Spatial memory requires new nerve cells.

At least this appears to be the case in mice. Here is the abstract from Dupre et al.

The dentate gyrus of the hippocampus is one of the few regions of the mammalian brain where new neurons are generated throughout adulthood. This adult neurogenesis has been proposed as a novel mechanism that mediates spatial memory. However, data showing a causal relationship between neurogenesis and spatial memory are controversial. Here, we developed an inducible transgenic strategy allowing specific ablation of adult-born hippocampal neurons. This resulted in an impairment of spatial relational memory, which supports a capacity for flexible, inferential memory expression. In contrast, less complex forms of spatial knowledge were unaltered. These findings demonstrate that adult-born neurons are necessary for complex forms of hippocampus-mediated learning.

(More specifically, the experiments involved generating transgenic mice that selectively overexpressed the pro-apoptotic protein Bax in neural precursor cells in an inducible manner. Overexpression of Bax removed newly born cells in the adult dentate gyrus and caused a strong deterioration in the relational processing of spatial information in the Morris water maze. Animals were unaffected when tested on simpler forms of spatial knowledge; nor were they affected in tasks where memory could be acquired without the hippocampus.)