GMATGuruNY wrote:

According to some botanists, invasive plants are the second most serious threat, after
habitat loss, to native species of plants and animals and to the maintenance of
biologically diverse ecosystems.

A. threat, after habitat loss, to native species of plants and animals and to the
maintenance of biologically diverse ecosystems
B. threat, after habitat loss, to native species of plants and animals and for
maintaining biologically diverse ecosystems
C. threat, after losing their habitat, to native species of plants and animals and
also to maintenance of biologically diverse ecosystems
D. threat to native species of plants and animals and for maintaining biologically
diverse ecosystems, after habitat loss
E. threat to native species of plants and animals as well as to maintaining
biologically diverse ecosystems, after losing their habitat

B and D: the second most serious threat...FOR maintaining biologically diverse ecosystems
Here, threat FOR is unidiomatic.
The correct idiom is X is a threat TO Y.
Eliminate B and D.

C: invasive species are the second most serious threat, after losing their habitat
Here, their seems to refer to invasive species, implying that INVASIVE SPECIES are losing their habitat.
The intended meaning is that NATIVE SPECIES lose their habitat.
Eliminate C.

E: maintaining biologically diverse ecosystems, after losing their habitat
Here, their seems to refer diverse ecosystems, implying that DIVERSE ECOSYSTEMS are losing their habitat.
The intended meaning is that NATIVE SPECIES lose their habitat.
Eliminate E.

The correct answer is A.

alanforde800Maximus wrote:Hello Mitch,

It's "Invasive plants" and not invasive species".

GMATGuruNY wrote:

According to some botanists, invasive plants are the second most serious threat, after
habitat loss, to native species of plants and animals and to the maintenance of
biologically diverse ecosystems.

A. threat, after habitat loss, to native species of plants and animals and to the
maintenance of biologically diverse ecosystems
B. threat, after habitat loss, to native species of plants and animals and for
maintaining biologically diverse ecosystems
C. threat, after losing their habitat, to native species of plants and animals and
also to maintenance of biologically diverse ecosystems
D. threat to native species of plants and animals and for maintaining biologically
diverse ecosystems, after habitat loss
E. threat to native species of plants and animals as well as to maintaining
biologically diverse ecosystems, after losing their habitat

in c, "their habitat' is problematic. pronoun refer to a noun in preceding phrase and seldom refer to a noun in the following phrase. "their " is INFERIOR already because we can think "their" refer to "plants" .
"their" if refering to "native species' is also redundant. "losing their habitat is a threat to species". we do not need "their". redundant is flagrantly wrong on gmat.
"losing their habitat" is not parallel to "invasive plants". "losing is a gerund , and "plants " is noun. this is inferior. inferior pattern can be correct if there is no better choice, but we have better choice A, in which "plants " is parallel to " "maintenance ". so, this inferior is wrong.

in D and e, "after losing/loss" is far from "threats" though it work as modifier of "threat" . this is wrong.
for maintaining/to maintaining is not parallel to "plants'. the first is gerund, the second noun.