Hedgerow rejuvenation management affects invertebrate communities through changes to habitat structure

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Abstract

Hedgerows are an important semi-natural habitat for invertebrates and other wildlife within agricultural landscapes. Hedgerow quality can be greatly affected either by over- or under-management. Neglect of hedgerows is an increasingly important issue as traditional management techniques such as hedgelaying become economically unviable. In the UK, funding for hedge management is available under agri-environment schemes but relatively little is known about how this impacts on wider biodiversity. We used a randomised block experiment to investigate how habitat structural change, arising from a range of techniques to rejuvenate hedgerows (including more economic/mechanised alternatives to traditional hedgelaying), affected invertebrate abundance and diversity. We combined digital image analysis with estimates of foliage biomass and quality to show which aspects of hedge structure were most affected by the rejuvenation treatments. All investigated aspects of habitat structure varied considerably with management type, though the abundance of herbivores and predators was affected primarily by foliage density. Detritivore abundance was most strongly correlated with variation in hedge gap size. The results suggest that habitat structure is an important organising force in invertebrate community interactions and that management technique may affect trophic groups differently. Specifically we find that alternative methods of hedgerow rejuvenation could support abundances of invertebrates comparable or even higher than traditional hedgelaying, with positive implications for the restoration of a larger area of hedgerow habitat on a limited budget.

Zusammenfassung

Hecken sind ein wichtiger halbnatürlicher Lebensraum für Wirbellose und andere Wildtiere in der Agrarlandschaft. Ihre Eignung als Habitat kann sowohl durch zu intensives Management als auch durch Vernachlässigung beeinträchtigt werden. Vernachlässigung von Hecken wird mehr und mehr zu einem Problem, da traditionelle Pflegemaßnahmen wie z.B. das “Knicken” wirtschaftlich nicht mehr tragbar sind. Im Vereinigten Königreich stellen Programme zur Förderung umweltgerechter Landwirtschaft Fördermittel für Hecken-Pflegemaßnahmen zur Verfügung, aber wenig ist darüber bekannt, wie solche Maßnahmen sich auf die Biodiversität von Hecken-Lebensräumen auswirken. Ein Block-randomisiertes Experiment diente uns dazu, zu erforschen, wie strukturelle Änderungen durch eine Reihe von Methoden der Hecken-Verjüngung die Häufigkeit und Diversität von Wirbellosen beeinflussen. Zu diesem Zweck kombinierten wir Methoden der digitalen Bildanalyse mit Schätzmethoden zur Bestimmung der Biomasse und Qualität des Blattwerkes, um zu bestimmen, welche Heckenstruktur-Aspekte am meisten von der Wahl der Verjüngungsmethode beeinflusst wurden. Alle untersuchten Aspekte der Habitatstruktur wurden durch die Art der Pflege deutlich beeinflusst. Hingegen wurden die Abundanzen von herbivoren und prädatorischen Wirbellosen primär durch die Dichte des Blattwerkes beeinflusst. Die Detritivoren-Häufigkeit korrelierte am stärksten mit der Variabilität der Lückengrößen der Hecken. Unsere Ergebnisse sind Beleg dafür, dass strukturelle Aspekte deutlichen Einfluss auf die Interaktionen innerhalb der Invertebraten-Zönose ausüben und dass Hecken-Pflegemaßnahmen verschiedene trophische Gruppen in unterschiedlicher Weise beeinflussen. Hierbei können alternative Methoden der Heckenverjüngung vergleichbare oder sogar höhere Abundanzen von Wirbellosen zur Folge haben als das traditionelle “Knicken” von Hecken. Dies wiederum hat bedeutende Konsequenzen für die großflächige Renaturierung von Hecken-Lebensräumen bei begrenzten finanziellen Mitteln.

Introduction

Habitat structure, defined as the composition and arrangement of objects in space (McCoy & Bell, 1991), is widely known to affect interactions within invertebrate communities (Langellotto & Denno, 2004). However, the direction and magnitude of these effects are dependent on the system in question, and the way in which structure is quantified. A meta-analysis of 67 manipulative studies found that enhancement of habitat structure resulted in a significant increase in predator and parasitoid abundance (Langellotto & Denno, 2004), concluding that increases in predators did not follow prey abundance but rather occurred through increased efficiency of prey capture. Predators may also be impaired by increased complexity of habitat structure, for example through reduced foraging efficiency (Legrand & Barbosa, 2003), or a higher number of refuges for prey (Sanders, Nickel, Grützner, & Platner, 2008).

At the within-habitat scale, structure may affect invertebrate interactions by altering the availability of resources for herbivores (Denno et al., 2002, Sanders et al., 2008), the ease with which predators are able to capture their prey (Schmidt & Rypstra, 2010), or the degree of interference among predators (Janssen, Sabelis, Magalhães, Montserrat, & van der Hammen, 2007). Alterations to habitat structure may concurrently alter resource quality. For example, the proliferation of young leaves resulting from mechanical disturbance have a decreased ratio of total carbon (C) to nitrogen (N; Havill and Raffa, 2000, Mediene et al., 2002), which can have effects on herbivores that cascade to other trophic levels (Chen, Olson, & Ruberson, 2010).

Hedgerows are a man-made linear habitat covering over 450,000 km in England alone (Norton et al., 2012), supporting a wide range of plants (Critchley, Wilson, Mole, Norton, & Smart, 2013), birds, mammals (Barr, Britt, Sparks, & Churchward, 2005), and over 1500 species of invertebrate (UK Steering Group, 1995). Traditional management by hedgelaying, whereby some stems are removed and those remaining are partially cut near the base and laid along the line of the hedge, has given way to intensive cutting by modern tractor and flail machinery or in some cases neglect. Resulting widespread changes in the structural quality of hedges (Croxton, Franssen, Myhill, & Sparks, 2004) include reductions in berry resources for wildlife (Staley et al., 2012) and ‘gappy’ hedges (Croxton & Sparks, 2002) or lines of trees (Croxton et al., 2004). A 6% decrease in the length of hedgerow between 1998 and 2007 was attributed largely to under-management, and in 2007 it was also estimated that only 48% of hedges were in ‘good’ structural condition (Norton et al., 2012). Valued as a priority habitat for conservation (JNCC & Defra, 2012), sensitive management of hedgerows, including rejuvenation, is promoted in the UK through agri-environment scheme funding (Natural England, 2013), making investigation into the potential of more economical methods pertinent.

Few formal comparisons have been made between the impacts of hedge rejuvenation management on invertebrates (Henry, Bell, & McAdam, 1994) though different methods lead to widely divergent habitat structures which are likely to impact differently on invertebrate community composition. In this study, we tested how invertebrate abundance and diversity in hedgerows was affected by changes in localised habitat structure (i.e. woody biomass distribution) and habitat quality (nutritional value of foliage for herbivores) using a multi-site manipulative field experiment at which hedgerow rejuvenation treatments were applied. We also measured foliage biomass, recognising that this represents both a structural and resource component of the system. We focussed on differences between trophic groups, hypothesising that increasing the spatial variation of (within-habitat) hedgerow structure would increase predator abundance but that herbivores would be more affected by the nutritional quality of food resources. Secondly, we hypothesised that hedges rejuvenated with more economical methods, used in place of traditional hedgelaying, will support a similar abundance and trophic diversity of invertebrates as those rejuvenated with traditional hedgelaying.

Section snippets

Experimental design

A randomised block experiment was established at four lowland arable sites in East and Southeast England; Newbottle Estate (NE; Buckinghamshire), Utcoate Grange (UG; Bedfordshire), Monks Wood (MW; Cambridgeshire) and Wimpole Hall (WH; Cambridgeshire). At each site, four rejuvenation techniques and an unmanaged control (Table 1) were randomly allocated and applied in October 2010 to 15 m contiguous sections (plots) of uniform hedgerows that had received little management for some years.

Results

In total 10,769 invertebrates were collected from beating the hedge canopy in 2011; no interactions were found between treatment and month so data were summed across months for further analysis. The most abundant taxa in decreasing order were Collembola (n = 4554), Acari (n = 1322), Coleoptera (n = 1197), Araneae (n = 811), Psocoptera (n = 597), Heteroptera (n = 570), Diptera (n = 447) and Psylloidea (n = 400). For all other taxa <250 individuals were sampled. Of the predators the most abundant taxa were

Hedgerow management affecting invertebrates

Hedge rejuvenation method resulted in considerable immediate differences in the structure and quality of hedgerow habitat which had knock-on effects on invertebrate communities. Techniques where the hedge was laid increased foliage biomass, though less so in the mechanical wildlife hedging. A positive relationship between foliage biomass and invertebrate abundance corroborates previous findings, particularly for spiders (Gunnarsson, 1990), and herbivores (Whitfeld, Novotny, Miller, & Hrcek, 2012

Acknowledgements

The experimental setup was funded as part of DEFRA grant BD2114, with additional data collection for this study supported by NERC Centre for Ecology and Hydrology core funding. Thanks to Marc Botham and Lucy Hulmes for help with invertebrate sampling, and Debbie Coldwell for assistance with foliar chemical analysis.

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