September 2009

KEY ANIMAL SPECIES

Last month we discussed key plant species and this month we discuss key animal species. Key species are those that have a very large positive effect (↑) or very large negative effect (↓) on the number of species in a community, i.e. its species diversity.

Can you tell by looking at the photos below whether the animal species increases (↑) or decreases (↓) species diversity?


Alligator in gator hole (↑)? (↓)? Hogs running very fast (↑)? (↓)?


NEGATIVE KEY SPECIES

Can you predict which species may cause a decline in overall diversity of native species? Which do you think are for species with negative effects () on diversity or present no problem (0)? See how you do with the following contrasts.

Characteristic (↓) (0)

Alien (non-native)

Native
Large (more than 200 lbs.)
Small (less than 2 lbs.)
Generalist diet & habitat
Specialist diet & habitat
High metabolic rate
Low metabolic rate
High reproductive rate
Low reproductive rate

The feral hog and Burmese python have invaded or may soon invade the Refuge. Which of the above characteristics does each have? And so which is more likely to be a negative key species?

FERAL HOG, Sus scrofa,
A NEGATIVE KEY SPECIES

 
Crossbow hunters have killed three hogs Hogs reproduce too fast to be controlled by hunters

Just as with alien plant species, we could have predicted that feral pigs could become a serious problem. As aliens, hogs have no natural diseases though they may be carriers of swine flu. They can grow to 200 - 500 pounds and the one native predator that can eat adults, the panther, is rare and endangered. Hogs are generalist herbivores that dig and root up vast areas of the forest floor in their search for edible roots and tubers. With the high metabolic rate of a mammal they eat a lot and grow fast. And they have a huge reproductive potential:  they mature at age two and have 6-12 piglets in a litter two to three times a year.

 
Hogs can grow to 500 pounds! Massive rooting damage done in one night

Feral hogs are out of control because control methods have been ineffective. The state pays trappers by the day and so there is no incentive to work hard. A better method would be to pay trappers and hunters by the hog!


BURMESE PYTHON, Python molurus bivittatus,
MAY BECOME A NEGATIVE KEY SPECIES

 
Pythons can be 23 feet long!

This huge top predator is listed as likely to become "out-of-control". But it only fits some of our criteria for becoming a negative key species. On the "yes" side it is an alien, very large, a habitat generalist and a diet generalist. On the "no" side it grows slowly, reproduces slowly, and has a low metabolic rate in the long times between big meals. It is known to prey on more than 20 native Florida species in both aquatic and terrestrial habitats. As it grows it can eat progressively larger prey that it kills by constriction. It eats the endangered Key Largo wood rat, muskrats, wading birds, bobcats, and even alligators. It could eat humans as Ogden Nash has written:

The python has, I fib no fibs,
318 pairs of ribs.
In stating this I place reliance,
On a seance with one who died for science.
This figure is sworn to and attested;
He counted them while being digested.


POSITIVE KEY SPECIES

Can you predict which native species are likely to increase the diversity of other native species? To increase local species diversity a positive key species: 1. Could be frequent and abundant enough to be food for many other species; 2. Could selectively eat any species that becomes abundant enough to severely compete with other species; and, 3. Could create new kinds of habitat.

ALLIGATOR, Alligator mississippiensis,
A POSITIVE KEY SPECIES

Alligators are positive key species because they perform "engineering" activities. But alligators' engineering is unlike the engineering by the US Army Corp of Engineers. When the Corp builds levees, reservoirs, and canals, species diversity declines. But when alligators excavate gator holes and build nests, species diversity increases. Laura Brandt, a Biologist formerly at our Refuge, has pointed out that alligators are responsible for the marsh's deepest and highest habitats. When they dig they create deep, wet gator holes. And when they pile up debris to make nests on islands they create high, dry habitat.

 
Gator holes have a greater diversity of plants than in the surrounding marsh

Gator holes have a higher diversity and
abundance of fish (seen here), frogs, birds, and mammals than in the surrounding marsh

 

Gator builds a nest on a tree island


To the tune of Kookaburra:

Alligator digs its gator hole
Promoting diversity is its role
Dig alligator, dig alligator
A place to swim and eat is your goal.


MOSQUITO FISH, Gambusia affinis,
A POSITIVE KEY SPECIES

Mosquito fish are eaten by virtually all other species and this is why they are a positive key species.
 
A 1.5 cm mosquito fish; the dark area by the
ventral fin are live babies about to be born.
Two baby mosquito fish
about to be born and younger embryos

How, you may ask, can a tiny, one inch, fish with a low metabolic rate be food for a diversity of other species? The answer is that mosquito fish are ubiquitous, fast reproducers, and abundant in all fresh-water habitats in south Florida.

Mosquito fish are dense because they are omnivores and have a very high reproductive rate. They bear live young, have 2-3 broods at different stages of development at one time, and reproduce all year.

 

A high density of mosquito fish;
about 1 x 1 meter area shown


Mosquito fish occur in all aquatic habitats at all times because they are adapted to live in even the warmest, shallowest, and lowest-oxygen water. As water level drops seasonally, only smaller and shallower ponds remain. With warming temperature, the oxygen in the deeper water drops to nearly zero and most fish die. But mosquito fish have access to the higher oxygen just at the water surface. With their flat head and upturned mouth they can "breathe" the top mm of water where oxygen is perhaps 10 parts per million since it is in contact with air with 200,000 parts per million oxygen.

 

Note the flat head with upturned mouth of these mosquito fish "breathing" oxygen-rich water at the water surface.


 

 

 

FLORIDA PANTHER, Panthera concolor coryi,
A POSITIVE KEY SPECIES

When asked to name their favorite species, both kids and adults most often name large, mammals like pandas, elephants, and panthers. We call these animals "charismatic mega-fauna."

We can hope that our Florida panther continues to recover and expand its range to the Refuge because it is a key species in the positive sense. It promotes species diversity because it is an umbrella species and because it is a key predator.

The panther is an umbrella species since, when we protect it, we protect many other species. The home range of a panther breeding unit, one male and 2-5 females, is about 200 square miles and encompasses many kinds of habitats and communities. As our largest predatory mammal at the top of many food chains it requires the largest area of reserves. So if we preserve an area large enough for panthers we also preserve other species that require smaller areas.

Panthers are a key predator in that they eat and scare the most abundant prey and thus limit their numbers. Because panthers are a large predator with a high metabolic rate, they have to eat a lot. They will eat the largest and most abundant prey. If hogs or deer become too common the panthers will especially eat the old, young, sick, and weak individuals because they are easiest to catch and kill. But panthers also scare away the healthy prey and so further limit their numbers. Where panthers are present, feral hogs do not root up all under-story plants and native deer do not over-browse all shrubs and small trees.

Until recently, Florida panthers were critically endangered. A major reason was that with a very small population there were genetic and developmental problems due to inbreeding. To solve this problem we introduced six female Texas panthers and this appears to have solved the problems: our Florida panther is now reproducing successfully.

As the panther expands its range it will come in contact with humans. We can coexist if we are careful. Ogden Nash writes:

The panther is like a leopard,
Except it hasn't been peppered.
Should you behold a panther crouch,
Prepare to say Ouch.
Better yet, if called by a panther,
Don't anther.

 

REVIEW QUESTIONS

a. Since feral hogs' closest relatives are domesticated it is unlikely that they could have negative effects on the diversity of native species.

b. As a fish with a low metabolic rate and a maximum length of about an inch, the mosquito fish is so little that it is unlikely to have a positive effect on many other native species.

c. Florida panthers are protected because they are regarded by humans as an example of "charismatic mega-fauna."

d. Douglas Florian writes, "The 'gator's smile is wide enough to stuff a hog inside. But did you know that alligators sometimes swallow second graders?"