Article

Monitoring Invasive Plants at Herbert Hoover National Historic Site

Two men walk through a grassland with spraying equipment. — Park workers spray invasive plants with herbicide
NPS Photo

Nonnative Species

The National Park Service’s management policies distinguish between native and exotic (i.e., nonnative) plant species (NPS 2006). Exotic plant species are typically characterized by their introduction due to human actions, whether intentional or not. Invasive plants, following the definition used in Executive Order 13112, are those plants that are exotic and cause ecological or economic harm. Finally, pest plants are defined less by their biology and more by their context in the same way that the term “weed” is defined (NPS 2006). Pest plants, which include native species, interfere with a specific management objective, including protecting human health. We refer to the collection of exotic, invasive, and pest plants as “potentially problematic”.

Management

Park managers, however, are only required to control any problematic plant that leads to “resource impairment.” For plant populations causing effects that fall short of the impairment threshold, park managers wield a high level of discretion in judging whether the population should be controlled or not. The standard for making this decision rests on five criteria: the origin of the species, prudence, feasibility, the harm (i.e., impact) that the plant causes to park resources, and the harm that removal causes (NPS 2006). As with impairment determinations, these decisions are based on professional judgment, environmental assessment, consultation with regulating agencies, evidence-based scholarship, subject matter expertise, and civic engagement with the public (NPS 2006).

Methods

For details on the survey, see Young et al. (2007a).

In 2006, 2009, and 2013, we sought plant species on two watch lists: early detection and park-established.
Observers used handheld GPS units to make three approximately equidistant passes through each search unit.
During each pass, observers estimated plant cover in a belt at least 3 meters wide, but used a larger belt width if conditions all wed. In 2006, the widest belt possible was observed. In 2009 and 2013, this maximum distance was capped at 12 m.
Coarse cover values (0=0, 1=0.1-0.9 m², 2=1-9.9 m², 3=10-49.9 m², 4= 50-99.9 m², 5=100-499.9 m², 6= 500-999.9 m², 7= ,000-4,999 m²) were attributed to each species per search unit.
To calculate the minimum end of the estimated cover range for each species, the lower endpoints associated with the assigned cover class values for that species were summed and then divided by the reference frame fraction observed assuming the widest possible survey belt (i.e., maximum fraction observed).
Maximum cover for each species was calculated similarly, using the upper endpoints of the cover values in each occupied search unit and assuming that a 3 meter belt was surveyed (i.e., minimum fraction of area observed ).
To provide additional information on the ecological impact and feasibility of control, the ecological impact and general management difficulty sub-ranks that constitute the invasiveness rank (I-rank), as determined by NatureServe (Morse et al. 2004), were listed when available.