Friday, February 13, 2015

A Naturalist's Thoughts on Invasive Species

Invasive species are defined as exotics that were brought or accidentally introduced to an ecosystem and wreak havoc on it because they have no native predators. Think kudzu in the American South.

A healthy ecosystem has checks and balances to keep each species in line. Visualize a very simple food cycle - let's say trees, white-tailed deer, black bear. Trees sprout and grow but the stand can't get too thick because of deer browsing the seedlings. The deer population is held in check by the bears. And the bears eat the berries on the trees and poop out the seeds, allowing new trees to grow.

Now, this is highly oversimplified, but you can see what I mean about checks and balances. You throw something non-native in the mix and the whole cycle suffers.

But it occurred to me yesterday that native species can become invasive. In reality, the simple food web that I described above doesn't work all that well where I live. Black bears (and other top predators) are rare and the white-tailed deer population has exploded. The understory of the forest here has almost no seedlings because the deer browse everything, leaving no baby trees to replace the ones that are sick or dying. 

In my opinion, white-tailed deer in the mid-Atlantic are an invasive species. Now, the very definition of an invasive species, at this point, implies that a species is non-native. But I would argue that when a native species population explodes out of control with nothing to check it, it's invasive too.

Interesting point of fact: not all exotics are invasive. Some, like the honeybee in the United States, were imported here and the local ecosystem adapted to them - and depend on them - over time.

 Honeybee on lavender
Photo credit: Ryan Wick, via Flickr (cc by 2.0)

Friday, February 6, 2015

A Naturalist's Thoughts on Winter Weather

February in the Mid-Atlantic can bring a wide range of weather - anything from sunny and balmy to freezing cold and snowy. This variability contributes to a wide range of winter precipitation. A wintry mix is a combination of rain, snow, freezing rain, and sleet.

Rain is liquid precipitation. The diameter of the droplets determines whether you have fog, mist, drizzle, light rain, moderate rain, heavy rain, excessive rain, or a cloudburst. Cloudburst droplets, although small at 2.85 mm in diameter, are over 200 times larger than fog droplets. (I wrote a previous post on this topic called How Big Are Raindrops?).

Snow consists of ice particles frozen into complex, six-sided patterns. Non-branching ice crystals - or diamond dust - form in the shapes of needles, columns, or plates.

Little Brother was very little when I took this photo of him playing in the snow.
He was very proud of his snowman!

Freezing Rain occurs when the atmosphere is warm enough for rain, but ground temperatures are 32 degrees F or lower. The rain freezes instantly when it hits the ground, coating everything in a layer of ice. Freezing drizzle is similar, but the individual drops of water are smaller. Freezing fog occurs when ice crystals are suspended in fog.

 I took this picture after an ice storm last winter. The rain froze the instant it hit the tree branches.

Sleet forms when snow melts in the atmosphere and then refreezes before it hits the ground. Sleet does not stick to objects the way freezing rain does.

This article (sans photographs) was first posted at the Audubon Naturalist Society. Come visit Woodend!