Transcript Topic 09 Lecture

Stream Ecology (NR 280)
Topic 9 – Species Interactions
Herbivory
Predation
Competition
Overview
Previously we considered fundamental
groups…
• Autotrophs and autotrophy
• Heterotrophs and heterotrophy
• Consumers: primary, secondary, tertiary
Now we want to consider interactions among
species and trophic levels
General Observation
Species interactions in aquatic systems
are similar to those in terrestrial systems,
with aquatic-specific characteristics.
Herbivory
• Most studies have focused on the effects of
grazing on periphyton.
• Literature on consumption of higher plants is
much more limited.
• Generalization: Grazers are attracted to the
periphyton on plants and not the plant itself.
• Obvious exceptions: higher animals (e.g.,
geese, moose) that graze on higher plants
Epilithic algal biomass
How does grazing affect the grazed?
Grazing by snails limits biomass accumulation
But…was primary production lower?
Steinman et al. (1996)
Top-Down vs Bottom-Up Controls
(Elimia, snail)
This appears to be an
N-limited system.
However, grazing can
overwhelm the nutrient
enrichment effect.
Fig. 9.5 Allan and Castillo (2007), from Rosemond et al. (1993)
Do grazers search for food randomly
or “purposefully”?
The grazing mayfly
(Baetis) searches
more thoroughly and
moves slower if food
quality is higher.
Implication: Baetis
lingers in food-rich
patches
Fig. 9.1 Allan and Castillo (2007), from Kohler (1984)
Do algivorous
fish benefit from
increased algal
productivity?
Rainy season
Dry season
Yes and no…
• Greater total biomass
• Greater population
density
• But basically same
individual production
Fig. 9.2 Allan and Castillo (2007), from Power (1983)
Does grazing
benefit the grazed
organisms?
A = ambient density
0 = grazer removed
# = experiment density
Biovolume = volume of an
average algal cell (biomass)
• Top: Even light grazing
reduces algal biovolume.
A grazing mayfly
• Bottom: Ratio CHLa to
biovolume increases with
grazing density
Fig. 9.3 Allan and Castillo (2007), from Hill and Knight (1987)
Hypothesized impacts of grazing
Photosynthesis
(some confidence)
GPP
(speculative)
Biomass
(high confidence)
Fig. 9.4 Allan and Castillo (2007), from Lambertti and Moore (1984)
Predation (including Cannibalism)
• One consumer eats another
• Size and quality of the “food” (prey) matter
as for grazers
• New variable: behavior
– Of the prey (e.g. avoidance, defense)
– Of the predator (e.g., foraging, feeding mode)
Abundance of predators
(caddisflies & alderflies) is
directly related to the
abundance of their prey
(chironomids & stoneflies).
More prey, more predators.
Caddisfly
# of predators / sample (N)
What does this tell you
about the relationship
between predator and
prey?
Alderfly
Biomass of
prey/sample (mg/m2)
Are predators
selective about
their prey?
Predators consumed
prey in approximate
proportion to their instream abundance
% Composition
Predators don’t appear
to be too selective
Benthic Samples
Gut Contents:
Alderfly
Gut Contents:
Caddisfly
Macroinvertebrate Prey Species
Individuals that are of similar
size eat similar things; i.e.,
they have similar “dietary
niches”.
Degree of overlap in what individual eat
Who eats what?
Benthic
macroinvertebrate
predators
Largest
Smallest
Fig. 9.8 Allan and Castillo (2007), from
Woodward and Hildrew (2002)
Difference in individual body size
Who eats whom?
• Small predators eat
small prey
• Large predators eat
large prey
Size Matters
• Larger prey means greater benefit per prey
item captured
• The “size refuge”: Too large to be eaten
• For invertebrates, size at different life stages
may differ considerably, thus a prey at one life
stage may be a predator at a later life stage,
and vice versa.
Prey defenses
• Physical
– Hard or spiny exoskeleton
• Chemical
– Distasteful
• Behavioral
– Nocturnal feeding
• Combinations
– Live in a habitat the predator can’t
access (e.g., shallows)
Distance fish will go to obtain food item
Do prey fish alter their behavior
in the presence of predators?
Juvenile coho salmon feeding
in presence or absence of
model rainbow trout.
“Risk” perceived by the predator
Fig. 9.9 Allan and Castillo (2007), from Dill and Fraser (1984)
Do lower organisms behave differently in the
presence of a predator?
Night vs Day drift by Mayfly species
Different
prey
species
Predation intensity
Different rivers
Fig. 9.10 Allan and Castillo (2007), from Flecker (1992)
Predator impacts on
the ecosystem may
be complex
- Competition
- Trophic cascades
Fig. 9.11 Allan and Castillo (2007), from Power (1990)
Competition
• Two species vie for a common resource
• One species disadvantages the other
– Reduces the other species’ fitness
– Reduces the other species abundance
• Modes of competition
– Exploitation: Dominant species uses more of the
critical resource to dominate the weaker species
– Interference: Dominant species directly interacts
to dominate the weaker species
Evidence for Competition
• Resource use
– Nutrient use efficiency in algae
– Assimilation efficiency in benthic
macroinvertebrates
• Resource partitioning
– In space: habitat use
– In time: day-night, seasonal
• “Niche exploitation”
But is this really competition? Or is it the “ghost of competition
past”; i.e., an evolutionary response to efficient resource use?
What happens if you eliminate a key grazer?
Natural collapse of grazing caddis fly Glossosoma
Fig. 9.15 Allan and Castillo (2007), from Kohler and Wiley (1997)
Summary
• Species are inter-connected through food
webs (herbivory, predation, competition)
• Bottom up effects: nutrient enrichment
• Top-down effects: trophic cascades
• Environmental stress may mask species
interactions (e.g. extreme nutrient scarcity,
frigid conditions)