August 2009

 KEY PLANT SPECIES

Key species are those that have a very large positive effect (↑) or very large negative effect (↓) on the species diversity of a community.

This month and next we will discuss key plant species and key animal species found at The Refuge or in The Everglades. Can you tell by looking at the photos below whether the species promote (↑) or inhibit (↓) the diversity of other species?
 

Alligator in gator hole (↑)? (↓)?	Hogs running (↑)? (↓)?
Dense climbing fern (↑)? (↓)?	Pond apple with birds' guano and nests (↑)? (↓ )?

We will start with plant key species and discuss animal key species next month.


PLANT SPECIES WITH KEY NEGATIVE EFFECTS

Here are some characteristics of key species. Which do you think are for species with negative effects (↓) on diversity or present no problem (0)? Usually it takes a combination of characteristics for a plant species to have large negative effects (↓) . Vote!  

Non-native species are more likely to be invasive and to take over communities because they often do not have natural enemies from their home lands. In south Florida ~ 40% of our plant species are non-native yet only less than 0.01 % of these are invasive and cause great declines in native plant and animal species diversity (↓). Why?

Species with a high growth and reproductive rate are most likely to be problems. We can look at how many months of a year a species reproduces and the number of seeds or offspring it has at one time. If the species also reproduces asexually by root sprouts or runners then it is even more likely to be a problem.

A corollary of reproductive rate is dispersal ability. If a species disperses far and often then it is more likely to spread and become a problem.

Here are two Refuge aliens with the characteristics that we hypothesize could result in negative effects on native plant diversity.

MELALEUCA TREE, (Melaleuca quinquenervia)
A NEGATIVE KEY SPECIES

Melaleuca has millions of windborne 1 mm seeds, can have root sprouts, occurs in dense stands of trees less than a meter apart, grows as much as 2 meters per year, sheds thick bark litter that covers the ground, and is fire resistant. These two photos support the hypothesis that Melaleuca is a negative key species.
 

An extremely dense stand burns but the trees are not killed and millions of seeds are released.	Almost no plants can grow in the thick litter of Melaleuca

OLD-WORLD CLIMBING FERN, Lygodium microphyllum
A NEGATIVE KEY SPECIES

Climbing fern has trillions of wind-borne 0.1 mm spores, spreads asexually by underground stems, has 100 per cent cover of fronds and stems, grows as much as 10 meters per year, and both promotes fire and is resistant to fire. This photo supports the hypothesis that climbing fern is a negative key species

Another support of our hypothesis that climbing fern and Melaleuca have negative effects on species diversity is that the greatest percent of our Refuge budget is spent to eradicate them.

Climbing fern covers everything and kills all other plants

PLANT SPECIES WITH KEY POSITIVE EFFECTS

To begin with we expect positive key plant species to be native and have characteristics listed as "no problem" in the pairs of characteristics at the beginning of this essay. These characteristics include natural enemies, a tree growth form, slow growth, and possible rewards to animals. As examples we will discuss strangler fig, cypress, and pond apple.


STRANGLER FIG, Ficus aurea,
A POSITIVE KEY SPECIES

Strangler fig is a key species because it provides fruit to animals year around. Figs reproduce all year because they have a specialized mutualistic relation with fig wasps (see my column "Plant-Animal Mutualisms I" on pollination). The fruit are especially nutritious for birds and mammals because they provide sugars in the fruit flesh, starch and fats in the seeds, and also protein and fat from the developing fig wasps. I find the fruit very tasty.
 

Strangler fig branch with fruit	Close-up of strangler fig fruit

Different species of figs grow throughout the tropics; they are especially important continuous sources of fruit in rain forests. Even though tropical forest fig trees are relatively rare and do not fruit in synchrony, a few fruiting trees are always present and all the birds and mammals learn where they are. It would be interesting to see if strangler figs are equally important key species at the Refuge and in the Everglades. It is possible that they are key species because they can start life anywhere. They can germinate and establish as epiphytes, on rocks, or in the ground.


CYPRESS, species of the Taxodium genus
A POSITIVE KEY SPECIES

In historic times, cypress swamps were larger around Lake Okeechobee and along the eastern fringe of Everglades marshes. Because the very rot-resistant cypress were excellent timber trees, virtually all were cut down. Our Refuge cypress swamp was clear-cut 70-80 years ago, so the trees are nowhere near their maximum age of over 1,000 years and maximum diameter of over 1.5 – 2 meters. Locally the only huge cypress are along the upper part of the Loxahatchee River.
 

Young cypress swamp at Refuge with lots of sun, many kinds of ferns and air plants, and a mix of other small trees, shrubs, and herbs	Old-growth cypress swamp with little sun and a relatively low diversity of under-story plants

One reason our cypress swamp supports so many plant species is that the understory light level is high. In our winter dry season, all cypress drop their leaves (deciduous) and the many air plants bloom and shed their wind-dispersed seeds at that time. In summer our swamp's pond cypress have leaves and foliage that cast very little shade so many plants grow in the under-story.

Another reason for the high plant diversity in our cypress swamp is that stumps provide a nursery for other plants that can not survive in water. Other than the many epiphytic lichens and bromeliads (see column "Lichens & Bromeliads"), virtually all the species of trees, shrubs, herbs, and ferns are on old decomposing stumps. No species can establish from seed in the 6 - 10 months that the swamp is flooded and cypress can only establish in a multi-year drought, when they grow tall enough to survive when next submerged.

In this photo, bushes and other plants grow on an old cypress stump; note that there are no plants in the bare ground that will soon be flooded during the rainy season.

There are not even aquatic plants in the cypress swamp, probably because the water is too acidic.


 

POND APPLE, Annona glabra,
A POSITIVE KEY SPECIES

Historically a one to three mile wide pond apple swamp occurred along the southern edge of Lake Okeechobee. These pond apples were responsible for development of a high nutrient muck soil in two ways. They slowed the water flow, allowing particulate organic matter to settle, and their dead leaves decomposed. The resulting rich soils were drained and cleared for agriculture.

Here is a portion of the remnant pond apple swamp with virtually no under-story plants.

Note the pond apple buttressing, multiple trunks, and contorted spreading branches. This makes them preferred nesting trees for wading birds. In addition, some epiphytes and vines, including the endangered Okeechobee gourd, grow on them.
 

Wading bird rookery in pond apples

POND APPLES SUPPORT MANY MORE ANIMAL SPECIES THAN CYPRESS

We observe that our young cypress swamp has high plant diversity but low animal diversity and pond apple swamp have low plant diversity but high animal diversity. Why? This author and Dr. Larry Harris have several complementary hypotheses that may explain these observations.

H1. Pond apples, like palms and maple, are Angiosperms (flowering seed plants). Angiosperms had an explosive co-evolutionary diversification with mammals and higher insects.

H2. Cypress, like pines, are Gymnosperms (non-flowering seed plants). Gymnosperms are more ancient than Angiosperms and so, perhaps like even more ancient ferns, they have evolved very effective defenses against herbivores.

H3. Angiosperms have coevolved rewards to pollinators and seed dispersers such as nectar, pollen, and palatable fruit.

Dr. Larry Harris' prose nicely captures the difference between Angiosperms and Gymnosperms that support our hypotheses.

Angiosperms are an absolute cornucopia for wildlife. Angiosperms produce products to hook myriad wildlife to do their work for them. Compare maple syrup for sapsuckers to pine turpentine -- ughh. Compare wheat and corn (grains) and cherries and apples (fruits) to pine cones and cypress balls. Compare bee-hive productivity of a gallberry and palmetto lease area to that of a cypress swamp -- not many bee-owners will opt for cypress!

In his best book, in my opinion,The Botany of Desire, Michael Pollan supports his thesis that four Angiosperms (tulip, apple, potato, and marijuana) have evolutionarily co-opted humans' desires.

In summary Angiosperm plants provides many more goods and services to humans and wildlife than Gymnosperm plants.


REVIEW QUESTIONS

a. An alien tree species with slow growth and few large seeds dispersed by mammals is likely to cause a decline in native plant species diversity.
b. Angiosperms are flowering seed plants that provide a cornucopia of goods and services to wildlife and humans.
c. The 80 year old cypress swamp at the Refuge has a low diversity of animal species but a high diversity of plant species.
d. Strangler fig is a positive key species because some large individuals are in fruit at all times and so provide a reliable source of very nutritious fruit to a wide variety of birds and mammals.