# Chapter 1 Introduction to R and lefko3

“Computers make excellent and efficient servants, but I have no wish to serve under them.”

— Spock, Star Trek, “The Ultimate Computer

R package lefko3 is devoted to the analysis of demographic data through matrix projection models (MPMs) . It serves as a full working environment for the construction and analysis of all kinds and styles of MPMs, including Lefkovitch (size-classified) MPMs, Leslie (age-based) MPMs, age × stage MPMs, and discretized integral projection models (IPMs). It can create and analyze both raw (empirical) and function-based forms of these models. It was originally developed to estimate and analyze historical size-classified matrix projection models (hMPMs), which are matrices designed to include the state of an individual in three consecutive times, in contrast to the two consecutive times that characterize most MPMs and IPMs. Such matrices are large, typically having dimensions several orders of magnitude higher than their standard, ahistorical counterparts (the latter will be hereafter referred to as ahistorical MPMs, or ahMPMs, while the acronym MPM will be used to refer to all matrix projection models, whether historical or not). As this package has developed, we have prioritized the development of core algorithms and methods to construct these models and the full suite of possible MPM types quickly, efficiently, and at least relatively painlessly. The result is a package that builds and analyzes MPMs of all types and all sizes quickly and efficiently, with enough flexibility that just about anyone interested in developing MPMs will find it useful.

This package introduces a complete suite of functions covering the MPM workflow, from dataset management to the construction of MPMs to their analysis. Dataset management functions standardize demographic datasets from the dominant formats demographers use into a format that facilitates MPM estimation while accounting for individual identity and other parameters. Demographic vital rate models may be estimated using demographic datasets with this standardized format, and these models take the form of generalized linear or mixed linear models using a variety of response distributions. Matrix estimation functions produce all necessary matrices from a single dataset, including all times, patches, and populations in a single shot, and do so quickly through core binaries engineered for speed and accuracy.

This manual assumes that the user has a very basic knowledge of R. We do not assume that users utilize or are even aware of any other packages, instead focusing on commands within lefko3 and base R (there are a few small exceptions, and these exceptions will include enough details to guide users without familiarity with those packages). We will begin our introduction with an overview of some required knowledge about R, for those who may be lacking even the very basic knowledge.

## 1.1 An Intro to R and RStudio

R is an object-oriented, open access programming language based on the S+ statistical programming language. It is available for free online (www.r-project.org), and operates at the command line. RStudio (www.rstudio.com) is a free development environment for R. It makes using R simpler by offering an organized space to see, write, and save code, view code outputs, track what is currently held in memory, and generally organize analyses. Use of RStudio is not required to use lefko3, but we encourage it, as it simplifies most analyses.

RStudio also allows the use of R Notebooks (file type ending with the .Rmd extension). This file type provides a means of mixing R code with text and output, as well as html and code for some other programming languages. One of the key initial advantages of using R Notebooks is that R automatically treats the directory in which the current R Notebook file is located in as the default directory for any file operations within the R Notebook. This is an advantage because R’s default directory for any code outside of an R Notebook is the R directory itself, and so using R Notebooks allows the user to skip resetting the directory every time they start programming.

To use R Notebooks, first make sure that you have downloaded and installed R. Then, download, install, and open R Studio. RStudio will open R command-line within the lower-left panel. The top-left panel will be a place for you to open, read, and write code. The top-right panel shows what is in memory (the Environment panel), as well as a history of commands entered (the History panel), and perhaps a few other odds and ends. The lower-right panel shows files in the current directory (Files panel), plots (Plots panel), installed packages (Packages panel), and help documents (Help panel), among potentially other things.

When you’ve installed R and R Studio, R Studio is set up to handle R analyses and script. However, it cannot yet properly handle R Notebooks. To write and read R Notebooks, we just need to perform one more step - to install the markdown and rmarkdown packages. To do so, make sure that you are connected to the internet, and enter the following commands at the R prompt:

install.packages("markdown", dependencies = TRUE)
install.packages("rmarkdown", dependencies = TRUE)

Now you’re ready to go! To start a new R Notebook, click R Notebook under the New File option in R Studio’s File menu. Make sure to save it in an appropriate place on your computer.

## 1.2 Basic mathematics, statistics, and programming operations in R

R allows us to do basic mathematical tasks, such as setting variables and doing basic arithmetic. Here we see such an example. First we ask R what 5+4 equals.

5+4
> [1] 9

Next, we create a variable x that we set to be equal to 5+4, and print the value of x to the screen.

x <- 5+4
x
> [1] 9

In the output above, R first shows us the answer to the problem 5+4. It denotes this answer after a [1] because R defaults to treating problems as though they were problems with vectors. Thus, a one-element vector with the element 5 is added to a one-element vector with a value of 4, and yields a one-element vector with a value of 9. In the next set of code, we create a variable named x and assign it the value of 5+4 using R’s main piping operator, <-. When we type x and press return, we see a one-element vector with the value of 9 in that element. If we had worked with a long vector, then each wrapped line of values would begin with a bracketed number corresponding to the element number within the vector of the first element in that row, making it a bit easier to see the structure of that vector.

The above example illustrates the use of objects. R is an object-oriented programming language, meaning that using R generally requires creating objects that act to hold data and other values, or to perform different functions. Objects always need names, with a few rules governing what names are possible: 1) object names are case-sensitive, meaning that objectx and object X are different objects; 2) object names may not begin with numbers, as well as a few special characters (such as *); and 3) objects should be named uniquely, to prevent existing functions or other objects being overwritten in memory. We can see what objects are in memory in the Environment panel in the top-right portion of the RStudio screen, where we see the names of the objects and some basic information about these objects.

Use of lefko3 also requires familiarity with certain key data classes. The most important of these are the vector, the matrix, the list, and the data frame. We can begin with the vector, which is probably the most important class and is defined using the c() function. A function is a special operation in a programming language, and in R, the function takes input provided within parentheses, as below. We will use the c() function to create two vectors, the first a numeric vector, and the second a string vector, with the elements of each vector given as input within parentheses.

simple_vector <- c(1, 2, 3, 4, 5, 6, 7, 8, 9)
text_vector <- c("1", "2", "3", "4", "5", "6", "7", "8", "9")

simple_vector
> [1] 1 2 3 4 5 6 7 8 9
text_vector
> [1] "1" "2" "3" "4" "5" "6" "7" "8" "9"

Vectors are atomic, meaning that all elements of a vector must be of the same type. The first is a vector of numeric class, meaning that all elements are assumed to be floating-point decimals. The second is of class character, meaning that they are assumed to be pure text. We can see this by checking the class of each vector, as below.

class(simple_vector)
> [1] "numeric"
class(text_vector)
> [1] "character"

Vectors are core building blocks for other structures in R. For example, we can use them to build matrices. Below, we use the matrix() function to create a simple numerical matrix. Note that this function takes specific arguments as input, and the names of these arguments come before each equal sign within parentheses. So, for example, the argument nrow refers to the number of rows in the matrix, and our statement nrow = 3 tells R to make a matrix with three rows.

simple_matrix <- matrix(c(1, 2, 3, 4, 5, 6, 7, 8, 9), nrow = 3, ncol = 3)
simple_matrix
>      [,1] [,2] [,3]
> [1,]    1    4    7
> [2,]    2    5    8
> [3,]    3    6    9

This matrix was built by taking a vector and filling the matrix by column using the matrix() function. Filling the matrix by column is standard practice in computing, and reflects how other key programming languages treat vectors. However, users can also use the byrow = TRUE option to fill the matrix by row preferentially, as they might in Matlab. We can also use a previously defined vector to fill the matrix, and all of these procedures work provided that the vector is of length equal to the number of elements in the matrix to be filled.

simple_matrix_byrow <- matrix(simple_vector, nrow = 3, ncol = 3, byrow = TRUE)
simple_matrix_byrow
>      [,1] [,2] [,3]
> [1,]    1    2    3
> [2,]    4    5    6
> [3,]    7    8    9

Matrices are also atomic, and so we can also build a text matrix, as below. If an attempt is made to combine numeric values and text values in a vector or matrix, then the resulting vector or matrix will treat all elements as text by default.

text_matrix <- matrix(text_vector, nrow = 3, ncol = 3)
text_matrix
>      [,1] [,2] [,3]
> [1,] "1"  "4"  "7"
> [2,] "2"  "5"  "8"
> [3,] "3"  "6"  "9"

Importantly, although the elements of these matrices are of either type numeric or character, the class() function does not allow us to differentiate here, instead telling us only that the object is a matrix and an array (an array is simply a multi-dimensional object that can be propagated with a vector, and a matrix is a two-dimensional array). To differentiate, we can use the class() function on the elements of the matrix.

class(simple_matrix)
> [1] "matrix" "array"
class(text_matrix)
> [1] "matrix" "array"
class(simple_matrix[1,1])
> [1] "numeric"
class(text_matrix[1,1])
> [1] "character"

Elements of matrices are denoted in square brackets, with the row on the left and the column on the right of the comma. However, because matrices are filled with vectors, they can also be accessed via a single number, in which case they correspond to the place of the element in the corresponding vector used to fill the matrix (note that this number always refers to the element number in the associated column vector, even if the matrix was filled by row). Thus, we can access the eighth element in simple_matrix in two ways.

simple_matrix[2,3]
> [1] 8
simple_matrix[8]
> [1] 8

R’s use of the vector as the default handling method for mathematical analysis means that even arithmetic operations handled by R are really done as problems in linear algebra. Thus, note that adding the scalar 3 to a numeric vector ends up adding 3 to each element in the vector, and adding 3 to a matrix adds 3 to each element in the matrix, as below.

simple_vector + 3
> [1]  4  5  6  7  8  9 10 11 12
simple_matrix + 3
>      [,1] [,2] [,3]
> [1,]    4    7   10
> [2,]    5    8   11
> [3,]    6    9   12

Preventing vectors from including multiple types of entries allows R to allocate memory efficiently. It also makes R’s vectors consistent with vector definitions in the major programming languages, making R vectors computationally passable to other languages. However, sometimes we wish to build a vector composed of multiple types of objects. In these cases, we can build an object of class list. Lists are powerful and flexible objects, where each element can be any kind of element. In fact, not only can these elements be numeric, character, integer (i.e. whole numbers), or logical (i.e. true of false values only), but they can also be entire vectors, matrices, or even other lists. Here, we will create a new list including some of the objects that we have created so far.

first_list <- list(my_fave_vector= simple_vector, my_fave_matrix= simple_matrix,
my_least_fave_matrix = text_matrix)
first_list
> $my_fave_vector > [1] 1 2 3 4 5 6 7 8 9 > >$my_fave_matrix
>      [,1] [,2] [,3]
> [1,]    1    4    7
> [2,]    2    5    8
> [3,]    3    6    9
>
> $my_least_fave_matrix > [,1] [,2] [,3] > [1,] "1" "4" "7" > [2,] "2" "5" "8" > [3,] "3" "6" "9" Here we see that our list has three objects, each of a different class. Each object has a name, and can be accessed using the $ operator or via the double square bracket, as below.

first_list$my_fave_matrix > [,1] [,2] [,3] > [1,] 1 4 7 > [2,] 2 5 8 > [3,] 3 6 9 first_list[[2]] > [,1] [,2] [,3] > [1,] 1 4 7 > [2,] 2 5 8 > [3,] 3 6 9 Finally, we come to the data frame. A data frame is essentially a dataset that meets R’s formatting requirements. Thus, columns are variables, and rows are data points. The variables that are part of a data frame are vectors of equal length, but do not need to be of the same class. Technically, a data frame is actually a list object in which each element of the list is a vector of the same length, and so the variables are accessible in the same way that list elements are. Here is an example using the cars data frame, which comes packaged with base R. Note that before we can look at the data frame, we need to load it into our working memory using the data() function. data(cars) cars > speed dist > 1 4 2 > 2 4 10 > 3 7 4 > 4 7 22 > 5 8 16 > 6 9 10 > 7 10 18 > 8 10 26 > 9 10 34 > 10 11 17 > 11 11 28 > 12 12 14 > 13 12 20 > 14 12 24 > 15 12 28 > 16 13 26 > 17 13 34 > 18 13 34 > 19 13 46 > 20 14 26 > 21 14 36 > 22 14 60 > 23 14 80 > 24 15 20 > 25 15 26 > 26 15 54 > 27 16 32 > 28 16 40 > 29 17 32 > 30 17 40 > 31 17 50 > 32 18 42 > 33 18 56 > 34 18 76 > 35 18 84 > 36 19 36 > 37 19 46 > 38 19 68 > 39 20 32 > 40 20 48 > 41 20 52 > 42 20 56 > 43 20 64 > 44 22 66 > 45 23 54 > 46 24 70 > 47 24 92 > 48 24 93 > 49 24 120 > 50 25 85 We can see that we have two variables in this data frame. Suppose we wished to access the sixth data point’s value for the speed variable. We can do so in the following way. cars$speed[6]
> [1] 9

Alternatively, we can see all of the values of the sixth data point by calling the row using single square brackets, and leaving the column blank, as below.

cars[6,]
>   speed dist
> 6     9   10

## 1.3 Control structures

As in other programming languages, R uses control structures to define the flow of a program. These structures are of invaluable use in developing code for analysis. These structures can be categorized as either selection structures, or repetition structures. The most commonly used selection structure is the if statement. This statement works by testing a condition, and then executing a set of instructions if the condition is true. Let’s see an example of this.

if (cars$speed[6] == 0) { writeLines("This car is not moving!") } else if (cars$speed[6] < 0) {
writeLines("This car is moving backward!")
} else {
writeLines("This car is moving forward!")
}
> This car is moving forward!

Here, R first sees if the 6th car’s speed in the cars dataset is zero. It tests this condition with the double equal sign, because the equal sign by itself is not used to test conditions but instead to assign values to variables and other objects. The else portion tells R what to do if the condition is not true. There are two elsestatements here, and the first tests another condition. The final else statement tells R what to do if none of the conditions tested is correct.

The most common repetition statement is the for loop. Here we see an example of such a loop.

total_dist <- 0

for (i in c(1:length(cars$speed))) { total_dist <- total_dist + cars$dist[i]
}

total_dist
> [1] 2149

In the chunk of code above, we first define a variable total_dist, and give it a starting value of 0. We then create a for loop that initializes an integer variable i. The initial value of i is 1, and the for loop will then add each value of dist (the stopping distance in the cars dataset) to total_dist. The loop runs with i incrementing to each successive value in the c() statement (c(1:length(cars$dist)) creates a vector of integers from 1 to the length of the speed variable in the cars dataset). When i increments to the last value, the loop ends. Other structures also exist, including the while statement, which runs a loop as long as a condition is met. ## 1.4 Data input and handling R deals primarily with statistical analysis and modeling. And, of course, statistical methods ultimately use data. There are numerous ways to get data into R, including importing spreadsheets or databases in various formats through a variety of functions. We can also create our own data frame, as below. Here, we create a series of atomic vectors of the same length using a version of the cbind() function which binds columns into a data frame (the cbind() function binds vectors of the same class together as columns within a matrix, while cbind.data.frame() binds vectors of any class together as variables within a data frame). Then, we use as.factor() to set one of these variables, gender, to be a categorical variable rather than a numeric variable. gender <- c(1,1,1,2,2,2) height <- c(1.5,1.4,1.6,1.5,1.6,1.5) our.data <- cbind.data.frame(gender, height) our.data$gender <- as.factor(our.data$gender) our.data > gender height > 1 1 1.5 > 2 1 1.4 > 3 1 1.6 > 4 2 1.5 > 5 2 1.6 > 6 2 1.5 Now let’s suppose that we would like to understand more about our data, for example the various measures of central tendency and variability. We can use the summary() function to get at some of this, most importantly at the mean, median, and the quartiles, as below. Note that because gender is a categorical variable, we only see counts of the categories rather than any measures of central tendency and variability. summary(our.data) > gender height > 1:3 Min. :1.400 > 2:3 1st Qu.:1.500 > Median :1.500 > Mean :1.517 > 3rd Qu.:1.575 > Max. :1.600 However, we would most likely want more than this. So, here are a few more useful functions. Note the use of the seq() function, which creates a sequence of values from the first number input to the second number input at increments of the third number input. This function is used to tell R the specific quantiles that we would like displayed. mean(our.data$height)
> [1] 1.516667
median(our.data$height) > [1] 1.5 sd(our.data$height)
> [1] 0.07527727
var(our.data$height) > [1] 0.005666667 range(our.data$height)
> [1] 1.4 1.6
quantile(our.data$height, probs = seq(0, 1, 0.25)) > 0% 25% 50% 75% 100% > 1.400 1.500 1.500 1.575 1.600 We might also wish to plot these data somehow. One of the most powerful functions to use to produce plots is plot(), which defaults to different styles of plot given different classes of input variables. Let’s use this function to explore the relationship between distance traveled as a function of speed in the cars dataset (figure 1.1). plot(dist ~ speed, cars) In the above plot, we utilized R’s standard linear notation to tell it the relationship between variables. Thus, dist ~ speed can be interpreted statistically as dist is a function of speed. Note also the reference to the cars dataset in the call - without this call, we would need to write cars$dist ~ cars$speed. We can alter this plot in many ways, including the kinds of points, the labels, the axes, etc. Here is an example (figure 1.2). plot(dist ~ speed, cars, pch = 2, col = "red", xlab = "Speed of car", ylab = "Distance until stop", xlim = c(0,40), ylim = c(0, 150)) We can also try using the hist() function to create a histogram, for example to look at height by gender in our own dataset (figure 1.3). hist(our.data$height)

The hist() function can also be modified in a variety of important ways, using similar inputs to plot().

Let’s now see what the default plot style is for our gender-height data (figure 1.4).

plot(height ~ gender, our.data)

We can see that the default here is a box-and-whisker plot, which is standard when we incorporate an x term that is categorical and a y term that is quantitative and continuous.

Finally, now that we’ve explored a simple method of getting data into R, what about importing spreadsheets of demographic data? Although other methods certainly exist, we are particularly partial to the read.csv() function approach. This function allows us to import our data as a comma-separated value (CSV) text file. To utilize this approach, first open your preferred spreadsheet program and make sure that the data file meets the following conformational characteristics:

1. The variables in your dataset are arranged by column, and the data points themselves are arranged by row.
2. The top row in your spreadsheet contains ONLY the names of your variables, and these variable names conform to R object naming rules (i.e. each variable name unique, each variable name starts with a letter or underscore only, each variable name includes no punctuation).
3. The data sheet itself does not contain ANY commas anywhere.
4. All columns intended to be numeric variables contain ONLY numeric entries, or blanks (even a single text entry in such a variable will cause the variable to be imported as a string variable).
5. The dataset must start with the very topleft cell in the spreadsheet. Starting everything lower than the first row will cause improper importing and formatting issues.
6. The dataset does not include any characters other than pure ASCII characters. While R can handle UTF-8 and some other kinds of text encoding, some methods produce unexpected behavior when handling values or objects named with non-ASCII encoding.

Once your dataset conforms to these parameters in the spreadsheet program, export your file to CSV format (we generally encourage MS-DOS CSV format, if such an option is provided, as it is the simplest version). Then, in R, use the following command to import your dataset, replacing the myfilenamehere portion with the real file name:

mydata <- read.csv("myfilenamehere.csv", header = TRUE)

Once done, you can try using the summary() function to see what everything looks like and whether the variables were imported properly.

Let’s now move on to using lefko3 itself.

## 1.5 Using lefko3

Users of lefko3 will first need to install the package itself. The following line of code will do that. The user should agree if prompted to install a number of other packages stated as dependencies. The specific dependencies currently include BH, glmmTMB, lme4, MuMIn, pscl, Rcpp, RcppArmadillo, SparseM, stringr, and VGAM, and they should be installed similarly if not prompted and if not installed. Note that lefko3 is also meant to work with the newest version of R. It has been tested with versions of R as far back as 3.5.0 and so should work with those, but it is possible that dependency upgrades will affect certain functions (most importantly modelsearch(), which is dependent on virtually all of the packages noted above).

install.packages("lefko3", dependencies = TRUE)

Alternatively, development versions of lefko3 are available on R Forge, where updates are more frequent than on CRAN. If the user has R set up to compile C++ and Fortran (this is not the default situation on Mac and Windows machines, so only advanced users are encouraged to do so), then the following line can also be used to install the latest development version of lefko3. Note that the CRAN version is stable and requires no compilation, and so DO NOT INSTALL THE R FORGE VERSION IF YOU ARE UNSURE OF WHAT COMPILING IS OR WHAT R NEEDS TO DO DURING INSTALLATION!

install.packages("lefko3", repos="http://R-Forge.R-project.org")

After that, use of lefko3 requires loading it into the working environment, which can be done with the following line.

library(lefko3)

Package lefko3 includes functions to handle the entire workflow of MPM construction and analysis. The functions themselves can be divided into at least 6 major categories, as below:

1. Data transformation and handling functions

1. add_lM(): Adds new matrices to existing MPMs.
2. create_lM(): Creates MPMs from matrices provided by the user.
3. delete_lM(): Deletes matrices from MPMs.
4. historicalize3(): Formats demographic datasets in vertical but not historical format.
5. subset_lM(): Creates MPMs from matrices in other MPMs.
6. verticalize3(): Formats demographic datasets in horizontal format.
2. Functions setting or determining population characteristics for analysis

1. density_input(): Summarizes density dependence inputs for population projection.
2. density_vr(): Summarizes density dependence inputs in vital rates for function-based population projection.
3. hfv_qc(): Tests assumptions of various response distributions in key vital rate variables in demographic datasets.
4. overwrite(): A less powerful version of function supplemental().
5. sf_create(): Creates a data frame summarizing the life history model.
6. sf_distrib(): Tests for overdispersion and zero-inflation in count-based size and fecundity.
7. start_input(): Creates a start vector for population projection.
8. summary_hfv(): Summarizes key information about properly formatted demographic datasets.
9. supplemental(): Summarizes extra inputs for MPM creation not found within the demographic dataset.
10. vrm_import(): Imports coefficients from linear vital rate models for use in developing discretized IPMs and function-based MPMs.
3. Vital rate model building and selection

1. create_pm(): Creates a skeleton data frame indexing variables used in vital rate modeling.
2. modelsearch(): Creates best-fit vital rate models for function-based MPMs, and to test the influences of individual history and other factors.
4. Matrix / integral projection model creation functions

1. aflefko2(): Constructs function-based ahistorical age-by-stage MPMs.
2. arlefko2(): Constructs raw ahistorical age-by-stage MPMs.
3. flefko2(): Constructs ahistorical function-based MPMs.
4. rlefko2(): Constrcuts ahistorical raw MPMs.
5. flefko3(): Constructs historical function-based MPMs.
6. rlefko3(): Constructs historical raw MPMs.
7. fleslie(): Constructs function-based Leslie MPMs.
8. rleslie(): Constructs raw Leslie MPMs.
9. hist_null(): Constructs historically-formatted MPM assuming no individual history.
10. lmean(): Develops element-wise arithmetic mean MPMs.
5. Population dynamics analysis functions

1. actualstage3(): Calculates actual population stage frequencies and proportions.
2. elasticity3(): Estimates the deterministic or stochastic elasticities of population growth rate to matrix elements.
3. f_projection3(): Core population projection function for time-by-time projection of function-based MPMs and IPMs from vital rate models.
4. lambda3(): Estimates deterministic population growth rate.
5. ltre3(): Conducts deterministic and stochastic life table response experiments.
6. projection3(): Core population projection function for already constructed MPMs and IPMs.
7. repvalue3(): Estimates the deterministic stage reproductive values or stochastic long-run reproductive values.
8. sensitivity3(): Estimates the deterministic or stochastic sensitivities of population growth rate to matrix elements.
9. slambda3(): Estimates stochastic population growth rate.
10. stablestage3(): Estimates the deterministic stable stage equilibrium or stochastic long-run stage distribution.
6. Functions describing, summarizing, comparing, or visualizing MPMs and derived structures.

1. cond_hmpm(): Extracts conditional ahistorical matrices from historical MPMs.
2. cond_diff(): Develops conditional ahistorical difference matrices, given two sets of historical or historically-formatted MPMs as input.
3. diff_lM(): Creates difference matrices between two MPMs wioth matrices of equal dimension.
4. image3(): Creates images of MPMs.
5. summary(): Summarizes a variety of different objects, including MPMs and analyses.
7. Other useful functions.

1. beverton3(): The two-parameter Beverton-Holt function for scalars.
2. logistic3(): The two-parameter logistic function for scalars.
3. ricker3(): The two-parameter Ricker function for scalars.
4. usher3(): The two-parameter Usher function for scalars.
5. plot.lefkoProj(): Displays plots of population projections resulting from functions projection3() and f_projection3().
6. summary(): Various versions of the summary() function have been created to provide useful summaries of output from many lefko3 functions.

In addition, lefko3 includes three datasets that can be used for educational purposes. These include cypdata, cypvert, and lathyrus.

### 1.5.1 The lefkoMat object: organized MPMs and IPMs

Package lefko3 produces a number of different classes of S3 objects, which can be thought of as standardized lists used as input and output for its functions. The most fundamental of these is the lefkoMat object, which holds the MPM and associated metadata. This list object is made of the following elements.

1. A: A list of full population projection matrices, in order of population, patch, and year. Each matrix is of R class matrix.
2. U: A list of matrices showing only survival-transition elements, in the same order as A.
3. F: A list of matrices showing only fecundity elements, in the same order as A.
4. hstages: A data frame showing the order of paired stages (given if matrices are historical, otherwise NA).
5. agestages: A data frame showing the order of age-stages (if an age-by-stage MPM has been created, otherwise NA).
6. ahstages: A data frame detailing the characteristics of the life history model used for construction of the MPM.
7. labels: A data frame showing the order of matrices, according to population, patch, and year.
8. matrixqc: A vector used in summary statements to describe the overall quality of each matrix.
9. dataqc: A vector used in summary statements to describe key sampling aspects of the dataset (in raw MPMs).
10. modelqc: A vector used in summary statements to describe the vital rate models (in function-based MPMs).

Objects within these lists may be called as in figure 1.5, below.

We will detail the creation and meaning of these objects in later chapters, but for now let us construct a simple MPM as a lefkoMat object. For this, we will input the first six matrices of the MPM analyzed in Davison et al. (2010). This example will focus on just two populations of Anthyllis vulneraria, an herbaceous plant studied in south-western Belgium. First we will create the stageframe, which summarizes the life history model of the plant (stageframes and life history models will be described in Chapter 2).

sizevector <- c(1, 1, 2, 3) # These sizes are not from the original paper
stagevector <- c("Sdl", "Veg", "SmFlo", "LFlo")
repvector <- c(0, 0, 1, 1)
obsvector <- c(1, 1, 1, 1)
matvector <- c(0, 1, 1, 1)
immvector <- c(1, 0, 0, 0)
propvector <- c(0, 0, 0, 0)
indataset <- c(1, 1, 1, 1)
binvec <- c(0.5, 0.5, 0.5, 0.5)
comments <- c("Seedling", "Vegetative adult", "Small flowering",
"Large flowering")

anthframe <- sf_create(sizes = sizevector, stagenames = stagevector,
repstatus = repvector, obsstatus = obsvector, matstatus = matvector,
immstatus = immvector, indataset = indataset, binhalfwidth = binvec,

class(anthframe)
> [1] "data.frame" "stageframe"
anthframe
>   stage size size_b size_c min_age max_age repstatus obsstatus propstatus
> 1   Sdl    1     NA     NA      NA      NA         0         1          0
> 2   Veg    1     NA     NA      NA      NA         0         1          0
> 3 SmFlo    2     NA     NA      NA      NA         1         1          0
> 4  LFlo    3     NA     NA      NA      NA         1         1          0
>   immstatus matstatus indataset binhalfwidth_raw sizebin_min sizebin_max
> 1         1         0         1              0.5         0.5         1.5
> 2         0         1         1              0.5         0.5         1.5
> 3         0         1         1              0.5         1.5         2.5
> 4         0         1         1              0.5         2.5         3.5
>   sizebin_center sizebin_width binhalfwidthb_raw sizebinb_min sizebinb_max
> 1              1             1                NA           NA           NA
> 2              1             1                NA           NA           NA
> 3              2             1                NA           NA           NA
> 4              3             1                NA           NA           NA
>   sizebinb_center sizebinb_width binhalfwidthc_raw sizebinc_min sizebinc_max
> 1              NA             NA                NA           NA           NA
> 2              NA             NA                NA           NA           NA
> 3              NA             NA                NA           NA           NA
> 4              NA             NA                NA           NA           NA
>   sizebinc_center sizebinc_width group         comments
> 1              NA             NA     0         Seedling
> 2              NA             NA     0 Vegetative adult
> 3              NA             NA     0  Small flowering
> 4              NA             NA     0  Large flowering

This object, called anthframe, is a data frame in R, and checking its class reveals that it is also a member of the class stageframe, which is a kind of data frame unique to lefko3 having the particular structure seen here. This object was created by developing a number of vectors describing the life history stages that this plant lives through, and using these as inputs into the stageframe creation function, sf_create().

Now let’s input three matrices each for two populations, both of which are part of a metapopulation. The three matrices show transitions across pairs of consecutive years, 2003-2004, 2004-2005, and 2005-2006. We input these matrices by row because the code supplied in Davison et al. (2010) was written in Matlab, which defaults to input by row.

# POPN C 2003-2004
XC3 <- matrix(c(0, 0, 1.74, 1.74,
0.208333333, 0, 0, 0.057142857,
0.041666667, 0.076923077, 0, 0,
0.083333333, 0.076923077, 0.066666667, 0.028571429), 4, 4, byrow = TRUE)

# POPN C 2004-2005
XC4 <- matrix(c(0, 0, 0.3, 0.6,
0.32183908, 0.142857143, 0, 0,
0.16091954, 0.285714286, 0, 0,
0.252873563, 0.285714286, 0.5, 0.6), 4, 4, byrow = TRUE)

# POPN C 2005-2006
XC5 <- matrix(c(0, 0, 0.50625, 0.675,
0, 0, 0, 0.035714286,
0.1, 0.068965517, 0.0625, 0.107142857,
0.3, 0.137931034, 0, 0.071428571), 4, 4, byrow = TRUE)

# POPN E 2003-2004
XE3 <- matrix(c(0, 0, 2.44, 6.569230769,
0.196428571, 0, 0, 0,
0.125, 0.5, 0, 0,
0.160714286, 0.5, 0.133333333, 0.076923077), 4, 4, byrow = TRUE)

# POPN E 2004-2005
XE4 <- matrix(c(0, 0, 0.45, 0.646153846,
0.06557377, 0.090909091, 0.125, 0,
0.032786885, 0, 0.125, 0.076923077,
0.049180328, 0, 0.125, 0.230769231), 4, 4, byrow = TRUE)

# POPN E 2005-2006
XE5 <- matrix(c(0, 0, 2.85, 3.99,
0.083333333, 0, 0, 0,
0, 0, 0, 0,
0.416666667, 0.1, 0, 0.1), 4, 4, byrow = TRUE)

Let’s take a quick look at one of these matrices.

XC3
>            [,1]       [,2]       [,3]       [,4]
> [1,] 0.00000000 0.00000000 1.74000000 1.74000000
> [2,] 0.20833333 0.00000000 0.00000000 0.05714286
> [3,] 0.04166667 0.07692308 0.00000000 0.00000000
> [4,] 0.08333333 0.07692308 0.06666667 0.02857143

Note that the structure is very conventional - the fecundity rates are up in the top-right corner, while the rest of the elements showing transition probabilities and so are bound between 0 and 1. All MPM matrices should contain only non-negative values, so we recommend inspecting your matrices when you construct them.

Once done, we will make a list of our matrices, and then use the create_lM() function to create our lefkoMat object. In addition to the list of matrices, we will supply vectors showing the order of patches in the list of matrices, and the order of year at time t in the list of matrices (ahistorical matrices show transition rates from time t to time t+1, so the matrix covering 2004-2005 is referred to by year 2004). Because lefko3 includes special methods to build and analyze historical MPMs, we also need to tell R that these matrices are not historical (historical = FALSE).

mats_list <- list(XC3, XC4, XC5, XE3, XE4, XE5)

anth_lefkoMat <- create_lM(mats_list, anthframe, historical = FALSE,
patchorder = c(1, 1, 1, 2, 2, 2),
yearorder = c(2003, 2004, 2005, 2003, 2004, 2005))

anth_lefkoMat
> $A >$A[[1]]
>            [,1]       [,2]       [,3]       [,4]
> [1,] 0.00000000 0.00000000 1.74000000 1.74000000
> [2,] 0.20833333 0.00000000 0.00000000 0.05714286
> [3,] 0.04166667 0.07692308 0.00000000 0.00000000
> [4,] 0.08333333 0.07692308 0.06666667 0.02857143
>
> $A[[2]] > [,1] [,2] [,3] [,4] > [1,] 0.0000000 0.0000000 0.3 0.6 > [2,] 0.3218391 0.1428571 0.0 0.0 > [3,] 0.1609195 0.2857143 0.0 0.0 > [4,] 0.2528736 0.2857143 0.5 0.6 > >$A[[3]]
>      [,1]       [,2]    [,3]       [,4]
> [1,]  0.0 0.00000000 0.50625 0.67500000
> [2,]  0.0 0.00000000 0.00000 0.03571429
> [3,]  0.1 0.06896552 0.06250 0.10714286
> [4,]  0.3 0.13793103 0.00000 0.07142857
>
> $A[[4]] > [,1] [,2] [,3] [,4] > [1,] 0.0000000 0.0 2.4400000 6.56923077 > [2,] 0.1964286 0.0 0.0000000 0.00000000 > [3,] 0.1250000 0.5 0.0000000 0.00000000 > [4,] 0.1607143 0.5 0.1333333 0.07692308 > >$A[[5]]
>            [,1]       [,2]  [,3]       [,4]
> [1,] 0.00000000 0.00000000 0.450 0.64615385
> [2,] 0.06557377 0.09090909 0.125 0.00000000
> [3,] 0.03278689 0.00000000 0.125 0.07692308
> [4,] 0.04918033 0.00000000 0.125 0.23076923
>
> $A[[6]] > [,1] [,2] [,3] [,4] > [1,] 0.00000000 0.0 2.85 3.99 > [2,] 0.08333333 0.0 0.00 0.00 > [3,] 0.00000000 0.0 0.00 0.00 > [4,] 0.41666667 0.1 0.00 0.10 > > >$U
> $U[[1]] > [,1] [,2] [,3] [,4] > [1,] 0.00000000 0.00000000 0.00000000 0.00000000 > [2,] 0.20833333 0.00000000 0.00000000 0.05714286 > [3,] 0.04166667 0.07692308 0.00000000 0.00000000 > [4,] 0.08333333 0.07692308 0.06666667 0.02857143 > >$U[[2]]
>           [,1]      [,2] [,3] [,4]
> [1,] 0.0000000 0.0000000  0.0  0.0
> [2,] 0.3218391 0.1428571  0.0  0.0
> [3,] 0.1609195 0.2857143  0.0  0.0
> [4,] 0.2528736 0.2857143  0.5  0.6
>
> $U[[3]] > [,1] [,2] [,3] [,4] > [1,] 0.0 0.00000000 0.0000 0.00000000 > [2,] 0.0 0.00000000 0.0000 0.03571429 > [3,] 0.1 0.06896552 0.0625 0.10714286 > [4,] 0.3 0.13793103 0.0000 0.07142857 > >$U[[4]]
>           [,1] [,2]      [,3]       [,4]
> [1,] 0.0000000  0.0 0.0000000 0.00000000
> [2,] 0.1964286  0.0 0.0000000 0.00000000
> [3,] 0.1250000  0.5 0.0000000 0.00000000
> [4,] 0.1607143  0.5 0.1333333 0.07692308
>
> $U[[5]] > [,1] [,2] [,3] [,4] > [1,] 0.00000000 0.00000000 0.000 0.00000000 > [2,] 0.06557377 0.09090909 0.125 0.00000000 > [3,] 0.03278689 0.00000000 0.125 0.07692308 > [4,] 0.04918033 0.00000000 0.125 0.23076923 > >$U[[6]]
>            [,1] [,2] [,3] [,4]
> [1,] 0.00000000  0.0    0  0.0
> [2,] 0.08333333  0.0    0  0.0
> [3,] 0.00000000  0.0    0  0.0
> [4,] 0.41666667  0.1    0  0.1
>
>
> $F >$F[[1]]
>      [,1] [,2] [,3] [,4]
> [1,]    0    0 1.74 1.74
> [2,]    0    0 0.00 0.00
> [3,]    0    0 0.00 0.00
> [4,]    0    0 0.00 0.00
>
> $F[[2]] > [,1] [,2] [,3] [,4] > [1,] 0 0 0.3 0.6 > [2,] 0 0 0.0 0.0 > [3,] 0 0 0.0 0.0 > [4,] 0 0 0.0 0.0 > >$F[[3]]
>      [,1] [,2]    [,3]  [,4]
> [1,]    0    0 0.50625 0.675
> [2,]    0    0 0.00000 0.000
> [3,]    0    0 0.00000 0.000
> [4,]    0    0 0.00000 0.000
>
> $F[[4]] > [,1] [,2] [,3] [,4] > [1,] 0 0 2.44 6.569231 > [2,] 0 0 0.00 0.000000 > [3,] 0 0 0.00 0.000000 > [4,] 0 0 0.00 0.000000 > >$F[[5]]
>      [,1] [,2] [,3]      [,4]
> [1,]    0    0 0.45 0.6461538
> [2,]    0    0 0.00 0.0000000
> [3,]    0    0 0.00 0.0000000
> [4,]    0    0 0.00 0.0000000
>
> $F[[6]] > [,1] [,2] [,3] [,4] > [1,] 0 0 2.85 3.99 > [2,] 0 0 0.00 0.00 > [3,] 0 0 0.00 0.00 > [4,] 0 0 0.00 0.00 > > >$hstages
> [1] NA
>
> $agestages > [1] NA > >$ahstages
>   stage_id stage_id stage original_size original_size_b original_size_c min_age
> 1        1        1   Sdl             1              NA              NA       0
> 2        2        2   Veg             1              NA              NA       0
> 3        3        3 SmFlo             2              NA              NA       0
> 4        4        4  LFlo             3              NA              NA       0
>   max_age repstatus obsstatus propstatus immstatus matstatus entrystage
> 1      NA         0         1          0         1         0          1
> 2      NA         0         1          0         0         1          0
> 3      NA         1         1          0         0         1          0
> 4      NA         1         1          0         0         1          0
>   indataset binhalfwidth_raw sizebin_min sizebin_max sizebin_center
> 1         1              0.5         0.5         1.5              1
> 2         1              0.5         0.5         1.5              1
> 3         1              0.5         1.5         2.5              2
> 4         1              0.5         2.5         3.5              3
>   sizebin_width binhalfwidthb_raw sizebinb_min sizebinb_max sizebinb_center
> 1             1                NA           NA           NA              NA
> 2             1                NA           NA           NA              NA
> 3             1                NA           NA           NA              NA
> 4             1                NA           NA           NA              NA
>   sizebinb_width binhalfwidthc_raw sizebinc_min sizebinc_max sizebinc_center
> 1             NA                NA           NA           NA              NA
> 2             NA                NA           NA           NA              NA
> 3             NA                NA           NA           NA              NA
> 4             NA                NA           NA           NA              NA
>   sizebinc_width group         comments alive almostborn
> 1             NA     0         Seedling     1          0
> 2             NA     0 Vegetative adult     1          0
> 3             NA     0  Small flowering     1          0
> 4             NA     0  Large flowering     1          0
>
> $labels > pop patch year2 > 1 1 1 2003 > 2 1 1 2004 > 3 1 1 2005 > 4 1 2 2003 > 5 1 2 2004 > 6 1 2 2005 > >$matrixqc
> [1] 44 12  6
>
> attr(,"class")
> [1] "lefkoMat"

Voilà! We are now free to use this MPM in our analyses. But before we do that we might wish to explore this structure a bit more. The first element of this lefkoMat object, named A, is a list of the actual projection matrices. The create_lM() function uses the stageframe input that we provided to ascertain which transition rates are survival probabilities and which correspond to fecundity rates. The element named U is a list including a survival-transition matrix for each A matrix, meaning that fecundity is removed. The F element is a list including a fecundity matrix for each A matrix, meaning that transition probabilities have been removed. The element ahstages is our stageframe, though slightly edited and reordered. Here, element hstages is just NA, but would include the order of historical stage pairs if this MPM was historical. Element agestages is also NA, but would show us the order of age-stage combinations if this were an age-by-stage MPM or a Leslie MPM. The final element, matrixqc includes some basic quality control data in vector format. All of this is summarized using the summary() function, as below.

summary(anth_lefkoMat)
>
> This ahistorical lefkoMat object contains 6 matrices.
>
> Each matrix is square with 4 rows and columns, and a total of 16 elements.
> A total of 44 survival transitions were estimated, with 7.333 per matrix.
> A total of 12 fecundity transitions were estimated, with 2 per matrix.
> This lefkoMat object covers 1 population, 2 patches, and 3 time steps.
>
> Survival probability sum check (each matrix represented by column in order):
>           [,1]  [,2]   [,3]   [,4]   [,5]  [,6]
> Min.    0.0667 0.500 0.0625 0.0769 0.0909 0.000
> 1st Qu. 0.0810 0.575 0.1708 0.1192 0.1334 0.075
> Median  0.1198 0.657 0.2106 0.3077 0.2276 0.100
> Mean    0.1599 0.637 0.2209 0.4231 0.2303 0.175
> 3rd Qu. 0.1987 0.720 0.2607 0.6116 0.3245 0.200
> Max.    0.3333 0.736 0.4000 1.0000 0.3750 0.500

Summaries such as this are quite useful. Here we can see how many matrices we have (corresponding to A matrices), how big each matrix is, how many elements were estimated on average per matrix and across the board (the numbers of estimated elements shown actually shows us the number of non-zero elements, and so it is possible that the number of estimated elements is actually larger if some elements were estimated as 0), and how many populations, patches / subpopulations, and time steps are covered. This is followed by a summary of the column sums of the corresponding U matrices - this is an important quality control check because all numbers within the U matrices should conform to $$0 \le a_{ij} \le 1$$, where $$a_{ij}$$ is the element at row $$i$$ and column $$j$$, and all column sums within these matrices should also conform to $$0 \le \sum_i a_{ij} \le 1$$, where $$\sum_i a_{ij}$$ is the sum of all elements in column $$j$$ and corresponds to the survival probability of stage $$j$$.

## 1.6 Datasets used in this book

This book will utilize three main datasets. The first dataset is a data frame holding individual-level monitoring data on a population of the North American orchid species Cypripedium candidum, also known as the white lady’s slipper. The second dataset is a data frame holding similar data for a population of the European perennial Lathyrus vernus. The final dataset is a lefkoMat object holding projection matrices from nine populations of from the European perennial Anthyllis vulneraria.

### 1.6.1Cypripedium candidum data

This dataset is available in two formats, as data frames called cypdata and cypvert. These datasets contain the exact same information and are in different formats only. They can be called with the following code.

data(cypdata)
data(cypvert)

The white lady’s slipper, Cypripedium candidum, is a North American perennial herb in the family Orchidaceae. It is long-lived and of conservation concern. This plant begins life by germinating from a dust seed, and then develops into a protocorm, which is a special subterranean life stage found in orchids and pyroloids. During this stage, the plant is non-photosynthetic and completely parasitic on its mycorrhizal fungi. It spends several years as a protocorm, and previous studies suggest that it typically spends 3 years before becoming a seedling. As a seedling, it may or may not produce aboveground sprouts, often remaining entirely subterranean and continuing its parasitic lifestyle. It may persist this way for many years before attaining adult size, at which point it may sprout with or without flowers, or may remain underground in a condition referred to as vegetative dormancy. The latter condition may continue for many years, with over a decade of continuous dormancy documented in the literature .

The population from which the dataset originates is located within a wet meadow in a state nature preserve located in northeastern Illinois, USA (Figure 1.6). The population was monitored annually from 2004 to 2009, with two monitoring sessions per year. Monitoring sessions took roughly 2 weeks each, and included complete censuses of the population divided into sections referred to as patches. Each monitoring session consisted of searches for previously recorded individuals, which were located according to coordinates relative to fixed stakes at the site, followed by a search for new individuals. Data recorded per individual included: the location, the number of non-flowering sprouts, the number of flowering sprouts, the number of flowers per flowering sprout, and the number of fruit pods per sprout (only in the second monitoring session per year, once fruiting had occurred). Location was used to infer individual identity. More information about this population and its characteristics is given in Shefferson et al. (2001) and Shefferson et al. (2017).

### 1.6.2Lathyrus vernus data

This dataset is available as a data frame called lathyrus, and can be called with the following code.

data(lathyrus)

Lathyrus vernus (family Fabaceae) is a long-lived forest herb, native to Europe and large parts of northern Asia. Individuals increase slowly in size and usually flower only after 10-15 years of vegetative growth. Flowering individuals have an average conditional lifespan of 44.3 years . L. vernus lacks organs for vegetative spread and individuals are well delimited . One or several erect shoots of up to 40 cm height emerge from a subterranean rhizome in March-April. Flowering occurs about four weeks after shoot emergence. Shoot growth is determinate, and the number of flowers is determined in the previous year . Individuals may not produce aboveground structures every year, but can remain dormant in one or more seasons. L. vernus is self-compatible but requires visits from bumble-bees to produce seeds. Individuals produce few, large seeds and establishment from seeds is relatively frequent . The pre-dispersal seed predator Bruchus atomarius often consumes a large fraction of developing seeds, and roe deer (Capreolus capreolus) sometimes consume the shoots .

Data for this study were collected from six permanent plots in a population of L. vernus located in a deciduous forest in the Tullgarn area, SE Sweden (58.9496 N, 17.6097 E), during 1988–1991 . The six plots were similar with regard to soil type, elevation, slope, and canopy cover. Within each plot, all individuals were marked with numbered tags that remained over the study period, and their locations were carefully mapped. New individuals were included in the study in each year. Individuals were recorded at least three times every growth season. At the time of shoot emergence, we recorded whether individuals were alive and produced above-ground shoots, and if shoots had been grazed. During flowering, we recorded flower number and the height and diameter of all shoots. At fruit maturation, we counted the number of intact and damaged seeds. To derive a measure of above-ground size for each individual, we calculated the volume of each shoot as $$\pi × (\frac{1}{2} diameter)^2 × height$$, and summed the volumes of all shoots. This measure is closely correlated with the dry mass of aboveground tissues ($$R^2 = 0.924$$, $$P < 0.001$$, $$n = 50$$, log-transformed values; Ehrlén 1995). Size of individuals that had been grazed was estimated based on measures of shoot diameter in grazed shoots, and the relationship between shoot diameter and shoot height in non-grazed individuals. Only individuals with an aboveground volume of more than 230 mm3 flowered and produced fruits during this study. Individuals that lacked aboveground structures in one season but reappeared in the following year were considered dormant. Individuals that lacked aboveground structures in two subsequent seasons were considered dead from the year in which they first lacked aboveground structures. Probabilities of seeds surviving to the next year, and of being present as seedlings or seeds in the soil seed bank, were derived from separate yearly sowing experiments in separate plots adjacent to each subplot .

### 1.6.3Anthyllis vulneraria data

Davison et al. (2010) reported stochastic contributions made by differences in vital rate means and variances among nine natural populations of Anthyllis vulneraria, also known as kidney vetch. This plant occurs in calcareous grasslands in the Viroin Valley of south-western Belgium. A. vulneraria is a grassland specialist and the unique host plant of the Red-listed blue butterfly (Cupido minimus). It is a short-lived, rosette-forming legume with a complex life cycle but no seedbank.

Nine populations (N = 27-50,000) growing in distinct grassland fragments were surveyed between 2003 and 2006, yielding three (4x4) annual transition matrices for each population. The populations occurred within grassland fragments, and were mostly managed as nature reserves through rotational sheep grazing. These surveys coincided with a summer heat wave (2003), followed by a spring drought (2005) and an even more extreme heat wave (2006). These populations have been subject to detailed study for aspects of their genetics and demography, and further details on the sites can be obtained through the resulting publications . We use the matrices published in Davison et al. (2010) to illustrate some of the features of lefko3, and so provide these matrices in a lefkoMat object (section 1.5.1).

## 1.7 Points to remember

1. R is an object-oriented language using the atomic vector as its basic unit of analysis.
2. R Studio provides an easy-to-use, convenient working environment for R analyses, including analyses using lefko3.
3. Package lefko3 provides R with convenient, standardized objects organizing all aspects of the MPM creation and analysis process. It also provides powerful functions that develop and analyze MPMs using lightning fast binaries.
4. Package lefko3 incorporates three key datasets for use in all examples, vignettes, and the chapters of this book.

### References

Davison, R., Jacquemyn, H., Adriaens, D., Honnay, O., Kroon, H. de & Tuljapurkar, S. (2010). Demographic effects of extreme weather events on a short-lived calcareous grassland species: Stochastic life table response experiments. Journal of Ecology, 98, 255–267.
Ehrlén, J. (1995). Demography of the perennial herb Lathyrus vernus. I. Herbivory and individual performance. Journal of Ecology, 83, 287–295.
Ehrlén, J. (2002). Assessing the lifetime consequences of plant-animal interactions for the perennial herb Lathyrus vernus (Fabaceae). Perspectives in Plant Ecology, Evolution and Systematics, 5, 145–163.
Ehrlén, J. & Eriksson, O. (1996). Seedling recruitment in the perennial herb Lathyrus vernus. Flora, 191, 377–383.
Ehrlén, J. & Lehtila, K. (2002). How perennal are perennial plants? Oikos, 98, 308–322.
Ehrlén, J. & Munzbergova, Z. (2009). Timing of flowering: Opposed selection on different fitness components and trait covariation. The American Naturalist, 173, 819–830.
Ehrlén, J. & Van Groenendael, J. (2001). Storage and the delayed costs of reproduction in the understorey perennial Lathyrus vernus. Journal of Ecology, 89, 237–246.
Honnay, O., Coart, E., Butaye, J., Adriaens, D., Glabeke, S. van & Roldan-Ruiz, I. (2006). Low impact of present and historical landscape configuration on the genetics of fragmented Anthyllis vulneraria populations. Biological Conservation, 127, 411–419.
Krauss, J., Steffan-Dewenter, I. & Tscharntke. (2004). Landscape occupancy and local population size depends on host plant distribution in the butterfly Cupido minimus. Biological Conservation, 120, 355–361.
Piessens, K., Adriaens, D., Jacquemyn, H. & Honnay, O. (2009). Synergistic effects of an extreme weather event and habitat fragmentation on a specialised insect herbivore. Oecologia, 159, 117–126.
Shefferson, R.P., Kull, T., Hutchings, M.J., Selosse, M.-A., Jacquemyn, H., Kellett, K.M., et al. (2018). Drivers of vegetative dormancy across herbaceous perennial plant species. Ecology Letters, 21, 724–733.
Shefferson, R.P., Kurokawa, S. & Ehrlén, J. (2021). lefko3: Analysing individual history through size-classified matrix population models. Methods in Ecology and Evolution, 12, 378–382.
Shefferson, R.P., Mizuta, R. & Hutchings, M.J. (2017). Predicting evolution in response to climate change: The example of sprouting probability in three dormancy-prone orchid species. Royal Society Open Science, 4, 160647.
Shefferson, R.P., Sandercock, B.K., Proper, J. & Beissinger, S.R. (2001). Estimating dormancy and survival of a rare herbaceous perennial using mark-recapture models. Ecology, 82, 145–156.