DOI: 10.5061/dryad.c59zw3r3v
title: Complexity within an oil palm monoculture: the effects of habitat variability and rainfall on adult dragonfly (Odonata) communities.

description: Recent expansion of oil palm agriculture has resulted in loss of forest
 habitat and forest-dependent species. However, large numbers of species –
 particularly insects – can persist within plantations. This study focuses
 on Odonata (dragonflies and damselflies): a charismatic indicator taxon,
 and a potentially valuable pest control agent. We surveyed adult Odonata
 populations biannually over three years within an industrial oil palm
 plantation in Sumatra, Indonesia. We assessed the effects of rainfall
 (including an El Niño Southern Oscillation-associated drought), the role
 of roadside ditches, and the importance of understory vegetation on
 Odonata populations. To assess the impacts of vegetation we took advantage
 of a long-term vegetation management experiment that is part of the
 Biodiversity and Ecosystem Function in Tropical Agriculture (BEFTA)
 Programme. We found 41 Odonata species, and communities varied between
 plantation core and roadside edge microhabitats, and between seasons.
 Abundance was significantly related to rainfall levels four months before
 surveys, probably indicating the importance of high water levels in
 roadside ditches for successful larval development. We found no
 significant effect of the BEFTA understory vegetation treatments on
 Odonata abundance, and only limited effects on community composition,
 suggesting that local understory vegetation structure plays a relatively
 unimportant role in determining communities. Our findings highlight that
 there are large numbers of Odonata species present within oil palm
 plantations, and suggest that their abundance could potentially be
 increased by maintaining or establishing waterbodies. As Odonata are
 predators, this could bring pest control benefits, in addition to
 enhancing biodiversity within intensive agricultural landscapes.
description: Site.—Fieldwork
 was conducted within oil palm plantations in Riau Province, central
 Sumatra, Indonesia. The plantations are owned and managed by PT Ivo Mas
 Tunggal (a subsidiary company of Golden Agri Resources, GAR), with
 technical input from Sinar Mas Agro Resources and Technology Research
 Institute (SMARTRI) (the research and development centre of GAR) (Fig. S1,
 full manuscript). The area is naturally mixed lowland dipterocarp forest,
 but was deforested ~30 years ago, and is currently heavily dominated by
 oil palm agriculture, with very little non-converted habitat remaining.
 The region has a tropical climate, with mean annual rainfall of 2350 mm,
 spread unevenly across the year, and is based on mineral soil (Tao et al.
 2016).             We
 surveyed sites that formed part of the Biodiversity and Ecosystem Function
 in Tropical Agriculture (BEFTA) Programme (Foster et al. 2014; Luke et al.
 2020). As part of this, the BEFTA Understory Vegetation Project tests the
 effects of understory vegetation complexity on the oil palm ecosystem,
 through varying levels of herbicide use and manual cutting of vegetation.
 BEFTA has established three understory vegetation treatments (see Luke et
 al. (2020) for full details):   <b>Reduced complexity</b>:
 this involves spraying all understory vegetation with herbicides.
 Hereafter referred to as ‘Reduced’. <b>Normal complexity</b>:
 this is the standard practice used within the SMARTRI estates and involves
 an intermediate level of herbicide spraying, with harvest paths and 1.5 m
 circles around palms being sprayed and woody vegetation being manually
 removed, but other vegetation being allowed to regrow. Hereafter referred
 to as ‘Normal’. 
 <b>Enhanced complexity</b>: this involves no spraying of
 herbicides and only limited hand-cutting of woody vegetation, and along
 harvesting paths and in palm circles. Hereafter referred to as
 ‘Enhanced’.               The three treatments
 represent the range of different management strategies that occur within
 oil palm plantations. There are eighteen BEFTA Understory Vegetation
 Project plots, split across two estates (Ujung Tanjung and Kandista), and
 organized into six triplets, each at least 800m from the next (Fig. S1;
 Fig. S2; Table S1, full manuscript). Each triplet contains one 150 x 150 m
 plot of each treatment, with the order allocated randomly. The middle plot
 in each triplet is 155 m from each of the outer plots within the triplet.
 Each plot is located at the end of a plantation block of 900 x 300 m, so
 that it is adjacent to an access road, and therefore includes both core
 and edge plantation microhabitat (hereafter referred to as ‘Core’ and
 ‘Edge’). Edge sites were located alongside both a dirt road and a drainage
 ditch filled with standing water (Fig. S2, full manuscript). These Edge
 sites occur around each of the 900 x 300m planting blocks. Core sites were
 within the interior of oil palm planting blocks, at least 50m from the
 edge. This led to differences in shading, exposure, and water occurrence
 in the two microhabitats, with consequent impacts on vegetation structure
 and diversity. Specifically, Edge areas had significantly higher plant
 biomass and cover, plant species richness, and higher occurrence of
 <i>Turnera ulmifolia</i> (planted along roadsides to promote
 populations of beneficial insects) than Core areas, whilst Core areas had
 significantly higher percentage cover of frond heaps (stacks of chopped
 palm fronds) (Luke et al. 2019, 2020) (and also see assessments of habitat
 structure presented in the full manuscript). All plots are in flat areas
 of the plantation, 10-30 m asl, and are in areas that were mature palm
 (palms planted between 1988 and 1993) at the time of survey. The SMARTRI
 estates are criss-crossed by a network of small streams (Fig. S1, full
 manuscript), and two plots had small streams running through them. There
 were no natural ponds or lakes nearby. Plots were marked out in October
 2012 and were all managed using the standard SMARTRI vegetation practices
 (Normal complexity) until treatments began in February 2014, after which
 plots were managed according to their allocated understory vegetation
 treatment. Refer to Luke et al. (2020), for more details.   Odonata
 surveying.—Within each plot, adult Odonata were surveyed
 along transects in the Core and Edge of the plot. Core transects followed
 the edge of a central 50 x 50 m sampling area within each plot and so were
 200 m long, whilst Edge transects followed the roadside edge of the plot
 and so were 150 m long (Fig. S2, full manuscript). Recorders walked at a
 steady pace along each transect and identified and counted the dragonflies
 seen within a 5 m sided cube of space in front of them. Survey duration
 was not fixed because the time required to complete a survey depended on
 the number of dragonflies present. When a species was not immediately
 identifiable by sight, they were caught using a hand-net and photographed,
 allowing later identification by an experienced taxonomist (Rory A. Dow)
 and using identification guides (Orr 2003; Tang et al. 2010). Surveys were
 conducted at each plot during two ‘Seasons’: February-April, and
 September, hereafter referred to as ‘March’ and ‘September’. Both seasons
 were surveyed in 2013, 2014, and 2015. Two repeat surveys of each plot
 were conducted within each season. Surveys conducted in March 2014 fell
 partly during the BEFTA Understory Vegetation Project pre-treatment
 (hereafter referred to as ‘pre-treatment’), and partly during the BEFTA
 Understory Vegetation Project post-treatment (hereafter referred to as
 ‘post-treatment’) time period, and were categorised accordingly in
 analyses. Surveys were conducted between 9am and 5pm.   Vegetation
 measurements.—Vegetation measurements were taken every 10 m
 along each of the transects, once in March-April 2013 (pre-treatment), and
 again in September 2014 (post-treatment). Vegetation percentage cover (of
 ferns, palm fronds, bare ground, empty fruit bunch (EFB – used husks of
 harvested palm fruits, applied by industry as a mulch), and the non-native
 shrub <i>Turnera ulmifolia</i> (planted by industry along
 roadways to promote pest control)) was assessed by eye within a 5 x 5 m
 area at each point. Vegetation height was measured with a metre rule. At
 each measurement point, the metre rule was held in a vertical position,
 with its base resting on the ground, and an A4 clipboard lightly rested on
 the vegetation (to push down any single tall stems) and the height
 measured accordingly. Canopy openness was measured using a densiometer
 (Lemmon 1956), averaging four measurements taken North, South, East and
 West at each point.   Rainfall
 measurements.—We recorded daily rainfall (mm) using rain
 gauges that each had a 100 cm<sup>2</sup> collecting area.
 Rainfall values from all rain gauges within each estate (three in Ujung
 Tanjung, and two in Kandista) were averaged to calculate a monthly
 rainfall value for each estate. In analyses, we used the estate rainfall
 value that was applicable to each plot. We used rainfall measures from
 March of the relevant year as the ‘monthly rainfall’ value for surveys
 conducted during February to April, and rainfall measures from September
 for September surveys. In addition to ‘monthly rainfall’, we also used the
 value of rainfall for the calendar month that was four months before
 (‘rainfall four months before’) the March and September dragonfly surveys
 in analyses. This is because dragonflies have an aquatic larval stage, and
 therefore presence of adults at a site is dependent on sufficient water
 for larval development before the survey date. Although different
 dragonfly species exhibit a range of reproductive strategies, the majority
 of tropical species are multivoltine (Corbet 1980). The development time
 for many tropical dragonfly larvae is between 60 and 200 days (Corbet
 1980), the mean of which is 130 days (4.3 months). Using this lagged
 rainfall value in analyses allowed us to consider the influence of
 rainfall on egg laying, egg survival, and larval development stages of the
 dragonfly lifecycle, in addition to assessing impacts of rainfall on adult
 presence.  
 Statistical
 analyses.—We conducted analyses using R statistical program
 version 3.5.1 (R Core Team 2018), and the program R Studio (R Studio Team
 2016). To compare 200 m-long Core transects and 150 m-long Edge transects,
 we standardised all abundance data to predicted values for 150 m-long
 transects by multiplying Core abundance values by 0.75. We included data
 for an unidentified teneral individual (‘Unknown teneral’) within overall
 abundance models, but excluded this data point from species richness
 curves and family level analyses, as its species and family were
 unknown.  
 <b>References</b>
 Corbet PS (1980) Biology of Odonata. Annu Rev Entomol 25:189–217 Foster WA, Snaddon JL, Advento AD, et al (2014) The Biodiversity and Ecosystem Function in Tropical Agriculture (BEFTA) Project. Plant Kuala Lumpur 90:581–591 Lemmon PE (1956) A spherical densiometer for estimating forest overstory density. For Sci 2:314–320 Luke SH, Advento AD, Aryawan AAK, et al (2020) Managing Oil Palm Plantations More Sustainably: Large-Scale Experiments Within the Biodiversity and Ecosystem Function in Tropical Agriculture (BEFTA) Programme. Front For Glob Chang 2: . doi: 10.3389/ffgc.2019.00075 Luke SH, Purnomo D, Advento AD, et al (2019) Effects of Understory Vegetation Management on Plant Communities in Oil Palm Plantations in Sumatra, Indonesia. Front For Glob Chang 2: . doi: 10.3389/ffgc.2019.00033 Orr AG (2003) A guide to the dragonflies of Borneo: their identification and biology. Natural History Publications (Borneo), Kota Kinabalu, Malaysia R Core Team (2018) R: A language and environment for statistical computing R Studio Team (2016) RStudio: Integrated Development for R Tang HB, Wang LK, Hämäläinen M (2010) A photographic guide to the dragonflies of Singapore. The Raffles Museum of Biodiversity Research, Singapore Tao H-H, Slade EM, Willis KJ, et al (2016) Effects of soil management practices on soil fauna feeding activity in an Indonesian oil palm plantation. Agric Ecosyst Environ 218:133–140 . doi: 10.1016/j.agee.2015.11.012  
description: In all datasets: Site = plantation block
 name Estate = plantation estate name
 Triplet = letter to indicate which sets of three plots are
 grouped together spatially CoreEdge = habitat within
 the core or on the edge of a plantation block Treatment
 = BEFTA Understory Vegetation Project (BEFTA UVP) understory vegetation
 treatment (Reduced, Normal, Enhanced) that has been assigned to that
 plot BeforeAfter = Before or After the experimental
 BEFTA UVP treatments were started Please see manuscript
 text, and also Luke et al. 2020, Managing Oil Palm Plantations More
 Sustainably: Large-Scale Experiments Within the Biodiversity and Ecosystem
 Function in Tropical Agriculture (BEFTA) Programme, Frontiers in Forests
 and Global Change, https://www.frontiersin.org/articles/10.3389/ffgc.2019.00075/full, for more details. Note that abundances within the tables presented have not been standardised to account for the difference in transect length between Core and Edge (see Methods). The data presented are the raw data.