I would like to introduce you to the yellow rattle (Rhinanthus minor). This little plant has pretty yellow flowers that belie a dark secret: it is a vampire plant. Okay, technically, it is a hemiparasite. The yellow rattle has green, photosynthetic leaves to make its own food, but its roots latch onto those of nearby plants to steal their water and nutrients. See? Vampire. I guess you could say that it hasn’t gone full parasite. So what’s wrong with a little leaching of material from your neighbor? Must be like stealing cable, right? Well, not exactly. Hemiparastitic plants can have impacts that are highly disproportionate to their small size. By stealing water and nutrients, they reduce the host plants’ photosynthetic rate and biomass, making them less competitive in their habitat. When you are less competitive in a highly-competitive environment then you don’t last all that long. But this is looking at it on an individual, plant-by-plant, level. Let’s scale it up a bit.
We’ll start by following the logic trail. Hemiparastic plants are widespread in ecosystems such as meadows where they attach to dominant grasses (in this case, dominant meaning numbers). As individual grass plants are weakened or killed, overall grass biomass decreases. When the dominants are gone there is more room for other types of plants to move in, increasing species richness. More types of plants attract more primary consumers (like insects) to feed on them. More insects attract more secondary consumers, and on and on. All in all that one little plant has caused indirect community level changes. A new paper by Hartley et al. in Ecology looks at how this one little plant can affect multiple trophic levels (or levels in the food chain).
The researchers laid out 13 blocks, each containing four 1 by 1 meter plots, in a field site in Sussex, UK. Three out of those four plots were allowed to be naturally colonized by yellow rattle. These “infected plots” were then randomly assigned to one of three treatments: yellow rattle removed, present, or enhanced. Then, during maximum vegetation and invertebrate abundance, plots were censused. Counts, ground cover, vegetation height, and species richness were measured for plants. Invertebrates were sampled with a Vortis suction sampler (a.k.a. bug vacuum) and hand counts. They were identified and classified into their trophic levels: herbivore, predator and detritivore.
They found that the density of yellow rattle had pronounced effects on the plant community. Grass cover was lower, plant diversity higher (although not richness), and plant height lower in plots where the hemiparasite was present or enhanced. Going up a trophic level, sap-feeding insects (Hemiptera) increased by 130%, caterpillars (Lepidoptera) by 217% and weevils (Curculionidae) by 188%. Whoa! Going up another level to the predators, spiders and harvestmen (Araneae and Opiliones) increased by 142% and bees and wasps (Hymenoptera) by 180%. Continuing with the whoa! Going down to the detritivore groups, wood lice (Isopoda) went up by 116% and springtails (Arthropleona) by 58%. Of course, not all animals fit into a single trophic group, but as you probably guessed by the trend you’ve been seeing, those groups went up too. Slugs and snails (Gastropoda) by 103% and mites (Acari) by 57%. That’s quite a list, but not everything increased. There were some non-significant groups including flies (Diptera), rove beetles (Staphylinidae), and springtails (Neelipleona and Symphypleona). It is also important to note that these numbers are from enhanced compared to removed plots. The present plots had increases in many of the same categories, though not as dramatic.
In scientist-speak these results showed that “the manipulation of a single sub-dominant plant species causes substantial changes in the abundance and diversity of organisms across four trophic levels in individual plots in a complex grassland community.” Basically, they demonstrated that grasses went down and just about everything else went up just by the addition of one, small and less numerous hemiparasite. This one little plant species caused striking (their term), rapid changes in the community. The less obvious effects of yellow rattle were how the effect was actually occurring. Was it on plant community composition or plant quality or stand height? The authors cite a greenhouse study that showed insects to prefer parasitized to unparasitized grasses when given the choice. Add to that that it is known that parasitic plants impact nutrient cycling, especially enhancing the nitrogen content of vegetation overall. This suggests that it may be plant quality that is the driving mechanism. So the observed, large increase of sap-sucking invertebrates, particularly the Hemiptera, with altered plant quality makes sense as does the increase in detritivores with newly nutrient-rich litter. At this point, perhaps you have considered the impact of these plant-attacking invertebrates as well. Good on you. They are probably themselves doing some damage on and affecting change on the plant community, but this work doesn’t go into that. Time for another study I think!
Hartley, S., Green, J., Massey, F., Press, M., Stewart, A., & John, E. (2015). Hemiparasitic plant impacts animal and plant communities across four trophic levels Ecology DOI: 10.1890/14-1244.1
(image via Urban Butterfly Garden UK)
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