The San Diego Barrel Cactus is found from N. Baja California, Mexico to the edge of San Diego County primarily along the coast in Coastal Sage scrub. This barrel cactus maintains extra-floral nectaries (EFNs - nectaries outside of the flower; floral nectaries attract pollenators in pollenation mutualisms). EFNs on the barrel cactus are modified spines that secrete carbohydrates. EFNs attract ants to protect the cactus from harmful herbivores (bugs). EFNs are found on a variety of plants not just cacti.
The following videos are of different ant species that occur on the cactus and presumably protect the cactus in exchange for a carbohydrate reward offered by the EFNs:
As you can see, these ants vary in their ability to protect the cactus. The larger Camponotus sp. does not recruit to food sources (apparently) as readily as Tapinoma sessile.
