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)