Inventory and Survey Methods for Nonindigenous Plant Species (PDF)

Chapter 5 • Tiered Sampling Method: Nevada’s ProtocolTier IIThe Tier II method is a stratified random survey ofareas that are not associated with disturbances, but maypotentially be infested with NIS. For example, areas notnecessarily considered impacted by disturbances constitutehuge geographic regions of Nevada; therefore, it is notfeasible to inventory such areas, and representative areascan only be spot checked or surveyed. Because sites forTier II data collection are selected using a stratified randomprocess, they are representative of the whole area; thus, iffew NIS patches are found it will not be necessary to samplemore of the area. Furthermore, collecting data in this waymeans that predictive maps can be made of the whole area(see Chapter 6).1. Random areas to survey are selected from 1:24,000 U.S.Geological Survey quadrangle maps where disturbanceshave not previously been found.2. The selected area should be stratified either by elevationor plant community, but not both. For example, randompoints should be selected either all within the sameelevation range on the map, or within a specific type ofplant community, such as a Wyoming sagebrush plantcommunity or a salt desert shrub plant community. Thisstratified random sampling method helps determinewhere NIS populations not associated with disturbancesare most likely to occur, and therefore where furthersurvey efforts should be concentrated. Such infestationsmight be occurring from random seed dispersed fromwildlife or from other unpredicted events.3. A representative number of sites to field check within thestratified area should be randomly selected.4. The stratified areas are then checked intensively for NISinfestations, according to the inventory protocol outlinedunder Tier I. As above, take care that the method of travelenables the surveyor to find any NIS populations alongthe route, so that survey results reflect the route traveledand approximate sight distances.5. These data are collected with the same GPS equipmentand database library as used in the Tier I method, and theinformation is handled in the same way.Tier IIIThe Tier III method was established to randomly checkat least 5% of all areas (both disturbed and undisturbed)previously sampled, and using the data stored in a database,to establish the accuracy of Tier I and Tier II efforts. Toincrease the confidence that most NIS populations werefound, the number of random checks can be increasedand accuracy assessed in more Tier I inventories or Tier IIsurveys. To accomplish Tier III, previously sampled areas arerandomly selected. Different observers then follow the sameroutes (transects, etc.) as followed by the initial crew, andrecord any NIS populations found. If the initial crew misseda population, then that crew receives additional training tolessen the chance of future errors (see sidebar). We have nottried to quantify the error rate or the decrease in error ratefrom this additional training.SafetyAn important consideration when conducting operationsin very remote areas such as rural Nevada is the safety andefficiency of personnel. Crews must be adequately trainedand supervised, and be provided with necessary equipment.Be sure that all members of the team can quickly and accuratelyidentify all growth stages of the NIS of interest, andthat they have a comprehensive knowledge of inventory/surveyprocedures, GPS operation, and database management.Adequate training takes about a week and a half, but thattime is quickly recouped in the speed and accuracy of theinventory/survey. (See training sidebar.)For safety, we prefer to work in crews of four, with crewmembers who are familiar with the area and are capableof handling themselves in adverse situations in remoteareas. In vehicles, travel in pairs is preferred; on foot, crewmembers should keep within reasonable distance of eachother. Vehicles should stay within reasonable distance, andwithin radio contact, of each other. One vehicle should bewithin radio contact of the base station as well.Advantages and Disadvantages of theTiered Sampling MethodThe Nevada methodology outlined here is applicable whenused to cover large acreages which have limited amounts ofdisturbances with a relatively high degree of reliability andconfidence that most of the NIS populations will be located.It would not work well in a more urban environment withsignificant percentages of total land area disturbed bydevelopment and construction, because its efficiency comesfrom the limited amount of disturbance relative to the totalarea. If the majority of the land has some form of anthropogenicdisturbance a different inventory/survey method mightbe more applicable.Managers within Nevada want to decrease the applicationof Tiers II and III, but to do so significantly reduces thechecks of the sampling methodology, thereby reducing theconfidence that most NIS populations have been or will belocated. Currently, when all three tiers are used together,this method locates more than 90% of noxious weeds in theareas searched in Nevada. However, this is a point-in-time47Inventory and Survey Methods for Nonindigenous Plant Species