Table 1.
Hypotheses to test the validity of species habitat specialization rankings with known species’ environmental and biogeographic ranges and to identify species’ traits that can be used to predict species location along the habitat specialization continuum.
Fig 1.
Hawaiian Islands on which plant species co-occurrence samples (1017 plots) were collected between 2004 and 2012.
Plots range from sea level to 3000 meters within Hawaiʻi Volcanoes National Park (563) and Mauna Kea state lands (79) on Hawaiʻi, Haleakalā National Park (239) on Maui, and Kalaupapa National Historical Park (138) on Molokaʻi. Plot frequency chart x-axis values indicate the upper end of each category (e.g., 1000 = plots located 501–1000 m).
Table 2.
Three data sets used to rank plant species’ habitat specialization.
Data were collected between 2004 and 2012 with plot size ranging from 400–1000 m2.
Fig 2.
Habitat specialization (Jaccard index) and the standard error around the estimator for 170 plant species.
The black center line indicates each plant species’ specialization index value (0 = complete specialist; 1 = complete generalist) and the grey lines are standard deviations. Species are ranked according to their specialization index.
Table 3.
The twenty most generalist and most specialist plant species (highest and lowest Jaccard index values, respectively).
The entire 170 plant species rank order and details are available in S2 Appendix. For each species, the Jaccard index value (standard deviation (SD) based on 100 permutations of selecting 25 plots), mean alpha richness per plot listed as μ(α), total number of plot occurrences out of 1019 possible, species origin, and life form are provided.
Fig 3.
A. Histogram of species specialization by biogeographic origin. Specialization defined by the Jaccard index (0 = complete specialist; 1 = complete generalist) and biogeographic origin (endemic, indigenous, and non-native) for 170 plant species in the Hawaiian Islands. Percent of species by origin is presented to equalize categories. B. Box plot of species specialization by biogeographic origin. Non-native plant species (n = 71) are more generalized, with higher Jaccard index values, than endemic Hawaiian plant species (n = 68). Habitat specialization values for indigenous plant species (n = 31) are intermediate between endemic and non-native species. Diamonds within box plots are group means and lines are median values. Categories that share the same letter are not significantly different (F(2,167) = 4.735, p = 0.01).
Table 4.
Findings from tests to validate species habitat specialization rankings and efforts to identify the predictability of species’ traits along the habitat specialization continuum.
Fig 4.
Species specialization by number of habitat moisture types.
Plant species ranked as generalists (higher Jaccard index values) occur in more habitat moisture types (dry, mesic, wet) as hypothesized according to the published Hawaiian plant literature, than those limited to a single habitat moisture type (p < 0.001; Table 5). Mean values and standard errors are reported.
Fig 5.
Species specialization by number of main Hawaiian Islands.
Plant species ranked as generalists (higher Jaccard index values) occur on more Hawaiian Islands according to the flora, supporting the hypothesis that habitat generalists are more likely to occur on multiple islands because they can persist on diverse substrate types (p < 0.001; Table 5). Mean values and standard errors are reported.
Fig 6.
Species specialization by minimum elevation.
A linear model was developed to determine if habitat specialization (Jaccard index) differs by minimum elevation and biogeographic origin. Plant species restricted to high elevations had lower Jaccard index values than those with low or no minimum elevation limit (p < 0.001; Table 6).
Table 5.
Conducted for each categorical variable (number of habitat moisture types, habitat moisture type, number of main Hawaiian Islands, life form, dispersal syndrome) to determine if habitat specialization rankings (Jaccard index) differed among categories while controlling for biogeographic origin.
Table 6.
Developed to examine if and to what extent habitat specialization (Jaccard index) differs across continuous variables (elevation minimum, elevation maximum, elevation range, leaf size variance) and biogeographic origin. Models test the null hypotheses that (1) the Jaccard index is zero (Intercept = Endemic, Indigenous, Non-native) when controlling for the continuous variable and (2) that there is no relationship (slope = 0) between the Jaccard index and the continuous variable. Bold p values indicate significant differences from the null hypotheses. The minimum elevation and elevation range models had significant slopes; however, only the minimum elevation model explained greater than twenty percent of the variance in the data (R2) with endemic species displaying the steepest slope (Fig 6).
Fig 7.
Species specialization of life form.
Species habitat specialization differs between herbaceous and woody plant life forms (p = 0.004) when controlling for biogeographic origin (Table 5). This pattern is most evident in non-native species. Mean values and standard errors are reported.
Fig 8.
Species specialization by dispersal syndrome.
As hypothesized, no significant difference was detected among dispersal syndromes across the specialization continuum (p = 0.071) when controlling for differences among biogeographic origin (Table 5). Mean values and standard errors are reported.