Seed dispersion by surface casting activities of earthworms in Colombian grasslands

Abstract

The effects of Martiodrilus sp. (Oligochaeta, Glossoscolecidae) on the soil seed banks was investigated in a Colombian savanna and two intensive pastures. Germination and washing–sieving methods were used to compare seed density, diversity, species composition and germination rates in earthworm casts and the surrounding soil. Although large amounts of seeds were present in casts (163.65, 156.84 and 60.36 seeds per 100 g of dry casts in the savanna, the old and young pastures, respectively), germination rates were 3–40 times lower than in the surrounding soil, likely due to degradation during the gut transit. The number of viable seeds present in casts was 0.40, 7.46 and 1.99 seeds per 100 g of dry casts in the savanna, the old and young pastures, respectively. Species composition of viable seeds was quite different in casts compared to soil, probably because of selective seed ingestion by earthworms. Viable seeds deposited in surface casts each year represented 0.65%, 16.17% and 8.24% of the total viable seed bank of the soil, in the savanna, the old and young pastures, respectively. In the savanna and the old pasture, species composition in casts was more similar to the vegetation than species composition in the soil was. This may indicate that ingested seeds that survive gut transit have a greater chance to germinate than those of the soil seed bank, providing vegetation cover is sufficiently opened to enable germination processes. Thus, casts may be considered as a regeneration niche for plant species, and earthworm activity a factor that enhances, in some cases, the expression of the soil seed bank in the standing vegetation.