Well, my experience has been that my system is remarkably stable, unless it isn't. And when it wasn't, I couldn't really pin-point why.
I have a couple of analogies from ecological science - the first comes from resilience theory which proposes that 'functioning' ecological systems have a series of feedback loops in place that resist perturbation. These perturbations can come in many forms - but most of them can be boiled down to species composition and nutrient cycling. The prime example cited in the literature are clear, low dissolved nutrient, high productivity, benthic lake ecosystems dominated by substrate-bound vascular plants. They adaptively respond to the increase or decrease of nutrients entering the water by increasing or decreasing the rate of plant growth, which in turn fuels an increase or a decrease in the rate of energy transfer up the trophic hierarchy in the lake, and maintains a constant low nutrient load in the water column. For some lakes, the 'window' of stability can be quite large, remaining stable with little observable change to the system across a wide range of nutrient inputs. However, if nutrient inputs exceed the system's max capacity to absorb it, the entire ecosystem will undergo a rapid state change from a clear, benthic, complex vascular plant dominant system to a turbid, pelagic, simple single celled algae dominant system, and once that's happened, the new 'system' is itself remarkably resilient and resistant to change, even if nutrient loading falls back to a range that the old system could previously cope with.
There are other examples from reclamation sciences, specifically in the grasslands. "Climax" grass plant communities in places like Alberta are remarkably resilient over time, and do a remarkable job of preventing invasive weeds that aren't native to this continent from gaining a foothold. However, very few of the ecological pathways and processes that lead to the development of what we see as a 'native' climax plant community exist any more, so once a climax community is physically disturbed (by say the installation of an oil well or pipeline), the presence of exotic weeds and the absence of things like bison and regular burning can fundamentally shift the successional sequence, resulting in a new climax plant community that looks nothing like what was there before, usually comprised of things we'd consider undesirable or weedy. The new state is incredibly stable and nearly impossible to change back without intensive intervention.
I think the analogies to be drawn to a tank is that any functioning reef system has a series of pathways and processes in place to reinforce the ecological 'state' you see. Some 'natural' (as in performed by things that live in the tank), but most are artificial through equipment you've installed and husbandry practices you engage in. Collectively, those work together to create the appearance of resilience that we see.
Tanks that get supposedly hard treatment with cold, unfiltered/treated tap water water changes are not experiencing large enough fluctuations over long enough periods of time to shift the system outside homeostasis. When you do a cold water water change, you drop the total temperature of the system by a few degrees C, but your heaters quickly kick on to bring the tank back up to the desired temp. Since that doesn't seem to hurt anything, then the total drop in temp and the amount of time the temp stays low must be within the acceptable metabolic range of your inhabitants. You also aren't introducing "bad" chemicals in concentrations high enough to harm anything, and chlorine rapidly off-gasses. If you didn't have a heater, or you dropped the temp too far with too large a water change, or if your local tap water was treated by the city differently, or even if you had a different mix of species with a different range of tolerances, the equation might be different. However, the bacteria and microfauna in captive systems have all demonstrated a clear ability to be shipped around the world on rocks covered in cold, wet newspaper. They're the most important part of the tank, and save for antibiotics and few genera specific poisons (prazipro, most dips, etc.), they probably have a range of tolerances far wider than those of the corals and fish you set the tank up for.
We also actively work to remove or eliminate 'weeds' that have the inherent capacity to change the ecological state of our systems, either by manually removing them, making the chemistry unfavourable for them, poisoning them, or introducing an appropriate predator for them. If certain pest species are present and the aquarist does nothing, the system will fundamentally shift towards those pest species, so we can't discount the aquarist's role in the perceived state of resilient stability of an established tank.
The caveat is that a lot of the resilience we see might be in spite of us, not because the system itself has an easy time dealing with the things we do to it. Regularly dropping the temp of your system by several degrees is taxing on your heaters, and if they fail and you're not there to see it, the whole system can crash in one night. Also, if you're doing things to it that have always seemed fine in the past, but rely on people you've never met and who don't care one bit about your aquarium (i.e. the people at the water treatment plant) to not make an unannounced change from chlorine to chloramines, or install copper fittings somewhere between your house and the treatment plant, you're only one water change away from a major problem.
|