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Currently, species are going extinct a thousand times faster than expected by fossil records (Millennium Ecosystem Assessment 2005) and we may already be entering the sixth mass extinction in the history of Earth (Barnosky et al. 2011). The largest threat to biodiversity and ecosystems is land use change and the subsequent loss of habitat (Sala et al. 2000, Millennium Ecosystem Assessment 2005). Loss of habitat also leads to fragmentation of the remaining patches with varying degrees of isolation (Andren 1994). The effect of landscape fragmentation on species has been studied by several approaches, of which metapopulation theory and island biogeography (Hanski 1999, Ricketts 2001) are among the most important. Island biogeography, focusing on the effect of island size and isolation on populations and species, has been extended to include habitat patches on land, surrounded by nonhabitat referred to as the matrix (Hanski 1998, Dover and Settele 2009, Fischer et al. 2009, Franzen et al. 2012). Other important factors that can affect species richness of fragmented habitats is the degree of isolation and the size of the habitat patch (Andren 1994, Benedick et al. 2006), as well as habitat patch quality (Thomas et al. 2001). To successfully conserve species richness, we need a better understanding of the responses to fragmentation. This is particularly true for species-rich groups such as insects, which include 1 million described species (IUCN Red List 2014) and contribute to important ecosystem services (Losey and Vaughan 2006). Understanding the patterns and responses of species in fragmented landscapes are essential for their conservation and should be a prioritized research task. Several studies have tried to identify the traits that make species vulnerable to fragmented landscapes (Henle et al. 2004). Studies of insects reveal that degree of specialization in habitat and food requirements, dispersal ability, body size, population size, and reproductive capacity are traits that can influence species vulnerability (Henle et al. 2004, Benedick et al. 2006, Cagnolo et al. 2009). Species that are highly specialized with narrow niches are most vulnerable, whereas species that are more general in their habitat and food requirements, breed in ephemeral habitats or have rapid growth and dispersal are more likely to be successful (McKinney and Lockwood 1999, Franzen et al. 2012). Species are categorized according to their extinction risk in international and national red lists (IUCN Red List 2014), but the number of species at risk are likely to be higher (McKinney and Lockwood 1999) and a large number of species are not evaluated due to information deficiency (Nieto et al. 2014). The vulnerability of specialization and the Holt’s hypothesis, which proposed that specialized species on top of the food chain should have a stronger response to fragmentation and habitat loss than generalists (Holt et al. 1999), have been confirmed in several studies (Komonen et al. 2000, Valladares et al. 2006, Cagnolo et al. 2009). Saproxylic species are species dependent on dead wood habitats or its inhabitants (Speight 1989), and they represent important decomposers with high species richness which is severely affected by land use changes. Modern forestry with clear-cutting and monocultures of planted trees has reduced the volumes of available dead wood and made old-growth forests rare or lacking in much of Europe (Grove 2002, Hanski 2008). Some species are highly specialized to certain host trees, whereas others can use a range of dead wood habitats independent of tree species (Grove 2002, Stokland et al. 2012). Oak (Quercus spp. L.) is a temperate broadleaved tree that can become close to 1,000 years old (Drobyshev and Niklasson 2010). As the tree grows old, the architectural diversity increases and a range of new microhabitats appear, such as dead branches, coarse bark, wood mould, and different types of rot (Alexander 2008). Oaks with hollows are normally older than 200 years (Ranius et al. 2009) and represent biodiversity hotspots (Kennedy and Southwood 1984, Sverdrup-Thygeson 2009, Bouget et al. 2014). The recruitment of such oaks is low. Most old-growth deciduous forests disappeared from Europe centuries ago, but the mature and ancient trees that still remain are threatened in large parts of Europe due to direct removal, intensification in forestry and agricultural landscapes, regrowth creating shade, lack of recruitment and pollution (Ranius et al. 2005, Gibbons et al. 2008, The Directorate for Nature Management 2012, Lindenmayer et al. 2012). Hollow oaks are well defined ‘patches’ with a large number of rare and red-listed species (Ranius 2002b, Sverdrup-Thygeson 2009, The Directorate for Nature Management 2012). Although areas of high density of ancient hollow oaks in Europe do exist, most hollow oaks are scattered and isolated, either in agricultural landscapes or in woodlands (Gibbons et al. 2008, Ranius et al. 2009). For dispersing insects, the distance between suitable trees is likely to represent a challenge, and some beetles associated with hollow trees are known to be poor dispersers (Ranius 2006). The current spatial distribution and connectivity of hollow oaks are therefore of major importance for the oak-dependent species living in hollow trees. To categorize the species into functional groups can help to identify if some groups are more vulnerable to fragmentation than others, which can in worst case lead to loss of important ecosystem functions. Therefore, to make sound management decisions it would be an advantage to know the most important aspects for maintaining species richness, rare species and ecosystem functions related to hollow oaks. Further, these aspects should be easily detectable in the field. Often, time or money does not allow expensive and time-consuming insect surveys. Recognizing important structures of the oak, or the surroundings, could provide valuable indication of which trees that are associated with high species richness or high number of rare species (Skarpaas et al. 2011). In our study, we have identified important structural variables related to patch size, habitat quality, and isolation, to test if these aspects affect the species richness and abundances in functional groups and red-listed species. We compared responses of different groups of beetles in highly isolated oaks to clustered hollow oaks (low isolation) while including variation in patch size and quality. The aim of our study was to evaluate the effect of patch size, habitat quality, and isolation on species richness and abundance of functional groups and red-listed oak species in hollow oaks. We expected a general positive effect of patch size and habitat quality on abundance and species richness, with open surroundings and low tree crown as proxies of high quality. We expected isolation to have strongest effect on the most specialized and vulnerable groups, such as the oak specialists and the red-listed species. |
