Functional roles affect diversity-succession relationships for boreal beetles

Abstract

Species diversity commonly increases with succession and this relationship is an important justification for conserving large areas of old-growth habitats. However, species with different ecological roles respond differently to succession. We examined the relationship between a range of diversity measures and time since disturbance for boreal forest beetles collected over a 285 year forest chronosequence. We compared responses of "functional" groups related to threat status, dependence on dead wood habitats, diet and the type of trap in which they were collected (indicative of the breadth of ecologies of species). We examined fits of commonly used rank-abundance models for each age class and traditional and derived diversity indices. Rank abundance distributions were closest to the Zipf-Mandelbrot distribution, suggesting little role for competition in structuring most assemblages. Diversity measures for most functional groups increased with succession, but differences in slopes were common. Evenness declined with succession; more so for red-listed species than common species. Saproxylic species increased in diversity with succession while non-saproxylic species did not. Slopes for fungivores were steeper than other diet groups, while detritivores were not strongly affected by succession. Species trapped using emergence traps (log specialists) responded more weakly to succession than those trapped using flight intercept traps (representing a broader set of ecologies). Species associated with microhabitats that accumulate with succession (fungi and dead wood) thus showed the strongest diversity responses to succession. These clear differences between functional group responses to forest succession should be considered in planning landscapes for optimum conservation value, particularly functional resilience.

Figure 1. Relationships between stand age and abundance and stand age and species richness for beetle functional groups based on : a) and e) threat status; b) and f) degree to which species are dependent on dead wood; c) and g) diet; and d) and h) trapping method.

Symbols are as follows: threat status: common species (●); red-listed species (○); saproxylic: non-saproxylic (Δ); facultative saproxylic (▲); obligatory saproxylic (▲); diet: cambium consumers (♦); detritivores (◊); fungivores (♦); predators (×); trap type: emergence = white bars; flight intercept = grey bars. Only significant or near-significant regression lines are shown. Lines may appear curved due to the log scale of the y-axis.

Figure 2. Relationships between stand age and Shannon’s H and stand age and Pielou’s evenness index for beetle functional groups based on: a) and e) threat status; b) and f) degree to which species are dependent on dead wood; c) and g) diet; and d) and h) trapping method.

Symbols as for Figure 1. Only significant or near-significant regression lines are shown. Lines may appear curved due to the log scale of the y-axis.

Figure 3. Relationships between stand age and Mouillot and Lepretre’s (1999) derived parameters for the Zipf-Mandelbrot distribution for beetle functional groups based on: a) and e) threat status; b) and f) degree to which species are dependent on dead wood; c) and g) diet; and d) and h) trapping method.

Functional groups where several age classes supported less than twenty species (red-listed species, detritivores and cambium consumers) were excluded. Symbols as for Figure 1. Only significant or near-significant regression lines are shown. Lines may appear curved due to the log scale of the y-axis.