In their paper ‘Unrecognized threat to global soil carbon by a widespread invasive species’ O'Bryan et al. (2021) suggested that wild boar (also named feral pigs or wild pigs) and their grubbing reduce global soil organic carbon (SOC) stocks. In this study, models were used to estimate global wild boar abundance and postulated additional CO₂ emissions due to wild boar bioturbation. However, the authors ignored experimental evidence about the effects of wild boar on SOC that points in a completely different direction altogether.

O'Bryan et al. based their study on the modelling of wild boar-induced CO₂ fluxes from soil derived from assumed enhanced decomposition of SOC. Soil CO₂ fluxes are only partly derived from SOC decomposition. Roughly half of CO₂ comes from plant roots as autotrophic respiration of recently fixed CO₂ (Högberg et al., 2001). Thus, this CO₂ flux part is not directly related to SOC stocks. Furthermore, wild boar cannot affect this flux directly if roots are mostly below the disturbed layer. Nevertheless, their modelling work was based on global CO₂ fluxes from the soil that included the autotrophic part. Failure to differentiate between autotrophic and heterotrophic respiration can lead to misleading conclusions being drawn. In plots disturbed by wild boar, higher soil respiration rates have been measured in forests in Central Europe (Risch et al., 2010), but ground vegetation was enhanced in these plots and the higher level of soil respiration could be explained by their autotrophic respiration.

Out of total soil respiration, O'Bryan et al. considered a relative fraction caused by wild boar grubbing. This fraction was assumed to be between 0% and 69.5% (with a uniform distribution of their occurrence). There is no experimental evidence of the minimum effect being zero since there are also studies showing negative effects of grubbing on soil respiration (Cuevas et al., 2012; Persico et al., 2017). The 69.5% comes from one Chinese study that was considered the highest reported additional CO₂ respiration due to wild boar disturbance (Liu et al., 2020). The higher soil respiration at the single study site in a warm temperate forest in central China was mostly explained by there being more SOC in the plot that was grubbed by wild boar than in the control plot. A close relationship between SOC stocks as a source of CO₂ and soil respiration is evident.

This demonstrates that: (i) Such soil respiration measurements alone are not necessarily suitable for estimating the bioturbation effects of wild boar on SOC stocks and (ii) it is much more appropriate to use data on SOC stocks to assess the bioturbation effects. Such data exist and, therefore, this is not an unexplored topic, as stated by O'Bryan et al., and it demonstrates that wild boar grubbing is not a threat to SOC at all:

A long-term wild boar exclosure study in tropical forests in Hawaii revealed no significant differences in SOC stocks due to the cessation of grubbing by wild boar. In contrast, recently disturbed plots contained significantly more SOC stocks (+12%) than undisturbed plots (Long et al., 2017). For SOC, it is irrelevant whether wild boar is native or invasive—the same processes will occur with wild boar grubbing, and it is therefore not logical to estimate wild boar effects only for areas in which wild boar is non-native.

In addition, Bueno et al. (2013) compared SOC content in wild boar-disturbed and undisturbed alpine grasslands and found an average increase of SOC content with wild boar grubbing of 6%, which was however not statistically significant. However, sampling was only 10-cm deep and therefore they may have missed the potential positive effects of wild boar mixing carbon into deeper soil layers. Thus, in ecosystems without an organic layer but with high SOC stocks, wild boar disturbance did not negatively affect soil carbon. Furthermore, in 11 hardwood forest stands in Switzerland, no significant effects of wild boar grubbing were detected when sampling recently disturbed (max. 3 years old) plots paired with non-disturbed plots. The main effect of wild boar grubbing and rooting was a redistribution of SOC from the forest floor to the mineral soils (Wirthner, 2011).

The detection of wild boar effects on SOC can be hampered by the heterogeneity of the soil and the grubbing pattern of wild pigs. In a controlled experiment, regular grubbing by wild boar was simulated in two forest areas in Germany in 24 plots for 6 years. Don et al. (2019) found no effects of grubbing disturbance on SOC stocks, but in contrast, labile SOC from the forest floor became protected and partly stabilized in the mineral soil. Thus, wild boar grubbing actually enhanced SOC stability.

In summary, the experimental evidence shows no or only positive effects of wild boar grubbing on SOC stocks across different climate regions and biomes. Bioturbation by mammals or other animals does not pose a threat to global SOC stocks. In contrast, bioturbation is the main process that develops Chernozems, which are mineral soils with the highest SOC stocks and thus making them most fertile. Their SOC rich, thick A-horizon is the result of bioturbation.

The invasion of non-native fauna and flora can considerably affect and damage ecosystems. However, in the case of wild boar and SOC, there is no experimental field evidence to suggest that ecosystem SOC stocks are under threat from the ongoing invasion of wild boar into new habitats.