  I introduce an axiomatic representation, called ecoset, to consider
interactions between species in ecological systems. For interspecific
competition, the exclusion conjecture (Gause) is put in a symbolic way and used
as a basic operational tool to consider more complex cases like two species
with two superposed resources (niche differentiation) and apparent competitors.
Competition between tortoises and invaders in Galapagos islands is considered
as a specific example. Our theory gives us an operative language to consider
elementary process in ecology and open a route to more complex systems.
