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Ecosystems
Population Responses to
Environmental Stress
Ecosystems

Response to Environmental Stress
– Homeostasis - the maintenance of
favorable internal conditions in a
system despite flucutations in external
conditions
Ecosystems

Response to Environmental Stress
– Feedback Loop - a circuit of sensing,
evaluating, and reacting to changes in
environmental conditions as a result of
information fed back into a system
Ecosystems

Response to Environmental Stress
– Feedback Loop
» Positive Feedback - a runaway cycle in
which a change in a certain direction
provides information that causes a system
to change further in the same direction
Ecosystems

Response to Environmental Stress
– Feedback Loop
» Negative Feedback - a cycle in which a
change in a certain direction leads to a
lessening of that change
Ecosystems

Response to Environmental Stress
– Ecosystem Disturbance
» Natural Changes

Catastrophic Changes
– Drought
– Flood
– Earthquake
– Disease
Ecosystems

Response to Environmental Stress
– Ecosystem Disturbance
» Natural Changes

Gradual Changes
– Climate change
– Immigration
– Evolution
Ecosystems

Response to Environmental Stress
– Ecosystem Disturbance
» Human-Caused Changes

Catastrophic
– Deforestation
– Plowing
– Pesticides
– Fires
Ecosystems

Response to Environmental Stress
– Ecosystem Disturbance
» Human-Caused Changes

Gradual
– Salinization
– Soil compaction
– Depletion of groundwater
– Tourism
Ecosystems

Response to Environmental Stress
– Ecosystem Stability
» Inertia
» Constancy
» Resilience
Ecosystems

Response to Environmental Stress
– Ecosystem Stability
» Inertia - ability of a living system to resist
being disturbed or altered
» Constancy - ability of a living system to
maintain a certain size
» Resilience - ability of a living system to
rebound from an external disturbance
Ecosystems

Response to Environmental Stress
– Ecosystem Stability
» Do ecosystems have to have high inertia
and high resilience to be considered
“stable”?

Most ecosystems lack high inertia and resilience
Ecosystems

Response to Environmental Stress
– Signs of Ecosystem Illness
» Drop in NPP
» Increased loss of nutrients
» Decline in indicator species
» Increased populations of pest species
» Decline in species diversity
» Presence of toxic chemicals
Ecosystems

Response to Environmental Stress
– Homeostasis and Time Delays
» Time delays between input of a stimulus
and the corresponding response



Ozone depletion
Global warming
CO2 scrubbing
Ecosystems

Response to Environmental Stress
– Homeostasis and Synergy
» Synergistic Reaction - a reaction that
occurs when two or more processes
interact so that the combined effect is
greater than the sum of their separate
effects
Ecosystems

Response to Environmental Stress
– Homeostasis and Synergy
» Synergistic Reactions


Beneficial [CO2 and photosynthesis]
Harmful [ozone depletion + global warming]
Ecosystems

Response to Environmental Stress
– Species Diversity and Ecosystem
Stability
» Ecosystems with higher species diversity
have higher NPP and are more resilient

Biodiversity provides “insurance” against
catastrophe
Ecosystems

Response to Environmental Stress
– Species Diversity and Ecosystem
Stability
» Ecosystems with higher species diversity
have higher NPP and are more resilient

Biodiversity provides “insurance” against
catastrophe
Ecosystems

Response to Environmental Stress
– Species Diversity and Ecosystem
Stability
» Tropical Rain Forests



High species diversity
High inertia
Low resilience
– Most nutrients stored in vegetation
Ecosystems

Response to Environmental Stress
– Species Diversity and Ecosystem
Stability
» Grasslands



Low species diversity
Low inertia
– Burn very easily
High resilience
– Most biomass is underground
Ecosystems

Response to Environmental Stress
– Population Responses to Stress
» Population Dynamics - the major abiotic and
biotic factors that tend to increase or
decrease the population size and the age
and sex composition of a species
Ecosystems

Response to Environmental Stress
– Population Responses to Stress
» Population Dynamics



Size
Density - number of individuals per area)
Dispersion - spatial distribution of species in
their habitat
Ecosystems

Response to Environmental Stress
– Population Responses to Stress
» Population Dynamics

Biotic Potential - the maximum rate at which a
population could grow if it had unlimited
resources

Environmental Resistance - all factors that act to
limit the growth a population
Ecosystems

Response to Environmental Stress
– Population Responses to Stress
» Population Dynamics

Carrying Capacity - the number of individuals of
a given species that can be sustained indefinitely
in a given area
Ecosystems

Response to Environmental Stress
– Population Responses to Stress
» Population Dynamics

Factors influencing carrying capacity
– Interspecific and intraspecific competetion
– Immigration
– Emigration
– Catastrophic events
– Food and water supply
– Habitat
Ecosystems

Response to Environmental Stress
– Population Density and Population
Growth
» Density-Dependent Population Controls

Increase in significance as population size
increases
– Competition for resources
– Predation
– Parasitism
Ecosystems

Response to Environmental Stress
– Population Density and Population
Growth
» Characteristics of dense populations:



Low birth rates
Slow growth rates
High death rates
Ecosystems

Response to Environmental Stress
– Population Density and Population
Growth
» Density-Independent Population Controls

Affect a population’s size regardless of size
– Natural disasters
– Deforestation
– Spraying of pesticides
Ecosystems

Response to Environmental Stress
– Population Density and Population
Growth
» Reproductive Strategies for Survival


r-strategists
K-strategists
Ecosystems

Response to Environmental Stress
– Population Density and Population
Growth
» Survivorship Curves

Show number of survivors of each age group for
a particular species
– Late Loss
– Constant Loss
– Early Loss