Most folks tend to think of "sustainable" in environmental terms, such as reducing energy and vehicle driving and promoting walkable neighborhoods. There are many examples with many of the advanced ones being in Western Europe. Others view it as an economic plan, such as for improved small town and rural areas, such as for expanded education and health clinics in Pennsylvania and Colorado. And for a coastal resort area such as South Padre Island, some would view it in the context of sustainable geo-tourism as opposed to mega-developments and high rises.
The concept of sustainability is simply one that says that rapid growth is not as important as being able to weather adverse conditions over the long term. A town that was built in a boom and became a ghost town was obviously not "sustainable." It's a fascinating topic and there is a ton of information about the subject, but nothing that is really comprehensive - which is why I like to write about it sometimes.
In classical Von Thunen terms, a city or town was a circle in the middle, surrounded by a fringe and then rural lands; the rural lands were cheaper so that is where the farming occurred. As cities grew with population increases, the cities expanded outward along with the fringe that pushed farm and ranchland further away from the city. This theory is still very much alive today with concepts such as a downtown, city limits, an ETJ, and surrounding areas that were under the control of counties, states, or the federal government. It meant a need for constant annexation as far as one could go - in the American West, seemingly forever.
The reason for annexation is because people who lived on the fringes enjoyed some of the benefits of the city, and therefore should eventually be taxed for that enjoyment. Plus in many cases, if the central city did not absorb those lands, other cities would claim the land and thus tax the residents and businesses, a net loss in revenue. Nowhere is the situation more evident than the cities of Port Isabel and Brownsville annexing all the land around South Padre Island.
If you look at sustainable areas such as those in Europe, population growth is fairly flat and the cities have grown all they can because of boundary issues. Our little SPI is remarkably similar, except for a few miles that can be developed and annexed to the north. However, our economic view even here on SPI requires growth for its future, a contradiction in terms. I'll write more about that in the next installment.