. The changing Illinois environment : critical trends : technical report of the Critical Trends Assessment Project. Man; Pollution; Environmental protection; Ecology; Environmental impact analysis. PRAIRIES 0 2 CO DC CO Q) o5 E *c CD Q. • • 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 Fraction of 1940 Area Remaining in 1988 Figure 22. The 1988 perimeter-to-area ratio plotted against the fraction of the 1940 area remaining in 1988 for nine Illinois hill prairies, indicating a tendency for sites with more edge to lose a greater proportion of is mainly influenced by plant
. The changing Illinois environment : critical trends : technical report of the Critical Trends Assessment Project. Man; Pollution; Environmental protection; Ecology; Environmental impact analysis. PRAIRIES 0 2 CO DC CO Q) o5 E *c CD Q. • • 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 Fraction of 1940 Area Remaining in 1988 Figure 22. The 1988 perimeter-to-area ratio plotted against the fraction of the 1940 area remaining in 1988 for nine Illinois hill prairies, indicating a tendency for sites with more edge to lose a greater proportion of is mainly influenced by plant mating systems and dispersal mechanisms. Several efforts have been made to summarize likely levels of genetic variability and population differentia- tion (Loveless and Hamrick 1984, Hamrick and Godt 1990, Hamrick et al. 1991). Observational evidence shows that obligate outcrossers have high levels of genetic variability, and that much of this genetic variability is distributed across populations such that individual populations are not genetically distinct or differentiated from one another. We can, in fact, visualize a continuum with self-fertilizing species with narrowly dispersed seed at one end and with outcross- ing, wind-pollinated species with widely dispersed seed at the other. At the first end, we expect to see much opportunity for population differentiation, and at the other, little opportunity. In any closed population, random genetic change tends to result in loss of genetic diversity over time. These effects are more pronounced in small populations than in large ones. It has long been noted that small popula- tion size can lead to inbreeding depression, or a reduced ability of the species to reproduce (Barrett and Kohn 1991). All organisms carry a complement of recessive alleles that are deleterious. Small population size (, < 50 individuals) increases the chances of creating individuals with a pair of homozygous recessive alleles, allowing the deleter
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