EEMB 142AL Final - Rivers Pt 2

disturbance and succession

• rivers are highly disturbed environments

• floods and droughts are common

• unpredictable reductions in biotic abundance

• colonization by some taxa rapid

• adaptation (good colonizer or disperser)

what is drift?

downstream transport of organisms (inverts) / detritus

what are the three types of invertebrate drift?

1. catastrophic: not choosing to move, forced to move

2. constant: an accident (slips/falls)

3. behavioral: choosing to let go

what time do organisms typically decide to drift?

diel periodicity (nocturnal) - to avoid predators

what kinds of adaptations do organisms that drift have?

flatten bodies, hooks, suckers, ballast

colonization cycle (Muller 1954)

larval invertebrates drift downstream and adults fly back upstream to mate

fish migration: anadromous

spawn in fresh water and live in salt water (ex. salmon)

fish migration: catadromous

spawn in salt water and live in fresh water (ex. eel)

African flood plains: white fish

river rises - white fish takes advantage of new resources - river decreases - returns

African flood plains: black fish

black fish travel from deep to shallow pools to access new resources

conditions of competitive interaction

1. resources must be limiting

2. density of organisms must be high

3. environment is stable

flashy streams

frequently disturbed, abiotic conditions control abundance and distribution of organisms (time is short)

stable streams

infrequently disturbed, biotic interactions control abundance and distribution of organisms (time is long)

sedentary invertebrates (interspecific competition) experiment - Hydropsyche vs Simulium

observations

• successional replacement over the summer

hypothesis

1. seasonality - abiotic factors change

2. competition for space (FPOM)

experiment

• remove H - increase of S throughout the summer

• remove S - no effect on either flies

• remove both - decrease in H and increase in S

results

• H out-competes S for space

• H is the superior competitor (K) & S is the superior colonizer (R)

successional replacement experiment - Hydropsyche vs Simulium

results

• H out-competes S for space, but outcome is modulated by disturbance

if moderately disturbed: intermediate disturbance leads to even distribution of H and S

trade offs in competitive vs dispersal ability: moderate disturbance leads to coexistence and high species diversity (K vs R)

competition for space - Montana Streams - Leucotrichia and Moss

• larger rocks = higher stability = K-selected = lower diversity

• larger rocks = higher % of space occupied

• uniform dispersion: aggressive interaction for space

disturbance is inversely proportional to rock size (flipping)

• small rocks flip too often for L to dominate space

even larger rocks?

• moss takes over as primary space holder over L

• BUT provides habitat for secondary space holders = increase in diversity

predation: consumption and behavioral modifications (4)

1. invertebrate predators reduce common prey

2. effects of fish on invertebrate prey variable

3. predators may have greater effects on prey behavior than consumptive effects on density and impasse may depend on predator movement (run/hide/fight)

4. some evidence for tropic cascades

interaction modification

the interaction between two species is modified by the presence of a third species

what are the two types of interaction modification?

1. facilitation (+): facilities prey capture

2. inhibition (-): prevents prey capture

interaction modification: channels with and without trout

trout grow faster in the presence of stoneflies

interaction modification: Southern California streams

treatments

1. added trout and larvae in isolation

2. added trout and larvae together

predictions

• predators will decrease the density of mayfly prey

• multiple predators will have a greater impact than single predators

results

• predators decrease prey density

• predators increase prey drift

• predators decrease prey exposure

BUT no significant difference in the number of prey consumed by multiple predators and predators in isolation

1. trout decrease foraging rates of larvae (inhibition)

2. larvae increase foraging rates of trout (facilitation)

no net effect = compensatory mechanism

trophic cascade

the effects of upper trophic levels cascade via direct and indirect effects through lower tragic levels

• simple food chains - should be more likely to show cascading effects

cascading effects of leaf litter decay: Utah Streams

• cages with and without invertebrate predators

• invertebrate predators reduced shredders causing a decrease in leaf decomposition rates

trophic cascade: Oklahoma stream pools - bass, herbivorous minnow, benthic algae

• algae on rocks moved into minnow pool decreased (was quickly eaten)

• tethered bass kept algae from being eaten (behavioral modification / consumption)

complex food webs: Northern California streams - Baetis, roach, trout

observations

• Baetis is dominant food of roach and trout in the Eel River

prediction

• removal of fish might decrease periphyton by increasing Baetis

results

• no effect on Baetis grazer and the algae increased in abundance

• missed a trophic level

conclusions

• fish do not eat chironomids because of retreats

• Baetis does not control algae

• Chironomids control algae

complex food webs: Northern California streams and disturbance

observations

• during drought years an increase or decrease in fish has no cascading effects on algae

experiment did NOT work

• another organism becomes dominant in the system that is not able to be eaten by the fish

conclusions

• no cascade (decoupled) because caddisfly is armored an inedible to predators

  • caddisfly larvae can not be eaten and reduces algae

• non flood years = 3 trophic levels

• flood years = 4 trophic levels

interactions between streams and riparian linkages: Japan

addition of trout decreases spider abundance via cascading effects (tropic cascade across ecosystem boundaries)

• add trout - decease grazers - increase algae - decrease emergent biomass - less prey for spiders

five factors affecting biological interactions

1. resource levels

2. omnivory

3. habitat heterogeneity (increases complexity)

4. environmental stress (disturbance)

5. prey defenses (flight, fight, hide)

patch dynamics

• dispersal ability of organisms within a shifting mosaic of environmental conditions

• different substrate types create a mosaic of habitats with different reliance to disturbance

• oscillating between stable and unstable

community structure of patch dynamics

• disturbance removes organisms and opens space

• new space is colonized

patch dynamics: biological trajectory (1)

competitive ability inversely correlated to colonization ability (R vs K)

patch dynamics: biological trajectory (2)

lottery competition

• new space - whoever gets there first gets to keep it

• only works for the first couple of weeks

patch dynamics: biological trajectory (3)

high dispersal rates (all r-selected)

community structure summary

• overall community exhibits regularity in species composition

  • environmental conditions are predictable as a whole

  • even though they are unpredictable in any given local place or time (patch)

• colonization and reproduction in variable environment determines community structure

• variable environment allows more species to co-occur