Trade-offs between nutrient and predator effects conceal the - - PowerPoint PPT Presentation

Trade-offs between nutrient and predator effects conceal the influence of canals on snails

Clifton B. Ruehl & Joel C. Trexler

Department of Biological Sciences Florida International University Miami, FL

Canals:

Panama canal Louisiana delta Suez canal Lake Okeechobee & canals

Canal effects

• Predator and nutrient gradients are correlated
• Near Canal: More nutrients &

More predators

• Far from Canal: Fewer nutrients &

Fewer predators

Nutrient effects:

• Thick floating and benthic periphyton mats
• Snails eat periphyton

Periphyton

Nutrient effects:

P P P P

Moderate phosphorous enrichment produces faster growing snails

Nutrient effects:

P P P P

Moderate phosphorous enrichment produces more snails

Predator effects:

Predator effects:

One individual eats another

Predator effects:

I’m scared

Effects on prey:

• Reduces growth rate
• Reduces reproductive output
• Reduces population growth

Predator effects:

Predator cues causing a shift in behavior

Individual growth rate

Trade-offs Confound

Canals

Predator cue Phosphorous

More predators More nutrients

Marsh

Fewer predators Fewer nutrients

Questions & Goals

• Characterize aquatic communities near and far from

the canal during the experiment?

• How do differences alter snail growth and reproduction

near and far from the canal?

• Why are these findings important?

The Everglades

• 2 sites near & 2 sites far from a canal in 2 blocks

Experimental Sites Marsh Canal

N

Block 1 Block 2

6.4 km 3.2 km

Aquatic Community Characterization

• 2 sites near & 2 sites far from a canal in 2 blocks
• Before and after the experiment at each site:
• Seven 1m2 throw traps
• Small fish & invertebrate abundance
• Periphyton volume
• Summed all snail predators

Aquatic Community Characterization

Sqrt snail pred (no./m2)

1 2 3 4 Near Far Block 1 Block2 Block 1 Block2

June: Before exp August: After exp

• Generally more snail predators near canal

Aquatic Community Characterization

• Before and after the experiment at each site:
• Seven 1m2 throw traps
• Small fish & invertebrate abundance
• Periphyton volume
• Twenty tethered snails & controls
• PVC tethers spaced 3-m apart
• 20 snails/site attached to 1 m of 6 # line with super glue
• 4 snails tethered inside control cage
• 2 sites near & 2 sites far from a canal in 2 blocks

Probability of consumption

1.00 0.75 0.50 0.25 0.00 Near Near Far Far

Tethering Near and Far

Proximity to canal

• More predation near the canal
• Equals more predator cues

Aquatic Community Characterization

• Before and after the experiment at each site:
• Seven 1m2 throw traps
• Small fish & invertebrate abundance
• Periphyton volume
• Twenty tethered snails & controls
• PVC tethers spaced 3-m apart
• 20 snails/site attached to 1 m of 6 # line with super glue
• 4 snails tethered inside control cage
• 2 sites near & 2 sites far from a canal in 2 blocks
• Before, during, and after the experiment at each site:
• Collected periphyton

C:P Ratio Near and Far

• C:P ratio for periphyton was lower near the canal

F3, 13 = 26.1; P = < 0.001

Periphyton C:P 1500 3000 4500 Near Far Block 1 Block 2

C:P Ratio Near and Far

• C:P ratio for periphyton was lower near the canal

F3, 13 = 26.1; P = < 0.001

Periphyton C:P 1500 3000 4500 Near Far Block 1 Block 2 Block 1 Block 2 sqrt chlorophyll a (ug/mgdry wt.) 1.0 1.2 1.4 1.6 1.8

Near Far

a b

• Chlorophyll-a in periphyton was higher near the canal

F2, 24 = 8.60; P = 0.0005

Individual growth rate

Trade-offs Confound

Canals

Predator cue Phosphorous

More predators More nutrients

Marsh

Fewer predators Fewer nutrients

?

Reciprocal Transplant Experiment

Reciprocal Transplant Experiment

• Snails: Present or Absent

Planorbella duryi

Reciprocal Transplant Experiment

• Added local periphyton to bags
• Transported periphyton between sites

FAR NEAR

Reciprocal Transplant Experiment

• Experiment ran for 39 days
• Measured snail growth on day 18 and day 39
• Sampled periphyton from bags on 18 & 39 d

RESULTS

Periphyton Consumption

• Snails reduced periphyton during the experiment

No snail Snail Sqrt drywt (g/container) 1 2 3 18 39

Snail grazing: F1, 30.2 = 23.1; P < 0.0001

c d Day a b

Snail grazing: F1, 30.1 = 21.8; P < 0.0001

Sqrt afdm (g/container) 1 2 3 18 39 b Day a c d

Within subjects Effect F P Day 154.8 < 0.0001 Day × site 0.4 0.9 Day × Peri origin 1.2 0.3 Day × Site × Peri origin 0.8 0.6 Between subjects Site 1.1 0.4 Peri Origin 1 0.3 Site × Peri origin 3.5 0.03

Snail biomass through time

• Repeated measures analysis of variance

Snail biomass

Snail biomass through time

Time (d) 10 20 30 40 Snail biomass (g) 0.04 0.08 0.12 0.16 0.20 0.24

Near 1 Far 1 Near 2 Far 2

• Snails had similar growth at sites near and far from the

canal.

Within subjects Effect F P Day 154.8 < 0.0001 Day × site 0.4 0.9 Day × Peri origin 1.2 0.3 Day × Site × Peri origin 0.8 0.6 Between subjects Site 1.1 0.4 Peri Origin 1 0.3 Site × Peri origin 3.5 0.03

Snail biomass through time

• Repeated measures analysis of variance

Snail biomass near and far

Periphyton Quality Growth Rate (mg/d)

3.0 3.3 3.6 3.9 4.2 4.5

• Far sites + Near periphyton = Fastest Growth

Canals

More predators

Marsh

Fewer predators

Snail biomass near and far

Periphyton Quality Low Growth Rate (mg/d)

3.0 3.3 3.6 3.9 4.2 4.5

• Far sites + Near periphyton = Fastest Growth

Block 1 Block 2

Canals

More predators

Marsh

Fewer predators

Snail biomass near and far

Periphyton Quality Low High Growth Rate (mg/d)

3.0 3.3 3.6 3.9 4.2 4.5

• Far sites + Near periphyton = Fastest Growth

Block 1 Block 2

Canals

More predators

Marsh

Fewer predators

Snail reproduction near and far

• More reproduction at far sites

Sqrt.(No. egg masses)

2 4 6 Near Far

Home Away

Block 1

Home Away

Block 2

Summary

High resource quality + More Predators

Near =

Predator s Snail s Periphyton Phosphorous

Summary

High resource quality + More Predators

Near = Far =

Low resource quality + Fewer Predators

Predator s Snail s Periphyton Phosphorous Predator s Snails Periphyton

Questions & Goals

• Characterize aquatic communities near and far from

the canal during the experiment?

• How do differences alter snail growth and reproduction

near and far from the canal?

• Why are these findings important?

Separating the effects

• Separating these effects leads to better understanding
• f biotic and abiotic drivers
• Understanding components of the net effect leads to

better forecasting of future environmental change

• Separating effects with experiments can aide

interpretation of monitoring data

Acknowledgements & Questions Acknowledgements & Questions

• Trexler lab
• Evelyn Gaiser
• Gaiser lab
• Lisa Jiang
• Adam Obaza
• Liz Huselid
• Liz Harrison

Within subjects Effects DF F P Day 1, 60 54.5 <.0001 Day × Site 3, 60 0.4 0.73 Day × Snail 1, 60 2.8 0.10 Day × RT 1, 60 0.7 0.40 Day × Site × Snail 3, 60 0.8 0.50 Day × Site × RT 3, 60 2.0 0.12 Day × Snail × RT 1, 60 0.5 0.50 Day × Site × Snail × RT 3, 60 0.4 0.74 Between subjects Site 3, 60 2.0 0.12 Snail 1, 60 39.4 <.0001

• Recip. Trans (RT)

1, 60 0.5 0.49 Site × Snail 3, 60 0.4 0.78 Site × RT 3, 60 4.4 0.01 Snail × RT 1, 60 0.0 0.93 Site × Snail × RT 3, 60 1.4 0.25

Resource quality across Space & Time

0.8 1.2 1.6 2.0 Home Away Home Away Near Far Block 2

sqrt Chlorophyll a (ug/mgdrywt.)

Block 1 Home Away Home Away Near Far

Resource quality across sites

Separating the effects Resources Human modifications _ + Population Stress

• Simplified food web
• Reciprocal transplant

experiments

• Survey multiple ecosystems
• Manipulation experiments
• Add/Remove stress
• Add resources

Summary

• Community composition was different near

compared to far from the canal

• Periphyton was more nutritious near compared to

far from the canal

• Snails grew fastest on periphyton that originated

near but placed far from the canal.

• Snail produced more egg masses far from the canal

Marsh Canal The Everglades

N

Marshes

• Lower Phosphorus (P) &

contaminants

• Fewer small consumers &

macroinvertebrates

Canals

• Higher P & contaminants
• More small consumers &

macroinvertebrates

• Refuge for large predators
• Characterizing anthropogenic effects

Rehage & Trexler 2008; Gaiser et al. 2005; Perry 2004; Turner et al. 1999

Per Capita effects b(s) d(s) S*

Adapted from Chase and Leibold 2003

Anthropogenic Effects

Biotic or abiotic stress Nutrient enrichment

Periphyton

• rigin

Far Near Proximity to canal Near Far Growth Rate (mg/d)

3.0 3.3 3.6 3.9 4.2

Summary Near =

High nutrients + High biotic

Far =

Low nutrients + Low biotic 4.5

Summary Near =

High nutrients + High biotic

Far =

Low nutrients + Low biotic

Periphyton

• rigin

Far Near Proximity to canal Near Far Growth Rate (mg/d)

3.0 3.3 3.6 3.9 4.2 4.5