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recap

53.5 recap

Species–area relationships, in which species diversity increases with increasing area, have been shown for islands and islandlike habitats. Smaller islands have higher extinction rates and lower immigration rates compared with larger islands. Earth’s biomes are becoming smaller and more fragmented by human activities, leading to lower diversity in the remaining habitats.

learning outcomes

You should be able to:

  • Explain the species–area relationship and use a conceptual model to describe it.

  • Describe specific ways in which habitat fragmentation negatively affects species diversity, using the Amazon rainforest as an example.

1.

Using Figure 53.17, explain how and why the small and far islands have lower species numbers than the large and near islands.

The premise of island biogeography is that the number of species on an island represents a balance between the immigration rate and the extinction rate. A far-away island will have fewer immigrants than an island close to the species pool. In addition, a smaller island will have fewer resources, and thus sustain fewer species and have higher extinction rates. Larger islands provide greater resources and can sustain larger populations (which tend to have lower extinction rates than small populations).

2.

Why is habitat fragmentation by human activities threatening species diversity? Besides ceasing the activity altogether, what can be done to reduce this threat?

Habitat fragmentation essentially creates isolated islands of habitat surrounded by unsuitable habitat. The smaller and more isolated the habitat fragment, the fewer species that can be supported within the habitat and the less likely it is that species can move to more suitable fragments. In addition, the fragment can be exposed to edge effects—hazards outside the habitat fragment that effectively reduce its size. Leaving habitat fragments that are large enough to sustain species can reduce the effects of habitat fragmentation or by creating corridors that species can use to move safely from one fragment to another.