Research Article

Journal of Orthoptera Research 2025, 34(2): 213-222

Initial response of Orthoptera fo beaver (Castor fiber) reintroductions in

post-arable enclosures

Tim GARDINER!, EMILY Crisp!

1 Fisheries, Biodiversity and Geomorphology, Environment Agency, Iceni House, Cobham Road, Ipswich, Suffolk, IP3 9JD, UK.

Corresponding author: Tim Gardiner (tim.gardiner@environment-agency.gov.uk)

Academic editor: Axel Hochkirch | Received 3 April 2025 | Accepted 28 May 2025 | Published 25 June 2025

https://zoobank.org/1E9F5658-393B-4FB3-B7C7-08F19EA26C6D

Citation: Gardiner T, Crisp E (2025) Initial response of Orthoptera to beaver (Castor fiber) reintroductions in post-arable enclosures. Journal of

Orthoptera Research 34(2): 213-222. https://doi.org/10.3897/jor.34.154843

Abstract

Rewilding aims to restore ‘self-willed’ ecosystems involving the creation of habitat subject to stochastic disturbance, connected by favorable corridors for dispersal of animals including insects. Herbivores such as the Eurasian beaver Castor fiber (Linnaeus, 1758) have been reintroduced in the UK to promote natural flood management as part of a rewilding strategy. At Spains Hall in Finchingfield (south-east England), beavers were released into two large enclosures (both c. 20 ha) in March 2023 which were concurrently reverting from arable farmland to riparian meadow. Monitoring of Orthop- tera 15-18 months after release in June-September 2024 revealed statisti- cal evidence that the abundance of the two localised groundhoppers, Tetrix subulata (Linnaeus, 1758) and Tetrix undulata (Sowerby, 1806), and the lesser marsh grasshopper Chorthippus albomarginatus (De Geer, 1773), was higher in the beaver enclosure meadows than in the control plots. The Essex Red Data List species, the common green grasshopper Omocestus viridulus (Lin- naeus, 1758), was also recorded within the beaver meadows. The damming of the watercourse in one enclosure led to the creation of new off-line lotic channels and wet ground which were important for these species, particular- ly bare mud margins which were used by groundhoppers (Tetrigidae). There was also evidence that meadows on the east side of the river had a greater abundance of long-winged conehead Conocephalus fuscus (Fabricius, 1793) and T: subulata than the west, perhaps due to the sheltered microclimate and wetland features on this side of the channel, particularly in the beaver enclo- sures. We conclude that in enclosures on former arable farmland Orthoptera may respond to damming which quickly creates beaver meadows in flooded areas within the first two years of release. However, only one enclosure had dams in this study so the effects of beavers may be dependent on variables such as habitat quality and watercourse characteristics.

Keywords

Acrididae, bush-cricket, grasshopper, groundhopper, natural flood man- agement, Tetrigidae, Tettigoniidae, rewilding

Introduction To reverse the decline of insects such as grasshoppers and bush-

crickets, rewilding of arable farmland may be highly beneficial (Tree 2017, van Klink and WallisDeVries 2018, Garrido et al. 2022).

The ideal aim of rewilding is to restore natural processes, often in- volving the creation of large areas of habitat subject to stochastic disturbance connected by favorable corridors for species to dis- perse along (Carver and Convery 2021, Gordon et al. 2021a, b). Re- wilding is well established at UK sites such as Knepp in West Sussex (Greenaway 2006, Wallace 2018, Dempsey 2021) and three sites in Suffolk — Arger Fen, Black Bourn Valley and the Somerleyton Es- tate (Gardiner and Casey 2022a, b, 2024). Research at Black Bourn Valley revealed that Orthoptera were in higher abundance and di- versity in fields >8 years post-arable cropping cessation with ripar- ian wet meadows being a key source of colonising insects (Broad 2023, Gardiner and Casey 2022a, b). Herbivores have a significant role in modifying vegetation structure and introducing habitat het- erogeneity into former arable river valleys, especially lagomorphs (Gardiner and Casey 2022a, b). Herbivores such as the Eurasian beaver Castor fiber (Linnaeus, 1758) also have significant impacts on habitats and species in riparian landscapes (Wright et al. 2002, Campbell-Palmer et al. 2015, Campbell-Palmer et al. 2016). In Eu- rope, beaver numbers declined to just 1200 individuals in the 20" century before recovering to over 1.2 million animals in 2020 due to the success of reintroduction programs (Wrobel 2020, Halley et al. 2021). Beavers have been reintroduced in the UK to promote natural flood management (NFM; Forbes et al. 2015).

Beavers became extirpated from the UK during the 16" century after the last known British record in 1526-7 (Raye 2015). Beavers are ecosystem engineers leading to significant modifications to river corridor structure, geomorphology, nutrient cycling and the storage of water on the floodplain (Rosell et al. 2005, Cunning- ham et al. 2007, Willby et al. 2018, Brazier et al. 2021, Larsen et al. 2021). Dam building and foraging on woody and herbaceous vegetation are the main beaver behaviours that influence river corridors and their biodiversity value (Law et al. 2014, Campbell- Palmer et al. 2015). Dam building leads to higher water levels and diversity of flow in river channels (Wright et al. 2002, Campbell- Palmer et al. 2016), while beaver lodges can also become biodi- versity hotspots in this environment with a high degree of habitat heterogeneity (Wilson and Bremner 2025).

Copyright Tim Gardiner & Emily Crisp. This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unre- stricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

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Geomorphological and hydrological reactions to beaver pres- ence have been studied (Hering et al. 2001), as have the positive effects on amphibians (Romansic et al. 2020), bats (Hooker et al. 2024) and aquatic macro-invertebrates (Nummi 1989, Bush and Wissinger 2016). Less well known is the impact of beaver dam building and foraging on the abundance and diversity of terrestrial invertebrates in riparian habitats. The studies that exist have fo- cused on increases in flying insects and moth diversity (Andersen et al. 2023), true bugs (Orazi et al. 2022) and saproxylic beetle as- semblages associated with dead wood habitats established due to the felling activities and flooding associated with beavers (Zahner et al. 2006, Horak et al. 2010, Mourant et al. 2018). Away from wooded beaver habitats, a single study of Orthoptera in beaver clearings in Germany has been undertaken (Dalbeck 2011).

Dalbeck’s study focused on the presence or absence of Orthop- tera in open beaver clearings (several hectares in size), recording 14 species, but noting the absence of otherwise abundant spe- cies such as Roesel’s bush-cricket Roeseliana roeselii (Hagenbach, 1822), possibly due to unsuitable, short vegetation structure. The relatively species-rich beaver clearings in the Eifel Mountains were populated by c. 18% of the native Orthoptera of Germany (Fart- mann et al. 2024) including stenotopic insects such as large marsh grasshopper Stenophyma grossum (Linnaeus, 1758) which require differing environmental niches (Dalbeck 2011).

Given the propensity in the UK for insects such as slender groundhopper Tetrix subulata (Linnaeus, 1758) to benefit from wet bare ground on the edges of ponds on riparian rewilding sites devel- oping on former arable fields (e.g. Black Bourn Valley, Gardiner and Casey 2022a, b), it is likely that these species may respond rapidly to early successional stages of wetland creation from the damming activities of beavers and any increase in habitat heterogeneity (e.g. formation of beaver meadows within first two years of beaver re- lease). Dalbeck (2011) only recorded the presence or absence of Or- thoptera and did not quantify the abundance or diversity of beaver clearings. The raising of water levels by impounded river water be- hind a dam, can lead to the formation of riparian beaver meadows where flooding occurs (Stringer and Gaywood 2016). In the first 1-2 years of beaver meadow development, wet grassland with pud- dles can form, with a mosaic of shallow mud shores on the marsh edge interspersed with taller vegetation (Andersen et al. 2024).

At Spains Hall in Finchingfield, south-east England, beavers were reintroduced to former arable land reverting naturally to open floodplain grassland and scrub mosaics. The two fenced enclosures were established in early 2023 and represented an excellent oppor- tunity to monitor the early development of beaver meadows within their first two years in a different context to the forest clearings of Dalbeck (2011) in Germany. Orthoptera diversity is low in north Es- sex due to the prevalence of intensive arable farming (i.e. high inor- ganic fertiliser and pesticide usage) and improved grassland which supports 7-11 species compared to the extensive grazed marshland and flood embankments of the coast (13-18 species, Fargeaud and Gardiner 2018). Therefore, the Spains Hall beaver enclosures and their developing post-arable grasslands could enhance Orthoptera abundance and diversity in an otherwise inhospitable landscape for insects or their isolation in an arable landscape may render colonisation slow or non-existent for less mobile species.

The aim of this paper is to report a study on the orthopteran assemblages of riparian grassland in fenced beaver enclosures at Spains Hall, Finchingfield, in Essex, UK two summers after rein- troduction. Results, specifically the abundance and diversity of Or- thoptera, are discussed in relation to the damming activities of bea- vers in the two enclosures compared to adjacent control meadows.

T. GARDINER AND E. CRISP

Materials and methods

Study site.—Spains Hall Estate in Finchingfield, south-east England (latitude/longitude 51 °58'52.0356"N, 0°27'9.6516"E) is a mosa- ic of lowland woodland, open farmland, old grassland, orchard and river valley (c. 800 ha total area). In 2019, beavers (one adult male and one female) were introduced into a 4 ha woodland area (The Moors) as part of a NFM scheme to store water and reduce flooding to the village of Finchingfield. Damming by beavers in The Moors has led to the storage of c. 2.4 million litres of water on the floodplain (Brown 2022). This unprecedented project by the Spains Hall Estate is jointly supported by a public and private partnership which includes Anglian Water, Environment Agency, Anglian Eastern Regional Flood and Coastal Committee (RFCC), Essex County Council and Essex and Suffolk Water.

The beaver reintroduction scheme was expanded in 2023 with the release of a pair of beavers (one adult male and female) into two large fenced enclosures (both c. 20 ha, Table 1) along the tree- lined river, Finchingfield Brook (river channel width bank top to bank top c. 2-4 m, water depth <1 m), which were formerly arable farmland (cropping ceased after 2018; Brown 2022) and are in the process of natural reversion to grassland floodplain meadows. The fields in the enclosures were previously cropped for agriculture with nitrogen (N) fertiliser applied to a range of crops including spring barley, winter beans and winter wheat. Soil types vary from clay loams with impeded drainage and freely drained sands/gravels. The intention of reverting the arable fields in the enclosures was to cre- ate natural wetland habitats alongside the river through the dam- ming behaviour of beavers, storing water on the floodplain. Graz- ing ungulates (deer) were removed from the enclosures before the release of beavers to ensure no overgrazing of enclosure habitats.

Narrow strips of grassland were present along Finchingfield Brook during the time of arable cropping, allowing some Orthoptera habitat to remain within the intensive arable cropping regime which would otherwise be highly unfavorable for insects (Gardiner et al. 2002). It is likely that after arable cropping ceased that Orthoptera colonised any reestablishing grassland before the enclosures were created. Therefore, this study measures the alteration of the reestab- lishing grassland by beaver activities and its effects on Orthoptera compared to control grasslands outside of the enclosures.

To contrast with the two fenced beaver enclosures, two unfenced open meadow control sites were selected for this study without bea- vers, Control A adjacent to the south of Enclosure A and Control B, immediately to the north of Enclosure B (Table 1, Figs 1, 2). The enclosure fences may influence the behaviour of grazing ungulates

Table 1. Characteristics of the four replicates demarcated by bea- ver enclosure and control treatment. In each replicate, there were four 100-m transects, two on either side of the river (total of 16 transects for the study).

ae No. No. River % river

Treatment/ Area Linking

Replicate (ha) habitat _‘“ansects transects length length

east west (m) _ tree-lined

Beaver enclosure rN 20 D,H,GL,W ps 2 958 84 B 20 DeH).W 2 2 895 62 Control ra 2 Wye AW. vi w 204 wire B 2 D,H,GLW wd 2: 283 98 Total 8 8 2340

Key: D = ditch, H = hedge, GL = green lane, W = woodland.

JOURNAL OF ORTHOPTERA RESEARCH 2025, 34(2)

T. GARDINER AND E. CRISP

(e.g. deer) along Finchingfield Brook, which will find their pathways across the landscape restricted, altering their behaviour (e.g. feeding areas) in the unfenced control meadows (Xu et al. 2021). The topog- raphy of the sampled enclosure and control meadows was almost completely even (c. slopes <1% gradient) within 10 m of Finch- ingfield Brook on the east and west sides. The control meadows to the south and north of the enclosures were post-arable grasslands (cropping ceased after 2018) unaffected by the activities of beavers. Adjacent to both post-arable control meadows were existing rough grassland areas (Brown 2022) which could be considered the donor sites for Orthoptera to colonise the establishing beaver enclosure meadows and bolster populations already existing in the reestab- lishing riparian grassland of the enclosures. Grassland of the enclo- sures and control meadows was predominantly tall (>50 cm height) and unmanaged before the commencement of the study. Throughout the duration of the 2024 summer study period there was no active conservation management of any of the sam- pled grassland. All fields were surrounded by dense hedgerows, ditches and/or woodland by Finchingfield Brook (Table 1, Fig. 2).

Transect surveys.—Four 1-m wide x 100-m long transects were estab- lished in each of the two beaver enclosure and two control meadow replicates and ran parallel to Finchingfield Brook within 10 m of the bank top (Table 1, Fig. 2). Transects were located along 100-m stretches of Finchingfield Brook which were representative of the veg- etation communities of the enclosures and control meadows. In En- closure B, the evidence of beaver activity was widespread (6 dams, 2 lodges and 6 felled trees, Fig. 3) leading to the creation of a new river channel where water had been impounded behind a dam and spilled

Fig. 1. Beaver enclosure B (A) with newly created wetland/lotic channel near to a dam and control meadow B (B) at Spains Hall © Tim Gardiner.

215

out onto the eastern floodplain (Fig. 1A). The transect in this area was along the edge of this lotic channel. Trees felled included willow Sa- lix spp. and aspen Populus tremula (Linnaeus, 1753). In Enclosure A, there were 5 felled trees but no dams or lodges (Fig. 1). Woody mate- rial was present in the channel and did impede flow but not so that water overflowed onto the floodplain similar to Enclosure B’s dams. The transect sampling closely followed the methodology of Gardiner et al. (2005), Gardiner and Hill (2006) and Gardiner and Casey (2024). Adult individuals of all Orthoptera species along all transects were recorded acoustically and visually to determine as- semblage composition and species diversity and richness. Each tran- sect was walked at a slow, strolling pace (2 km/hr) once in June, July, August and September 2024 (4 surveys). Nymphs flushed from a 1-m wide band in front of the observer were recorded along all transects. As it is difficult to distinguish between species in the early instars (though not impossible, see Thommen 2021), nymphs of all species were lumped together for recording purposes. With prac- tice, it was relatively easy to ascertain the species of adults without capture (Gardiner and Hill 2006) although some species such as the long-winged conehead Conocephalus fuscus (Fabricius, 1793) and R. roeselii are significantly under-recorded using visual transects (Gardiner and Hill 2006). A dual visual and acoustic monitoring method has been used by Weiss et al. (2013) to ensure complete coverage of the orthopteran fauna of sites. In the current study, a stridulation monitoring technique was used to record adult males of species which stridulated along the transects at the same time as visual monitoring by flushing. Stridulation monitoring has been used to record cryptic species in Essex and has been found to be effective compared to visual sighting transects and pitfall traps (Har- vey and Gardiner 2006, Gardiner et al. 2010). A Magenta Mké4 bat detector (Magenta Electronics Ltd., Burton-on-Trent, Staffordshire, UK) was used in the current study (set at 28 khz to detect ultrasonic bush-crickets, Meyer and Elsner 1996) by the first author (TG) to allow reliable detection of stridulating males up to 20 m away either

Finchingfield Brook Beaver dam Beaver lodge Beaver-felled tree

= New beaver channel Flooded area Transect location

« Ditch/hedgerow Road

I” i] I : meadow A

Duck End

Fig. 2. Map of the features of the two fenced beaver enclosures and unfenced control meadows at Spains Hall in Finchingfield, UK.

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side of the transect. The weather conditions on survey days were favorable for insect activity, being largely sunny and warm (>17°C).

Environmental surveys.—A total of 40 vegetation heights were re- corded at random positions along each of the 16 Orthoptera tran- sects using a 1-m ruler in September 2024. The vegetation species the 1-m ruler touched at ground level at each of the 40 random locations on every transect was recorded to determine the com- parative number of hits for different species.

Statistical analysis. —All data were square-root transformed to cor- rect for non-normality before analysis (Heath 1995). Significance for all tests was accepted as evidence on the following scale in accordance with Muff et al. 2022: p-value >0.1 little or no evi- dence, 0.05-0.1 weak evidence, <0.05 moderate evidence, <0.01 strong evidence or <0.001 very strong evidence.

Orthoptera.—To determine species preferences between beaver en- closure and control meadows all detections of Orthoptera (visual or acoustic) were summed for each replicate for the survey period (4 surveys) to determine the relative abundance of adults of each spe- cies in accordance with previous studies (notably, Weiss et al. 2013). Independence of replicate was assumed and data pooled for each one in a similar way to data analysis in other monitoring studies (Nur et al. 1999). Additionally, to test the relationship between dif- ferent species and vegetation height, all detections of Orthoptera (visual or acoustic) were summed for each 100-m transect section. Species richness was calculated for each 100-m transect. Assem- blage diversity estimates were also calculated using Version 4.1.2. Species Diversity and Richness software (Pisces Conservation Ltd, IRC House, The Square, Pennington, Lymington, Hampshire) from collated data. The Shannon-Wiener Diversity Index (H’, Kent and Coker 1992) was calculated using the total number of individuals re- corded for each Orthoptera species in each of the 16 transect sections. To determine whether the abundance of adults (of all species) and nymphs and vegetation height differed between beaver enclo- sure and control meadows (treatment) and east and west aspects (as- pect), a two-way ANOVA was used for all comparisons. For aspect, data from transects on the east and west side of Finchingfield Brook were compared. To determine the influence of beaver enclosures fur- ther, species richness and Shannon-Wiener diversity were compared for beaver enclosures and control transects (treatment) and east and west aspects (aspect) using a two-way ANOVA for all comparisons. Additionally, to determine whether adults of each species, nymphs and species richness and diversity had significant relationships with vegetation height, linear regression models were run (Heath 1995).

T. GARDINER AND E. CRISP

Results

Nine species of Orthoptera (33% of British native species) were recorded in both the beaver enclosures and control mead- ows. Two localised (T. subulata and common groundhopper Tetrix undulata (Sowerby, 1806) and one Essex Red Data List species, common green grasshopper Omocestus viridulus (Linnaeus, 1758) were detected in beaver enclosure and control meadows (Fig. 4). The most commonly recorded species were R. roeselii (n = 684, 41%) and Pseudochorthippus parallelus (Zetterstedt, 1821) (n = 564, 33% of adult detections), followed by Chorthippus brunneus (Thun- berg, 1815) (n = 130, 8%). Both dark bush-cricket Pholidoptera griseoaptera (De Geer, 1773) (n = 86, 5%) and C. fuscus (n = 68, 4%). were more infrequent. Scarcer species (<4% of detections) included both groundhoppers (T. subulata n = 62, and T: undulata n = 57), O. viridulus (n = 17) and lesser marsh grasshopper Chort- hippus albomarginatus (De Geer 1773) (n= 16).

There was moderate evidence (p < 0.05) that the abundance of C. albomarginatus, T. subulata and T. undulata was significantly higher in the beaver enclosures compared to the control mead- ows (Table 2, Fig. 4). Weak evidence existed for higher species diversity (F = 6.7, p = 0.06) in the beaver enclosures compared to the controls. For C. fuscus and T. subulata, there was statistical evidence (weak) of higher abundance on the east side of Finching- field Brook. In the case of T. subulata, abundance was significantly higher on the east side of the watercourse in the beaver enclosures than on the west (F = 11.2, p = 0.03).

In the beaver enclosures, R. roeselii was the most abundant or- thopteran, comprising c. 38% of the total number of Orthoptera overall (total no. = 843), while P. parallelus formed c. 25%. How- ever, in the control meadows P. parallelus comprised c. 42% of adult detections and R. roeselii c. 43%. In the beaver enclosures, the two groundhoppers, T: subulata and T. undulata (both spe- cies enclosure: c. 7%, control <1%), were in higher abundance in the beaver enclosure assemblages compared to the control grass- lands. In Enclosure B (east transects), we noted groundhoppers (both Tetrix spp. combined) in June colonising the dry mud of a new river channel established by a beaver dam (Fig. 6), only to disappear after flow returned to the watercourse (in July) during a period of heavy rain (Table 3). Groundhoppers then reappeared later in the summer (August and September) after flows dropped and there was exposed mud once more. On the edge of pooled water in Enclosure B (east side), groundhoppers were consist- ently observed throughout the surveys (June-September, Table 3).

The combined abundance of the two co-dominant species in the beaver enclosures (R. roeselii and P. parallelus enclosure:

Fig. 3. Beaver enclosure B dam (A) and patchy vegetation cover near a lodge (B) at Spains Hall © Tim Gardiner.

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Table 2. Two-way ANOVA F values, significance and statistical evidence shown for differences between means for each Orthoptera species/nymphs and Shannon-Wiener species diversity and richness for treatment (beaver enclosure and control meadow) and aspect

(east/west side of river).

Species F Treatment P Evidence F Aspect P Evidence Acrididae (grasshoppers) Chorthippus albomarginatus 9.94 0.03 Moderate 1.90 0.24 - Chorthippus brunneus 2.81 0.17 - Palle: 0.15 - Omocestus viridulus 13 0.35 - 0.01 093 - Pseudochorthippus parallelus 2,25 0.17 - 0.07 0.80 - Tettigoniidae (bush-crickets) Conocephalus fuscus 4.06 0.11 - 57 0.07 Weak Pholidoptera griseoaptera 0.26 0.64 - Las? 0.29 - Roeseliana roeselii 0:73 0.44 - 0.18 0.69 - Tetrigidae (groundhoppers) Tetrix subulata 16.33 0.02 Moderate 7.01 0.06 Weak Tetrix undulata 8.00 0.05 Moderate 1.57 0.28 - Nymphs (all species) 0.02 0.89 - 0.44 0.54 - Species richness 2.0 0.23 - 0 1.00 - Species diversity 6.7 0.06 Weak 4 0.12 - Vegetation height 9,50 0.04 Moderate 2575 0.17 - Table 3. Total number of Tetrigidae (both Tetrix species combined) ‘e in Enclosure B on east transects parallel to a new river channel cre- _ lace

ated by beaver damming and by pools of water on the floodplain.

Month River margins Pool margins Total June 10 7 17 July 0 rat u August ES 31 50 September 2 16 18 Total 31 65 96

63%, control: 85%) may have accounted for their higher species diversity. Vegetation height was significantly lower (F = 9.5, p = 0.04) in the beaver enclosures compared to the control meadows (Tables 2, 4). Vegetation height was typically homogenously tall in the control meadows (only 25 (12.5%) recorded heights < 50 cm) which contrasted with the beaver enclosures where it was lower (87 (43.5%) recorded heights < 50 cm).

At a more localised level, vegetation height influenced the abundance and diversity of Orthoptera. There were moder- ately significant negative relationships detected for C. brunneus (Fig. 5) and T: subulata (Table 5), indicating a preference for short- er vegetation in both cases. For species diversity there was also sta- tistical evidence for a significant negative relationship with vegeta- tion height (Table 5, Fig. 5), suggesting more diverse assemblages at lower sward heights (<50 cm).

In both the beaver enclosures in this study, the grasses false oat Arrhenatherum elatius, timothy Phleum pratense and common bent Agrostis capillaris were more frequent in the beaver enclo- sures, while cock’s-foot Dactylis glomerata (Linnaeus, 1753), York- shire fog Holcus lanatus (Linnaeus, 1753) and tall fescue Lolium arundinacea (Darbyshire 1993) were more numerous in the con- trol meadows (Table 6). Common comfrey Symphytum officinale (Linnaeus, 1753) was found in wetter patches along Finchingfield Brook in the beaver enclosures in some abundance (Table 6). The declining Essex Red Data plant, sneezewort Achillea ptarmica (Lin- naeus, 1753) was found outside of the standard survey in Enclo- sure A while water mint Mentha aquatica ((Linnaeus), Tucker and Naczi (2006)) and great willowherb Epilobium hirsutum (Linnaeus, 1753) were observed in both enclosures.

B Tetrix undulata

Beaver enclosure Control

8 Chorthippus albomarginatus

B Chorthippus brunneus

100 5 O Omocestus viridulus

80 B Pseudochorthippus parallelus

Mean detections/replicate

Beaver enclosure Control

@ Conocephalus fuscus 100 4 @Pholidoptera griseoaptera

BD Roeseliana roeselii

West East

East

West

Beaver enclosure Control

Treatment/aspect Fig. 4. Mean detections (+ standard error) in beaver enclosures and controls (east and west aspect shown) for groundhoppers (Tetrigidae - top); grasshoppers (Acrididae - middle) and bush- crickets (Tettigoniidae - bottom).

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Table 4. Total species count, mean Shannon-Wiener species diver- sity, richness and mean vegetation height for treatment (beaver en- closure and control meadow) and aspect (east/west side of river), standard error shown.

Variable Beaver enclosure Control East West East West Total species 3) ) ¢ 9 Species richness 8.5405 7.5415 6041.0 7.0+1.0 Species diversity 1.8401 13403 12+00 1.2+0.0

Vegetation height (cm) 54.54+8.2 74.14+2.0 81.944.7 78.042.4 Table 5. Linear regression (all models 1 DF) values for vegetation height (independent variable) paired with Orthoptera nymphs and adults of each species, species diversity/richness and habitat depen- dent variables. Significance evidence shown for regression model.

Species R F p Evidence Chorthippus brunneus -0.69 12.70 ~=<0.01 Strong Tetrix subulata -0.50 4.69 0.05 Moderate Tetrix undulata -0.41 2.86 0.11 - Chorthippus albomarginatus -0.31 1.51 0.24 - Pseudochorthippus parallelus 0.25 0.93 0.35 - Omocestus viridulus 0.22 0.68 0.42 - Roeseliana roeselii 0.18 0.48 0.50 - Pholidoptera griseoaptera -0.11 0.18 0.68 - Conocephalus fuscus -0.01 <0.01 0.98 - Nymphs (all species) -0.41 2.81 0.12 - Species diversity -0.52 59 0.04 Moderate Species richness -0.31 1.50 0.24 -

The mean height of the tussock-forming grasses, D. glomerata, P. pratense and L. arundinacea was >85 cm in both beaver enclo- sures and control meadows forming homogenously tall grassland where they occurred (Table 6). However, A. capillaris and H. lana- tus had much shorter heights in the beaver enclosures (<50 cm) compared to the control meadows (>60 cm). The height of S. of- ficinale was similarly low (<45 cm) in the beaver enclosures where it was much more frequent than the control meadows (Table 6).

Discussion

Rewilding on arable farmland can lead to fast colonisation by Or- thoptera (Gardiner and Casey 2022a, b). Field edge habitats (Cher- rill 2015, Arnoczkyné Jakab and Nagy 2022) and ditch banks (Torma et al. 2018) form important corridors in intensively managed farm- land maintaining assemblage diversity for Orthoptera and facilitat- ing quick colonization of new beaver meadows in arable landscapes. At Spains Hall, several ditches connected the enclosure transects with surrounding grassland habitats (Fig. 2) which are likely corridors of dispersal to the beaver meadows for ditch bank species such as C. brunneus, C. fuscus, P. parallelus and R. roeselii (Torma et al. 2018).

The 9 species recorded at Spains Hall in the controls and beaver enclosures were all recorded at Black Bourn Valley, a Suffolk rewild- ing site 40 km to the north, which had the same species richness (9 species). Orthopterans inhabiting Black Bourn Valley's post-arable rewilded fields (Gardiner and Casey 2022a, b), were the grasshopper O. viridulus and groundhoppers T. subulata and T. undulata, all species found in the beaver enclosures at Spains Hall in the current study. The similarity of rewilding river valley assemblages to the beaver en- closures suggests that the influence of the mammals may bring posi- tive benefits to damp riparian habitats within just two years. Before grassland reversion, the intensively managed arable fields of the

T. GARDINER AND E. CRISP

2.0 -——_i——1 1.8 -— 1 a £. 23 7 32 “4 1.6 a y = -0.0082x + 1.8327 s aa R?= 0.2497 zs. te Sb & oOo e o Q x,

y = -0.4201x + 38.415 R?= 0.4149

Total no. grasshoppers

30 50 70 90 Mean vegetation height (cm)

110

Fig. 5. The relationship between vegetation height and the abun- dance of the field grasshopper Chorthippus brunneus (bottom) and assemblage diversity (top), standard error bars shown along with line of best fit and slope statistics. Green circle: control meadow, blue triangle: beaver enclosure.

enclosures and control meadows would've been generally denuded of Orthoptera with most species completely absent or in low abun- dance due to annual ploughing and fertiliser application (Gardiner et al. 2002, Gardiner 2007), leaving narrow (<5 m) unploughed strips either side of Finchingfield Brook and connecting ditches as the only favourable habitat for a baseline population pre-beaver re- lease from which they could colonise the new beaver meadows.

In Germany, a total of 14 species of Orthoptera were record- ed in beaver clearings in forests, although R. roeselii was absent (Dalbeck 2011), which contrasts with 9 species in the post-arable riparian enclosures in the current study. In both Spains Hall enclo- sures, R. roeselii was the most frequently recorded species perhaps suggesting that open river valley landscapes may be more favorable for this species which was abundant in its preferred tall grassland swards dominated by A. elatius and D. glomerata (Gardiner 2009).

The initial damming by beavers in Enclosure B (Fig. 3), led to the creation of wetland pools and new river channels running par- allel to Finchingfield Brook (Fig. 1). The higher water table led to standing water in this enclosure in particular, although there was wet ground and wetland pools in Enclosure A (Fig. 2). Groundhoppers were observed in some abundance in these areas, largely restricted to wet ground where flooding, occasional tractor wheel ruts and a high- water table led to the dieback of tall grassland swards which were replaced with bare mud. These muddy margins have been noted as a feature of beaver dammed areas in the first two years of activity (Andersen et al. 2024) and are likely important for orthopterans of early successional stages such as groundhoppers which are well known from floodplains (Gardiner and Haines 2008). Groundhop- pers may be adapted to the riparian habitats responding to falls in atmospheric pressure by moving to dense vegetation to avoid floods from incoming rainfall (Musiolek and Koéarek 2016), even though they can survive being washed away (Musiolek and Koéarek 2017).

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Table 6. Frequency of hits (touches) and mean height (+ s.e.) for the most abundant plant species (>10 hits) in beaver enclosures and controls.

No. hits (%) Mean height (cm)

and Eicam Cour! ee ante! Arrhenatherum elatius 39 (20) 12(6) 85.6464 105.6+7.7 Dactylis glomerata 23:12) 56'(28)> “904 357:8 —92.0'+3:0 Holcus lanatus 32(16) 79(40) 48.2448 73.942.5 Symphytum officinale 38 (19) 10(5) 341424 42.943.1 Phleum pratense 22*(119) 12(6) 896449 83.247.6 Lolium arundinaceum 11 (6) 17 (9) 852792 95853.9 Agrostis capillaris 10 (5) 7(4) 35.3450 61443.2

These adaptations make them ideally adapted to tolerate the spo- radic flooding associated with beaver damming activities. In Enclo- sure B, we noted groundhoppers in June colonising the dry mud ofa new river channel established by a beaver dam (Table 3, Fig. 6), only to completely disappear after flow returned to the watercourse (in July) during a period of heavy rain. Groundhoppers then reappeared later in the summer (August and September) after flows dropped and there was exposed mud once more, a preferred tetrigid habitat around ponds on rewilding sites (Gardiner and Casey 2022a, b).

In Enclosure A where there was beaver felling of trees but no dams or lodges, groundhoppers were much less frequent, being found on wet, muddy ground where a wetland pool had formed in winter and around a few flooded tractor ruts. Small numbers of groundhoppers were also found on wet ground with bare mud around felled trees. In Enclosure A there was woody debris in Finchingfield Brook which appeared to be backing up the flow although it is hard to attribute this to beaver activity with any certainty. The differences in beaver activity between Enclosures A and B indicates that their impacts on riparian environments are patchy in the initial years after reintroduc- tion. This may not be a disadvantage as Orthoptera respond quickly to the patchiness of the floodplain environment, colonising suitable wetland as it develops. If the beavers move on to other areas of the enclosure floodplains, then the wetlands abandoned may develop tall grassland and scrub as succession progresses in the absence of flooding from beaver dams (Andersen et al. 2024).

Tall vegetation (>50 cm) is important in the floodplain mosaic established by beaver damming as habitat for locally scarce damp grassland species such as O. viridulus which was found within the first two years of enclosure. Early colonists of beaver enclosure ri- parian habitats include the bush-crickets, C. fuscus and R. roeselii, the grasses A. elatius, D. glomerata, L. arundinacea and P. pratense providing the tall vegetation cover both species require. The vari- ations in vegetation height facilitated by flooding and destruction of plant cover by high water levels creates patchy grassland which provides shelter for C. albomarginatus. Heterogeneity of vegetation height was often seen around beaver dams and and lodges (Fig. 3), where P. parallelus and R. roeselii could be abundant in taller veg- etation while C. brunneus inhabited shorter grass.

Assemblage diversity was higher in the shorter swards of the beaver enclosures where early successional Orthoptera species thrive when flooding creates heterogeneity of vegetation cover and suppresses grass growth (Table 4). Livestock grazing (e.g. cattle or sheep) or mosaic grassland mowing can produce a similar diversity in vegetation height (Gardiner 2015). Future management of the en- closures should consider complementing the flooding impacts with grazing or mosaic mowing to produce high quality insect habitats.

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Fig. 6. Bare mud in Enclosure B colonised by groundhoppers where a new river channel created by beaver damming had dried out in June 2024 © Tim Gardiner.

Both C. brunneus and T: undulata were negatively associated with vegetation height being more abundant in short vegetation where vegetation had been suppressed by flooding of the beaver meadows (Table 5, Fig. 5). Habitat preferences of Orthoptera may relate to choice of oviposition site, food preferences and veg- etation height (Clarke 1948, Richards and Waloff 1954, Gardiner 2006, 2009).

Short sward patches established by flooding will have exces- sively hot temperatures (>40°C) similar to hay meadows after cutting (Gardiner and Hassall 2009), unlikely to be favorable for grasshoppers in the absence of ‘cool’ tussocks in close proximity. Both beaver enclosures had mean vegetation height >50 cm which will have provided Orthoptera with numerous sheltered ‘cool’ ar- eas of tall vegetation when temperatures may be excessively hot during summer. This behavioural thermoregulation may have ac- counted for the persistence of species such as C. brunneus where basking and egg-laying sites (bare soil) were in close proximity to cooler tall vegetation for shade seeking orthopterans. Along with C. brunneus, P. parallelus was in relatively high abundance in the beaver enclosures, perhaps preferring the lush D. glomerata vegetation for feeding, shelter and oviposition in grass-covered soil (Waloff 1950, Gardiner and Hill 2004), even though the veg- etation height was well above the optimal level (10-20 cm) for both grasshoppers (Gardiner et al. 2002). Scrub and trees along the river could be important for species of woody vegetation such as P. griseoaptera although they may be affected by removal of their wooded habitat by beaver tree felling in the open valley landscape.

Aspect of the meadows had some influence over C. fuscus and T. subulata abundance (Table 2), where weak statistical evidence revealed higher abundance in meadows on the east side of the tree-lined river corridor (>60% tree cover, Table 1), particularly for T. subulata in the beaver enclosures. It is suggested that shelter from the wind and the exposure to early morning sunlight for Or- thoptera on the east side of hedgerows and tree lines are impor- tant factors governing their occurrence (Gardiner and Dover 2008) while research on the common spittlebug Philaenus spumarius (Linnaeus, 1758) found that temperatures were generally higher on the south-east side of hedgerows compared to the north-west which led to earlier peaks in the abundance of spittle-masses (Ry- ley and Cherill 2024). Beavers could impact on tree cover in en- closures opening up potentially unfavorable shaded habitat for Orthoptera to colonise when the microclimate becomes warmer which could be significant where the watercourse is tree-lined with a dense overhanging canopy. Further research is needed to inves- tigate the influence of aspect on terrestrial insects along river cor- ridors with beavers.

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Study limitations

Beaver enclosures. —The main source of error in this study relates to the presence of only two replicates each for the beaver enclosures and control meadows and the survey only covering one adult Or- thoptera season. Future studies should incorporate several enclo- sures monitored over more than one adult season, although on a field scale this will be expensive due to the length of secure fencing needed and the regular checks of it to ensure there are no escape points. The UK government has also recently authorised the wild release of beavers; therefore, the future focus is likely to move away from captive reintroductions (DEFRA 2025). Enclosure B, which had abundant evidence of beaver activity including 6 dams (dam density 6.7/km), was upstream of Enclosure A where there was no evidence of damming, only tree felling. Therefore, the alteration of water velocity and storage on the floodplain with slower flow due to damming by the upstream Enclosure B beavers may reduce the river favourability for beavers downstream in Enclosure A. Dam density is typically 0.4-1.6 dams/km although it can be as high as 30 dams/km (Graham et al. 2022), usually on rivers <6 m wide and <0.7 m deep (Hartman and Térnl6év 2006). Therefore, Enclosure B has a high density of dams, particularly interesting beaver territo- ries are in areas with substantive woodland cover, although not al- ways (Stringer et al. 2018). Beavers in the open grassland enclosures at Spains Hall may therefore by in uncharacteristic environments with little riparian woodland where activity will be unpredictable.

The observed variability of beaver activity in the two enclosures suggests it is hard to predict how important the influence of dam- ming, lodge building and tree felling will be, consequently desired outcomes for insect assemblages may not be achieved quickly, if at all. A greater independence of enclosures along a river with con- trol plots in between could test interactions between beavers in upper and lower parts of a river catchment providing further evi- dence of how they react in post-arable environments.

Enclosures also limit beaver impact on the landscape, confin- ing it to a relatively small area (in this case 20 ha in each enclo- sure) which could prevent successional stages from fully develop- ing due to the widespread flooding of riparian grassland. Beavers can abandon areas in the wider landscape which allows beaver meadows to dry out and develop into tall grassland and scrub, while newly dammed parts of rivers can revert to early succes- sional floodplain habitats with plenty of pools and bare mud in beaver meadows. The absence of deer grazing in the enclosures meant that their pathways and laydown areas with shorter vegeta- tion favourable to grasshoppers (Gardiner and Casey 2022a), were absent. The enclosure fences may also have altered the pathways and grazing areas available to deer in the surrounding control meadows (Xu et al. 2021). Therefore, we should be cautious about applying the study results to wild beaver release locations where herbivore grazing and passage will be unimpeded.

The study also reports the results of the first two years after the beavers were reintroduced which is in the beaver meadow devel- opment phase. How the beaver meadows mature in the coming seasons will determine changes to the Orthoptera assemblages. Caution should therefore be exercised in declaring new beaver en- closures successful without longer-term data.

Survey limitations.—The visual and acoustic surveying technique used at Spains Hall further developed that used at other sites utiliz- ing acoustic detectors applied for bat surveys (e.g. Gardiner and Ca- sey 2024) to record species in a standardized way (see Newson et al. 2017 and Walcher et al. 2022), which ensured a strong measure of

T. GARDINER AND E. CRISP

repeatability between observers and surveys (Diwaker et al. 2007). All seven stridulating species were detected using both visual and acoustic survey techniques at Spains Hall. The conehead C. fuscus generally has a peak stridulation frequency (30 kHz) beyond the upper range of human hearing (c. 20 kHz, Diwaker et al. 2007) and is virtually undetected by ear without acoustic detection equipment (Gardiner and Casey 2024). It was recorded with a bat detector at Spains Hall in low abundance (17 acoustic detections vs. 51 visual). Estimates of C. fuscus abundance are also impacted by visual detec- tion methods where the cryptic nature of the species leads to under- recording and lower species richness estimates (Gardiner and Hill 2006), particularly problematic in taller vegetation (>50 cm) which was prevalent in the control and beaver meadow enclosures.

Other species, such as R. roeselii (peak frequency c. 17 kHz), were commonly detected with the bat detector on the surveys (570 acoustic detections vs. 114 visual), suggesting that the method was useful for species with call frequencies close to or within the range of human hearing (the three grasshopper species in this study have a peak frequency limit < 21 kHz; Meyer and Elsner 1996) which may be hard to detect visually in the tall vegetation of the beaver enclosures and control meadows. The use of a bat detector for the acoustic component of the surveying technique ensured stand- ardization, repeatability and detection of orthopterans which may be hard to visually detect in tall grassland (Gardiner et al. 2005).

Acknowledgemenis

The authors would like to thank the Spains Hall Estate for sup- porting the project and giving permission to survey the enclosures and controls, specifically, Sarah Brockless and Archie Ruggles-Brise. The Environment Agency also supported the authors during the project, particularly Christine Freeland. Thanks are extended to the reviewers, Anton Kristin and Thomas Zuna-Kratky, and editors, Axel Hochkirch and Tony Robillard, for constructive feedback on the paper.

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