A community is held together by shared values, beginning with the lives of its members, and extending to what makes those members happiest. Happiness (and survival) requires resources, which, while being consumed, are not available to anyone – or anything – else.
If the definition of the community is extended to include more members, the community must either make more resources available (such as acquiring the resources already used by the new members), get more use out of the resources already consumed (increase efficiency), or settle for a lower level of happiness. If the amount of happiness is increased, statistical trends indicate that consumption – and the amount of available resources – will need to increase exponentially to compensate.
If the community defines itself more restrictively, then it may view former members as competitors or even resources to be "consumed." This could result in an increase in happiness, an increase in longevity (the amount of time that resources can last), or both, by decreasing the happiness and longevity of the members it shed. A more humane way to shed members is to send them to other areas, voluntarily or otherwise, which has the potential benefit of providing access to additional resources.
Changing values is especially costly if a community reclassifies someone or something it typically consumes (uses and throws away) as a member. Slaves and species used for food are obvious examples. The community's resource base must be redefined in fundamental ways that can fundamentally change how the community deals with itself and its environment. What was good is now evil, and what could be counted on to enhance happiness must either be replaced or happiness must be reduced. Understandably, a reduction in happiness (which is proportional to life expectancy) would be considered unacceptable by many people, so we could expect a lot of resistance to a such a change in values.
With humanity's ecological impact rising so high that it threatens the habitability of our planet, our values must change in such a way as to reduce that impact to a safe level, and this must happen soon – in much less than 20 years. Arguably, all of the alternatives are already being tried. Environmentalists are promoting the redefinition of community to include more of the biosphere (other species), while limiting population growth as a multiplier on individual impact/consumption; this potentially has the additional benefit of growing the resource base, especially when combined with efficiency-increasing measures. Those who favor restricting communities are promoting the "shedding" of members of the population, most without acknowledging the threat, by favoring perpetual warfare and declining working conditions as "others" either become slaves or die. Space enthusiasts are pushing for the settlement of other planets, which is the equivalent of humane population-shedding – though most of the places people might go are uninhabitable wastelands, just as the Earth is in danger of becoming.
Given the urgency of the task of reducing our ecological impact, the first two options have the best chance of working, while the third (space settlement) is at best a desperate insurance policy against total extinction. If all human life is valued, the second option must be ruled out, though it is arguably built into human nature (and some humans more than others). Unfortunately, the environmentalist's approach requires the most change, and is therefore prone to facing the most resistance – which it is getting.
For those of us who favor an emphasis on the environmentalist's approach, reducing resistance to it is critical. Based on this discussion, it should be clear that attempting to counter the resistance without addressing the associated values is likely doomed to failure.