If there is judgement and creativity used in the selection of the list, that selection is protectable by copyright under US law, and under the laws of many other countries. The standards vary, but in most cases the degree of creativity required is low.
In the US the "time and effort" put into the list is not relevant, it is the degree of originality and creativity that matters. This is not true in all countries.
If the list includes comments on individual list items written by the list creator, those comments would normally be protected by copyright, even when the list as a whole is not.
If the list is ordered in some creative way, the ordering would also be protected, such as dividing the list into categories. an "obvious" ordering such as alphabetical is not subject to copyright protection.
However, that these various web sites exist at particular addresses is a fact (or a set of facts) and facts are never protected by copyright. If someone produces a different list, that includes many but not all of the same sites, and some other sites, it is probably not infringement even if your list was used as one source for the newer list.
If your list makes a serious attempt to include every nature website, and no selection is done, there is actually less creativity in the selection than if some judgement is used on what to include and what to omit. In that case, another list that also includes every nature website may not be considered an infringement.
See Feist Publications, Inc., v. Rural Telephone Service Co., 499 U.S. 340 (1991) in which one company copied a telephone book published by another company. Because there was no selection (everyone in the designated area was included) and the order was obvious (alphabetical), the US Supreme Court rules that there was no originality, and there fore no copyright protection in the book, and copying it was not copyright infringement. Feist is now a key case on copyright law in the US, and is followed in some other countries.