Among the parasites of the world, parasitic angiosperms are relatively understudied. Past research tends to focus on the study of those that parasitize agricultural crops such as Striga species. These studies sought to understand the nature of the parasites so as to devise more effective methods of control. More recently, some researchers have begun to study the ecological impacts of parasitic plants within their native communities. Many have been restricted to experimenting with simple interactions between parasites and their hosts, looking at nutrient transfer and changes in biomass with parasitic connection. However, several studies have expanded to include interactions at the community level.
The family Orobanchaceae contains almost 2000 parasites in 78 genera, which are distributed in a wide variety of ecosystems from extreme northern latitudes to tropical regions. As a result they make an excellent subset of parasites in which to study community effects. In recent years a collection of researchers have looked at several aspects of parasitic Orobanchaceaes’ influence on community dynamics and these are summarized in Phoenix and Press’ 2005 paper: “Linking physiological traits to impacts on community structure and function: the role of root hemiparasitic Orobanchaceae (ex-Scrophulariaceae).” The paper is divided into sections based on aspects of hemiparasitic Orobanchaceae.
High transpiration rates of hemiparasitic Orobanchaceae help to maximize carbon uptake from hosts, and as a result can change competition within a community by reducing host plants’ productivity. It has been suggested that this trait could also cause reductions in soil water and create cooler microclimates with high levels of leaf transpiration, but these have yet to be tested.
There is still much research to be done on heterotrophic carbon and nutrient acquisition by hemiparasites. Haustorial connections allow the uptake of carbon and nutrients from the host, but it seems that there may be variation in the amount of selectivity and regulation that occurs in this process. Since hemiparasites have the ability to gain carbon autotrophically, it has been suggested that nutrients such as nitrogen and potassium gained from hosts may be more beneficial to hemiparasites than carbon. In fact, any studies have shown that nitrogen-fixing species such as lupines are often superior hosts in terms of hemiarasite growth and reproduction, although hemiparasitic Orobanchaceae are generalist parasites and have a wide range of potential hosts. Community effects of carbon and nutrient acquisition by hemiparasites can include reductions in overall biomass and productivity, increased ecological diversity, and possibly increased numbers of pollinators attracted by increased inflorescences of hemiparasites.
Autotrophic nutrition by hemiparasites can be greatly influenced by uptake of nitrogen from hosts. Reduction in host height can also greatly benefit the hemiparasite by allowing more light and thus opportunity for autotrophic carbon gain.
Uptake of secondary metabolites such as alkaloids from host can greatly improve the ability of the hemiparasite to withstand attacks by pests. In one study, uptake of alkaloids decreased pest impact, which increased the number of flowers available, which then increased number of pollinator visits to the hemiparasite. At the community level this may give the hemiparasite additional abilities to outgrow host and non-host species and compete for light resources.
Root functioning of hemiparasites is limited, but hosts’ roots can be affected by infection. In some cases, hemiparasite attachment can reduce the host’s mycorrhizal connections, but mycorrhizal connections of the host can also improve host quality of the hemparasite.
Foliar nutrients and retention during senescence can have major impacts on nutrient cycling within a community. Hemiparasites have relatively high-nutrient litter, which may enhance decomposition of other litter with higher C:N ratios. The speedy cycling of nutrients through litter can increase soil microbial ac9tivity and create microsites that enhance seedling establishment of parasites, hosts, and non-hosts within the community.
Overall this article shows a wide variety of ways in which hemiparasites can affect their hosts and communities and makes excellent suggestions for future research in the field. Parasitic plants add unique functional diversity to their communities and can contribute to valuable ecosystem services such as nutrient cycling. Their effects on communities allow them to act as drivers of both succession and shifts in community composition.
February 19, 2013