Stream Ecology (NR 280)
Topic 9 – Species Interactions
HerbivoryPredation
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
How does grazing affect the grazed?
Grazing by snails limits biomass accumulationBut…was primary production lower?
Steinman et al. (1996)
Epi
lithi
c al
gal b
iom
ass
Top-Down vs Bottom-Up Controls
(Elimia, snail)
Fig. 9.5 Allan and Castillo (2007), from Rosemond et al. (1993)
This appears to be an N-limited system. However, grazing can overwhelm the nutrient enrichment effect.
Do grazers search for food randomly or “purposefully”?
Fig. 9.1 Allan and Castillo (2007), from Kohler (1984)
The grazing mayfly (Baetis) searches more thoroughly and moves slower if food quality is higher.
Implication: Baetis lingers in food-rich patches
Do algivorous fish benefit from increased algal productivity?
Fig. 9.2 Allan and Castillo (2007), from Power (1983)
Yes and no…
• Greater total biomass
• Greater population density
• But basically same individual production
Rainy season
Dry season
Does grazing benefit the grazed organisms?
Fig. 9.3 Allan and Castillo (2007), from Hill and Knight (1987)
• Top: Even light grazing reduces algal biovolume.
• Bottom: Ratio CHLa to biovolume increases with grazing density
A grazing mayfly
A = ambient density0 = grazer removed# = experiment density
Biovolume = volume of an average algal cell (biomass)
Hypothesized impacts of grazing
Biomass (high confidence)
Photosynthesis (some confidence)
GPP(speculative)
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)
What does this tell you about the relationship between predator and prey?
Caddisfly
Alderfly
# of
pre
dato
rs /
sam
ple
(N)
Biomass of prey/sample (mg/m2)
Abundance of predators (caddisflies & alderflies) is directly related to the abundance of their prey (chironomids & stoneflies). More prey, more predators.
Are predators selective about their prey?
Predators don’t appear to be too selective
Predators consumed prey in approximate proportion to their in-stream abundance
% C
ompo
sitio
n
Benthic Samples
Gut Contents:Alderfly
Gut Contents:Caddisfly
Macroinvertebrate Prey Species
Who eats what?
Benthic macroinvertebrate
predators
Deg
ree
of o
verla
p in
wha
t in
divi
dual
eat
Difference in individual body size
Individuals that are of similar size eat similar things; i.e., they have similar “dietary niches”.
Largest
Smallest
Fig. 9.8 Allan and Castillo (2007), from Woodward and Hildrew (2002)
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)
Do prey fish alter their behavior in the presence of predators?
Fig. 9.9 Allan and Castillo (2007), from Dill and Fraser (1984)
Dis
tan
ce fi
sh w
ill g
o to
ob
tain
food
ite
m
“Risk” perceived by the predator
Juvenile coho salmon feeding in presence or absence of
model rainbow trout.
Do lower organisms behave differently in the presence of a predator?
Night vs Day drift by Mayfly species
Fig. 9.10 Allan and Castillo (2007), from Flecker (1992)
Different rivers
Predation intensity
Differentprey
species
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)