In UofTCoders/eeb430.2017.JuniorCoders: Team Junior Coders Final Report for EEB430 at UofT

Brose et al. 2006: Consumer-Resource Body-Size Relationships in Natural Food Webs

Introduction

• Differences in body size between predators and prey important for determining strength of interactions, population dynamics, food web structure, function, evolution. Linked to locomotion, dispersal, niche use, biological rates.
• Trophic links and interaction strengths in communities can reflect size constraints on who eats who -- body size relationships can explain food web structure and stability.
• Do predator-prey relationships vary among habitats or taxonomic groups?
• They show that the mean body size ratios of herbivores, detritivores, and carnivores are different.
• Predators tend to be larger than prey, but how does this relationship vary across ecosystems and predator types?
• Size constraints on feeding could be related to habitat structure -- for this reason, predator-prey relationships could differ across habitats.

Methods

• Use average body masses of predator-prey links.
• Data includes: different consumer and resource types across different habitats.
• For interactions, calculate the log ratio of predator mass over prey mass. The log ratios were analyzed for significant differences using an ANOVA among feeding types (I guess they calculated the mean for each feeding type?).
• Excluded outliers of the dataset with residuals that deviated more than 3 SDs.
• Three regressions were carried out:
  • log predator mass as a function of log prey mass
  • log prey mass as a function of log predator mass
  • major axis regression that treats both variables symmetrically (?)

Results

• Consumer-resource log body mass ratios differ among feeding types.
  • Smaller consumer versus larger resource
  • Equally sized
  • Large consumer consumer versus small resource
• Figure 1: Log predator mass versus prey mass
  • Major axis regression line explains most of the variation (86%)
• Figure 2: Predator-prey body size ratios across different habitat categories and predator types.
  • Interaction between predator type and habitat category explaining the variance in body mass ratios.
  • Size differences according to predator type tend to be bigger in certain habitats.
• Table 1: Mean measures of ratios, log predator mass, log prey mass across different habitats and feeding types.
  • Compared different TYPE OF PREDATOR (e.g. invertebrate predators) in different habitats to see if the mean ratios were significantly different (using a Scheffé post hoc test).
  • The mean ratios also depended on PREY TYPE (i.e. feeding type, like planktivorous).

Discussion

• Consumer-resource body-mass ratios
• Predators on average larger than prey.
• Different body mass ratios might suggest that predator-prey dynamics are different.
• Aquatic systems: consumers are often gape-limited, ingest whole-particles, small, short generation times, shorter growth rates.

• Terrestial systems: usually large with long-generation times.

• Predator-prey body-mass ratios

• Most predators larger than prey, prey and predator size increase together.
• Linear models assume that predictor variables were measured without error, normal distributions.
• Body mass ratios higher for:
  • Vertebrate predators than for invertebrates predators
  • Ectotherm vertebrate predators in freshwater habitats than in terrestial or marine habitats.
• Higher ratios reflect higher prey body-masses.
• Author says: More information is needed for pelagic marine ecosystems (although they don't really provide any information about other marine ecosystems) to analyze differences between marine, freshwater, benthic...
  • Note: We can do marine and possibly benthic?
  • They also didn't consider non-independence of data? idk

- Caveats

Gibert and DeLong, 2014: Temperature alters food web body-size structure

Introduction

• Body size influences: landscape use and locomotion, energetic requirements, prey selection. For the reasons below, likely important for species persistence, food web complexity and stability.
• Large organisms tend to eat large prey. The ratio affects:
  • predator-prey dynamics
  • interaction strengths
  • trophic position
  • size structure (in food webs)
  • function of food webs
• Body size often declines with temperature -- this is the temperature size rule. Could affect the way species interact -- smaller organisms tend to eat smaller prey.
• Increasing temperature can decrease average body in food webs and have important consequences for food web stability and species persistence:
  • reduces the number of trophic levels
  • reduces overall food web connectivity
• Does tempreature alter relationship between predator and prey body size?

Methods

• Three linear mixed effects models aimed at controlling hierarchical structure of the data.
  • Model 1: prey body size is the response variable, habitat type is the random intercept, predator identify (species) is the random slope.
  • Model 2: same model as above (i.e. same random effects, I think?) but with additive effect of temperature.
  • Model 3: interaction effect of predator body mass and temperature as the random effects.
• Used AIC to determine the best model.

Results

• Prey size increases with predator size and depends on the effect of temperature.
• Temperature is increasing the size of the relationship but decreases the slope. The reverse is true for large predators.
• A slope close to 1 suggests that body sizes remain constant across predator mass sizes.

Discussion

• They show that the relationship between prey and predator size is depenent on the interaction between temperature and predator body size.
• Magnitude of relationship changes depending on the temperature, but the direction doesn't. Reveal that body size and temperature have a strong interactive effect on food web body size structure.
• Consequences of differences in body size change and structure:
  • Range of body sizes will be different depending on temperature.
  • Trophic level of a species can vary across temperature because trophic level icreases with body size, and temperature affects body size.
  • Intermediate tophic levels (intermediate body sizes) likely least affect by body size-temperature interaction.
  • Differential effect of temperature on predator and prey body sizes -- with warming, predators are becoming smaller faster than their prey.
• Temperature effects predator-prey body sizes could scale up to interaction and food web scaling.
  • Interactions strenghts are typically large at high trophic levels because body sizes are large.

Tucker and Rogers 2014: Examining predator-prey body size, trophic level and body mass across marine and terrestial mammals

Introduction

• Mammals exploit variety of habitats, niches, feeding ecologies.
• Feeding ecology -- two relationships are used to investigate this: trophic level and body mass.
• Energetic requirements -- large mammalian carnivores tend to feed on large prey. Larger carnivores also tend to have higher trophic levels.
  • This is the idea that food webs are size structured: large predators tend to feed on large prey and vice versa.
• Productivity differs between marine and terrestial environments.
  • Marine:
    • Lots of single celld plants like phytoplankton are primary producers -- energy flow is faster and more readily accessible: high turnover rate.
    • Aquatic systems tend to be really size structured -- trophic interactions driven by large consumers feeding on smaller species.
  • Terrestial:
    • Most primary producers are here, turnover rate is much slower.
• Three patterns we might expect in the relationship between trophic level and body mass: (1) Differences in environmental variables driving trophic patterns across mammals, expectation of positive relationship between trophic level and body mass. (2) Body mass is driving trophic level patterns. (3) Quadratic trophic-level-body-mass relationship.
• Predator-prey body mass ratios: info about food web complexity, ecosystem stability, community structure.
  • More stable with large predators because its harder for new invasive predators to come in and outcompete native predators.
• Investigating the relationship between body size, trophic level and PP ratio across mammals: (1) How do differences between consumers within terrestrial and marine environments influence trophics and food web structure of mammamls (2) test whether there is a negative relationship between PP ratio and predator trophic level across mammals.

Materials and methods

Dataset

• Trophic level and species body masses from a bunch of carnivorous mammals in marine and terrestrial environments (give sample size).

Predator-prey body mass

• PP was calculated for carnivorous species by dividing the average mass of each predator species by their average prey mass.

Analysis

• They made some models.
• The likelihood of a given model explaining species differences in trophic levels determined by AIC.
• Lowest AIC = highest support. Any models within two units of the lowest model were also likely candidates (delta AIC <2).
• For the model that got the most support, extracted 95% confidence intervals of the slope values, assessed the allometric effects associated with predictor variables.

Discussion

The effects of body mass on trophic position

• The evolution of trophic position in mammalian carnivores appears to be driven by the environment they live in.
• Marine carbivores are usually higher in trophic level than mammals -- likely driven by complexities in nature of marine food webs where there are higher levels of trophic levels and interactions between trophic levels.
• Strong relationship between body mass and trophic level in mammals only.
• No positive relationship between body mass and trophic level for terrestiral mammals.

Environmental effects on herbivory

• Productivity seems to drive differences in trophic level patterns and food web structures across two environments.
• Marine productivity -- driven by high quantities of small single celled primary producers -- reverse of trophic level patterns and food web structures found on land.
  • Single-celled phytoplankton: don't really have chemical defenses, fast growth rates, lots of photosynthetic tissue high in P and N.
  • This allows marine herbivores to feed on lots of nutrient-rich phytoplankton at once that is easily digestible.

Environmental effects on carnivory

• Lots of nutrients in marine environments means more mammalian carnivores in marine ecosystems can be supported.
• Phytoplankton support zooplankton (primary consumers) which scale up trophic levels. The small size and fast generation times of zooplanktonic species means they can reproduce quickly -- you get lots of primary productivity. This is pretty much an algal bloom.

Body size, diet and environment

• Mammals with increasing body mass probably have less prey items to consume -- this is for terrestrial habitats.
  • Large carnivores need lots of resources but there are only so many resources available. This is the case for large mammals, which are generally herbivorous.
  • Most primary productivity has lignin in it which is hard for mammals to digest -- mammals have lots of adaptations to be able to break these things down.
  • Resource availability limits how large body sizes can get -- energetic costs, need for reliable resources.
• Marine environments see an increase in body size:
  • Thermal advantages, prey availability, hunting efficiency = larger bodies.
  • Marine carnivores are better hunters, waste less energy than terrestrial carnivores which spend a lot of energy.
  • Marine vegetation contains small amounts of lignin = more nutrients for marine consumers which passes through the food web.
    • Higher population sizes, more stability in terms of its feeding behaviour. E.g. terrestrial mammals change their diet depending on food availability, vs. marine mammals usually stick to a single diet.
• And so you see changes in body mass based on different diets, and there are also changes in the minimum and maximum body size trends.
  • Driven by: environmental characteristics, physiology, resource availability.

Trophic level and predator-prey ratio relationships

• General rule: As trophic level increases, carnivores feed on prey with body mass more similar to their own (equal to 1).
  • increasing body mass with trophic level for endotherms, increasing prey mass with increasing carnivore body mass.
• But in this study:
  • No relationship between trophic level and carnivore body mass.
  • Positive relationship between carnivore body size and prey body size seen in terrestrial mammals, but NOT IN MARINE CARNIVORES.
• Large PP ratios in marine systems not seen in terrestrial systems.
  • Small prey do not form huge abundant groups as they do in marine systems: bulk feeding, minimal capture and processing time.
• Trophic efficiency -- exchange of energy between trophic levels based on predator-prey interactions.
  • Large PP ratios: trophic efficiency decreases.
• Trophic efficiency has important implications for energy flow through an ecosystem, community structure and dynamics.
  • Not a big issue in marine systems because large PP tends to stabilize marine food webs. Large PPs reduce interaction strengths between predators and prey
• Bias in data:
  • Information reliant on published dietary studies which biases data to well-studied species.
• Broad study of how environment affects ecological patterns in marine systems.
• Results demonstrate primary productivity and its availability important for trophic structure of food webs, body size, prevalence of carnivory and herbivory. - Pattern of trophic level and PP ratio stroner within each environment.

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