Here’s the first chapter following the prologue in the draft of the urban ecosystems book.
First Street
Systems ecologists operate like engineers let loose on the natural world [1]. We like to envision things as storage boxes, but our boxes have machines inside. Matter and energy go into the box, are transformed by the machine, some is tucked away for later, some is sent back out. Think of a “thing”, say, a dog. Food and water in, heat, feces, urine, and, (often what seems like too much), energy out. To a systems ecologist, a dog is a storage box (for dog flesh and bones) with a food transformer inside.
It is no random coincidence that the field of systems ecology is related to engineering. Systems ecology was born with the enormous engineering feats of the nuclear weapons programs of World War Two and the Cold War. One outcome of nuclear and thermonuclear detonations was the sudden appearance of otherwise rare radioactive materials in the environment. These chemicals acted as radiotracers—they could be tracked through the ecosystem. Known half-lives and other chemical properties allowed us to quantify biological processes that had previously been intractable. Suddenly, we could see the biological world as an organized, interactive, hierarchical, integrated system. Cells, organisms, populations of organisms, communities of populations all had a place in this new view of the world. The term ecosystem, coined in the 1930s, applied to the relationship of organisms and their physical and chemical environments [2]. By the 1950s, it was clear that ecosystems had far broader meaning and many more dimensions.
This was something of a shock. Until that time, ecologists operated in subfields—plant or animal, fish or mammal, worm or microbe. And our primary tool was counting. How many animals or plants (or species of mammals or worms or…) are found in this habitat? If the habitat is changed, how do the numbers of species and numbers of individuals of each species change? Now we were thrown into a world where deconstructing biology into component parts was inadequate study design.
We no longer did ecology by enumeration. In addition to radiotracers, we had begun to use calorimetry—measurement of energy content—to explore ecosystems. We realized that physical laws—the laws of thermodynamics—applied not just generally, they applied very specifically to the living world. We could track the flux of energy through ecosystems, from sun to plants to herbivores to carnivores. We were able to understand, as one ecologist put it, “why big fierce animals are rare” [3]. In part it is because energy transfer is inefficient. In moving from plants at the bottom to carnivores at the top of the food chain, roughly 50% to 90% of energy potentially available is lost at each transformation. There is a “heat penalty” paid when matter or energy transforms. Heat is the great energy sink of the universe. To get a unit of energy from the next step down in the food chain, you have to invest half or more of the energy to pay that penalty. Physics places fundamental constraints on biology.
And puts new tools in the scientific toolkit. Input/output analysis, where we learn about the machine-in-the-box by measuring what happens to ins when they come out. Feedback quantification, finding processes in the box that increase (positive feedback) or decrease (negative feedback) inputs. Forcing functions, parameters that change the system from the outside.
In research labs in the 1960s and 1970s, there were ongoing debates over what constituted a proper ecosystem. We argued over endless pizzas and pitchers of beer. Ponds and lakes were ecosystems, we agreed. They had definable boundaries, and processes in the ecosystem (under water) were different from those outside. Could a mud bar where a stream entered a pond be an ecosystem? It wasn’t clear. A forest watershed could be an ecosystem. Could a standing dead tree be one? How about a fallen tree trunk on the forest floor? An area of wind-blown dune in a dry desert? The arctic tundra is an ecosystem. Is an iceberg?
To this day, there is no generally-agreed definition of an ecosystem. This is probably as close as possible: an ecosystem is an area with identifiable boundaries, with processes more active within and beyond than across the boundaries. The boundary acts as a filter, the environment within the boundary differs in measurable ways from that outside.
This is a very empirical definition. And it is not foolproof. A single cell could be an ecosystem under this definition, but is generally not thought of as such. However, a road-killed opossum, decomposing on grassy shoulder, probably is. Ponds, lakes and watersheds? For sure. Cities? Definitely.
Notes
[1] Parts of this chapter appeared in different form in Ludwig and Iannuzzi 2011.
[2]
[3]
No comments:
Post a Comment