Chapter 28
Community Interactions
Community Defined
• An ecological community consists of all the interacting populations in an ecosystem.
Community
• All the populations that live together in a habitat
• Habitat is the type of place where individuals of a species typically live
• Type of habitat shapes a community’s structure
Community Structure
• Determined by species composition and abundance
Community Structure
Why Are Interactions in Ecological Communities Important?
• Community Interactions Help Limit Population Size
– Populations come into a balance due to predation and competition
– This balance can be undermined by introduced “exotic” species
Exotic Species
• Species that has left its home range and become established elsewhere
• Becomes part of its new community
• Can have beneficial, neutral, but mostly harmful effects on a community: leaves predators, etc. behind
Examples:
Kudzu in Georgia
• Imported for erosion control
• No natural herbivores, pathogens, or competitors
• Grows over landscapes and cannot be dug up or burned out
• May turn out to have some commercial use
Rabbits in Australia
• Rabbits were introduced for food and hunting
• Without predators, their numbers soared
• Attempts at control using fences or viruses have thus far been unsuccessful
Endangered Species
• A species that is extremely vulnerable to extinction
• Close to 70 percent of endangered species have been negatively affected by exotic competitors
Bullfrogs vs Native Frogs
Why Are Interactions in Ecological Communities Important?
• Community Interactions Influence Evolutionary Change
– Coevolution: interacting species influence one another’s evolution by acting as agents of mutual selection
Yucca and Yucca Moth
• Example of an obligatory mutualism
• Each species of yucca is pollinated only by one species of moth
• Moth larvae can grow only in that one species of yucca
Yucca-Moth Coevolution
• The moths emerge when the yucca flowers open. The female gathers pollen from one flower, rolls it into a ball, flies to another flower, lays four or five eggs, and inserts the pollen mass in the opening thus formed. The larvae eat about half the approximately 200 seeds produced by the plant.
• The yucca can be fertilized by no other insect, and the moth can utilize no other plant.
Competition
• Interspecific - between species
• Intraspecific - between members of the same species
• Intraspecific competition is most intense
Niche
Sum of activities and relationships in which a species engages to secure and use resources necessary for survival and reproduction
Realized & Fundamental Niches
• Fundamental niche
– Theoretical niche occupied in the absence of any competing species
• Realized niche
– Niche a species actually occupies
• Realized niche is some fraction of the fundamental niche
• When the Balanus sp.were removed from the rocks Chthamatus sp. were able to move down onto the unoccupied surface (so part of fundamental niche, not realized)
But when the Chthamatus sp. was removed from the upper areas of the rock, Balanus sp. was not able to move up and occupy the upper areas (so NOT part of fundamental niche).
Why ? Balanus sp. cannot stand to be exposed to the air for a long time.
What Are the Effects of Competition among Species?
• The Ecological Niches of Coexisting Species Never Overlap Completely
– Called the “competitive exclusion principle”
– Interspecific competition helps determine how populations are distributed: fundamental vs. realized niche
– Species evolve to reduce niche overlap through resource partitioning
Resource Partitioning
• Apparent competitors may actually have slightly different niches
• Species may use resources in a different way or time
• Minimizes competition and allows coexistence
Types of predator-prey interactions
• Predators consume prey
• Parasites feed on, but do not try to kill their host
• Herbivores feed on, but do not necessarily kill, the plants they eat
Predation
• Predators are animals that feed on other living organisms
• Predators are free-living; they do not take up residence on their prey
Coevolution
• Natural selection promotes traits that help prey escape predation
• It also promotes traits that make predators more successful at capturing prey
Predators and Prey Coevolve
• Camouflage
• Warning coloration
• Mimicry
• Startle coloration
• Chemical warfare
Predator Responses
• Any adaptation that protects prey may select for predators that can overcome that adaptation
• Predator adaptations include stealth, camouflage, and ways to avoid chemical repellents
Camouflage
Visual and behavioral mimicry
Startle coloration
Chemical warfare
Plant Defenses
• Chemical warfare
• Thorns, thick bark
• Silica in grass
Symbiosis
• Living together- an intimate, prolonged interaction between organisms of two different species
Types:
• Commensal – helps one, other indifferent
• Parasite- One hurt, one helped
• Mutualism
Parasitism
• Parasites drain nutrients from their hosts and live on or in their bodies
• Natural selection favors parasites that do not kill their host too quickly
Parasites
Mycorrhizae
• Obligatory mutualism between fungus and plant root
• Fungus supplies mineral ions to root
• Root supplies sugars to fungus
Lubchenco Experiment
Keystone Species
• An organism that plays a pivotal role in shaping community structure
• Removal of a keystone species can cause drastic changes in a community; can increase or decrease diversity
Types of Succession
• Primary succession - new environments
• Secondary succession - communities were destroyed or displaced
Pioneer Species
• Species that colonize barren habitats
• Lichens, small plants with brief life cycles
• Improve conditions for other species who then replace them
Climax Community
• Stable array of species that persists relatively unchanged over time
• Succession does not always move predictably toward a specific climax community; other stable communities may persist
Primary succession
Secondary succession
Succession in a freshwater pond
Restoration Ecology
• Natural restoration of a damaged community can take a very long time
• Active restoration is an attempt to reestablish biodiversity in an area
• Ecologists are actively working to restore reefs, grasslands, and wetlands
Community Instability
• Disturbances can cause a community to change in ways that persist even if the change is reversed