Trees Know Who Nibbles

Not being able to escape form your predators makes for an interesting challenge. Plants, being the static organisms that they are, have risen to this challenge in some amazing ways. From thorns, to hairs, and even chemical warfare, there is no end to the strategies plants have evolved to discourage herbivores.

These adaptations come at a cost. Whether its physical or chemical, defenses require ample resources to produce. That is why so many plant species have evolved induced defense mechanisms. They don't bother producing their chemical cocktails until they sense that herbivores are munching on their tissues. Whereas the literature is rife with examples of insect-induced defenses in plants, few studies have ever investigated whether plants can sense mammalian herbivory. 

A 2016 paper published in Functional Ecology suggests that some trees certainly can. By looking at the responses of two different tree species, sycamore maple (Acer pseudoplantanus) and European beech (Fagus sylvatica), the research team found that trees seem to be able to distinguish between being clipped and being browsed, in this case, by deer. Deer populations are at historically high densities. This has had severe ramifications on forest health and composition. Tree saplings are especially vulnerable, so much so that we are witnessing an alarming decrease in tree replacement over time. 

The research team tested if trees could sense herbivory in a pretty ingenious way. They set out into the forest with pruners. Saplings were subjected to two different treatments - simple pruning and pruning followed by the addition of deer saliva. The team then took a look at how each tree responded on a molecular level. What they found was quite startling. Trees that were subjected to pruning alone began producing a class of hormones called jasmonates. This was not surprising as jasmonates are involved in some generic plant defenses. The most interesting results came from the treatments in which deer saliva was added.

With the addition of deer saliva to pruned beech twigs, it was discovered that the trees increased their production of metabolites related to growth of buds and leaves. They also found that the addition of deer saliva caused an increase in the production of defense compounds, specifically tannins. Tannins bind to proteins in animal guts, making them harder to digest. In maples specifically, they also found an increase in certain types of flavanols, which have shown to have anti-herbivory properties in insects and humans, however, more work is needed to see if they do in fact deter other mammalian herbivores.

Although we still don't know what exactly the trees are responding to in deer saliva, these results nonetheless offer the first evidence of trees not only being able to perceive mammalian herbivores but also responding with an increase in defense compounds. Although they only looked at two tree species, it stands to reason that such responses are wide spread throughout many plant lineages. It also calls into question previous research that used simple pruning as a proxy for herbivory. Taken together, the picture of plants being unresponsive backdrops to more charismatic fauna is entirely erroneous. Plants are proving to be quite "aware" of their environment.

Photo Credit: [1]

Further Reading: [1]

 

 

Hyperabundant Deer Populations Are Reducing Forest Diversity

Synthesizing the effects of white-tailed deer on the landscape have, until now, been difficult. Although strong sentiments are there, there really hasn't been a collective review that indicates if overabundant white-tailed deer populations are having a net impact on the ecosystem. A recent meta-analysis published in the Annals of Botany: Plant Science Research aimed to change that. What they have found is that the overabundance of deer is having strong negative impacts on forest understory plant communities in North America.

White-tailed deer have become a pervasive issue on this continent. With an estimated population of well over 30 million individuals, deer have been managed so well that they have reached proportions never seen on this continent in the past. The effects of this hyper abundance are felt all across the landscape. As anyone who gardens will tell you, deer are voracious eaters.

Tackling this issue isn't easy. Raising questions about proper management in the face of an ecological disaster that we have created can really put a divide in the room. Even some of you may be experiencing an uptick in your blood pressure simply by reading this. Feelings aside, the fact of the matter is overabundant deer are causing a decline in forest diversity. This is especially true for woody plant species. Deer browsing at such high levels can reduce woody plant diversity by upwards of 60%. Especially hard hit are seedlings and saplings. In many areas, forests are growing older without any young trees to replace them.

What's more, their selectivity when it comes to what's on the menu means that forests are becoming more homogenous. Grasses, sedges, and ferns are increasingly replacing herbaceous cover gobbled up by deer. Also, deer appear to prefer native plants over invasives, leaving behind a sea of plants that local wildlife can't readily utilize. It's not just plants that are affected either. Excessive deer browse is creating trophic cascades that propagate throughout the food web.

For instance, birds and plants are intricately linked. Flowers attract insects and eventually produce seeds. These in turn provide food for birds. Shrubs provide food as well as shelter and nesting space, a necessary requisite for healthy bird populations. Other studies have shown that in areas that experience the highest deer densities songbird populations are nearly 40% lower than in areas with smaller deer populations. As deer make short work of our native plants, they are hurting far more than just the plants themselves. Every plant that disappears from the landscape is one less plant that can support wildlife.

Sadly, due to the elimination of large predators from the landscape, deer have no natural checks and balances on their populations other than disease and starvation. As we replace natural areas with manicured lawns and gardens, we are only making the problem worse. Deer have adapted quite well to human disturbance, a fact not lost on anyone who has had their garden raided by these ungulates. Whereas the deer problem is only a piece of the puzzle when it comes to environmental issues, it is nonetheless a large one. With management practices aimed more towards trophy deer than healthy population numbers, it is likely this issue will only get worse.

Photo Credit: tuchodi (http://bit.ly/1wFYh2X)

Further Reading:
http://aobpla.oxfordjournals.org/content/7/plv119.full

http://aobpla.oxfordjournals.org/content/6/plu030.full

http://www.sciencedirect.com/science/article/pii/S0006320705001722