Week 8: Assembly of Continental Biota

BROWN CHAPTER SIX

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Chapter six discusses patterns of geographic range. I summarize the chapter in semi-bullet point format below.

There are three general constraints to range limits:

1) Maximum size is limited by available space (a.k.a. a North American terrestrial organism is limited by the size of the continent)

2) An apparently constant minimum population density as illustrated in figure 5.7a

3) The relative limits associated with the niche requirements of a species

Species with large ranges tend to be more abundant within their ranges than species with smaller ranges.

Body size constrains geographic range because body size constrains abundance: species with large body size have lower abundance.

However, small populations are generally at greater risk of extinction, so large animals tend to have larger ranges. Most large animals with small ranges are considered threatened or endangered.

There appears to be a limit on minimum range size, and this seems to positively correlate with body size.

Rapoport (1982) found that there is a consistent perimeter to area ration of animal ranges of 10, suggesting some fractal properties of range size.

For North American vertebrates, smaller ranges tend to be larger north-south and larger ranges tend to be larger east-west. This is likely related to the north-south orientation of most North American mountain ranges, river valleys, and coastlines. There is an opposite trend in Europe where major land formations run east-west.

Rapoport’s rule: the bigger the average latitude of a species’ range, the larger the latitudinal range.

Similarly, the higher the center of the elevational range, the larger the elevational range.

There are several hypotheses for the two above trends:

1) The North American continent is narrows as lower latitudes, “squishing” habitats and associated organisms’ ranges.

2) Lower latitudes are generally more stable and abiotically favorable. This does not restrict this hypothesis to North America.

3) Because lower latitudes have less yearly temperature variation, organisms are more specialized to specific climates. Therefore, elevational gradients on mountain ranges provide a greater barrier to population movement. (Mountains are “higher” in the tropics)

4) Climatic variation in the Pleistocene has resulted in species that are adapted to climate variation. Because organisms at higher latitudes have been exposed to more climate variation, they are more adapted to climate variation, and therefore occupy habitats with a wider range of climates.

5) Diversity increases at lower latitudes. Therefore there is more competition at lower latitudes and species are restricted to the habitats that they are most adapted to. At higher latitudes, there is less competition so species can occupy areas at the limits of their ideal habitats.

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Spatial Response of Mammals to Late Quaternary Environmental Fluctuations

Graham et. al.

Science vol. 272 no. 5268 p. 1601-1606

June 14, 1996

This study looks at general patterns of range change in mammals in the Holocene and Pleistocene. Generally, species’ ranges moved north or south in correlation with major warm and cool periods, but different species ranges changed at different rates and not necessarily to the same degree. This means that community species compositions were not consistent.

Of the species studied, the nine-banded armadillo, northern pygmy mouse, hispid cotton rat, meadow jumping mouse, and least weasel still seem to be adjusting their ranges to the current climate.

Some species moved longitudinally instead of latitudinally in response to postglacial warming: the least shrew and other moved east, and the northern pocket gopher and others moved west.

Other species do not appear to have undergone any substantial range changes in the last 20,000 years including the eastern woodrat, and the northern pygmy mouse. These species tend to live in the southern United States.