Discontinuity in the responses of ecosystem processes and multifunctionality to altered soil community composition

Abstract

Ecosystem management policies increasingly emphasize provision of multiple, as opposed to single, ecosystem services. Management for such "multifunctionality" has stimulated research into the role that biodiversity plays in providing desired rates of multiple ecosystem processes. Positive effects of biodiversity on indices of multifunctionality are consistently found, primarily because species that are redundant for one ecosystem process under a given set of environmental conditions play a distinct role under different conditions or in the provision of another ecosystem process. Here we show that the positive effects of diversity (specifically community composition) on multifunctionality indices can also arise from a statistical fallacy analogous to Simpson's paradox (where aggregating data obscures causal relationships). We manipulated soil faunal community composition in combination with nitrogen fertilization of model grassland ecosystems and repeatedly measured five ecosystem processes related to plant productivity, carbon storage, and nutrient turnover. We calculated three common multifunctionality indices based on these processes and found that the functional complexity of the soil communities had a consistent positive effect on the indices. However, only two of the five ecosystem processes also responded positively to increasing complexity, whereas the other three responded neutrally or negatively. Furthermore, none of the individual processes responded to both the complexity and the nitrogen manipulations in a manner consistent with the indices. Our data show that multifunctionality indices can obscure relationships that exist between communities and key ecosystem processes, leading us to question their use in advancing theoretical understanding--and in management decisions--about how biodiversity is related to the provision of multiple ecosystem services.

Keywords: aboveground–belowground interactions; ecosystem functioning; plant-soil feedbacks; soil biodiversity; soil fauna.

Fig. 1.

Loss of soil community functional complexity decreases multifunctionality when measured as (A) average multifunctionality or (B) the number of functions exceeding threshold levels of maximum process rates. Data points in A are jittered to visualize vertical spread, and represent average multifunctionality of each replicate at each of the four time points at which measures were taken. The horizontal line shows the median, the box the 25th and 75th percentiles of the data, and the extent of the whiskers 1.5 times the interquartile range. (B) Lines represent the slope between soil fauna loss and the number of functions greater than or equal to a threshold value. Separate statistical models were fit at each threshold ranging from 5 to 99% of maximum functioning, with blue lines indicating high percentages of maximum functioning and red lines indicating low percentages of maximum functioning. The curve in C indicates the change in the slope of the relationship described in B across all thresholds with the gray area showing SE. When the error does not cross the zero line (starting at 72%), the loss of soil community functional complexity is associated with a statistically significant decrease in ecosystem multifunctionality.

Fig. 2.

Loss of soil community functional complexity and nitrogen fertilization effects on the rates of five biogeochemical processes, compared with the response of average multifunctionality (Multifunc). The biogeochemical processes are net primary productivity (NPP); net ecosystem productivity (NEP); decomposition of a standard litter (S-litter); decomposition of litter returned within each replicate community (R-litter); and total community respiration (Respn). Each symbol represents the mean response to the complexity by fertilization treatments, and lines connect low- and high-complexity plots within control or nitrogen-fertilized subplots to facilitate the visualization of community effects. Values are standardized using a z-score transformation to permit comparison with the response of average multifunctionality and between processes with different units and absolute magnitudes. The statistical significance and coefficients of the complexity by fertilization treatments are given in Table 1 and the full process data in Fig. S1. Only standard litter decomposition responds in a qualitative manner (i.e., there is a positive complexity effect and no nitrogen or interaction effect) that is consistent with multifunctionality, but the absolute magnitude of the decomposition response to the community manipulation is much greater than for multifunctionality. The discontinuity between the individual process and multifunctionality responses raises questions about the practical and mechanistic interpretation of multifunctionality indices.