Napompeth, Banpot

Nishida, T.

Proceedings of the Hawaiian Entomological Society

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11QVol. XXII, No. 1, August, 1975 'Effect of Age-Specific Predation on Age Distribution and Survival ofthe Giant African Snail, Achatina fulica1T. NlSHIDA AND BANPOT NAPOMPETH2' 3UNIVERSITY OF HAWAIIHONOLULU, HAWAIIThe giant African snail, Achatina fulica Bowdich, one of the mostdestructive molluscan pests of many tropical areas of the world, becameestablished in Hawaii November 30, 1936 (Pemberton, 1938). To controlthis pest predaceous snails, Gonaxis quadrilateralis (Preston), G. kib-weziensis (E. A. Smith), and Euglandina rosea (Ferussac) were introduced (Davis and Butler, 1964; Davis and Krauss, 1964) . According toDavis (personal communication, 1973) the population of A. fulica hasdeclined markedly in recent years and numerous empty shells have beenobserved in many areas of Oahu. Using these shells a demographic analysis was made in this study to determine the role of introduced predators on the biological control of this mollusk.Method of StudyThe life table approach was used in this study. To construct lifetables it was necessary to obtain shell samples in the field and to estimatethe age of each individual at the time of death.Collection and Measurement of Shells. The shells were collected fromtwo gulches, designated A and B, at Kailua, Oahu, March 18, 1973. Thegulches, undisturbed by man, were covered with dense growth of guava(Psidiurn guajava L.), koa haole (Leucaena glauca L. Benth.) and monkey pod (Samanea saman (Jacq.) Merr.). All unbroken shells within anarea of approximately 700 m2 were collected by searching through thelitter of decaying leaves and twigs. The number of shells collected fromgulch A and B was 185 and 154, respectively. The length of each shellwas measured to the nearest mm using a measuring rack built for thispurpose.Age Estimation. Since it was not possible to estimate age by appearance,shell length was used. Estimation of age was made by fitting the logisticgrowth curve to the shell length-age data of Kondo (1964) :Y = K / 1 + C ert1 Published with the approval of the Director of the Hawaii Agricultural ExperimentStation as Journal Series No. 1690. .2 Present address: Department of Entomology, Kasetsart University, Bangkok, Thailand.3 The helpful information and comments by C. J. Davis, State Department of Agriculture, and Frank Olson, Plant Protection Division, ARS, USDA, are gratefullyacknowledged.120 Proceedings, Hawaiian Entomological Societywhere Y = age; t = shejl length; e = base of natural logarithm; andK, C and r = constants. The method of curve fitting was the same asthat described by Pearl (1944). The calculated curve shown in fig 1indicates that although small numbers were used the observed and calculated values agreed quite well.140120 -50 ♦ 105H-IO8e-°O4t0 5 10 15 20 25 30 35 40 45 50 55 60AGE IN WEEKSfig. 1. The relationship between shell length and age of Achatina fulica. The calculatedequation is based on data of Kondo (1964).Construction of Life Table. The method of construction of life table wassimilar to that used for ungulates by Murie (1944), Deevy (1947) andSpinage (1972). Although complete life tables were constructed, data ononly age distribution and survivorship, considered most pertinent to thisstudy, are presented in this paper.ResultsAge Distribution. The age-distribution pyramids shown in fig. 2 indicatethat all age groups were not represented. In gulch A, individuals in agegroups younger than 12 weeks were missing and in gulch B, individualsin age groups younger than 9 weeks. The data also indicate that the agerange of shells in gulch A was higher than that of gulch B; 11-25 weeksfor A and 8-20 weeks for B.Field observations made during this study showed that predation byGonaxis spp. and E. rosea on young A. fulica was the major cause of thisVol. XXII, No. 1, August, 1975 121anomolous age distribution. In gulch A there were numerous livingGonaxis spp. and in gulch B, E. rosea. Although small numbers of largeliving A. fnlica were found, it was difficult to find small individuals. Similar observations were made by Davis (personal communication, 1973)and Davis and Butler (1964) who found that these molluscan predatorsare specific to the eggs and juvenile stages of A. fulica.Survivorship. The survivorship curves, shown in fig. 3, were based partlyon data from field collected shells and partly on estimates from Davis(personal communication, 1973) and Davis and Butler (1964). Theestimates were made because it was not possible to obtain a completearray of individuals in all age groups.2520* 15*50GULCH-A z--i I I i 1 I I 1 i i i i i i i IGULCH -B<Illlli—1 1 1 1 1 1 1 114 12 8 4 0 4PERCENT8 12 14 8 4 0 4PERCENTfig. 2. Age-distribution pyramids based on age estimated from length of dead fieldcollected shells of Achatina fulica.Fig. 3 indicates that in gulches A and B there was a marked decreasein survival at two points; one in young age groups and the other in theold age groups. In both cases the survival rate was lowest among youngage groups. It is also evident from fig. 3 that the overall survival ratewas lower in gulch B than in A. There appears to be no explanationfor this difference, except that in the former gulch the predominantpredator was Gonaxis spp. while in the latter it was E. rosea.122 Proceedings, Hawaiian Entomological Society1000800600iT\"\- \\- \_2<4OOOO2 2001000a:800^§ 600400200-0 -GULCH-A•—•—• OBSERVEDESTIMATED_J I L I I I I I I IGULCH -B\-\\- \J I I I L J I I I L0 2 4 6 8 10 12 14 16 18 20 22TIME IN WEEKSfig. 3. Survival curves of Achatina fulica based partially on data calculated from fieldsamples and partially from Davis (personal communication, 1973) and from Davisand Butler (1964) . The standard life table notation was used in this study; lx refersto the number of individuals surviving in each age class.DiscussionIn this study shell length was used in estimating the age of A. fulica.It should be pointed out that in using shell dimensions we disregardedaestivation, which appears to be a common phenomenon. Presumablyshell growth ceased during aestivation and therefore the predicted agescorresponded to physiological rather than chronological ages. Unlesscharacters aside from shell dimensions are used, it would be difficult toestimate the true chronological age.Among A. fulica populations throughout the world there is a greatvariation in the size of the shells (Mead, 1961) . This variation may bedue to genetic, nutritional and environmental factors. The variation insize and growth rate that might be caused by these factors would affectVol. XXII, No. 1, August, 1975 123age prediction. To improve prediction further studies are needed.One of the problems in a study of this kind is the differential perishability between shells of old and young snails. Shells of young snailsbeing thin and fragile do not withstand weathering and predation. Frequent sampling of the shells would partially eliminate this problem.The absence of individuals in young age groups in field samples was,however, not due entirely to perishability. Davis (personal communication, 1973) and Davis and Butler (1964) stated that Gonaxis spp. andE. rosea feed on eggs and young individuals of A. fulica. Therefore, thesepredators, acting as age-specific mortality factors, are capable of causingthe type of age distribution observed in this study. Furthermore, Davisand Butler found that in a living population of A. fulica there were 197individuals less than half inch in length, out of a total of 424, in theabsence of predators. After the introduction of predators they were notable to find any individual less than 2 inches long among a total of 250live snails collected. Such data also support the viewpoint that Gonaxisspp. and E. rosea are effective age specific mortality factors that can alterage distribution as well as reduce the survival of A. fulica.SummaryAge distribution and survival rate of the giant African snail, Achatinafulica Bowdich, were studied by means of life tables which were constructed by estimating the age of field collected shells of dead A. fulicaby shell length. The lack of individuals in young age groups was thecharacteristic feature of the age-distribution pyramids. This feature wasattributed to age specific predation by Gonaxis spp. and Euglandinarosea. The low survival rate among young individuals was also attributedto predation.LITERATURE CITEDDavis, C. J. 1973. Personal communication.Davis, C. J. and G. D. Butler, Jr. 1964. Introduced enemies of the giant African snail,Achatina fulica Bowdich in Hawaii (Pulmonata: Achatinidae). Proc. Hawaii.Entomol. Soc. 18(3): 377-389.Davis, C. J. and N. L. H. Krauss. 1964. Recent introductions for biological control inHawaii IX. Proc. Hawaii. Entomol. Soc. 18(3): 391-397Deevey, E. S. 1947. Life tables for natural populations of animals. Quart. Rev. Biol.22: 283-314.Kondo, Y. 1964. Growth rates in Achatina fulica Bowdich. Nautilus 78(1): 6-15.Mead, A. R. 1961. The giant African snail: a problem in economic malacology. University of Chicago Press. 257 pp.Murie, A. 1944. The wolves of Mount McKinley (Fauna of the National Parks of theU. S., Fauna series No. 5) . U. S. Dept. Inst. Nat. Park Service, Washington, D.C.Pearl, Raymond. 1940. Introduction to medical biometry and statistics. 3rd edition.Philadelphia, W. B. Saunders Co.Pemberton, C. E. 1938. The giant snail Achatina fulica (Fer.) discovered in Hawaii.Hawaiian Plant. Rec. 42(2): 135-140.Spinage, C. A. 1974. African ungulate life tables. Ecology 53(4): 645-652.

Effect of Age-Specific Predation on Age Distribution and Survival of the Giant African Snail, Achatina fulica

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Effect of Age-Specific Predation on Age Distribution and Survival of the Giant African Snail, Achatina fulica

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