A few weeks ago, I wrote about Indian Cucumber-root and why it occurs in some places and not others. The short answer is that available resources (light, soil conditions, etc.) are important in explaining where it occurs and where it does not – besides the fact that deer like to eat it.
What I did not address in that post was competition from other species. That’s right. Just like Pennsylvanians competing for milk and bread before a winter storm, all living things are competing with other living things for resources.
The end result? You are more likely to find a species in one place rather than another! There are many different types of competition, but one thing is guaranteed – one species tends to be a winner and the other a loser.
In wildlife, we think about competition quite a bit. In fact, wildlife biologists have devised methods to measure whether one species is excluding another.
For example, we know that coyotes and fox don’t quite get along. If a coyote has an opportunity it will kill a fox. Fortunately, gray fox can climb trees (coyotes can’t) so they can vertically avoid each other when necessary. Red fox are grounded and cannot climb trees. So, red fox tend to hang out in the margins, the empty spaces between coyote territories where they are less likely to encounter them.
We can measure whether one species excludes another using what are called “occupancy models”. Occupancy models simply estimate the probability that a species occurs in a given location. If you monitor two species, you can estimate the probability that one species is present given that the other species is present or absent.
In the coyote-fox example, fox would more likely be present at a site if coyotes are absent. Speaking for myself [not for Jeannine], that is pretty cool! I can even calculate measures of precision about the estimate to indicate my confidence in the estimate [Only one of us gets excited about confidence intervals].
Coming back to plants…their competition is usually not as dramatic as the relationship between coyote and red fox, but plants locked in battle nonetheless are doing their best to exclude one another.
How are deer involved? You know they are. This is a blog about deer. Deer have been accused of causing a legacy effect in our forests. Munching their way through our forest eating their favorite plants, they open the door for less favorable plants to take their place. For example, perhaps deer eating all their favorite foods allowed less tasty plants, like mountain laurel and hay-scented fern, to take over.
Once they gain ground, mountain laurel and fern aren’t going to give up territory without a fight. They shade out sunlight preventing those tasty plants – like Indian cucumber-root – from returning. Thus, the deer browsing “legacy effect.”
Danielle Begley-Miller, a former Ph.D. student on The Deer-Forest Study, set out to study this “fight”? Is mountain laurel excluding Indian cucumber-root? She used occupancy models to learn if the presence of Indian cucumber-root was influenced by the presence of mountain laurel.
The answer? Mountain laurel can’t be bothered with Indian cucumber-root. And neither can huckleberry, another ericaceous shrub that grows densely in the forest understory.
Her conclusion? “This study demonstrates the importance of soil chemistry in shaping plant community composition in the north-central Appalachians, and suggests soil as an alternative, or additional, explanation for deer vegetation legacy effects.”
Is her work the end of the story? No. You know that over-layered, multi-pronged, tangled-up web Jeannine is always talking about? Well, this is another example. Life only gets more complicated after this.
-Duane Diefenbach
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