Dynamics of extinction of a small population of the three-spined stickleback (Gasterosteus aculeatus L) caused by habitat modification

Abundance of a population of three-spined stickleback, Gasterosteus aculeatus L., in a small backwater of Afon Rheidol in mid-Wales (UK) was estimated annually each October from 1972 to 1999 by mark-recapture. 2. The population became extinct in 2000, because of land-use changes in 1995, which modified the drainage pattern through the backwater, causing the backwater to eventually dry up. 3. The final decline to extinction started from an estimated abundance of 1550 in 1998 and the abundance in 1999, the year before extinction, was 85. The smallest abundance from which the population showed an increase was 670. 4. Two years before extinction (1998), the population was characterized by an anomalously high proportion of small fish. 5. There was a significant power relationship between the years to extinction and population size, but if the data were analysed in two periods, 1972-1989 and 1990-1999, the relationship was only significant for the second period. 6. There was no relationship between time to extinction and per capita annual rate of increase (k), although values of k were unusually low in the last two years before extinction. 7. Time to extinction was not related to mean length, mean mass or the condition of the fish. 8. The results suggest that the indicators of impending extinction may vary with the causes of extinction and may be ambiguous, even when a long time-series of demographic data is available.