(The contents of this post are harder to find images for than my lazy soul can stand – instead, listen to this while you read because it sounds pretty)
You didn’t think we’d be getting away from ecology THAT easily, did you? Man, I have enough notes from that course to choke a rhino. A rhino with two sets of esophagi. And a scuba tank. There’s a new bit of terminology every two inches and a new concept every three feet. Luckily, most of it is devoted to over-explaining simple, easy-to-grasp concepts. So that’s nice!
Welcome to our what the hell is this thing again?
Our first simple, easy-to-grasp concept that will be overexplained is the community. Ecological communities differ from yours: they never ask you to help participate in a clean-up-the-streets initiative, they don’t take hours of your service in exchange for minor and obnoxious crimes, and very few of them mandate human participation in any way. Instead, they can best be defined as something like this: a bunch of populations of different species in the same spot, interacting in some way (predating, fleeing, flirting, rummaging, bartering, raconteuring, outright ignoring, etc). A bit vague, but serviceable. Just not serviceable enough for some people, which is why we had two big honkin’ concepts of what an ecological community was clashing against each other for a few whiles.
First up to bat, we had Fred Clements, who described the community as a superorganism, an organized and discrete unit whose contents interact with one another in specific ways to produce specific outcomes. Each species only makes sense in terms of how they fit into the whole system – you can’t understand a tiger without understanding sambar without understanding small woody shrubs without understanding soil quality without blur de blar de blah. This is known as the holistic concept.
Second but making up for it in enthusiasm (and in large part as a deliberate, pointed, not-far-short-of-‘screw-you’ response to Clements) we had Hank (Henry) Gleason, who basically summed up communities as “a big old pile of bullshit and luck.” Well, not in quite those words – though he did describe a plant association as “not an organism, scarcely even a vegetational unit, but merely a coincidence.” Lifeforms get thrown together all willy-nilly and hurly-burly by the tangled and cruel winds of fate, and what works together stays together. There’s no real organization above species level in this, which is why it’s termed the individualistic concept.
Of course, history being what it is, we’ve since decided that they were both totally wrong – but it sounds more polite to say that we’ve ‘integrated their premises’ and they’re therefore both right. We’ve taken onboard Gleason’s notion that communities don’t have firm boundaries since if there’s one thing species love it’s to encroach on each others private business, and we’ve accepted Clement’s idea that species interaction plays a huge part on how the community ends up working because if stuff doesn’t poke the stuff around it nothing interesting happens and it all ends up extinct.
They’d both probably be livid.
Open and Shut
Two fun ways communities can work out geographically are open and closed. In a closed community, the species that make up the community share a closely overlapped distribution that is firmly separate from others species, forming a segregated, quasi-racist lump. Y’know, sort like a human gated community. You can spot their boundaries quite easily – they’re the places where the environment changes sharply, like going from a British Columbian temperate rainforest to the seashore. Those borders are called ecotones, and the places where they’re sharpest are usually where big physical changes happen, like going from land to water, or the soil completely changing, or switching from one side of a mountain to the other. Like tracking a floor’s texture going from a carpet to hardwood boards to that one spot underneath the stove that hasn’t seen the light of day since the house was built that could be made out of precambrian rock for all you know.
Open communities, by contrast, have no natural boundaries and the species that make them up can end up all over the place, falling into more than one community grouping themselves. Because of this, they don’t really have ecotones. Too laid-back. Species distribution in these communities follows underlying gradients in the environment – rain levels, altitude, blah blah blah – and thus the communities never quite become homogenous enough to be given clear boundaries where area A ends and area B begins. This sort of distribution ends up being called the continuum concept, and has no sharp-edged ecotones, just a varying pattern. Like a big patchwork quilt gone all wrong.
Succession
This is how both royalty and ecology transition from one (head of) state to another. In both cases, violence is usually the cause – your old ruler/community is dead/disturbed, better initiate succession and get him/it replaced/reinvigorated.
There are two broad types of ecological succession – primary and secondary. Primary succession kicks in when a place has been scrubbed bare of life – a glacier scraped it all off, or a volcano buried it, or a landslide smothered it, or or or or or you get the idea. There’s not usually a lot of soil left. Primary succession begins where everything has ended, and puts down something new.
Secondary succession occurs where a community’s been disturbed but not annihilated – though the degree of damage is pretty flexible. A few trees fall over, that’s secondary succession. A wildfire scorches out hundreds of acres, crisps the soil, and flash-fries a majestic chunk of the flora? Also secondary succession, since it’s still a repair job rather than a make-something-out-of-nothing. Obviously, it’s a lot more common than primary succession, and it takes a lot less time. Not that succession is a particularly hasty process in any case – grasslands in North America are a two-to-four-decade wait on secondary succession, and they’re pretty fast about it. Imagine how long it takes to wait for trees to grow back.
And now, a hypothetical succession.
Let’s say we clear-cut a little strip of a North American deciduous forest. Just for the giggles.
The first stuff on the scene will be the little quick-growing buggers that were being stifled by the shade of the big trees, the annuals and such. Soon enough (relatively speaking) shrubs will come in and show their appreciation for the renewed soil and shade by crowding out most of them and bumping ’em off. After that come the pines, which bump off the shrubs, and then WOAH OOPS it turns out that all this recolonization just left the perfect environment for the deciduous trees to step in and retake their old ground back, grinding the proletariat under their merciless leafy regime for all eternity.
In the above example, we’ve got three things.
First, obviously, we’ve got secondary succession. The forest is down but all life isn’t out, it’s a rebuilding job rather than a ‘well, guess we’d better start weathering this bedrock into soil’ project. A fixer-upper.
Second, we’ve got a new term, something that can be applied to each of the stages our succession went through from annuals-shrubs-evergreens-deciduous: sere. Annoying to pronounce, but useful to remember, a sere is any of the ‘steps’ in an ecological succession. Each sere alters the conditions of the habitat and sets the stage for the next one – encouraging some species through facilitation succession (annuals helpfully rebuilding and shading up the soil for their shrub overlords) and denying others the opportunity through inhibition succession (the pines don’t exactly leave the annuals room to get re-established).
Third, we’ve got the climax. Notice how nothing came in after the deciduous forest staked its claim? It’s the final sere, the last word, the buck-stops-here-er, the head honcho, the king of the hill, the enormous brie the gargantuan cheddar AND the massive mozzarella. The climax community is the be-all end-all stage of a particular succession sequence.
…Although they vary substantially within themselves due to environmental gradients, and are thus much less homogenous than I just described.
…And if the habitat’s unstable it can just be a transient climax, because the environmental conditions are going to change like crazy and make an entirely new climax the popular favorite for ‘most likely to survive flash floods followed by prolonged submersion’ or something.
…And if it’s a cyclic climax, predominance of one species in a climax will just cause another climax species to naturally rise up and eclipse the first one and vice versa in an endless see-saw where dominance of one merely leads to its inevitable downfall ad nauseum.
….Oh, and if there’s a lot of herbivores that really like to eat the climax vegetation (I mean REALLY LIKE, like elephants really like), then they can force it into a different climax through delicious, hunger-based attrition.
Look, it’s all VERY COMPLICATED, all right? This is always what happens when you try and explain simple, easy-to-grasp concepts.