Cliffhanger

July 8th, 2016

CliffhangerI think I was on a long-haul flight across the Pacific when I succumbed to jet-lag induced doldrums and watched Sylvester Stallone’s Cliffhanger which stars him (surprise) as a mountaineer who gets himself out of a dastardly plot by climbing around and flexing his muscles. So if there’s a Rocky of the rare plant world, it’s Appalachian avens, or Geum radiatum.

Like Stallone, Geum likes to hang on cliffs in the Southern Appalachians. And talk about hang!  Eric Menges sent me a few photos of their field work–they use ladders to census the populations.  Now there are also a few populations found on so-called grassy balds (I am fond of this name), but the cliffside populations tend to be more common.  And these cliffs occur at high elevations where the distinction between cloud and mist dissipates.

Geum radiatum

Geum radiatum (Wikimedia)

And like Stallone Geum is threatened–so much so it’s listed on the US Endangered Species Act.  In fact Pedro Quintana-Ascencio‘s models predict that the overall growth rate of the populations for which they have census data is currently below replacement level.  But–intriguingly–growth rate is highly positively linked to relative humidity.

Which is really cool in a technical way–because I was able to extract coarse-scale humidity data from the ClimateNA dataset, then relate this to their fine-scale measurements.  And this in turn allowed Pedro to predict population growth rates under future scenarios of climate change.

Stallone

“Hang in there, Geum!”

Alas, Geum needs a strongman like Stallone… even though we predicted relative humidity will drop by just a few percent, that’s enough to exacerbate the species’ current downward trajectory.  It’s a slow fall, but it’s still a fall.

 

Citations

Ulrey, C., Quiantana-Ascencio, P.F., Kauffman, G., Smith, A.B., and Menges, E.S.  2016.  Life at the top: Long-term demography, microclimatic refugia, and responses to climate change for a high-elevation southern Appalachian endemic plant.  Biological Conservation 200:80-92. (article page)

Species distribution models not for species

July 20th, 2016
SDMs - Not just for species

Mathematically these are all the same.

Have you ever see the number of people who drowned by falling into a swimming pool–films starring Nicholas Cage model?  You might also know it as “linear regression.”  Have you ever seen a species distribution model?  By calling it thus we make the same limiting semantic complexification as in the first case.

This post is not about the debate over whether we should be calling it species distribution modeling or ecological niche modeling (notice the participle form of each term–adding an ing refers to the act of modeling).  I’m talking about calling them species distribution models.

To put it bluntly, the underlying mathematics of a SDM doesn’t care whether it’s depicting a species or anything else.  In fact there are numerous examples of applications of species-less “SDMs”:

I even once met a person who uses Maxent to locate opportune places for underwater archaeology!

The fundamental commonality that allows all of these phenomena to be modeled by “SDM” algorithms is the nature of the response data–either unary (i.e., just presences) or binary (presences and absences).  Ergo, if you can describe a pattern with unary or binary data, you can also likely apply an “SDM”.

So feel free to refer to “my species distribution model” to reference your particular model of a species’ distribution.  But don’t let the moniker box you into thinking they’re just for species!

The fine limits of climate data

July 13th, 2016

Modeling Geum radiatum was very challenging compared to modeling I’ve done before because we know the species specializes on small habitats with microclimates.  And these particular microhabitats are are not well reflected by the coarse-scale climate data that is available.  I used ClimateNA for the basic climate layers. ClimateNA purports to be “scale free” but actually isn’t (and its creators acknowledge this).  Indeed, I found that the relative humidity measured by Eric’s team was far higher than the values predicted by ClimateNA for the same years and locations.  Ergo, ClimateNA (and other similar climate data sets) cannot really reflect microclimate on the order of of 100s of meters or less.

So how can one capture microclimate when one doesn’t have microclimate rasters? One option is to use microtopography, fine-scale measures of slope and aspect that hopefully help create microclimatic conditions relevant to the species.  This gives us the “topoclimate” of Guitiérrez Illán  et al. (2010).  So, I coupled fine-scale topography data at 90-meter resolution with climate data from ClimateNA at 1-km resolution.

Now the normal way to combine data sets of differing resolutions would be to resample the coarser-scaled rasters so they have the same resolution as the fine-scaled rasters.  But I felt this would create an illusion of precision in how well we know climate at fine scales.

So instead I wrote an R script to extract values from each data set in its native resolution and fed that to the predict() function.  Essentially, the script goes cell-by-cell across the fine-scaled topography raster stack and extracts the matching coarse-scaled data from the climate raster stack.  Then it writes a fine-scaled prediction raster from the ENM.

Geum radiatum (zoomed prediction)

Geum radiatum (zoomed prediction)

The output doesn’t look especially blocky when you see it in the publication (Ulrey et al. 2016), but if you look carefully at the zoom-ins you can indeed see the effect of matching coarse- and fine-scaled data sets. The larger reddish cells reflect appropriate macroclimatic conditions (i.e., at 1000-m scales), while the smaller dark red areas indicate appropriate macro- and topoclimatic conditions.

I admit the map isn’t visually pleasing–it’s obviously an artifact that the predictions are blocky.  But by keeping the data sets at their native resolution I believe the output better reflects the limit of our knowledge about the predictors, and in turn, the geographic realization of Geum‘s microclimatic niche.

Citations

Guitiérrez Illán, J., Gutiérrez, D., and Wilson, R.J.  2010.  The contributions of topoclimate and land cover to species distributions and abundance: Fine-resolution tests for a mountain butterfly fauna.  Global Ecology and Biogeography 19:159-173. (article page)

Ulrey, C., Quiantana-Ascencio, P.F., Kauffman, G., Smith, A.B., and Menges, E.S.  2016.  Life at the top: Long-term demography, microclimatic refugia, and responses to climate change for a high-elevation southern Appalachian endemic plant.  Biological Conservation 200:80-92. (article page)