Trade is a particular type of exchange.  If we exchange Christmas gifts, another 
species might interpret our behavior as trade.  If monkeys groom each other that 
might be trade if each paid the other $5.  Otherwise, it is more likely sociability 
rather than trade.  Symbiotic species are not trading anymore than the plant and 
animal life that make up a coral reef are trading.
Admittedly, trade can expand beyond market relations.  Kids can trade baseball 
cards.  But the examples proposed here do not appear to be trade -- an activity in 
which both parties are aware of a mutual advantage.

Michael Perelman