New properties of the first‐order Riemann–Liouville fractional integrals are proved and first‐order fractional derivatives for the equivalence class of functions in
are analyzed. Solutions, normal solutions, and generalized normal solutions of the initial value problems (IVPs) for nonlinear first‐order fractional differential equations (FDEs) with fraction
are introduced. The equivalences in
between the first‐order FDEs and the corresponding (Volterra) integral equations are established based on normal solutions and generalized normal solutions. New results on the existence and uniqueness of generalized normal solutions or nonnegative generalized normal solutions in
of the IVPs are obtained for
. These results are applied to study the existence and uniqueness of generalized normal nonnegative solutions in
of the IVP for the nonlinear first‐order FDE with nonlinearities related to the population models with growth rates of Ricker type.