tidysdm overview

SDMs with tidymodels

Species Distribution Modelling relies on several algorithms, many of which have a number of hyperparameters that require turning. The tidymodels universe includes a number of packages specifically design to fit, tune and validate models. The advantage of tidymodels is that the models syntax and the results returned to the users are standardised, thus providing a coherent interface to modelling. Given the variety of models required for SDM, tidymodels is an ideal framework. tidysdm provides a number of wrappers and specialised functions to facilitate the fitting of SDM with tidymodels.

This article provides an overview of the how tidysdm facilitates fitting SDMs. Further articles, detailing how to use the package for palaeodata, fitting more complex models and how to troubleshoot models can be found on the tidisdm website. As tidysdm relies on tidymodels, users are advised to familiarise themselves with the introductory tutorials on the tidymodels website.

When we load tidysdm, it automatically loads tidymodels and all associated packages necessary to fit models:

library(tidysdm)
#> Loading required package: tidymodels
#> ── Attaching packages ────────────────────────────────────── tidymodels 1.2.0 ──
#> ✔ broom        1.0.7     ✔ recipes      1.1.0
#> ✔ dials        1.3.0     ✔ rsample      1.2.1
#> ✔ dplyr        1.1.4     ✔ tibble       3.2.1
#> ✔ ggplot2      3.5.1     ✔ tidyr        1.3.1
#> ✔ infer        1.0.7     ✔ tune         1.2.1
#> ✔ modeldata    1.4.0     ✔ workflows    1.1.4
#> ✔ parsnip      1.2.1     ✔ workflowsets 1.1.0
#> ✔ purrr        1.0.4     ✔ yardstick    1.3.1
#> ── Conflicts ───────────────────────────────────────── tidymodels_conflicts() ──
#> ✖ purrr::discard() masks scales::discard()
#> ✖ dplyr::filter()  masks stats::filter()
#> ✖ dplyr::lag()     masks stats::lag()
#> ✖ recipes::step()  masks stats::step()
#> • Use tidymodels_prefer() to resolve common conflicts.
#> Loading required package: spatialsample

Accessing the data for this vignette: how to use rgbif

We start by reading in a set of presences for a species of lizard that inhabits the Iberian peninsula, Lacerta schreiberi. This data is taken from GBIF Occurrence Download (6 July 2023) https://doi.org/10.15468/dl.srq3b3. The dataset is already included in the tidysdm package:

data(lacerta)
head(lacerta)
#> # A tibble: 6 × 3
#>          ID latitude longitude
#>       <dbl>    <dbl>     <dbl>
#> 1 858029749     42.6     -7.09
#> 2 858029738     42.6     -7.09
#> 3 614631090     41.4     -7.90
#> 4 614631085     41.3     -7.81
#> 5 614631083     41.3     -7.81
#> 6 614631080     41.4     -7.83

Alternatively, we can easily access and manipulate this dataset using rbgif. Note that the data from GBIF often requires some level of cleaning. Here we will use a simple cleaning function from the CoordinateCleaner; in general, we recommend to inspect the data that are flagged as problematic, rather than just accepting them as we do here:

# download presences
library(rgbif)
occ_download_get(key = "0068808-230530130749713", path = tempdir())
# read file
library(readr)
distrib <- read_delim(file.path(tempdir(), "0068808-230530130749713.zip"))
# keep the necessary columns and rename them
lacerta <- distrib %>%
  select(gbifID, decimalLatitude, decimalLongitude) %>%
  rename(ID = gbifID, latitude = decimalLatitude, longitude = decimalLongitude)
# clean up the data
library(CoordinateCleaner)
lacerta <- clean_coordinates(
  x = lacerta,
  lon = "longitude",
  lat = "latitude",
  species = "ID",
  value = "clean"
)

Preparing your data

First, let us visualise our presences by plotting on a map. tidysdm works with sf objects to represent locations, so we will cast our coordinates into an sf object, and set its projections to standard ‘lonlat’ using the proj4 string “+proj=longlat”.

library(sf)
#> Linking to GEOS 3.12.1, GDAL 3.8.4, PROJ 9.3.1; sf_use_s2() is TRUE
lacerta <- st_as_sf(lacerta, coords = c("longitude", "latitude"))
st_crs(lacerta) <- "+proj=longlat"

It is usually advisable to plot the locations directly on the raster that will be used to extract climatic variables, to see how the locations fall within the discrete space of the raster. For this vignette, we will use WorldClim as our source of climatic information. We will access the WorldClim data via the library pastclim; even though this library, as the name suggests, is mostly designed to handle palaeoclimatic reconstructions, it also provides convenient functions to access present day reconstructions and future projections. pastclim has a handy function to get the land mask for the available datasets, which we can use as background for our locations. We will cut the raster to the Iberian peninsula, where our lizard lives.

For this example:

library(pastclim)
download_dataset(dataset = "WorldClim_2.1_10m")
land_mask <-
  get_land_mask(time_ce = 1985, dataset = "WorldClim_2.1_10m")

# Iberia peninsula extension
iberia_poly <-
  terra::vect(
    "POLYGON((-9.8 43.3,-7.8 44.1,-2.0 43.7,3.6 42.5,3.8 41.5,1.3 40.8,0.3 39.5,
     0.9 38.6,-0.4 37.5,-1.6 36.7,-2.3 36.3,-4.1 36.4,-4.5 36.4,-5.0 36.1,
    -5.6 36.0,-6.3 36.0,-7.1 36.9,-9.5 36.6,-9.4 38.0,-10.6 38.9,-9.5 40.8,
    -9.8 43.3))"
  )

crs(iberia_poly) <- "+proj=longlat"
# crop the extent
land_mask <- crop(land_mask, iberia_poly)
# and mask to the polygon
land_mask <- mask(land_mask, iberia_poly)
#> Loading required package: terra
#> terra 1.8.21
#> 
#> Attaching package: 'terra'
#> The following object is masked from 'package:tidyr':
#> 
#>     extract
#> The following object is masked from 'package:scales':
#> 
#>     rescale
#> [1] TRUE

For plotting, we will take advantage of tidyterra, which makes handling of terra rasters with ggplot a breeze.

library(tidyterra)
#> 
#> Attaching package: 'tidyterra'
#> The following object is masked from 'package:stats':
#> 
#>     filter
library(ggplot2)
ggplot() +
  geom_spatraster(data = land_mask, aes(fill = land_mask_1985)) +
  geom_sf(data = lacerta) +
  guides(fill = "none")

Map projection

Before we start thinning the data we need to make sure that all our data (points and rasters) have the same geographic coordinate reference system (CRS) by projecting them. In some of the pipeline steps (e.g. thinning data, measuring areas) using an equal area projection may make a significant difference, especially for large-scale projects.

You can use the website projectionwizard.org (https://link.springer.com/chapter/10.1007/978-3-319-51835-0_9) to find an appropriate equal area projection for any region.

To define our projection within the code, we will use a proj4 string, which provides information on the type of projection, its parameters and the units of distance in which the new coordinates will be expressed (if you are using projectionwizard.org it will provide yo with the string as well).

In this case, we will use a Albers Equal Area Conic projection centred on the Iberian peninsula, with km as units. The proj4 string is:

iberia_proj4 <-
  "+proj=aea +lon_0=-4.0 +lat_1=36.8 +lat_2=42.6 +lat_0=39.7 +datum=WGS84 +units=m +no_defs"

For rasters (maps), we use the terra function project to change the CRS. We pass the raster object and the proj4 string as arguments:

land_mask <- terra::project(land_mask, y = iberia_proj4)

Now we need to project the data points to the same CRS as the raster. We will do so using the appropriate sf function:

lacerta <- st_transform(lacerta, iberia_proj4)

Plotting the data, we will see that the shape of the land mask has slightly changed following the new projection.

ggplot() +
  geom_spatraster(data = land_mask, aes(fill = land_mask_1985)) +
  geom_sf(data = lacerta) +
  guides(fill = "none")

Thinning step

Now, we thin the observations to have one per cell in the raster (given our project, each cell is approximately the same size):

set.seed(1234567)
lacerta <- thin_by_cell(lacerta, raster = land_mask)
nrow(lacerta)
#> [1] 233
pres_data <- terra::extract(land_mask, lacerta)
summary(pres_data)
#>        ID      land_mask_1985
#>  Min.   :  1   land:233      
#>  1st Qu.: 59                 
#>  Median :117                 
#>  Mean   :117                 
#>  3rd Qu.:175                 
#>  Max.   :233
ggplot() +
  geom_spatraster(data = land_mask, aes(fill = land_mask_1985)) +
  geom_sf(data = lacerta) +
  guides(fill = "none")

Now, we thin further to remove points that are closer than 20km. As the units of our projection are m (the default for most projections), we use a a convenient conversion function, km2m(), to avoid having to write lots of zeroes:

set.seed(1234567)
lacerta_thin <- thin_by_dist(lacerta, dist_min = km2m(20))
nrow(lacerta_thin)
#> [1] 112

Let’s see what we have left of our points:

ggplot() +
  geom_spatraster(data = land_mask, aes(fill = land_mask_1985)) +
  geom_sf(data = lacerta_thin) +
  guides(fill = "none")

We now need to select points that represent the potential available area for the species. There are two approaches, we can either sample the background with sample_background(), or we can generate pseudo-absences with sample_pseudoabs(). In this example, we will sample the background; more specifically, we will attempt to account for potential sampling biases by using a target group approach, where presences from other species within the same taxonomic group are used to condition the sampling of the background, providing information on differential sampling of different areas within the region of interest.

We will start by downloading records from 8 genera of Lacertidae, covering the same geographic region of the Iberian peninsula from GBIF https://doi.org/10.15468/dl.53js5z:

library(rgbif)
occ_download_get(key = "0121761-240321170329656", path = tempdir())
library(readr)
backg_distrib <- readr::read_delim(file.path(
  tempdir(),
  "0121761-240321170329656.zip"
))
# keep the necessary columns
lacertidae_background <- backg_distrib %>%
  select(gbifID, decimalLatitude, decimalLongitude) %>%
  rename(ID = gbifID, latitude = decimalLatitude, longitude = decimalLongitude)

In this case as well, we need to use the appropriate projection (the same defined before) for the background. If the projections do not correspond the analyses will stop giving an error message.

# convert to an sf object
lacertidae_background <- st_as_sf(lacertidae_background,
  coords = c("longitude", "latitude")
)

st_crs(lacertidae_background) <- "+proj=longlat"
lacertidae_background <- st_transform(lacertidae_background, crs = iberia_proj4)

We need to convert these observations into a raster whose values are the number of records (which will be later used to determine how likely each cell is to be used as a background point). We will also mask the resulting background raster to match the land mask of interest.

lacertidae_background_raster <- rasterize(lacertidae_background,
  land_mask,
  fun = "count"
)
lacertidae_background_raster <- mask(
  lacertidae_background_raster,
  land_mask
)
ggplot() +
  geom_spatraster(data = lacertidae_background_raster, aes(fill = count)) +
  scale_fill_viridis_b(na.value = "transparent")

guides(fill = "none")
#> <Guides[1] ggproto object>
#> 
#> fill : "none"

We can see that the sampling is far from random, with certain locations having very large number of records. We can now sample the background, using the ‘bias’ method to represent this heterogeneity in sampling effort:

set.seed(1234567)
lacerta_thin <- sample_background(
  data = lacerta_thin, raster = lacertidae_background_raster,
  n = 3 * nrow(lacerta_thin),
  method = "bias",
  class_label = "background",
  return_pres = TRUE
)

Let’s see our presences and background:

ggplot() +
  geom_spatraster(data = land_mask, aes(fill = land_mask_1985)) +
  geom_sf(data = lacerta_thin, aes(col = class)) +
  guides(fill = "none")

We can use pastclim to download the WorldClim dataset (we’ll use the 10 arc-minute resolution) and extract the bioclimatic variables that are available (but you do not have to use pastclim, you could use any raster dataset you have access to, loading it directly with terra).

download_dataset("WorldClim_2.1_10m")
climate_vars <- get_vars_for_dataset("WorldClim_2.1_10m")
climate_present <- pastclim::region_slice(
  time_ce = 1985,
  bio_variables = climate_vars,
  data = "WorldClim_2.1_10m",
  crop = iberia_poly
)

Note that the dataset covers the period 1970-2000, so pastclim dates it as 1985 (the midpoint). We have also cropped it directly to the Iberian peninsula.

Note that, in this vignette, we focus on continuous variables; most machine learning algorithms do not natively cope with multi-level factors, but it is possible to use recipes::step_dummy() to generate dummy variables from factors. A worked example can be found in the article on additional features of tidymodels with tidysdm.

And now we project the climate variables in the same way as we did for all previous spatial data:

climate_present <- terra::project(climate_present, y = iberia_proj4)

Next, we extract climate for all presences and background points:

lacerta_thin <- lacerta_thin %>%
  bind_cols(terra::extract(climate_present, lacerta_thin, ID = FALSE))

Before going forward with the analysis, we should make sure that there are no missing values in the climate that we extracted:

summary(lacerta_thin)
#>         class              geometry       bio01            bio02       
#>  presence  :112   POINT        :448   Min.   : 3.748   Min.   : 5.920  
#>  background:336   epsg:NA      :  0   1st Qu.:12.334   1st Qu.: 8.955  
#>                   +proj=aea ...:  0   Median :14.048   Median : 9.793  
#>                                       Mean   :13.737   Mean   :10.009  
#>                                       3rd Qu.:15.751   3rd Qu.:10.948  
#>                                       Max.   :18.194   Max.   :13.911  
#>      bio03           bio04           bio05           bio06       
#>  Min.   :32.96   Min.   :320.6   Min.   :20.17   Min.   :-7.722  
#>  1st Qu.:38.25   1st Qu.:463.2   1st Qu.:24.83   1st Qu.: 1.137  
#>  Median :40.35   Median :547.9   Median :27.73   Median : 3.374  
#>  Mean   :40.45   Mean   :533.8   Mean   :27.81   Mean   : 2.945  
#>  3rd Qu.:42.46   3rd Qu.:609.0   3rd Qu.:30.90   3rd Qu.: 5.087  
#>  Max.   :47.59   Max.   :734.9   Max.   :35.95   Max.   : 8.396  
#>      bio07           bio08            bio09            bio10      
#>  Min.   :14.74   Min.   :-2.000   Min.   : 2.829   Min.   :12.25  
#>  1st Qu.:21.95   1st Qu.: 7.953   1st Qu.:17.969   1st Qu.:18.66  
#>  Median :25.02   Median :10.048   Median :19.817   Median :20.68  
#>  Mean   :24.86   Mean   :10.332   Mean   :19.479   Mean   :20.66  
#>  3rd Qu.:27.75   3rd Qu.:12.424   3rd Qu.:22.681   3rd Qu.:22.96  
#>  Max.   :33.70   Max.   :20.716   Max.   :25.852   Max.   :25.90  
#>      bio11            bio12            bio13            bio14      
#>  Min.   :-3.481   Min.   : 243.5   Min.   : 33.00   Min.   : 0.00  
#>  1st Qu.: 6.233   1st Qu.: 553.9   1st Qu.: 81.83   1st Qu.: 6.00  
#>  Median : 8.144   Median : 740.9   Median :106.29   Median :15.03  
#>  Mean   : 7.670   Mean   : 838.0   Mean   :116.89   Mean   :19.45  
#>  3rd Qu.: 9.820   3rd Qu.:1119.0   3rd Qu.:144.27   3rd Qu.:28.13  
#>  Max.   :12.647   Max.   :1732.6   Max.   :249.79   Max.   :73.84  
#>      bio15           bio16           bio17            bio18       
#>  Min.   :15.98   Min.   : 94.8   Min.   : 14.60   Min.   : 18.61  
#>  1st Qu.:34.43   1st Qu.:221.3   1st Qu.: 36.78   1st Qu.: 41.41  
#>  Median :50.53   Median :296.3   Median : 75.04   Median : 80.16  
#>  Mean   :46.89   Mean   :324.3   Mean   : 83.89   Mean   : 93.95  
#>  3rd Qu.:57.32   3rd Qu.:404.4   3rd Qu.:123.58   3rd Qu.:142.16  
#>  Max.   :77.95   Max.   :695.0   Max.   :260.54   Max.   :264.12  
#>      bio19           altitude      
#>  Min.   : 71.39   Min.   :  20.12  
#>  1st Qu.:171.93   1st Qu.: 170.51  
#>  Median :277.38   Median : 419.39  
#>  Mean   :298.73   Mean   : 509.95  
#>  3rd Qu.:383.86   3rd Qu.: 698.54  
#>  Max.   :694.98   Max.   :2056.80

We can see that there are no missing values in any of the extracted climate variables. If that was not the case, we would have to go back to the climate raster and homogenise the NAs across layers (i.e. variables). This can be achieved either by setting the same cells to NA in all layers (including the land mask that we used to thin the data), or by interpolating the layers with less information to fill the gaps (e.g. cloud cover in some remote sensed data). interpolate the missing

Based on this paper (https://doi.org/10.1007/s10531-010-9865-2), we are interested in these variables: “bio06”, “bio05”, “bio13”, “bio14”, “bio15”. We can visualise the differences between presences and the background using violin plots:

lacerta_thin %>% plot_pres_vs_bg(class)

We can see that all the variables of interest do seem to have a different distribution between presences and the background. We can formally quantify the mismatch between the two by computing the overlap:

lacerta_thin %>% dist_pres_vs_bg(class)
#>      bio09      bio12      bio16      bio13      bio10      bio19      bio05 
#> 0.45046089 0.42716809 0.40839385 0.40784275 0.40752637 0.40267151 0.39335505 
#>      bio02      bio07      bio08      bio04      bio17      bio15      bio18 
#> 0.37600379 0.35861798 0.33268356 0.32472686 0.27106995 0.26347515 0.25802874 
#>      bio01      bio14      bio03   altitude      bio11      bio06 
#> 0.25403942 0.24900839 0.13560214 0.11578352 0.10760145 0.08284886

Again, we can see that the variables of interest seem good candidates with a clear signal. Let us then focus on those variables:

suggested_vars <- c("bio06", "bio05", "bio13", "bio14", "bio15")

Environmental variables are often highly correlated, and collinearity is an issue for several types of models. We can inspect the correlation among variables with:

pairs(climate_present[[suggested_vars]])

We can see that some variables have rather high correlation (e.g. bio05 vs bio14). We can subset to variables below a certain threshold correlation (e.g. 0.7) with:

climate_present <- climate_present[[suggested_vars]]

vars_uncor <- filter_collinear(climate_present,
  cutoff = 0.7,
  method = "cor_caret"
)
vars_uncor
#> [1] "bio15" "bio05" "bio13" "bio06"
#> attr(,"to_remove")
#> [1] "bio14"

So, removing bio14 leaves us with a set of uncorrelated variables. Note that filter_collinear has other methods based on variable inflation that would also be worth exploring. For this example, we will remove bio14 and work with the remaining variables.

lacerta_thin <- lacerta_thin %>% select(all_of(c(vars_uncor, "class")))
climate_present <- climate_present[[vars_uncor]]
names(climate_present) # variables retained in the end
#> [1] "bio15" "bio05" "bio13" "bio06"

Fit the model by cross-validation

Next, we need to set up a recipe to define how to handle our dataset. We don’t want to do anything to our data in terms of transformations, so we just need to define the formula (class is the outcome, all other variables are predictors; note that, for sf objects, geometry is automatically replaced by X and Y columns which are assigned a role of coords, and thus not used as predictors):

lacerta_rec <- recipe(lacerta_thin, formula = class ~ .)
lacerta_rec
#> 
#> ── Recipe ──────────────────────────────────────────────────────────────────────
#> 
#> ── Inputs
#> Number of variables by role
#> outcome:   1
#> predictor: 4
#> coords:    2

Note that step_ functions from recipes that filter data rows (e.g step_naomit()) can be problematic (see the relevant section from the man page for step_naomit()). We would recommend that you filter your data before setting up the recipe, rather than attempting to use steps in the recipe.

In classification models for tidymodels, the assumption is that the level of interest for the response (in our case, presences) is the reference level. We can confirm that we have the data correctly formatted with:

lacerta_thin %>% check_sdm_presence(class)
#> [1] TRUE

We now build a workflow_set of different models, defining which hyperparameters we want to tune. We will use glm, random forest, boosted_trees and maxent as our models (for more details on how to use workflow_sets, see this tutorial). The latter three models have tunable hyperparameters. For the most commonly used models, tidysdm automatically chooses the most important parameters, but it is possible to fully customise model specifications (e.g. see the help for sdm_spec_rf). Note that, if you used GAMs with sdm_spec_gam(), it is necessary to update the model with gam_formula() due to the non-standard formula notation of GAMs (see the help of sdm_spec_gam() for an example of how to do this).

lacerta_models <-
  # create the workflow_set
  workflow_set(
    preproc = list(default = lacerta_rec),
    models = list(
      # the standard glm specs
      glm = sdm_spec_glm(),
      # rf specs with tuning
      rf = sdm_spec_rf(),
      # boosted tree model (gbm) specs with tuning
      gbm = sdm_spec_boost_tree(),
      # maxent specs with tuning
      maxent = sdm_spec_maxent()
    ),
    # make all combinations of preproc and models,
    cross = TRUE
  ) %>%
  # tweak controls to store information needed later to create the ensemble
  option_add(control = control_ensemble_grid())

We now want to set up a spatial block cross-validation scheme to tune and assess our models. We will split the data by creating 3 folds. We use the spatial_block_cv function from the package spatialsample. spatialsample offers a number of sampling approaches for spatial data; it is also possible to convert objects created with blockCV (which offers further features for spatial sampling, such as stratified sampling) into an rsample object suitable to tisysdm with the function blockcv2rsample.

library(tidysdm)
set.seed(105)
lacerta_cv <- spatial_block_cv(lacerta_thin, v = 5)
autoplot(lacerta_cv)

We can check that the splits are reasonably balanced with:

check_splits_balance(lacerta_cv, class)
#> # A tibble: 5 × 4
#>   presence_assessment background_assessment presence_analysis
#>                 <int>                 <int>             <int>
#> 1                  91                   246                21
#> 2                  91                   276                21
#> 3                  73                   256                39
#> 4                 102                   278                10
#> 5                  91                   288                21
#> # ℹ 1 more variable: background_analysis <int>

We can now use the block CV folds to tune and assess the models (to keep computations fast, we will only explore 3 combination of hyperparameters per model; this is far too little in real life!):

set.seed(1234567)
lacerta_models <-
  lacerta_models %>%
  workflow_map("tune_grid",
    resamples = lacerta_cv, grid = 3,
    metrics = sdm_metric_set(), verbose = TRUE
  )
#> i    No tuning parameters. `fit_resamples()` will be attempted
#> i 1 of 4 resampling: default_glm
#> ✔ 1 of 4 resampling: default_glm (127ms)
#> i 2 of 4 tuning:     default_rf
#> i Creating pre-processing data to finalize unknown parameter: mtry
#> ✔ 2 of 4 tuning:     default_rf (836ms)
#> i 3 of 4 tuning:     default_gbm
#> i Creating pre-processing data to finalize unknown parameter: mtry
#> ✔ 3 of 4 tuning:     default_gbm (3s)
#> i 4 of 4 tuning:     default_maxent
#> ✔ 4 of 4 tuning:     default_maxent (1s)

Note that workflow_set correctly detects that we have no tuning parameters for glm. We can have a look at the performance of our models with:

autoplot(lacerta_models)

Now let’s create an ensemble, selecting the best set of parameters for each model (this is really only relevant for the ML algorithms, as there were not hype-parameters to tune for the glm). We will use the Boyce continuous index as our metric to choose the best random forest and boosted tree. When adding members to an ensemble, they are automatically fitted to the full training dataset, and so ready to make predictions.

lacerta_ensemble <- simple_ensemble() %>%
  add_member(lacerta_models, metric = "boyce_cont")
lacerta_ensemble
#> A simple_ensemble of models
#> 
#> Members:
#> • default_glm
#> • default_rf
#> • default_gbm
#> • default_maxent
#> 
#> Available metrics:
#> • boyce_cont
#> • roc_auc
#> • tss_max
#> 
#> Metric used to tune workflows:
#> • boyce_cont

And visualise it

autoplot(lacerta_ensemble)

A tabular form of the model metrics can be obtained with:

lacerta_ensemble %>% collect_metrics()
#> # A tibble: 12 × 5
#>    wflow_id       .metric     mean std_err     n
#>    <chr>          <chr>      <dbl>   <dbl> <int>
#>  1 default_glm    boyce_cont 0.443  0.0899     5
#>  2 default_glm    roc_auc    0.786  0.0391     5
#>  3 default_glm    tss_max    0.556  0.0745     5
#>  4 default_rf     boyce_cont 0.537  0.0778     5
#>  5 default_rf     roc_auc    0.810  0.0421     5
#>  6 default_rf     tss_max    0.559  0.0841     5
#>  7 default_gbm    boyce_cont 0.550  0.120      5
#>  8 default_gbm    roc_auc    0.805  0.0349     5
#>  9 default_gbm    tss_max    0.542  0.0613     5
#> 10 default_maxent boyce_cont 0.665  0.0922     5
#> 11 default_maxent roc_auc    0.820  0.0398     5
#> 12 default_maxent tss_max    0.590  0.0712     5

Projecting to the present

We can now make predictions with this ensemble (using the default option of taking the mean of the predictions from each model).

prediction_present <- predict_raster(lacerta_ensemble, climate_present)
ggplot() +
  geom_spatraster(data = prediction_present, aes(fill = mean)) +
  scale_fill_terrain_c() +
  # plot presences used in the model
  geom_sf(data = lacerta_thin %>% filter(class == "presence"))

We can subset the ensemble to only use the best models, based on the Boyce continuous index, by setting a minimum threshold of 0.5 for that metric (this is somewhat low, for a real analysis we would recommend a higher value of 0.7 or higher). We will also take the median of the available model predictions (instead of the mean, which is the default). The plot does not change much (the models are quite consistent).

prediction_present_boyce <- predict_raster(lacerta_ensemble, climate_present,
  metric_thresh = c("boyce_cont", 0.5),
  fun = "median"
)
ggplot() +
  geom_spatraster(data = prediction_present_boyce, aes(fill = median)) +
  scale_fill_terrain_c() +
  geom_sf(data = lacerta_thin %>% filter(class == "presence"))

Sometimes, it is desirable to have binary predictions (presence vs absence), rather than the probability of occurrence. To do so, we first need to calibrate the threshold used to convert probabilities into classes (in this case, we optimise the TSS):

lacerta_ensemble <- calib_class_thresh(lacerta_ensemble,
  class_thresh = "tss_max",
  metric_thresh = c("boyce_cont", 0.5)
)

And now we can predict for the whole continent:

prediction_present_binary <- predict_raster(lacerta_ensemble,
  climate_present,
  type = "class",
  class_thresh = c("tss_max"),
  metric_thresh = c("boyce_cont", 0.5)
)
ggplot() +
  geom_spatraster(data = prediction_present_binary, aes(fill = binary_mean)) +
  geom_sf(data = lacerta_thin %>% filter(class == "presence")) +
  scale_fill_discrete(na.value = "transparent")

Projecting to the future

WorldClim has a wide selection of projections for the future based on different models and Shared Socio-economic Pathways (SSP). Type help("WorldClim_2.1") for a full list. We will use predictions based on “HadGEM3-GC31-LL” model for SSP 245 (intermediate green house gas emissions) at the same resolution as the present day data (10 arc-minutes). We first download the data:

download_dataset("WorldClim_2.1_HadGEM3-GC31-LL_ssp245_10m")

Let’s see what times are available:

get_time_ce_steps("WorldClim_2.1_HadGEM3-GC31-LL_ssp245_10m")
#> [1] 2030 2050 2070 2090

We will predict for 2090, the further prediction in the future that is available.

Let’s now check the available variables:

get_vars_for_dataset("WorldClim_2.1_HadGEM3-GC31-LL_ssp245_10m")
#>  [1] "bio01" "bio02" "bio03" "bio04" "bio05" "bio06" "bio07" "bio08" "bio09"
#> [10] "bio10" "bio11" "bio12" "bio13" "bio14" "bio15" "bio16" "bio17" "bio18"
#> [19] "bio19"

Note that future predictions do not include altitude (as that does not change with time), so if we needed it, we would have to copy it over from the present. However, it is not in our set of uncorrelated variables that we used earlier, so we don’t need to worry about it.

climate_future <- pastclim::region_slice(
  time_ce = 2090,
  bio_variables = vars_uncor,
  data = "WorldClim_2.1_HadGEM3-GC31-LL_ssp245_10m",
  crop = iberia_poly
)

Project the climatic raster with the same projection that we have been using for the analysis:

climate_future <- terra::project(climate_future, y = iberia_proj4)

And predict using the ensemble:

prediction_future <- predict_raster(lacerta_ensemble, climate_future)

ggplot() +
  geom_spatraster(data = prediction_future, aes(fill = mean)) +
  scale_fill_terrain_c()

Dealing with extrapolation

The total area of projection of the model may include environmental conditions which lie outside the range of conditions covered by the calibration dataset. This phenomenon can lead to misinterpretation of the SDM outcomes due to spatial extrapolation.

tidysdm offers a couple of approaches to deal with this problem. The simplest one is that we can clamp the environmental variables to stay within the limits observed in the calibration set:

climate_future_clamped <- clamp_predictors(climate_future,
  training = lacerta_thin,
  .col = class
)
prediction_future_clamped <- predict_raster(lacerta_ensemble,
  raster = climate_future_clamped
)

ggplot() +
  geom_spatraster(data = prediction_future_clamped, aes(fill = mean)) +
  scale_fill_terrain_c()

The predictions seem to have changed very little.

An alternative is to allow values to exceed the ranges of the calibration set, but compute the Multivariate environmental similarity surfaces (MESS) (Elith et al. 2010) to highlight areas where extrapolation occurs and thus visualise the prediction’s uncertainty.

We estimate the MESS for the same future time slice used above:

lacerta_mess_future <- extrapol_mess(
  x = climate_future,
  training = lacerta_thin,
  .col = "class"
)

ggplot() +
  geom_spatraster(data = lacerta_mess_future) +
  scale_fill_viridis_b(na.value = "transparent")

Extrapolation occurs in areas where MESS values are negative, with the magnitude of the negative values indicating how extreme is in the interpolation. From this plot, we can see that the area of extrapolation is where the model already predicted a suitability of zero. This explains why clamping did little to our predictions.

We can now overlay MESS values with current prediction to visualize areas characterized by spatial extrapolation.

# subset mess
lacerta_mess_future_subset <- lacerta_mess_future
lacerta_mess_future_subset[lacerta_mess_future_subset >= 0] <- NA
lacerta_mess_future_subset[lacerta_mess_future_subset < 0] <- 1

# convert into polygon
lacerta_mess_future_subset <- as.polygons(lacerta_mess_future_subset)

library(ggpattern)

# plot as a mask
ggplot() +
  geom_spatraster(data = prediction_future) +
  scale_fill_terrain_c() +
  geom_sf_pattern(
    data = lacerta_mess_future_subset,
    pattern = "stripe",
    fill = "transparent",
    pattern_fill = "black",
    pattern_density = 0.02,
    pattern_spacing = 0.05,
    pattern_angle = 45,
    alpha = 0.1,
    linewidth = 0.5
  )

Note that clamping and MESS are not only useful when making predictions into the future, but also into the past and present (in the latter case, it allows us to make sure that the background/pseudoabsences do cover the full range of predictor variables over the area of interest).

The tidymodels universe also includes functions to estimate the area of applicability in the package waywiser, which can be used with tidysdm.

Visualising the contribution of individual variables

It is sometimes of interest to understand the relative contribution of individual variables to the prediction. This is a complex task, especially if there are interactions among variables. For simpler linear models, it is possible to obtain marginal response curves (which show the effect of a variable whilst keeping all other variables to their mean) using step_profile() from the recipes package. We use step_profile() to define a new recipe which we can then bake to generate the appropriate dataset to make the marginal prediction. We can then plot the predictions against the values of the variable of interest. For example, to investigate the contribution of bio05, we would:

bio05_prof <- lacerta_rec %>%
  step_profile(-bio05, profile = vars(bio05)) %>%
  prep(training = lacerta_thin)

bio05_data <- bake(bio05_prof, new_data = NULL)

bio05_data <- bio05_data %>%
  mutate(
    pred = predict(lacerta_ensemble, bio05_data)$mean
  )

ggplot(bio05_data, aes(x = bio05, y = pred)) +
  geom_point(alpha = .5, cex = 1)

It is also possible to use DALEX,to explore tidysdm models; see more details in the tidymodels additions article.

Repeated ensembles

The steps of thinning and sampling pseudo-absences can have a bit impact on the performance of SDMs. As these steps are stochastic, it is good practice to explore their effect by repeating them, and then creating ensembles of models over these repeats. In tidysdm, it is possible to create repeat_ensembles. We start by creating a list of simple_ensembles, by looping through the SDM pipeline. We will just use two fast models to speed up the process, and use pseudo-absences instead of background.

# empty object to store the simple ensembles that we will create
ensemble_list <- list()
set.seed(1234) # make sure you set the seed OUTSIDE the loop
for (i_repeat in 1:3) {
  # thin the data
  lacerta_thin_rep <- thin_by_cell(lacerta, raster = climate_present)
  lacerta_thin_rep <- thin_by_dist(lacerta_thin_rep, dist_min = 20000)
  # sample pseudo-absences
  lacerta_thin_rep <- sample_pseudoabs(lacerta_thin_rep,
    n = 3 * nrow(lacerta_thin_rep),
    raster = climate_present,
    method = c("dist_min", 50000)
  )
  # get climate
  lacerta_thin_rep <- lacerta_thin_rep %>%
    bind_cols(terra::extract(climate_present, lacerta_thin_rep, ID = FALSE))
  # create folds
  lacerta_thin_rep_cv <- spatial_block_cv(lacerta_thin_rep, v = 5)
  # create a recipe
  lacerta_thin_rep_rec <- recipe(lacerta_thin_rep, formula = class ~ .)
  # create a workflow_set
  lacerta_thin_rep_models <-
    # create the workflow_set
    workflow_set(
      preproc = list(default = lacerta_thin_rep_rec),
      models = list(
        # the standard glm specs
        glm = sdm_spec_glm(),
        # maxent specs with tuning
        maxent = sdm_spec_maxent()
      ),
      # make all combinations of preproc and models,
      cross = TRUE
    ) %>%
    # tweak controls to store information needed later to create the ensemble
    option_add(control = control_ensemble_grid())

  # train the model
  lacerta_thin_rep_models <-
    lacerta_thin_rep_models %>%
    workflow_map("tune_grid",
      resamples = lacerta_thin_rep_cv, grid = 3,
      metrics = sdm_metric_set(), verbose = TRUE
    )
  # make an simple ensemble and add it to the list
  ensemble_list[[i_repeat]] <- simple_ensemble() %>%
    add_member(lacerta_thin_rep_models, metric = "boyce_cont")
}
#> i    No tuning parameters. `fit_resamples()` will be attempted
#> i 1 of 2 resampling: default_glm
#> ✔ 1 of 2 resampling: default_glm (114ms)
#> i 2 of 2 tuning:     default_maxent
#> ✔ 2 of 2 tuning:     default_maxent (1s)
#> i    No tuning parameters. `fit_resamples()` will be attempted
#> i 1 of 2 resampling: default_glm
#> ✔ 1 of 2 resampling: default_glm (110ms)
#> i 2 of 2 tuning:     default_maxent
#> ✔ 2 of 2 tuning:     default_maxent (982ms)
#> i    No tuning parameters. `fit_resamples()` will be attempted
#> i 1 of 2 resampling: default_glm
#> ✔ 1 of 2 resampling: default_glm (113ms)
#> i 2 of 2 tuning:     default_maxent
#> ✔ 2 of 2 tuning:     default_maxent (960ms)

Now we can create a repeat_ensemble from the list:

lacerta_rep_ens <- repeat_ensemble() %>% add_repeat(ensemble_list)
lacerta_rep_ens
#> A repeat_ensemble of models
#> 
#> Number of repeats:
#> • 3
#> 
#> Members:
#> • default_glm
#> • default_maxent
#> 
#> Available metrics:
#> • boyce_cont
#> • roc_auc
#> • tss_max
#> 
#> Metric used to tune workflows:
#> • boyce_cont

We can summarise the goodness of fit of models for each repeat with collect_metrics(), but there is no autoplot() function for repeated_ensemble objects.

We can then predict in the usual way. We will take the mean and median of all models, without filtering by performance, and plot the results:

lacerta_rep_ens <- predict_raster(lacerta_rep_ens, climate_present,
  fun = c("mean", "median")
)
ggplot() +
  geom_spatraster(data = lacerta_rep_ens, aes(fill = median)) +
  scale_fill_terrain_c()

Note that the predictions are quite similar to the ones we obtained before, but the predicted suitable range is somewhat larger, probably because we included models that are not very good (as we did not filter by performance) in the ensemble.