Effects of cattle grazing, insect interactions, and population ecology of Snake River goldenweed (Haplopappus rediatus)

Background

Snake River goldenweed (Haplopappus radiatus) is a rare plant of eastern Oregon and adjacent western Idaho. The species is listed as endangered by the State of Oregon Department of Agriculture and the Oregon Natural Heritage Program. It is considered a Sensitive Species by the Bureau of Land Management. The species occurs in arid shrub-steppe rangeland and many populations are on federally managed lands that are subject to livestock grazing. This report is the result of a long-term study intended to document the effects of livestock grazing on Snake River goldenweed and identify other factors affecting populations of the species.

The Study

This study was initiated in 1991 in Baker County, Oregon on lands managed by the Vale District, BLM. Five pairs of study plots were established, one plot fenced to exclude cattle and the other left unfenced. In each year from 1991 through 2001, plants within the study plots were mapped and measured. Plant size and reproduction was compared for grazed and protected plots each year of the study, factors such as seasonal precipitation were examined as factors affecting the species biology, and a population viability analysis utilizing on demographic models was conducted.

Effects of Grazing

Plants exposed to grazing were smaller, flowered less, and had lower population growth rate than protected plants by the end of the study. However, there was a substantial time-lag (six to eight years) between when fences were constructed and when this effect was detected. There were also negative correlations between the frequency of grazing on Snake River goldenweed and plant flowering and population growth. Despite these effects, there was no detected difference in population viability as measured by extinction probability in grazed vs. fenced plots. More sophisticated models that take into account seasonal precipitation and intensity of grazing in any given year may yield different results.

Effects of Seasonal Precipitation

Seasonal precipitation had significant positive effects on Snake River golden weed flowering, seed production, and population growth, but negative effects on seed predation and grasshopper damage.

Seed Predators and Grasshoppers

Seed predation by insect larvae (weevils, moths and midges) was intense in some years, killing at least half of the seeds in four of the ten years sampled. Grasshoppers also had substantial impacts, consuming over 60% of plant foliage in 2 of eleven years sampled and lowering seed set. These levels of seed damage and herbivory may have long-term impacts on this species.

Recommendations

Livestock grazing in populations of Snake River goldenweed should be minimized to allow plants to achieve large size and reproduce well. It is likely that other populations of this species that occur in similar habitats subject to grazing have smaller plants that reproduce less and have lower population growth rates, on average, than populations in non-grazed areas. Efforts to improve conditions for this species should be given long periods of time to be effective. The exclosures established for this study should be left in place for detection of longer-term effects of fencing on this species. To acquire additional long-term information, the study plots should be resampled at 3-5 year intervals.