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Unit 8
10–15% of examEcology is the study of how living things interact with each other and their surroundings. In this unit you'll follow energy and matter through ecosystems, model how populations grow, look at how species affect each other in communities, and predict what happens when an ecosystem is disrupted.
Longer videos that cover the whole unit. Good for a first pass or a final review.
Organisms react to changes around them through behavior, like moving toward light or away from danger, and through changes in their bodies. They also communicate with visual, sound, touch, electrical or chemical signals to warn others, defend territory and find mates, and behaviors that boost survival and reproduction are favored by natural selection.
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Endotherms make their own body heat from metabolism, while ectotherms depend mostly on their surroundings; producers capture energy from sunlight or chemicals, and consumers pass it up trophic levels, losing much of it at each step. Matter, unlike energy, is recycled, moving between living things and the environment in the water, carbon, nitrogen and phosphorus cycles.
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A population is all the members of one species living in an area, and how fast it grows depends on birth rate, death rate and current size. With no limits, a population grows exponentially (dN/dt = rₘₐₓN, where rₘₐₓ is the maximum growth rate per individual), adding more individuals the bigger it gets.
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As a population gets crowded, density-dependent factors like competition, predation and disease slow its growth, giving an S-shaped logistic curve that levels off at the carrying capacity, K (dN/dt = rₘₐₓN(K − N)/K). Density-independent factors, like floods or a hard freeze, hit a population no matter how crowded it is.
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A community is all the populations of different species living together in one place, described by which species are there and how diverse it is (measured with Simpson's diversity index). Competition, predation and symbioses (mutualism, commensalism and parasitism) shape how species get energy and resources, sometimes through niche partitioning or trophic cascades.
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Ecosystems with more species and more variety are usually more resilient, so they handle disturbances better. Keystone species have an effect far bigger than their numbers suggest: remove sea otters from a kelp forest and sea urchins can multiply and wipe out the kelp.
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Ecosystems change when new pressures arrive: invasive species can spread without predators and outcompete native ones, human activities cause problems like biomagnification and eutrophication, and geological or weather events such as El Niño reshape habitats. Which individuals survive depends on variation already in the population, since mutations don't happen because they're needed, and sometimes heterozygotes survive best (heterozygote advantage), as with the sickle cell allele where malaria is common.
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