A Case in Point
The lack of population data over long periods of time (decades or hundreds of years) means that our understanding of population viability and vulnerability is largely based on theoretical concepts and population modeling. These theories and models predict that population extinction is more likely to occur in smaller populations and that the dispersal of individuals between populations is important for the maintenance of genetic viability and for maintaining local and regional populations in the face of population extinctions (Leigh 1981, Shaffer 1981, Fahrig and Merriam 1985, Shaffer and Samson 1985, Hanski and Gilpin 1991).
One recent study provides an excellent illustration of the impact of fragmentation in riverine systems. The study by Kentaro Morita and Shoichiro Yamamoto (2002) focused on populations of white-spotted charr (Salvelinus leucomaenis) occupying mountain streams in Japan. The white-spotted charr is a salmonid fish that occurs as both large migrant individuals and small resident fish that normally interbreed in unaltered streams. Many of the mountain streams used by charr have been fragmented by small erosion-control dams that, while they don't impound the stream, do prevent fish from moving upstream. Above these dams, charr populations are sustained only by the smaller, resident fish.
Morita and Yamamoto surveyed both dammed and undammed stream segments for the presence of charr in appropriate habitat. Based on habitat conditions, they concluded that charr should have been able to establish populations in all dammed sites. Charr populations were found in all undammed sites that they surveyed, but were absent in 32.7 percent of dammed sites. The results indicated that the probability of charr occurring in dammed stream segments decreased with decreasing watershed area and increasing isolation period. Further, this study also found evidence of genetic deterioration in populations above dams, including lower genetic diversity, higher morphological asymmetry, and genetically-based lower growth rates, compared to populations below dams.
Results of this study of white-spotted charr are consistent with predictions of increased vulnerability for smaller and more isolated populations. Genetic and population consequences due to fragmentation occurred over a relatively short period of time (30-35 years). The fact that the probability of occurrence was related to watershed size suggests that the smallest populations were the most vulnerable. The relationship between isolation period and probability of occurrence suggests that additional populations may well be lost over time.
The situation of small dams on headwater streams in Japan may be comparable to watersheds in the U.S. that contain road crossings with sub-standard culverts. Culverts that block the upstream movement of fish and other organisms effectively isolate populations above these crossings. Areas with relatively small amounts of habitat upstream of the crossing will be most vulnerable to population loss. Over time, it would be expected that more and more populations will fail and, due to the disruption of metapopulation dynamics, these areas of suitable habitat are likely to remain unoccupied.
[From Stream Simulation: An Ecological Approach to Providing Passage for Aquatic Organism at Road-Stream Crossings, US Forest Service, May 2008]
Fahrig L. and G. Merriam. 1985. Habitat patch connectivity and population survival. Ecology 66(6):1762-1768.
Hanski, I. and M.E. Gilpin. 1991. Metapopulation dynamics: brief history and conceptual domain. Biological Journal of the Linnean Society 42:3-16.
Leigh, E.G., Jr. 1981. The average lifetime of a populations in a varying environment. J. Theor. Biol. 90:213-239.Lowe, W.H. and D.T. Bolger. 2002. Local and landscape-scale predictors of salamander abundance in New Hampshire headwater streams. Conservation Biology 16(1):183-193.
Morita, K. and S. Yamamoto. 2002. Effects of habitat fragmentation by damming on the persistence of stream-dwelling charr populations. Conservation Biology 16(5): 1318-1323.
Shaffer, M.L. 1981. Minimum population sizes for species conservation. BioScience 31(2):131-134.
Shaffer, M.L. and F.B. Samson. 1985. Population size and extinction: a note on determining critical population sizes. Am. Nat. 125:144-152.
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