Trends in counts of Slickspot Peppergrass (Lepidium papilliferum) on Habitat Integrity and Population Monitoring transects, 2005 - 2015

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Slickspot peppergrass (Lepidium papilliferum, LEPA) is a small white-flowered forb endemic to southwest Idaho and recently classified as threatened by the US Fish and Wildlife Service (USFWS)1. This plant is found in slickspots, areas of varying size with fine soil and salts that maintain a habitat quite different from the soil and vegetation that surround them. In an effort to better understand the biology and status of the plant over time, the Idaho Natural Heritage Program, in the Idaho Department of Fish and Game, has been monitoring the plant on transects throughout southwest Idaho annually since 2004. The annual monitoring effort, termed “Habitat Integrity and Population (HIP) Monitoring”, collects counts of LEPA on the slickspots, in addition to several slickspot attributes that may be associated with the robustness of the plant’s population. <br>

This study was commissioned by USFWS to address specific questions related to the HIP transect data:
1) To understand whether the plant’s population was increasing, decreasing, or holding steady over the years of monitoring, and to identify slickspot attributes that might be associated with these trends,
2) To incorporate annual variation in weather and local climate into these population assessments,
3) To evaluate the habitat quality outside of the slickspots, as determined by in-depth plant community and ground cover sampling that occurred prior to 2010. <br>

As it is understood that plant populations generally vary annually with weather, the USFWS wanted to evaluate the existence of short-term weather changes which might independently be associated with population changes in LEPA. Thus the fourth request from USFWS was to identify the presence or absence of short term trends in the weather data used for the analysis of LEPA populations. <br>

To address the first two questions, a generalized linear mixed statistical model was fit to the slickspot peppergrass counts, slickspot attributes, and year of observation to identify annual change, controlling for the slickspot attributes and the seasonal weather. To evaluate the habitat quality, plants species were classified into broad categories and a principle components analysis was used to describe co-occurrence of plant types across the three geographical areas of study. Lastly, a regression model incorporating temporal autocorrelation was used to analyze annual trends in the weather data summaries used in the statistical model of LEPA counts. <br>

Broadly slickspot peppergrass declined region-wide over 10 (2005 – 2014) years of monitoring, with the steepest declines in the Jarbidge region. Accounting for weather, introduced plant species, anthills, soil crust, fire, and livestock, slickspot peppergrass decreased at an average rate of 18% (95% confidence interval of 9%, 26%) per year in the Foothills, 10% (6%, 14%) in the Snake River Plain, and 27% (22%, 32%) in the Jarbidge. Anthills, soil crust, fire livestock and some introduced plant species were all associated with reductions in slickspot peppergrass plant counts, with some plant species associated with an increase in plant counts. <br>

Despite the steeper declines in slickspot peppergrass in the Jarbidge region, the habitat of these transects appears to be healthier in terms of having higher cover of native shrubs and native perennial grasses, and lower cover of introduced annual grasses. The Foothills region is characterized by high cover of introduced annual grasses and low cover of native perennial grasses. The Snake River Plain is intermediate between these two regions. <br>

Several seasonal weather summaries were used in the slickspot peppergrass population models, but only one of them, growing degree days over February and March in Twin Falls, demonstrated a statistically significant increase over the 11 years of monitoring. This analysis is separate and distinct from the analysis of slickspot peppergrass counts. Any connection between increasing degree days per year with decreasing slickspot peppergrass counts is likely spurious; in fact, late winter growing degree days is associated with an increase in slickspot peppergrass counts, based on the statistical model. <br>

The appendices at the end of the report describe the data collected at the slickspots, both over time, and their relative values.

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Reference Code: U17BON01IDUS <br>

Full Citation: Bond L. 2017. Trends in counts of Slickspot Peppergrass (Lepidium papilliferum) on Habitat Integrity and Population Monitoring transects, 2005 - 2015. Boise State University, Boise, Idaho. p.44 plus appendixes <br>

Location: ELECTRONIC FILE - BOTANY: PLANT SPECIES: Lepidium papilliferum <br>

Keywords: trend, Lepidium papilliferum, slickspot peppergrass, LEPA, Habitat Integrity and Population monitoring, HIP, statistical analysis, statistics <br>