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CHAPTER 53
Community Ecology
Lecture Outline
• What is a community?
• Interspecific interactions
• Competition
• Predation
• Mutualism
• Community structure
• Feeding patterns
• Patterns in the change of species over time
• Geographic patterns in species diversity

What is a community?
• Community = assemblage of species living close enough together for potential interaction
• Communities differ in biodiversity:
• in species richness = the number of species they contain
• and in the relative abundance of different species

Species richness & relative abundance

Interspecific interactions
= interactions between species
• Possible interspecific interactions can have positive or negative effects on individual populations:

Competition
• Negative for both sides
• Species may be competing for:
• food or access to mates (animals)
• nutrients and water (plants)
• Competition occurs between species with overlapping requirements (food, habitat and abiotic factors).
-> a species’ set of requirements is referred to as its …

e.g. Darwin’s finches / Galpagos Islands were competing for food resources:
-> specialization on different kinds of food resources
= …
-> morphological change that
enabled food specialization
= …
-> natural selection
leading to speciation:
new, even more
specialized species

Predation
• A predator eats prey
• Herbivory: animals eat plants
• Parasitism: predators live on/in a host and depend on the host for nutrition

• Predatory interaction: bad for one – good for the other
• but, the prey is not necessarily killed:
-> herbivores benefit & plants suffer
-> parasites consume resources provided by host;
they harm but do not kill the host

• Protection from predation:
• Plants:
- mechanical defenses: spines, thorns etc.
- chemical defenses: toxic compounds

• Animals:
- behavioral defenses: fleeing, hiding, self-defense, noises, and mobbing
- mechanical defenses: hard shell, spines, etc.
- chemical defenses: odors and toxins
- visual means: camouflage, mimicry etc.

- visual means: camouflage, mimicry etc.

- camouflage:
cryptic coloration,
deceptive markings

- aposematic colours
= warning colors,
often combined
with toxins

- Mimicry: …

- Batesian mimicry
-> harmless species
mimics a
harmful one

- Müllerian mimicry
-> two or more
unpalatable species resemble each other

• Predation causes strong natural selection:
• in the predator for traits that help catch and consume prey:
- faster, stronger, more agile
- sharper senses
- bigger sharper teeth & claws
-> catch more food, survive & reproduce more
• in the prey for traits that confer protection from predation:
- cryptic
- poisonous
- faster
-> increased survival rate & offspring production

Mutualism
->…
e.g. pollination:
-> pollinator gets food
-> plant gets pollinated

Interspecific interactions
= interactions between species

• Populations within a community have varying degrees of interactions with each other,
- from none (for example lichen and lynx)
- to very intimate ( plants and their specialist pollinators, animals and their parasites)

Trophic structure
= feeding patterns
• The trophic structure of a community is determined by the feeding relationships between organisms.

fundamental rule:
…
= photosynthetic organisms

-> they are where energy enters the living system

• Food chain:
the transfer of food energy
from its source in
photosynthetic organisms
through herbivores and
carnivores

• usually 4-5 links
= trophic levels

• food chains are not isolated units but are hooked together into food webs

-> who eats whom
in a community?

-> frequently consumers
have more than one
food source

Patterns in the change in species through time
-> by disturbance and succession
• disturbance
= discrete events that alter resource availability, and change which species live in a community

-> most communities are in a state of nonequilibrium due to disturbances

• succession
= progression of species that occupy a site

Disturbance
• Examples for disturbances
- natural events: storms, floods & fires
- periods of drought and overgrazing
- animal activities:
- human activities: logging, plowing, dams, roads, buildings…

-> human activities cause more disturbance than natural events and usually reduce species diversity in communities

• disturbances are not just negative - in many cases they are necessary for community development and survival
• many animal activities increase diversity of species
e.g. plants colonizing dirt piles around animal burrows etc.
• fires remove accumulated dead plants and reinvigorate the vegetation
e.g. prairies need to burn periodically, otherwise they will be
overtaken by brush --> loss of prairie species

Succession

= sequence of community changes after a disturbance
• Primary succession:
begins in a lifeless
area where soil has
not yet formed

e.g. retreating glacier:
mosses & lichens first,
-> development of soil

• Secondary succession:
occurs where an existing community has been
cleared by some event, but the soil is left intact
e.g. hurricane, volcanic eruption, fire …

-> herbaceous plants grow first, then trees and other organisms colonize the area

Secondary succession changes the growing conditions:
e.g. soil concentrations of nutrients

Biodiversity

= species diversity in a community
• Communities differ in
biodiversity:

• in species richness
= the number of species
they contain

• and in the relative
abundance of different
species

-> species richness may be equal, but relative abundance
may be different

Biodiversity
• biodiversity of
a community depends
on biogeography:

-> tropical habitats support
much larger numbers
of species than do temperate
and polar regions

WHY??

• What causes these gradients in biodiversity?
-> the more energy available, the more species and the more biomass
• Energy depends on availability of water & temperature:
-> can be measured as evapotranspiration
= evaporation from soil + transpiration from plants

Biodiversity
• biodiversity of a community depends also
on the size of a community’s geographic area:

Species-area curve:
-> the larger the area,
the more species

-> habitat fragmentation
results in loss of species

Key terms 53
Community
Species richness
Relative abundance
Interspecific interactions
Competition
Ecological niche
Resource partitioning
Character displacement
Predation
Herbivory
Parasitism, parasite & host
Cryptic coloration
Aposematic coloration
Batesian mimicry
Müllerian mimicry
Mutualism
commensalism
trophic structure
trophic level
food chain
food webs
disturbance
succession
primary succession
secondary succession
biodiversity
biomass
species-area curve