  Owing to their isotropy, it is generally believed that ultrahigh energy
cosmic rays (UHECRs) are extragalactic in origin. It is then expected that
interactions of these cosmic rays with photons of the cosmic background
radiation (CBR) should produce a drastic reduction in their flux above and
energy of about $5 \times 10^{19}$ eV (50 EeV), the so-called ``GZK effect''.
At present, the existence of this effect is uncertain owing to conflicting
observational data and small number statistics. We show here that a small
amount of Lorentz invariance violation (LIV), which could turn off photomeson
interactions of UHECRs with the CBR, could explain the UHECR spectrum as
measured by {\it AGASA} which shows an excess of UHECRs at energies above 100
EeV. If new results from the {\it Auger} array agree with the {\it AGASA}
spectrum, this may be interpreted as evidence for a small amount of LIV. If, on
the other hand, the new results are consistent with the {\it HiRes} results
favoring a GZK effect, this would place severe constraints on LIV and, by
implication, on some Planck scale quantum gravity models. We also discuss the
power requirements needed to explain the UHECR spectrum for a range of
assumptions, including source evolution and LIV and show that in all cases our
results disfavor a $\gamma$-ray burst origin for the UHECRs.
