Jeff Holmquist


Effects of pack stock grazing on subalpine meadows and streams:

Funding: National Park Service J8R07080005, J8C07100004, P12AC15057, P12AC15021 and P12AC15022
examples of meadow fauna

Comparisons of persisting versus annual effects of a given long-term disturbance are less common than might be expected.  Grazing management provides a good laboratory for such studies, because of detailed, long-term stock use records and the presence of de facto long-term exclosures. Pack stock grazing in the subalpine wetlands of Sequoia, Kings Canyon, and Yosemite National Parks, in the Sierra Nevada mountains of California, presents an interesting disturbance regime, in that there are short pulses of disturbance, followed by long periods of winter recovery that persist for about nine months while these wet meadows lie under several meters of snow. The primary objective of this component of our work was to compare long-term and annual effects of pack stock grazing on arthropods and vegetation, and this disturbance regime allowed us to establish a long-term and large-scale experiment designed to accomplish this goal.  We sampled after meadow greenup, but just before stock grazing, and also after wetlands had been grazed by stock for most of the growing season, but just before vegetation senescence.  This design, using paired ungrazed and grazed wetlands, allows us to separate long-term effects, that carry over through many winter recovery periods, from short-term, current-year effects that might be caused by stock during individual growing/grazing seasons. (S Haultain photo)

Grazing management necessarily emphasizes the most spatially extensive vegetation assemblages, but landscapes are mosaics, often with more mesic vegetation types embedded within a matrix of dryer vegetation. We are thus also contrasting effects of equine grazing, on both subalpine vegetation structure and associated arthropods, in a dryer reed grass dominated habitat versus a wetter, more productive, sedge habitat.

(J Schmidt-Gengenbach photo)

This work effectively addresses effects at the landscape scale but has the potential to underestimate effects at the patch scale. We are comparing heavily-grazed patches with areas of lesser use within individual wetlands—and with reference wetlands-- in order to detect maximum levels of impact at smaller scales.

Stock cross streams throughout the Sierra, and an additional component of our work is an investigation of potential effects on streams via nutrient input and sedimentation.  We are sampling above and below replicate stream fords in early and late season to determine effects on stream macroinvertebrate assemblages, algal characteristics, and detrital deposition.

Influence of invasive palms on terrestrial arthropod assemblages in desert spring habitat

Funding: National Park Service J8R07090004

larger scale view of spring habitat with native shrubs and live and dead invasive palms.

Invasive plants can eliminate native flora and ultimately have negative indirect effects on fauna and the functional ecology of ecosystems, but understanding of these cascading effects on arthropod assemblages is poor.  Desert spring habitats are small, isolated landscape elements that are literal oases for flora and fauna and support high diversity assemblages; invasive palms can colonize desert springs and form monocultures. In an effort to understand effects of these invasive trees on higher terrestrial trophic levels at springs, we are contrasting assemblage structure of terrestrial arthropods in native vegetation versus invasive palm habitat.  We are vacuum sampling arthropods in paired palm and native habitat at replicate springs in Death Valley National Park, California, across multiple growing seasons, both before and after Park Service palm removal efforts.  (J Schmidt-Gengenbach photo)

Efficacy of low and high complexity restoration treatments for reestablishing assemblage structure of wetland arthropods in a montane ecosystem

Funding: National Park Service J8C07100023 and P12AC10901

The ultimate goal of restoration is re-creation of ecological function, versus achieving a targeted plant density or general appearance. Restoration inherently alters the meso-landscape, and the nature of these changes to landscape configuration may have important influences on faunal reassembly. Habitat heterogeneity (compositional differences among patches) and complexity (density of structural components), shape invertebrate assemblage structure and may be especially important during restoration. Increasing connectivity via reduced patch boundary contrast or provision of corridors might be expected to increase recolonization of restored habitats. Our objectives were to: 1) assess recovery rate of the arthropod assemblage to wetland restoration in the harsh montane environment of Sequoia National Park, 2) assess faunal reassembly in dispersed plug plantings versus an additional treatment that incorporated additional heterogeneity, complexity, and connectance to source habitats.

Click here for larger scale view of one portion of study area

Role of dam discharge characteristics in structuring aquatic invertebrate assemblages along a montane river corridor

Funding: National Park Service J8R07070007, J8C07110016, and P12AC10902

(J Schmidt-Gengenbach photo)

We have been able to use a series of experimental releases from the Hetch Hetchy reservoir in Yosemite National Park to investigate effects of spring floods on macroinvertebrates in both the river and nearby lentic habitats in the floodplain. Faunal samples are being collected at a number of locations before floods, immediately after floods, and over the ensuing months of each year in an effort to understand the complex ways in which fauna respond to these events. We have been tracking changes in faunal assemblage structure of riffle habitats, stream drift (both aquatic and terrestrial components), and in ephemeral ponds, which are inundated for as little as a few days or as long as five months. We are also sampling terrestrial insects, with both terrestrial and aquatic origins, in an effort to understand insect interactions with bird and bat predators.

Click here for overview of river corridor and sampled habitats

Throw trap toss video

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