Properties of the structures created by ecological engineers at the soil surface of a Colombian savanna.

Fourteen types of physical structures produced by ecosystem engineers were sampled at the surface of a savanna soil of Colombia. Invertebrates implicated in the creation of these structures were identified. Some physical (aggregate size and stability, bulk density) and chemical (C, N, P contents, pH, etc) properties of structures were assessed. Three large groups of structures were identified: (i) earthworm casts characterised by a high bulk density (1.3-1.4 g.cm(-3)), constituted of aggregates (7-10mm), high in organic C (3-4%) and assimilable nutrients; (ii) termite mounds with low bulk density (0.6-0.9 g.cm(-3)), constituted of aggregates (8-9 mm), high in organic C (3.5-10 %) and assimilable nutrients; and (iii) slightly compact (0.4-0.7 g.cm(-3)) and granular (aggregate size < 1.5 mm) termite superficial channels and ant mounds low in organic C (less than 1.5 %) and assimilable nutrients. These results underline the large diversity of the biogenic structures produced at the surface of the studied soil. They suggest the feasibility of a functional classification of engineer organisms that would take into account simultaneously the different functional attributes reflected by these structures. (C) 2001 Academie des sciences/Editions scientifiques et medicales Elsevier SAS.