Document Type

Dissertation - Open Access

Award Date


Degree Name

Doctor of Philosophy (PhD)

Department / School

Plant Pathology

First Advisor

W.S. Gardner

Second Advisor

Gerald Thorne


In an attempt to elucidate the role of nematodes in a grassland ecosystem nematode taxa, number and biomass data was gathered over a three year period at the Cottonwood International Biological Program Grassland Biome site in western South Dakota. The effects of grazing intensity, sampling date and sampling depth were studied. Nematicide treatment of range grasses in field and greenhouse studies was also used to evaluate the role of nematodes. Biomass data was used to estimate the intake of plant feeding nematodes. Results obtained demonstrate that soil inhabiting nematodes constitute a significant proportion of the consumer biomass at the Cotton[1]wood site. Biomass of plant feeding forms was significantly greater in the ungrazed treatment due mainly to the high numbers of dagger nematode, Xiphinema americanum. Biomass of predacious forms was also greater in the ungrazed treatment and overall nearly equaled that of the plant feeding forms, thus indicating their potential as agents of biological control. Biomass of saprophagous forms was considerably less than that of the other trophic levels and also showed little treatment response. An inverse relationship between numbers of Tylenchorhynchus spp. and Helicotylenchus spp. was noted in the grazed treatment. Tylenchorhynchus spp. appeared nearly limited to the upper 10 cm of soil with Helicotylenchus spp. predominating with increasing depth, indicating a possible antagonistic relationship between these taxa. The diversity of taxa was found to decrease with increasing sampling depth, a response attributed to a decrease in variety of food sources. Total number of nematodes also decreased with increasing sampling depth and approximately 70% of the nematodes occurred above 20 cm. Nematicide treatment of range grasses in field and greenhouse studies significantly reduced nematode populations, increased above ground herbage weight, and further, provided a demonstration of the importance of nematodes as controllers of productivity in range. A formula was used to estimate nematode intake at the Cottonwood site and, surprisingly, plant feeding nematodes were found to consume more range grass than cattle. In addition, comparisons of nematode intake with that of several other consumer populations indicated that nematodes are major consumers at the Cottonwood site. The large biomass, high metabolic rate and indigenous nature of the nematode populations were suggested as probable reasons for the high intake. Overall, it is apparent that soil inhabiting nematodes constitute a significant pathway of energy flow in a grassland ecosystem. They also are probably responsible for a significant proportion of the belowground nutrient recycling.

Library of Congress Subject Headings

Nematode diseases of plants
Grasses -- South Dakota




South Dakota State University