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Population limits and species interactions


How big will the population of a species become?

Reproductive potential is a population’s maximum size, if:

all of the species found mates,

all had offspring

none of the offspring get eaten, die from disease, or run out of resources.

In real life this doesn’t occur – there are always limiting factors.

Notice how the population of this fish changes over time.

Fish population growth

Changes in predator (wolf) and prey (moose) populations

Notice how the size of one population affects the size of the other population.

Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates and WH Freeman.

Wolf Moose predator prey graph

Human population growth

Our growth is not like that of most other animals. For thousands of years the human population has shown exponential growth.

This growth obviously will not last forever.

“When resources are unlimited, a population can experience exponential growth, where its size increases at a greater and greater rate.

Are there any noticeable dips in the graph below?

Source: Boundless. “Exponential Growth.” Boundless Biology. Boundless,  https://www.boundless.com/biology/textbooks/boundless-biology-textbook/population-and-community-ecology-45/environmental-limits-to-population-growth-251/exponential-growth-929-12185/

Human population growth

Eventually a population stops growing exponentially, because organisms die due to starvation, disease, predation, bad weather, etc.

Image from http://andyarthur.org/topics/experiences/the-woods/carrying-capacity.html

Deercarrying_capacity_chart surplus decreased

Andy Arthur.org, Creative Commons Attribution 3.0 License.

Learning Standards

Massachusetts Science and Technology/Engineering Curriculum Framework

7.MS-LS2-6(MA). Explain how changes to the biodiversity of an ecosystem—the variety of
species found in the ecosystem—may limit the availability of resources humans use.

7.MS-LS2-2. Describe how relationships among and between organisms in an ecosystem can be competitive, predatory, parasitic, and mutually beneficial and that these interactions are found across multiple ecosystems

7.MS-LS2-3. Develop a model to describe that matter and energy are transferred among living and nonliving parts of an ecosystem and that both matter and energy are conserved through these processes.
Clarification Statements:
• Cycling of matter should include the role of photosynthesis, cellular respiration, and decomposition, as well as transfer among producers, consumers (primary, secondary, and tertiary), and decomposers. Models may include food webs and food chains.

HS-LS2-2. Use mathematical representations to support explanations that biotic and abiotic factors affect biodiversity, including genetic diversity within a population and species diversity within an ecosystem.
Clarification Statements:
• Examples of biotic factors could include relationships among individuals (feeding relationships, symbiosis, competition) and disease.
• Examples of abiotic factors could include climate and weather conditions, natural disasters, and availability of resources.
• Examples of mathematical representations include finding the average, determining trends, and using graphical comparisons of multiple sets of data.

HS-LS2-6. Analyze data to show ecosystems tend to maintain relatively consistent numbers and types of organisms even when small changes in conditions occur but that extreme fluctuations in conditions may result in a new ecosystem. Construct an argument supported by evidence that ecosystems with greater biodiversity tend to have greater resistance to change and resilience.
Clarification: Examples of changes in ecosystem conditions could include modest biological or physical changes, such as moderate hunting or a seasonal flood; and extreme changes, such as volcanic eruption, fires, the decline or loss of a keystone species, climate changes, ocean acidification, or sea level rise

Benchmarks, American Association for the Advancement of Science

In all environments, organisms with similar needs may compete with one another for limited resources, including food, space, water, air, and shelter. 5D/M1a*
The world contains a wide diversity of physical conditions, which creates a wide variety of environments: freshwater, marine, forest, desert, grassland, mountain, and others. In any particular environment, the growth and survival of organisms depend on the physical conditions. 5D/M1b*


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