Facilitation and predation structure a grassland detrital food web: the responses of soil nematodes to isopod processing of litter

Abstract

Summary1. Detritus can support successive consumers, whose interactions may be structured by changes in the condition of their shared resource. One model of such species interactions is a processing chain, in which consumers feeding on the resource in a less processed state change the resource condition for subsequent consumers. 2. In a series of experiments, the hypothesis was tested that a common detritivore, the terrestrial isopod Porcellio scaber, affects soil nematodes through the processing of plant litter. Different detrital resources were added to soil from a California coastal prairie in order to simulate litter processing by the detritivore. Treatments that included only whole grass litter corresponded to detrital food webs lacking detritivores, while treatments that included mixtures of P. scaber faeces and grass litter corresponded to different densities or feeding rates of P. scaber. 3. Simulated litter processing by P. scaber increased the abundance of bacterivorous nematodes by between 32% and 202% after 24-44 days in laboratory experiments, but had no effect on fungivorous or predaceous nematodes. 4. In a subsequent field experiment, however, fungivorous nematodes were suppressed by isopod litter processing while bacterivores showed no response. Instead, P. scaber processing of litter increased the abundance of predaceous nematodes in the field experiment by 176%. 5. When simulated litter processing of litter was crossed in laboratory experiments with predaceous nematode addition (comparable to the response of predators in the field experiment), the abundance of bacterivores was increased by isopod processing of litter (by an average of 122%), but suppressed by elevated densities of predaceous nematodes (by an average of 41%). 6. This suggests that litter processing by P. scaber facilitates the bacterial channel of the soil food web, but that predaceous nematodes suppress the response of bacterivores in the field. Processing chain interactions may, therefore, be important in understanding the relative importance of bacterial and fungal channels in the soil food web, while top-down effects of predators determine the resulting changes in population abundance and biomass.

Litter conversion into detritivore faeces reshuffles the quality control over C and N dynamics during decomposition

“…Using a wide variety of leaf litter obtained from seven tree species, we found that litter conversion into faeces was generally accompanied by the homogenisation of initial litter quality differences into a quality more favourable for further decomposition (e.g., lower C:N ratio, less condensed tannins, Figures b,a) in line with our first hypothesis. These findings are in accord with previous studies that showed lower C:N (Bastow, ) and tannin concentrations (Coulis et al., ) in faeces compared to leaf litter. With regard to physical characteristics, the specific area was strongly affected, but the changes depended on the scale considered.…”

Resource quality in a soil food web

“…Bacterivorous nematodes are more strongly affected by the N content than the C lability of their substrates, while fungivorous nematodes showed inconsistent responses to C lability. An abundant detritivore at the Bodega Marine Reserve, the isopod P. scaber, facilitates bacterivorous nematodes in laboratory microcosm experiments through its processing of grass litter (Bastow 2011). P. scaber produces feces with higher N contents than the food it consumes, as do many other detritivores (Teuben and Verhoef 1992;Szlávecz 1993;Torres 1994;Rawlins et al 2006;Hedde et al 2007) and herbivores (Cochran et al 2000;Zimmer and Topp 2002;Ikpe et al 2003;Magid et al 2006;Reijs et al 2007).…”

“…Previous experiments showed that litter processing by the terrestrial isopod Porcellio scaber facilitates bacterivorous nematodes in a California coastal prairie (Bastow 2011). Although the reduction of litter particle size (e.g., comminution) by isopods stimulates bacterivores, differences in resource quality between grass litter and isopod feces also appear to contribute to this interaction.…”

“…Soil food webs can be affected by aboveground herbivores (Hunter 2001; Wardle 2003, 2010) and detritivores (Hedde et al 2007;Bastow 2011) through their effects on vegetation and litter dynamics, as well as the conversion of plant matter to feces. Herbivores and detritivores alter detrital resource quality in predictable ways.…”

Contrasting latitudinal clines of nematode diversity in Spartina alterniflora salt marshes between native and introduced ranges

“…This suggests that the belowground food web in Chinese salt marshes dominated by S. alterniflora may not be functioning normally. Changes to nematode community composition may alter the microbial community structure, carbon cycling, decomposition, and the abundance of species that compete with or feed on nematodes (Alkemade, Wielemaker, Herman, & Hemminga, 1994; Bastow, 2011; Kreuzinger‐Janik, Brüchner‐ Hüttemann, & Traunspurger, 2019). Given the vast extent of the S. alterniflora invasion in China, these potential consequences of the altered nematode communities could have important implications for coastal ecosystem structure and functions, and hence deserve further attention.…”