One symbol is the infestation of crabs in Pelayo's house. During the strong rains, the land crabs hide indoors so they won't drown; Pelayo kills them and throws them into the water. The crabs symbolize both Pelayo's poverty -- he can't afford a better house -- and the instinct of all animals to survive under pressure. When the angel shows up, it is lying in the mud, trying to move but stuck. Just like the crabs, the angel has the instinct to survive, but it is too old:
...before going to bed [Pelayo] dragged him out of the mud and locked him up with the hens in the wire chicken coop.
(Márquez, "A Very Old Man with Enormous Wings," salvoblue.homestead.com)
Although they do not treat it well, Pelayo and his wife offer the angel sanctuary to live through the winter; its instincts, if it sought them out specifically, were sound. After gaining money, Pelayo builds a new house, this time with barriers to keep the crabs out; the symbol of the crabs has been replaced by the angel, which now lives in the unchanged chicken coop, representing the poverty that Pelayo and his wife used to live in. The crabs, therefore, have served their purpose, and are no longer necessary.