Hollow oaks and beetle functional diversity: Significance of surroundings extends beyond taxonomy

Abstract

Veteran hollow oaks (Quercus spp.) are keystone structures hosting high insect diversity but are declining in numbers due to intensification of land use and the abandonment of traditional management. The loss of this vital habitat is resulting in a reduction of biodiversity, and this likely has consequences for ecosystem functioning, especially if functional diversity is reduced. A considerable amount of research has been done on predictors of beetle taxonomic diversity in veteran oaks, but predictors of functional diversity have remained largely unexplored. The aim of this study was to establish whether the features and surroundings of veteran oaks are related to functional diversity within three functional groups of beetles (decomposers, predators, and flower visitors) and determine whether species richness and functional diversity within the groups are dependent on the same predictors. Sampling was carried out intermittently between 2004 and 2011 on 61 veteran oaks in Southern Norway. Of the 876 beetle species that were collected, 359 were determined to be decomposers, 284 were predators, and 85 were flower visitors. Species richness and functional diversity in all groups were consistently higher in traps mounted on veteran oaks in forests than in open landscapes. However, additional predictors differed between groups, and for species richness and functional diversity. Decomposer species richness responded to tree vitality, while functional diversity responded to habitat connectivity, predator species richness responded to regrowth of shrubs while functional diversity responded to tree circumference, and flower visitor richness and functional diversity did not respond to any additional predictors. Previous studies have found that the features and surroundings of veteran oaks are important for conservation of taxonomic diversity, and the results from this study indicate that they are also important for functional diversity within multiple functional groups. K E Y W O R D Sbiodiversity and ecosystem functioning, coleoptera, functional diversity, invertebrate conservation, Quercus spp., saproxylic beetles, veteran trees

Veteran trees are a source of natural enemies

“…Although this study focused on the dichotomy between veteran and young trees at rather small spatial scales (200 m or less), other research has established that the immediate surroundings and the wider landscape around the veteran trees can influence invertebrate diversity 22 , 38 – 41 and that large scale anthropogenic factors such as urbanization gradients influence predation rates by natural enemies 12 , 26 . In contrast, research within agroecosystems has found that strips of non-agriculture area with resources for natural enemies reduce pests and crop plant damage independent of landscape complexity 42 .…”

“…Habitat heterogeneity is important for predation by natural enemies, as it increases the possibility of multiple invertebrate enemies to coexist 11 , 12 and has been linked to a greater diversity of invertebrate predators 22 , 33 . The coexistence of natural enemies with complementary traits has been found to be the most important predictor of pest control by natural enemies 10 .…”

“…Large trees in agroforestry systems have been found to enhance functional biodiversity and promote beneficial NCP including invertebrate pest control 21 . Veteran oaks provide food resources and shelter for a diverse set of species 19 , including arthropod predators 22 , 23 and may be a source of a natural enemies of invertebrate pests.…”

“…Natural enemies are complementary when they attack different pest species, have differences in their phenologies and diurnal/nocturnal activities, and have different hunting behaviors 11 . Although it has been established that veteran trees increase the structural complexity of the landscape 15 and are a source of diverse predator 22 , the link between biodiversity and ecosystem functioning has received less attention and it is unknown to what extent the presences of these trees influences the beneficial NCP of predation by natural enemies.…”

“…The first prediction was based on the fact that veteran oaks have high diversity of arthropods associated with them 19 . We defined diversity as including both taxonomic and functional diversity, and for our measure of functional diversity we chose to focus on traits that were identified as being complementary for natural enemies 10 , 22 , 32 . The next prediction was rooted in the first: a community of predators with diverse complementary traits (ie: predators with different hunting strategies, prey species and phenologies) will have high predation levels.…”