Lecture 13

Question 1

What is resilience in ecosystems?

Answer 1

The ability of an ecosystem to absorb disturbances and return to its original state.

Question 2

What is hysteresis in ecosystems?

Answer 2

When an ecosystem requires greater effort to return to its original state after a shift.

Question 3

What are tipping points?

Answer 3

Critical thresholds where small changes in conditions lead to significant and often irreversible changes.

Question 4

What do NPZ models simulate?

Answer 4

Nutrient-Phytoplankton-Zooplankton interactions within ecosystems.

Question 5

What are resilience metrics?

Answer 5

Metrics that measure an ecosystem’s ability to recover, such as recovery time and resistance to perturbations.

Question 6

How do scaling laws relate to ecological processes?

Answer 6

Ecological processes often follow power-law relationships, where variables such as metabolism scale with body size.

Question 7

Why is a larger system sometimes chosen for models?

Answer 7

To avoid effects from migration and boundary issues in the model.

Question 8

How can different time frames affect model dynamics?

Answer 8

A longer timespan can capture important oscillations, while shorter timespans may miss key patterns.

Question 9

What is a forcing function and how is it used in models?

Answer 9

Forcing functions, like fishing mortality or catch rates, are external inputs to models and are not simulated but influence state variables.

Question 10

How are unknowns in linear systems solved?

Answer 10

By fixing some parameters using literature values and solving for the remaining unknowns.

Question 11

What is EE in Ecopath, and why is it challenging to measure?

Answer 11

EE (Ecotrophic Efficiency) represents how efficiently energy is transferred through a trophic level and is often estimated through model-solving rather than direct measurement.

Question 12

What type of model does Ecopath use?

Answer 12

A snapshot, non-dynamic model that represents averaged values over a defined period.

Question 13

How does vulnerability impact predator-prey interactions?

Answer 13

Vulnerability parameters control whether prey are easily accessible or hidden, influencing energy transfer in Lotka-Volterra equations.

Question 14

What is an end-to-end model?

Answer 14

A comprehensive ecosystem model that integrates multiple levels of the food web, often requiring significant computational time to run.

Question 15

What challenge do end-to-end models face?

Answer 15

They can take decades to fully simulate due to their complexity.

Question 16

How can MATLAB code be used in ecosystem modelling?

Answer 16

MATLAB scripts are used to parameterize, solve equations, and visualize ecosystem model outputs efficiently.

Question 17

How are trophic flows solved in complex systems?

Answer 17

By applying a combination of literature values, forcing functions, and model-derived estimates, ensuring all flows are balanced.

Question 18

How many observations per fitted parameter are typically required to avoid overfitting?

Answer 18

3-9 observations per parameter. For example, if there are 23 yearly observations (e.g., 2000-2023), dividing this by 5 indicates that 4 parameters can be fitted (e.g., fishing mortality, temperature, carrying capacity).

Question 19

What can be done if the number of observations is insufficient?

Answer 19

Additional observations can be sourced, such as size-based data, to improve parameter fitting.

Question 20

Why is it important to avoid small system boundaries in modelling?

Answer 20

To prevent migration effects from distorting model predictions.

Question 21

Why might two models be created for different time spans?

Answer 21

To capture dynamics before and after a regime shift, as system characteristics may change.

Question 22

Why are oscillations significant in ecosystem models?

Answer 22

Oscillations reflect meaningful predator-prey and environmental interactions rather than noise, so excluding them may oversimplify the system.

Question 23

How can forcing functions such as catch rates influence the model?

Answer 23

Forcing functions provide external inputs, like fishing effort, that affect variables but are not directly simulated.

Question 24

What is meant by solving a linear system with unknowns?

Answer 24

By setting known values from literature for some variables, the remaining unknowns (e.g., Ecotrophic Efficiency) can be solved.

Question 25

How is Ecotrophic Efficiency (EE) used in Ecopath?

Answer 25

EE represents the proportion of energy flow used within a trophic level and is often the primary unknown solved by Ecopath.

Question 26

Why is Ecopath considered a non-dynamic model?

Answer 26

It provides a snapshot for a given period, averaging values instead of simulating changes over time.

Question 27

What approach does Ecopath use for mass balance?

Answer 27

It calculates an average value for each component (e.g., prey availability) to achieve equilibrium across trophic levels.

Question 28

What do Lotka-Volterra predator-prey equations describe?

Answer 28

The interactions between predators and prey, with terms for vulnerability controlling the accessibility of prey.

Question 29

What does the vulnerability parameter signify?

Answer 29

A high vulnerability value indicates that prey are readily available, whereas a low value indicates that prey are hidden or inaccessible.

Question 30

How does the flow proportionality influence prey dynamics?

Answer 30

Bottom-up control dictates that prey availability drives energy flow and influences predator population sizes.

Question 31

How is vulnerability incorporated into Lotka-Volterra models?

Answer 31

It can be easily added to simulate different prey availability scenarios.

Question 32

What are end-to-end models, and why are they significant?

Answer 32

They are comprehensive models that integrate the entire food web, from primary producers to top predators, often requiring long computation times.

Question 33

What challenge is associated with running end-to-end models?

Answer 33

These models can take decades to run due to their complexity and detailed representation.

Question 34

What insights do slides about diet composition provide in ecological modelling?

Answer 34

They illustrate the importance of diet data in parameterizing models and highlight potential uncertainties in trophic flow estimates.

Question 35

Why are average values important for non-dynamic snapshots?

Answer 35

By calculating averages over a defined period, the model represents stable conditions rather than short-term fluctuations.

Question 36

How does MATLAB support ecosystem modelling?

Answer 36

MATLAB code is used to fit parameters, solve equations, and visualize system outputs, making the process more efficient.

Question 37

What is the significance of slide 60’s equation Q(ij)?

Answer 37

This equation incorporates Lotka-Volterra principles and includes terms for prey availability and predator-prey interactions

Question 38

Why might vulnerability be adjusted in a model?

Answer 38

To account for changes in prey accessibility due to environmental conditions or predator behavior.

Question 39

How can linear systems be solved when equations have unknowns?

Answer 39

By fixing known variables (e.g., three out of four unknowns from literature), the final unknown (e.g., Ecotrophic Efficiency) can be derived.

Question 40

What challenge did slide 71 highlight about end-to-end models?

Answer 40

The long runtime required for such comprehensive simulations.

Question 41

What is the difference between dynamic and non-dynamic models?

Answer 41

Dynamic models simulate temporal changes, while non-dynamic models provide a static average representation for a specific period.

Question 42

Why might trophic flow models rely on estimate-based unknowns?

Answer 42

Certain variables, like EE, are difficult to measure directly and are derived through model-based calculations.