A few weeks ago I wrote about pollination syndromes – sets of floral triats that are said to attract specific groups of pollinators. In that post I discussed how pollination syndromes have largely fallen out of favor as a reliable method of predicting the pollinators that will visit particular flowers. In this post I review a recent study involving a species of cactus in Argentina that, as the authors state in their abstract, “adds another example to the growing body of mismatches between floral syndrome and observed pollinator.”
Denmoza rhodacantha is one of many species of cacti found in Argentina. It is the only species in its genus, and it is widely distributed across the east slopes and foothills of the Andes. It is a slow growing cactus, maintaining a globulous (globe-shaped) form through its juvenile phase and developing a columnar form as it reaches maturity. D. rhodacantha can reach up to 4 meters tall and can live beyond 100 years of age. Individual plants can begin flowering in their juvenile stage. Flowers are red, nectar rich, scentless, and tubular. The stigma is lobed and is surrounded by a dense grouping of stamens. Both male and female reproductive organs are extended above the corolla. The flowers have been described by multiple sources as being hummingbird pollinated, not based on direct observation of hummingbirds visiting the flowers, but rather due to the floral traits of the species.
In a paper entitled, Flowering phenology and observations on the pollination biology of South American cacti – Denmoza rhodacantha, which was published in volume 20 of Haseltonia (the yearbook of the Cactus and Succulent Society of America), Urs Eggli and Mario Giorgetta discuss their findings after making detailed observations of a population of D. rhodacantha in early 2013 and late 2013 – early 2014. The population consisted of about 30 individuals (both juveniles and adults) located in the Calchaqui Valley near the village of Angastaco, Argentina. At least three other species with “hummingbird-syndrome flowers” were noted in the area, and three species of hummingbirds were observed during the study periods. Over 100 observation hours were logged, and during that time “the studied plants, their flowering phenology, and flower and fruit visitors were documented by means of photographs and video.”
The flowers of D. rhodacantha only persist for a few short days, and in that time their sexual organs are only receptive for about 24 hours. The flowers are self-sterile and so require a pollinator to cross pollinate them. Despite their red, tubular shape and abundant nectar, no hummingbirds were observed visiting the flowers. One individual hummingbird approached but quickly turned away. Hummingbirds were, however, observed visiting the flowers of an associated species, Tecoma fulva ssp. garrocha. Instead, a species of halictid bee (possibly in the genus Dialictus) was regularly observed visiting the flowers of D. rhodacantha. The bees collected pollen on their hind legs and abdomen and were seen crawling across the lobes of the stigma. None of them were found feeding on the nectar. In one observation, a flower was visited by a bee that was “already heavily loaded with the typical violet-coloured pollen of Denmoza,” suggesting that this particular bee species was seeking out these flowers for their pollen. Small, unidentified beetles and ants were seen entering the flowers to consume nectar, however they didn’t appear to be capable of offering a pollination service.
D. rhodacantha populations have been observed in many cases to produce few fruits, suggesting that pollination success is minimal. The authors witnessed “very low fruit set” in the population that they were studying, which was “in marked contrast to the almost 100% fruit set rates of the sympatric cactus species at the study site.” This observation wasn’t of great concern to the authors though, because juvenile plants are present in observed populations, so recruitment appears to be occurring. In this study, dehisced fruits were “rapidly visited by several unidentified species of ants of different sizes.” The “scant pulp” was harvested by smaller ants, and larger ants carried away the seeds after “cleaning them from adhering pulp.”
The authors propose at least two reasons why hummingbirds avoid the flowers of D. rhodacantha. The first being that the native hummingbirds have bills that are too short to reach the nectar inside the long tubular flowers, and often the flowers barely extend beyond the spines of the cactus which may deter the hummingbirds from approaching. The second reason is that other plants in the area flower during the same period and have nectar that is easier to gather. The authors acknowledge that this is just speculation, but it could help explain why the flowers are pollinated instead by an insect (the opportunist, generalist halictid bee species) for whom the flowers “could be considered to be ill adapted.” The authors go on to say, “it should be kept in mind, however, that adaptions do not have to be perfect, as long as they work sufficiently well.”