SCIENCE LITERACY WORKING WITH DATA

Habitat loss is currently the main driver of species endangerment and extinction, but habitat loss need not be complete to cause a problem; habitat fragmentation may also be an insurmountable problem for some species. Islands that are created when a river is dammed to form a reservoir provide instant habitat fragments. Luke Gibson and his team evaluated the number of small mammal species in large (10-56 hectares [25-140 acres]) and small (<10 hectares [<25 acres]) forested islands in Chiew Larn Reservoir of Thailand. Island sampling was done shortly after the reservoir was formed (about 6 years after isolation); the islands were sampled again about 26 years after isolation. Their results are below. (For comparison, on average, nine species were found on mainland (pre-reservoir) plots; the richness did not change in this mainland forest over the study period.)

Number of Small Mammal Species (Richness) Found on Islands 6 and 26 Years after Isolation

Interpretation

Question 12.17

Before evaluating the data, draw a graph that compares species richness of large islands 6 years after isolation versus 26 years after isolation and that also shows the same for small islands. (Hint: Calculate the average species richness values for each group and draw a bar graph that allows you to directly compare the richness of large islands after both sampling periods to the richness of small islands after both sampling periods.)

There are various ways to show this data. A bar graph that plots species richness on the y-axis for the two time periods (6 and 26 years) and shows 2 bars (one for large islands and one for small islands) would allow direct comparison of the total number of species in each time period for each island size. The authors of the paper show it as a box plot (see: http://www.cfbiodiv.org/userfiles/2013%20Near-Complete%20Extinction%20of%20Native%20Small%20Mammal%20Fauna%2025%20Years%20After%20Forest%20Fragmentation.pdf )

Question 12.18

Consider that species richness before isolation was nine. How does the species richness compare in large islands before isolation (use “mainland” data), 6 years after isolation, and 26 years after isolation? How does it compare for the small islands over those two sampling periods?

Species richness did not drop in the initial 6 years after isolation on large islands but it did drop dramatically from an average of 9 to 3 in the final 20 years (26 years after isolation). It dropped much faster on small islands which lost 78% (7 out of 9) of their species in the 1st 5 years. Richness was then halved (from 2 to 1) in 20 years that followed the 1st sampling period.

Advance Your Thinking

Question 12.19

What might lead to the difference in species richness losses in large islands compared to small islands?

The starting populations on smaller islands might have been smaller with less genetic diversity, making them more vulnerable to extinction. Fragmentation into a smaller habitat might also have reduced the availability of needed resources such as food.

Question 12.20

On all of the islands, the most common (and sometimes only) small mammal 26 years after isolation was a non-native rat. Could this have had an influence on the loss of the other, native species? Explain.

Yes — if this non-native rat was a better competitor or more adaptable, it could have driven the other species to extinction. Since it is the sole remaining species on many of the small islands, it is likely that it played a role in the extermination of the other species.

Question 12.21

What conclusion can be drawn regarding the value of leaving behind fragmented forest landscapes for protecting species that live in the habitat fragments?

The size of the fragment is important in protecting biodiversity and smaller fragments may not be large enough to be of any value. Larger habitat areas will be needed to protect many species from extinction.