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Density‐Dependent Population Regulation in a Texas Grassland
Author(s) -
Fowler N. L.
Publication year - 1986
Publication title -
ecology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.144
H-Index - 294
eISSN - 1939-9170
pISSN - 0012-9658
DOI - 10.2307/1938597
Subject(s) - density dependence , population density , population , grassland , perennial plant , biology , competition (biology) , ecology , reproduction , population size , vital rates , population growth , demography , sociology
This study examines the extent to which plant population dynamics are affected by density—dependent processes, and at what stage(s) in the life cycle density—dependent effects occur. A natural population of Bouteloua rigidiseta, a perennial grass, was subjected to two changes of density, the addition of seeds and the removal of 30—40% of the adult plants. Censuses were conducted before and after these treatments, one year apart. Survival, growth, and reproduction were all highly size dependent. Seed additions increased proportionately the number of new recruits to the population, indicating that the size of this age class was limited by seed input; seed additions decreased the size of new recruits. Adult removal tended to increase the sizes of both new recruits and adults. Thus competition among adults, among recruits, and by adults against recruits, but not competition by recruits against adults, affected this population in a density—dependent fashion. No density—dependent treatment effects involving reproduction were found. There was a high level of environmental heterogeneity in the magnitude of treatment responses. The magnitudes of all responses to changes in density were much less than the magnitudes of the changes themselves, indicating that density—dependent effects were quite weak. The most probable cause was the decline of the B. rigidiseta population following a drought after the first census and before the treatments were effectively imposed. The highly variable climate of this region may make the relaxation or absence of density—dependent population regulation common. The size of new recruits to the population of Aristida longiseta, another perennial grass, was decreased by the increase in the number of B. rigidiseta new recruits, and the survival of A. longiseta recruits may have been increased. No other effects of the treatments on A. longiseta were found, and the treatments had no effect on the total densities of three other grasses. These results may also be a product of the decline in the B. rigidiseta population. The changes in abundances of the five species suggest that temporal partitioning of niche space is important in this community.