From ecological menace to roadside attraction: 28 years of evidence support successful biocontrol of purple loosestrife
Data files
Oct 18, 2024 version files 1.17 MB
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dispersal_distance_in_montezuma.csv
1.01 KB
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formatted.5.tallest.csv
668.98 KB
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formatted.plquad.csv
487.15 KB
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Loosestrife_Decimal_Degrees_GPS_Coordinates.csv
1.30 KB
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README.md
13.86 KB
Abstract
Introduction and spread of non-native plants provide ecologists and evolutionary biologists with abundant scientific opportunities. However, land managers charged with preventing ecological impacts face financial and logistical challenges to reduce threats by introduced species. The available toolbox (chemical, mechanical, or biological) is also rather limited. Failure to permanently suppress introduced species by mechanical and chemical treatments may result in biocontrol programs using host-specific insect herbivores. Regardless of the chosen method, long-term assessment of management outcomes not just on the target species, but also on associated biota, should be an essential component of management programs. However, data to assess whether management results in desirable outcomes beyond short-term reductions of the target plant are limited. Here we use implementation of a biocontrol program targeting a widespread wetland invader, Lythrum salicaria (purple loosestrife), in North America to track outcomes on the target plant over more than two decades in New York State. After extensive testing, two leaf-feeding beetles (Galerucella calmariensis and G. pusilla; hereafter ‘Galerucella’), a root-feeding weevil (Hylobius transversovittatus) and a flower-feeding weevil (Nanophyes marmoratus) were approved for field releases. We used a standardized monitoring protocol to record insect abundance and L. salicaria stem densities and heights in 1m2 permanent quadrats at 33 different wetlands and followed sites for up to 28 years. As part of this long-term monitoring in 20 of these wetlands, we established a factorial experiment releasing either no insects (control), only root feeders, only leaf beetles, or root- and leaf-feeders. We document reduced L. salicaria occupancy and stem densities following insect releases over time, irrespective of site-specific differences in starting plant communities or L. salicaria abundance. We could not complete our factorial experiment because dispersal of leaf beetles to root-feeder-only and control sites within five years invalidated our experimental controls. Our data show that it took time for significant changes to occur, and short-term studies may provide misleading results, as L. salicara stem densities initially increased before significantly decreasing. Several decades after insect releases, pre-release predictions of significant purple loosestrife declines have been confirmed.
doi:10.5061/dryad.8pk0p2nt9
Description of the data and file structure
Data are typically summarized by unique quadrat by time treatment combinations (i.e., data rows). Which csv file is needed for each analysis is explained within the annotations of the associated R scripts. “NA” represents ‘missing’ values in the dataset. Some important variables of note in the data files include area (region of state where site was located), month (month of data collection; either ‘June’ for summer monitoring or ‘Sept’ for fall monitoring), and pl.density (number of stems per permanent 1m2 quadrat). Descriptions of other variables are also found in the main text in addition to the detail provided herein.
Column descriptions by csv file:
dispersal_distance_in_montezuma.csv
Note: this represents summary data from only a subset of the monitored sites. This file only contains data from sites within the Montezuma region, as this was the only region in which we established a factorial experiment releasing either no insects (control), only root feeders, only leaf beetles, or root- and leaf-feeders. We further subset the data in this file to look only at sites where leaf-feedings (Galerucella) had not been intentionally released, to better assess how long it takes Galerucella to disperse from neighboring release sites. Column descriptions are as follows:
- area - region of state where site was located (overall, we sampled four regions: ‘hudson’ [Hudson River Valley, NY], ‘mont’ [Montezuma, NY], ‘tnc’ [Western New York sites under stewardship of The Nature Conservancy], and ‘west’ [Western Lake Ontario sites in NY]; this subset dataset only includes data from Montezuma, NY)
- site - name of sampled site (i.e., unique site identifier)
- latitude - latitude of site in decimal degrees
- longitude - longitude of site in decimal degrees
- insect treatment - which insects were intentionally released as part of purple loosestrife biocontrol program (this dataset is subset, so only contains two treatments: control = no insects were released, hyl = only Hylobius transversovittatus were released)
- closest.site.with.gal - name of closest neighboring site where Galerucella sp. were intentionally released
- closest.lat - the latitude (in decimal degrees) of the closest neighboring site where Galerucella sp. were intentionally released
- closest.long - the longitude (in decimal degrees) of the closest neighboring site where Galerucella sp. were intentionally released
- distance.in.meters - the distance (in meters) between the site and the closest neighboring site where Galerucella sp. were intentionally released
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# of quads - the number of permanent 1m2 quadrats that were monitored within the site - date of insect releases - if applicable, the year of insect releases at the site; The ‘H’ following the year indicates that only Hylobius transversovittatus were released at the site; many rows do not have data for this column, as there were no insects released at those sites
- first year Galerucella confirmed to be established? - first year that Galerucella were observed at the site
- date since earliest Gal. release - Number of years since Galerucella were first released at the site identified in the “closest.site.with.gal” site
formatted.5.tallest.csv
Note: Only the five-tallest stems within each permanent 1m2 quadrat were measured for this dataset; each unique sampling year by quadrat combination therefore has five rows. If there were less than five stems within a quadrat during a specific sampling time, we still included a row for the ‘missing’ stems, but left the columns labeled “pl.height”, “pl.fertile”, “pl.number.inflor”, “pl.inflorescence.length”, and “pl.number.flowers” as “NA” for this row in the dataset.
- row.order - the order of the rows in the original, unformatted dataset
- area - region of state where site was located (we sampled four regions: ‘hudson’ [Hudson River Valley, NY], ‘mont’ [Montezuma, NY], ‘tnc’ [Western New York sites under stewardship of The Nature Conservancy], and ‘west’ [Western Lake Ontario sites in NY])
- site - name of sampled site (i.e., unique site identifier)
- trt - which insects were intentionally released as part of purple loosestrife biocontrol program (four treatments: control = no insects were released, gal = only Galerucella sp. were released, gal.hyl = both Galerucella sp. and Hylobius transversovittatus were released, gal.hyl.nan = Galerucella sp., Hylobius transversovittatus, and Nanophyes marmoratus were all released [note: this only applies to one site], hyl = only Hylobius transversovittatus were released)
- year - year data was collected
- year.of.first.release - year insects were first released at the site
- num.years.after.release - number of years at time of data collection after the first insects were released at the site (referred to as ‘time’ within the manuscript)
- quad - unique, within-in site identifier for each permanent 1m2 quadrat
- pl.height - height of measured purple loosestrife stem in cm
- pl.fertile - whether the measured stem had an inflorescence
- pl.number.inflor - the total number of inflorescences on the measured stem
- pl.inflorescence.length - the length of the longest inflorescence (in cm) on the measured stem
- pl.number.flowers - the number of individual flowers in the middle 5 cm of the longest inflorescence. Note: if inflorescence was <5cm, all flowers were counted.
- notes - pertinent notes pertaining to that row of data
- latitude - latitude of site in decimal degrees
- longitude - longitude of site in decimal degrees
- initial.density - the number of purple loosestrife stems within the permanent 1m2 quadrat the first year of sampling (coincided with the year insects were first released at that site or [for the ‘control’ sites at Montezuma] the years insects were first released at neighboring sites)
formatted.plquad
- database.row - the order of the rows in the original, unformatted dataset
- area - region of state where site was located (we sampled four regions: ‘hudson’ [Hudson River Valley, NY], ‘mont’ [Montezuma, NY], ‘tnc’ [Western New York sites under stewardship of The Nature Conservancy], and ‘west’ [Western Lake Ontario sites in NY])
- site - name of sampled site (i.e., unique site identifier)
- trt - which insects were intentionally released as part of purple loosestrife biocontrol program (four treatments: control = no insects were released, gal = only Galerucella sp. were released, gal.hyl = both Galerucella sp. and Hylobius transversovittatus were released, gal.hyl.nan = Galerucella sp., Hylobius transversovittatus, and Nanophyes marmoratus were all released [note: this only applies to one site], hyl = only Hylobius transversovittatus were released)
- quad - unique, within-in site identifier for each permanent 1m2 quadrat
- month - month of data collection (either ‘June’ for summer monitoring of insects or ‘Sept’ for fall monitoring of plants)
- date of insect release - first year insects were released at the site
- year - the year the data was collected
- num.years.after.release - the number of years between the first year insects were released at the site and the year the data was collected
- pl.density - number of purple loosestrife stems per permanent 1m2 quadrat
- num.infl - total number of inflorescences produced by all purple loosestrife stems within the permanent 1m2 quadrat
- pl.damage - estimated percent of foliage removed by Galerucella sp. feeding (note: this data was collected in the Spring only, and was estimated as 0, 5%, 10%, or in similar 5% increments thereafter)
- gal.eggs.actual.count - the number of Galerucella sp. eggs observed in a 1-minute screening of the permanent 1m2 quadrat
- gal.eggs.categorical - the number of Galerucella adults observed in a 1-minute screening of the permanent 1m2 quadrat (estimated in categorical ‘bins’, with 5, 55, 300, 750, and 1500 representing the midpoint of each category)
- gal.larvae - number of Galerucella sp. larvae observed in a 1-minute screening of the permanent 1m2 quadrat
- gal.larvae.presence - binary measure of whether or not any Galerucella larvae were observed in a 1-minute screening of the permanent 1m2 quadrat
- gal. adults - the number of Galerucella sp. adults observed in a 1-minute screening of the permanent 1m2 quadrat
- water.cover - the percent of the permanent 1m2 quadrat covered in water (note: the sum of the values in water.cover, litter.cover, and soil.cover should equal 100)
- litter.cover - the percent of the permanent 1m2 quadrat covered in litter (note: the sum of the values in water.cover, litter.cover, and soil.cover should equal 100)
- soil.cover - the percent of the permanent 1m2 quadrat where bare soil is visible (note: the sum of the values in water.cover, litter.cover, and soil.cover should equal 100)
- water.depth - the average of the water depth (in cm) measured at each of the four corners of the permanent 1m2 quadrat
- litter.depth - the average of the litter depth (in cm) measured at each of the four corners of the permanent 1m2 quadrat
- vertical.cover - lowest number (0.5 dm) seen on a 4”-wide board held vertically; observed from 4m distant from the board
- forb.cover - percent cover of forbs (excluding purple loosestrife) in permanent 1m2 quadrats (note: estimated in large categories at Montezuma)
- gram.cover - percent cover of graminoids (excluding purple loosestrife) in permanent 1m2 quadrats (note: estimated in large categories at Montezuma)
- ld.cover - total estimated percent cover of all forbs and graminoids in permanent 1m2 quadrats (note: not used in the analyses - highest number is 100)
- veg.cover.by.species - sum of each percent cover for estimated for each species (note: sum may exceed 100)
- num.species.with.pl - number of species in permanent 1m2 quadrat (including purple loosestrife)
- typha.cover - percent cover of Typha angustifolia within the permanent 1m2 quadrat (categories included: 0 [no Typha angustifolia], 3 [<5% Typha angustifolia], 10 [5-15% Typha angustifolia], and 10% increments thereafter)
- typha.density - the number of Typha angustifolia stems within the permanent 1m2 quadrat
- initial.density - the number of purple loosestrife stems within the permanent 1m2 quadrat the first year of sampling (coincided with the year insects were first released at that site or [for the ‘control’ sites at Montezuma] the years insects were first released at neighboring sites)
- latitude - latitude of site in decimal degrees
- longitude - longitude of site in decimal degrees
- presence - whether or not purple loosestrife was present in permanent 1m2 quadrat
Loosestrife_Decimal_Degrees_GPS_Coordinates.csv
- area - region of state where site was located (we sampled four regions: ‘hudson’ [Hudson River Valley, NY], ‘mont’ [Montezuma, NY], ‘tnc’ [Western New York sites under stewardship of The Nature Conservancy], and ‘west’ [Western Lake Ontario sites in NY])
- site - name of sampled site (i.e., unique site identifier)
- latitude - latitude of site in decimal degrees
- longitude - longitude of site in decimal degrees
- insect treatment - which insects were intentionally released as part of purple loosestrife biocontrol program (four treatments: control = no insects were released, gal = only Galerucella sp. were released, gal.hyl = both Galerucella sp. and Hylobius transversovittatus were released, gal.hyl.nan = Galerucella sp., Hylobius transversovittatus, and Nanophyes marmoratus were all released [note: this only applies to one site], hyl = only Hylobius transversovittatus were released)
Code/Software
We used R to run the included scripts for data analyses.
Brief explanation of R scripts related to analyses
In order of how they are presented in Table 1 in the associated manuscript:
- GAM.presence.R - model selection approach of models fit to data of occupancy (i.e., whether or not purple loosestrife was present or not within monitored quadrats) over time
- GAM.pl.density.negative.binomial.R - for quadrats where purple loosestrife was present, this script includes the model selection approach for models fit to data of purple loosestrife stem density over time
- GAM.five.tallest.height - for data of five tallest stems within each quadrat, this script includes the model selection approach for models fit to data of purple loosestrife stem height over time
- GAM.fertile.R - for data of five tallest stems within each quadrat, this script includes the model selection approach for models fit to data of proportion of fertile stems over time
- GAM.numinflorescences.R - for data of five tallest stems with each quadrat, this script includes the model selection approach for models fit to data of number of inflorescences over time
- GAM.5.tallest.inflor.length - for data of five tallest stems with each quadrat, this script includes the model selection approach for models fit to data of inflorescence length over time
- GAM.number.of.flowers.R - for data of five tallest stems within each quadrat, this script includes the model selection approach for models fit to data of flower density over time
Note: more detailed explanations of the response variables are available in the preceding column descriptions section and in the associated manuscript
We worked at 33 sites (mix of federal, state, and private ownership) varying in size from a few hundred square meters to >10 ha across New York State. Initially, most sites (old fields, wetlands, marshes, and impoundments) were dominated by L. salicaria, although some releases occurred where managers documented only small patches. Water levels varied through time, reflecting natural variation in precipitation patterns, beaver activity, and intentional manipulations by wetland managers and private landowners. We monitored each site for a period of 15 to 28 years, starting the year insects were first released. Releases included one or several of the following species: two leaf-feeding beetles (Galerucella calmariensis and G. pusilla), a root-feeding weevil (Hylobius transversovittatus), and a flower-feeding weevil (Nanophyes marmoratus).
At each site we established 1 to 15 1 m2 permanent quadrats spaced 5–50 m apart. Presence of L. salicaria was a pre-requisite for initial quadrat placement, as the region had well established long-term presence of L. salicaria and populations were stable and not undergoing short-term fluctuations. The number of quadrats and distance between quadrats was dictated by L. salicaria population size. We used 1.5 m tall PVC (2.53 cm diameter) poles to mark each corner of each quadrat, and recorded GPS position and hand-drew maps indicating distance and azimuth to help relocate quadrats. Over the 28 years, individual poles and occasionally all poles of a quadrat disappeared; we used GPS and field notes to re-establish quadrats as close to the original position as possible.
We initially visited all sites twice annually. At each site, we recorded detailed data about L. salicaria growth and reproduction, presence and abundance of released insects and co-occurring plant species. In June we counted L. salicaria stem density, estimated L. salicaria cover (%), and recorded Galerucella abundance using timed counts (1 min each for eggs, adults, and larvae), and made visual assessments of leaf area removed (%). We did not record presence for H. transversovittatus as larvae feed in roots, and adults are night active, preventing detection without destructive sampling. We also did not record presence of N. marmoratus. However, during our last sampling in 2019, we recorded presence of flower feeders and assessed root-feeder presence at all sites, except those that were permanently flooded, by excavating rootstocks and checking for larval feeding damage. In September, in each quadrat we recorded L. salicaria stem density and cover (%) and, for the five tallest stems, height, number of inflorescences, length of the tallest inflorescence, and number of flower buds in the central 5cm of that inflorescence. Depending on insect releases, we recorded data for variable time periods at each site between 1996 and 2007, and again at all 33 sites in August and September 2019.
Data was entered into excel, formatted, and then analyzed using R. Please see the associated manuscript for more details.
Excel or Text-editor
R or R studio