Energy and Nutrient Dynamics of Spider and Orthopteran Populations in a Grassland Ecosystem

Population dynamics, energy budgets, and nutrient concentrations were used to develop a compartment model for evaluating energy flow and nutrient fluxes in the spider and orthopteran components of an eastern Tennessee grassland ecosystem. The arthropod community consisted of; herbivores–Melanoplus sanguinipes (Acrididae), Conocephalus fasciatus (Tettigoniidae), and several of the Homoptera—Hemiptera; omnivore–Pteronemobius fasciatus (Gryllidae); and predators–Lycosa spp. (Lycosidae). Weekly estimates of arthropod density and biomass and monthly estimates of vegetation biomass were made during the 1965—69 growing season. Supporting studies included: (1) determination of caloric equivalents; (2) measurements of arthropod metabolism; (3) determination of whole—body concentrations of Na, Ca, and K; (4) estimation of biological turnover rates of these nutrients with radioactive analogues; and (5) field studies of arthropod feeding habits and food consumption. Total net primary production of the grassland ecosystem was 1,274 kcal/m²; 89% of this total was by grass species (Festuca arundinacea and Andropogon virginicus) and 11% by green forbs. Herbivores and omnivores consumed 9.6% (122.9 kcal/m²) of net primary production. Annual net secondary production by the insect community was 32.05 kcal/m². Total net tertiary production by spiders was 2.26 kcal/m². Total annual energy flow (= total assimilation) through the community was 75.6 kcal/m². Herbivores accounted for 79.8% of this total, omnivores 12.2%, and predators 8.1%. Whole—body concentrations of sodium increased with higher trophic position in the food chain: vegetation (0.43 mg/g ash free dry wt = AFDW), prey (1.18 mg/g AFDW), and predators (1.94 mg/g AFDW). Calcium concentrations decreased from vegetation (5.82 mg/g AFDW) to prey (0.85 mg/g AFDW) and remained constant to predators (0.88 mg/g AFDW). Potassium concentrations were much higher in vegetation (14.28 mg/g AFDW) than in either the prey (1.87 mg/g AFDW) or predators (1.96 mg/g AFDW), which were essentially the same. Sodium and potassium behaved similarly in their movement through arthropod food chains. The low concentrations of calcium in arthropod trophic levels and the rapid elimination of this element from each trophic level suggest that calcium is not as limiting as are sodium and potassium in this arthropod community. Herbivores, the dominant members of the arthropod fauna, attained a maximum standing crop of 927 mg/m² (Melanoplus = 705 mg/m², Conocephalus = 200 mg/m², and the Homoptera—Hemiptera = 22 mg/m²). This trophic level was responsible for 85% of the sodium turnover, 76% of the calcium turnover, and 78% of the potassium turnover by the arthropod component of the ecosystem. The omnivore Pteronemobius fed equally well on fresh vegetation and on litter. It reached a maximum biomass of 194 mg/m² and utilized 10% of the sodium, 22% of the calcium, and 20% of the potassium which passed through the arthropod community. The predator (Lycosa spp.) biomass reached a maximum of 146 mg/m² in late fall and was responsible for 5% of the sodium, 2% of the calcium, and 2% of the potassium utilized by the arthropod component of the ecosystem. Wolf spiders consumed 21% of total net secondary production. Absence of other predator species in collections from the arthropod community suggests that Lycosa was the dominant invertebrate predator in this grassland ecosystem.