Population Growth of the Pitcher Plant, Sarracenia Purpurea L., at Cranberry Bog, Licking County, Ohio1

Ohio J. Sci. GROWTH OF RED-TAILED HAWKS 19

POPULATION GROWTH OF THE PITCHER PLANT, SARRACENIA PURPUREA L., AT CRANBERRY BOG, LICKING COUNTY, OHIO1

KENT E. SCHWAEGERLE, Department of Botany, University of Illinois, Urbana, IL 61801

ABSTRACT. A population of the pitcher plant, Sarracenia purpurea L., at Cranberry Bog Nature Preserve, Licking County, Ohio, was founded by a single individual transplanted to the bog in 1912. From an observation of the population in 1921, the intrinsic rate of increase for this population was estimated to be between .0016 and .0023 per day. A recent survey of the present size of the population revealed approximately 157,000 pitcher plants. These observations suggest that the population has not only reached the carrying capacity of Cranberry Bog, but that it did so as early as 1942. Comparisons between the intrinsic rate of increase for S. purpurea and the few other plant species that have been studied suggest that pitcher plants have a relatively slow rate of population increase for an herbaceous species.

OHIOJ. SCI. 83 (1): 19-22, 1983

INTRODUCTION have been arrived at through demographic Despite the importance of population studies of age-specific birth and death rates growth to an understanding of the ecology whlch samPle population growth rates and evolution of plant populations, few during only a short period of a populations studies have presented reliable estimates of history. the growth rates of plant populations in A uniclue opportunity to directly mom- nature. The few estimates that do exist tor population growth over an extended period has arisen from an unusual historical 'Manuscript received 28 May 1981 and in revised event. The population of pitcher plants, form 25 September 1981 (#81-22). Sarracenia purpurea L., at Cranberry Bog 20 K. E. SCHWAEGERLE Vol. 83

Nature Preserve, Licking County, Ohio, time (Nt) is a function of the number at was founded by a single individual plant time zero (No), time (t), and the intrinsic brought to the bog in 1912 by Freda rate of increase (r). Such an estimate will Detmers, graduate student in botany at only be accurate for a population during a The Ohio State University (Schaffner period of exponential growth. Density- 1922). The population was observed by dependent, rate limiting factors which Schaffner in 1921 to consist of hundreds of would cause deviation from exponential individuals, and today pitcher plants cover increase can be assumed to be negligible the bog. during the first years of colonization. In- Sarracenia purpurea L. (Sarraceniaceae) isterpreting Shaffner's words "hundreds of an insectivorous, herbaceous perennial vigorous plants" conservatively as 200 found in low nutrient, usually bog soils plants, results in an average intrinsic rate along the eastern coast of the United States of increase of .0016 per day for this period and west to British Columbia (Crow of the population s history. A generous in- 1969). It was reported to have existed in terpretation of 2000 individuals modifies several Ohio bogs before their destruction. the estimate of r only slightly (.0023 per Unlike pitcher plant sites originating day). The intrinsic rate of increase for through natural processes, Cranberry Bog S. purpurea at Cranberry Bog almost cer- was accidentally created with the construc- tainly lies between these 2 values. tion of Buckeye Lake in 1832. Con- Since a 67—year period of population sequently, pitcher plants did not grow growth at this rate would produce an unre- there until their introduction in 1912. De- alistically large number of plants for the spite some vegetative propagation by rhi- limited area of Cranberry Bog, an estimate zomes (personal observation), most new of current population size (1978) becomes plants appear to develop from the small interesting. Such an estimate was obtained seeds (2 mm) and probably spend several by sampling the bog with a 0.50 m2 years in a juvenile period of vegetative frame. Fifty casts of the square at regular growth. A flowering plant may produce intervals along 2 transects of the popu- 10,000 seeds in one year, indicating a very lation approximates a random sample high potential for population growth. A of plant density. A mean density of 6—week period of stratification followed by 6.12 ± 1.00 SE individuals per 0.50 m2 warm temperatures results in nearly 100% was calculated using this procedure. The germination in the laboratory. These ob- area of the bog was estimated from mea- servations suggest high rates of mortality surements obtained with a Silva range before and during seedling establishment. finder to be approximately 12,000 m . The continuous blanketing growth of the The 95% confidence interval for popula- sphagnum may account for much of this tion size is 157,000 ± 50,000 individuals. mortality. The effects of the genetic bottle- From the above minimum and maxi- neck resulting from the founding event of mum estimates of intrinsic rate of increase S. purpurea at Cranberry Bog has been ex- (.0016 and .0023) and population size, amined in another paper (Schwaegerle and the carrying capacity for S. purpurea at Schaal 1979). Cranberry Bog can be calculated. Popu- lation growth can be examined using POPULATION GROWTH RATES Verhulst's logistic model of population An estimate of the intrinsic rate of in- growth, dN/dt = rN(l-N/K), where K crease for the first 9 years of population represents the carrying capacity of the en- growth at Cranberry Bog can be based vironment. The number of individuals in a upon Shaffner's (1922) observation. The population at time t is expressed by: Lotka model of exponential population K growth, N, = Ntf"', states that the num- N, = ber of individuals in a population at any [(K-N0)/N0] OhioJ. Sci. PITCHER PLANT POPULATION GROWTH 21

Substituting either the minimum or the a bog offers. The doubling time for maximum estimate of r in the equation, the population during the early colo- results in a value for K equal to present nization period is 1.2 and .82 years for population size, 157,000. Since population the minimum and maximum estimate of size would be expected to be fairly constant r, respectively. at Cranberry Bog due to the slow growth In contrast to the scarcity of information and longevity of established plants, this is on plant populations, many estimates of r probably a good estimate of the carrying for animal species are available. Animals capacity of the bog. The logistic growth exhibit a wider range of values and often model with the minimum estimate of r of much higher rates of increase than plants. .0016 also suggests that population num- For example, the beetle, Tribolium con- bers approached their present level as early fusum, at optimal conditions has an intrin- as 1942. This unique situation offers an sic rate of increase of .120 per day, while excellent opportunity to study the demo- Homo sapiens has an r of .0003 per day graphic processes in plant populations (Pianka 1974). Pitcher plants fall toward which result in density-dependent limits the lower end of this continuum. to growth. In conclusion, Cranberry Bog appears to Estimates of the intrinsic rate of increase have been a favorable environment for the for populations of other plant species pitcher plant. The observations discussed (table 1) demonstrate that the estimate de- here suggest the population expanded rived for S. purpurea is a reasonable one. throughout the bog and probably saturated However, this value falls towards the lower the habitat after about 30 years. Sarracenia end of the growth rates exhibited by the purpurea has an intrinsic rate of increase in herbaceous species that have been studied. agreement with other herbaceous species A low intrinsic rate of increase in the that have been studied. That S. purpurea pitcher plant would be consistent with falls in the lower end of these values is several other life history features such as: consistent with other features of its life 1) populations at the carrying capacity of history which suggest a generally more K- available habitat, 2) infrequent opportuni- selected strategy than might be expected ties for colonization of new habitat, and for an herbaceous plant. The great number 3) high interspecific competition in the of pitcher plants at Cranberry Bog reflects predictable, fairly constant environment the impact of exponential growth in an open environment and suggests that even non-weedy plant populations can respond TABLE 1 surprisingly rapidly to new resources. Maximum intrinsic rates of increase observed in field populations of 8 plant species (Leverich and Levin 1979,ACKNOWLEDGMENTS. The author thanks Barbara Werner and Caswell 1977, Sarukhan andGadgil 1974, A. Schaal, Barbara L. Benner, the 2 anonymous re- Hartshorn 1972, 1975). viewers, and the members of the Gleason Memorial Plant Ecology Laboratory at the University of Illinois for their support and helpful suggestions in Maximum the preparation, of this manuscript. This study was Species Habit observed r supported by a Sigma Xi Grant-In-Aid and by NSF grant DEB 76-09709- Phlox drummondii annual herb .0024/day Dipsacus sylvestris biennial herb .0026 LITERATURE CITED Ranunculus repens perennial herb .0016 R. bulbosus perennial herb .0058 Crow, G. E. 1969 A phytogeographical analysis R. acris perennial herb .0023 of a southern Michigan bog. Mich. Bot. 8: 51—61. Sarracenia purpurea perennial herb .0016-.0023 Hartshorn, G. S. 1972 The ecological life his- Pentaclethra tory and population dynamics of Pentaclethra macroloba tropical tree .0001 macroloba, a tropical wet forest dominant and Stryphnodendron Stryphnodendron excelsum, an occasional associate. excelsum tropical tree <.0001 Unpub. Ph.D. dissert. Univ. Washington, Seat- tle. 119 p. 22 K. E. SCHWAEGERLE Vol. 83

197 5 A matrix model of tree population model incorporating multiple modes of repro- dynamics, p. 41-51. In: KB. Golley and E. duction. J. Ecol. 62: 921-936. Medina (eds.) Tropical ecological systems: Trends Schaffner, J. H. 1922 Additions to the catalog in terrestrial and aquatic research. Springer- of Ohio vascular plants for 1921. Ohio J. Sci. Verlag, NY. 22: 92. Leverich, W.J. and D. A. Levin 1979 Age- Schwaegerle, K. E. and B. A. Schaal 1979 Gene- specific survivorship and reproduction in Phlox tic variability and founder effect in the pitcher drummondii. Amer. Nat. 113: 881-903. plant, Sarracenia purpurea L. Evolution 33: Pianka, E. R. 1974 Evolutionary ecology. 1210-1218. Harper & Row, NY. 356 p. Werner, P. A. and H. Caswell 1977 Population Sarukhan, J. and M. Gadgil 1974 Studies on growth rates and age versus stage-distribution plant demography: Ranunculus repens L., R. bul- models for teasel (Dipsacus sylvestris Huds.). bosus L., and R. acris L. III. A mathematical Ecology 58: 1103-1111.