Inventory and Survey Methods for Nonindigenous Plant Species (PDF)

Chapter 4Digital Aerial Sketch-Mapping for EarlyDetection and MappingJason W. Karl and Mark PorterIntroductionAn ideal technique for conducting cost-effective andtimely nonindigenous species (NIS) inventories/surveysin large, rugged landscapes would allow detectionof multiple species at a time, would not be overlysensitive to environmental conditions, and could bereadily performed by most experienced weed managerswithout extensive training. Digital aerial sketch-mapping(DASM) was pioneered by the USDA Forest Serviceto detect infestations of forest insects and pathogens(Morris 2001). Using DASM, a trained observer inan aircraft draws locations of infected stands on atouch-sensitive, digital, moving-map computer display(McConnell et al. 2000), downloads the informationimmediately upon landing, and gives it to land managersfor treatment of infected areas. Here we proposea standard protocol for conducting DASM for nonindigenousplant species, review technical requirements forconducting DASM, and discuss practical considerationsaffecting the efficacy of DASM as an inventory/surveytechnique.Survey ProtocolProtocols for digital aerial sketch-mapping ofnonindigenous plant species (NIS) were derived fromthose used in mapping forest pathogens (Pywell et al. 2001;B.C. Ministry of Forests 2000) and surveying big gamepopulations (Unsworth et al. 1999), with modificationsspecific to plants. Since survey protocols aid researchers incontrolling for factors that could confound results, deviatingfrom the protocols can seriously affect the quality of datacollected. Once an inventory/survey protocol has been set,such factors as the number of observers, type of aircraftused, flight pattern, and survey subunits should not bechanged during successive flights (Unsworth et al. 1999).Simply put, DASM is a method for rapidly identifying andrecording NIS infestations over large areas. Hence, DASMcan be employed within the context of either a full inventoryor a (biased or unbiased) survey. Due to flight costs,however, DASM is most commonly used in a biased surveyapproach where areas with a high likelihood of target NISare prioritized for searching, and lower priority areas areeither not searched or are searched with less effort.Selecting Targets and Timing theInventory/SurveyFlights should be designed around one or a few NIS toaid delineation of survey areas and timing. It is importantto understand the life history and habitat associations oftarget NIS so that the detection reliability can be judged andthe appropriate time of year for conducting surveys can beidentified.Aerial mapping should be timed to coincide with thephenological stage of the target species that is most visible tothe human observer. Many plants have certain phenologicalstages that are more distinct than others (see Lass andCallihan 1997), while other species may be difficult to detectregardless of their growth stage. Likewise, the minimumpatch size (i.e., survey resolution) that can be reliably identifiedby observers may also vary by species, although it canbe assumed that smaller patches and individual plants willbe more difficult to detect than larger patches. Almost anyplant species or any size patch is detectable if the observercan get close enough to it. However, the closer to the groundand slower the flight required to identify target species(i.e., to obtain finer resolution), the less cost efficient DASMsurveys become because less area per unit time is covered.One advantage of DASM over other techniques forcovering large areas is that the observer is able to detectoccurrences of nontarget NIS over the course of the survey.It is important, however, to recognize that detections ofnontarget NIS during DASM may not be reliable indicatorsof their actual distribution within the study area for severalreasons. First, detection rates of NIS patches may decreasewith patch size (i.e., it is harder to see small patches than tosee large ones). Second, flights designed around maximumvisibility of one target NIS may not be optimal for nontargetspecies. In a trial DASM flight for hoary cress (Cardariadraba) conducted in Hells Canyon, Idaho, on May 17, 2004,we also detected 26 patches of Dalmatian toadflax (Linariadalmatica). A subsequent flight over the same area targetingDalmatian toadflax in full bloom on June 14, 2004, found a350% increase of Dalmatian toadflax (91 patches) detected.Despite the potential limitations of mapping nontarget NIS,such incidental observations can be quite useful and shouldbe recorded when observed.SubunitsA DASM inventory/survey area may be divided intosubunits, making it easier to track progress during the flight,quantify effective area sampled, and estimate confidenceintervals of detection rates and population sizes. Properlydefined subunits help focus the sampling and increase itsefficiency. Subunits can be any size, but Unsworth et al.33Inventory and Survey Methods for Nonindigenous Plant Species