An experimental test of the habitat‐amount hypothesis for saproxylic beetles in a forested region

Abstract

The habitat-amount hypothesis challenges traditional concepts that explain species richness within habitats, such as the habitat-patch hypothesis, where species number is a function of patch size and patch isolation. It posits that effects of patch size and patch isolation are driven by effects of sample area, and thus that the number of species at a site is basically a function of the total habitat amount surrounding this site. We tested the habitat-amount hypothesis for saproxylic beetles and their habitat of dead wood by using an experiment comprising 190 plots with manipulated patch sizes situated in a forested region with a high variation in habitat amount (i.e., density of dead trees in the surrounding landscape). Although dead wood is a spatio-temporally dynamic habitat, saproxylic insects have life cycles shorter than the time needed for habitat turnover and they closely track their resource. Patch size was manipulated by adding various amounts of downed dead wood to the plots (~800 m³ in total); dead trees in the surrounding landscape (~240 km ) were identified using airborne laser scanning (light detection and ranging). Over 3 yr, 477 saproxylic species (101,416 individuals) were recorded. Considering 20-1,000 m radii around the patches, local landscapes were identified as having a radius of 40-120 m. Both patch size and habitat amount in the local landscapes independently affected species numbers without a significant interaction effect, hence refuting the island effect. Species accumulation curves relative to cumulative patch size were not consistent with either the habitat-patch hypothesis or the habitat-amount hypothesis: several small dead-wood patches held more species than a single large patch with an amount of dead wood equal to the sum of that of the small patches. Our results indicate that conservation of saproxylic beetles in forested regions should primarily focus on increasing the overall amount of dead wood without considering its spatial arrangement. This means dead wood should be added wherever possible including in local landscapes with low or high dead-wood amounts. For species that have disappeared from most forests owing to anthropogenic habitat degradation, this should, however, be complemented by specific conservation measures pursued within their extant distributional ranges.

Habitat amount, not patch size and isolation, drives species richness of macro‐moth communities in countryside landscapes

“…A study in the naturally heterogeneous savanna landscape of the Chapada Diamantina in Brazil did so for Euglossini bees (Moreira et al., ), a study within five major industrial sites in Europe did so for five out of seven tested taxa (Piano et al., ), while a study on fluvial islands in Amazonia did so for arboreal mammals (Rabelo, Bicca‐Marques, Aragón, & Nelson, ). Both patch size and habitat amount in the local landscape independently affected species numbers of saproxylic beetles in German forests, without an interaction effect, hence consistent with the habitat amount hypothesis and refuting the island effect (Seibold et al., ). A study on woodland small mammals in the Brazilian cerrado (i.e.…”

Importance of spatial configuration of deadwood habitats in species conservation

“…In a study of saproxylic beetles, Seibold et al. () did not find interactions between habitat amount in the landscape and response of local biodiversity to local habitat amount, indicating that it does not matter where local conservation efforts are conducted. In contrast, Rubene et al.…”

“…(). Another difference was that Seibold et al.’s () study was conducted in a national park, whereas the other 2 were in forest landscapes with either small or highly varying amounts of unmanaged forest.…”

Minimal effects on genetic structuring of a fungus‐dwelling saproxylic beetle after recolonisation of a restored forest

“…However, in a forest experimental setup, Seibold et al. () found little support for isolation effects in a forested landscape, beyond the limiting effect of the amount of available deadwood habitat for deadwood‐dwelling beetles. The data in Seibold et al.…”

“…The data in Seibold et al. () are likely dominated by flight‐active species, but it shows that we require greater understanding of how saproxylic beetles can disperse around the landscape. This will also benefit management strategy development, where it is important to know what drives the origin of new colonisations by saproxylic species in an area (see Drag & Cizek, ).…”