Life history traits in microarthropods: Evidence for a soil animal economics spectrum

Evolution optimizes the performance of living organisms through budgeting of limited resources, leading to life-history trade-offs. Many life-history traits are related to body size with larger species typically exhibiting a slower pace of life and lower fecundity. However, soil-living organisms may exhibit size-independent life-history strategies due to habitat space constraints, but this has never been tested. Here, we synthesize life-history traits in springtails (Insecta: Collembola) and mites (Acari: Oribatida, Astigmata, Mesostigmata), the most abundant microarthropods worldwide, living mainly in litter and the pore space of soil. We related life-history traits to body size and individual metabolic rate, and showed that life-history traits of soil microarthropods display a trade-off between lifespan and reproductive rate, spanning a continuum from fast to slow life-history strategies. Oribatida exhibit remarkably slow life-histories and long lifespans with lower reproductive rates than Collembola, Astigmata and Mesostigmata. Despite fresh body mass of soil microarthropods varying by three orders of magnitude, fast and slow life-history strategies occurred in all size classes suggesting largely size-independent life-history strategies. Overall, these findings indicate a soil animal economics spectrum that bears key implications for understanding local biodiversity and the coexistence of soil animal species, such as how Collembola and Oribatida coexist worldwide.