There were similar patterns of seed preference in most of the small mammal species in this study. Most species showed a distinct preference for lodgepole pine seeds,
intermediate consumption of white spruce seeds, and avoidance of subalpine fir seeds. Many individuals did not consume even a single subalpine fir seed during trials. Most small mammal species also consumed the different species of seeds in relatively similar proportions. Phenacomys intermedius behaved very differently in that it consumed very few seeds in total. Most P. intermedius individuals were correctly classified by the DFA, demonstrating the disparity in their seed consumption behaviour from the other small mammal species. The inaccurate classification of a large percentage of P. maniculatus,
M. gapperi, M. longicaudus, and M. pennsylvanicus individuals is based on low discrimination of their seed consumption patterns, and further demonstrates the high degree of similarity in their seed consumption behaviour.
The excess amounts of seeds provided, the availability of other palatable food sources and the maintenance of body mass by all species indicated that individuals were not forced to consume seeds in the laboratory trials, but were instead selecting them. Seed selection was partial rather than absolute, which is consistent with previous studies testing all-or-nothing optimal diet strategies (Vickery 1984; Krebs and Kacelnik 1991). This sampling behaviour is believed to benefit rodents, especially with novel foods, by allowing them to assess the nutritional value of each food-type available to them (Vickery 1984). While most species did not differentially consume seeds over the three
trial days, M. gapperi sampled the subalpine fir seeds on the first trial day, and subsequently avoided them for the remainder of the study.
Côté et al. (2003) showed that the main food items of P. intermedius are berries and leaves from vascular plants, and that P. intermedius consume a very small percentage of seeds available to them. Although the difference in total daily seed consumption between
P. intermedius and M. longicaudus was not statistically significant, M. longicaudus
consumed over 6.5 times more seeds than P. intermedius; we believe this marked difference is biologically relevant. Microtus longicaudus are not very abundant in the Kananaskis Valley (Millar et al. 1985) and are rarely found in dense populations (Van Horne 1982), making them difficult to trap. A larger sample size of M. longicaudus could further discriminate seed-consumption patterns in this species. Low overall seed
consumption also explains why P. intermedius did not show a preference among seed species, as they prefer alternative food options. A study providing seeds as the only food available would further elucidate their seed preference. Furthermore, P. intermedius and
M. longicaudus show very high niche overlap and low microhabitat segregation in the Kananaskis Valley, and their primarily herbivorous food habits are also similar (Millar et al. 1985). It is possible that their co-existence is partially explained by differential seed consumption.
Myodes gapperi and M. pennsylvanicus showed a greater relative preference for
lodgepole pine seeds over white spruce seeds than P. intermedius. However, because P. intermedius did not show an overall preference among seed species, this result is very misleading. There was no disparity in the degree to which lodgepole pine seeds were preferred over white spruce seeds among P. maniculatus, M. gapperi, M. longicaudus,
and M. pennsylvanicus, all of whom showed similar overall preferences in seed consumption.
Findings from the field seed selection trials were consistent with laboratory results.
Peromyscus maniculatus and M. gapperi showed very similar patterns of total daily seed consumption and consumption of each species of seed, as well as preference among the different species of seeds in the laboratory and the field. This is in agreement with Drożdż (1966), who used field food selection experiments and stomach-content analysis to show that cafeteria-style feeding experiments accurately represent the actual food habits of small mammals. Field seed selection trials were not conducted on P. intermedius, M. longicaudus, and M. pennsylvancius because of their relatively low abundance in the Kananaskis Valley (Millar et al. 1985), which made trapping difficult. However, our results suggest that seed consumption by these species in the laboratory would be similar in the field.
The basis for seed selection by animals has been shown to involve characteristics such as size, seed coat, digestibility, palatability, nutritional content, and secondary compounds (Janzen 1971; Kerley and Erasmus 1991; Vickery et al. 1994; Ramos 1996; Lewis et al. 2001). Lodgepole pine seeds contain high-energy storage tissues (Despain 2001). The size of lodgepole pine and white spruce seeds are relatively similar and are both smaller than subalpine fir seeds. Larger seeds are generally favoured by rodents (Price 1983; Hulme 1998). However, the increase in handling time associated with larger seeds is a possible deterrent (Kerley and Erasmus 1991). Fir seeds are also thought to possess a natural repellence to small mammals (Abbott 1962), likely because of the presence of secondary compounds, and individuals often ignore these seeds if other species are
available (Abbott 1962; Drożdż 1966; Schreiner et al. 2000). The relative importance of these factors is not known for the selection of lodgepole pine, white spruce, and subalpine fir seeds and should be the focus of future studies.
The results of our study predict that post-dispersal seed predation by small mammals would not be a major problem in the regeneration of subalpine fir stands through either natural or artificial (e.g., direct seeding) processes. Subalpine fir should also be
successful in recruitment and colonization of new sites in forests. Conversely, the white spruce consumption patterns reiterate findings by Peters et al. (2004), who showed that post-dispersal seed predation by small mammals negatively affects recruitment success. Abbott (1962) suggests that avoidance of fir seeds by predators may contribute to the high ratio of fir to spruce seedlings in mixed forests.
The distinct preference for lodgepole pine seeds by small mammals should severely decrease its regeneration and recruitment success, especially in mixed forests. Direct seeding coupled with diversionary foods has been shown to be an effective strategy to curtail seed predation by rodents; Sullivan and Sullivan (1982) dramatically reduced the loss of lodgepole pine seeds to rodents using a ratio of two sunflower seeds to one pine seed. Lodgepole pine (subspecies latifolia) trees in the Kananaskis Valley produce
predominantly serotinous cones (remain closed at maturity until they have been subjected to temperatures in excess of 50 ºC), which has been suspected to be a natural defence against post-dispersal seed destruction by small mammals (Despain 2001; Sjöberg and Danell 2001). Crown fires cause the release of seeds stored in cones, which flood newly available sites and overwhelm most coniferous competitors (Despain 2001). Herbaceous vegetation and small mammal populations are also reduced long enough to allow the
successful establishment of seedlings (Despain 2001). Other forms of disturbance (e.g., clearcutting) are also beneficial to lodgepole pine; small mammal populations often respond in a similar way as they would to a fire (Simon et al. 2002), and a fast growth rate and the ability to tolerate dry, low nutrient soil allow seeds released by the non- serotinous cones to be more successful (Despain 2001).
The overall effect of seed predation may not be very important when other stages of recruitment are limiting (Hulme 1998), especially when seed production and dispersal varies in competing species. While the knowledge of seed preference allows us to make predictions of how successful certain conifer species will be over others in mixed forests and new areas, further studies are required to evaluate the direct effects of species- specific differences in seed predation by small mammals on recruitment and natural and artificial regeneration.