Chapter 11: the Detritus-Based Trophic System

The Detritus-Based Trophic System S. F. MacLean, Jr. INTRODUCTION The detritus-based trophic system is composed of animals that use energy only after it has passed from living components through the pool of dead organic matter. The system includes animals feeding directly upon dead organic matter (detritivores), upon microbial tissue (microbi- vores), or upon other animals (carnivores) (Figure 10- 1). This chapter considers the abundance, energetics , and ecological function of animals in the detritus-based trophic system , and the contribution that they make to the decomposition of organic matter and cycling of mineral nutrients in the coastal tundra at Barrow. Even in years with high lemmiug populations , assimilation of energy by herbivores amounts to only about 6"70 of net primary production (Chapter 10). Another 13"1 of net primary production is returned to the tundra as feces , while about 80"1 passes directly to the dead organic matter pool when unconsumed vegetation (including moss and vascular plant roots) senesces and dies. Thus, each year , 93 to 99"1 of the annual primary production enters the pool of dead organic matter and becomes available to microorganisms and invertebrate detritivores, which form the first link in the detritus-based trophic system. The rate at which soil and litter organisms use this energy source is limited by the quality of the organic matter, the length and temperature of the period of biological activity I and other factors such as soil moisture and aeration. These influence both the population density and the rate of activity of individual organisms. The cumulative effect of these rate-limiting factors is seen in the large accumulation of organic matter in the soil which indicates that, by and large , the processes involved in the decomposition of organic matter have been more limited by arctic conditions than have the processes involved in the synthesis of organic 411 412 S. F. MacLean , Jr. matter by green plants. As a result , soil invertebrates live in an environ- ment that is energy-rich, with up to 80"1 of the soil dry weight consisting of organic matter (/ 1675 J cm ) in the top 5 em , where most of the animals are found. As in other functional units of the coastal tundra at Barrow , the diversity of animals in the detritus-based trophic system is low compared to that found in more temperate and tropical ecosystems; however . the diversity of soil invertebrates is not as limited as is that of herbivores. Some taxa that are important soil organisms in other ecosystems are missing altogether (earthworms , isopods, milipedes, ants, termites) or are poorly represented (beetles) in the fauna of the coastal tundra at Barrow. Other taxa, for example mites (Acari), springtails (Collembola), fles (Diptera), and enchytraeid worms (Enchytraeidae), show only a modest reduction in diversity compared with temperate ecosystems , and commonly are quite abundant in the coastal tundra ecosystem. Invertebrate carnivores are few; the most conspicuous are the predatory cranefly larvae, Pedicia hannai and beetles of the families Cara- bidae and Staphylinidae. Only two families of spiders , Linyphiidae and Lycosidae, are found, the latter with only a single species. During the summer months soil invertebrate communities support an abundant and diverse group of breeding birds, especially shorebirds or waders. ABUNDANCE AND BIOMASS OF SOIL INVERTEBRATES The array of microtopographic units that compose the coastal tundra was described in Chapter I. Sample plots for the study of soil invertebrates were established in representative units of polygonal and meadow terrain (Table 11- 1). The characteristics of the study areas and more de- tailed reports of the data are given by Douce (1976), Douce and Crossley (1977) and MacLean et al.(1977). This discussion draws heavily from these data, and emphasizes higher taxonomic categories. Community or- ganization at the level of species is considered by Douce (1976), Douce and Crossley (1977), and MacLean et al. (1978). Energy budgets are cal- culated based upon abundance and biomass estimated in 1972 , for which data were most complete. Large differences among microtopographic units are apparent in mean abundance and biomass of the major faunal groups during the period of biological activity (Table II- I). A two-way analysis of variance, using density as the dependent variable and plot and sample date as inde- pendent variables, was performed for free-living and plant-parasitic nematodes , Enchytraeidae , Collembola, and three major suborders of soil Acari (Prostigmata, Mesostigmata, and Oribatei). Location made TABLE 11- Abundance and Biomass of Soil Microfauna in Various Microtopographic- Vegetation Units of the Coastal Tundra at Barrow Arranged Along a Moisture Gradient (Wet-Dry) Nematoda Enchytraeidae Acari' Col1crnboJa Unit (no. rn (I'gind- (mgm (no. m ' (I'gind ' (rngm (no. m ' (I-gind- (mg rn (no. (I-gind- (rng m ' Curex-Oncophorus 199, 50.4 2620 18, 200 ,s6 346 mcadow Curex- Oncopho(us 226 500 200 51.0 2255 26,900 126 270 mcadow Dupontia meadow 304, 500 400 32. 2716 500 100 244 Polygon trough 276 700 600 44. 4132 162 800 651 Dupont/a meadow 307 700 50, 100 32.4 1622 46, 145 171 6SA Polygon basin 46, 200 200 73.3 1847 600 400 Polygon rim 448 400 50. 651 700 147 S3, 700 335 Carex-Pua 723 800 40, 700 2203 100 2.4 197 800 343 mesic meadow High-centered 361 300 11,400 90. IOU 500 224 400 290 polygon Mean 321 (0. 1)' 46, 100 46- 2119 500 700 (4)' 363 Numbers as extracted and counted; biomass assuming 400fu extraction efficiency. Numbers , and hence biomass, differ slightly from those reported by Douce and Crossley (1977), being based upon 28 rather than !4 cores per pial. Two different Carex- Oncophurus meadow plots located within rhe Biome intensive study area (site 2). Average values used to estimate biomass on all of the study locarions. 414 S. F. MacLean , Jr. the greatest contribution to total variation in five of the seven cases. The exceptions were mesostigmatid mites and Collembola, in which within- sample variation was greatest. Sampling date made a relatively small contribution to total variation , indicating that spatial variation is more important than temporal variation in determining the abundance of tundra soil invertebrates. A similar pattern is seen in the soil micro flora (Chapter 8). Mean annual density of nematodes , estimated by soil sieving fol- lowed by concentration through sugar solution centrifugation , varied be- tween about 50 00 individuals in the basins of low-centered polygons and 724 000 m in the mesic meadow. These values are very low , but have been confirmed by the use of three different extraction procedures. In a review of data on nematode density and biomass in a variety of terrestrial ecosystems, Sohlenius (1980) found that only in deserts were mean values below one milion individuals with coniferous forest, deciduous forest, and temperate grasslands averaging over three six , and nine million individuals m , respectively. Procter (1977) reported densities in the range of one to almost five milion individuals m in the tundra of Devon Island, N. T., Canada. Thus, the low densities found at Barrow are not characteristic of tundra. As at Barrow , the wet meadow at Devon Island supported the lowest density of nematodes. Trophic function of nematodes was determined by examination of mouthparts. Although abundance varied by a factor of 15 among sample points, trophic structure of the population was remarkably constant. The free-living nematodes, which are largely bacterial and algal feeders, were most abundant overall. The plant-parasitic nematodes became relatively more abundant on the drier units, and were dominant on the mesic meadow. Predatory nematodes made up a small proportion of the population in all cases. The abundance of all three nematode groups was in- versely correlated with soil moisture; the Spearmann rank correlation be- tween total nematode abundance and moisture was - 65 (p 05). Drier units contain more dicotyledonous plants , many of which are mycorrhizal (Miler and Laursen 1978). This is particularly so for Sa/Ix on the mesic meadow. The much greater abundance of plant-parasitic nematodes here may indicate that the root systems of dicotyledonous plants are more susceptible to the attack of nematodes than are roots of the graminoids that dominate wetter areas, or that the nematodes are feeding directly upon mycorrhizal fungi. Biomass of nematodes was not directly determined. Based upon studies of nematodes at a variety of other locations (Sohlenius 1980), a mean biomass estimate of 0. g dry weight per individual was used to approximate population biomass (Table 11- 1). More elaborate functions relating density to biomass, for example based upon differences in trophic function, might be used, but hardly seem justified in light of the p? The Detritus-Based Trophic System 415 low abundance. These results indicate that nematodes are relatively un- important in the coastal tundra at Barrow compared with other ecosystems. The northern Coastal Plain near Barrow is totally lacking in earthworms (Annelida: Lumbricidae), but is rich in smaller worms of the fam- ily Enchytraeidae. These worms are largely aquatic, living in the soil in- terstitial water, and are less abundant in the drier areas. Mean densities ranged from 11 000 and 13 00 individuals m ' on the well- drained polygon top and rim to 94 00 worms m-' in the moist polygonal trough. The Spearmann rank correlation of enchytraeid abundance and soil moisture across the nine micro topographic units (Table 11- 1) was positive and significant (r = + 0. 75; -( 0.05). Biomass of Enchytraeidae was determined from the distribution of body lengths of each species , using the geometric equations of Abraham- son (1973).

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