Host preference of the hemiparasite Struthanthus flexicaulis (Loranthaceae) in ironstone outcrop plant communities, southeast Brazil

Abstract

Struthanthus flexicaulis is a hemiparasite abundant in ironstone outcrops in southeast Brazil. We evaluated its host preference among species of the plant community, taking into account the abundance and foliage cover of the hosts. The importance of each species in the community and the mortality caused by the parasite were assessed based on a quantitative survey in 10 strips measuring 1m x 50m. The 10,290 individuals belonged to 42 species. Only 15 had a relative abundance in the plant community greater than 1%, of which 12 showed vestiges of parasitism. More than 80% of deaths in the community were associated with parasitism. Non-infected individuals had significantly less mortality rates (7%) than those infected (83%) (²= 1102.4, df = 1, p < 0.001). The observed infestation was different from the expected both regarding relative host abundance (²= 714.2, df = 11, p<0.001) and foliage cover (²= 209.2, df = 11, p<0.001). Struthanthus flexicaulis preferred Mimosa calodendron, a legume attractive to avian seed dispersers. The interaction is maintained and intensified not only by the birds, who deposit innumerous seeds on the hosts branches, but also very likely by the ability of M. calodendron to fix nitrogen, thereby enhancing the mistletoe's development.

The mistletoe Struthanthus flexicaulis reduces dominance and increases diversity of plants in campo rupestre

“…Then, these patches may become more visible and represent important resting sites to birds in disturbed areas, increasing the deposition of seeds and consequently the abundance of parasitic plants [46], [19]. In a study carried out in ferruginous campo rupestre, [9] found that Mimosa calodendron was the host plant with the highest proportion of parasitized individuals. The authors suggested that the aggregate populations of this host plant within the community, forming green patches in the landscape, determined its greater use as perches by seed-dispersing birds and, consequently, a greater deposition of hemiparasite seeds.…”

“…On a local scale, abiotic factors have an important influence on the structuring of plant communities, such as the availability of water and soil nutrients [34] in addition to interactions with other organisms, such as microbes and arbuscular mycorrhizal fungi [5, 6, 7]. In turn, the distribution of plant communities strongly affects the community structure of other organisms that directly depend on them as resources, such as herbivores and parasitic plants [8, 9, 10].…”

“…Once the hemiparasitic plants are established, they can affect the host plant both directly and indirectly [21, 22, 23, 9] and, consequently, affect the structure of plant communities [14]. Directly, the hemiparasite decreases the growth and branching of the host plant [23], as they obtain water and nutrients directly from the xylem, decreasing the resources available for the metabolism of the host itself [11, 24, 25], which may ultimately lead to death [19, 26, 9]. Indirectly, the parasite can make the host plant more susceptible to attack by natural enemies [23] and to environmental stress [12, 11], which can also accelerate the death of the host.…”

“…In the Cerrado biome, 36 species have been described, all of which are hemiparasites [33]. Among them, the mistletoe Struthanthus flexicaulis is a common generalist hemiparasite in campos rupestres, with more than 60 species recorded as hosts within at least 24 families of woody plants [25, 9]. The campo rupestre ecosystem is subject to significant anthropic pressure such as mining and road construction [3, 34], and in degraded areas the native pioneer shrub, Baccharis dracunculifolia, whose populations form patches, often dominates the plant community [3, 10].…”

A holoparasitic plant severely reduces the vegetative and reproductive performance of its host plant in the Caatinga, a Brazilian seasonally dry forest

“…Once parasitic plants have severely reduced host performance, parasitism may lead to changes in competitive interactions between host and non-host plants and impact the community structure, diversity, vegetation cycling and zonation (Pennings & Callaway 2002;Aukema 2003;Grewell 2008;Graffis & Kneitel 2015;Mourão et al 2016). These effects on community structure are often dynamic and may change depending on the performance of the parasite itself or due to environmental conditions (Pennings & Callaway 2002;Press & Phoenix 2005;Grewell 2008;Irving & Cameron 2009;Graffis & Kneitel 2015).…”