The Hitchhiker’s Guide to the Herbivore

Image Credit: Jorg Hempel, CC BY-SA 2.0

Can plant traits predict seed dispersal probability via red deer guts, fur, and hooves? (2019) Petersen and Bruun, Ecology and Evolution, https://doi.org/10.1002/ece3.5512

The Crux

Large animals are key players in structuring both the physical structure and the species compositions of plant communities. They eat some plants, but not others, they trample vegetation, they deposit nutrients through feces. However, they can also affect plant communities by transporting seeds (a process called zoochory) – either by eating them and defecating later on or by acting as vehicles for seeds stuck in their fur or on their feet. As large plant eaters are found in most of the world, and several populations are actually increasing, a deeper insight into these processes could turn out to be of great importance.

Today’s authors (myself and former colleague Hans Henrik Bruun) looked at the transport of plant seeds by red deer in Denmark: whether the different kinds of seed dispersal are significantly different with regards to what species are transported, and if certain plant and seed traits can be used to predict whether a seed is more likely to be found on the outside or inside of a deer.

What They Did

We collected seeds transported by red deer during the annual hunt at four different sites in Denmark. We did so by brushing the fur and the hoofs, and by collecting feces from the guts of 57 shot deer. Approximately 5500 viable seeds from the samples were identified (preferably) to species level, and the species compositions of the three dispersal routes (guts, fur and hooves) were compared. We then made statistical models to see if certain plant and seed traits could be used to predict which dispersal route the different plant species were most likely to take. The analyzed traits were: seed mass, releasing height of the seeds, number of seeds per plant, seed shape, a measure of abundance of the plant in the landscape, habitat association and whether the seeds had certain appendages (such as hooks, hairs, wings, etc.) or not.

Did You Know?

In 1899, the ecologist Clement Reid stated what is known as Reid’s Paradox (or more correctly: “Reid’s Paradox of Rapid Plant Migration”). The paradox is that if we look at the average dispersal distance of most plants (aka the distance a seed moves away from the mother plant), most plant migration should be quite slow. However, when we look around, both now and in the fossil record, most plants have colonized areas much earlier than what should be possible, e.g. colonization of norther Europe after the last Ice Age.

Generally, the solution to the paradox is that occasionally a few seeds will be transported an incredible distance – but as this a very rare chance event, we are unlikely to observe it happening! One potential “chance event” could be seeds being transported by animals – having legs or wings are a great help, when it comes to moving long distances.

What They Found

At least 66 different plant species were transported by the red deer. The models showed that the species compositions from each dispersal route overlapped greatly, but they were significantly different. In particular, the species transported through the guts were unlike the ones carried on the outside of the animals. Fur and hoof samples did not differ to a large extent.

The plant and seed traits could be used to predict what dispersal route the different species were most likely to take. Surprisingly, appendages on the seeds did not influence the dispersal route. However, only transport through the gut and in the fur could be predicted – hoof-dispersal could not be modelled correctly. The habitat association of the plan also influenced how the seeds were transported: forest species were unlikely to be transported through the guts, but likely to be stuck in the fur, whereas ruderal species (e.g. species that are very quick to colonize abandoned land, most would think of these as “weeds”) were often found in the guts. In general, most species transported by deer were grassland species.

Screenshot from 2019-08-21 12-28-49

Whilst we thought that the presence of appendages such as these burs on seeds would influence our results, our models didn’t seem to indicate it had any effect (Image Credit: Max Pixel, CC0)

Problems?

It would have very interesting to investigate whether the study sites differed, as two of them were fenced reserves, and two of them were completely open, allowing the animals to roam in agricultural fields. But as the study depended on hunting success, the sampling sizes from each site were too different to incorporate this aspect.

So What?

When talking discussing animal transport of seeds, it is important to distinguish between “inside” and “outside” – and both should be studied, as they do not cover the same plant species.

The finding that appendages did not determine what dispersal route the plants could take show that this characteristic is seemingly less important than what has been thought so far.

The study shows that red deer can potentially transport a great variety of plant species – potentially over long distances (this was unfortunately not assessed in the study). Deer are more likely to transport species from the arable landscape into forested areas than the other way around. Considering that red deer populations are increasing in most of Europe, deer-mediated dispersal of plants could become an increasingly important mechanism.

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