a collection vial. A similar method, the Baermann funnel, is used for nematodes. The Baerman funnel is wet, however (while the Berlese funnel is dry) and does not depend on a light/heat gradient. Nematodes move out of the soil and to the bottom of the funnel because, as they move, they are denser than water and are unable to swim. Soil microbial communities are characterized in many different ways. The activity of microbes can be measured by their respiration and carbon dioxide release. The cellular components of microbes can be extracted from soil and genetically profiled, or microbial biomass can be calculated by weighing the soil before and after fumigation. Types of food webs There are three different types of food web representations: topological (or traditional) food webs, flow webs and interaction webs. These webs can describe systems both above and below ground. Topological webs Early food webs were topological; they were descriptive and provided a nonquantitative picture of consumers, resources and the links between them. Pimm et al. (1991) described these webs as a map of which organisms in a community eat which other kinds. The earliest topological food web, made in 1912, examined the predators and parasites of cotton boll weevil (reviewed by Pimm et al. 1991). Researchers analyzed and compared topological webs between ecosystems by measuring the web’s interaction chain lengths and connectivity. One problem faced in standardizing such measurements is that there are often too many species for each to have a separate box. Depending on the author, the number of species aggregated or separated into functional groups may be different. Authors may even eliminate some organisms. By convention, the dead material flowing back to detritus is not shown, as it would complicate the figure, but it is taken account in any calculations. Flow webs Miosis build on interconnected food chains , adding quantitative information on the movement of carbon or other nutrients from producers to consumers. Hunt et al. (1987) published the first flow web for soil, describing the short grass prairie in Colorado, USA. The authors estimated nitrogen transferral rates through the soil food web and calculated nitrogen mineralization rates for a range of soil organisms. In another landmark study, researchers from the Lovinkhoeve Experimental Farm in the Netherlands examined the flow of carbon and illustrated transfer rates with arrows of different thicknesses. In order to create a flow web, a topological web is