Acacia constricta gains novel benefits from ants while minimizing potential conflicts

Abstract

The sum of costs and benefits in an interspecific interaction determines whether the relationship is mutualistic, neutral, or antagonistic. We investigate novel benefits Acacia constricta may gain from ant visitors and how A. constricta may minimize potential costs of ant visitation. A. constricta attracts ants onto its foliage and encourages nesting at its base with extrafloral nectaries. Plants with basal nests have significantly greater soil nutrients and produce twice as many seeds as plants lacking basal nests (Wagner 1997). Along side these benefits, however, ants can interfere with plant reproduction. This study tests whether augmented soil nutrients increase A. constricta's defenses and ant attractants. We further test mechanisms A. constricta may have to reduce the potential costs of ants to reproduction. We found that increased soil nutrients elevated defense mechanisms in A. constricta and increased extrafloral nectary number, suggesting ants that provide plants with nutrients may indirectly increase plant defense as well as participate in a feed back cycle where ants increase soil nutrients allowing plants to increase ant attractants. In addition, plants have at least two mechanisms to keeps ants separated from flowers, minimizing ant costs to reproduction.