Elsevier

Forest Ecology and Management

Volume 445, 1 August 2019, Pages 146-153
Forest Ecology and Management

Insect herbivory and avian insectivory in novel native oak forests: Divergent effects of stand size and connectivity

https://doi.org/10.1016/j.foreco.2019.05.018Get rights and content

Highlights

  • Trophic interactions in novel forests vary with stand size and connectivity.

  • Insect herbivory varied with stand size and stand connectivity.

  • Bird abundance but not richness increased with stand size and decreased with stand connectivity.

  • Avian predation increased with stand size, regardless of connectivity.

  • Avian predation was not determined by bird richness or abundance in novel forests.

  • Insect herbivory was not affected by avian predation in novel forests.

Abstract

The value of novel native broadleaf woodlands for biodiversity conservation is important to consider for adequate forest management in rural landscapes. Passive reforestation has been proposed as a cost-efficient tool for creating networks of novel native forest stands that would help restoring biodiversity and associated ecosystem services. Yet to date the ecological functioning of such stands remains strongly understudied compared to forest remnants resulting from longer-term fragmentation. We assessed how the size and connectivity of newly established Pedunculate oak (Quercus robur L.) stands in rural landscapes of SW France affect rates of herbivory by different insect guilds as well as rates of avian insectivory and the abundance and richness of insectivorous birds. Comparing 18 novel forest stands along a gradient of size (0.04–1.15 ha) and cover of broadleaf forests in the surroundings (0–30% within a 500 m radius), we found that even the smallest stands are colonised by leaf miners and chewers/skeletonizers, and that rates of herbivory are globally comparable to those reported from older and larger oak forests. The size of stands had a relatively minor effect on herbivory, whereas it increased the abundance of insectivorous bird. It also determined rates of avian insectivory as estimated by an experiment with plasticine caterpillars. These rates were however rather low and unrelated with the extent of herbivory in the stand. Overall, our study indicates that insect herbivores tend to react more rapidly to the establishment of novel native forests than their avian predators as the latter may depend on the development of larger patches of suitable habitat in the surrounding landscape. To favour a rapid build-up of diverse, and hence stable, trophic networks involving insect herbivores and their predators, woodland creation schemes should therefore primarily focus on habitat size and quality.

Introduction

Forest fragmentation is well-known to alter patterns of species distribution and abundance, relationships between organisms and resulting ecosystem processes (Ewers and Didham, 2006, Fahrig, 2017, Haddad et al., 2015, Lindenmayer and Fisher, 2013). Among others, it exerts strong effects on trophic cascades such as plant-herbivore-predator interactions, eventually affecting rates of tree damage and health (Bagchi et al., 2018, Chávez-Pesqueira et al., 2015, Rossetti et al., 2017). While forest fragmentation continues to occur in many regions of the world, forest cover is increasing in many others as a consequence of active planting and passive afforestation following rural abandonment (Fuchs et al., 2015, Hansen et al., 2013). For instance, Europe has experienced a steady increase of forested surfaces by 0.8 million ha per year since 1990 (Forest Europe, 2015), a trend that is expected to continue in the coming decades (Fuchs et al., 2015, Schroter, 2005). Habitat defragmentation through passive afforestation has been proposed as an effective tool to reinforce biodiversity and ecosystem functioning in rural and urban landscapes where forest stands were formerly sparse and isolated (Fischer et al., 2006, Rey Benayas et al., 2008, Rey Benayas and Bullock, 2012). Yet little ecological research has to date focused on newly established native forest stands and we largely ignore whether trophic interactions in such stands underlie similar mechanisms as in remnants of similar sizes but resulting from forest fragmentation.

Novel native forest stands establish from a few founder trees that colonize an available habitat patch within an unsuitable matrix through long-distance dispersal and fill their neighbourhood with their offsprings (Gerzabek et al., 2017, Sezen et al., 2005). Such stands share certain characteristics that set them apart from those created by fragmentation: (i) they typically are quite small-sized – even smaller than the smallest fragments of remnant forest; (ii) they are dominated by young trees, resulting in a reduced amount and range of habitats available to forest-dwelling species (Franklin, 1988, Fuller et al., 2018); and (iii) all their species necessarily originate from colonization events over a limited period of time, implying that these systems are triggered by immigration credit instead of extinction debt (Jackson and Sax, 2010). Recent studies on insect and bird species richness along chronosequences of novel native forest development have shown that these are rapidly colonized by woodland generalists whereas specialists can still remain absent even 150 years after forest establishment (Fuentes-Montemayor et al., 2015, Fuller et al., 2018, Whytock et al., 2018). These studies also revealed that local stand characteristics are relatively more important than landscape characteristics for successful colonization by insects and birds. Similar findings have been reported for planted forests (reviewed in Burton et al., 2018). However, their consequences for trophic relationships between plants, insect herbivores and insectivores remain unknown.

Despite the differences between novel native forest stands and remnant forest fragments, the ecological mechanisms underlying trophic cascades involving trees, insect herbivores and birds can to some extent be inferred from fragmentation studies. These have documented that the size and connectivity of forest stands can shape trophic cascades very differently depending on the relative importance of the bottom-up and top-down effects involved (De La Vega et al., 2012, Rossetti et al., 2014). Thus, small and isolated forest stands provide less and possibly lower-quality resources to herbivores (Chávez-Pesqueira et al., 2015) and their colonization requires longer-distance movements that increase energetic and fitness costs (O’Rourke and Petersen, 2017), eventually resulting in lower herbivore abundance (De La Vega et al., 2012, Simonetti et al., 2007). However, small stands also experience greater edge effects which typically go along with increased herbivory (Bagchi et al., 2018, De Carvalho Guimarães et al., 2014). On the other hand, insect herbivores are more likely to colonize small but closer novel forest stands while their predatory vertebrates are more likely to colonize more distant but larger ones (Barbaro et al., 2014, Bereczki et al., 2014, Cooper et al., 2012, Maguire et al., 2015).

There is broad consensus that, generally, predators can notably reduce insect herbivory by regulating herbivore populations (Böhm et al., 2011, Letourneau et al., 2009, Maguire et al., 2015, Rosenheim, 1998). However, their actual relevance in novel native forest stands depends strongly on how both prey and predators respond to stand size and connectivity (Gripenberg and Roslin, 2007). This study investigated how levels of insect herbivory, avian predation and the abundance and diversity of insectivorous birds in recently established native Pedunculate oak (Quercus robur) forest stands are influenced by their size and the cover of broadleaf forest in the surrounding landscape. Specifically, we addressed the following questions: (i) Does herbivory increase or decrease along gradients of increasing stand size and connectivity? (ii) Does avian predation increase or decrease along the same gradients? (iii) Are the observed trends related with the local abundance and diversity of insectivorous birds? We contrast our findings with those reported from studies of forest fragmentation and discuss implications in a context of increasing forest connectivity following ongoing changes in landscape use and management (Burton et al., 2018, Rey Benayas and Bullock, 2012).

Section snippets

Study area and selection of study sites

The study was carried out in the Landes de Gascogne region (south-western France) about 40 km southwest of Bordeaux (44°41′N, 00°51′W). The region is characterized by an oceanic climate with mean annual temperature of 12.8 °C and annual precipitation of 873 mm over the last 20 years. The area is covered by extensive plantations of maritime pine (Pinus pinaster Ait.) interspersed with small stands of broadleaved forests that are dominated by Pedunculate oak (Quercus robur) and contain Pyrenean

Results

Insect herbivory was on average (± se, n = 72) 8.02 ±4.51% (Table A1). The effect of stand size on herbivory depended on the connectivity of the stand (significant Size × Connectivity interaction, Table 1): herbivory tended to increase with stand size in landscapes with a low stand connectivity whereas it decreased in landscape where broadleaf forests where more abundant (Fig. 1). Neither avian predation on plasticine caterpillars nor bird abundance or richness had a significant effect on

Discussion

Our study revealed that the size and connectivity of novel native forest stands affect herbivorous insects and insectivorous birds in different ways. While the abundance of leaf miners depended on stand connectivity alone, herbivory by chewers and skeletonizers was influenced by an interplay between stand size and connectivity, and bird abundance (but not species richness) showed consistent independent and opposite responses to stand size and connectivity. This divergence of relationships is

Acknowledgements

We thank Christophe Poileux, Victor Rébillard, Fabrice Vetillard and Elias Garrouj for their technical assistance in the field and in the laboratory. We also thank two anonymous reviewers for helpful comments. E.V.C was founded by the project SPONFOREST (grant BIODIVERSA 2015-58).

Author contributions

E.V.C., B.C., A.H and I.V.H conceived the study and acquired the data. E.V.C and B.C analysed the data. E.V.C., B.C and A.H drafted the first version of the manuscript. All authors wrote the final version of the manuscript.

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