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Species Interactions, Ecological Succession,

and Population Control

Lecture 6

How Do Species Interact?

• Five types of species interactions affect resource use and species population sizes in an ecosystem

• Competition • Predation • Parasitism • Mutualism • Commensalism

Competition for Resources

• Most common interaction is competition

• Interspecific competition • Competition between

different species to use the same limited resources

• Resource Partitioning • Intraspecific competition

Predation

• Predator feeds directly on all or part of a member of another species (prey)

• Strong effect on population sizes and other factors in ecosystems

• Methods of predation • Walk, swim, or fly • Camouflage • Chemical warfare

• Coevolution

Parasitism, Mutualism, and Commensalism

• Parasitism • One species (parasite) lives on another organism • Parasites harm but rarely kill the host • Examples: tapeworms, sea lampreys, fleas, and ticks

• Mutualism • Interaction that benefits both species • Nutrition and protective relationship • Not cooperation—mutual exploitation • Example: clownfish live within sea anemones

• Gain protection and feed on waste matter left by anemones’ meals

• Clownfish protect anemones from some predators and parasites

Parasitism, Mutualism, and Commensalism

• Commensalism • Benefits one species and has little effect on the other • Examples:

• Epiphytes (air plants) attach themselves to trees (Pitcher Plant) • Birds nest in trees

• https://www.youtube.com/watch?v=doB6fyzoO68

How Do Communities and Ecosystems Respond to Changing Environmental Conditions?

• Ecological succession • Normally gradual change in structure and species

composition in a given system

• Primary ecological succession • Involves gradual establishment of communities of

different species in lifeless areas • Need to build up fertile soil or aquatic sediments to

support plant community • Takes hundreds to thousands of years • Pioneer species such as lichens or mosses quickly spread

and release acids

Ecological Succession Creates and Changes Ecosystems

• Secondary ecological succession • Series of terrestrial communities or ecosystems develop in places with soil

or sediment • Examples: abandoned farmland, burned or cut forests, and flooded land

Ecological succession is an important ecosystem service enriching biodiversity

https://www.youtube.com/watch?v=8ceDE01iWLE

What Limits the Growth of Populations?

• Population • Group of interbreeding individuals of the same species

• Population size • May increase, decrease, or remain the same in response to

changing environmental conditions • Scientists use sampling techniques to estimate

What Limits the Growth of Populations?

• Population distribution varies over their habitats • Most populations live together in clumps or groups

• Organisms cluster for resources • Protection from predators

• Variables that govern changes in population size • Births, deaths, immigration, and emigration

Several Factors Can Limit Population Size

• Each population has a range of tolerance • Variation in physical and chemical

environment under which it can survive

• Limiting factors • Precipitation (on land) • Water temperature, depth, clarity, and

other factors (in aquatic environments)

• Population density • Density-dependent factors (parasites and

diseases spread easily, higher death rates; finding mates in sexually reproducing individuals is easy)

No Population Can Grow Indefinitely: J-Curves and S-Curves

• Some species can reproduce exponentially

• Reproduce at an early age • Have many offspring each time they

reproduce • Short intervals in between

reproductive cycles • Produces J-shaped curve of growth • Examples: bacteria and many insect

species

No Population Can Grow Indefinitely: J-Curves and S-Curves

• Population growth in nature always limited • Environmental resistance

• Sum of all factors that limit population growth

• Carrying capacity • Maximum population of a given species that a particular habitat can sustain

indefinitely • Overshoot results in population crash

No Population Can Grow Indefinitely: J-Curves and S-Curves

Reproductive Patterns

• r-Selected species • Species with capacity for a high rate

of population growth • Examples: algae, bacteria, frogs,

most insects, and many fish • May go through irregular and

unstable cycles in population sizes

Reproductive Patterns

• K-Selected species • Species that reproduce later in life • Have few offspring • Have long life spans • Examples: large mammals, whales,

humans, birds of prey, and long-lived plants

• Can be vulnerable to extinction

Species Vary in Their Life Spans

• Survivorship curve • Shows the percentages of members of population surviving at different ages

• Late loss (K-selected species) • Early loss (r-selected species) • Constant loss (many songbirds)

Humans Are Not Exempt from Nature’s Population Controls

• Ireland • Potato crop destroyed by fungus in 1845 • Killed one million people

• Bubonic plague • Killed 25 million during the 14th century in densely populated European

cities

• Technological, social, and cultural changes have expanded earth’s carrying capacity for the human species today