Preface

In this vignette, we show how to run MGDrivE2 simulations using a variety of sampling techniques, including deterministic integration with ODE solvers from the deSolve package. We demonstrate a logistic (carrying capacity) form of larval mortality as well as the Lotka-Volterra form. While the behavior of the two types of density-dependent mortality will be nearly identical, because they are both quadratic in the number of larvae, this is meant to demonstrate the relative ease at which new density-dependent (and in general, any hazard, see “MGDrivE2: Simulation of Time-inhomogeneous Stochastic Processes (Seasonality)” for a more complex example) functions can be swapped as necessary for the simulation task at hand.

Parameterization

Next, we set entomological parameters that dictate life-history characteristics, as well as the equilibrium number of adult female mosquitoes. The density dependent per-capita larval mortality is thus: $\mu_{L}(1 + \frac{L}{K})$, where L is the current larval population size. Note that when the population is zero there is no effect of density dependence.

The parameters qE, qL, and qP are the mean dwell times in the egg, larval, and pupae stages, respectively. The Erlang distributed aquatic stages allow users to select the mean and variance of the dwell time distribution for each stage. For example, $\frac{1}{q_{E}}$ will give the mean dwell time in the egg stage, and variance $\frac{1}{n_{E} \cdot q_{E}^{2}}$. A table of (case-sensitive) biological parameters the user needs to specify is given below (Note that all parameters must be specified as rates per day). The equilibrium constant for larval density dependence (K) is returned when equilibrium is calculated.

The nu parameter gives the rate at which unmated female mosquitoes mate when males become available again. Ordinarily, female mosquitoes are assumed to mate upon emergence if males are present. However if male mosquitoes become locally extinct and are later replenished either by currently developing egg batches or releases (or for metapopulation simulations, inbound male migration), then there will be some time where newly emerging adult females are unable to find mates. These unmated female mosquitoes experience mortality at the same rate as mated females, muF. Because they are unmated, they do not produce eggs, nor do they blood-feed (which is done in order to provision developing egg batches). While blood-feeding behavior doesn’t matter for lifecycle simulations, this means that for the epidemiological simulations we don’t need to worry about unmated females becoming infected, because they will not be biologically primed for this behavior until after they have mated and need to produce egg batches. Once males become available, they will mate at rate nu.

# number of adult female mosquitoes
NF <- 500

# entomological parameters
theta <- list(
  qE = 1/4,
  nE = 2,
  qL = 1/3,
  nL = 3,
  qP = 1/6,
  nP = 2,
  muE = 0.05,
  muL = 0.15,
  muP = 0.05,
  muF = 0.09,
  muM = 0.09,
  beta = 16,
  nu = 1/(4/24)
)

Initialization of the Petri Net

The next thing we need to do is set up the structural properties of the SPN model (technically just a Petri Net, PN, at this point). First, we set all of the “places” in the model. As this is a compartmental model, “places” are simply distinct life stages (and locations for metapopulation versions). Thus, the “places” include all aquatic stages (number of bins in each egg, larval, and pupal stages), adult males, and adult females (further separated by mate genotype). All of these stages are unique for each genotype as well, so the number of “places” (i.e., compartments) grows with the number of genotypes and Erlang-distributed stage length.

Once the places for the SPN are setup, we define the transitions between each place. Transitions are how objects move from one compartment to another. These are not the rates that objects change compartments, only whether or not such a move is possible. The rate of movement from one “place” to another are calculated later by the spn_hazards() function.

Finally, as not all transitions apply to all “places”, we create a summary of possible transitions to and from each “place”. This is handled by the spn_S() function.

# "Places"
#  These are defined by Erlang-distributed life stages and genotypes
SPN_P <- spn_P_lifecycle_node(params = theta,cube = cube)

# Transitions
# This is a list of viable transitions from one "place" to another
SPN_T <- spn_T_lifecycle_node(spn_P = SPN_P,params = theta,cube = cube)

# Stoichiometry matrix
#  A sparse matrix representing the effect of each transition on each place
S <- spn_S(spn_P = SPN_P, spn_T = SPN_T)

Equilibrium Conditions and Hazard Functions

Once the “places” are defined, we use equilibrium_lifeycle() to get initial conditions (M(0)), solved at the dynamic equilibrium for NF number of adult females, as well as the density-dependent constants for larval competition, either K (logistic dependence) or gamma (Lotka-Volterra dependence).

# calculate equilibrium and setup initial conditions
#  outputs required parameters in the named list "params"
#  outputs intial equilibrium for adv users, "init
#  outputs properly filled initial markings, "M0", vectorized over:
#    all patches
#    any genotypes( includes XX/XY inheritance patterns)
#    allows initial ratios to vary
#    properly mates males/females
initialCons <- equilibrium_lifeycle(params = theta, NF = NF, phi = 0.5,
                                    log = TRUE, spn_P = SPN_P, cube = cube)

The transitions in our SPN define possible movement from one compartment to another, but they do not define the rate that these changes occur. The rate of change from one compartment to another is called a “hazard”, calculated by the spn_hazards() function. This calculates the rate of change from one compartment to another. Below, we make both a set of exact hazards, for sampling algorithms with integer valued state spaces, and a set of approximate hazards, for algorithms using a continuous approximation.

# approximate hazards for continous approximation
approx_hazards <- spn_hazards(spn_P = SPN_P, spn_T = SPN_T, cube = cube,
                              params = initialCons$params, type = "life",
                              log = TRUE, exact = FALSE, tol = 1e-8,
                              verbose = FALSE)

# exact hazards for integer-valued state space
exact_hazards <- spn_hazards(spn_P = SPN_P, spn_T = SPN_T, cube = cube,
                             params = initialCons$params, type = "life",
                             log = TRUE, exact = TRUE, tol = NaN,
                             verbose = FALSE)

Simulation of Fully Specified SPN Model

Finally, before we run simulations, we set up a release scheme to generate interesting dynamics. We will release 50 adult females with homozygous recessive alleles 5 times, every 10 days, starting at day 20. Releases are possible at every life-stage, so long as the event name matches the place in the simulation. It is critically important that the event names match a place name in the simulation. The simulation function checks this and will throw an error if the event name does not exist as a place in the simulation. This format is used in MGDrivE2 for consistency with solvers in deSolve.

# releases
r_times <- seq(from = 20, length.out = 5, by = 10)
r_size <- 50
events <- data.frame("var" = paste0("F_", cube$releaseType, "_", cube$wildType),
                     "time" = r_times,
                     "value" = r_size,
                     "method" = "add",
                     stringsAsFactors = FALSE)

# max simulation time
tmax <- 125
# time-step for output return, not the time-step of the sampling algorithm
dt <- 1

# run deterministic simulation
ODE_out <- sim_trajectory_R(x0 = initialCons$M0, t0 = 0, tt = tmax, dt = dt, S = S,
                            hazards = approx_hazards, sampler = "ode", method = "lsoda",
                            events = events, verbose = FALSE)

# summarize females by genotype
ODE_out_f <- summarize_females(out = ODE_out$state, spn_P = SPN_P)

# summarize males by genotype
ODE_out_m <- summarize_males(out = ODE_out$state)

# add sex for plotting
ODE_out_f$sex <- "Female"
ODE_out_m$sex <- "Male"

# plot
ggplot(data = rbind(ODE_out_f, ODE_out_m)) +
  geom_line(aes(x = time, y = value, color = genotype)) +
  facet_wrap(facets = vars(sex), scales = "fixed") +
  theme_bw() +
  ggtitle("SPN: ODE Solution")

# delta t
dt_stoch <- 0.1

# tau sampling
PTS_out <- sim_trajectory_R(x0 = initialCons$M0, t0 = 0, tt = tmax, dt = dt,
                            dt_stoch = dt_stoch, S = S, hazards = exact_hazards,
                            sampler = "tau", events = events, verbose = FALSE)

# summarize females/males
PTS_out_f <- summarize_females(out = PTS_out$state, spn_P = SPN_P)
PTS_out_m <- summarize_males(out = PTS_out$state)

# add sex for plotting
PTS_out_f$sex <- "Female"
PTS_out_m$sex <- "Male"

# plot adults
ggplot(data = rbind(PTS_out_f, PTS_out_m)) +
  geom_line(aes(x = time, y = value, color = genotype)) +
  facet_wrap(facets = vars(sex), scales = "fixed") +
  theme_bw() +
  ggtitle("SPN: Tau-leaping Approximation")

Equilibrium Conditions and Hazard Functions

To use Lotka-Volterra density dependence, we need to recalculate the equilibrium and then rebuild the hazard functions. The per-capital larval mortality is μ_L + γL, noted by log = FALSE in the options to equilibrium_lifeycle().

# using the same parameters as above, along with the SPN_P object already created
# calculate equilibrium for lotka-volterra dynamics
initialCons <- equilibrium_lifeycle(params = theta, NF = NF, phi = 0.5,
                                    log = FALSE, spn_P = SPN_P,cube=cube)

We now follow the same steps as before to make the vector of hazards (setting log = FALSE).

# approximate hazards for continous approximation
approx_hazards <- spn_hazards(spn_P = SPN_P, spn_T = SPN_T, cube = cube,
                              params = initialCons$params, type = "life",
                              exact = FALSE, tol = 1e-8, verbose = FALSE,
                              log = FALSE)

# exact hazards for integer-valued state space
exact_hazards <- spn_hazards(spn_P = SPN_P, spn_T = SPN_T, cube = cube,
                             params = initialCons$params, type = "life",
                             exact = TRUE, tol = NaN, verbose = FALSE,
                             log = FALSE)

Simulation of Fully Specified SPN Model

We will use the releases defined above, and again begin with an ODE solution to our system, before testing a different stochastic sampler.

Deterministic: ODE Solutions

As before, we run an ODE simulation, but this time we will plot the aquatic stages in two ways: looking at their genotypes over time, and looking at the progress through Erlang-distributed stages.

# deterministic simulation
ODE_out <- sim_trajectory_R(x0 = initialCons$M0, t0 = 0, tt = tmax, dt = dt,
                            S = S, hazards = approx_hazards, sampler = "ode",
                            events = events, verbose = FALSE)

# summarize aquatic stages by genotype
ODE_out_e <- summarize_eggs_geno(out = ODE_out$state, spn_P = SPN_P)
ODE_out_l <- summarize_larvae_geno(out = ODE_out$state, spn_P = SPN_P)
ODE_out_p <- summarize_pupae_geno(out = ODE_out$state, spn_P = SPN_P)

# add stage name
ODE_out_e$stage <- "Egg"
ODE_out_l$stage <- "Larvae"
ODE_out_p$stage <- "Pupae"

# plot by genotype
ggplot(data = rbind(ODE_out_e, ODE_out_l,ODE_out_p)) +
  geom_line(aes(x = time, y = value, color = genotype)) +
  facet_wrap(facets = vars(stage), scales = "free_y") +
  theme_bw() +
  ggtitle("SPN: ODE Solution - Genotypes")

Notice the vastly different equilibrium number for the aquatic stages. These values are parameterized from the adult female value, and the death rates at each stage. Notice that the “releases” here appear to be Aa individuals - this is because our released females (aa) mated with wild-type males (AA), and produced heterozygous (Aa) offspring. Also notice how the releases are smoothed out from Egg to Larvae and Larvae to Pupae stages, due to the death in each stage. We can also see a small dip in the pupal stage, stemming from the over-compensation of density-dependent mortality in the larval stage, before returning to equilibrium.

We can also explore the aquatic stages by dwell time instead of genotype.

# summarize aquatic stages by Erlang stage
ODE_out_e <- summarize_eggs_stage(out = ODE_out$state, spn_P = SPN_P)
ODE_out_l <- summarize_larvae_stage(out = ODE_out$state, spn_P = SPN_P)
ODE_out_p <- summarize_pupae_stage(out = ODE_out$state, spn_P = SPN_P)

# add stage name
ODE_out_e$stage <- "Egg"
ODE_out_l$stage <- "Larvae"
ODE_out_p$stage <- "Pupae"

# plot by Erlang stage
ggplot(data = rbind(ODE_out_e, ODE_out_l,ODE_out_p)) +
  geom_line(aes(x = time, y = value, color = `Erlang-stage`)) +
  facet_wrap(facets = vars(stage), scales = "free_y") +
  theme_bw() +
  ggtitle("SPN: ODE Solution - Erlang Dwell Stage")

This plot shows how individuals are moving through each aquatic stage. There are significantly more individuals in the first Erlang-stage, compared to the second Erlang-stage, and we clearly see how the large increase in eggs is attenuated by death through the larval stage, and the effect of density-dependence actually lowering the amount of larvae that make it to the pupal stage.

Finally, for comparison with the logistic density-dependent dynamics above, and a different stochastic sampler below, we plot the adult stages.

# summarize females/males
ODE_out_f <- summarize_females(out = ODE_out$state, spn_P = SPN_P)
ODE_out_m <- summarize_males(out = ODE_out$state)

# add sex for plotting
ODE_out_f$sex <- "Female"
ODE_out_m$sex <- "Male"

# plot adults
ggplot(data = rbind(ODE_out_f, ODE_out_m)) +
  geom_line(aes(x = time, y = value, color = genotype)) +
  facet_wrap(facets = vars(sex), scales = "fixed") +
  theme_bw() +
  ggtitle("SPN: ODE Solution - Adult Stages")

Notice the similarity with the logistic density-dependent solution. Since there are no fitness effects or complex dynamics happening here, the solutions are effectively the same.

# chemical langevin sampler
CLE_out <- sim_trajectory_R(x0 = initialCons$M0, t0 = 0, tt = tmax, dt = dt,
                            dt_stoch = dt_stoch, S = S, hazards = approx_hazards,
                            sampler = "cle", events = events, verbose = FALSE)

# summarize females/males
CLE_out_f <- summarize_females(out = CLE_out$state, spn_P = SPN_P)
CLE_out_m <- summarize_males(out = CLE_out$state)

# add sex for plotting
CLE_out_f$sex <- "Female"
CLE_out_m$sex <- "Male"

# plot adults
ggplot(data = rbind(CLE_out_f, CLE_out_m)) +
  geom_line(aes(x = time, y = value, color = genotype)) +
  facet_wrap(facets = vars(sex), scales = "fixed") +
  theme_bw() +
  ggtitle("SPN: Chemical Langevin Approximation")