The Stacks project

Proof. A base change of a locally separated morphism is locally separated, see Morphisms of Spaces, Lemma 67.4.4. Hence the direct implication in Definition 74.10.1.

Let $\{ Y_ i \to Y\} _{i \in I}$ be an fppf covering of algebraic spaces over $S$. Let $f : X \to Y$ be a morphism of algebraic spaces over $S$. Assume each base change $X_ i := Y_ i \times _ Y X \to Y_ i$ is locally separated. This means that each of the morphisms

is an immersion. The base change of a fppf covering is an fppf covering, see Topologies on Spaces, Lemma 73.7.3 hence $\{ Y_ i \times _ Y (X \times _ Y X) \to X \times _ Y X\}$ is an fppf covering of algebraic spaces. Moreover, each $\Delta _ i$ is the base change of the morphism $\Delta : X \to X \times _ Y X$. Hence it follows from Lemma 74.12.1 that $\Delta$ is a immersion, i.e., $f$ is locally separated. $\square$