Consequences of floral visits by ants and invasive honeybees to the hummingbird‐pollinated, Caribbean cactus Melocactus intortus

Abstract

The pink, tubular, nectariferous flowers of Melocactus intortus (Cactaceae) in Puerto Rico are visited by native hummingbirds (Anthracothorax dominicus), but also by invasive honeybees (Apis mellifera) and ants (Solenopsis sp.). We sought to determine if the bees and ants significantly alter the pollination of M. intortus by measuring pollinator effectiveness. Using traditional estimates of effectiveness (visitation rate and seed set), our results show that hummingbirds were the most effective pollinators as expected. Bees and ants were less effective, and their contributions were one-fourth to one-tenth of that observed for hummingbirds. We then modified this measure of effectiveness by adding two components, fitness of progeny and temporal availability of visitors, both of which refine estimates of flower visitor effectiveness. With these new estimations, we found that the effectiveness values of all three animal visitors decreased; however, the role of hummingbirds as the principal pollinator was maintained, whereas the effectiveness values of bees and ants approached zero. By these new measures of overall pollinator effectiveness, the invasive honeybees and ants have little effect on the reproductive success of M. intortus.

How specialised is bird pollination in the Cactaceae?

“…; Nassar et al . ; Fagua & Ackerman ). These traits are also present within other cacti of the tribe Trichocereeae, such as Oreocereus celsianus , whose main pollinators are hummingbirds (Larrea‐Alcázar & López ).…”

“…In the case of the species studied here, the narrow tubular shape of the flowers could restrict access to the nectar to other animals beyond hummingbirds. In the Cactaceae, reddish tubular flowers have been cited for various bird-pollinated species (Locatelli & Machado 1999;Scobell & Scott 2002;Nassar & Ram ırez 2004;Colac ßo et al 2006;Nassar et al 2007;Fagua & Ackerman 2011). These traits are also present within other cacti of the tribe Trichocereeae, such as Oreocereus celsianus, whose main pollinators are hummingbirds (Larrea-Alc azar & L opez 2011).…”

“…In some South American cactus genera, such as Oreocereus, Cleistocactus, Matucana and Denmoza, ornithophilous flowers are a common characteristic (Anderson 2001;Hunt 2006), suggesting that these lineages have evolved toward bird specialisation. However, little is known of the relationship between birds and the cactus flowers (D ıaz & Cocucci 2003;Nassar & Ram ırez 2004;Colac ßo et al 2006;Saleme Aona et al 2006;Nassar et al 2007;Fagua & Ackerman 2011;Larrea-Alc azar & L opez 2011).…”

A general framework for effectiveness concepts in mutualisms

“…), fruit and seed set (Mayfield et al . ; Sahli & Conner ; Fagua & Ackerman ; Robertson & Leavitt ; Sánchez‐Lafuente et al . ; Maldonado et al .…”

“…; Rodríguez‐Rodríguez et al . ; Bruckman & Campbell ), surrogates of seed vigour such as seed size and germination (Fagua & Ackerman ), and offspring survival to flowering (Gómez ). Diverse estimates for total effect range from dimensionless scores of combined surrogate metrics (e.g.…”