Findings from a Survey of Native and Exotic Species on Midway Atoll National Wildlife Refuge

FINDINGS FROM A SURVEY OF NATIVE AND EXOTIC SPECIES ON MIDWAY ATOLL NATIONAL WILDLIFE REFUGE

TECHNICAL REPORT – 4 OCTOBER 2017

Robert V Taylor

Restoration Ecologist National Wildlife Refuge Association Midway Atoll National Wildlife Refuge 1082 Makepono St, Honolulu, HI 96819 [email protected]

Cover photo: Conducting a plant survey on Eastern Island, Midway Atoll National Wildlife Refuge (Credit: RV Taylor)

Disclaimer

The findings and conclusions in this article are those of the author and do not necessarily represent the views of the U.S. Fish and Wildlife Service. Furthermore, any mention of specific software products do not constitute an endorsement of those products or companies by the author or the US Fish and Wildlife Service.

Suggested citation

Taylor, Robert V. 2017. Findings from a Survey of Native and Exotic Species on Midway Atoll National Wildlife Refuge. Technical Report. US Fish and Wildlife Service, Honolulu, HI.

SUMMARY

A vascular plant survey of exotic, invasive and native species was conducted across 186 sites on Midway Atoll National Wildlife Refuge (Refuge) in 2017 providing valuable baseline data on 67 species (16 native and 51 exotic) as well as continuing to document trends in Verbesina encelioides ( golden crownbeard or golden crownbeard). The most abundant native species found was Boerhavia repens (alena) with a frequency of 33% and the most abundant exotic species was Lobularia maritima (sweet alyssum), which occurred at 58% of sites. Verbesina was detected at 18% of the survey sites and had a density of 0.0066 (± 0.016 Std Dev) plants / m2 on Sand Island and 0.0062 (± 0.24 Std Dev) plants / m2 on Eastern Island. Four new exotic species were detected during the 2017 surveys. Data revealed a decrease in golden crownbeard from 2016 to 2017 on Sand Island; whereas, abundance on Eastern Island was stable. Expanding the scope of the surveys in 2017 from their prior focus on golden crownbeard should be evaluated in light of overall refuge resource management objectives.

INTRODUCTION

Midway (Pihemanu) is a coral atoll within the northwestern Hawaiian Islands (28°11'41"–28°16'50"N and 177°18'38"–177°25'38" W). It consists of three small islands – Box 1. Definitions Sand, Eastern, and Spit – and has a total land area of 592 ha (1461 acres). The first vascular plant inventory of Midway was Not all exotic species are weeds and not all weeds are exotic species. Every weed has conducted in 1904 by William Bryan who documented just 13 some characteristic which makes it species (Bryan 1905). Since then, over a dozen visits have been undesirable (noxious) in certain made by botanists to these remote islands to investigate the plant circumstances. Not all weeds are equally life occurring there (e.g., Christophersen and Caum, 1931, invasive. Invasiveness, in itself, can make a Apfelbaum, et al. 1983; Bruegmann, 1998; Conant, 1983; Neff and plant noxious. DuMont, 1955). Past surveys have documented the presence of Exotic species – A plant species not 389 species on Midway, 350 of which were exotic (Figure 1). indigenous to Midway but occurs there Introductions of exotic species (Box 1) to Midway have occurred because of its introduction (intentionally via a variety of means, beginning with the planting of gardens to or otherwise) by humans. Weed – A native or exotic species having provide food for residents of the Pacific Commercial Cable Station th one or more undesirable characteristics during the first decade of the 20 century. Other species were that make it necessary to manage (i.e., brought in accidentally during the war years, where a soldier control) in some places at some times. might arrive with mud in his boots containing seeds. With few Invasive species – A weed that has been biosecurity precautions in place, exotic plants continue to make shown to spread easily and quickly from their way to Midway with human-assistance; every time a plane one place to another displacing other or ship arrives there is a possibility that yet another new species species during the process. Some weeds are more invasive (spread more quickly) will have made its way here. Not all plant species introduced to than others Midway persist or thrive. Less than half of the exotic plants once observed on Midway, approximately 170 species, occur there today. High rates of colonization and extinction make the flora of Midway, perhaps, one of the most dynamic anywhere; this presents a formidable challenge to those charged with the responsibility of abating the threat

3 that exotic species pose to wildlife and the habitat that sustains it. Periodic plant surveys, thus, provide invaluable information in this regard.

Botanists Forest and Kim Starr have conducted botanical surveys at Midway since 1999, shortly after the atoll became a part of the National Wildlife Refuge System (Starr and Martz 1999, Starr and Starr 2008, Starr and Starr 2015). As with earlier investigators, the bulk of the information produced by the Starr’s surveys has consisted of lists of the plant species observed, collection of voucher specimens for new occurrences, and locations of some of the plants they observed with qualitative notes on distribution and abundance. Because the Starrs made multiple visits to the Refuge over the course of two decades, they also were able to note trends they perceived in certain species and, therefore, advice on the invasiveness of many weed species. Although this information has undoubtedly been of great value, it is not, however, easily utilized for mapping the occurrence of species across the atoll and in quantitatively evaluating trends in distribution and abundance over time. Missing from these earlier efforts were data on the locations of areas surveyed and accompanying “negative data” – i.e., sites where plant species were not found.

Annual weed monitoring has been conducted on Midway since 2012 and has focused almost exclusively on detecting trends in cover of golden crownbeard (Verbesina encelioides), a highly invasive plant species that has been the subject of a refuge-wide eradication campaign. Past monitoring was conducted at 100 sites (50 sites each on Sand and Eastern islands) and entailed visually estimating the foliar cover of golden crownbeard within a 4.7 m radius plot. Although this monitoring effort served the Refuge well in documenting the steep decline of golden crownbeard (decreasing from approximately 50% cover to under 1% over a 5-year period), it is less well- suited for detecting further reductions of this weed species. The problem stems from the low accuracy of estimates within the plot (i.e., using visual estimation, it is nearly impossible to distinguish between 1% and 0.25% cover) compounded by small sample size which limits precision. Detecting future changes in the abundance of golden crownbeard requires that the methods used in monitoring the species accommodate its current sparse and patchy distribution.

As golden crownbeard has declined on Midway, other plant species have taken its place. Although native species have been re-introduced into some areas via active restoration efforts (e.g., seeding and outplanting), the vast majority of areas formerly dominated by golden crownbeard are now dominated by other exotic species. This has compelled refuge staff to consider a much broader range of exotic plant species with respect to invasive plant management and to prioritize species for control. Three elements are needed for good decision-making in this arena: (1) information about which exotic plant species occur on the Refuge along with data on their abundance and distribution; (2) information on the ecological characteristics of those species; and (3) a framework utilizing this information (Appendix 1).

Although refuge staff was slated to again to conduct annual golden crownbeard monitoring in 2017, the decision was made to revamp the monitoring protocols to rectify the aforementioned issues. With funding from the FWS Invasive Species Strike Team, they were able to enlist the assistance of Forest and Kim Starr and expand the scope of the surveys to consider both exotic and native plant species. As the importance of conducting these types of surveys consistently across the Refuge System has become increasingly apparent, the re-design of the survey for Midway incorporated existing guidance (e.g., Elwood and Block 2017) and followed state-of-the art methodologies already being implemented broadly within FWS (e.g., Booker et al. 2017). The proposed methods

4 for the 2017 surveys were drafted (Taylor et al. 2017) and reviewed by FWS personnel at both the Refuge and within the Inventory and Monitoring and Invasive Species programs prior to being finalized for implementation. The proposed methodology includes several different types of data collections to be performed at different time intervals (Appendix 2).

METHODS

The following sampling objectives were established for the 2017 project (Taylor et al. 2017):

 Find and document any novel exotic species that may have recently arrived on the atoll  Compare abundance of golden crownbeard over time to evaluate trends. Specifically, data should have 80% power to detect a 20% absolute change in the frequency of golden crownbeard with 90% confidence for both Sand and Eastern islands from one year to the next.  Collect baseline data on the distribution and abundance of known priority weed species over time, especially those species posing the greatest potential impact to wildlife conservation and habitat restoration efforts. Specifically, data should have 80% power to detect a 40% absolute change in the frequency of all other Class A, Class B.1, and C.1 weed species with 90% confidence across Sand and Eastern islands from one year to the next.  Collect baseline data on the distribution and abundance of native plant species

To accomplish these sampling objectives, the following three types of surveys were performed.

 Focal Area and Corridor Monitoring – Botanists Forest and Kim Starr surveyed for new invaders and existing high priority weed species along corridors (e.g., roads) and in certain, pre-defined areas (e.g., roads, residences, etc.) where novel exotic species were deemed most likely to appear.  Annual Weed Monitoring – FWS technicians conducted surveys for priority weed species and native plant species at random locations across Sand, Eastern, and Spit islands providing presence/absence data for high priority weed species as well as density data for golden crownbeard.  Random-plot botanical Inventory – Botanists Forest and Kim Starr compiled a complete list of all plant species within fixed-area plots at random locations across Sand, Eastern, and Spit islands. Counts of golden crownbeard were also made at the sites.

For all surveys, the Midway Plant List, which utilizes the taxonomy and nomenclature of the Integrated Taxonomic Information System (ITIS) served as the sole source of information regarding the current status and classification of all plant species.

FOCAL AREA AND CORRIDOR MONITORING

Focal area and corridor monitoring sought primarily to identify exotic plant species that have recently arrived at the Refuge so that they may be targeted by the Early Detection Rapid Response (EDRR) program. These surveys also served to supplement to what was already known of the occurrence of certain priority weed species. For these surveys to be conducted properly, observers must possess expert knowledge of the flora of the Refuge

5 and must also be familiar with weed species from the main Hawaiian Islands and other tropical and sub-tropical islands. In 2017, these surveys were conducted by Forest and Kim Starr of the University of Hawaii at Manoa.

Survey Area

Focal area surveys were conducted in subjectively selected areas, focusing on those parts of the Refuge having highest likelihood of harboring recent plant invaders. Such areas were around residences, piers, and airport facilities on Sand Island. Because Eastern and Spit islands are not typically visited by island visitors and have no facilities other than a single pier used by refuge staff and contractors, they were not be subject to these surveys. Three distinct areas, totaling 30.3 ha (74.8 acres) were selected for focal area sampling (Figure 2).

Corridor sampling included certain roads on Sand Island (Figure 2) and areas immediately adjacent (within 5-10 m). Experienced botanists walked along the road observing all plant species with a focus on novel and target plant species (see below), detouring into roadside areas, when necessary. Eastern and Spit island were excluded from corridor surveys due to lack of visitation and low likelihood that they would be the entry point for new species. The total length of the roads selected for corridor sampling was 32.3 km (20.1 miles).

Field Methods

Botanists meandered through focal areas searching for any novel plant species and species on the incidental species list (Table 1). No specific routes were followed or plots visited, rather the botanists use their best judgement as to which portions of the focal area would have the highest likelihood or harboring new invaders. These included gardens, areas around houses, loading/unloading areas. Corridor surveys were performed in very similar fashion, the only difference being that a route (typically, a road) was followed and the area adjacent to the route (e.g., roadsides) were inspected for occurrences of the target species.

Prior to initiating a focal area or corridor survey, the GPS tracklog was cleared and a new track was initiated. Any novel or incidental weed species encountered during these surveys were documented by recording a GPS waypoint and notes were recorded on the incidentals data sheet (Appendix 3). For novel species, a voucher specimen was collected along with detailed notes regarding the occurrence – including site characteristics, the number of plants, and other nearby species – for eventual deposition at the herbarium of the Bishop Museum in Honolulu . Upon completion of surveys within an assigned area, the GPS track was saved and the name of the tracklog and the GPS unit is recorded on the data sheet, prior to leaving the survey area. The tracklog served to document the area surveyed.

Data Processing, Analysis, and Storage

Data collected in the focal area and corridors consisted mainly of incidental plant observations. Information regarding these observations were transcribed from paper data sheets into an into an MS Access database (Midway-Weed Monitoring.accdb). Details on the use of this database can be found in the accompanying users guide (Taylor and Stockenberg 2018). Paper data sheets should be scanned to convert them into Adobe PDF format and stored in the project archive (…\Midway THM I&M\Invasive Plant Management\Monitoring\Annual Weed Monitoring\Archive).

GPS waypoints for incidental locations and tracks were downloaded from the GPS units using the DNRGPS application (Minnesota Department of Natural Resources 2014). Metadata for incidental observations (Observer Name and Comments) were exported from the Access database using the Incidentals_Export query and then merged with the downloaded GPS data using the Import Incidental Waypoints and Merge/Join Incidental Waypoints tools (Inventory.gdb : Toolbox) before being appended to the PlantPts feature class (Inventory.gdb) using the Append Incidental Waypoints tool. Tracks were imported into the Midway GIS (InvasivePlantMgmt.gdb: Tracks) using the Add New Tracks to Tracklog Database tool (InvasivePlantMgmt.gdb : Toolbox).

These data were then compiled to create the following products:

 A list of all novel and target species observed across all focal areas and corridors surveyed  A map of all novel and target species observed within the focal areas and corridors.  A map of all paths transited by botanists within the focal areas and corridors  An updated version of the Midway Plant List (Midway-Plant List.xlsx).

All data were stored on ServCat (Codes: 85208, 85209, 85493, 85528).

ANNUAL WEED MONITORING

The annual weed monitoring effort consisted of surveys for a select suite of plant species within 5 m radius (78.54 m2) plots at randomly located sites within a Weed Survey Area. A small list of priority weed species were also documented as incidentals if observed between sites. Details on how the survey area was delimited, how survey sites were located, how data were collected in the field, and how they were analyzed are described in the paragraphs below.

Survey Area

The Weed Survey Area describes the universe from which samples are collected and to which any information resulting from the analysis of the data can be applied. Survey areas were defined separately for Sand, Eastern and Spit islands using the ArcGIS 10.4 Geographical Information System (GIS) with the Model Builder tool and then merged into a single ArcGIS feature class (InvasivePlantMgmt.gdb : feature class GridMonWeedSurveyAreas). Excluded from the survey areas were any locations pre-determined to be virtually devoid of plants which included paved areas (e.g., roads, runways), buildings, tanks, and ponds. Also excluded were areas covered by the focal area and corridor surveys. Weed survey areas for Sand, Eastern, and Spit islands totaled 271.9 ha (671.7 acres), 110.1 ha (272.0 acres), and 4.3 ha (10.5 acres), respectfully. See Appendix 4 for additional details on how these areas were defined.

Site Selection

Random sites were established by overlaying the weed survey area with a regular, rectangular 200 m x 200m grid (Figures 3 and 4). Within each 4 ha grid cell, a single random survey site was located using the Create Random Points tool, which ensured that the site was at least 5 m from the edge of the grid cell. Survey sites

7 were generated separately for each of the three islands. A total of 187 survey sites were established: 133 for Sand Island; 51 for Eastern, and 3 for Spit. Detailed methods for establishment of the grid and selection of survey sites can be found in Appendix 4.

Field Methods

Annual weed monitoring was conducted by trained technicians, proficient in the identification of all native plant and priority weed species of Midway Atoll1. On a given day, each technician or team of two was assigned an area Box 2. Equipment needed for field survey containing multiple survey sites to cover. Before heading out  Plant identification materials  GPS with survey sites pre-loaded into the field, the team ensured they had necessary equipment  5-m measuring tape (or longer) (Box 2) and a general plan for visiting the sites was made  1 stake for anchoring tape taking into consideration locations of roads, etc.  4 pin flags for outlining plot boundary  Clicker-counter (for golden crownbeard A GPS unit was used to navigate from one survey site to the counts; monitoring surveys only) next. Survey sites were typically within 200 m of one another  Data sheets and pencils and technicians walked from site to site.

Box 3. Qualitative abundance categories Once initial transport to a location near the first survey site  Absent – The plant species does not occur was accomplished, the GPS tracklog was cleared and a new in the plot track started. Surveys were then conducted as follows:  Few – The plant species is sparse within the plot having just one or several individuals.  Technicians navigated to the survey site using the For species (e.g., grasses) where individuals GPS (site locations were pre-loaded as waypoints). Because are not easily discerned, the plant covers GPS error makes it virtually impossible to locate the exact <5% of plot and is a minor component of survey site location, navigation to the site was considered the plant community. complete once the GPS indicated that the technician was  Moderate – The species is neither sparse, nor dominant/co-dominant but somewhere within 1 m of the site. in-between.  A stake was placed between the technician’s feet;  Dominant – Dozens or hundreds of this stake then served as the center point of a 5-m radius individuals of the species occur within plot circular plot (Figure 5). and it represents a major component of the plant community (dominant or co-  A measuring tape was used to delineate the 5m dominant). For species (e.g., grasses) where radius circular plot. The observer thoroughly searched the individuals are not easily discerned, cover plot for the target plant species, which included all native values exceed 25%. species plus Midway Class A, B.1, and C.1 weed species (Table 2). Plot searches were done by first subdividing the plot into quadrants and then searching each of the quadrants (Figure 5).  Each target plant species found rooted within the plot was noted on the data sheet along with a qualitative measure of its abundance (Box 3).

1 Ann Humphrey, David Dow, Weiteke Holthuijzen, and Rob Taylor all participated in data collection in 2017.

 A count was made of the total number of stems of golden crownbeard rooted within the plot. If a plant’s stems, leaves, or inflorescences overhung the plot boundary but was not rooted in the plot it was not counted. Dead plants that were the result of a recent herbicide application were counted.

 Estimates of the fraction of the plot occupied by a set of predefined land cover types was made (totaling to 100%). These were: Native Plants, Exotic Plants, Other (sand/gravel/cobble, pavement or concrete, litter/duff/dead plants). Note that both the native and exotic plant categories included both live plants and recently senesced stems (e.g., brown leaves on living grasses). If plants were entirely dead they were included in the Other category.  All data were recorded on the Annual Weed Monitoring Data Sheet (Appendix 3).

When travelling from one survey site to the next, observers searched for any novel weed species or any species on the Incidental Species List (Table 1). If observed, these were documented by recording a GPS waypoint; associated data were recorded on the Incidentals Data Sheet (Appendix 3).

Upon completion of surveys within an assigned area, the GPS track was saved and the name of the tracklog and the GPS unit was recorded on the data sheet, prior to leaving the survey area.

Data Processing, Analysis, and Storage

Data collected within plots as well as those related to incidental plant observations were transcribed from paper data sheets into an into an MS Access database (Midway-Weed Monitoring.accdb). Paper data sheets were scanned and hard copies stored on the refuge. Details on the use of this database can be found in the accompanying users guide (Taylor and Stockenberg 2018). Paper data sheets should be scanned to convert them into Adobe PDF format and stored in the project archive (…\Midway THM I&M\Invasive Plant Management\Monitoring\Annual Weed Monitoring\Archive).

GPS waypoints for incidental locations and tracks were downloaded from the GPS units using the DNRGPS application (Minnesota Department of Natural Resources 2014). Associated data for incidental observations, consisting basically of brief field notes, were exported from the Access database using the Incidentals_Export query and then merged with the downloaded GPS waypoint data using the Import Incidental Waypoints and Merge/Join Incidental Waypoints tools (Inventory.gdb : Toolbox) before being appended to the PlantPts feature class (Inventory.gdb) using the Append Incidental Waypoints tool. Tracks were imported into the Midway GIS (InvasivePlantMgmt.gdb: Tracks) using the Add New Tracks to Tracklog Database tool (InvasivePlantMgmt.gdb : Toolbox).

Plot data were exported from MS/Access using the SiteResults_Export query and then imported into the InvasivesPlantMgmt geodatabase to create the GridMon_SiteResults feature class (InvasivePlantMgmt.gdb) as well as tables containing results from the plot surveys (GridMon_SiteResults_Spp, GridMon_SiteResults_VERENC, GridMon_SiteResults_LC). Relationships were created within ArcGIS linking the site results layer to each of the tables.

These data were compiled and analyzed to create the following products:

 Map of routes transited by technicians while doing surveys (from feature class Tracks)  List of survey sites where monitoring was conducted (Sites Surveyed; InvasivePlantMgmt.gdb : feature class GridMon_SiteResults)  List of target species observed at each plot along with qualitative abundance (InvasivePlantMgmt.gdb : table GridMon_SiteResults_Spp)  Estimates of abundance (frequency), including a 90% confidence interval for each target species (R Statistics: prop.test function)  Estimates of the abundance (density) of golden crownbeard, including a 90% confidence interval (R command (R Statistics: see script Verbesina.Density.R).  Results of tests for differences (either T-test or Wilcoxon signed rank test, depending on data characteristics) of the abundance of golden crownbeard (both density and proportion) for Sand Island and Eastern Island using 2017 data and those from prior years (R commands: t.test, wilcox.test)  A map of the distribution of each target species detected during the surveys across the refuge (InvasivePlantMgmt.gdb : GridMon_A_o) using both survey site and incidental data (ArcGIS: see model Create_A_o).  An updated version of the Midway Plant List (Midway-Plant List.xlsx).

Statistics were calculated using the R program for statistics (R Core Team 2017) using scripts Sites_Species_Data.R, Verbesina-Density.R, and Verbesina-Frequency.R. All data, scripts, GIS models, etc. have been archived on ServCat (Codes: 85208, 85209, 85493, 85528).

RANDOM-PLOT BOTANICAL INVENTORY

Random survey site sampling complemented focal area and corridor sampling by providing data on plant species occurrence across all three islands of the Refuge. Methods for conducting these surveys were identical to those used for Annual Weed Monitoring except that all plant species within the plot were recorded (rather than just the target species in Table 2). Originally, half of the Annual Weed Survey sites were designated to be surveyed using the Random-Plot Botanical Inventory method, however, because botanists Kim and Forest Starr performed nearly all the surveys in 2017, the Random-Plot method was used even at sites originally slated to be subject to less intensive methods. Data from the Random Plot-Botanical Inventory surveys were processed and analyzed together with the Annual Weed Survey data. When estimating the abundance of any plant species, whether on the target list or otherwise, the actual number of survey sites for which that species was searched for was utilized in the calculation.

RESULTS AND DISCUSSION

Weed survey areas were defined

A total of 129 species were detected on the Refuge during at least one survey (Table 3). Of the 61 species that were specifically targeted by the surveys (Tables 1 and 2), only 40 were observed. All of the focal areas and corridors were searched (Figure 6) and within those areas 43 occurrences of 16 species were recorded (Table 3, Figure 6). Four species were new records for the atoll: Cucurbita maxima, Luffa acutangula, Persicaria odorata, and Thymus vulgaris. All of these were growing either in outdoor gardens near residences or, in the case of T

10 vulgaris, were in the hydroponic greenhouse and none were considered to be especially harmful (see Starr and Starr 2017 for additional information).

Of the 187 proposed survey sites, 186 were actually surveyed2. The Random Plot Inventory method was used at 154 sites; 32 plots were surveyed using the Annual Weed Monitoring Method. Fifteen native species were detected during the plot surveys (Table 3, Figure 7). The most abundant native plants were Boerhavia coccinea (scarlet spiderling; 33.3%), Tribulus cistoides (nohu; 24.7%), Scaevola taccada (beach naupaka; 19.4%), and Solanum americanum (glossy nightshade; 15.6%). The least abundant species detected were Chenopodium oahuense (aweoweo), Ipomoea indica (oceanblue morning-glory), Lepidium bidentatum var. owaihiense (Kunana pepperwort), Solanum nelsonii (popolo), and Sporobolus virginicus (akiaki), all of which were present at just one of the 186 survey sites.

Native plants have been propagated on the Refuge for over two decades and the widespread distribution of these species indicates the success of these efforts. The results from these surveys constitute the first known quantification of the abundance of native plants across all habitats (excluding runways, roads, etc.) on the Refuge and may prove useful as a baseline against which future studies may gauge long-term restoration effects. Two native species, nohu and glossy nightshade, are also classified as weeds on the Refuge and, therefore, information on their abundance could prove useful for informing weed management. For each native species that was detected in at least one plot during the 2017 surveys, a georeferenced PDF map was created (e.g., Figure 9), which can be loaded onto a smartphone or tablet and used with a GPS-enabled mapping application (e.g., Avenza). These maps are also included in the Midway Plant Atlas (Taylor 2017), which is available as a separate document.

Of the 61 exotic species detected in the plots, 12 were target species (weed classes A, B.1, and C.1; Figure 8), where the remaining 49 were detected during Random Plot Inventory surveys (Table 3). The most abundant exotic weed species were Lobularia maritima (sweet alyssum; 57.8%), Eleusine indica (goose grass; 38.7%), Cynodon dactylon (bermuda grass; 28.7%), Casuarina equisetifolia (ironwood; 21.0%), and Verbesina encelioides (17.7%). Bounded estimates of all species detected in the plots were calculated (Table 4) and used to evaluate sampling objectives. Both lower and upper confidence intervals for every weed species were < 10% absolute frequency, suggesting that objectives were met. A georeferenced PDF was created for each of the target weed species that was detected either within a survey plot or as an incidental observation. Maps are also included in the Midway Plant Atlas (Taylor 2017). Note that because incidental locations of most weed species were not recorded during the surveys, the map for a species does not include all areas where it exists on the refuge.

Verbesina was detected at 33 of the 186 survey sites across the Refuge. Although observed frequency was higher on Sand Island (20% [14-26%, 90% confidence interval]) than on Eastern Island (14% [7-25%]), differences were not significant (Figure 11). Density was calculated from the counts of golden crownbeard within the plots. Mean density across plots on Sand Island was 0.0066 plants / m2 (0.0042 – 0.0089, 90% confidence interval) while on Eastern measured density was 0.0062 plants / m2 (0.0006 – 0.01119). The overlap in these confidence intervals suggests these differences may be due to sampling error. No golden crownbeard was found in any of the three plots on Spit Island.

2 Site X5Y5, on Sand Island, was unintentionally skipped.

Data on golden crownbeard abundance in 2017 was compared to those from previous years to reveal any trends over time. For Sand Island both density3 and frequency4 in 2017 were lower than in 2016 (Figure 2). Frequency data for Eastern Island indicate a decrease in golden crownbeard abundance from the 2012-2014 to the 2015- 2017 period (Figure 12), however, no difference was found in either frequency5 or density6 from 2016 to 2017. Whether golden crownbeard abundance has stabilized on Eastern Island or whether it continues to decline as a result of eradication efforts, remains an important question. Lower sample sizes on Eastern Island along with a low fraction of sites having golden crownbeard results in low statistical power to detect differences.

CONCLUSION

The first comprehensive and quantitative plant surveys done in 2017 on Midway Atoll provide valuable baseline data on the abundance of both native and priority weed species which may be used for a variety of purposes including: tracking the success of native plant restoration efforts, assessing the capacity of certain weed species to spread, and evaluating the effectiveness of weed control efforts. These surveys also served to detect novel species arriving on the atoll, allowing for rapid control and eradication.

Unfortunately, these surveys did not solve the problem of increasing the power to detect change in the abundance of golden crownbeard on Eastern Island. Two possible, non-mutually exclusive, solutions would be (1) to increase the size of the survey plots and/or change the shape of the plot from circular to rectangular; (2) increase the number or survey sites. Prior to making a change in the size/shape of the survey plot pilot data should be collected and compared to data from this study.

ACKNOWLEDGEMENTS

This study could not have been accomplished without the assistance of many people. Thanks to Ann Humphrey Weiteke Holthuijzen, and Kevin Kilbride for invaluable feedback during the planning process. Botanists Forest and Kim Starr performed the vast majority of the field work for this project and also provided useful suggestions regarding the analysis and interpretation of the data. David Dow assisted in conducting field surveys.

3 Differences in mean densities found using Welch two-sample t-test (t = 2.0849, df = 55.497, p-value = 0.04169) which assumes a normal distribution; also significant using non-parametric Wilcoxon signed rank (W = 4054, p- value = 0.002). 4 X-squared = 25.147, df = 3; p < 0.001; Pairwise.prop.test, p = 0.020 5 X-squared = 61.158, df = 5, p-value; Pairwise.prop.test, p = 0.640 6 Differences in mean densities not found using Welch two-sample t-test (t = 0.23079, df = 78.869, p-value = 0.818) assuming a normal distribution and equal variance; however test for differences using non-parametric Wilcoxon signed rank was significant (W = 1460, p-value = 0.070).

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Taylor, RV. 2017. Map Atlas for Midway Atoll National Wildlife Refuge. Technical Report. US Fish and Wildlife Service. Honolulu, HI.

Taylor, RV, A Humphrey, W Holthuijzen. 2017. Weed Monitoring Plan for Midway Atoll National Wildlife Refuge. Technical Report. US Fish and Wildlife Service, Honolulu, HI (ServCat Code: 85487).

Taylor, RV and E Stockenberg. 2018. Midway Weed Monitoring Database User’s Guide. US Fish and Wildlife Service, Region 1 Inventory and Monitoring Group, Portland, OR.

TABLES

Table 1. Incidental weed species. When encountered, these species were documented by recording the location using a GPS waypoint.

PlantID Scientific name (common name) ITIS TSN Origin Weed Class ACAFAR Acacia farnesiana (klu)7 26427 Exotic Watch List AGASIS Agave sisalana (sisal) 182694 Exotic Watch List ANTLEP Antigonon leptopus (Mexican creeper) 20994 Exotic Watch List BOECOC Boerhavia coccinea (scarlet spiderling) 501005 Exotic A.3 CAEBON Guilandina bonduc (yellow knickers) 514312 Native A.3 CENECH Cenchrus echinatus (sand bur) 40575 Exotic Watch List CESNOC Cestrum nocturnum (night cestrum) 30498 Exotic Watch List CHLBAR Chloris barbata (swollen finger grass) 565064 Exotic Watch List COCGRA Coccinia grandis (ivy gourd) 22358 Exotic Watch List DIGINS Digitaria insularis (sour grass) 40636 Exotic Watch List EUGUNI Eugenia uniflora (Suriname cherry) 27224 Exotic Watch List FICELA Ficus elastica (Indian rubber tree) 19085 Exotic Watch List FICMAC Ficus macrophylla (Moreton bay fig) 823749 Exotic D FICMIC Ficus microcarpa (Chinese banyan) 502616 Exotic D MEGMAX Megathyrsus maximus (Guinea grass) 783700 Exotic Watch List PASEDU Passiflora edulis (passion vine) 22223 Exotic Watch List PITDUL Pithecellobium dulce (opiuma, Manila tamarind) 26871 Exotic Watch List PLUIND Pluchea indica (Indian pluchea) 36072 Exotic Watch List PROPAL Prosopis pallida (kiawe) 504620 Exotic Watch List PSIGUA Psidium guajava (guava) 27240 Exotic Watch List SCHTER Schinus terebinthifolius (Christmas berry) 28812 Exotic Watch List SOLTOR Solanum torvum (turkey berry) 30460 Exotic Watch List SPACAM Spathodea campanulata (African tulip tree) 34342 Exotic Watch List TETTET Tetragonia tetragonioides (New Zealand spinach) 19938 Exotic Watch List THEPER Thevetia peruviana (be still tree) 30228 Exotic Watch List TRIPRO Tridax procumbens (coat buttons) 38575 Exotic Watch List UROMUT Urochloa mutica (California grass) 523027 Exotic Watch List VITTRI Vitex trifolia (pohinahina) 32223 Exotic Watch List ZIZYPH Ziziphus sp. (jujube) 28594 Exotic Watch List

7 This species is now known as Vachellia farnesiana var. farnesiana (TSN 821007)

Table 2. Plant species for which data was collected within plots for the 2017 Annual Weed Monitoring effort.

Natives Weed PlantID Scientific name (common name) ITIS TSN Origin Class BOEREP Boerhavia repens (alena) 501007 PAC U CAPSAN Capparis sandwichiana (Hawaiian caper) 22609 HI U CHEOAH Chenopodium oahuense (aweoweo) 20640 HI-MA U CYPLAE Cyperus laevigatus (smooth flatsedge) 39935 PAC-MA U CYPPOL Cyperus polystachyos (manyspike flatsedge) 39957 PAC U ERAPAU Eragrostis paupera (Oahu lovegrass) 40737 PAC U ERAVAR Eragrostis variabilis (variable lovegrass) 40739 HI U FIMCYM Fimbristylis cymosa (button sedge, hurricanegrass) 40123 PAC-MA U HIBTIL Hibiscus tiliaceus (sea hibiscus) 845742 PAC-MA B.2 HIBWAI Hibiscus waimeae (white kauai rosemallow) 21652 HI-NWHI U IPOIND Ipomoea indica (oceanblue morning-glory) 30775 PAC U IPOPES Ipomoea pes-caprae ssp. brasiliensis (beach morning 30787 PAC U LEPBID glory;Lepidium brazilian bidentatum bayhops) var. owaihiense (Kunana 528714 HI U LEPREP pepperwort)Lepturus repens (Pacific Island thintail) 42301 PAC- U PRIHIL Pritchardia hillebrandii (molokai fan palm) 42475 HI+NWHIHI-NWHI U PRIREM Pritchardia remota (Nihoa fan palm) 42490 NWHI-MA U PRITCH Pritchardia sp. (loulu palm) 42461 N/A U PSESAN Pseudognaphalium sandwicensium var. 780946 HI-NWHI U SCATAC sandwicensiumScaevola taccada (ena'ena) (naupaka kahakai) 34782 PAC U SESPOR Sesuvium portulacastrum (shoreline seapurslane, 19910 PAC U SIDFAL cencilla)Sida fallax (yellow ilima) 21746 PAC U SOLNEL Solanum nelsonii (popolo) 30483 HI U SPOVIR Sporobolus virginicus (akiaki, beach dropseed) 42127 PAC U THEPOP Thespesia populnea (portia tree) 21762 PAC-MA U TRICIS Tribulus cistoides (nohu) 29056 PAC C.1 VITROT Vitex rotundifolia (pohinahina) 505725 PAC-MA C.2 WALIND Waltheria indica (uhaloa) 21561 PAC-MA B.2

Class A Weeds Weed PlantID Scientific name (common name) ITIS TSN Origin Class ABUGRA Abutilon grandifolium (hairy Indian mallow) 21679 Exotic A.2 AMASPI Amaranthus spinosus (spiny pigweed) 20748 Exotic A.2 BOECOC Boerhavia coccinea (scarlet spiderling) 501005 Exotic A.3 CROINC Crotalaria incana (fuzzy rattle pod) 26571 Exotic A.2 CYPINV Cyperus involucratus (umbrella plant) 501929 Exotic A.2 DESPER Desmanthus pernambucanus (slender mimosa) 507825 Exotic A.2 FICMAC Ficus macrophylla (Moreton bay fig) 823749 Exotic A.2 FICMIC Ficus microcarpa (Chinese banyan) 502616 Exotic A.2 CAEBON Guilandina bonduc (yellow knickers) 514312 Native A.3 INDHEN Indigofera hendecaphylla 515084 Exotic A.3 LANCAM Lantana camara (lantana) 32175 Exotic A.2 LEULEU Leucaena leucocephala (white leadtree) 26766 Exotic A.2 MALPAR Malva parviflora (cheese weed) 21838 Exotic A.2 MIRJAL Mirabilis jalapa (four o'clock) 19648 Exotic A.2 PLUCAR Pluchea carolinensis (sour bush) 196238 Exotic A.2 RICCOM Ricinus communis (castor bean) 28393 Exotic A.2 VERLIT Verbena litoralis (vervain) 32130 Exotic A.2 VERENC Verbesina encelioides (golden crown-beard) 38601 Exotic A.1

Class B.1 Weeds Weed PlantID Scientific name (common name) ITIS TSN Origin Class ANDGLO Andropogon glomeratus var. pumilus (broomsedge) 182522 Exotic B.1 BRANIG Brassica nigra (black mustard) 23061 Exotic B.1 CASUAR Casuarina spp. (ironwood) 19516 Exotic B.1 EUPCYA Euphorbia cyathophora (wild poinsettia) 28060 Exotic B.1 SOLAME Solanum americanum (glossy nightshade) 565523 Native B.1 SONOLE Sonchus oleraceus (sow thistle) 38427 Exotic B.1

Class C.1 Weeds Weed PlantID Scientific name (common name) ITIS TSN Origin Class BIDALB Bidens alba var. radiata (beggarticks)8 196224 Exotic C.1 BROCAT Bromus catharticus (prairie grass) 501066 Exotic C.1 CENCIL Cenchrus ciliaris (buffel grass) 504198 Exotic C.1 CONBON Conyza bonariensis (hairy horseweed) 37112 Exotic C.1 CONCAN Conyza canadensis var. pusilla (horseweed) 527478 Exotic C.1 CONYZA Conyza sp. (horseweed) 37108 Exotic C.1 CORDID Coronopus didymus (swine cress)9 23105 Exotic C.1 DACAEG Dactyloctenium aegyptium (beach wire grass) 41630 Exotic C.1 ELEIND Eleusine indica (goose grass) 41692 Exotic C.1 SETVER Setaria verticillata (bristly foxtail) 41232 Exotic C.1 TRICIS Tribulus cistoides (nohu) 29056 Native C.1

8 This species is now known as Bidens pilosa (TSN 35731) 9 This species is now known as Lepidium didymum (TSN 515763)

Table 3. Plant species searched for and/or detected during all surveys on Midway Atoll in 2017. For each species, the detection column indicates whether and how detection was made: I = Incidental, FC = Focal/Corridor survey, P = Survey plot, Not Detected.

Weed PlantID Scientific name (common name) ITIS TSN Origin Class Detection ABUGRA Abutilon grandifolium (hairy Indian mallow) 21679 Exotic A.2 I ACAFAR Acacia farnesiana (klu) 26427 Exotic Watch List Not Detected AGASIS Agave sisalana (sisal) 182694 Exotic Watch List Not Detected AMASPI Amaranthus spinosus (spiny pigweed) 20748 Exotic A.2 I, FC AMAVIR Amaranthus viridis (slender amaranth) 20751 Exotic C.2 P ANAARV Anagallis arvensis (scarlet pimpernel) 24043 Exotic U.2 P ANDGLO Andropogon glomeratus var. pumilus (broomsedge) 182522 Exotic B.1 Not Detected ANTLEP Antigonon leptopus (Mexican creeper) 20994 Exotic Watch List Not Detected BIDALB Bidens alba var. radiata (beggarticks) 196224 Exotic C.1 P, I BOECOC Boerhavia coccinea (scarlet spiderling) 501005 Exotic A.3 I, FC BOEREP Boerhavia repens (alena) 501007 Native U P BOTPER Bothriochloa pertusa (pitted beard grass) 41480 Exotic U.2 P BRANIG Brassica nigra (black mustard) 23061 Exotic B.1 P BROCAT Bromus catharticus (prairie grass) 501066 Exotic C.1 I, FC CALVIA Calyptocarpus vialis (calyptocarpus) 501164 Exotic U.2 P CAPSAN Capparis sandwichiana (Hawaiian caper) 22609 Native U Not Detected CASEQU Casuarina equisetifolia (ironwood) 19516 Exotic B.1 P CASGLA Casuarina glauca (longleaf ironwood) 19517 Exotic B.1 Not Detected CENCIL Cenchrus ciliaris (buffel grass) 504198 Exotic C.1 I CENECH Cenchrus echinatus (sand bur) 40575 Exotic Watch List Not Detected CENERY Centaurium erythraea (bitter herb) 30045 Exotic U.2 P CESNOC Cestrum nocturnum (night cestrum) 30498 Exotic Watch List Not Detected CHEMUR Chenopodium murale (goosefoot) 20622 Exotic B.2 P CHEOAH Chenopodium oahuense (aweoweo) 20640 Native U P CHLBAR Chloris barbata (swollen finger grass) 565064 Exotic Watch List Not Detected COCGRA Coccinia grandis (ivy gourd) 22358 Exotic Watch List Not Detected COCUVI Coccoloba uvifera (sea grape) 21039 Exotic D P CONBON Conyza bonariensis (hairy horseweed) 37112 Exotic C.1 P CONCAN Conyza canadensis var. pusilla (horseweed) 527478 Exotic C.1 P

Weed PlantID Scientific name (common name) ITIS TSN Origin Class Detection CONYZA Conyza sp. (horseweed) 37108 Exotic C.1 P CORDID Coronopus didymus (swine cress) 23105 Exotic C.1 P CROINC Crotalaria incana (fuzzy rattle pod) 26571 Exotic A.2 I CUCMAX Cucurbita maxima (winter squash) 22369 Exotic U FC CICLEP Cyclospermum leptophyllum (fir-leaved celery) 507572 Exotic C.2 P CYNDAC Cynodon dactylon (bermuda grass) 41619 Exotic D P CYPINV Cyperus involucratus (umbrella plant) 501929 Exotic A.2 I, FC CYPLAE Cyperus laevigatus (smooth flatsedge) 39935 Native U Not Detected CYPPOL Cyperus polystachyos (manyspike flatsedge) 39957 Native U P CYPROT Cyperus rotundus (purple nut sedge) 39900 Exotic C.2 P DACAEG Dactyloctenium aegyptium (beach wire grass) 41630 Exotic C.1 P DESPER Desmanthus pernambucanus (slender mimosa) 507825 Exotic A.2 I DIGCIL Digitaria ciliaris (Henry's crab grass) 40619 Exotic U.1 P DIGINS Digitaria insularis (sour grass) 40636 Exotic Watch List I ELEIND Eleusine indica (goose grass) 41692 Exotic C.1 P ERAPAU Eragrostis paupera (Oahu lovegrass) 40737 Native U Not Detected ERAAMA Eragrostis tenella (Japanese love grass) 40765 Exotic U.1 P ERAVAR Eragrostis variabilis (variable lovegrass) 40739 Native U P EUGUNI Eugenia uniflora (Suriname cherry) 27224 Exotic Watch List Not Detected EUPCYA Euphorbia cyathophora (wild poinsettia) 28060 Exotic B.1 P EUPHIR Euphorbia hirta (hairy spurge) 28081 Exotic C.2 P EUPHYP Euphorbia hypericifolia (graceful spurge) 28083 Exotic C.2 P EUPMAC Euphorbia maculata (spurge) 28034 Exotic C.2 P EUPPEP Euphorbia peplus (petty spurge) 28117 Exotic C.2 P EUPPRO Euphorbia prostrata (small ground fig) 28124 Exotic C.2 P EUPSER Euphorbia serpens (matted sandmat) 28248 Exotic C.2 P EUPPR2 Euploca procumbens (heliotropium) 835414 Exotic C.2 P EUSPET Eustachys petraea (eustachys) 41743 Exotic B.2 P FICELA Ficus elastica (Indian rubber tree) 19085 Exotic Watch List Not Detected FICMAC Ficus macrophylla (Moreton bay fig) 823749 Exotic A.2 Not Detected FICMIC Ficus microcarpa (Chinese banyan) 502616 Exotic A.2 I, FC FIMCYM Fimbristylis cymosa (button sedge, hurricanegrass) 40123 Native U P

Weed PlantID Scientific name (common name) ITIS TSN Origin Class Detection CAEBON Guilandina bonduc (yellow knickers) 514312 Native A.3 Not Detected HIBTIL Hibiscus tiliaceus (sea hibiscus) 845742 Native B.2 Not Detected HIBWAI Hibiscus waimeae (white kauai rosemallow) 21652 Native U Not Detected INDHEN Indigofera hendecaphylla (creeping indigo) 515084 Exotic A.3 I, FC IPOIND Ipomoea indica (oceanblue morning-glory) 30775 Native U P IPOPES Ipomoea pes-caprae ssp. brasiliensis (beach morning glory) 30787 Native U P LANCAM Lantana camara (lantana) 32175 Exotic A.2 Not Detected LEPBID Lepidium bidentatum var. owaihiense (Kunana pepperwort) 528714 Native U P LEPVIR Lepidium virginicum (pepper grass) 22955 Exotic U.1 P LEPREP Lepturus repens (Pacific Island thintail) 42301 Native U P LEULEU Leucaena leucocephala (white leadtree) 26766 Exotic A.2 I, FC LOBMAR Lobularia maritima (sweet alyssum) 23236 Exotic C.2 P LUFACU Luffa acutangula (sinkwa towelsponge) 503569 Exotic U FC MALPAR Malva parviflora (cheese weed) 21838 Exotic A.2 I MALCOR Malvastrum coromandelianum (false mallow) 21702 Exotic C.2 P MEDLUP Medicago lupulina (black medic) 503721 Exotic U.1 P MEGMAX Megathyrsus maximus (Guinea grass) 783700 Exotic Watch List Not Detected MELIND Melilotus indicus (yellow sweet clover) 503741 Exotic U.2 P MIRJAL Mirabilis jalapa (four o'clock) 19648 Exotic A.2 I, FC MOMCHA Momordica charantia (bitter melon) 22399 Exotic U.2 I, FC OENLAC Oenothera laciniata (evening primrose) 27371 Exotic C.2 P OXACOR Oxalis corniculata (yellow wood sorrel) 29067 Exotic U.2 P PASSET Paspalum setaceum (paspalum) 41042 Exotic U.1 P PASEDU Passiflora edulis (passion vine) 22223 Exotic Watch List Not Detected PERODO Persicaria odorata (Vietnamese coriander) N/A Exotic U FC PHYNOD Phyla nodiflora (turkey tangle fog fruit) 32197 Exotic C.2 P PITDUL Pithecellobium dulce (opiuma, Manila tamarind) 26871 Exotic Watch List Not Detected PLUCAR Pluchea carolinensis (sour bush) 196238 Exotic A.2 I PLUIND Pluchea indica (Indian pluchea) 36072 Exotic Watch List Not Detected POLMON Polypogon monspeliensis (annual ditch beard grass) 41171 Exotic U.1 P POROLE Portulaca oleracea (common purslane) 20422 Exotic N P PRIHIL Pritchardia hillebrandii (molokai fan palm) 42475 Native U Not Detected PRIREM Pritchardia remota (Nihoa fan palm) 42490 Native U Not Detected

Weed PlantID Scientific name (common name) ITIS TSN Origin Class Detection PRITCH Pritchardia sp. (loulu palm) 42461 Native U Not Detected PROPAL Prosopis pallida (kiawe) 504620 Exotic Watch List Not Detected PSESAN Pseudognaphalium sandwicensium var. sandwicensium (ena'ena) 780946 Native U P PSIGUA Psidium guajava (guava) 27240 Exotic Watch List Not Detected RICCOM Ricinus communis (castor bean) 28393 Exotic A.2 I SCATAC Scaevola taccada (naupaka kahakai) 34782 Native U P SCHACT Schefflera actinophylla (octopus tree) 505047 Exotic U.2 I, FC SCHTER Schinus terebinthifolius (Christmas berry) 28812 Exotic Watch List Not Detected SESPOR Sesuvium portulacastrum (shoreline seapurslane, cencilla) 19910 Native U P SETVER Setaria verticillata (bristly foxtail) 41232 Exotic C.1 P, I SIDFAL Sida fallax (yellow ilima) 21746 Native U Not Detected SOLAME Solanum americanum (glossy nightshade) 565523 Native B.1 P SOLNEL Solanum nelsonii (popolo) 30483 Native U P, I SOLTOR Solanum torvum (turkey berry) 30460 Exotic Watch List I, FC SONOLE Sonchus oleraceus (sow thistle) 38427 Exotic B.1 P, I SPACAM Spathodea campanulata (African tulip tree) 34342 Exotic Watch List Not Detected SPEMAR Spergularia salina (saltmarsh sand spurry) 507251 Exotic C.2 P SPOIND Sporobolus indicus (Indian dropseed) 42140 Exotic C.2 P SPOPYR Sporobolus pyramidatus (sporobolus) 42148 Exotic C.2 P SPOVIR Sporobolus virginicus (akiaki, beach dropseed) 42127 Native U P STESEC Stenotaphrum secundatum (St. Augustine grass) 42157 Exotic C.2 P TETTET Tetragonia tetragonioides (New Zealand spinach) 19938 Exotic Watch List Not Detected THEPOP Thespesia populnea (portia tree) 21762 Native U Not Detected THEPER Thevetia peruviana (be still tree) 30228 Exotic Watch List Not Detected THYVUL Thymus vulgaris (garden thyme) 505501 Exotic U FC TOUARG Tournefortia argentea (tree heliotrope) 505540 Exotic N P TRICIS Tribulus cistoides (nohu) 29056 Native C.1 P TRIPRO Tridax procumbens (coat buttons) 38575 Exotic Watch List Not Detected UROMUT Urochloa mutica (California grass) 523027 Exotic Watch List Not Detected VERLIT Verbena litoralis (vervain) 32130 Exotic A.2 I, FC VERENC Verbesina encelioides (golden crown-beard) 38601 Exotic A.1 P VITROT Vitex rotundifolia (pohinahina) 505725 Native C.2 Not Detected VITTRI Vitex trifolia (pohinahina) 32223 Exotic Watch List Not Detected WALIND Waltheria indica (uhaloa) 21561 Native B.2 Not Detected ZIZYPH Ziziphus sp. (jujube) 28594 Exotic Watch List Not Detected

Table 4. Abundance estimates for all exotic and native species detected in the Annual Weed Survey and Random Plot Inventory plots. Frequency, calculated as the total number of plots in which the plant was present divided by the total number of plots sampled, is provided along with lower and upper bounds of the 90% confidence interval. Note that for non-target exotic species, the number of plots sampled is lower than for the target species.

Class A

PlantID Scientific name (common name) N_sites Present Frequency Lower90 Upper90 VERENC Verbesina encelioides (golden 186 33 17.74% 13.37% 23.09% crown-beard)

Class B.1

PlantID Scientific name (common name) N_sites Present Frequency Lower90 Upper90 BRANIG Brassica nigra (black mustard) 186 1 0.54% 0.04% 2.79% CASEQU Casuarina equisetifolia 186 39 20.97% 16.25% 26.56% (ironwood) EUPCYA Euphorbia cyathophora (wild 186 2 1.08% 0.23% 3.59% poinsettia) SOLAME Solanum americanum (glossy 186 29 15.59% 11.48% 20.75% nightshade) SONOLE Sonchus oleraceus (sow thistle) 186 3 1.61% 0.50% 4.34%

Class C.1

PlantID Scientific name (common name) N_sites Present Frequency Lower90 Upper90 BIDALB Bidens alba var. radiata 186 6 3.23% 1.49% 6.46% (beggarticks) CONBON Conyza bonariensis (hairy 186 12 6.45% 3.86% 10.40% horseweed) CONCAN Conyza canadensis var. pusilla 186 30 16.13% 11.95% 21.34% (horseweed) CONYZA Conyza sp. (horseweed) 186 2 1.08% 0.23% 3.59% CORDID Coronopus didymus (swine cress) 186 7 3.76% 1.86% 7.14% DACAEG Dactyloctenium aegyptium 186 25 13.44% 9.62% 18.38% (beach wire grass) ELEIND Eleusine indica (goose grass) 186 72 38.71% 32.78% 44.98% SETVER Setaria verticillata (bristly foxtail) 186 7 3.76% 1.86% 7.14% TRICIS Tribulus cistoides (nohu) 186 46 24.73% 19.67% 30.56%

Native

PlantID Scientific name (common name) N_sites Present Frequency Lower90 Upper90 BOEREP Boerhavia repens (alena) 186 62 33.33% 27.67% 39.50% CHEOAH Chenopodium oahuense 186 1 0.54% 0.04% 2.79% (aweoweo) CYPPOL Cyperus polystachyos (manyspike 186 6 3.23% 1.49% 6.46% flatsedge) ERAVAR Eragrostis variabilis (variable 186 16 8.60% 5.57% 12.92% lovegrass) FIMCYM Fimbristylis cymosa (button 186 4 2.15% 0.80% 5.06% sedge, hurricanegrass) IPOIND Ipomoea indica (oceanblue 186 1 0.54% 0.04% 2.79% morning-glory) IPOPES Ipomoea pes-caprae ssp. 186 4 2.15% 0.80% 5.06% brasiliensis (beach morning glory) LEPBID Lepidium bidentatum var. 186 1 0.54% 0.04% 2.79% owaihiense (Kunana pepperwort) LEPREP Lepturus repens (Pacific Island 186 2 1.08% 0.23% 3.59% thintail) PSESAN Pseudognaphalium 186 4 2.15% 0.80% 5.06% sandwicensium var. sandwicensium (ena'ena) SCATAC Scaevola taccada (naupaka 186 36 19.35% 14.80% 24.83% kahakai) SESPOR Sesuvium portulacastrum 186 6 3.23% 1.49% 6.46% (shoreline seapurslane, cencilla) SOLAME Solanum americanum (glossy 186 29 15.59% 11.48% 20.75% nightshade) SOLNEL Solanum nelsonii (popolo) 186 1 0.54% 0.04% 2.79% SPOVIR Sporobolus virginicus (akiaki, 186 1 0.54% 0.04% 2.79% beach dropseed) TRICIS Tribulus cistoides (nohu) 186 46 24.73% 19.67% 30.56%

Non-target exotic species

PlantID Scientific name (common name) N_sites Present Frequency Lower90 Upper90 AMAVIR Amaranthus viridis (slender 154 2 1.30% 0.28% 4.32% amaranth) ANAARV Anagallis arvensis (scarlet 154 4 2.60% 0.97% 6.09% pimpernel) BOTPER Bothriochloa pertusa (pitted 154 5 3.25% 1.38% 6.94% beard grass) CALVIA Calyptocarpus vialis 154 1 0.65% 0.04% 3.36% (calyptocarpus) CENERY Centaurium erythraea (bitter 154 1 0.65% 0.04% 3.36% herb) CHEMUR Chenopodium murale (goosefoot) 154 1 0.65% 0.04% 3.36% CICLEP Cyclospermum leptophyllum (fir- 154 4 2.60% 0.97% 6.09% leaved celery) COCUVI Coccoloba uvifera (sea grape) 154 1 0.65% 0.04% 3.36% CYNDAC Cynodon dactylon (bermuda 154 44 28.57% 22.69% 35.23% grass) CYPROT Cyperus rotundus (purple nut 154 13 8.44% 5.18% 13.26% sedge) DIGCIL Digitaria ciliaris (Henry's crab 154 8 5.19% 2.72% 9.38% grass) ERAAMA Eragrostis tenella (Japanese love 154 9 5.84% 3.19% 10.18% grass) EUPHIR Euphorbia hirta (hairy spurge) 154 8 5.19% 2.72% 9.38% EUPHYP Euphorbia hypericifolia (graceful 154 1 0.65% 0.04% 3.36% spurge) EUPMAC Euphorbia maculata (spurge) 154 4 2.60% 0.97% 6.09% EUPPEP Euphorbia peplus (petty spurge) 154 5 3.25% 1.38% 6.94% EUPPR2 Euploca procumbens 154 13 8.44% 5.18% 13.26% (heliotropium) EUPPRO Euphorbia prostrata (small ground 154 2 1.30% 0.28% 4.32% fig) EUPSER Euphorbia serpens (matted 154 12 7.79% 4.67% 12.50% sandmat) EUSPET Eustachys petraea (eustachys) 154 12 7.79% 4.67% 12.50% FICMIC Ficus microcarpa (Chinese banyan) 154 1 0.65% 0.04% 3.36% LEPVIR Lepidium virginicum (pepper 154 21 13.64% 9.44% 19.18% grass) LOBMAR Lobularia maritima (sweet 154 89 57.79% 50.84% 64.46% alyssum) MALCOR Malvastrum coromandelianum 154 3 1.95% 0.60% 5.22% (false mallow) MEDLUP Medicago lupulina (black medic) 154 1 0.65% 0.04% 3.36% MELIND Melilotus indicus (yellow sweet 154 1 0.65% 0.04% 3.36%

clover) OENLAC Oenothera laciniata (evening 154 4 2.60% 0.97% 6.09% primrose) OXACOR Oxalis corniculata (yellow wood 154 1 0.65% 0.04% 3.36% sorrel) PASSET Paspalum setaceum (paspalum) 154 7 4.55% 2.25% 8.58% PHYNOD Phyla nodiflora (turkey tangle fog 154 7 4.55% 2.25% 8.58% fruit) POLMON Polypogon monspeliensis (annual 154 1 0.65% 0.04% 3.36% ditch beard grass) POROLE Portulaca oleracea (common 154 5 3.25% 1.38% 6.94% purslane) SPEMAR Spergularia salina (saltmarsh sand 154 4 2.60% 0.97% 6.09% spurry) SPOIND Sporobolus indicus (Indian 154 6 3.90% 1.81% 7.77% dropseed) SPOPYR Sporobolus pyramidatus 154 4 2.60% 0.97% 6.09% (sporobolus) STESEC Stenotaphrum secundatum (St. 154 6 3.90% 1.81% 7.77% Augustine grass) TOUARG Tournefortia argentea (tree 154 13 8.44% 5.18% 13.26% heliotrope)

FIGURES

Figure 1. Changes in the number of native and exotic plant species on Midway Atoll from 1902-2015. Also shown are names of botanists who have conducted significant surveys over the years. 26

Figure 2. Focal area and corridor survey areas on Sand Island. Roads of priorities 1, 2, and 3 will be included..

Figure 3. Weed survey area on Sand Island (271 ha; 671 acres)overlaid with 4 ha grid and proposed random survey sites (n = 133).

Figure 4. Weed survey areas on Eastern Island (10 ha; 272 acres)and Spit Island (4.2 ha; 10.5 acres) overlaid with 4 ha grid and proposed random survey sites (n = 54). 29

Figure 5. Detailed diagram of the 5-m radius survey plot. The plot must be searched thoroughly for the target list of plants, which can be accomplished in a variety of ways. Two suggested methods include: Left – Divide the plot into 3 concentric sub-areas searching first the area within 1m of the plot center, then concentric ring located 1-3 m from the center, and finally, the outermost ring (3-5 m); dashed lines represent the path that the observer walks when surveying the outer plots, which is accomplished using a measuring tape affixed to the center stake. Right – Divide the plot into 4, equal sized wedges using pin flags to mark the four outer corners.

Figure 6. Results from focal area and corridor surveys. Lines show routes taken by surveyors and points indicate locations of weeds observed during the surveys. See Table 3 for explanation of species codes. 31

Figure 7. Estimated abundance of native plants across Sand, Eastern, and Spit Islands based on presence/absence data. Frequency is calculated as the number of plots in which the plant was observed divided by the total number or plots sampled.

Figure 8. Estimated abundance of priority weeds (classes A.1, B.1, and C.1) across Sand, Eastern, and Spit Islands based on presence/absence data. Frequency is calculated as the number of plots in which the plant was observed divided by the total number or plots sampled.

Figure 9. Map of the native shrub Scaevola taccada (beach naupaka) resulting from the 2017 survey data. Note that as this species is widespread across the atoll and the number of survey locations was relatively small, the mapped area likely underestimates the actual distribution of this species.

Figure 10. Map of the invasive weed Bidens alba var. radiata (beggarticks) resulting from the 2017 survey data along with incidental point locations from 1999-2016. Note that as this species is widespread across the Sand Island and the number of survey locations was relatively small, the mapped area likely underestimates the actual distribution of this species.

Figure 11. Locations of plots where the invasive weed Verbesina encelioides (golden crown-beard) was found within monitoring plots on Sand (top; n = 152), Eastern (bottom; n = 51), and Spit (bottom; n = 3) Islands.

Figure 12. Changes in the abundance of Verbesina encelioides (golden crownbeard) expressed as density (top) and frequency (bottom) at Midway Atoll NWR for Sand Island (left) and Eastern Island (right) across the monitoring period. Surveys for golden crownbeard have been conducted on Sand Island each year since 2014; on Eastern Island surveys began in 2012. Density monitoring began in 2016. Error bars represent 90% confidence intervals.

APPENDIX 1 – MIDWAY WEED CLASSIFICATION

Weed Wildlife Native Plant Ability to Recommended History Abundance Invasiveness Ecosystem Impact Monitor Comments Class Impact Impact Control Action

These are weeds that spread quickly and have the ability to form dense populations across a wide area that cause significant harm to wildlife; may also affect native plant recruitment and ecosystem function (e.g., Widespread; Low, reducing stability of the site through increased becoming locally Low, Moderate, A.1 Long High High Moderate, Good Yes Eradicate erosion); and that cover (or once covered) dense over time or High or High significant portions of the atoll and for which without control good control methods exist. The highest priority for management action. A good example of this is Verbesina encelioides .

These weeds have been on the atoll for a significant amount of time (> 20 years) and have shown the ability to spread rapidly but due to past control efforts have been greatly reduced in extent leaving only a few remnant Good - A.2 Long Localized High High in at least one category Yes Suppress populations which could reasonably be Moderate eradicated with continued low effort. An example of this would be Ricinus communis which can grow up to 5 feet tall and can form dense stands.

These EDRR (Early Detection Rapid Response) or "incipient" weeds should be detected and eliminated as quickly as possible, before they become widespread and while effective control is feasible and can be accomplished with little A.3 Short Localized Unknown Low, Moderate or High Good Yes Eradicate effort. Surveys to detect these weeds should be done systematically on a regular basis.

These species are thought to be extirpated on Midway due to past control efforts but are considered very invasive elsewhere. These Watch Short or Good or Currently extirpated High Low, Moderate or High Yes Eradicate species may persist as seeds in soil or and have List Long Poor the potential to be reintroduced.

Weed Wildlife Native Plant Ability to Recommended History Abundance Invasiveness Ecosystem Impact Monitor Comments Class Impact Impact Control Action

These widespread weeds spread quickly and can form dense stands that are deleterious to wildlife or native plant communities. Keeping these species in check in areas where they will Widespread; cause greatest harm is necessary to meet Low, becoming locally Low, Moderate, resource management objectives. B.1 Long High High Moderate, Good Yes Suppress dense over time or High Recommendation is to suppress below or High without control threshold level above which significant impacts occur. Monitoring of these species over time will be necessary to ensure they are kept to benign levels.

These widespread weeds are slow/moderate to spread and can have deleterious impacts to wildlife species if left unchecked. Keeping these species in check in areas where they will cause Widespread; greatest harm is necessary to meet resource Low, becoming locally Low, Moderate, Good - management objectives. Recommendation is to B.2 Long Moderate to High High Moderate, Yes Suppress or Contain dense over time or High Moderate suppress below threshold level above which or High without control significant impacts occur. Monitoring of these species over time will be necessary to ensure they are kept to benign levels.

These are weeds that spread quickly and have the ability to form dense populations which, though they do not constitute a direct threat to Localized or priority wildlife species (e.g., seabirds, Laysan Short or Low, Moderate, Low to Suppress where C.1 widespread; Dense High High Good Yes ducks) at high levels will preclude the ability of Long or High Moderate impacts are greatest or sparse native plants to survive and reproduce.

Weed Wildlife Native Plant Ability to Recommended History Abundance Invasiveness Ecosystem Impact Monitor Comments Class Impact Impact Control Action

These are weeds are slow to spread but eventually have the ability to form dense populations which, though they do not constitute a direct threat to priority wildlife Localized or Suppress and/or species (e.g., seabirds, Laysan duck) could Low, Moderate, Low to Good - C.2 widespread; Dense Low to Moderate Low No eliminate from compete with native species. Moderate priority or High Moderate Moderate or sparse certain areas for management action (Suppress to low level) within native plant restoration or historical use areas.

These are weeds are slow to spread and cause at most moderate impact but care should be taken to prevent populations from increasing Suppress and/or Widespread but over time. An example is coconut palm which D Long Low Low to Moderate Good No eliminate from sparse has few negative (and perhaps some positive) certain areas effects when at very low densities.

Although these weeds occur over a wide area, they never attain an abundance high enough to Localized or Good or N Long Low Low in all categories No Do nothing cause significant impacts. Recommended action widespread; Sparse Poor is to do nothing.

Weed Wildlife Native Plant Ability to Recommended History Abundance Invasiveness Ecosystem Impact Monitor Comments Class Impact Impact Control Action

These species are suspected of being moderately/highly invasive and/or having moderate/high impacts but information on important characteristics of these species is lacking. Recommended that research be Localized or U.1 Long Unknown but suspected to be invasive and having significant impact Yes Investigate conducted which may consist of some Widespread combination of queries to other land managers or organizations, literature review, Weed Risk Assessment (WRA) ranking, and monitoring.

Information on important characteristics of these species is lacking but little evidence to Localized or date that they are problematic. Recommended U.2 Long Unknown but not suspected to be invasive and having significant impact Yes Defer Widespread that more observation be done before ranking.

N/A Not Applicable Reserved for native species

APPENDIX 2 – OVERVIEW OF PLANT SURVEYS AND PRELIMINARY SCHEDULE

Weed surveys performed in 2017 were part of multi-year monitoring scheme that was developed for Midway10. The following describes the types of monitoring proposed for Midway and the frequency with which each activity should be performed.

 Every year …  Annual Weed Monitoring – Each year, technicians will conduct surveys for priority weed species at random locations across Sand, Eastern, and Spit islands providing presence/absence data for high priority weed species as well as density data for golden crownbeard.  Every 3 – 5 years …  Focal Area and Corridor Monitoring – surveys for new invaders and existing high priority weed species along corridors (e.g., roads) and in certain, pre-defined areas (e.g., roads, residences, etc.) where novel exotic species are most likely to appear.  Random-plot botanical Inventory – botany technicians compile a complete list of all plant species within fixed-area plots at random locations across Sand, Eastern, and Spit Islands.  Several times per year …  Restoration Area Surveys – Occasional, qualitative, monitoring of weeds within habitat restoration areas.

In those years when both annual monitoring and random-plot botanical inventory are performed, surveys may be combined. That is, the botany technicians conducting the inventory surveys will also collect monitoring data thus reducing overall effort (i.e., transportation to the site and plot set up need to be done just once) as well as creating synergies between the data (e.g., relationships between weed species and other plant species can be investigated).

10 Taylor, RV, A Humphrey, W Holthuijzen. 2017. Weed Monitoring Plan for Midway Atoll National Wildlife Refuge. Technical Report. US Fish and Wildlife Service.

APPENDIX 3 – DATA SHEETS

APPENDIX 4 – DETAILED METHODS FOR SELECTING RANDOM SURVEY SITES

Sand Island

The weed survey area for Sand Island (SIMA) was determined using the model builder feature in the ArcPro 1.4 Geographic Information System as follows (model shown at right).

The entirety of SIMA was considered for sampling

From within this area, the following unsuitable areas were merged and then buffered by 10 m

 Structures (included buildings, tanks, piers, etc.)  Miscellaneous unvegetated/paved areas (layer Misc_Poly in the Miscellany.gdb geodatabase)  The “Boneyard” (this is a paved junkyard of sorts in Sector 35)  The edges of the island (1 m buffer)  Existing all vehicle roads  Open water areas (e.g., seeps)  Urban areas (these would be surveyed using the Focal Area Survey Method)

These unsuitable areas where “erased” from SIMA, with the remainder then representing the SIMA Weed Survey Area.

This area was then overlaid with a 4 ha (200 m x 200 m) square grid and within each grid cell a random survey site was selected, the only condition being that the site had to be at least 8 m from the edge of the grid cell. This was accomplished using the Grid Sampling Design Tool (see below) within ArcGIS Pro 1.4.1.

Eastern Island

The weed survey area for Eastern Island (EIMA) was determined using the model builder feature in the ArcPro 1.4 Geographic Information System as follows (model shown at right).

The entirety of EIMA was considered for sampling

From within this area, the following unsuitable areas were merged and then buffered by 10 m

 The edges of the island (1 m buffer)  Existing all vehicle roads  Open water areas (e.g., seeps)

These unsuitable areas where “erased” from EIMA, with the remainder then representing the EIMA Weed Survey Area.

Spit Island

The weed survey area for Spit Island (Spit) was determined using the model builder feature in the ArcPro 1.4 Geographic Information System as follows (model shown at right).

The entirety of Spit was considered for sampling

From within this area, the following unsuitable areas were merged and then buffered by 10 m

 The edges of the island (1 m buffer)

These unsuitable areas where “erased” from Spit, with the remainder then representing the Spit Weed Survey Area.

Grid Sampling Design Tool

The following ArcPro 1.4.1 model was used to create the grid and random survey sites. This model can be found within the Midway Invasive Plant Management geodatabase which resides in the Midway GIS folder.