Step 1: Understand the causes of biodiversity loss.
- Over exploitation (A) leads to depletion of species due to excessive hunting or harvesting.
- Co-extinction (B) occurs when one species goes extinct, leading to the extinction of others dependent on it.
- Habitat loss (C) is the destruction or alteration of habitats, leading to a decline in biodiversity.
Step 2: Analyze forestation.
- Forestation (D), which refers to the creation of new forests, is not a cause of biodiversity loss. In fact, it can help increase biodiversity by providing new habitats.
Final Answer:
\[
\boxed{(D) \; Forestation}
\]