IUCN / SSC Cat Specialist Group - Digital Cat Library |
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| Liberg, O.; Andr‚n, H. | |
| Lodjursstammen i Sverige 1994-2004. En utv„rdering av inventeringresultat och metoder | |
| 2006 Full Book | |
This is a report from an assignment by the Swedish Environmental Protection Agency and the Swedish Association for Hunting and Wildlife Management to evaluate the national lynx censuses in the period 1993 - 2004 including evaluation of census methods and, on basis of the evaluation results, to estimate figures for the total Swedish lynx population during the period. The reindeer husbandry area in Sweden covers the northern 40 % of the country. In this area, the county boards have had the responsibility for censusing lynx since 1996. They have used an accumulation method, which means accumulation of records of tracks and observations of lynx during the whole snow tracking period, with an evaluation at the end of the season. South of this region, the Swedish Association for Hunting and Wildlife Management was responsible for the censuses in the period 1993 - 2001. Censuses were performed according to linebacktracking method at one occasion on fresh snow: Lynx tracks are detected by tracking a dense net of census lines and tracks found backtracked until snow from the last snowfall is found in the track. From 2003, the county boards were responsible for the censuses also in this part of the country; they used both methods to a varying extent. In this report we only consider census of lynx family groups, i.e., breeding females with accompanying kittens of the year. The most important source of error in the accumulation method occurs when the so-called distance criterion is used to separate different family groups. In high densities this error causes underestimation, at low densities overestimation. The exact magnitude of the error could not be calculated. The most important error in the line-backtracking method is caused by lynx staying put during the period between snowfall and census. Censuses performed on one night snow underestimated the number of family groups by 20 %, censuses on two nights snow by 5 %. Thereafter, the error was negligible. One problem for both methods is that approximately 30 % of all born litters disappear before the winter census period. This figure is the same in both parts of the country. On basis of census results, considering areas not covered and bias caused by passive family groups, we calculated that the breeding lynx population outside the reindeer area increased from 111 family groups in 1994 to 208 groups in 2000, after which it again decreased in this part of the country to 165 groups in 2003. The census made in 2004 outside the reindeer area was too poor to base any estimate on. In the reindeer area, there were 167 family groups in 1998 and 173 groups the year after, but thereafter the number has steadily decreased to 88 groups in 2004. Total number of family groups in the whole country was 338 in 1998, and peaked in 2000 with 340 groups. Thereafter, it decreased to 265 groups in 2003, which was the last year with useable figures for the whole country. This corresponds to 1 300 - 1 600 lynx. 8 An effort was also made to calculate a long time sustainable lynx population, based on estimated sustainable prey populations. In the reindeer area, political and economical constraints will force the managers to limit the lynx population to 80 family groups, which is well below any biological constraints. In central Sweden, the maximum biological carrying capacity for lynx was estimated to be 200 family groups, provided that no roe deer, the main prey for lynx, is taken by human hunters. The carrying capacity in the southern third of the country is high, estimated to just over 300 family groups. If these estimates are approximately correct, it means that, to reach the national political goal of 300 family groups, lynx must to a large extent start to colonize the southern third of the country, an area where we today have less than ten family groups. Some data on spread into these southern part based on radio-marked lynx are presented, and constraints for lynx colonization of this part of the country are discussed. The report finishes with some suggestions as how to make the censuses more efficient. |
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