Different years - the environmental conditions at the time of the photography (wet vs. drought years, flooding, etc.) may affect the densities seen during the on-site field visits.
(the environmental conditions at the time of the photography (wet vs. drought years, flooding, etc.; may affect; the densities seen during the on-site field visits) (the densities; seen; T:during the on-site field visits)
the environmental conditions at the time of the photography (wet vs. drought years, flooding, etc. the densities
may affect seen
the densities seen during the on-site field visits
T:during the on-site field visits