Seedling‐Scale Environmental Heterogeneity Influences Individual Fitness and Population Structure

This study examines the potential of small-scale environmental heterogeneity to influ- ence population structure and fitness of individuals within a population. Two populations of Abutilon theophrasti were established at each of three densities in the greenhouse. At each density, one population was grown on substrate made heterogeneous before planting, with the addition of stones, localized nutrients, or sand, and soil compactions on a scale corresponding to individual seedlings. These were termed "patch types" and were chosen to represent factors that would be unpredictable in both time and space to seedlings emerging in nature. Another population was established on homogeneous substrate at each density. Seedling height, final height, biomass, and seed number were measured. The variation in biomass of individuals within a population was significantly greater on heterogeneous than on homogeneous substrate at the lowest density. When the data were analyzed with ANOVA, there was a significant interaction of patch type with presence or absence of heterogeneity. Patch type explained 47-62% of the variance in 2-d seedling height and 27-33% or 30-37% of the variance in seed output or final biomass, respectively. A multiple regression model including patch type, seedling size, and random block together explained 58-76% of the total variance in final biomass, depending on density. Size of immediate neighbors showed no correlation with individual size. The data support our hypothesis that small-scale stochastic events can influence the position of an individual in a population hierarchy. This conclusion implies that genetic determinants of fitness may be overridden by commonly occurring stochastic events. The resulting lack of predictable selection against certain genotypes should contribute to the maintenance of genetic diversity in populations.

[1]  P. T. Spieth.  Environmental Heterogeneity: A Problem of Contradictory Selection Pressures, Gene Flow, and Local Polymorphism , 1979, The American Naturalist.

[2]  S. Levin,et al.  The role of mosaic phenomena in natural communities. , 1977, Theoretical population biology.

[3]  M. Posno,et al.  A COMPARISON OF THE REACTION TO A LOCALIZED SUPPLY OF PHOSPHATE IN PLANTAGO MAJOR, PLANTAGO LANCEOLATA AND PLANTAGO MEDIA* , 1979 .

[4]  J. Harper,et al.  The Demography of a Sand Dune Annual: Vulpia Fasciculata: I. The Natural Regulation of Populations , 1978 .

[5]  R. Allard,et al.  Microgeographical Variation in Allozyme Frequencies in Avena barbata. , 1972, Proceedings of the National Academy of Sciences of the United States of America.

[6]  John L. Harper,et al.  INTERFERENCE IN DUNE ANNUALS: SPATIAL PATTERN AND NEIGHBOURHOOD EFFECTS , 1977 .

[7]  O. T. Solbrig,et al.  STUDIES ON THE POPULATION BIOLOGY OF THE GENUS VIOLA. II. THE EFFECT OF PLANT SIZE ON FITNESS IN VIOLA SORORIA , 1981, Evolution; international journal of organic evolution.

[8]  W. Elberse,et al.  Germination of Six Grassland Herbs in Microsites with Different Water Contents , 1976 .

[9]  P. Hedrick,et al.  Genetic Polymorphism in Heterogeneous Environments , 1976 .

[10]  R. Sharitz,et al.  Population Dynamics of Two Competing Annual Plant Species , 1973 .

[11]  F. Bazzaz,et al.  THE POPULATION DYNAMICS OF ERIGERON CANADENSIS, A SUCCESSIONAL WINTER ANNUAL , 1979 .

[12]  J. Harper.  Population Biology of Plants , 1979 .

[13]  H. Levene,et al.  Genetic Equilibrium When More Than One Ecological Niche is Available , 1953, The American Naturalist.

[14]  R. N. Mack.  Survivorship of Cerastium atrovirens at Aberffraw, Anglesey , 1976 .

[15]  C. Blom.  Germination, seedling emergence and establishment of some Plantago species under laboratory and field conditions. , 1978 .

[16]  L. Gottlieb.  GENOTYPIC SIMILARITY OF LARGE AND SMALL INDIVIDUALS IN A NATURAL POPULATION OF THE ANNUAL PLANT STEPHANOMERIA EXIGUA SSP. CORONARIA (COMPOSITAE) , 1977 .

[17]  J. Ogden.  Plant population structure and productivity. , 1970 .

[18]  J. White,et al.  CORRELATED CHANGES IN PLANT SIZE AND NUMBER IN PLANT POPULATIONS , 1970 .

[19]  F. A. Bazzaz,et al.  Plant Species Diversity in Old‐Field Successional Ecosystems in Southern Illinois , 1975 .

[20]  R. Weiss,et al.  A search for genetic effects of high natural radioactivity in South India. , 1966, Special report series (Medical Research Council (Great Britain)).

[21]  John L. Harper,et al.  OCCUPATION OF BIOLOGICAL SPACE DURING SEEDLING ESTABLISHMENT , 1972 .

[22]  J. Harper,et al.  The Demography of Plants , 1974 .

[23]  O. T. Solbrig,et al.  The Population Biology of the Genus Viola: I. The Demography of Viola Sororia , 1980 .

[24]  A. P. Hartgerink.  Effects of Some Events Experienced by Seedlings in Competition , 1981 .

[25]  J. Silander,et al.  Microevolution and Clone Structure in Spartina patens , 1979, Science.

[26]  J. Burdon.  INTRA-SPECIFIC DIVERSITY IN A NATURAL POPULATION OF TRIFOLIUM REPENS , 1980 .

[27]  R. Paine,et al.  Disturbance, patch formation, and community structure. , 1974, Proceedings of the National Academy of Sciences of the United States of America.