Table 2.
Mixed model effects of white-tailed deer a) access/exclusion and b) population density and native vegetation on introduced plant richness and abundance (per cent cover and stem density)a. Results are based on floristic composition data collected from deer-access (unfenced) and deer-exclusion (fenced) plots at 23 sites in east-central and north-eastern USA. The ratio of introduced to native vegetation was evaluated based on the interaction of the deer effect with native vegetation; the lack of significant interactions indicates that the ratio of introduced to native vegetation does not vary with the deer effect. See Supporting Information—Fig. S2 for the relationships between introduced and native vegetation. For random effect results, see Supporting Information—Table S3. P values are in bold print if significant at the alpha level α < 0.05.
| Introduced species richness | Introduced cover | Introduced stem density | |
|---|---|---|---|
| a) Deer access/exclusion | |||
| Intercept (SE) | 0.8 (0.2) | 1.3 (0.2) | 1.2 (0.3) |
| DA/DE coefficient (SE) | –0.01 (0.08) | –0.2 (0.1) | 0.05 (0.1) |
| F statistic (DFn,DFd) | 0.028 (1,222) | 2.2 (1,193) | 0.22 (1,171) |
| P value | 0.9 | 0.1 | 0.6 |
| Native vegetation coefficient (SE) | 0.036 (0.006) | 0.012 (0.005) | 0.020 (0.006) |
| F statistic (DFn,DFd) | 39 (1,388) | 11 (1,346) | 9.3 (1,257) |
| P value | <0.001 | <0.001 | 0.002 |
| DA/DE * Native vegetation coefficient (SE) | –5 × 10−4 (0.005) | –0.003 (0.004) | –0.010 (0.007) |
| F statistic (DFn,DFd) | 0.008 (1,216) | 0.56 (1,228) | 1.9 (1,194) |
| P value | 0.9 | 0.4 | 0.2 |
| n | 404 | 392 | 290 |
| # Sites | 18 | 17 | 11 |
| b) Deer density | |||
| Intercept (SE) | 1.3 (0.3) | 1.1 (0.6) | 0.5 (0.7) |
| DD coefficient (SE) | 0.003 (0.005) | –3 × 10−4 (0.01) | 0.01 (0.01) |
| F statistic (DFn,DFd) | 0.54 (1,23) | 0.0011 (1,16) | 1.4 (1,11) |
| P value | 0.5 | 1 | 0.3 |
| Native vegetation coefficient (SE) | 0.013 (0.008) | 0.023 (0.009) | 0.03 (0.02) |
| F statistic (DFn,DFd) | 2.3 (1,154) | 6 (1,127) | 2.8 (1,140) |
| P value | 0.1 | 0.01 | 0.09 |
| DD * Native vegetation coefficient (SE) | 3 × 10−4 (2 × 10−4) | 1 × 10−4 (3 × 10−4) | –3 × 10−4 (2 × 10−4) |
| F statistic (DFn,DFd) | 2.4 (1,185) | 0.32 (1,177) | 1.7 (1,139) |
| P value | 0.1 | 0.6 | 0.2 |
| n | 190 | 184 | 145 |
| # Sites | 17 | 16 | 11 |
aNative species richness was used as the native vegetation predictor variable for introduced species richness and native species cover and stem density were used as the native vegetation predictor variables for introduced cover and stem density, respectively. Square-root transformations of species richness and natural log +1 transformations of species cover and stem density were used to meet statistical assumptions. SE = standard error; DA = deer access; DE = deer exclusion; DFn = degrees of freedom, numerator; DFd = degrees of freedom, denominator; n = number of observations; DD = deer density.