Fire Effects on Aquatic
Ecosystems
guest lecture by
Christine May
Fire Effects on Aquatic Systems
• How can fire affect fish?
• When and where does fire or fire management
pose a threat?
• What management alternatives are most likely
to benefit aquatic systems?
Dunham et al. 2003
Direct Effects
• Mortality (causes are largely unknown)
• Water temperature
• Chemical toxicity from smoke or ash
• Absorption of smoke gases into surface waters can
cause ammonium levels to increase > 40-fold.
• Leaching of aerially deposited ash can increase
phosphorus levels.
• Fire retardant is highly toxic to many aquatic
organisms
Indirect Effects
1. Physical
2. Chemical
3. Biological
Physical Effects
• Hydrological
• Increased water yield
• Geomorphic
• Accelerated erosion rates
• Changes in channel morphology
• Elevated water temperatures
• Dependant upon removal of riparian canopy cover
Factors that Influence Watershed
Responses
• Burn severity
• Proportion of the watershed burned
• Relative proximity of the burned area to the
stream channel
• Slope steepness
• Soil type / erosivity
Chemical Effects
• Rivers: increases in nutrient and chemical
concentrations typically have a short duration
and are flushed through the system with the
first pre-fire precipitation events.
• Lakes: inputs are often diluted but may be
more persistent.
Biological Effects
• Often associated with a short-term increase in
biological productivity:
• Increased light and nutrient availability = greater
primary productivity.
• Food web dynamics = algae → invertebrates → fish
• Shift in functional feeding groups from shredders and
collectors (associated with litter input) to grazers.
Adapt ed from Minshall (1989)
Why is the Historic Range of
Variation Important?
Without prior exposure to a particular
frequency, magnitude, or type of disturbance
there is no evolutionary basis for an individual
or a community to respond.
Vulnerability of Fish to Fire
1. Quality of the affected habitats
2. Amount and spatial distribution of habitat
(habitat fragmentation)
3. Position in the drainage network
4. Habitat specificity
5. Mobility
6. Life history diversity
Which populations are the most
vulnerable?
Relatively immobile species with a narrow range
of habitat requirements in highly degraded or
fragmented systems.
Metapopulation Dynamics
Metapopulation Dynamics
• Network of habitat patches potentially
interconnected by dispersal.
• Driven by local extinction and recolonization.
• Population recovery is faster in sites closer to
sources of recolonization and free from
migration barriers.
Dunham et al. 2003
Dunham et al. 2003
Dunham et al. 2003
Dunham et al. 2003
Dunham et al. 2003
Dunham et al. 2003
Dunham et al. 2003
Isolated Populations
• In some cases, local extinctions have been
observed in response to fire.
• Particularly in small, headwater streams.
• Example: fire-related mortality halted de-
listing of the endangered Gila trout.
Dunham et al. 2003
Dunham et al. 2003
HABITAT
FRAGMENTATION
MOBILITY
HABITAT SPECIFICITY
ISOLATION
HABITAT SIZE
HABITAT DEGRADATION
modified from Dunham et al. 2003
Dunham et al. 2003
Pre-fire Management
• A proactive approach, which addresses factors
that render fish populations vulnerable to fire-
related disturbance
• Likely to be the most effective!
Fire Management
• Consideration for vulnerable populations in
fire suppression or let burn policies.
• Placement of fire lines.
• Toxicity of fire fighting chemicals.
Post-fire Management
• Reactive approach that attempts to speed
recovery of a system.
• Most expensive and outcomes are uncertain.
• Salvage logging.
Research & Monitoring
Adaptive management recognizes that
management plans are made with imperfect
information and understanding, and
management decisions often lead to
unintended or unsuspected consequences.
Trajectories of Disturbance &
Reorganization
Intermediate Disturbance Hypothesis
Species Diversity
Disturbance Frequency
Intermediate Disturbance Hypothesis
Species Diversity
Fast Recolonizers &
Rapid Reproducers
(Inferior Competitors
Disturbance Frequency
Intermediate Disturbance Hypothesis
Competitive
Exclusion by
A Few Species
Species Diversity
Fast Recolonizers &
Rapid Reproducers
(Inferior Competitors
Disturbance Frequency
Intermediate Disturbance Hypothesis
Competitive
Exclusion by
A Few Species
Species Diversity
Fast Recolonizers &
Rapid Reproducers
(Inferior Competitors
Disturbance Frequency
Stochastic, Abiotic Biotic Interactions
Processes Dominate Dominate
Patterns of Recovery
• Dependant upon the frequency, magnitude,
and composition of the disturbance.
• Population size
• Species pool
Questions??
Question for the class:
If you are planning a prescribed fire, what are
some factors that should be considered for
protecting or restoring aquatic ecosystems?
Question for the class:
Do you think large, low severity fires or small,
high severity fires have a greater affected on
aquatic ecosystems?