Understanding Ecosystems: From Organisms to Ecological Succession

Ecologists Study Relationships

Ecologists study the environment at different levels of organization.

What is Ecology?

• Ecology is the study of the interactions among living things and between living things and their surroundings.

Levels of Organization

• An organism is an individual living thing, such as an alligator.
• A population is a group of the same species that lives in one area.
• A community is a group of different species that live together in one area.
• An ecosystem includes all of the organisms as well as the climate, soil, water, rocks, and other non-living things in a given area.
• A biome is a major regional or global community of organisms characterized by the climate conditions and plant communities that thrive there.

Ecological Research Methods

Ecological research methods include observation, experimentation, and modeling.

• Observation is the act of carefully watching something over time.
• Observations of populations can be done by visual surveys.
• Direct surveys for easy-to-spot species employ binoculars or scopes.
• Indirect surveys are used for species that are difficult to track and include looking for other signs of their presence.

Biotic and Abiotic Factors

An ecosystem includes both biotic and abiotic factors.

Biotic Factors

Biotic factors are living things.

• Plants
• Animals
• Fungi
• Bacteria

Abiotic Factors

Abiotic factors are non-living components of an ecosystem.

Cycling of Matter

The Water Cycle

• The hydrologic, or water, cycle is the circular pathway of water on Earth.
• Organisms all have bodies made mostly of water.

Nutrient Cycling

• A biogeochemical cycle is the movement of a particular chemical through the biological and geological parts of an ecosystem.
• Oxygen cycles indirectly through an ecosystem by the cycling of other nutrients.

The Carbon Cycle

Carbon is the building block of life.

• The carbon cycle moves carbon from the atmosphere, through the food web, and returns to the atmosphere.
• Carbon is emitted by the burning of fossil fuels.
• Some carbon is stored for long periods in areas called carbon sinks.

The Nitrogen Cycle

The nitrogen cycle mostly takes place underground.

• Some bacteria convert gaseous nitrogen into ammonia through a process called nitrogen fixation.
• Some nitrogen-fixing bacteria live in nodules on the roots of plants; others live freely in the soil.

The Phosphorus Cycle

The phosphorus cycle takes place at and below ground level.

• Phosphate is released by the weathering of rocks.
• Phosphorus moves through the food web and returns to the soil during decomposition.
• Phosphorus leaches into groundwater from the soil and is locked in sediments.
• Both mining and agriculture add phosphorus to the environment.

Energy Flow in Ecosystems

Energy Pyramids

• Energy pyramids compare energy used by producers and other organisms on trophic levels.
• Between each tier of an energy pyramid, up to 90 percent of the energy is lost to the atmosphere as heat.

Other Ecological Pyramids

• Biomass is a measure of the total dry mass of organisms in a given area.
• A pyramid of numbers shows the number of individual organisms at each trophic level in an ecosystem.

Niches and Community Interactions

What is a Niche?

• A niche is composed of all the physical, chemical, and biological factors that a species needs to survive.
• Niche includes: the type of food a species eats, tolerance to certain abiotic conditions, and a species' behavior.
• Habitat is like an address in an ecosystem, and a niche is like a job in an ecosystem.

Resource Availability

• Resource availability provides structure to communities.

What is a Community?

• A community consists of different groups of species.
• Many species share similar habitats and use some of the same resources.

Competitive Exclusion

• Competitive exclusion can present a problem if two species use the same resource in the same way.
• One species will always be better adapted than others.
• The competitive exclusion principle states that when two species are competing for the same resources, one species will be better suited to the niche, and the other species will be pushed into another niche or become extinct.

Example:

In Great Britain, the introduced North American gray squirrel (better suited) outcompeted and displaced the native European red squirrel.

Other Outcomes of Competitive Exclusion

• Niche partitioning: Species divide resources based on competitive advantages.
• Evolutionary response: Different sizes of teeth could affect the way species eat, leading to niche differentiation.
• Ecological equivalents: Species that occupy similar niches but live in different geographical regions.

Community Interactions

• Competition and predation are two important ways in which organisms interact.
• Competition occurs when two organisms fight for the same limited resources.

Two Types of Competition:

• Interspecific competition: Competition between different species.
• Intraspecific competition: Competition within the same species.

Predation

• Predation is the process by which one organism captures and feeds upon another organism.

More Community Interactions: Symbiosis

• Symbiosis is any relationship where two species live closely together and interact.

Three Types of Symbiosis:

• Mutualism: Both species benefit from the relationship.
• Commensalism: One organism benefits while the other is unaffected.
• Parasitism: One organism benefits while the other is harmed.

Two Types of Parasites:

• Ectoparasite: Lives on the outside of an organism, attaching itself to the outside of the host.
• Endoparasite: Found on the inside of living organisms, feeding on nutrients ingested by their host.

Population Ecology

What is a Population?

• A population is a group of the same species of an organism living in the same area.

How Do Populations Grow?

• Geographic distribution describes the area inhabited by a population.

Population Dispersion Patterns:

1. Clumped dispersion
2. Uniform dispersion
3. Random dispersion

• Population density: Number of individuals per unit area
  • Population Density = # of individuals / area (units²)
• Growth rate: The rate at which a population grows.
• Age structure: The distribution of individuals of different ages in a population.

Survivorship Curves

• Type I: Low level of infant mortality and a population that generally will survive until old age.
• Type II: At all times, these species have equal chances of living or dying.
• Type III: Organisms have a very high birth rate and also very high infant mortality.

Factors Affecting Population Growth

• Number of births
• Number of deaths
• Number of individuals that enter and leave the population
• Immigration: Movement into an area.
• Emigration: Movement out of an area.

Population Growth and Resource Availability

Population growth is a function of the environment.

Exponential Growth

• Occurs when a population has abundant space and food.
• Happens when individuals in a population reproduce at a constant rate.
• Lack of limiting factors causes population growth to decrease.

Biotic Potential

• When a population has unlimited resources and continues to grow to its full living potential, it is called its biotic potential.
• A population that has reached biotic potential has reached its maximum reproductive capacity.

Limiting Factors

A limiting factor is a factor that causes population growth to decrease.

Density-Dependent Limiting Factors

• Predation: Population size is often controlled by predation, which takes place in the predator-prey relationship. It is the best mechanism of population control.
• Parasitism and Disease: Parasites and diseases can limit the growth of a population. They take nourishment at the expense of their hosts, often weakening them and causing disease and death.

Density-Independent Limiting Factors

• Natural causes, not caused by animals within the population (e.g., natural disasters).

Age Structure and Population Growth

• Population growth depends on how many individuals of different ages make up a given population.
• Populations with large numbers of young offspring have greater potential for rapid growth.

Ecological Succession

• Succession is the sequence of biotic changes that regenerate a damaged community or create a community in a previously uninhabited area.
• Succession can result from slow changes in the physical environment.
• Sudden natural disasters or human activities also play a role in ecological succession.

Primary Succession

• The establishment and development of an ecosystem in an area that was previously uninhabited.
• Occurs on land where no soil exists.
• Occurs on surfaces formed as volcanic eruptions build new islands or cover the land with lava rock or volcanic ash.
• Can also occur on bare rock.

Secondary Succession

1. Begins in a place that already has soil and was once the home of living organisms.
2. Occurs faster and has different pioneer species than primary succession.
3. Often seen after a natural disaster such as a forest fire or volcano.

Example of Succession:

Glacier -> Soil formation -> Lichens -> Grass -> Annual plants -> Biennial plants -> Pines