Resource allocation in

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Oecologia

Oecologia (Berlin) (1985) 65 :229-235

© Springer-Verlag 1985

Resource allocation in Tomocerus flayescens (Insect a, Collembola): a study with C-14-labelled food Volkmar Wolters II. Zoologisches Institut der Universitiit Gi:ittingen, Abteilung Okologie, Berliner Strasse 28, D-3400 Gi:ittingen, Federal Republic of Germany

Summary. Individuals of the collembolan species Tomocerus flavescens from a beech wood on limestone near Gottingen (West Germany) were fed with C-14-labelled algae in the laboratory. On an average, T. flavescens exchanged almost all of its endogenous carbon after 3.4 moults. In periods of starvation the interval between two ecdyses increased from 5.2 to 6.1 days (17° C). The assimilation efficiency was determined in three independent ways (carbon use of different algal components; A= C- FU; C = P + R +FU, A= P + R): it ranged from 0.34 to 0.40 (converted to energy values 0.30 to 0.45). It could be raised in phases of high mobility and in periods of food shortage by lowering the gut passage rate. The decrease in feeding activity (demonstrated by gut contents analyses) in case of a detoriation of the food quality and the food availability can be partly compensated in this way. For individuals which did not reproduce, the ratio of production to assimilation was 0.24 (converted to energy values 0.31). T. flavescens showed an increased body growth in connection with reaching sexual maturity. Specific demands of nutritive substances in periods of physiological changes could possibly be compensated by a more efficient resource use as well as by differences in resource allocation. The studied population of T. flavescens was compared both with that of a Danish beech forest and general data from the literature. The production of few eggs rich in energy, the high weight of newly hatched individuals, the high growth as well as the storage of carbon connected with egg production, the comparatively low respiration metabolism and the high amount of energy invested in search for food have to be regarded as an adaptation to the habitat. According to conservative estimates climbing individuals of this species consume 0.83 g of the algal dry mass growing on one beech tree during one summer. T. flavescens mainly uses the ethanol-soluble components of the algae.

Collembola occur in high density in almost every soil of the earth, but often constitute only a small fraction of decomposer biomass. The springtails of a beech forest on limestone near Gottingen for example had an average dry weight of 150 mg m - 2 with a mean annual density of 38,000 ind. m- 2 (Wolters 1983a). The biomass of Lumbricidae of the same area was 9,800 mg m- 2 (Poser 1981), that of Diplopoda 600 mg m - 2 (Sprengel 1981 ), that of Gastropoda 430 mg m- 2 (Corsmann 1981), and that of dipterous

larvae 166 mg m - 2 (Hovemeyer 1984). In general, the direct influence of soil fauna on energy flow (and thus on the carbon cycle) is limited. However, the indirect influence on soil properties and microorganisms can be considerable (Schaefer 1982; Wolters in prep.). In a beech forest on moder soil the mesofauna covers approximately 50% of the production capacity of all mycophages and reduces the standing crop of fungal biomass to 66% of its possible maximum (Beck 1983). Seastedt (1984), in a literature review estimates, that microarthropods cause an average increase in decomposition rates of 23%. In current research attention has turned from energy flow to the regulation of nutrient cycling and the indirect effects which do not become apparent in the magnitude of energy flow (Kitchell et al. 1979). Thus the object of investigation of classical production ecology gains importance on a new level (via the terms 'translocation ', e.g. storage or 'transformation', e.g. egestion). In this paper the results of feeding experiments with 14 C-labelled algae on Tomocerus flavescens (Tullberg 1871) are discussed on the basis of field data. According to its biomass T. flavescens is the dominant species of the collembolan population of the investigated beech wood. It is shown that this species responds to changes in food availability by increasing its assimilation efficiency. Materials and methods

Individuals of T. flavescens were sampled by means of an aspirator in a beech forest on limestone. The study site is situated on a plateau in the 'Gottinger Wald', about 420 m above sea level, and mainly consists of approximately 100- to 120-year-old beech trees (Fagus sylvatica) (Bachmann et al. 1983). There is no scrub layer, the forest floor is covered by a herb layer. The beech wood belongs to the Melico-fagetum hordelymetosum. The soil is a rendzina, the C horizon consists of lower Muschelkalk. 250 individuals of T. flavescens were collected off and near trees in July 1982. In the laboratory 25 of these animals (body length 0.6 to 6.8 mm) were kept at 17° C in permanent darkness, in glass vessels half filled with a mixture of plaster of paris and activated charcoal. They were fed with 14 C-labelled green algae (Chlorella fusca 211-8b of the Gottingen algae collection) grown in non-synchronised liquid cultures. Glassfibre filter disks were imbibed with 5 ml of the algae suspension and offered to the animals as food. The amount of live food offered to the Collembola

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was determined by using a counting chamber. The 14 C 0 2 released was absorbed in a special scintillator. The following values were determined: a) the course of 14 C 0 2 release by differing amounts of algae in containers without Collembola. b) the 14C 0 2 release during the time while the animals were feeding c) the 14 C 0 2 release in vessels with algal disks and faeces (after removal of the animals) d) the 14 C 0 2 respired by the animals (after withdrawal of the labelled food) e) the 14 C 0 2 loss due to the absorption in the plaster of paris/charcoal mixture. Counting was carried out with a Phillips PW 4700 liquid scintillation analyser. The organically bound carbon-14 was released by wet oxidation or by burning in an Intertechnique IN 4101 L.S. sample oxidizer. The total amount of carbon was determined by titration or by gas chromatography and if necessary, combined with scintillation counting. For special questions the ethanol-soluble compounds of the algae were extracted using standard methods. The vessels were checked daily. Faeces, exuvia, and eggs were removed and the 14 C content was determined. When greater amounts of dry substance were necessary for comparative measurements, animals in control vessels were also fed with labelled algae. The live weight of 70 T. flavescens was determined before the start of the experiments. 45 of these animals were dried in a dry freezer immediately after weighing and the water content was determined. The remaining animals were placed into separate vessels. They were dried and weighed after the experiments had been completed. Thus an estimate of the increase in dry weight was possible. All weighing was carried out on a Sartorius 4431 Electronic Micro Balance (10- 7 g). For the determination of the 14 C 0 2 respired, the animals were placed into new vessels after having been completely labelled. They were provided with unlabelled food and the 14 C 0 2 was collected (3 x 8 h). For gut contents analysis the animals were cleared in lactic acid and glycerin and guts were dissected. The percentage of gut filling was determined exclusively from animals caught in pitfall traps. For the determination of the different components of the contents, additional animals were used which were sampled in soil cores (extraction method: a modified Kempson infra-red extractor) and an arboreal photoeclector (2 m above ground level) (Wolters 1983a). The following conversion factors were used: whole individuals of T.flavescens 20.97 J mg- 1 DW (Petersen 1975); eggs of 0. cincta 24.83 J mg- 1 DW, exuviae of the same 22.06 J mg- 1 DW, and gut epithelia species 21.12 J mg- 1 DW (Testerink 1983); 20 J mg- 1 0 2 (Engelmann 1966); C content of marine green algae 34% (Bowen 1979); epiphyte material 20.1 J mg- 1 (Turner1983); RQ= 0.85; Q10 =5.2 (Petersen 1981). The length measurements were carried out in a way similar to the method given by Petersen (1975). Results

Labelling of the algae

Figure 1 shows the labelling of the non-synchronised liquid cultures of Chlorella fusca. After the labelling was stopped an approximate equilibrium resulted between the specific activity of the ethanol-soluble extract and the remaining

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Fig. 1. Example for the incorporation of

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C into the ethanolsoluble components of Chlorella fusca used as food for T. flavescens. - - : total activity of this fraction in DPM. -----:percentage of 14 C in this fraction. · · · · ·: the same corrected for the density of the slowly growing liquid culture. The arrow indicates the moment when labelling was stopped

components of the algae. The share of 46% of the entire 14 C in the easily soluble compounds corresponds to the share of this fraction in the total carbon contents of algae (44 ± 3 % ). For many investigations with radioactive tracers an even distribution of the activity is desired. With regard to the special questions in the present study, labelling was stopped when 14 C had already been incorporated into the cell walls and compounds of higher molecular weight, the specific activity of the ethanol-soluble extract being still distinctly higher. When labelling was stopped (not identical with the example given in Fig. 1) the specific activity was: 0.081 µCi mg- 1 ethanol-soluble compounds 0.033 µCi mg- 1 other cell constituents. Incorporation of 14 C-labelled compounds into the body substance

The accumulation of carbon in the bodies of T. jlavescens could be followed through the exuviae (Fig. 2). After the fifth ecdysis no further increase in the average radioactivity occurred. At this point a balance of the 14 C/C ratio of the animals is reached. It is identical with that of the ingested food. It is evident that an average of 29.4% of the carbon found in the exuviae is replaced by the new carbon taken in with food. After the fifth or sixth ecdysis respectively, the animals were taken out of the experimental containers and transferred into new vessels with unlabelled Chlorella fusca for 24 h to allow them to empty their guts. The specific activity of the animals was 0.080 µCi mg- 1 C and thus nearly identical to the ethanol-soluble extract of the algae. It can be concluded that the animals use carbon from the easily soluble fraction of the algae. T. flavescens in control containers were fed 4-5 additional weeks with labelled algae. These animals had a specific activity of 0.080 µCi mg- 1 C as well (see Fig. 2). This result confirms the conclusion that the total carbon metabolism of the animals to a large extent proceeds parallel to the production of the exuvivae. On an average T. flavescens exchanges all of its endogenous carbon after 3.4 ecdyses.

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contents of the faeces and the fluctuations in the amount of faeces produced between two moults are taken into account:

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CFu=0.34 DW 0 · 96 (p