Question: a species that has an essential role in maintaining ecosystem structure is called a
[{'answer': 'A keystone species', 'provenance': [{'wikipedia_id': '230342', 'title': 'Keystone species', 'start_paragraph_id': 1, 'start_character': 0, 'end_paragraph_id': 1, 'end_character': 18, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '230342', 'title': 'Keystone species', 'start_paragraph_id': 1, 'start_character': 0, 'end_paragraph_id': 1, 'end_character': 589, 'bleu_score': 0.9623384567935761, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 4312229711431475320}, 'section': 'Section::::Abstract.'}]}]