It is common to assume that buildings are simply inanimate physical entities, relatively stable over time. This implies that there is little interaction between the triad of the building, what’s in it (occupants and contents), and what’s around it( the larger environment). We commonly see the overwhelming majority of the mass of material in a building as relatively unchanged physical material over time. In fact, the true nature of buildings can be viewed as the result of a complex set of dynamic interactions among their physical, chemical, and biological dimensions. Buildings can be described and understood as complex systems. Research applying the approaches ecologists use to the understanding of ecosystems can help increase our understanding. “Building ecology “ is proposed here as the application of those approaches to the built environment considering the dynamic system of buildings, their occupants, and the larger environment.
Buildings constantly evolve as a result of the changes in the environment around them as well as the occupants, materials, and activities within them. The various surfaces and the air inside a building are constantly interacting, and this interaction results in changes in each. For example, we may see a window as changing slightly over time as it becomes dirty, then is cleaned, accumulates dirt again, is cleaned again, and so on through its life. In fact, the “dirt” we see may be evolving as a result of the interactions among the moisture, chemicals, and biological materials found there.
Humans are covered with bacteria on all surfaces exposed to the environment around us – our skin, respiratory, and digestive tracks. Roughly 2,000 organisms occupy each square centimeter of these surfaces (roughly 15,000 organisms per square inch). We shed our outer skin layer each two weeks. The skin cells and the oils and other chemical in and on them as well the bacteria hitch-hiking a ride on them end up on the floor, furniture, and even the walls and windows.
When bacteria undergo the transformation from a nomadic life (in air) and become sedentary (settle on surfaces), “they undergo a reversible lifestyle switch.” They “…lose motility and become enclosed in a gooey extracellular matrix,” a kind of film on the surface. There they “sense” their neighbors in the “society” where they find themselves and develop specialized strains to take on different tasks in the community where they find themselves. These evolved bacteria secrete chemicals as part of the “community”  
Of course these chemicals are not occurring independent of the conditions surrounding them, the moisture, chemicals, and particles that are also on the surface or in the air immediately adjacent to it.
This type of diverse coating is present on virtually all indoor surfaces, and the particles, chemicals, and microbes that comprise it are in a sense, each a dynamic ecosystem.. While the window glass itself may remain largely unchanged by the processes on its surface, many other surfaces are not as stable. Flooring materials or floor coverings become worn over time, and this wear depends on their material composition, the use that is made of them as well as the maintenance they are given. The wear may result in release of chemicals and particles into the air, and some of these may end up on other surfaces such as the window.
While most of these processes may occur rather slowly, there are some processes that occur much more rapidly, especially those associated with human activities or ventilation with air from outdoors. Chemical interactions produce new chemicals, and moisture on many surfaces support the life, reproduction and evolution of microorganisms. The microorganisms themselves produce chemicals, some of which can alter the pH of the surface and subsequent surface chemistry.
Buildings are designed or intended to respond actively to some of these changes in and around them with heating, cooling, ventilating, air cleaning or illuminating systems. .We clean, sanitize, and maintain surfaces to enhance their appearance, performance, or longevity. In other cases, such changes subtly or even dramatically alter buildings in ways that may be important to their own integrity or their impact on building occupants through the evolution of the physical, chemical, and biological processes that define them at any time. We may find it useful to combine the tools of the physical sciences with those of the biological sciences and, especially, some of the approaches used by scientists studying ecosystems, in order to gain an enhanced understanding of the environments in which we spend the majority of our time, our buildings.
 John Harte suggests “…that particularity and contingency, which characterize the ecological sciences, and generality and simplicity, which characterize the physical sciences are miscible, and indeed necessary, ingredients in the quest to understand humankind’s home in the universe.” (Harte, 2002. Toward a synthesis of Newtonian and Darwinian World Views. Physics Today, October 2002.
 Roberto Kolter and E. Peter Greenberg 2006. The superficial life of microbes. Nature 443:300-302. 18 May 2006.
 Yumiko Sakuragi and Roberto Kolter 2007. Quorum-Sensing Regulation of the Biofilm Matrix Genes (pel) of Pseudomonas aeruginosa. Journal of Bacteriology. July 2007, p. 5383–5386 Vol. 189, No. 14.
 David G. Davies, Matthew R. Parsek, James P. Pearson, Barbara H. Iglewski, J. W. Costerton, E. P. Greenberg 1998. The Involvement of Cell-to-Cell Signals in the Development of a Bacterial Biofilm. Science 280: 295-298. 10 April 1998.