Given the task definition and input, reply with output. In this task, you're given passages that contain mentions of names of people, places, or things. Some of these mentions refer to the same person, place, or thing. Your job is to write questions that evaluate one's understanding of such references. Good questions are expected to link pronouns (she, her, him, his, their, etc.) or other mentions to people, places, or things to which they may refer. Do not ask questions that can be answered correctly without understanding the paragraph or having multiple answers. Avoid questions that do not link phrases referring to the same entity. For each of your questions, the answer should be one or more phrases in the paragraph, and it should be unambiguous.

Passage: The red-necked grebe has a large range, estimated at 1–10 million square kilometres (0.4–3.8 million square miles), and a global population of 150,000–370,000 individuals, with P. g. holboellii about twice as numerous as the nominate race. The population trend has not been quantified, but it is not believed to meet the thresholds for the population decline criterion (declining more than 30% in ten years or three generations) of the IUCN Red List. For these reasons, the species is evaluated as Least Concern.The red-necked grebe is one of the species to which the Agreement on the Conservation of African-Eurasian Migratory Waterbirds (AEWA) applies. Parties to the Agreement are required to engage in a wide range of conservation strategies which are describes in a detailed action plan. The plan is intended to address key issues such as species and habitat conservation, management of human activities, research, education, and implementation.The Red-necked Grebe was hunted by humans in northern Europe in the Mesolithic and Paleolithic periods, but there is no evidence that there is any significant level of hunting at the present time. In North America, there are potential threats from pollutants such as polychlorinated biphenyls (PCBs) and pesticides such as DDT which cause reduced reproductive success due to egg sterility and eggshell thinning. Breeding areas may be threatened by the modification and degradation of lakes and by human disturbance from water-based recreational activities. There is no evidence to suggest that these threats could result in a significant risk to the overall population; more than 70% of North American red-necked grebes breed in Canada, where the population is stable or increasing.
What is P.g. holboelli twice as numerous as?