The results show that Pardosa prativaga is able to respond to short-term nutrient deprivation after the stress treatment is alleviated. After a period of nutritionally stressful treatment P. prativaga responded to the availability of high-quality flies by increasing the growth rate. This compensatory response occurred regardless of the quality of the diet given during the stress period, but the strength of the response varied according to the diet's nutritional quality and the duration of the stress period. Statistically, surviving spiders in Experiments 1 and 2 exhibited weight compensation after 6 weeks. However, the weight curves for starvation, F. candida and R. padi, lie below the curves for N- and E-flies almost throughout the experiment, indicating that growth compensation was actually slightly incomplete. In Experiment 3 all individuals that survived more than 13 weeks exhibited weight compensation. Comparison of weights of animals that died during the first 70 days and those that survived for more than 70 days revealed that small spiders generally had a higher mortality than larger ones.
In many details the responses of the spiders comply well with the compensatory growth model of Broekhuizen et al. (1994) based on fish studies. Thus, the reduction of growth rates was particularly rapid in the initial phase of a stress period. Under nutritional stress animals may experience a considerable decrease in reserves. Spiders respond by reducing their basal metabolic rate during starvation (Anderson 1974), thus reducing the rate of weight loss. Recovery did not start immediately upon introduction of high-quality food but took several days to be initiated. Presumably it takes some time to re-establish a fully functioning metabolism (Broekhuizen et al. 1994). However, after recovery had begun, growth accelerated and remained high until most or all the weight deficit had been regained, after which the previously stressed animals followed the same growth trajectory as the control group.
In Experiment 3 a slower rate of compensation was observed in association with starvation, which corroborates the hypothesis of Nicieza & Metcalfe (1997) that growth reduction during starvation is more severe than that associated with other stress treatments. Since F. candida is toxic to P. prativaga, it was expected to cause greater mortality than starvation, as observed by Toft & Wise (1999), and slower compensation. In Experiment 1, the starved spiders and spiders fed F. candida seemed to compensate equally fast, while in Experiment 3, spiders from the F. candida group compensated faster than the starved spiders. This might be because even food of low quality contains some nutrients (Miyashita 1968b) and because the starved spiders, which survived the stress period in Experiment 3, had a greater size deficit to recover from. A low amount of food or availability of low-quality food (e.g. F. candida or R. padi) may be insufficient for growth and development, but may allow survival for extended periods (Miyashita 1968b; Toft 1995). Thus, spiders fed F. candida did not lose weight during the stress periods, but actually gained a little weight (10–19% of initial weight). In some respects, F. candida is toxic to P. prativaga (E. N. Fisker & S. Toft, unpublished observations) and influences the activity of detoxification enzymes in the spider (Nielsen & Toft 2002). A doubling of the respiration rate was found in P. prativaga fed a single-prey diet of F. candida (Toft & Nielsen 1997).
The spiders fed R. padi all gained weight during the stress periods (1–15% of initial weight), but at a lower rate than the spiders fed fruit flies (107–1772% of initial weight). Shortly after the stress period the spiders fed R. padi accelerated their growth and it remained high until the weight deficit had been recovered after 3–5 weeks. This quick recovery is probably due to the fact that R. padi is only deterrent, but not toxic to P. prativaga in the amounts eaten (Toft 1995). In Experiments 1 and 2 spiders fed R. padi compensated faster than spiders fed F. candida. In Experiment 3 the two groups seemed to compensate equally fast, though statistically F. candida compensated faster than R. padi. This is certainly a sample-size effect since only two spiders from the F. candida group survived throughout the experiment. Spiders fed N-flies for 7 or 14 days had weight curves similar to those of control animals fed E-flies though the spiders fed N-flies for 70 days gained weight at a lower rate during the stress period. In this experiment (Experiment 3) growth accelerated initially after the stress period and remained high until the weight deficit had been recovered.
Early nutritional stress can affect many parameters of development. Total development time is a function of the number of instars and the instar duration (Higgins & Rankin 1996). Significant differences in total development time to maturity were observed only between spiders from the groups of F. candida and R. padi and the control with a stress period of 14 days. In Lepthyphantes tenuis (Blackwall) fed R. padi until the first moult, Beck & Toft (2000) observed that the total development time increased, though only slightly, compared with controls. No increases were observed in the number of instars to maturity with a reduction of nutrients, so the increased time for total development in Experiment 2 must be an increase of the instar duration. Miyashita (1968a) showed that in Pardosa astrigera L. Koch (syn. Lycosa T-insignita (Boes. et Str.)) the number of instars to maturity increased with a decrease in food supply. Duration of the first instar, which was partly within the stress period, was longer for individuals from the starvation, F. candida and R. padi groups than for spiders fed fruit flies. For Leptyphantes tenuis fed R. padi until the first moult the same tendency was found (Beck & Toft 2000).
The hypothesis that spiders should respond differently to various types of nutritional stress was confirmed. It was expected and largely confirmed that the N-flies would be the least stressful of the treatments, that the toxic F. candida would be the most stressful and the deterrent R. padi intermediate. However, the expectation that a toxic prey should be a stronger stress factor than starvation was not supported. The results presented pertain to a specific selection of one of each type of stressful prey. It is possible to imagine that other prey types may be more seriously deficient in certain nutrients than the N-flies used here, and other toxic prey may have toxins with more serious effects than F. candida. Experiments with more prey types need to be done before final generalizations are possible.
The ability for growth compensation probably evolved from strong selection on the spiders to synchronize their reproduction to a specific season. From an analysis of geographical patterns of spider life cycles, Toft (1976) concluded that, within a species the mating and egg-laying periods were always the same in annual and biennial parts of the distribution area and in annual and biennial fractions of a population in one area, even if the spiders completed their maturation moult at different seasons. This is true whether the species is spring, summer or autumn breeding (Toft 1976). Thus, the alternatives to compensatory growth with reproduction at the ‘normal’ time and at the ‘normal’ size are (1) reproduction at a subnormal size, or (2) adding a full year to the developmental period. The first option has large fecundity costs (Schaefer 1987). The second option may allow the spider to mature at a larger than normal body size and thus give a fecundity advantage but has costs in terms of increased mortality by predation and other risks. As long as the costs of compensatory growth are smaller than the costs of either of these alternatives, compensation should be the chosen response. However, it is possible that spiders can be stressed at a time during their development or for such a long period that they are unable to recover sufficiently in due time, and are therefore forced to add a year to their development. In this case they might not utilize their capacity for compensatory growth and they may save these costs. On the contrary, they may take advantage of the time available and use a slower-than-normal growth trajectory, and reach the maturation size at the proper season without having to pay the costs of a high foraging rate and a high growth rate. This may be the reason for the frequent occurrence of mixed annual–biennial life cycles in spiders (Toft 1976).