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Don't Run that Way!

Posted: May 17, 2018

The escape route of a screaming teen being chased by chainsaw-wielding madman is always questionable.
https://pixabay CC0

https://pixabay CC0

Ever groan in frustration as the would-be victim in a horror movie running from death trips in what can only be a groundhog hole in a deserted field?  It’s a little more believable when trees are involved.  A plethora of twigs, branches, and downed debris play right into the hands of pursuing death.  

If you are a fawn, it’s very believable.  Those branches aren’t just a tripping hazard – they are giant road blocks when running from a bear, i.e. death! 

The first few months of a deer’s life can easily be equated to a horror movie.  Everything is big.  Everything is scary.  And everything is trying to kill you.  Fawns living in forested habitats may be less likely to escape capture during a chase than fawns in more open habitats. 

We know fawn survival increased as the amount of agricultural land cover increased and predation was still the leading cause of mortality (darn groundhog holes!).  But there are obviously fewer fawns dying when agriculture increases.  So what changes?  

Whoohoo – let’s categorize some more!  For this analysis, there are three types of landscapes that fawn studies were conducted in: forested, agricultural, and mixed. 

Mixed landscapes contained approximately equal amounts of forest and agriculture. Forested and agricultural landscapes were dominated by a single cover type. Mixed landscapes likely have more “edge” habitat than those dominated by one cover type. 

Predation is the leading source of mortality in all landscapes when compared to human and other natural mortalities.  The question is does the proportion of fawns that die from predation change among the three landscape types. 

Previous studies have shown that predator density is greater near forest edges…but other studies have shown that predation may be reduced near forest edges if predator species have access to alternative food sources such as crops. Conflicting research regarding how landscapes influence fawn predation can be overwhelming.  Like a panicked, nameless horror movie character trying to decide which escape route to take! 

Taking a deep breath, I hypothesized the number of fawns that die from predation would be lowest in mixed landscapes. Fawns in mixed landscapes may have access to good hiding cover in forests and high quality food in agricultural food simultaneously.  

Did I choose wisely?  Eh.  The proportion of fawns that died from predation was lower in mixed landscapes when compared to forests, but not when compared to agriculture. 

In the graph, we see the three types of landscapes in relation to the proportion of fawns that died from predation (Each has an associated 95% confidence interval and letters indicate which are similar to each other).

Don't run-graph1

Keep in mind that while the proportion of predation was lower in mixed landscapes when compared to forested landscapes, predation was still the leading cause of mortality in all three landscapes.

There is a limitation of this meta-analysis.  I can’t study the WHY.  It could be that predators in mixed landscapes used alternative food sources such as crops and this reduced fawn predation as originally suggested. Or it could be that fawns were more likely to escape predation in these landscapes that were broken up by forest and agricultural cover. 

But let’s not ignore the other causes of fawn mortality. 

Many farmers have stories of running over fawns with equipment and it is mentioned in the literature for many young ungulates as well.  So human-caused deaths might be greater in mixed landscapes. 

Indeed, the proportion of fawns that died from human causes was greater in mixed landscapes when compared to forests. (Yes!)  But not when compared to agriculture. (Gah!)

Don't run-graph2

Despite the many stories of unlucky fawns being run over by an unlucky farmer on tractor, it’s not mentioned much in the literature. Most human-caused fawn deaths were from hunter harvest and roadkill. 

Well, we still have one category of death to look at. 

Landscapes devoted to agriculture generally have more productive soils in comparison to forests and can potentially produce higher quality and quantities of forage for fawns and their moms. If this is the case, then natural deaths such as starvation may be fewer in agricultural landscapes. 

Wrong!  Natural deaths accounted for equal proportions of fawn deaths in the three landscapes (See super exciting graph below). 

Don't run-graph3

Keep in mind that agricultural landscapes are still associated with a higher fawn survival than forested landscapes.

Dang! 

After a little soul searching on the failure of detecting a difference in natural deaths among landscapes, we took the Vizzini approach and “went back to the beginning.”   Maybe the categories could be partially to blame. 

Starvation, all diseases, abandonment from mom, hypothermia, and a few random others like drowning in a river were all placed in the natural death category. What if agricultural landscapes did provide fawns with a bit more food and it reduced starvation related deaths but it didn’t reduce hypothermia, abandonment, and disease? If that was the case then a reduction in starvation related deaths would not be detected as the number of other natural deaths like disease and drowning stayed the same. 

Unfortunately, there was not enough data to break down each cause of natural death nor was it possible to measure the forage quality and quantity in the agricultural landscapes. 

Double Dang!

Other studies will have to tackle the question of WHY agricultural landscapes are associated with greater fawn survival!

Deer density, landscape type, survival, causes of death – all cool and relevant stuff.  But we all know those are not the questions people ask when discussing fawn survival.  

The most asked question is always WHO kills the most fawns.  Who is the star of this horror movie? 

-Tess Gingery
M.S. Graduate Student
Department of Ecosystem Science and Management
Pennsylvania Cooperative Fish and Wildlife Research Unit

Previous articles in this series:

Fawns, Predators, and Deer Populations-what's the bottom line on fawn survival?
Bunches, Heaps, and Gobs of Fawns
Land of the Living
A Million Ways to Die

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