In this post I will talk about my recently published paper on the increase of predation rates with plant diversity. You can access the full-text article here.
Background
20 years ago researchers started to wonder about the consequences of diversity loss for the functioning of natural system. By functioning we usually mean two things: (i) stocks of matter such as plant productivity or bird abundance and (ii) transfer of stuff such as transfer of nutrients from the soil to the roots of the plants or transfer of matter from a caterpillar being eaten by a bird to the happy birdie. One way to explore this relationship is to completely destroy some patch of land, replant plant mixtures of varying levels of diversity, wait some time, measure a bunch of variables on the different plant mixtures.
My PhD was about the effect of plant diversity on arthropods (insects, spiders, millipedes …), the main idea being that the effect of diversity loss at a particular level (plant) will affect other levels (insect).
The experiment
One of my project was to measure predation rates on the different plant mixtures. Basically trying to measure how much predators eat in relation to plant diversity. To do that we used a technique called sentinel prey, it consists in setting different type of baits, and the baits being either immobilized or unable to move, we could come back after a day and observe which baits were eaten and which were not.
The experiment lasted for two times two weeks, in total we exposed 17600 mealworms, 8800 dummy caterpillars and 4400 aphids.
What came out
On the first day of the experiment we quickly realized that we had super high predation rates on our mealworms, in some area of the field site more than 90% of the mealworms were being eaten. We found out that starlings were very happy to find immobilized and tasty mealworms. This is one of these field work downturn that appear logical with hindsight but that we really did not expected. We continued our sampling as stubborn researchers and conducted an extra experiment on a subset of mixtures this time ensuring that birds could not access the mealworms.
After analysis the results showed that across the different type of baits predation rates increased with plant diversity and this both in spring and in summer.
Also the pattern observed in spring for the mealworm was strongly driven by bird predation, as shown by the results of the extra experiment. Basically mealworm baits unaccessible to birds showed the same pattern as the other baits.
Conclusion
Here we showed that plant diversity increase predation rates. This is important results in times where policy-makers and farmers try to reduce their dependence on insecticide. Indeed our results imply that increasing plant diversity will provide stronger biological control potentially keeping insect herbivore population at low levels.