Ecology

Introduction

• Ecology is the study of interactions between species and with the non-living environment.

Levels

• Population: all members of one species living in a given area.

• Community: all living organisms (many different species) living in a given area

• Ecosystem: the organisms in a given area along with the non-living environment (rain, temperature, soil, etc.). – large scale ecosystems are called biomes.

• Biosphere: the entire living world and its non-living environment. The portion of Earth that supports life. Roughly 2 miles below the surface to 6 miles above the surface, over the entire planet.

Population Dynamics• How a species is distributed and how it

changes over time. • populations can stay steady with time• or, they can undergo exponential

growth (a J-shaped curve, 1-2-4-8-16-32-...). Ends up increasing very quickly, but the rate can vary. The doubling time of a population is the critical parameter: How long it takes to double the population size.

• or, they can grow exponentially for a while and then level off as limits are approached (logistic growth).

• carrying capacity : how many of a species can a given area support. If the population grows much past the carrying capacity, it will have a die off.

– limiting factors: food, waste disposal, predators, nesting/mating sites

Population Age Structure

• Relative number of individuals of different ages and sexes.

• How many are of reproductive age vs. before it or after it?

• predicts future structure: groups move up in age but relative sizes of groups stays the same.

r-Selected vs. K-Selected Species• Two fundamentally different strategies.• K-selected, or equilibrium species.

Produce a small number of offspring, enough to replace each generation or have a slow population growth rate.

– Lots of energy into each offspring. – Late maturity, many live to old age,

parents care for their offspring. – Population size stays near the carrying

capacity of the environment. – Humans and most other large

mammals are K-selected.• r-selected, or opportunist species.

Produce vastly more offspring than can survive.

– If times are good, many will survive and if times are bad only a few will survive.

– Allows rapid exploitation of new resources.

– Small organisms, early maturity. cheap to make, short life expectancy, no parental care.

– Mosquitoes, flies, many fish

Community Interactions• A community is all the organisms that live is a

particular region.• Habitat: physical surroundings inhabited by a

community of organisms. The type of place where they live. Influenced by temperature, soil, rainfall, etc.

• Niche. How an individual species makes a living: What it eats, where it lives, how it competes with other species. its food resources, mating and offspring rearing resources, defenses against predators and diseases.

– All species in a community share a common habitat, but each species has its own niche.

– fundamental niche vs. realized niche. Fundamental niche is what a species could occupy in the absence of competition and other constraints. realized niche is the niche the species actually sues. realized niches change over time as the species responds to pressures.

• The important thing about the niche is that it is heavily influenced by interactions between species.

Competition• Different species often have

similar requirements for resources, but never as similar as members of the same species.

– competition within a species is the driving force in microevolution: individuals with more fit alleles take over the population.

• competitive exclusion: each niche can have only 1 occupying species. Others will be driven out.

– the diagram: 2 species of paramecium will grow well alone, but one dies of if they are mixed.

• resource partitioning: dividing up a niche as a method of co-existing

Predation and Parasitism

• predator-prey cycles: too many predators means prey population size crashes, leading to predator crash.

• successful parasites don’t kill all of their hosts.

• large amount of evolutionary change driven by arms race between predators (or parasites) and prey

Evolutionary Arms Race• Co-evolution: species influence

each other’s ability to survive and reproduce.

• Camouflage: hiding from predators• Defenses: painful sting, sharp

claws, bad taste.• Warning coloration: Warn the

predator about prey’s defenses so it learns to stay away.

• Mimicry as a tool: fool predators by looking like another species that is unpleasant or dangerous to attack.

• Of course, predators evolve ways to get around these defenses: sharper senses, chemical detoxification.

Mutually Beneficial Interactions• Symbiosis: two species living together.

Some symbioses are mutually beneficial and others (parasitism) are not.

• mutualism: both species benefit. – Pollination of flowers is a good

example: the flower supplies nectar (sugary nutrients) and the bee spreads the flower’s pollen from one individual to another. Many such interactions between flowering plants and their pollinators.

– Nitrogen-fixing bacteria invade root nodules on certain plants: the plants get fixed nitrogen and the bacteria get nutrients and shelter from oxygen, which would kill them.

• commensalism: one benefits, the other neither gains nor loses.

– Birds nesting in trees. The birds benefit, but the trees are unaffected.

Community Succession• How does a community come into

existence? • Classical model of succession:

– a barren lifeless area starts out with a few very tough species

– These pioneer organisms improve conditions so that other species can move in.

– a series of different communities takes over in turn, each one improving conditions for the following community

– ends in a steady state, self-sustaining climax community

• Example:– bare rock is colonized by lichens and

mosses. As they decay, they build up a thin layer of soil.

– Grasses and other small plants can grow on this thin soil.

– They in turn improve the soil, allowing shrubs to grow.

– These are followed by small trees which shade the ground.

– Other trees can grow well in the shade, and they eventually take over.

More on Succession• primary vs. secondary.

– Primary succession is taking over a lifeless area: new land formed after volcanic eruption or a glacier’s retreat.

– Secondary succession is an area whose climax community has been disrupted, such as after a fire, or an abandoned farm field. Starts with some life already present. Many communities require periodic fires to stay healthy.

• The environment doesn’t stay stable forever: no climax community lasts forever.

• New species can radically alter a community.

– grasslands are now farmlands– exotic species can take over an area:

zebra mussels, kudzu, chestnut blight, starlings, and many more.

Energy Flow in Ecosystems• Energy flow is a one-way process: from

high quality sunlight to low quality waste heat. Energy is lost to heat at every step.

• Three basic actors:1. primary producers: convert energy from

sunlight into chemical bond energy in sugars and other organic molecules.

2. Consumers: eat plants and each other. Herbivores eat plants, carnivores eat other animals.

3. Decomposers eat dead organisms.• Trophic levels. We can classify

organisms according to a hierarchy of feeding relationships: who eats who. How close an organism is to the primary producers.

– Several levels of consumer, starting with primary consumers (herbivores), then several levels of predator, ending at a top-level predator.

– Similar levels among the decomposers, starting with large decomposers such as carrion-eating birds, then going down through insects, fungi and finally bacteria.

Energy Pyramid

• Each level of consumption above the primary producers involves about a 90% loss of energy. This implies that the biomass at each stage is 1/10 the biomass of the stage below it.

• Different areas of the world have different levels of primary productivity, which is the rate at which biomass accumulates.

Nutrient Cycles

• Unlike the one-way flow of energy, matter is continuously recycled.

• Elements such as carbon and nitrogen often change their state: for instance carbon can exist as organic compounds, as carbon dioxide in the atmosphere, or as calcium carbonate rock.

• Environmental reservoirs, especially geological reservoirs, can hold large amounts of nutrients for very long times

• Nutrients cycle very rapidly through living organisms • We are going to talk about a few cycles, but every

element used in living organisms has a cycle.

Carbon Cycle• Most of the carbon on Earth is found

in rocks, as calcium carbonate or as coal and oil.

• The next largest pool of carbon is the oceans, where carbon dioxide is dissolved in the water.

• The atmosphere holds a significant amount as carbon dioxide

• Plants and cyanobacteria (especially in the oceans) convert carbon dioxide into organic carbon compounds.

• Other organisms convert organic compounds back into carbon dioxide.

• Also, significant amounts of organic carbon is converted to carbon dioxide by humans burning fuel.

• The carbon dioxide level is increasing in the atmosphere. Since carbon dioxide reflects heat back to the surface (it’s a greenhouse gas), the Earth’s temperature is slowly increasing.

Nitrogen Cycle• The largest pool of nitrogen is in the

atmosphere, which is about 80% nitrogen gas.

• Nitrogen is “fixed”, converted to a form usable by living organisms, by nitrogen-fixing bacteria. Several different types of bacteria interconvert three main forms of fixed nitrogen: ammonia, nitrate and nitrite.

• Lightning in the atmosphere fixes some nitrogen.

• Artificial nitrogen fixation: the industrial production of fertilizer, is also an important factor.

• Most living organisms keep nitrogen in fixed form. However, denitrifying bacteria convert it back into nitrogen gas.

• Nitrogen fertilizer leaching out of fields into ground water sometimes ends up over-fertilizing bodies of water. This leads to blooms of algae which use up all the oxygen in the water and kill most other organisms.

Hydrological Cycle• Water is composed of oxygen and

hydrogen, but it can be considered as a single cycle for the most part.

• Water evaporates from the oceans and other surfaces, then precipitates out as rain or snow.

• Some of it goes into the ground, but eventually nearly all of it ends up in bodies of water and ultimately the oceans.

• The primary problem: getting enough fresh (non-salty) water to where it is needed.

– One obvious solution: don’t live in the desert.

• However, water does decompose into hydrogen and oxygen. Hydrogen gas is very light, and at the top of the atmosphere hydrogen gas is slowly escaping into space.

– Because the Moon has less gravity, most of its hydrogen has already escaped. This lack of hydrogen is the primary difficulty that any Moon colony will face.

Climate and Atmosphere• Our climate is created by interactions of the

atmosphere, the oceans, and the land.– the Sun heats the equatorial region more than the

poles.– the Earth rotates– land absorbs and gives up heat faster than the

oceans• Mountains create rain shadows: winds blowing

over them have to release their water before the air can get over the top. Thus the far side is often very dry: a major cause of deserts.

• Monsoons: dry land in tropical places gets heated by the sun. Warm air is low pressure, and this draws in moisture-laden air from surrounding oceans.

• Bodies of water increase the amount of moisture in the air and moderate temperature changes.

– It is colder in the winter and warmer in the summer here in DeKalb than it is near Chicago

– Chicago’s lake effect snow: warm moisture-laden air meets cold air over the lake and water precipitates out.

• At our latitude (roughly 42o North of the equator) the winds mostly blow from the west. Other latitudes have different directions of prevailing winds.

– Big effects on yearly temperatures and precipitation

Ocean Circulation• Water in the oceans moves more slowly

than air in the atmosphere, but water circulates much more heat throughout the world.

• Also, ocean currents move nutrients and oxygen around the world.

• The oceans are one continuous body of water: Earth is more water than land.

• The ocean waters flow in a continuous conveyor belt powered by temperature and salt content.

• Warm, less salty water flows near the surface.

– The Gulf Stream is part of this: warm water from tropical America keeps Europe warm. For instance, London England is a t51oN. That latitude is about 700 miles north of us, a very cold part of Canada.

• As it reaches colder areas near Greenland and Antarctica, it sinks down to become a cold, saltier current.

• The cold current picks up nutrients from the ocean bottom. The oceans at high latitudes (north and south) are very productive.

• The warm current picks up oxygen from the atmosphere and mixes it in with all ocean water

Terrestrial Biomes• We are now going to briefly examine several of the major

ecosystems on Earth, the biomes.• Each biome has characteristic temperature and rainfall

levels, and corresponding characteristic plant life and productivity.

• tundra• taiga (boreal forest)• temperate forest• grasslands• tropical rain forest• desert

Tundra• tundra: cold, low lying. • Far north and high on mountains.

– going south, Antarctica is mostly covered in ice, and the tips of South America, Australia, and Africa are not far enough away from the equator

• Cold is the dominating factor. • Also dryness: less than 10 inches of

rain per year. • Dominated by lichens, low shrubs,

grasses. Low productivity, short growing season.

• Animals have adaptations to the cold, including antifreeze in the bodily fluids of some insects.

• Massive blooms of mosquitoes and biting flies in the summer.

• Permafrost (permanently frozen ground) just under the surface: leads to soggy melted zone on top in summer.

Taiga

• Also called “boreal forest”. • Cold, less dry than tundra.

Some of it quite swampy even.• Dominated by conifer forests.

– Not losing their needles means thy can do photosynthesis whenever it is warm and sunny enough, not just during a short growing season.

– Needles covered with wax to prevent drying out.

• A number of large mammals.• Harsh in winter: animals either

migrate south or hibernate.

Temperate Forest

• Also called deciduous forest, meaning that the trees lose their leaves in the winter.

• Our local biome, stretching eastward to the Atlantic Ocean.

• Hot in the summer, cold in the winter, abundant rainfall. Relatively long growing season, but not year-round.

• Many animal species. And plants too. Very rich habitat easily inhabited by humans.

Grasslands• Also called prairie, steppes, pampas,

veldt: different names for the same phenomenon.

• Drier than forested regions. • Plants need to survive droughts. • Fire is another problem: it’s dry.

Grasses survive fire much better than trees because the growing point of grasses is below the ground.

• Stretches west from Illinois to the Rocky Mountains, gradually getting drier.

• Dominated by grasses. • Largely converted to farming in many

areas, due to very fertile soil. – Add some trees and shrubs =

savannah, dry shrublands, dry woodlands

Tropical Rain Forest

• Heavy rain, lots of vegetation.

• Continuous canopy of trees, with very little light reaching the ground.

• Soil is very poor: all the nutrients are tied up in living things. – On the other hand, many

animal species never go down to the ground because there is so much nutrition available up high.

• Very high species diversity and productivity.

Desert• Less than 10 inches of rain per

year—very dry. • Some are hot (such as the Sahara

or the Mohave in California) and some are cold (such as the Gobi desert in Mongolia).

• Sparse vegetation, but it can grow very quickly in response to rain.

• Adaptations to saving water: thick fleshy plant leaves, animals often live underground, are most active at night, and have urine-concentrating mechanisms.

– some use for crops: Imperial Valley in California. But irrigation is necessary and the soil tends to get salty.

• Deserts usually have grasslands next to them, as rainfall levels increase.

Aquatic Ecosystems

• freshwater: lakes, rivers, wetlands

• estuaries: river meets ocean, giving water that is less salty than sea water.

• oceans

• Freshwater ecosystems are often very productive

Oceans• Intertidal and coastal zones. The most

productive parts of the ocean are near the shore, due to wave action carrying in fresh nutrients and lots of sunlight

– also coral reefs (secreted by cnidarian animals) are very rich communities.

• Ocean surface: lots of sunlight, but little nutrients in many places. Most productive in polar regions.

– Photosynthesis by “phytoplankton” = algae, seaweed.

– Fed upon by “zooplankton”, mostly crustaceans and the larval stages of ocean invertebrates.

– Fish and sea mammals and birds prey on zooplankton.

• Deep ocean: unlighted below 100 meters, so all nutrients fall from above.

– Very low productivity. Almost all organisms are decomposers or predators.

– But, there are thermal vents that release high energy inorganic compounds that Archaea and other chemotrophs can use. These form the basis of communities in the ocean deeps.