Fig 1.
Map depicting sampling area and sampling design.
A spatially nested sampling design was employed. Thirty sites (represented by black circles) were sampled twice, once in each season of peak insect activity (autumn and spring). Groups of five sites were spatially aggregated to form six clusters (small grey squares). Two clusters were present in each of the three mountain blocks sampled (large rectangles). Sites consisted of two 10 x 10 m squares situated 10–50 m apart (insert on the right). Each square contained five 2.5 x 2.5 m plots (four corners and centre of the square). These plots were sampled both for Restionaceae plants and all insect herbivores present on Restionaceae plants.
Table 1.
Number of insect species per plant species for various regions (i.e. the Cape Floristic Region, temperate zones, tropical zones).
The number of insect species per plant species is significantly higher in the tropics than the CFR, although this is entirely confounded by plant growth form. The CFR does not have significantly more insect species per plant species than other temperate zones, suggesting regional plant richness and growth form are better predictors of regional insect richness than environmental variables.
Fig 2.
The morphospecies richness and abundance per insect order.
Total morphospecies richness (black bars) and abundance (grey bars) for each insect order captured during vacuum sampling surveys of Restionaceae communities across the Cape Floristic Region. Hemiptera dominated herbivore communities on restios. Only herbivorous insects are included. The mean (± SD) insect morphospecies richness for plots was 9.47 ± 4.76 (range: 0–27), for squares 29.25 ± 10.09 (range: 12–60), for sites 44.80 ± 12.80 (range: 26–70), for clusters 126.17 ± 9.06 (range: 118–144) and for mountains 192.67 ± 22.55 (range: 171–216) (Table 2).
Table 2.
The mean insect and plant richness for each sampling scale.
The mean (± SD) insect and plant richness of each mountain block (HH: Hottentots-Holland; KB: Kogelberg; CP: Cape Peninsula) is shown for the various sampling scales.
Table 3.
Insect-plant diversity relationship at various spatial sampling scales.
While the relationship between insect and plant diversity components was always positive, it was only significant at the smaller sampling scales with the most statistical power, and plant diversity components explained a maximum of 11% of variance in insect diversity.
Fig 3.
Relationship between plant and insect richness at various sampling scales.
The plot scale is represented by a solid line and circles, the square scale is represented by a dashed line and crosses, and the site scale is shown by a dotted line and triangles. The association is positive at the plot and square scales (p < 0.001), but not significant at the site scale.
Table 4.
Results of GLM models with insect species richness (at the square level) as the response variable.
The model was first run with the entire dataset (i.e. all species for both seasons), and then on subsets of the data (i.e. seasons separately, and main insect orders separately). Data from all 60 sampled squares were included.