Answers to Analyzing Data 18.1 Questions

1. There was a steeper slope (z) and lower y intercept (c) for the species­area relationship of invaded communities compared with uninvaded communities. These results suggest that invaders have strong negative effects on species richness at the smallest spatial scales and little or no effect at large spatial scales.

2. To convert log x values to x values, solve for 10^x. The approximate range of area values is from (at the smallest scale) 1 m² to (at the largest scale) 500 m². The approximate range in species richness for invaded plots is 0.6 (smallest scale) to 16 species (largest scale). For uninvaded plots, it is 3 (smallest scale) to 20 species (largest scale).

3. One hypothesis is that the invaded areas turn into island-like habitat where native species occur within a sea of invaders. As we saw in the example in Ecological Toolkit 18.1, island-like systems tend to have steeper slopes and lower y intercepts than mainland-like habitats. The equilibrium theory of island biogeography posits that smaller areas and those more distantly connected to the sources of species will have higher extinction rates and lower immigration rates and thus fewer species per given area. At the largest spatial scale, however, even though invasions may have negative effects on native species, the area may be large enough that immigration of species from uninvaded areas may rescue species from extinction. At some point, though, if large enough areas are invaded and/or immigration from uninvaded areas ceases, one could see a decline in species richness.