Ecological strategies of soil fauna to survive drought

Soil fauna is essential for the functioning of agricultural systems and ecosystems. Droughts and flooding are a threat to soil fauna and thereby to the services it provides. To what extent soil organisms can cope with these extremes partly depends on the combination of their traits. If we can find out which traits determine their reaction to drought, we can extrapolate this knowledge beyond the context of the study. Besides traits, interactions with other organisms in the community may also determine the reaction of soil fauna to climate extremes. This thesis investigates how soil fauna, especially earthworms and springtails, react to climate extremes and which traits determine this reaction. It also tests whether earthworms, being ecosystem engineers, influence the effect of drought on springtails. In a field experiment, we collected springtails at different levels of flooding intensity near large rivers. We identified all springtails and used per-species traits (body size, preferred depth in soil) to explain changes in the community (chapter 2). To test whether earthworm body size determines survival time under drought conditions, we exposed 98 individuals of 14 earthworm species to 85% relative humidity and recorded survival time (chapter 3). We then tested whether the same relationship between earthworm body size and soil moisture conditions was present in the field, by sampling earthworms in dry to wet grasslands (chapter 4). Next, we tested whether earthworms could facilitate downward migration of soil fauna and whether this might alleviate drought stress (chapter 5). For this experiment, we used thin, transparent terraria with endogeic and anecic earthworms and springtails (la{Folsomia candida}). From these terraria, we measured depth and extent of earthworm burrows using computer vision AI, and we measured depth and survival of springtails. We found communities of larger species of springtails in the moistest and most frequently flooded places near the river, but no relationship between soil moisture and average preferred depth of species in the community (chapter 2). We found that in the lab, large earthworms survived drought up to 35 times as long as the smallest individuals (chapter 3). In the field, we found a similar—though less strong—correlation: earthworms in dry places were generally larger (chapter 4). This means the relation between moisture and body size is opposite for earthworms compared to springtails. We found evidence that the downward migration of springtails is facilitated by earthworm burrowing, but we found no concurrent increase in survival of springtails in dry conditions (chapter 5). Our results show that body size is probably a very relevant trait in determining desiccation resistance. In earthworms, this may be partly due to the fact that smaller individuals have a less favorable weight:surface ratio, but this cannot explain why we found smaller springtails in dry places. We discuss the possible relevance of r/K-selection. Our results also indicate that climate change may lead to shifts in average body sizes of soil organisms. This could—in the case of earthworms—affect the ability of other soil fauna to migrate and—in general—have an effect on the functioning of soil fauna.