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 o Predator and nutrient gradients are correlated o Near Canal: More nutrients & More predators o Far from Canal: Fewer nutrients & Fewer predators
Nutrient effects: o Thick floating and benthic periphyton mats o Snails eat periphyton Periphyton
Nutrient effects: Moderate phosphorous enrichment produces faster growing snails P P P P
Nutrient effects: Moderate phosphorous enrichment produces more snails P P P P
Predator effects:
One individual eats another Predator effects:
Predator effects:
Predator effects: Predator cues causing a shift in behavior I’m scared Effects on prey: • Reduces growth rate • Reduces reproductive output • Reduces population growth
Trade-offs Confound Phosphorous Individual growth rate Predator cue Marsh Canals Fewer predators More predators Fewer nutrients More nutrients
Questions & Goals o Characterize aquatic communities near and far from the canal during the experiment? o How do differences alter snail growth and reproduction near and far from the canal? o Why are these findings important?
The Everglades
Experimental Sites o 2 sites near & 2 sites far from a canal in 2 blocks Marsh N 3.2 km Block 2 Block 1 Canal 6.4 km
Aquatic Community Characterization o 2 sites near & 2 sites far from a canal in 2 blocks o Before and after the experiment at each site: • Seven 1m 2 throw traps - Small fish & invertebrate abundance - Periphyton volume - Summed all snail predators
Aquatic Community Characterization o Generally more snail predators near canal June: Before exp August: After exp 4 Sqrt snail pred (no./m 2 ) Near Far 3 2 1 0 Block 1 Block2 Block 1 Block2
Aquatic Community Characterization o 2 sites near & 2 sites far from a canal in 2 blocks o Before and after the experiment at each site: • Seven 1m 2 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
Tethering Near and Far o More predation near the canal o Equals more predator cues 1.00 consumption Probability of 0.75 0.50 0.25 0.00 Near Near Far Far Proximity to canal
Aquatic Community Characterization o 2 sites near & 2 sites far from a canal in 2 blocks o Before and after the experiment at each site: • Seven 1m 2 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 o Before, during, and after the experiment at each site: • Collected periphyton
C:P Ratio Near and Far o C:P ratio for periphyton was lower near the canal Near 4500 Far Periphyton C:P 3000 1500 0 Block 1 Block 2 F 3, 13 = 26.1; P = < 0.001
C:P Ratio Near and Far o C:P ratio for periphyton was lower near the canal o Chlorophyll- a in periphyton was higher near the canal sqrt chlorophyll a (ug/mgdry wt.) 1.8 Near Near 4500 Far Far a Periphyton C:P 1.6 3000 b 1.4 1500 1.2 0 1.0 Block 1 Block 2 Block 1 Block 2 F 2, 24 = 8.60; P = 0.0005 F 3, 13 = 26.1; P = < 0.001
Trade-offs Confound Phosphorous Individual growth rate ? Predator cue Marsh Canals Fewer predators More predators Fewer nutrients More 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 o Experiment ran for 39 days o Measured snail growth on day 18 and day 39 o Sampled periphyton from bags on 18 & 39 d RESULTS
Periphyton Consumption o Snails reduced periphyton during the experiment 3 3 No snail c Sqrt afdm (g/container) Sqrt drywt (g/container) c Snail d d 2 2 a a b b 1 1 0 0 18 39 18 39 Day Day Snail grazing: Snail grazing: F 1, 30.2 = 23.1; P < 0.0001 F 1, 30.1 = 21.8; P < 0.0001
Snail biomass through time o Repeated measures analysis of variance Snail biomass 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 o Snails had similar growth at sites near and far from the canal. 0.24 Near 1 Far 1 Snail biomass (g) Near 2 0.20 Far 2 0.16 0.12 0.08 0.04 0 10 20 30 40 Time (d)
Snail biomass through time o Repeated measures analysis of variance 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 near and far o Far sites + Near periphyton = Fastest Growth 4.5 Periphyton Quality Growth Rate (mg/d) 4.2 3.9 3.6 3.3 3.0 Marsh Canals Fewer predators More predators
Snail biomass near and far o Far sites + Near periphyton = Fastest Growth 4.5 Periphyton Block 2 Block 1 Quality Growth Rate (mg/d) 4.2 3.9 3.6 3.3 Low 3.0 Marsh Canals Fewer predators More predators
Snail biomass near and far o Far sites + Near periphyton = Fastest Growth 4.5 Periphyton Block 2 Block 1 Quality Growth Rate (mg/d) 4.2 High 3.9 3.6 3.3 Low 3.0 Marsh Canals Fewer predators More predators
Snail reproduction near and far o More reproduction at far sites 6 Sqrt.(No. egg masses) Block 2 Block 1 Near Far 4 2 0 Home Away Home Away
Summary Predator s High resource quality Snail Near = + s More Predators Periphyton Phosphorous
Summary Predator s High resource quality Snail Near = + s More Predators Periphyton Phosphorous Predator s Low resource quality Far = + Snails Fewer Predators Periphyton
Questions & Goals o Characterize aquatic communities near and far from the canal during the experiment? o How do differences alter snail growth and reproduction near and far from the canal? o Why are these findings important?
Separating the effects o Separating these effects leads to better understanding of biotic and abiotic drivers o Understanding components of the net effect leads to better forecasting of future environmental change o 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
Resource quality across Space & Time 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 sites 2.0 Block 2 Block 1 sqrt Chlorophyll a (ug/mgdrywt.) 1.6 1.2 0.8 Home Away Home Away Home Away Home Away Near Far Near Far
Separating the effects Human modifications o Simplified food web Stress o Survey multiple ecosystems _ o Manipulation experiments Population - Add/Remove stress - Add resources + o Reciprocal transplant experiments Resources
Summary o Community composition was different near compared to far from the canal o Periphyton was more nutritious near compared to far from the canal o Snails grew fastest on periphyton that originated near but placed far from the canal. o Snail produced more egg masses far from the canal
The Everglades o Characterizing anthropogenic effects Marshes Marsh N • Lower Phosphorus (P) & contaminants • Fewer small consumers & macroinvertebrates Canals • Higher P & contaminants • More small consumers & Canal macroinvertebrates • Refuge for large predators Rehage & Trexler 2008; Gaiser et al. 2005; Perry 2004; Turner et al. 1999
Anthropogenic Effects d(s) Per Capita effects S* b(s) Nutrient Biotic or enrichment abiotic stress Adapted from Chase and Leibold 2003
Summary 4.5 Periphyton origin High nutrients Near = + Growth Rate (mg/d) 4.2 High biotic Near 3.9 3.6 3.3 Far Low nutrients Far = + 3.0 Far Near Low biotic Proximity to canal
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