General Biology 101
Chapter Outlines
Starr & Taggert
10th edition
Chapter 47:
Community Interactions
The introduction to the chapter hi-lights the concept of resource partitioning of pigeons
found in New Guinea. Twelve species of pigeons each specializing on a certain food
source co-exist because they have very finely defined resource preferences.
Section 1
Which factors shape community structure?
Habitat – A place characterized by physical and chemical features and by the array of
species within it.
Community – the populations of all species in the habitat that associate with one
another.
Factors that affect community structure.
1) Interactions among climate and topography
2) Kinds and amounts of food and other resources
3) Adaptations for survival
4) Interactions e.g. predation, competition etc.
5) Overall pattern of population size.
Niche – the role/profession of a given organism in a community, sum of all activities and
relationships to secure resources.
Fundamental niche – the “ideal” situation without competition.
Realized niche – the “actual” or “constrained” situation depending upon the
prevailing conditions.
Different types of species interations:
1)
2)
3)
4)
5)
Commensalism*
Mutualism*
Competition
Predation
Parasitism*
*Examples of symbiosis, which means “living together.”
Table 47.1 demonstrates the positive and negative effects of these interactions.
Section 47.2 Mutualism
Mutualism – interactions between individuals of different species where each benefits by
the interaction.
Example: Pollination of flowers by their pollinators. Both benefit and neither is harmed.
Obligatory mutualism – individuals of one speices cannot grow nad reproduce without
the other species presence. “Intimate dependency on the other.”
Examples:
1) Yucca and its yucca moth.
2) Mycorrhizae and roots of young plants
3) Endosymbiotic origin of eukaryotes (See Sec 20.4 for further details).
Section 47.3
Competition – occurs whenever there are limited resources.
Intraspecific competition – occurs between individuals of the same species.
Interspecific competition – occurs between individuals of different species. This
form tends to not be so intense as intraspecific competition.
Types of competition interaction
Exploitative – when a competitor has some selective advantage at using limiting
resources.
Interference – when access to resources is limited because of actions of one member
involved in the competition. E.g. rufous hummingbirds outcompete broadtail
hummingbirds in the Rocky Mountains because they tend to be more aggressive and have
greater likelihood of procuring resources (of nectar).
Competitive Exclusion – Theory that two or more species requiring identical resources
cannot co-exist indefinitely. Demonstrated by Gause with an experiment of Paramecium
(single celled organisms) in a flask of food (bacteria – their prey item).
Where populations of two different populations coexist in nature, competitive interactions
suppress the growth rate of the both of them. Demonstrated by Hairston’s studies with
salamanders.
Resource Partitioning – the subdividing of some category of similar resources that lets
competing species coexist.
> Species might co-exist in the same habitat even if their niches overlap. They are better
able to do this if they share the resource in different ways or different times e.g. annual
plants/weeds growing in a field.
Section 47.4 predator-Prey Interactions
Predators – consumers feeding on living organisms, includes carnivores and omnivores.
Coevolution – the joint evolution of two or more species that exert selection pressure on
each other as an outcome of close ecological interation. E.g. Crab and its prey the snails.
Models of Predator–prey interactions:
Type I: Each predator consumes a constant number of prey individuals over time
regardless of the prey’s abundance. Graph is shaped like: /
Type II: the consumption of prey increases with an increase in prey density, but
not as fast as the increasing prey density. Graph is shaped like an asymptote.
Type III: Predator response is slowest when prey density is lowest, similar to a
logarhythmic curve.
Factors that can affect predator/prey models/responses:
1)
2)
3)
4)
Predator and prey reproductive rates.
Hiding places for prey i.e. shelter
Presence of alternative prey species
Shifts in environmental conditions
 Stability depends on carrying capacity and population densities.
Classic example:
Canada lynx and prey of snowshoe hare. Their populations oscillate in synch
with one another. What was found by Charles Krebs is that availability and abundance of
plants also plays a significant role/factor i.e. a 3-fold model necessary to explain the
observations was needed.
Section 47.5
Evolutionary Arms Race
Camouflage – some prey gain protection from predators by the adaptation of
concealment e.g. living stone plants in the desert.
Warning Coloration – conspicuous patterns on potential prey that warn the would be
predators that they taste bad or are poisonous e.g. Wasp/bee colors, poison dart frogs,
skunks.
Mimicry – when prey species resemble dangerous or bad tasting species. Can be done
with form, behavior or coloration e.g. beetles that resemble wasps but are in fact edible.
Moment of Truth Defenses – when animals are cornered they may act defensively by
snarling, growling, showing their bare teeth, spewing odors etc. to deter the would be
predator.
Predator Responses to Prey – in the evolutionary arms race predators counter prey
defenses in a variety of ways with special adaptations such as:
1) Stealth (quickness and being sneaky)
2) Camouflage e.g. Scorpion fish laying on the sea floor.
3) Ways of avoiding repellents e.g. mice that behaviorally avoid spraying beetles
by disabling their spray ability.
Section 47.6
Evolution of Parasitism
Parasites – live in (inhabit) or live in other organisms, benefit by gaining nutrients.
Hosts – those organisms that carry parasites. It may or may not die from the interaction.
Parastitic infections can lower the birth rate of the host, raise the death rate, and influence
intraspecific and interspecific competition of the hosts.
Kinds of Parasites
Ectoparasites – live on a host’s body surface.
Endoparasites – live within the body of the host.
Microparasites – microscopic in size and fast reproducers.
Macroparasites – include parasitic worms e.g. flatworms, mites, ticks, fleas.
Holoparasitic plants – nonphotosynthetic plants that withdraw nutrient and water from
young roots.
Hemiparasitic plants – retain some ability to photosynthesize, but also gain nutrients
from another plant that it resides upon e.g. mistletoe.
Social parasites – alter the social behavior of another species to complete their life cycle
e.g. cuckoos and north American cowbirds.
Parasitoids – organisms that are part parasitic and part predatory e.g. larvae of insects
that live inside and consume all of the soft tissues of the host. E.g. parasitoid wasps that
lay their eggs on larvae of sawflies.
Parasites as Biological Control Agents: Some parasites have been “employed” to fight
pest species (of insects) but this is difficult to control and not always successful.