Physical and biotic factors driving the diversity of spider assemblages in tree hollows of MediterraneanQuercusforests

Abstract

Tree hollows are keystone structures that promote forest biodiversity. This study analyses the spatio‐temporal diversity of spiders in these microhabitats. Forty‐eight emergence traps were installed in tree cavities of Quercus pyrenaica forests in the Iberian Peninsula. Traps were collected monthly during a complete year. Generalised linear models and canonical correspondence analysis were used to evaluate the effect of two physical (hollow volume and height above ground) and three biotic variables (beetle richness and abundance and isopod abundance) on the spider diversity and composition of the entire assemblage and of the foraging guilds. Moreover, we examined the temporal segregation of the species using beta diversity metrics. Tree hollows hosted a rich spider assemblage (87 species) including 10 endemic and one vulnerable species. We registered four tree‐bark obligate and one hollow‐dependent species. Hollow height and beetle richness were the variables that better explained richness, while all variables analysed influenced spider abundances. However, contrary to the observed effect for other taxa (i.e. beetles), bigger cavities did not host more spider species. Temporal beta diversity throughout the year was very high and primarily explained by species turnover suggesting a temporal species segregation. We conclude that tree hollows constitute a key habitat for forest spiders and that special attention must be paid to the preservation of this microhabitat in forest ecosystems.

Spatio‐temporal species aggregations do not rule out interspecific competition in tree hollow spider assemblages

“…In fact, recent studies conducted with predatory arthropods (Coleoptera and Odonata) in tree hollows show that communities segregate as a consequence of habitat preferences rather than competition (Oliveira-Junior et al, 2021;Sánchez-Galván et al, 2018). Moreover, knowing that hollows are physically and chemically heterogeneous from one another (Mic o et al, 2015) and that the Order Araneae is a very sensitive group to variations in environmental conditions (Jiménez-Valverde & Lobo, 2007;Košulič et al, 2016;Malumbres-Olarte et al, 2018;Müller et al, 2020), it is understandable that spiders are distributed in these saproxylic microenvironments according to their ecological preferences (Hernández-Corral et al, 2021). For instance, in the H. gredensis-T. denticulata pair (Tables 2 and 3), the former species prefers humid environments (Bidegaray-Batista et al, 2014) and requires little space for hunting (Hernández-Corral et al, 2021), while the latter species is usually found in sunny environments (Hernández-Corral, 2020) and needs larger spaces to build its hunting webs (Hernández-Corral et al, 2021).…”

“…Moreover, knowing that hollows are physically and chemically heterogeneous from one another (Mic o et al, 2015) and that the Order Araneae is a very sensitive group to variations in environmental conditions (Jiménez-Valverde & Lobo, 2007;Košulič et al, 2016;Malumbres-Olarte et al, 2018;Müller et al, 2020), it is understandable that spiders are distributed in these saproxylic microenvironments according to their ecological preferences (Hernández-Corral et al, 2021). For instance, in the H. gredensis-T. denticulata pair (Tables 2 and 3), the former species prefers humid environments (Bidegaray-Batista et al, 2014) and requires little space for hunting (Hernández-Corral et al, 2021), while the latter species is usually found in sunny environments (Hernández-Corral, 2020) and needs larger spaces to build its hunting webs (Hernández-Corral et al, 2021). Likewise in the E. atrica-T. denticulata segregation (Table 2 and Figure 2l), the difference in their sizes may mean that they prefer larger or smaller hollows, respectively.…”

“…Very few studies have addressed relationships in predatory groups, which is the case of Odonata in phytotelmata hollows in tropical systems (Oliveira‐Junior et al, 2021). Spiders also constitute a suitable model to test the contribution of interspecific competition factors because they are a very diverse group in tree hollows (Barrientos et al, 2020; Barrientos et al, 2022; Hernández‐Corral et al, 2017; Machač et al, 2018), they are exclusively predatory (Coddington & Levi, 1991; Nyffeler, 1999; Turnbull, 1973) and they are influenced by the physical and biotic variables of tree hollow (Hernández‐Corral et al, 2021).…”

On how the abandonment of traditional forest management practices could reduce saproxylic diversity in the Mediterranean Region

“…Moreover, the different types and characteristics of each microhabitat constrain species' functional traits and trophic guild distribution (Micó et al, 2020), which means that their presence and heterogeneity drive species diversity in different ways. Tree management (i.e., pollarding) promotes tree hollow formation, which are keystone elements for forest diversity, especially for arthropods (Müller et al, 2013;Micó, 2018;Hernández-Corral et al, 2021). The presence of tree hollows favours species richness, and doubling the mean number of tree hollows per hectare (from 10 to 20 trees) has been reported to increase species richness by more than 7%.…”

Sex specificity of dispersal behaviour and flight morphology varies among tree hollow beetle species

“…One of the most interesting communities for studying insect dispersal behaviour within forest ecosystems are the saproxylic assemblages that inhabit tree hollows. 1 First, tree hollows are considered a keystone microhabitat for European forest biodiversity conservation [ 16 – 18 ]. Second, cavity availability and spatial connectivity are currently jeopardized by several factors, such as forest fragmentation, climatic change, forestry and the abandonment of cultural practices such as tree pollarding (note that tree pollarding accelerates the formation of tree cavities) [ 16 , 19 , 20 ].…”