Ecological Energetics of the Spider Tarentula kochi (Araneae: Lycosidae)1

Abstract

Tarentula kochi Keyserling had a 2-year life cycle and 2 populations which matured in alternate years and utilized 2 size classes of prey. The spiders had 12 instars, and each was 35–40 days long in the field at a temperature of 15°C. The molting of spiders from one instar to the next was not synchronized with other spiders of the same instar in the field. Spiders overwintered twice (6th instar and adult). The spiders matured in the fall in response to shorter day length. The shortage of food in the field probably limited growth. The duration of an instar was decreased 1.65 days per degree between 12 and 30°C and decreased with increasing feeding rate. The rate of development in the field was increased by higher microclimate temperatures and reduced by lower feeding rates. The surface temperatures of the litter were a mean 1–4°C warmer than air temperature with maximum difference of 23°C. The temperature about 1 inch below the surface of the litter was 4°C lower than surface temperature in less shaded areas and 8°C lower in more shaded areas. There was a direct correlation between the amount of litter and spider density. Spider activity was greatest during the coolest part of the daylight hours. The ecological “zero” was about 10°C, since spiders could not molt at this temperature. The population densities of eggs, 7th instar, and male spiders were estimated at 72,000, 4914, and 1170 per 1.5 acre, respectively, and mortality approximated a logarithmic decrease. The sex ratio was 1:1 T. kochi occupied 45% of the 1.5-acre study area, and all stages were distributed throughout the area occupied. The adult males and 7th-instar spiders traveled 12 and 3 ft per day respectively. Females produced a mean 48 mg of eggs and this fact accounted for the increase in weight of the females between November and April and the increase in weight of the females over the males. The individual efficiency (wt gain/ingestion) and ecological efficiency (wt gain/prey captured) were 32.5 and 19.1%, respectively. The young spiders had a smaller capacity for ingestion, required more frequent meals, and had a lower chance of survival than larger spiders. The mortality balanced the increased ingestion with age and seemed to be optimal for a population living on a fixed level of available prey biomass. The population energy budget indicated that the biomass of T. kochi dying equaled only 20% of the biomass of the prey captured by T. kochi . Cannibalism could have been the only mortality factor. Cannibalism would allow a population to adjust its density to available prey and maintain itself during periods of low prey density, and would shunt the biomass of small prey captured by the 1st-year population into the 2nd-year population, where it would contribute to reproduction.