size is a very important character with which many morphological, physiological, behavioral, and ecological traits are correlated (Brown, 1995; Calder, 1984). A decrease in body size and abundance of an entire assemblage of molluscs in a community can potentially have drastic effects on the viability of the entire system. This is especially true for molluscs, which gain fecundity with an increase in body size. The effect of removing the most fecund individuals from a population can be dramatic (Branch and Moreno, 1994). Since human impacts of intertidal communities are an ongoing process, it is essential to understand how these pressures are affecting the viability of individual species. The limpets of California are ideal organisms with which to study the effect of human exploitation on the ecology of rocky intertidal gastropods. They are easy to follow and tag, and for most species, the life-history characteristics have already been described. Lottia gigantea, the Owl Limpet, has an excellent history of exploitation as a food source (Lindberg et al, 1998). Both the susceptibility of L. gigantea to human predation and its unique life history features make it a useful study organism for this project. L.gigantea has indeterminate growth with a maximum-recorded length of 116mm and can reach an age of about 30 years (personal observation). Like most limpets, L.gigantea also has planktonic larval dispersal. As an aggressively territorial limpet, L. gigantea is one of the dominant space occupiers of the rocky intertidal of California and thus has a direct influence on the abundance of other invertebrates and algae. Moreover, L. gigantea is a protandric hermaphrodite that sequentially changes sex from male to female, with the large size classes being predominantly territorial females (Wright and Lindberg, 1982). The human effect of removing the territorial individuals can have a large impact on both the local community structure of invertebrates and algae and also on the population viability of L. gigantea. Exploitation of L. gigantea continues, and mainland populations at most localities have been greatly reduced (Pombo and Escofet, 1996). Using Lottia gigantea as a model organism to study the effect size-selective harvesting has on the ecology and life-history of this species I am addressing the following questions: 1. Historical Changes: To what extent have humans altered the size structure, growth rates, and longevity of L.gigantea within the past few thousand years? Data from this portion of the study will come from a combination of archaeological Indian midden specimens, museum specimens, and live specimens. 2. Sex Ratio: How is size-selective harvesting affecting the sex ratio of L.gigantea? Because large females are preferentially collected and territory acquisition is the main factor controlling sex change (Wright, 1985), I hypothesize that males are changing sex at an earlier age at impacted sites. 3. Reproductive Output: How is size-selective harvesting affecting the reproductive output of female L.gigantea, and thus the population as a whole? I hypothesize that females at impacted sites (which constitute the vast majority of sites in California) are contributing far fewer gametes to the gene pool than females at non-impacted sites. Rationale The diverse biota of the rocky intertidal ecosystems of California is under an increasing and continuous threat from various human activities. As described above, reduced body-size can potentially have deleterious effects on both the viability of individual species, but also on the community structure of the entire ecosystem. However, there is evidence that some of these impacts have the potential to reverse themselves to more natural conditions given enough time (Roy et al, 2003). California law prohibits the collection of invertebrates at nearly every rocky intertidal site in California, yet enforcement of these laws is virtually non-existent (Murray et al, 1999). I intend to completely investigate just how much damage is being done by focusing my study on one of the most exploitable species of mollusc in California (Lottia gigantea). I anticipate both technical and societal benefits from the results of this work. Technically, this research will help policy-makers and law enforcement officials such as the California Fish and Game Department develop a more vigorous strategy to curb the illegal collection of invertebrates. In a more social context I believe that with the results of this project will come a better understanding of how people are impacting such a valuable resource that has very much been a part of humanity for thousands of years. Approach I will be conducting fieldwork for this project at exploited and non-exploited rocky intertidal sites throughout the geographic range of Lottia gigantea (Mid-Baja to northern California). The University of California reserve system will be a valuable resource for this project. I intend to have field sites at 5 UC reserve sites: Scripps Coastal reserve, Santa Cruz Island reserve, Coal Oil Point reserve, Ken Norris reserve and the Landels-Hill Big Creek reserve. Each reserve has a different history of human predation on L.gigantea. While it is illegal to collect invertebrates at each reserve without a permit, poaching is still common at some sites (personal observation at Scripps Coastal reserve; personal communication from reserve manager of the Ken Norris reserve). I will employ the same field techniques at each reserve. Fieldwork will be intermittent starting in Fall 2003 and lasting until 2007. Historical Changes: Lottia gigantea has been exploited as a food source since the peopling of the west coast, approximately 13,000 years ago (Vedder and Norris, 1963; Lindberg et al, 1998). Shell middens are common along the California coast, with L.gigantea shells constituting a large proportion of the assemblages, especially on the Channel Islands (Meighan, 2000). Also, museum collections for this species date back to the mid-19th century. I have recorded the dimensions of body size, date and locality for every specimen at four different museums in California (UC museum of Paleontology, San Diego museum of Natural history, Santa Barbara museum of Natural History, and the Los Angeles County Museum) totaling 2,300 specimens throughout its geographic range. A study comparing historical change in maximum body size using modern and museum occurrences of L.gigantea indicate that average length has decreased significantly in southern California (Roy et al, 2003). I will expand the data set of this study to include pre-historic, historic, and modern specimens of L.gigantea throughout its geographic range. Besides measurements of body size, I will also be collecting data on longevity, growth rates, and changes in morphology through time. When a Lottia gigantea shell is sectioned length-wise, one can observe small growth increments arranged parallel to one another inside the thickness layer of the shell. These growth increments are similar to the growth rings of a tree. As in dendrochronology, I can count the number of growth increments of a limpet shell to approximate the age of the individual (sclerochronology). Analysis of field data on growth rates compared with sclerochronology indicates that the observed growth increments represent one year of growth. I will be confirming this result through the use of stable oxygen isotopes as they relate to fluctuations in seasonal water temperatures (growth increments are separated by distinct lines due to a cessation of growth during the winter months). In addition, average annual growth rate can be determined by dividing the length of the shell by the number of growth increments. By comparing growth rate, age and morphology from specimens ranging over thousands of years to the present, I will be able to quantify the extent of human impacts on these life history characters. Sex Ratio: The main factor controlling sex change in L. gigantea is acquisition of a territory (Wright, 1985; Wright, 1989). Therefore, most of the small individuals in un-impacted sites are non-territorial males. Territorial limpets will keep their approximate 1 ft2 ?garden? free of intruders by bulldozing newly settled larvae (mussels and barnacles) and other limpets (intra and interspecifically) (Stimson, 1973). Once a territory is opened up either through natural mortality or from poaching of a territorial female, the surrounding males are given the chance to set up their own territory. Once a male has established a territory, it takes approximately 1-2 years for sex change to take place (Wright, 1985). Therefore, males in exploited populations are likely to have more opportunities to acquire a territory and change sex compared to males in non-exploited populations where territorial females occupy much of the available habitat. I will measure the size-frequency distribution, age-frequency distribution (using sclerochronology) and sex ratio of impacted and non-impacted sites in order to test the hypothesis that males at exploited sites are changing sex earlier than would be expected under natural conditions. Preliminary data suggest that at least in southern California, most of the males are not larger than 45 mm. Given this information, analysis of size-frequency distributions at impacted and non-impacted sites in southern California demonstrate that most of the limpets at the highly impacted sites are below the approximate threshold for being a female. Therefore, either there is a serious sex ratio skew, or males are taking advantage of the reduced intraspecific competition from females and are starting to change sex at an earlier age. If males are indeed changing sex at an earlier age, this result could be evidence of modification of an important life history feature in response to human predation. This would be similar to the examples of fish changing their age at sexual maturity in response to size-selective harvesting (Trexler and Travis, 2000; Ricker, 1981; Rowell, 1993). Reproductive Output: In order to test the hypothesis that human impacts are causing an overall decrease in reproductive output of exploited populations, I will compare the reproductive output of exploited and non-exploited populations of L. gigantea. I will measure reproductive output as the ratio of gonad mass just before spawning to the weight of the limpet minus the shell. Gonad mass scales positively with size in L. gigantea (Kido 2000; Wright, 1985). I will sample individual limpets from the population equally across the size-frequency distribution to determine the relationship between size and gender. Once this has been established, I will only sample females because male fecundity is not a controlling factor in the overall fecundity of the population. It may seem obvious that reproductive output at exploited populations are generally lower, however the possibility exists that reproductive output will not be significantly different from non-impacted sites for a number of reasons. In the short term, harvesting will decrease the abundance of a population. However, with a lower density also comes reduced intraspecific competition for space by territorial limpets. A reduction in competition could trigger an increase in the settlement of juvenile limpets. The resulting increase in density of small limpets may be sufficiently high for their collective reproductive output per unit area to approach, or even exceed that of the larger limpets (Hockey, 1994). However, since the number of females determines the overall fecundity of the population in L. gigantea, this increase in small limpets may simply just be an increase in the number of males. Unless sex change is occurring at a higher frequency in exploited populations, this increase in density would not cause an increase in reproductive output. Field Methods I will be collecting size frequency distributions at each site using a quadrat and transect method. Once the size frequency distributions have been established, I will sample individual limpets equally across the distribution to determine the relationship between size, age (using sclerochronology), and sex. The number of limpets to be collected at each site will depend on the abundance and density of the population. For example, the density of limpets at Scripps coastal reserve is approximately 12 limpets per meter2 and the density of limpets at Coal Oil Point reserve is approximately 4 limpets per meter2(personal observations). Therefore, I will be collecting fewer individuals at Coal Oil Point than at Scripps because the density is much lower there. Limpets will be measured to the nearest mm using vernier calipers. Field measurements of growth rates will be measured by attaching very small numbered bee tags to the shells of limpets with a non-toxic super glue adhesive. Limpets do not need to be removed from the substrate to perform these measurements. Besides collection of limpets, no other field manipulation at the UC reserves will take place. I am very conscious about not over collecting limpets for this project. I will consult with the reserve manager on the number of limpets that will be collected prior to any removal once I have collected sufficient field data on density and size-frequency distributions. In addition, whenever I can find empty shells, I will section them to determine age and growth rate. I am confident that the number of limpets collected will not affect the health of the population of limpets at each site since a relatively small number will ultimately be collected. In addition, each limpet collected will provide data on a number of life history characters such as gender, reproductive output, age, and growth rate. Therefore, no limpet will go to waste for this project. ?When properly prepared, it is delicious, having finer meat and a more delicate flavor than abalone??.It is much too interesting an animal to be carelessly tossed down a gourmet?s gullet? A quote referring to Lottia gigantea from Between Pacific Tides References Branch, G.M. and Moreno C.A. Intertidal and Subtidal Grazers. Rocky Shores: Exploitation in Chile and South Africa. 1994 ed. Siegfried R.W. New York Brown, J. Macroecology. Chicago: University of Chicago Press, 1995. Calder, W.A. Size, function, and life history. Cambridge, Mass: Harvard University Press, 1984. Hockey, PAR. Man as component of the littoral predator spectrum. Rocky Shores: Exploitation in Chile and South Africa. 1994 ed. Siegfried R.W. New York. Kido, J. Variations in the Structure of Lottia gigantea (Owl Limpet) populations among an within sites on southern California rocky shores. M.S. Thesis. California State University, Fullerton 2000. Lindberg, D.R.; Estes, J.A.; Warheit, K.I.; Human influences on trophic cascades along rocky shores. Ecological Applications 1998 8(3): 880-890. Meighan, C.W. Overview of the Archaeology of San Clemente Island, California. Pacific Coast Archaeological Society Quarterly. 2000. 36: 1-17. Murray, S.N.; Denis, T.G.; Kido,J.S.; Smith, J.S. Human visitation and the frequency And potential effects of collecting on rocky intertidal populations of southern California marine reserves. Calif. Coop. Ocean. Fish. Invest. Report, 1999. 40: 100-106. Pombo, O.A.; Escofet, A.; Effect of exploitation on the limpet Lottia gigantea: a field study in Baja California (Mexico) and California (U.S.A.). Pacif. Sci., 1996. 50: 393-403. Ricker, W.E. Changes in the average size and average age of Pacific Salmon. Can. J. Fish. Aquat. Sci. 1981 (38) 1636-1656. Rowell, C. The effects of fishing on the timing of maturity in North Sea Cod (Gadus morhua). In: Stokes, T.K.; McGlade, J.M.; Law, R (eds). 1993. The exploitation of evolving resources. Springer-Verlag, Berlin, p 44-61. Roy, K; Collins, A.G.; Becker, B.J; Begovic, E; Engle, J. Anthropogenic impacts and historical in body size of rocky intertidal gastropods in southern California. Ecology Letters 2003. 6: 205-211. Stimson, J. The role of the territory in the ecology of the intertidal limpet Lottia gigantea. Ecology. 1973. 54 (5). 1020-1030. Trexler, J.C; Travis, J. Can marine protected areas restore and conserve stock attributes of reef fishes? Bulletin of Marine Science 2000. 66(3). 853-873. Vedder, J.G.; Norris, R.M.; Geology of San Nicolas Island California. U.S. Geological Survey Professional Paper. 1963. 369: 1-65. Wright, W.G.; Lindberg, D.R.; Direct observation of sex change in the patellacean limpet Lottia gigantea. J. Mar. Biol. Assoc. U.K. 1982. 62: 737-738. Wright, W.G. The behavioral ecology of the limpet Lottia gigantea: interaction between territoriality, demography, and protandric hermaphroditism. Ph.D thesis: Scripps 1985. Wright, W.G. Intraspecific density mediates sex-change in the territorial patellacean limpet Lottia gigantea. Marine Biology 1989. 100(3): 353-363.

Visit #2018 @Kenneth S. Norris Rancho Marino Reserve

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Under Project # 1610 | Research

University of California San Diego - Ecology, Behavior, Evolution. Division of Biology

graduate_student - University of California, San Diego

Reservation Members(s)

Phillip Fenberg Jan 1 - 6, 2004 (6 days)
Phillip Fenberg Jan 1 - 6, 2004 (6 days)

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