Internationally speaking, both natural parks and natural monuments are defined as protected areas, which, according to its current definition from the International Union for Conservation of Nature (ICUN), means:
A protected area is a clearly defined geographical space, recognised, dedicated and managed, through legal or other effective means, to achieve the long term conservation of nature with associated ecosystem services and cultural values.
The ICUN states that this is the most widely used definition internationally. Indeed, any protected area listed on the World Database of Protected Areas must comply with it.
Moreover, the ICUN divides protected areas into 7 categories, in which we can find natural parks and natural monuments:
- Category Ia — Strict Nature Reserve
- Category Ib — Wilderness Area
- Category II — National Park
- Category III — Natural Monument or Feature
- Category IV — Habitat/Species Management Area
- Category V — Protected Landscape/Seascape
- Category VI – Protected Area with sustainable use of natural resources
These categorical distinctions provide guidelines to the purpose and regulation to be applied to different areas. Quoting from the ICUN website:
II: National Park
Similar to the objectives of Wilderness Areas, National Parks provide protection for functioning ecosystems, but tend to be more lenient with human visitation and the supporting infrastructure. National Parks are managed in a way that may contribute to local economies through promoting educational and recreational tourism on a scale that will not reduce the effectiveness of conservation efforts. The surrounding areas of a National Park may be for consumptive or non-consumptive use, but should nevertheless act as a barrier for the defence of the protected area's native species and communities to enable them to remain sustainable in the long term.
III: Natural Monument or Feature
These are comparatively smaller areas that are specifically allocated to protect a natural monument and its surrounding habitats. Natural Monuments or Features can be natural in the wholest sense, or include elements that have been influenced or introduced by humans. The latter should hold biodiversity associations or could otherwise be classified as a historical or spiritual site, though this distinction can be quite difficult to ascertain. As such,The classification then falls into two subcategories, those in which the biodiversity in uniquely related to the conditions of the natural feature, and those in which the current levels of biodiversity are dependent on the presence of the sacred sites that have created an essentially modified ecosystem. Natural Monuments or Features have a high cultural or spiritual value which can be utilised to gain support for conservation challenges.
USA National Monuments and Parks
According to Wikipedia, in the USA the difference between a national park and a national monument lie in the fact that the former can be declared by the President of the USA, without him having to seek the approval of Congress.
In terms of the general general regulatory differences between monuments and parks in the USA, I think that a a single answer cannot be formulated. This is partly due to the fact that protected areas in the USA are not managed by a single entity. Indeed some are county-, other are state-managed. Moreover, some protected areas are used for commercial exploitation. Here is a bit more info on protected areas in the USA. Bottom line is that different areas have different rules, so always check in advance.
As a final thought I would say that the USA regulations tend to follow the guidelines set by the ICUN. For example, this is true for national parks, which are often not only open to tourists but also "exploitable" (fishing, camping, etc.).