The blank should be filled with:
Preventable
Here's why:
* The statement emphasizes that researchers and conservationists believe biodiversity loss due to invasive species can be prevented.
* The example provided further strengthens this point by highlighting simple actions individuals can take to mitigate the introduction of invasive species.
* Using "preventable" aligns with the overall message of taking proactive measures to address the issue.
Therefore, "preventable" is the most appropriate word to fill the blank in the context of the sentence.