What are factors that may affect diversity levels in a given community?
Factors that may affect diversity in a community include the heterogeneity of the landscape and the age of the community and its environment.
Diversity can be defined in multiple ways. Assuming a general definition of diversity, as opposed to genetic diversity for example, there are multiple factors that may affect the diversity of a community.
The heterogeneity of the landscape and the age of the community and its environment may affect diversity.
The diversity in the landscape or habitat itself is going to affect diversity. If you have a habitat that is continuously boreal forest, with the same species distribution and composition throughout, there isn't the opportunity for much variability. If you have a habitat that has multiple habitat types, there's the opportunity for more ecological niches. Different species can occupy different niches and adapt specifically to them.
The diversity of the community below is going to be higher than one that was only made up of dense shrubs:
Older communities and landscapes may be more diverse because they've had time to evolve and adapt according to the specifics of their environment.
Diversity is a complex topic though and multiple factors affect it. If we had two communities that were identical, but we introduced a disturbance to one community, that community may be more diverse. Disturbances often, temporarily at least, increase diversity because they increase the number of ecological niches to be filled. It's worth remembering that diversity is a complex issue that scientists are still studying.