HECHOS POSTERIORES

I investigated three reproductive characteristics and three measures of survival of PL and PMG to assess differences between species that might contribute to total population growth. Shifts in

67 community structure are ultimately caused by differences in reproductive success of the species that make up the community. Total annual reproductive output of a species consists of

reproduction of OW and YOY females. Reproductive output is influenced by reproductive traits such as the rate at which breeding individuals have litters and the age of an individual at first reproduction. Reproductive success also depends on winter survival and the length of time that an individual persists during the breeding season (Figure 3-1).

Figure 3-1: Conceptual model of species traits investigated in this study that influence population growth due to reproduction. Traits influencing reproduction are represented in boxes, and the effect of each factor (either directly on another factor or indirectly on another effect) is indicated with directional arrows.

Reproductive output of OW and YOY females was estimated as the per-capita number of known pregnancies and compared between species and cohorts using two-sample t-tests. The frequency

68 with which breeding females have litters is an important contributor to overall population growth because individuals that reproduce quickly during a short life may be equally successful as those that live longer and reproduce at a slower rate (Stearns 1976). The frequency of litters for each breeding female was estimated as the number of known pregnancies divided by annual

persistence (ATG) on the grid. I compared the frequency of litters between species and between OW and YOY females to determine whether there was variation in the length of time between litters.

The age at which reproduction begins significantly impacts population growth via reproduction, effectively shortening generation time (Cole 1954). The age at which female mice reach sexual maturity is not reported to differ between PL and PMG (King 1968; Millar et al. 1979); however, there may be individual variation in the timing of first reproduction (Lusk and Millar 1989). While I was not able to determine the exact age of individuals in this study, I investigated the proportion of YOY mice that were known to breed in the year of their birth to make broad comparisons between PL and PMG.

Enhanced winter survival increases the abundance of breeding OW females in the spring and thus has direct influence on reproduction by this cohort. Winter survival was estimated as the proportion of mice that were trapped in both the fall prior to and the spring after a winter. Fisher’s Exact test was used in lieu of Chi-squared analysis due to small sample sizes.

Much of the difference between potential and observed reproductive output in some Peromyscus populations is attributable to survival (Schug et al. 1991), so that females that survive longer have proportionally more litters than short-lived females (Millar et al. 1992). No direct measures of longevity were possible; however I used the total time that an individual was known to spend on the grid (TTG) as a rough estimate. Average TTG was compared between groups (species, gender, age at first appearance, reproductive status) and years using two-sample t-tests.

Persistence within a single season was assessed using three methods – the proportions of mice that became residents on the grid, annual persistence (ATG), and the probability of loss of an individual between one trapping session and the next. Differences in the proportion of mice that established residency on the grid was investigated using either Chi-squared analysis or Fisher’s Exact test when sample sizes were low. Annual persistence (ATG, the length of time an

69 individual was known to be present during a single year) was compared between species and cohorts using two-sampled t-tests.

The probability of loss of an individual from the marked population was calculated for each two week period during the study. Because mortality and emigration may depend on factors such as predation pressure and population density, it is likely that the proportion of the population lost changes within a season. A time interval of two weeks was chosen to coincide with the average length of gaps between trapping sessions. Individuals were assumed to have been present on the grid for the entire duration between their first and last capture, and an individual was considered lost only when it disappeared permanently from the marked population. To detect more general seasonal differences in the probability of loss, time periods were grouped into two categories: early season, or before July 15; and late season, or after July 15. The proportion of mice lost from the population was compared between species, genders, years, and seasonal categories using chi-squared analysis.

I additionally investigated two factors that could influence survival. The average weight of mice in the fall was compared between overwintered and non-overwintered mice with two-sample t- tests. Average dates of first appearance on the grid (FD) were compared among residents and between residents and non-residents to investigate possible density dependent effects that could affect persistence and disappearance.

Table 3-3 provides a summary of the traits investigated, how characteristics were estimated, the statistical tests used, and restrictions on the datasets used in analyses. Datasets were often restricted to certain subgroups for analysis; the most common was exclusion of OW-NR mice. Because OW-NR mice were known to have been alive in the fall prior to their first capture, the timing of their first appearance on the grid (FD) and their age at the time (AF) were unknown. It is likely that there were other mice present in the fall that escaped detection altogether, so the inclusion of OW-NR mice may bias analyses in which proportions of mice are compared (i.e., out of all known incoming immigrants or new births, how many became residents or

overwintered). Comparisons of reproductive characteristics were restricted to residents because longitudinal records were required. Some analyses of survivorship were also restricted to resident mice in order to better distinguish the effects of mortality from emigration.

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