Under conditions where food resources are available in sufficient quantity, and other abiotic factors are also favourable, the presence of active nematodes is restricted to thin water films and water-filled pores. They are excluded from dry pores and viscous films, as determined by the soil water matric potential. Nematodes tend to accumulate in moist pockets as the soil dries, providing isolated refuges. Nematodes survive dry periods by anhydrobiosis in the humid air of soil pores even when there is insufficient film water. The air-filled pores favour the locomotion of microarthropods and Collembola, which are usually reduced in numbers in wetter sites. This becomes an opportunity for microarthro-pod consumers of nematodes. Different nematode species tolerate different soil water matric potentials (Wallace, 1958; Neher et al., 1999). This was investigated in a series of experiments with intact soil cores kept under increasingly dry conditions (-3 to -50 kPa) (Neher et al., 1999). The cores were sampled four times over 21-58 days when conditions mimicked natural seasonal conditions. The main conclusions of this study can be summarized as follows. For some species (not all), the effect of drying was the same between seasons. Nematode abundance
did not correlate with estimates of the fungal or bacterial biomass, under these microcosm conditions. Predacious and omnivorous nema-todes increased in abundance when there were more nematodes in general. This study also observed that under wetter conditions (-3 kPa), the bacterial biomass was greater than under drier conditions. In contrast, fungal species preferred drier conditions. This is not unlike previous conclusions from similar studies with primary saprotrophs (see earlier in this chapter).
The natural heterogeneity of soil with depth and across the horizontal plane creates many distinct microsites for nematodes, which maintain species diversity at the landscape level. Within microsites and at small scales, samples tend to be dominated by very few species. For example, when comparing nematode species composition between samples within one field site, there is usually little overlap between samples (Ettema, 1998). Each soil core contains few dominant species, with several infrequent species. This reinforces the idea that the soil consists of small overlapping (three-dimensional) patches. One effect of patchy species distribution is to separate, spatially, species that would otherwise be competitors. The idea was verified by monitoring the coexistence of five species of the bacterivorous Chronogaster in a riparian wetland (Ettema et al., 2000). A map of the occurrence and abundance of each of five species of Chronogaster over four seasons at this site was established using statistical methods (spatiotemporal variograms, cross-correlograms and log-normal kriging). This predicted the abundance probability of each species across the landscape, between sampled cores. The map depicts location and abundance changes of each species relative to each other, over the seasons. The results of the analysis emphasize the effect of sample size, and of distance between samples, on the resolution of the data. When the sample contains too many microsites or patches, the population spatial distributions are blurred and may appear homogeneous. However, the study identifies patches of species that are more aggregated. The data suggest that patch dynamics concepts can be applied to explain the ecology of this genus at this site. However, the authors remark that it is not clear which aspect of the species biology is responsible for the spatial segregation, since the biology appears to be very similar between these species. Differences in prey species preferences or slight variations in life history can be implicated, as well as purely stochastic explanations due to unpredictable microsite disturbances.
The parameters that affect soil nematode distribution locally are numerous, and include soil bulk density and mean pore diameter, as well as moisture, temperature, soil air CO2 concentration, preferred prey species abundance, chemical irritants or inhibitors (see Chapter 1 and Ettema, 1998). Predation and parasitism will also impact population dynamics. The effect of these parameters can be easily demonstrated experimentally in the laboratory and in the field. For example, the entomophagic Steinernema riobrave (Steinernematidae) was shown to occur and chose soil depths that reflected its moisture preferences (Gouge et al., 2000). In the field, individuals are encountered at depths which remain within a preferred soil moisture range over the seasons.
In another field experiment, Bakonyi and Nagy (2000) manipulated temperature and moisture content on a series of 2 X 2 m plots. The experiment was only conducted over 4 months, but the authors provide several conclusions based on this initial trend. Significant changes in nematode species composition between plots were detected near the end of the experiment. It is difficult based on short field studies to distinguish between the magnitude of the experimental effect and of the underlying seasonal effect. However, it is clear that over this time period, there were shifts in species composition that were due to moisture and temperature changes at the manipulated plots, compared with control plots. One such shift in composition occurred in the ratio of bacterivorous to fungivorous taxa. The abundance of fungivores was reduced in wetter plots, showing a preference for drier sites. It is unclear whether the fungal biomass between wet and drier plots was mostly active or inactive. That would affect grazing preferences (see Sohlenius and Wasilewska, 1984). The activity of bacterial and fungal species varies greatly with these changes (see earlier in this chapter). Other factors such as dissolved O2 content of the soil solution at wetter sites and changes in predation on the nematodes were not considered. The authors attempted to explain the results based simply on the nematode genera, not considering the effect of the manipulations on other species in the food web. Imposing an abrupt change in soil moisture and temperature inevitably caused a decrease in nematode abundance in all manipulated plots. It would also affect species in other phyla that are competitors with or predators on nematodes. It would be interesting to see the results of follow-up studies from this experiment after three annual cycles.
These factors are important and must be considered along with the biology of other coexisting species. During normal seasonal fluctuations, the abundance of all active species in the soil is affected by these factors. There are changes due to gradual changes in abiotic conditions (such as pH, temperature and moisture), as well as litter substrate species and chemistry, and the abundance of new competitors and predators. For example, in a description of seasonal changes in the composition of species and edaphic parameters, Hanel (2000) observed that the biological factors (food and species composition) influenced the nematode populations more than abiotic parameters. Neither soil temperature nor moisture were the main drivers of the changes in species composition directly. The seasonal fluctuations in species numbers correlated well with other coexisting taxa of enchytraeids, rotifers or tardi-grades, which were competitors or predators of nematodes (Fig. 5.12). Correlations were found between predacious nematodes and tardi-grades (r = 0.673) which are both competitors for small invertebrates, fungivorous nematodes and enchytraeids (r = 0.830), omnivorous nematodes and enchytraeids (r = 0.919), and bacterivorous nematodes
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