While an MPA or an NTR may have a defined boundary, those limits have not been, shall we say, "communicated" to the marine life below the surface and so spatial density, or spillover as it is sometimes called, becomes a critical component. A healthy zone that generates populations of species that extend beyond its borders and provides a reasonable commercial yield, does not do so in a vacuum. There must be a proper flow of incoming influences including plankton, coral, and fish larval stages and other biosystem factors - all of which pay no attention to a zone's arbitrary boundaries.
While preliminary results appear positive, there is a considerable amount of challenging research taking place:
- Studying the impact of political/public use influences on the size (reduction) of a zone versus initial environmental recommendations (preliminary research indicates the negative effect is disproportionately larger than the amount of size reduction).
- Researchers are often challenged by a lack of extensive baseline studies of ecosystems prior to the zone for use in evaluating against post-zone studies.
- Much research needs to be done to document the relationship/effect of multiple MPAs or NTRs and how they interact with each other.