UBC Theses and Dissertations
Conifer seed predation by the deer mouse : a problem in reforestation Sullivan, Thomas Priestlay
Reforestation of cutover forest land by direct application of seed has long been a goal of foresters in North America. Destruction of the seed supply by small mammals and birds has resulted in a general failure of these reforestation projects-Numerous techniques have been developed for protecting tree seed from rodents and seed-eating birds. Conventional control methods include mechanical devices, poison baits, and chemical repellents applied directly to seeds. None of these techniques has been successful. Little research has been done on methods involving biological control, and consequently, this thesis will describe the development of a biological technique for reducing conifer seed predation in deer mice. The effects of forestry practice on populations of deer mice and other small mammals have been well documented. Most studies have concluded that there are higher densities of deer mice and related small mammals on clearcut areas than in forested regions. The first chapter in this thesis was designed to test the widely-held hypothesis that clearcut (logged) habitats support higher density populations of Peromyscus than do forested habitats. Deer mouse populations were live-trapped in forest and clearcut habitats at Maple Bidge, British Columbia, from May 1975 to April 1978. Another clearcut population was monitored from April 1977 to April 1978. The average density of mice per ha in the forest was: 19.6 (1975), 15.8 (1976), 22.3 (1977) and on the clearcut areas was: 23.3 (1975), 16.6 (1976), 29.9 and 20.2 (1977). The slightly higher number of animals on the clearcut in 1975, and on one of the clearcut areas in 1977, reflected a burst of recruitment in the late summer and fall of each year- The density of mice on clearcut areas declined during each winter to a level comparable to that in the forest- More female mice were breeding on the clearcut in all years than in the forest, but there was little difference in male reproduction. Forest animals were consistently heavier than those on the clearcut. Juvenile male deer mice were able to enter into the clearcut populations during the breeding season in the first year after logging. The second chapter was designed to test the hypothesis that removal of all deer mice from a given area would result in satisfactory survival of conifer seed. Areas of 1.1 ha were cleared of deer mice and related small mammals. Colonization of these depopulated areas by deer mice and the survival of Douglas fir seed dispersed over the clearcut were monitored. During the fall, a total of 99 mice colonized a depopulated area. Animals continually moved onto the area, and 95% of the conifer seed was lost within a 3-day period. Similarly, during a removal experiment in the spring, a total of 48 mice immigrated into the vacant habitat from surrounding regions. These animals destroyed 92.6% of the conifer seed within a 5-day period. This intensive study of seed survival and movements of deer mice on control and removal areas has shown the futility of baiting owing to the rapid reinvasion by mice and consequent destruction of the seed supply. The third chapter discusses the use of alternative foods as a biological control technique which has successfully reduced conifer seed predation by deer mice. The upper limits to the number of mice and number of seeds taken per mouse were determined over a wide range of densities of Douglas fir seed. The number of seeds taken stabilized at about 860,000 seeds/ha. Experiments with alternative foods were done at this seed density in ratios of 7 sunflower to 1 Douglas fir and 5 sunflower to 2 oats to 1 Douglas fir seed. These mixtures produced the best results: 70% survival of conifer seed after 2 weeks and 50% survival after 4 weeks compared with 5% survival of control Douglas fir by itself. These results were obtained in the early spring (March to April) with similarly favourable results obtained in the fall (November to December). Seeding experiments with alternative foods during other times of the year and with lower densities of sunflower seeds were not successful. The fourth and fifth chapters discuss the application of this biological control technique to reforestation by direct seeding. The very best time for seeding with respect to low populations of seed-eating small mammals (chipmunks and deer mice) and birds (several species) is in the late winter-early spring in southwestern D.C. This period fits in well with the presumed most favourable time for conifer seed germination. The recommended mixture of seed to be aerially seeded over a clearcut area is as follows: 0.10 kg Douglas fir seed or other conifer seed (equivalent number of seeds by weight) to 56 kg sunflower seed and 7 kg oats per ha.
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