Whether this difference in impact is due primarily to the lack of insect or other predators of the plant in North America, or whether the difference is due to a combination of factors, such as different agricultural practices, weather conditions, or competing species, is unclear (Selleck et al ., 1962).
(this difference in impact; is; due primarily to the lack of insect or other predators of the plant in North America) (the difference; is; due to a combination of factors, such as different agricultural practices, weather conditions, or competing species)
this difference in impact the difference
is is
due primarily to the lack of insect or other predators of the plant in North America
due to a combination of factors, such as different agricultural practices, weather conditions, or competing species