Predator Poop Propagating Plant Persistence
An omnivorous mesopredator modifies predation of omnivore-dispersed seeds (2021) Bartel & Orrock, Ecosphere, https://doi.org/10.1002/ecs2.3369.
The evolution of different methods of seed dispersal has played a huge role in shaping plant diversity and distribution. Earlier plants could only use the water or wind to disperse their offspring, but eventually plants evolved the ability to harness the movement of animals, letting their seeds disperse often further and more efficiently than before.
Seeds are also a vital form of food for many species, including small rodents and insects. Larger animals too, including wild boars, bears, and coyotes who will get stuck into berries when there’s plenty around. This leads to them leaving berry seeds mixed in with their faeces. We might be deterred by the idea of picking dinner out of another animals poop, but many of those rodents and insects don’t mind.
But what about when those faeces are from one of your predators? Do you still want that seed, or should you get the hell out of an area clearly inhabited by a threat to your livelihood? The answers to these questions can determine which seeds get left where, which in turn can determine where plants end up taking root and spreading to. That’s the focus of today’s study.
What They Did
The researchers set up a series of plots in a forest in South Carolina in the USA. Each plot had four feeding stations housed in small plastic containers. Two of the stations allowed rodents to enter, two had doors that were too small for the rodents to get in (but insects could still enter). Two had coyote scat (fancy word for poo) placed inside, two didn’t. This created four different types of feeding stations. One without scat which rodents could access, one without scat that they couldn’t, one with scat that rodents could access, and one with scat that they couldn’t.
Each stations was filled with 20 seeds of two different sizes. Smaller seeds that the insects could manage to carry off, and larger seeds theorised to be too big for the insects to manage but fine for the rodents.
The researchers also checked what the scat was made up of. If there was more meat inside the scat, would the rodents recognise this and avoid that feeding station? Additionally, a second series of plots were set up with both coyote scat and wild boar scat to make sure it was the scat of the predator that scared off the rodents and not just scat in general.
Did You Know: Carnivores Eating Berries?
It’s odd to think of, but many species that we traditionally think of as carnivores, like bears or wolves, spend a lot of time eating plant matter. European brown bears are up to 80% herbivorous, and many populations of wolves will often turn more to foraging when prey species get scarce. Some populations of wolves in North America forage extinsively on berries, and will have diets at some points of the year that consist of blueberries up to 80%.
What They Found
As the researchers guessed, insects and other arthropods didn’t take the larger seeds at all. The rodents did take them, though much less frequently when predator scats were present, especially when those scats were made up of more meat. That side experiment also showed that rodents didn’t react when wild boar scat was used, implying that the fact that the scat came from a coyote was what was scaring the rodents off.
The removal of small seeds was consistent across all the stations, except for those where no scats were present and rodents couldn’t get in, in which case much less seeds were removed.
The obvious problem here is that while you can keep rodents out of a station by narrowing the entry, there’s no way to keep smaller arthropods out so you can see what happens when only rodents can get in. What has to be made is an assumption that whatever the difference is between seed removal in stations with and without rodents is caused by the rodents. It’s a bit dicey, but not something that’s easily avoided.
A food web is a complex, dynamic structure that drives all the processes in an ecosystem. Experiments like this give us a little bit more insight into that structure. They let us piece together how an ecosystem works, and what drives species distributions and abundances. Importantly here, the fact that coyote presence influences the seed consumption of rodents gives us an insight into how our attitudes to predators may affect different aspects of a forest, including its plant life.
Sam Perrin is a freshwater ecologist and climate data analyst who is quite sure that at this point there are enough scat-related paper breakdowns on this site to start up a scat-focussed subsite. You can read more about his research and the rest of the Ecology for the Masses writers here, see more of his work at Ecology for the Masses here, or follow him on Twitter @samperrinNTNU.