Communities & Ecosystems

Communities

• A biological community is an assemblage of the populations of all the organisms that live close enough together to interact.

• South Florida communities include: pinewoods (upper right) and tropical hardwoods hammocks (lower right)

• The characteristics of communities include:– Species diversity– Dominant species– Trophic structure– Response to disturbance

Competition

• In a community there may be interspecific competition for resources, such as food, light, living space, and nutrients.

• A resource that is in short supply may act as a limiting factor.• The competitive exclusion principle states that if two species are

competing for the same limited resource, any difference in the ability to efficiently use this resources, will lead to a reproductive advantage for one species with the eventually elimination of the other species.

• Each species has a niche or specific role in the community.• If two species have exactly the same niche, then either one will

become extinct or it will evolve to use a different set of resources. • The latter is known as resource partitioning.

Predators and Prey

• Predation is an interaction between species in which one species, the predator, kills and eats another species, the prey.

• Natural selection has lead to many adaptations to both predator and prey species. For predators This may include speed and agility, poison injecting fangs and stingers, sharp claws and talons, ir keen vision to spot the prey. Likewise, prey species may be swift, may have keen senses of smell, hearing, or vision to detect the presence of predators.

• Beyond these obvious adaptations, there are some others that merit special attention. On the next several pages, we will look at four of these: cryptic coloration, warning coloration, Bastesian mimicry and Mullerian mimicry.

Batesian Mimicry

• Batesian mimicry can be best seen in insects. It occurs when an edible species closely resembles a distasteful, unpalatable species. Predators that learn to avoid the distasteful form also avoid the edible mimic. This presumes that the model is usually more abundant than the mimic.

• The Queen butterfly (upper right), whose larva feed on poisonous milkweed plants has an edible mimic, the Viceroy (lower right).

• The Queen butterfly is closely related to the unpalatable Monarch butterfly.

Mullerian Mimicry

• The larva of all of the Heliconid

butterflies (seen to the right) feed on passion flower vines and store the toxic compounds found in their leaves. Mullerian mimicry occurs when a number of distasteful species, found in the same area, have a similar color pattern. A predator becoming imprinted on one heliconid, tends to avoid other butterflies with similar patterns. Thus all receive protection from predation by their mutual resemblance.

Warning Coloration

• Bright colors, such as reds and oranges, can be used as warning colors. Predators quickly learn to recognize and avoid unpalatable or toxic brightly colored animals.

• The colors of this poison arrow frog are one example.

• In South Florida the larva of the Atala butterfly are easily seen, due to their red colored body and yellow spots. They also feed on the toxic leaves of the coontie (a native cycad).

Cryptic Coloration

• Cryptic coloration (camouflage) allows a predator to wait, unseen by its prey.

• It also allows animals to hide from predators by blending into the background.

• Normally this Sphinx moth would have its wings closed, hiding the abdomen and lighter colored hind wings.

• Can you find the stone fly larvae on the rock to the right?

Predation – Keystone species

• A keystone species is a predator that maintains strong control over community structure. A keystone species of the everglades ecosystem and an indicator of its health is the American alligator. This ancient reptile builds "alligator holes" that provide an important food and water source for many other animals in times of drought.

Coevolution of plants and herbivores

• Plants have evolved many physical and chemical defenses against herbivores. Sharp spines, thorns, bad tasting chemical are but a few.

• Another interesting example is the coevolution of ant and bull-horn acacia trees. The ants live in the hollow horn (thorns) and eat sugar-rich nectar secreted by the plants. In turn they quickly attack any herbivorous creature that tries to dine on the acacia.

• Studies have shown that the removal of ant colonies from the acacia trees, negatively affects the overall health of the trees.

Symbiotic relationships

• Symbiotic relationships are interactions between two or more species that live in direct contact with each other. These relationships can take many forms, though the commonest are commensalism, mutualism and parasitism.

• The example to the right is commensalism. The Cape Buffalo stir up insects upon which cattle egrets feed. Small tick birds have a mutualistic relationship with the buffalo. They feed upon ticks and other pests of the large herbivore. They get food, while the parasites are removed from the buffalo.

• In South Florida, cattle egrets can often be seen following tractors and lawn mowers. We are now part of the symbiotic pathways!

Symbiotic relationships 2

• In the next slide there is an example of both parasitism and mutualism. Aphids are a major plant pest. They such the sap out of the plant and their sweet secretions may cause a heavy growth of sooty mildew on the leaf surfaces. These same aphids are ‘farmed’ by ants. The ants feed on the sugar rich secretions of the aphids, move them to better sites, and protect them from predators.

• Ants and aphids can be seen in the right illustration.

Parasitism and Mutualism

Disturbance and Succession

• Communities are constantly changing in response to disturbances.

• This leads to ecological succession, where species are gradually replaced by a succession of other species.

• If this begins in a bare, lifeless area, it is called primary succession. Secondary succession occurs when an existing community is destroyed or disrupted.

• In South Florida, disturbances may be fire, a hurricane, lowered water table, or even agriculture.

• The final stage of succession is called the climax. The climax community in South Florida is the tropical hard-wood hammock, however periodic fire kept much of the area in pinewoods. This is called a fire climax.

• Old abandoned farmlands in North Carolina will undergo secondary succession and slowly return to temperate forest.

Succession (in Alaska)

Trophic levels• Feeding levels are called trophic levels. At the base of every food chain and

food web are the primary producers. These are photosynthetic organisms that fix solar energy into sugar. Primary consumers feed on the primary producers. In turn primary consumers fall prey to secondary consumers. Tertiary and quaternary consumers are found at successively higher trophic levels.

Trophic levels continued

• Another important trophic level is that of the decomposers or detritivores. These are the organisms that consume dead and decaying organic matter. They are critical to the recycling of nutrients and minerals. The materials that they release are available

• Animals, such as, crayfish, vultures, earthworms and mice often play this role.

• However, the most important recyclers are fungi and bacteria. If you turn over a pile of leaves that have been on the ground for a long time, many of the leaves will be held together by white thread-like fungal filaments. Did you know that when you see a mushroom in a field you are only seeing the reproductive structure of a fungus? The rest of it is spread out through the dead organic matter in the soil.

• Food chains show the steps in the energy flow pathway.

• While it is a useful concept, the feeding relationships within a real community are rarely that simple.

• Think of your supper. Unless you are a Vegan (pure vegetarian) your meal probable represents several trophic levels. That doesn’t fit neatly into a food chain like the one you see at the right.

Food Chains

Food Webs

• Food webs can be used to study the complexity of energy flow through a community or ecosystem.

• The web at the right could be even more complex. What kinds of fishes? What is their relative abundance? Would it matter if a particular species of copepod were eliminated by disease? Does the loss of one species of birds affect the web or do the others increase in abundance once it is eliminated?

Energy Flow and Chemical Cycling

Primary Production

• Three terms used in ecology are biomass, standing crop and productivity. Don’t confuse them!

• Biomass is a measure of the amount of biological material.• Standing crop is the amount of biomass at a point in time.• Productivity is a measure of the rate at which biomass is

produced.• Imagine two one acre fields of grass. You cut the grass in both

fields one inch from the ground, and then you weight the grass. Field A has 1000 lbs of biomass, while Field B has 800 lbs of biomass.

• Is field A more productive? You can’t say unless you know the rate of production of grass. It is July 20th. Field A hasn’t been mowed since May 5th . Field B was mowed May 20th, June 15th, and July 1st. Even though the standing crop in Field A was greater, the productivity in Field B was much greater.

Energy Pyramids and Energy Flow

• Pyramids are a way to show energy relationships. These may take the form of a pyramid of biomass or a pyramid of numbers of organisms. The general rule is that there is about a 10% transfer of energy from trophic level to the next.

• In the example to the right, the upper pyramid is typical of most energy pyramids.

• How could you explain the lower pyramid? It is very

atypical.

Water Cycle

• Freshwater is critical for plants and animal communities. The water cycle can be see at the right. There is a net evaporation of water over the oceans. Water vapor moves over land, driven by atmospheric patterns of circulation. Rainfall on the plants is taken up by plants, but also sinks into the soil and/or forms lakes and rivers, eventually returning to the oceans. A small amount of available water is trapped in polar and high elevation ice caps.

Carbon Cycle

• Life on earth is based on carbon compounds. All of the energy-rich organic molecules of living systems are constructed using carbon.

• The carbon cycle can be seen at the right. The oceans can serve as a carbon sink, however it is a limited sink. The huge amounts of carbon dumped into the atmosphere by the burning of fossil fuels threaten to upset the atmospheric patterns of temperature and rainfall. More about that in our last lesson.

Nitrogen cycle

• Nitrogen is an essential nutrient for plants.

• Although over 78% of the atmosphere is comprised of nitrogen, it is not available to plants in the form of N2.

• As seen in the diagram at the right, nitrogen fixation and cycling depends upon the action of species of bacteria. They fix atmospheric nitrogen, decompose the amino groups of proteins to ammonia, convert ammonia to nitrite and nitrite to nitrates.

Phosphorus cycle

• Phosphorus is needed By both plants and animals. It is an essential part of ATP and the nucleic acid molecules that carry genetic information.

• Mosat phosphorus comes from rocks, and is released through the process of weathering.

• Phosphorus can be a limiting factor, due to the difficulty in releasing from rocks and sediments.

• The lack of phosphorus in lakes often limits the growth of algae and other plant life.

Eutrophication

• Runoff from man’s activities, such as agriculture or lawn fertilizing, can remove phosphorus as a limiting factor and lead to tremendous algal blooms.

• Eutrophication occurs when nutrients accumulate in a lake and the growth of vegetation is accelerated. This can be a natural aging process, but more frequently is caused by the addition of nutrients directly or indirectly.

Deforestation• When land is cleared and the soil is denuded of vegetation, there is nothing

to hold the soil in place when rain falls. As a result, silt and nutrients are washed into streams and rivers, and alter the aquatic communities found there

Deforestation

The loss of forests in Haiti, often leads to massive flooding and erosion during the heavy rains associated with tropical storms.

Massive scale land clearing for oil palm plantations in Indonesia threatens biodiversity and leads to polluted waterways.

Scenes like the one to the right can be found throughout the tropical regions of the world.

In our next and last lesson, man’s impact on the environment will be examined.