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The L Component

Models for Immature Mosquito Ecology

Source: vignettes/L-interface.Rmd
L-interface.Rmd

The L component was designed to model adult mosquito ecology and infection dynamics.

This vignette takes a deep dive into the design and structure of the L component. It is useful for anyone who wants to learn more about how the code works.

Required Functions

Each L module includes XX required functions and some optional ones.

Some of these required functions are S3 class functions in adult-L-interface.R. Others are defined for each module.

One good example is the SI module, posted in the ramp.xds github repository human-L-SI.R.

The required functions deal with various tasks for model building and solving: constructing the L model object; the dynamics; the parameters; the variables and their initial values; computing terms and standard outputs; and consistency checks. Optional outputs include other metrics; functions to compute steady states; and module specific functions to visualize the outputs.

Each module is defined by a string, generically called Lname, that identifies the module: e.g. SI.

Dynamics

  1. The dynamics are defined by at least one of the following is required, depending on whether the model family is a system of differential equations or a discrete time system:

    • dLdt.Lname :: differential equations are defined by a function that computes the derivatives. In ramp.xds these are encoded in a function called dLdt. The function is set up to be solved by deSolve::ode or deSolve::dede.

    • Update_Lt.Lname :: discrete time systems are defined by the function that updates the state variables in one time step. In ramp.xds these are encoded in a function called Update_Lt that computes and returns the state variables. The forms mimic the ones used for differential equations.

L Model Object

Each module has a pair of functions that set up a structured list called the L model object. The object is a list that is assigned to a class that dispatches the S3 functions described below. It is a compound list, where some of the sub-lists are assigned their own class that dispatch other S3 functions.

  1. make_L_obj_Lname :: returns a structured list called an L model object:

    • bionomic parameter values or bionomic parameter objects. In most models, the value of baseline bionomic parameters are functions of time, or exogenous variables that vary with time.

    • class(L_obj) = Lname

    • the indices for the model variables are stored as L_obj$ix

    • the initial values are stored as L_obj$inits

    • anything else that is needed can be configured here

  2. setup_L_obj.Lname is a wrapper that calls make_L_obj_Lname and (for the \(i^{th}\) species) attaches the object as xds_obj$L_obj[[i]]

Parameters

  1. change_L_pars.Lname changes the values of some parameters. It is designed to be used after setup. New parameter values are passed by name in a list called options.

  2. get_L_pars.Lname is a utility to inspect the values of the parameters.

Variables

Since the L component is one of three, a function sets up the indices for all the variables in a model.

Two other functions use those indices: one pulls the variables from the state variable vector \(y\); the other one pulls the variables by name from an output matrix returned by xds_solve.

After pulling, both functions return the variables in by name in a list to make it easy to inspect or use.

  1. setup_L_ix.Lname - is the function that assigns an index to each variable in the model, and stores it as xds_obj$L_obj[[i]]$ix. The indices are returned as a named list.

  2. get_L_vars.Lname - retrieves the value of variables from the state variables vector \(y\) at a point in time and returns the values by name in a list; the function gets called by dLdt and by change_L_inits and it can be useful in other contexts.

  3. parse_L_orbits.Lname - this function is like get_L_vars but it parses the matrix of outputs returned by xds_solve.

Initial Values

A set of functions is sets up or changes the initial values for the state variables.

  1. make_L_inits_Lname - each model must include a function that makes a set of initial values as a named list. This function does not belong to any S3 class, so it can take any form. The function should supply default initial values for all the variables. These can be overwritten by passing new initial values in options.

  2. setup_L_inits.Lname - is a wrapper, that gets called by xds_setup and that calls make_L_inits_Lname. The setup options are passed to overwrite default values. The initial values are stored as L_obj$inits.

  3. change_L_inits.Lname - a utility to change the initial values.

Dynamical Terms

These functions compute dynamical terms – the outputs passed to an interface.

  1. F_emerge.Lname -

Standard Outputs

Each module must output a few key quantities:

  1. F_larvae.Lname - compute the density of all larvae

  2. F_capacity.Lname - compute the carrying capacity for each habitat

Consistency Checks

Some modules in ramp.xds or ramp.library have been included for various reasons. Not all of those models are capable of being extended. To help users avoid using models in ways that are not appropriate, we developed two function classes:

  1. skill_set_L.Lname :: describes model capabilities and limitations

  2. check_L.Lname :: at the end of xds_setup and at the beginning of xds_solve, this function gets run to ensure that some quantities have been properly updated, and to see if anything has been added to a model that is not in its skill set.

Optional Functions

The L interface also sets up S3 classes for some optional functions, but these might not be appropriate for all models. If a function is not in the skill set of the module, then the limitation should be noted in the documentation of skill_set_L.Lname with information in the list.

Steady States

Methods are defined to compute various steady states under static parameter values.

  • steady_state_L.Lname :: pass the egg laying rate of and compute steady states for the L component.

Visualization

Functions have been developed to plot the standard terms, but each module can define its own method for plotting:

  • xds_plot_L.Lname is a wrapper that calls xds_lines_X

  • xds_lines_L.Lname defines a default method for plotting orbits