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Nested Intensity Monitoring: Tracking Rare Plant Populations at Multiple Levels Improves Efficiency

Contributed by Thomas N. Kaye, Executive Director and Senior Ecologist at the Institute for Applied Ecology, Corvallis, Oregon

The need for monitoring

The number of rare plant species is daunting. In the United States alone, 942 plant species are listed by the US Fish and Wildlife Service as Threatened or Endangered (USFWS 2017), and globally the number of plant taxa categorized as near threatened or worse exceeds 30,000 (IUCN 2017). Rare species may occur as one remaining population with few individuals or as many populations with thousands of individuals, and they face a wide range of causes for their imperilment. Threats to rare species include habitat loss, invasive species interactions, changes in land use or land management, overexploitation, and other pressures (Oostermeijer 2003, Wilcove et al. 2009). Rare plants may occur on the landscape as wild or reintroduced populations, and in remnant or restored habitat, or all combinations. Understanding their population trends is necessary to making management decisions about individual populations, often in the context of the species’ overall range or abundance. Population monitoring can be a useful tool for gathering the data necessary to support population management, but resources for monitoring rare plant populations are often limited. Therefore, improving monitoring efficiency to meet overall information needs is crucial.

Some of the typical objectives for rare plant monitoring are to:

  • Document the number and locations of populations in a given area (e.g., entire range, sub-region, selected ownership, etc.)
  • Track population trends of a species
  • Provide information to managers that informs them when management actions are needed (e.g., populations drop below a previously identified threshold, populations are in decline, or specific threats are present)
  • Document progress toward endangered species recovery goals
  • Evaluate effectiveness of management treatments
  • Understand the basic biology of a species and its life history
  • Provide data for a population viability analysis, climate change model, or other data-intensive analysis

A common challenge is to develop a monitoring strategy that meets the needs of managers and researchers while keeping costs within available resources. My goal here is to describe a comprehensive approach to rare plant monitoring, discuss some pros and cons, and provide an example of the approach in action.

Nested intensity monitoring

Nested intensity monitoring is one approach to tackling data collection for species with numerous occurrences to provide information on total number of populations, threats, population trends, as well as sophisticated information for documenting and modeling population dynamics. Since rare plant monitoring can be costly and time consuming, it is reasonable to scale monitoring according to the resources available and the monitoring objectives. Nested intensity monitoring uses a combination of low, medium and high intensity monitoring to collect multiple types of data, at a variety of spatial scales. The combination of data collected using this method reduces overall monitoring costs while simultaneously delivering a fairly robust result. The idea behind nested intensity monitoring is to first collect low intensity information from all or many populations of a species; then to collect moderate intensity data for population estimates from a subset of those populations; and, finally, to collect high intensity, detailed demographic data from a still smaller subset of populations (Figure 1). This hierarchical approach has been used successfully to evaluate population trends and management needs of multiple species (e.g., Menges and Dolan 1996, Philippi et al. 2001). 

Figure 1.  Nested intensity monitoring samples all or many populations at low intensity (e.g., site visits), a subset of those at moderate intensity (e.g., population size estimates), and a further subset of those at high intensity (e.g., demographic sampling). 

Low intensity monitoring may take the form of a site visit conducted by a biologist or technician to document the presence/absence of a population or a rough estimate of population size (e.g. classifying estimates into categories like 0 individuals, 1-10, 11-100, 101-1000, etc.), and to describe the habitat and any existing population threats (Table 1). Site visits may be conducted at many or all of the known populations, or at sites in a particular geographic region or ownership, and at a low frequency such as every three to five years.

Moderate intensity monitoring can be a formal population estimate derived from randomly placed plots with extrapolation to the entire population area, or a population census. Protocols for population size estimation are well developed and widely available (e.g. Elzinga et al. 1998, 2009). Population estimates can be conducted at randomly selected sites or locations prioritized for sampling based on ownership, geographic area, or other criteria, at a frequency of every one to three years.

High intensity monitoring can emphasize demographic sampling where permanent plots are established to track individual plants and recruitment. This method may be implemented annually at only a few randomly selected sites, or sites of specific populations, such as those of a size higher than a particular threshold, in a specific habitat type or condition, experiencing a management treatment of interest, or suitable for a specific research question.

Pros and cons

The levels of monitoring intensity and the monitoring methods proposed here have their pros and cons (Table 2). For example, low intensity site visits are low cost (relatively little training required, and fast – possibly only minutes at each site) and provide a coarse count of extant populations and their relative sizes, locations and conditions. Downsides to this approach are that the resulting data are low resolution, with potentially very low accuracy and no estimate of uncertainty. Moderate intensity monitoring based on formal population estimates can be conducted at a moderate cost and provide accurate, precise estimates of population size and long-term trends, with low variation among observers. On the other hand, it requires a larger time investment (typically one day or less per site) than site visits and may or may not detect juvenile plant recruitment (unless documenting seedlings is part of the protocol). Demographic sampling used for high intensity monitoring can provide all of the benefits of a formal population estimate, plus basic life-history information on the target species as well as quantitative measures of vital rates such as seedling recruitment, and size- or age-specific mortality. But demographic monitoring is like adopting a puppy – it requires a commitment to frequent attention, care, and follow through, with the associated costs and training.

Table 2.  Pros and cons of each level of monitoring intensity.

An example:  Cook’s lomatium

A real-world example of nested intensity monitoring comes from a rare plant, Cook’s lomatium (Lomatium cookii), of southwestern Oregon (Figure 2a). This species is listed by the US Fish and Wildlife Service as Threatened, and is managed as a Sensitive Species by the Bureau of Land Management (BLM). The species has two population centers, one in the Rogue River Basin, and the other in the Illinois River Valley. 

Figure 2. Cook’s lomatium (Lomatium cookii), a threatened plant of southwestern Oregon (a). Sampling high intensity demographic plots for Cook’s desert parsley at the French Flat Area of Critical Environmental Concern in southwestern Oregon (b) (Photos: Tom Kaye).


BLM manages several of the 36 populations of Cook’s lomatium that are scattered across public and private lands in the Illinois River Valley. BLM conducts low intensity monitoring via site visits to locations on public lands as staff are available, resulting in a re-visit frequency of about three to five years. Sites on private land are revisited only in cases where strong, positive relationships are established with private land owners who provide access voluntarily. Information from site visits has provided basic information to support the development of a Recovery Plan (USFWS 2009), as well as location information for seed collection to initiate a reintroduction program for the species. 

At the moderate intensity level, BLM staff visit seven of those sites annually to provide estimates of population size. These sites represent the largest populations on BLM land and are considered the most significant for species recovery. Four of these sites also have demographic plots established to collect data on stage-specific mortality, growth, and reproduction. This data is used in population viability models, which provide estimates of extinction probability. Sites were selected for high intensity monitoring (Figure 2b) based on potential threats from mining, large population size, or involvement in species reintroduction trials. The objectives of the moderate and high intensity monitoring are to document population trends and provide management recommendations, inform reintroduction techniques, and assess the potential effects of climate change on the species.

Moderate intensity monitoring subsamples the Cook’s lomatium populations with randomly placed long, narrow permanent plots that are 10-cm wide and up to 40-m long to estimate plant density, overall population size, and track the population’s perimeter. Much smaller (0.5 x 0.5 m) demographic plots are randomly located along the density plots (Figure 3) for high intensity monitoring at selected sites. This multistage approach (Philippi et al. 2001) combines monitoring intensities into a nested set of sampling plots.

Figure 3. Locations of 36 populations of Cook’s lomatium in the Illinois Valley of southwestern Oregon.  Most locations receive site visits every 3-5 years, and seven of those receive moderate intensity monitoring through annual population estimation or census.  A subset of three of those sites also receives high intensity monitoring in the form of annual demographic sampling. 


Moderate intensity monitoring has also been used to identify and address threats at four locations, including off-road vehicle damage and trash dumping, as well as shrub and tree encroachment into grassland habitats. Demographic sampling at two sites, French Flat South and French Flat Middle, has documented population declines and low population viability (Pfingsten et al. 2016) due to low seedling recruitment, which motivated managers to prescribe field burning in 2015 as a method of improving conditions for plant establishment from seed (Figure 4)In addition, demographic monitoring has been used to track the performance of reintroduced patches at four sites (Kaye et al. 2016), and the effects of climate change on this species are currently being evaluated with integral projection models.

Figure 4.  Combined sampling for medium intensity (density plots) and high intensity (demographic plots) for Cook’s lomatium at French Flat South in the French Flat Area of Critical Environmental Concern in southwestern Oregon .  


Nested intensity monitoring is intended to focus resources where they are most needed to address specific objectives. This approach will not fit all species and situations. For example, where the remaining populations of a species are very few and small, managers may choose to use only high intensity monitoring. If there is no need for carefully collected demographic data (i.e., for a PVA or assessment of forecasted climate change impact), managers may wish to skip high intensity monitoring altogether. But in cases where monitoring objectives require information from multiple scales – across multiple populations and within single populations – nested intensity monitoring can provide a balance between high-cost techniques and rapid, low cost site visits with defensible results at all scales. Monitoring across multiple scales in this way can identify when populations need management intervention – including habitat restoration – and inform reintroduction techniques.


Many thanks to the Medford BLM, particularly Rachel Showalter, Bryan Wender, and Mark Mousseaux for their support of population monitoring for Cook’s lomatium, as well as staff at the Institute for Applied Ecology, especially Denise Giles.

Literature cited

 Elzinga, C.L., D.W. Salzer, and J.W. Willoughby. 1998. Measuring and monitoring plant populations. BLM Technical Reference 1730-1.

Elzinga, C.L., D.W. Salzer, J.W. Willoughby, and J.P. Gibbs. 2009. Monitoring plant and animal populations: a handbook for field biologists. John Wiley & Sons.

IUCN. 2017. IUCN RED list of threatened species. Information accessed online 5 August 2017.

Kaye, T.N., I.A. Pfingsten, D.E.L. Giles, and I.S. Silvernail. 2016. Developing reintroduction techniques for Lomatium cookii. Institute for Applied Ecology, Corvallis, Oregon and USDI Bureau of Land Management, Medford District.

Menges, E.S., and D.R. Gordon. 1996. Three levels of monitoring intensity for rare plant species. Natural Areas Journal 16:227-237.

Oostermeijer, J. G. B. 2003. Threats to rare plant persistence. Pages 17-58 in C.A. Brigham, and M.W. Schwartz (eds). Population Viability in Plants: Conservation, Management, and Modeling of Rare Plants. Springer, Berlin.

Pfingsten, I.A., D.E.L. Giles, and T.N. Kaye. 2016. Lomatium cookii population monitoring in the Illinois Valley, Josephine County, Oregon. Institute for Applied Ecology, Corvallis, Oregon and USDI Bureau of Land Management, Medford District.

Philippi, T., B. Collins, S. Guisti, and P.M. Dixon. 2001. A multistage approach to population monitoring for rare plant populations. Natural Areas Journal 21:111-116.

[USFWS] US Fish & Wildlife Service. 2006. Draft Recovery Plan for Listed Species of the Rogue Valley Vernal Pool and Illinois Valley Wet Meadow Ecosystems. Region 1, Portland, Oregon.

[USFWS] US Fish & Wildlife Service. 2017. Endangered Species. Information accessed online 5 August 2017.

Wilcove, D. S., D. Rothstein, J. Dubow, A. Phillips, and E. Losos. 1998. Quantifying threats to imperiled species in the United States. BioScience 48:607-615.

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