PSU Deer Study

This week I am on Cumberland Island National Seashore collecting bobcat scat with a highly-trained crew!

The scat collection team (left to right): Lisa Diefenbach, Lacey Williamson, Danielle Begley-Miller, Alyssia Church, Leslie Hansen, and Tess Gingery (not pictured, the crew’s cook, Bill Weaver).

In case you didn’t know, ‘scat’ is the fancy term scientists use for animal poop. That and ‘fecal material’ (or ‘faecal’ if you’re European).

What’s that got to do with deer? I’m sure you have an idea, but we’ll get to that in a minute.

Just over 25 years ago, I was part of a team (including Leslie Hansen – see photo above) that reintroduced bobcats to the island.

Why? Well, bobcats were native to the island and the mission of the National Park Service is to protect and manage the ecosystem for future generations. That means keeping or restoring all parts of those ecosystems.

It’s why the NPS reintroduced wolves to Yellowstone National Park, Ozark hellbenders to the Ozarks National Scenic Riverways, and the Sierra Nevada yellow-legged frog to Yosemite National Park, among many other species.

When bobcats were reintroduced back in 1988, we predicted they would prey primarily on small mammals (rabbits, mice, squirrels). All the research on bobcats (on the mainland) indicated deer were only a small part of their diet.

But what actually happened was a bit different.

It turns out that deer became an important part of bobcat’s diets on the island. All year long. Why?

It’s because island deer are tiny. How tiny you ask? I think one of my first experiences on the island tells it all.

To monitor deer populations, we conducted spotlight surveys. We were conducting our first survey in early October 1988 and a small deer stepped out from behind a saw palmetto. I declared, “There’s a fawn!”

Then an even smaller deer stepped out from behind the saw palmetto – the actual fawn – of this adult doe. Oops – my bad.

Here I am photo stalking an adult doe who had 2 fawns following her (of course, readers of this blog know they may not be her fawns!). Photo credit: A. Church.

A field-dressed adult buck (4.5 years old) on Cumberland Island weighed about 75 lbs. An adult doe field-dressed about 55 lbs.

What’s really interesting is that 25 years later the deer on Cumberland Island are substantially larger. An adult buck now field dresses at about 100 lbs.

All because of what is called a “trophic cascade.” There are 3 basic “trophic levels.” Predators (bobcats) are at the top, then herbivores – things that eat plants (deer) – are next, and finally plants are the bottom trophic level.

The cascade that occurred was that the reintroduction of bobcats reduced the number of deer, which reduced the browsing pressure on plants. This resulted in more food available to the remaining deer. The remaining deer enjoyed better habitat and grew larger.

But Cumberland Island deer are still not that large – a 4.5-year-old buck field dressing at 100 lbs is no monster.

So why are island deer tiny?

It’s a function of resources. An island has finite resources and no way for animals to disperse – but it can support more, smaller animals. For populations to persist they need a minimum number of animals. Consequently, species that successfully inhabit islands evolve to become smaller (to result in larger populations).

For deer, the best example is the Key deer (Odocoileus virginianus clavium). In this subspecies of white-tailed deer, an adult male weighs about 55-75 lbs (and that is not field dressed!).

In Pennsylvania, we have found that bobcats most commonly prey on young-of-the-year when they begin traveling with their mother and are <1 year old. Those fawns generally weigh 75 lbs or less. How interesting!

In hindsight, maybe I should have expected bobcats to have an effect on the island deer population. Regardless, it’s pretty exciting to observe ecosystem-level changes simply by reintroducing a missing part!

-Duane Diefenbach

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