GEOG 442

Biogeography

Lab: Distribution of Taxa and Environmental Factors

The lecture introduced the concept of environmental gradients in which any environmental factor can be scaled from low to high and a species' abundance can be scaled from absent through limited to abundant. Any such gradient can be divided into five sub-ranges:

Lower zone of intolerance, beyond the species' ability to endure: The species is absent in such areas
Lower zone of physiological stress, in which the environmental factor is too low to promote healthy survival and reproduction: The species has a low population or sporadic individuals in the area
Optimum range, in which the factor is close to ideal for the promotion of survival and reproductive success: The species is abundant in such areas
Upper zone of physiological stress, in which the environmental factor is too high to promote healthy survival and reproduction: The species again has a low population or just a few individuals in the area
Upper zone of intolerance, in which the factor is now too high for the species to endure: There are no members of the species that can persist here

We also saw that species have multiple environmental factors important to their survival, for example, temperature (which can act several ways, e.g., average annual temperature, average annual range, average January and July temperatures, the highest daytime high and the lowest nighttime low, etc.), moisture (e.g., immersion in water, amount and variability of precipitation, ability to extract water vapor from the air), salt (e.g., highly tolerant of or requiring salt or intolerant of salt), acidity (e.g., some plants require acidic soils and others require basic soils), critical elements (e.g., nitrogen, phosphorous, potassium, selenium, copper, iron). Complicating the picture is the interaction possible among two or more of these factors: Sometimes abundance of one can partially offset shortages in another. An example discussed in class was the interaction between temperature and humidity, in which some species can tolerate drier conditions if the temperatures are low.

The interaction of two or even three such variables can be visually imagined as an X-Y or an X-Y-Z graph with the interacting tolerances shown as a kind of data cloud or envelope of interacting conditions. Past three of these and we are way beyond our own visual imaginations, but regression and factor analytic techniques, among others, can allow us to "see" such multidimensional interactions. Sort of.

Further complicating the already difficult-to-visualize picture is the observation popularized by Justus von Liebig: Liebig's Law of the Minimum. If several factors act to limit a species' productivity and, therefore, distribution, it is the one factor in limited supply (or out of the species' tolerance) that limits actual productivity and distribution. No chain is stronger than its weakest link. Von Liebig used the analogy of a badly made barrel that someone cobbled together with uneven staves (that represent one or another of the critical factors): Whichever stave (factor) is the shortest (most critical) is the one that determines the water level in the barrel (or productivity or yield):

[ Justus von Liebig's barrel made of staves of uneven lengths ]

Geographers get pretty excited when such concepts can be mapped. We saw how single limiting factors constrain the distribution of several species:

The northern boundary in North America of the eastern phoebe (Sayornis phoebe) during December and January coïncides pretty closely with the -4° C isotherm (see p. 59 in the MacDonald text or p. 53 in the Cox and Moore text)
The global distribution of palms (Arecaceæ aka Palmæ) corresponds roughly to the 15° C monthly mean minimum temperature isotherm (see p. 53 in MacDonald or p. 35 in Cox and Moore)
Spruce (Picea spp.) in Canada seem to coïcide roughly just over the 10° C July mean temperature isotherm to the north and the 17.5° C isotherm to the south (see p. 59 in MacDonald)
Piñon pines (Pinus edulis and P. monophylla) are desert upland species limited to stay above the 40 cm annual precipitation isohyet (see p. 62 in MacDonald)
Sugar maple (Acer saccarum) is restricted both by temperatures and by precipitation. The northern boundary closely coïncides with the -18° C mean January isotherm, while the southern border roughly follows the 27° C mean July isotherm. These interact with moisture, such that the western boundary is limited by dryness but cooler temperatures to the north mean that it can tolerate drier conditions, about 50 cm annual precipitation to the north while, in the hotter conditions of the south, it needs at least 100 cm annually.

Lab A: Figuring out the California Gnatcatcher's Distribution

The California gnatcatcher (Polioptila californica) is a small bird that is associated with coastal sage scrub in Southern California and northern Baja California.

The California gnatcatcher has been listed by the State of California as a Species of Special Concern and by the U.S. Fish and Wildlife Service as a Threatened species. In 1998, Patrick J. Mock wrote a study entitled, "Energetic constraints to the distribution and abundance of the California gnatcatcher," which was published in Western Birds in 1998 (Vol. 29, pp. 413-420). In it, he collected weather stations' data for mean January temperature in degrees C and mean annual precipitation in centimeters. He also noted whether the California gnatcatcher had a population reported in the vicinity. He presented these data in a scatterplot from which I have interpolated the original readings (there are probably minor rounding errors). The data table is located here: https://home.csulb.edu/~rodrigue/geog442/labs/gnatcatchertable.html. If you print it, be sure to do Print Preview and then select Landscape Orientation.

Construct a scatterplot of these data on the graph at https://home.csulb.edu/~rodrigue/geog442/labs/gnatcatchergraph.html. Again, be sure you format your printer for landscape orientation. For each station where the gnatcatchers have been reported, put a filled-in dark mark (perhaps a square or a diamond) above its January mean temperature and to the right of its annual precipitation. Do the same for each station not reporting gnatcatcher populations, but use an open mark (e.g., an open circle or triangle), or otherwise clearly differentiate them graphically.

Based on your graph, answer the following questions:

What is the lowest precipitation receipt for the stations reporting gnatcatchers?


__________ cm/year  What would that be in inches/year? __________  (divide cm by 2.54)

What is the lowest precipitation receipt for the stations not reporting gnatcatchers?
```
__________ cm/year   In inches/year? __________ 
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What is the highest precpitation receipt for the stations reporting gnatcatchers?
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__________ cm/year   In inches/year? __________ 
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What is the highest precipitation receipt for the stations not reporting gnatcatchers?
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__________ cm/year   In inches/year?  __________ 
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Compare, first, the two minima with one another and, then, the two maxima with one another. Which pair seems more conspicuously divergent? The more divergent pair is likelier to be a locally noticeable constraint on the species' range.
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_____ the two precipitation minima are more divergent  _____ the two precipitation maxima are more different
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What is the coldest average minimum temperature among those stations reporting gnatcatchers nearby? (usually happens in January)
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__________ ° C   What is that in ° F? __________  (multiply centigrade by 9, divide answer by 5, add that answer to 32)
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What is the coldest mean minimum temperature among those stations not reporting gnatcatchers nearby?
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__________ ° C   What is that in ° F? __________
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What is the warmest mean minimum temperature among those stations reporting gnatcatchers nearby?
```
__________ ° C   What is that in ° F? __________
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What is the warmest average minimum temperature among those stations not reporting gnatcatchers nearby?
```
__________ ° C   What is that in ° F?  __________
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Again, compare the minima and the maxima. Which pair is more divergent and likelier to be a more important driver of the species' range?
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_____ the minima are more divergent   _____ the maxima are more divergent
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Focussing only on the stations reporting gnatcatchers nearby, pencil in an ellipse around the group of stations doing so: You are trying to construct a two-dimensional envelope of environmental constraints on the gnatcatchers.
What seems to be the cutoff January mean temperature below which you don't find gnatcatchers (the leftmost boundary of the envelope ellipse)?
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__________ ° C   What would that be in ° F? __________ (round to one decimal place of  accuracy)
```

What seems to be the cutoff annual precipitation above which you don't find gnatcatchers?


__________ cm/yr   What would that be in in./yr? __________ (rounded to nearest one decimal place)

What seems to be the cutoff annual precipitation below which you don't find gnatcatchers, where it's too dry for them?
```
__________ cm/yr   What would that be in in./yr? __________ (rounded to one decimal place)
```

Why would birds be constrained by January temperatures? Enumerate at least two reasons (one direct and one indirect) that birds would "care" about precipitation (i.e., find their range constrained by it)? What is going on to explain the precipitation upper limit and the temperature lower limit here?


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Lab B: Mapping Potential California Gnatcatcher Habitat

Examine the following maps:

Southern California annual precipitation in inches
California annual precipitation in inches on a shaded relief map
Average minimum (January) temperatures in California in ° F
Blank relief map of Southern California, onto which you'll transfer your map of the constraints on California gnatcatchers' distribution

Try to infer where the minimum January temperature limit of the gnatcatcher falls and lightly pencil in that isotherm on the blank relief map. Since the maps are in English units (though the data were in metric), construct the using the isotherm in the English units into which you converted the answers in Lab A, where you estimated the lower temperature limit of the gnatcatcher. Tentatively pencil in that isotherm on the January average minimum temperature map.

Now, try to infer where the maximum precipitation tolerance for the gnatcatcher is found. Again, you'll be using the English version of your answer to map the critical isohyet. Now, pencil boundaries enclosing the areas wetter than that rounded maximum.

Do the same for the minimum precipitation tolerance, metric-English conversion, rounding, and all. On the isohyet map, pencil in the boundaries beyond which it's just too dry for the gnatcatcher.

By examining the critical isotherm and isohyets, you are in a position to identify the California gnatcatcher's potential or fundamental niche, at least with regard to just these two interacting environmental factors. The species' realized niche is going to be different, smaller than what the geography of its apparent tolerances would imply.

Now, using the blank relief map, transfer the critical isotherm and the critical isoyets, first, carefully in pencil. Only use Santa Barbara, Ventura, Los Angeles, Orange, and San Diego counties and a straight line going across San Bernardino County eastward from L.A. County's northern border and another straight line through western San Bernardino and Riverside counties due north of San Diego's eastern boundary. I don't want you worrying about Kern, Inyo, and San Luis Obispo counties or the vast eastern halves of Riverside and "San Berdoo."

When you're happy with your transferred isolines, shade in the parts of Southern California where the bird is not excluded by too cold January minimum temperatures or by excessive or insufficient precipitation. This will represent the gnatcatcher's potential habitat. There will be strong overlap between the temperature and the excessive precipitation areas, but there are a few small areas where it's one and not the other that excludes the bird.

By comparing your X-Y graphs of potential habitat, you'll find no sites outside the potential habitat that report gnatcatchers. You'll also find 4 sites that do seem to be within the potential habitat and yet the birds are not reported there: Alpine, Beaumont, Newhall, and San Jacinto. You can find them in Google Maps by typing their name, followed by "CA" and then scale it so you can see where it is at the scale of your paper map. Why do you suppose the potential habitat is not fully realized habitat for California gnatcatchers? If you're morbidly curious, you might try locating these on Google Maps, with "satellite" active. Alpine's a little weird, but the other three share common landscape issues that will tell you exactly what's happened to this bird's world.


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