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Title: Population dynamics and energetics of Trochosa terricola (Thorell) (Araneae : Lycosidae).
Authors: Workman, Christopher.
First Published: 1975
Award date: 1975
Abstract: An East Anglian Breckland grass heath was selected for field studies of lycosid spiders over the period, November 1972 to August 1975. A floristic analysis of the area showed differences between rabbit grazed and non-grazed (tussock) areas. A preliminary analysis of the spider fauna of four sites using pitfall traps and heat extraction techniques was under-taken. Similarity between the sites was assessed with Sorensen and Mountford indices, and Kendall's rank correlation coefficient. Four sampling methods were used to assess the population distribution, density and activity of Trochosa terricola (Thorell). Heat extraction methods were found to be the most efficient for T. terricola and were used from April 1973 to August 1975. Carapace width was used to differentiate the instars of T. terricola but only the first three instars could be assessed accurately from field population data. The population data were divided into 8 (male) or 9 (female) instars from laboratory rearing data combined with the polymodal carapace width field distribution. The number of instars in the field varied with feeding condition. The life cycle of T. terricola was described from sequential sampling data during the study period, different cohorts being separated in the population. Individuals emerged from the egg sac in July and overwintered in the third or fourth instar. This life cycle was similar to that found for T. terricola in central Europe and in Finland. The activity periods of male Alopecosa accentuata (Latreille) and A. cuneata (Clerck) were similar to those found in southern England and central Europe. A. accentuata and T. terricola had similar, diplochrone activity periods, but A. cuneata exhibited a stenochrone pattern. T. terricola juveniles were diurnally active whereas subadults and mature individuals were nocturnal. T. terricola females did not sun their egg sacs unlike other lycosid species. Sward structure was assessed subjectively on the basis of height, age, density, moisture and litter. The population density of T. terricola ranged from 14.8 m-2 to 75.5 m-2. Density was generally high during summer and autumn, after breeding, but low in spring. Population biomass was greatest during summer and autumn and smallest in spring. The changes were associated with reproduction and growth of each cohort. Cannibalism may have been the major population control factor. The survivorship curve varied between type I and II in different years depending on cohort recruitment, small for the former and large for the latter type. The system could measure rates under fluctuating temperature conditons. Data of individual respiration at constant temperatures were obtained from the electrolytic, Gilson and Cartesian Diver respirometers. An increase in metabolic rate was recorded during moulting. A decrease in metabolic rate to approximately 50% of the fed condition was measured after feeding. The decrease was dependent on the quantity of food eaten during the meal. A decline in metabolic rate was found in juveniles at night whereas adult males showed an increase at night at both 10 and 15°C. The respiration rate of individual T. terricola increased logarithmically with weight. A significant difference in respiration rate was found at 25°C between individuals from con- and diurnally fluctuating culture temperatures. The metabolic rate of eggs in the egg sac increased with age reaching a peak at emergence. Linear live-dry weight regression analyses were calculated for laboratory reared and field collected individuals. A stepwise effect of individual growth was apparent and equations for single instars or instar groups were used in the quantitative feeding study. T. terricola did not actively hunt for food but responded to prey movement. Under unlimited prey conditions peaks of feeding were found for the first 50% of each instar and after the brood left the female's abdomen. The actual feeding pattern was dependent on food availability and a threshold quantity of food was necessary for completion of each instar. The time taken to reach maturity was similar under constant and fluctuating temperature conditions. The consumption efficiency increased with size of spider. A decrease in efficiency with age was depicted but variations occurred dependent upon time spent in each instar. The energy extracted from the food was greater than that predicted by weight. A mean individual energy budget for T. terricola from egg sac emergence to maturity was developed for males and females. Female T. terricola had a greater growth rate and increased consumption than the male. Laboratory growth rates represented those in the field when account was taken of temperature differences. Life cycles differed from year to year depending on the annual temperature pattern. Annual population production was calculated by a graphical method and from the relationship of population respiratory loss to population production by the Engelmann line for both study years. (Abstract shortened by UMI.).
Rights: Copyright © the author. All rights reserved.
Appears in Collections:Theses, College of Medicine, Biological Sciences and Psychology
Leicester Theses

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