Species Interactions in Ecosystems (College Board AP® Environmental Science): Study Guide

What are ecosystems?

Ecosystems definition

• An ecosystem refers to a community of living organisms, along with their physical environment, interacting as a system within a specific area

  • This includes the living, biotic components (such as plants, animals, fungi and microorganisms) interacting with the non-living, abiotic components (such as soil, water, air, sunlight, temperature, humidity and minerals)

  • These abiotic components provide the essential resources and conditions needed for the survival and functioning of the biotic community

Species definition

• A species is a group of organisms sharing common characteristics that interbreed to produce fertile offspring

  • This is known as the biological species concept

  • Members of the same species share a gene pool, meaning that they can breed and produce offspring with similar traits

Populations definition

• A population is a group of organisms of the same species living in the same area at the same time

  • One species may consist of any number of populations, from a single populations to many populations

Community definition

• A community includes all of the different populations (of different species) living in the same area at the same time

  • A community is a collection of interacting populations within an ecosystem

  • For example, each species within a community depends on other species for food, shelter, pollination, seed dispersal, etc.

Habitat definition

• A habitat is the local environment in which an organism, species, population or community normally lives

  • E.g. badgers, deer, oak trees and ants are all species that would live in a woodland habitat

Ecological niche definition

• The role that species plays within an ecosystem is its niche

  • This includes where in the environment the organism is, how it gets its energy, how it interacts with other species and its physical environment

• A niche can only be occupied by one species, meaning that every individual species has its own unique niche

  • If two species try to occupy the same niche, they will compete with each other for the same resources

  • One of the species will be more successful and out-compete the other species until only one species is left

  • The other is either forced to occupy a new, slightly different niche or to go extinct from the habitat or ecosystem altogether

Predator-prey relationships

• Consumers that kill and eat other animals are known as predators, and those that are eaten are known as prey

• In a stable community, the numbers of predators and prey rise and fall in cycles

  • This limits the population sizes of both predators and prey

• The graph below shows some of the key patterns of predator-prey cycles:

  • The number of predators increases as there is more prey available

  • The number of prey then decreases as there are now more predators

  • The number of predators then decreases as there is now less prey available

  • The number of prey then increases as there are now fewer predators

  • The cycle now repeats

Symbiosis in ecosystems

• Symbiosis refers to a close and long-term interaction between two different species in an ecosystem

Types of symbiosis

Mutualism

• Both species benefit from the interaction

  • Example: Bees pollinate flowers by transferring pollen

  • This enables the plant to reproduce

  • The bee collects nectar as an energy source

Commensalism

• One species benefits while the other is unaffected

  • Example: Barnacles attach to whales, providing them with a habitat and transporting them to food-rich areas

  • The whale is unaffected

Parasitism

• One species benefits (the parasite), while the other is harmed (the host)

  • Example: Fleas feed on a dog’s blood for nourishment

  • This causes irritation and potential health issues to the dog (such as the transfer of diseases)

Competition & resource partitioning in ecosystems

Types of competition

• Competition can be divided into:

  • Intraspecific competition

  • Interspecific competition

Intraspecific competition

• This is competition within species (between individuals of the same species)

Interspecific competition

• This is competition between individuals from different species

Resource partitioning

• Resource partitioning is the division of resources among species to reduce competition

  • This can reduce the negative impact of competition on survival

Morphological partitioning

• Species evolve physical differences to use resources differently

  • Example: Birds with different beak shapes feeding on various parts of the same plant

Temporal partitioning

• Species use the same resource at different times

  • Example: Nocturnal and diurnal animals hunting at separate times

Spatial partitioning

• Species use different areas of a shared habitat

  • Example: Plants with shallow and deep root systems accessing different soil layers

• The three North American warbler species shown below all occupy the same habitat (spruces and other conifer trees)

• However, they occupy slightly different niches as each species feeds at a different height within the trees

  • This spatial partitioning avoids competition between the three species

  • This allows them to co-exist closely with each other in the same habitat