Transcript lecture.13 - Cal State LA

Community Structure
A biological community is an association of
interacting populations
Community Diversity & Community
Structure
• Community diversity relates the number of
different species in a community
• A community’s structure is based on the
interactions between all of its species
Individualistic
holistic
Community Structure
• Consumer-resource interactions are a
significant component of community
structure
• Feeding relationships are often used to
describe community structure
Community Structure
• Consumer-resource (feeding) interactions
are critical to community structure,
because they represent the travel of energy
through the ecosystem
Community Structure – Trophic
Levels
• Primary producers - autotrophs (plants and
other ‘self-feeders’)
• Primary consumers – herbivores
• Secondary consumers – carnivores that eat
herbivores
• Tertiary consumers – carnivores that eat
secondary consumers…
Community Structure – Food Web
• Food web = a summary of all the feeding
relationships within a community
Effects of species richness on food
web structure
Is there a relationship between the species
richness of a community and the complexity
of its feeding interactions?
Complexity of a food web can be
characterized based on:
• number of feeding interactions
• number of trophic levels
Effect of species richness on food
web complexity
• The number of feeding interactions that each
species has with other species is independent
of the overall diversity of the community
a
b
Effect of species richness on food
web complexity
• However, the number of trophic levels does
tend to increase with species richness, so
higher community diversity is usually
associated with increased food web
complexity
Effect of species richness on food
web complexity
Effect of food web structure on
species diversity
• feeding relationships (and other species
interactions) can affect species diversity within
a community
• for example: when a predator controls the
population of an otherwise dominant
competitor, it may allow other less
competitive species to persist
Effect of food web structure on
species diversity
• The number of species at one trophic level
may depend on the presence of consumer
species at a higher trophic level
There are different ways to
visualize community structure
• Robert Paine – noted that some feeding
relationships are more important than others
in terms of shaping community diversity
• “strong interactions”
• defining criterion not necessarily the
quantity of energy flow, but the degree of
influence on population sizes and
community structure
There are different ways to
visualize community structure
Effect of food web structure on
species diversity: Keystone species
• Keystone species are those species whose
interactions (usually feeding) have a
disproportionately large influence on the
structure of their community
• influence is disproportionate relative to the
keystone species’ actual biomass
• distinct from dominant species (whose
influence is due to high biomass, e.g.,
redwoods)
Community structure - alternative
stable states
• Similar to population sizes in predator-prey
cycles, community structure can exist at different
stable equilibria
• e.g., kelp forest / urchin barren
Community structure & trophic
cascades
• Trophic levels are influenced from above by
predation and from below by production
• Top-down control (predator-mediated)
• Bottom-up control (producer-mediated)
• Trophic Cascade – the activity of one trophic
level has far-reaching effects on the biomass
of distant trophic levels
Community structure & trophic
cascades
Community structure & trophic
cascades
Community structure & trophic
cascades
Community structure & trophic
cascades
• Trophic cascades can have far-reaching and
unpredictable effects
Introduced Norway rats indirectly alter the intertidal community in the Aleutian Islands
through direct predation on birds that forage in the intertidal
Kurle C. M. et.al. PNAS 2008;105:3800-3804
©2008 by National Academy of Sciences
Introduced species & community
structure
• Introduced species tend to reduce the
complexity of community structure by
simplifying trophic structure / eliminating
trophic levels
Energy flow constrains community
structure
• Trophic dynamics: as energy is transferred
from one trophic level to the next, some of it
is lost due to:
• limited assimilation
• respiration
• heat production
Energy flow constrains community
structure - Demo
• paper clips = energy units
• must have at least 2 energy units to survive
• front row = primary producers (3 clips)
• second row = primary consumers (forage 3
clips from primary consumers, pay 2 clips
for respiration)
• third row = secondary consumers (get 3
clips worth of energy from prey, pay 2 clips
for respiration)
Energy flow constrains community
structure
• Energy transfer
between trophic levels
is inefficient enough to
make numerous
trophic levels an
impossibility
• The most productive
ecosystems usually
max out at six trophic
levels
Energy Pyramids