  Theoretical data obtained from physically sensible field and string theory
models suggest that gravitational Effective Field Theories (EFTs) live on
islands that are tiny compared to current general bounds determined from
unitarity, causality, crossing symmetry, and a good high-energy behavior. In
this work, we present explicit perturbative and nonperturbative $2 \to 2$
graviton scattering amplitudes and their associated low-energy expansion in
spacetime dimensions $D\geq 4$ to support this notion. Our new results include
a first nonperturbative example consisting of a $D=4$, $\mathcal{N}=1$
supersymmetric field theory that is coupled weakly to gravity. We show that
this nonperturbative model lies on the same islands identified using
four-dimensional perturbative models based on string theory and
minimally-coupled matter circulating a loop. Furthermore, we generalize the
previous four-dimensional perturbative models based on string theory and
minimally-coupled massive spin-0 and spin-1 states circulating in the loop to
$D$ dimensions. Remarkably, we again find that the low-energy EFT coefficients
lie on small islands. These results offer a useful guide towards constraining
possible extensions of Einstein gravity.
