Promoting Mouth Grab
They have an extremely bad reputation, for they parasitize at the expense of their host, weakening them or even sucking the life out of them - all without the involuntary carrier gaining any benefit. However, the entire community as such could perhaps benefit from the reviled parasites.
The louse in the fur, the tick and maybe even the tapeworm are probably the first things that come to mind when the keyword "parasite" comes up. But such - at first glance - unwelcome fellows are equally in the plant kingdom. Here they feast on the juices of trees, bushes and even herbs such as grasses, but in return they often do without the formation of chlorophyll for photosynthesis: Why green when you can also tap into other production systems?
Among these beneficiaries of outside services is the native rattler (Rhinanthus minor) – a common species in the figwort family found in many grasslands of Europe and North America. Rhinanthus minor is only a so-called facultative semi-parasite, because it can also carry out photosynthesis through its leafy greens and thus provide for its own livelihood. Nevertheless, it also attacks the roots of many grasses, including important types of grain, in order to secure its share of their nutrient and water supply. Root exudates from the host even promote the germination of its lodger.
Science already knows a lot about the interactions between the parasites and their involuntary hosts. However, what influence the parasites have on the entire ecosystem in which this mouth-robbing takes place has so far largely eluded them. Richard Bardgett from the University of Lancaster and his colleagues therefore specifically investigated this aspect and considered the influence of the Little Rattlepot on its environment.
To do this, they planted large boxes with grassland communities of various grasses such as rough bluegrass (Poa trivialis), perennial ryegrass (Lolium perenne), common trembling grass (Briza media), and red bentgrass (Agrostis capillaris) and herbs such as the Creeping buttercup (Ranunculus repens), common trefoil (Lotus corniculatus) or cranesbill (Geranium sylvaticum), typical of traditional hay meadows. As substrates, the researchers used soils that ranged from nutrient-poor to averagely fertile, depending on the availability of the nutrient element phosphorus. Some of the mini-pastures were either not fertilized at all, fertilized once a year or every two years during the following observation period in order to simulate common agricultural practices.
Then, the gardening researchers then seeded some of the boxes with rattle pots at varying densities, while leaving others free of the species. Lo and behold, no matter how meager or fertile the soil was: Without rattle pots, the communities were significantly poorer in species than the communities in which the parasite was present. Because Rhinanthus minor preferentially attacked the very fast-growing perennial lark, which usually dominates through dense grass carpets, and whose competitiveness fell sharply as a result. This in turn gave some herbs a chance to spread or reestablish themselves, significantly increasing the percentage of these plants in the grassland.
The effect was maintained even with continued fertilization, which actually favors fast-growing grasses and generally reduces the biodiversity of such ecosystems. Consequently, the presence of the parasite balanced or even outweighed all other influencing factors such as the original soil fertility or fertilization.
This was also reflected in another phenomenon: The denser the rattle pot came up, the lower the overall biomass production of the other plants in the beds, which in turn was primarily at the expense of the lark. The scientists attributed this effect to changes in the decomposition processes and species communities of the soil organisms, which could also be triggered by Rhinanthus minor.
The researchers therefore used the so-called phospholipid fatty acid analysis to measure whether and how the reaction of the soil microbes to the presence of the boarder changed. All in all, the same result was obtained from all of the experimental setups: the overall size of these communities did not seem to change, but their structure did. For example, soil fungi had a significantly larger share of metabolism in lean soils than in richer ones, where bacteria became more important. Under the influence of the rattle pot, however, this shift took place much earlier; the presence of the plant could therefore favor bacteria.
Such changes are also generally associated with improved matter fluxes and increased turnover of nutrient elements in the soil. Another indication that this upheaval is actually taking place is, according to the researchers, also the large increase - in some cases by more than 170 percent - in nitrogen mineralization when rattle pots were present. The nitrogen was then also present in the soil in the dissolved form preferred by grassland species, making it more receptive to greens.
Critics could now argue that this fertilization from below should actually increase the productivity of the community again in the medium term and reduce biodiversity. But that was not the case, which further underlines the extremely strong negative influence of the green Nassauer on the growth and dominance performance of certain grass species.
For the biologists, this once again confirms the important role of parasites in biodiversity. So far, they have primarily been regarded as an evolutionary motor that drives the formation of defense mechanisms. However, the example shows that they can also indirectly contribute their mite to species-rich habitats. Maybe we should take a stand for these creatures after all.
