Realms & Foibles: Towards an atlas of autecological and synecological contingencies

“And yet I have constructed in my mind a model city from which all possible cities can be deduced,” Kublai said. “It contains everything corresponding to the norm. Since the cities that exist diverge in varying degree from the norm, I need only foresee the exceptions to the norm and calculate the most probable combinations.”

“I have also thought of a model city from which I deduce all the others,” Marco answered. “It is a city made only of exceptions, exclusions, incongruities, contradictions. If such a city is the most improbable, by reducing the number of abnormal elements, we increase the probability that the city really exists. So I have only to subtract exceptions from my model, and in whatever direction I proceed, I will arrive at one of the cities which, always as an exception, exist. But I cannot force my operation beyond a certain limit: I would achieve cities too probable to be real.”

p69 Invisible Cities; Italo Calvino 1972 [translated from the Italian by William Weaver 1974]; (the cover design by Louise Fili is the featured image on this blog post)

In the above passage from Calvino’s series of vignettes, surreal, fictionalized versions of Marco Polo and Kublai Khan exchange observations about fantastic cities. Khan’s approach, attempting to envision an idealized, hypothetical city is analogous to the approach we take in many of our discussions in this class. You’ll hear me say things like, “all else being equal,” or “to a first-order approximation,” or, “let’s assume…” The last of which is essentially me asking you to suspend your disbelief, and forget things you might know about individual ecosystems and organisms that might contradict what I'm about to say. Setting up “rules” that apply to a hypothetical, idealized ecosystem is a step along the way towards a more integrative, predictive ecology, and I think it’s a justifiable pastime.

But as Marco reminds us in the second paragraph from the top, some people revel in strangeness and surprises. Some systems in ecology and evolution are so chalk-full of exceptions, exclusions, incongruities, and contradictions, it seems like it might feel mentally easier for us to start with the ridiculous and move towards the mundane.

So, the subtitle of this post: “Towards an atlas of autecological and synecological contingencies.” What is that?

Autecology is an organism-out perspective on the ecology of a system that tends to encompass things like physiology, life history strategies, and some population ecology (as opposed to “synecology,” which focuses on interactions among different types of organisms, and the community as a whole). Many ecologists would like to believe that we can describe generalizable patterns in ecology that will be applicable across many systems. However, anyone with some knowledge of a system will be aware of some exceptions to “the rules.” Salamanders and seals have peaks of species richness pretty far outside of the tropics, for example. Many rules of body size ecology have dubious applicability to sessile colonial animals and fungi. Organisms like shrews and hummingbirds are often flirting with starvation, whereas organisms capable of undergoing dormancy or cryptobiosis (myxomycetes, tardigrades, lupines…) are a bit less likely to become locally extinct, even if they’re not metabolically a part of the ecosystem.

How has evolutionary biology shaped community assembly? In some ways, this is a question about priority effects on the grandest scale: are cloud forests that are dominated by tree ferns different than cloud forests that are dominated by angiosperms? How different are the tetrapod communities in Australia vs. Africa as a result of the different types of mammals that you can find in the two respective continents? From the perspective of community-level properties (species richness, relative abundance distributions, resilience, resistance, ecological interaction network topology), what’s weird about __[insert clade or functional group here]__ ? What differences are there among the tropical and semi-tropical deserts of the world as a result of being colonized by different types of plants?

This is a place we could potentially address stuff like niche construction, ecological engineering, keystone spp., etc. Are there types of organisms that play important facilitative roles in an ecosystem? Alternate stable states? From a carbon storage perspective, how do the different guilds of plants fare? What about different types of plankton? Sea grasses vs. algae? Bivalves vs. brachiopods? Cephalopods vs. fishes? Birds vs. bats? Eusocial insects vs. non-eusocial insects? Different types of epiphytes (ferns, bromeliads, orchids…)

The organisms researchers focus on might influence the way they view ecology and evolutionary biology, and so too might the broad categories of habitats these organisms live in. How are things different for taxa in freshwater, saltwater, and terrestrial systems (people sometimes refer to these major habitat types as “realms”)? What about subsurface (e.g., hypogaeic, hyporheic) and extreme habitats? Are there broad statements we can make about “environments of structure B” ?

Some differences among the realms are the inevitable outcome of the physical and chemical properties of the systems: freshwater ecosystems are more disconnected from each other than marine systems; only terrestrial habitats are likely to be precipitation-limited; the world’s marine systems have much more uniform elemental availability than some lakes and soils. There are not big reefs built of sessile invertebrates on land or freshwater, and animal-mediated pollination is rare or absent in marine and freshwater systems.

However, some differences among the realms are less obvious, and perhaps contingent upon the evolutionary history of the organisms that happened to colonize the different realms. Fishes are an extremely important component of freshwater and marine ecological interaction networks, and they are often gape-limited. The base of the foodchain in marine system is often unicellular phytoplankton (as opposed to large, vascular plants), which, combined with fishes’ gape limitation often results in long-ish food chains, especially in open-water systems (e.g., not kelp forests, sea grass beds, mangroves, etc.). However, on land, arthropods and tetrapods are less frequently gape-limited (picture a spider eating a slightly larger fly, or a weasel eating a slightly larger rabbit), and the primary producers are often large and/or configured in such a way so as to be effectively eaten by herbivores of many sizes, from aphids to elephants, so it is rare to find organisms on land that are specialized in eating things as high up the trophic ladder as an orca that is specialized on eating seals or tuna. Some phenomena like trophic cascades may be more rare on land than they are in marine and freshwater systems.

The marine realm is certainly the largest and most well-connected of the three, although there are marine lakes that are analogous to islands or freshwater lakes. Freshwater systems come in two primary varieties: lotic and lentic (moving and relatively still), and freshwater organisms in different watersheds may be fairly isolated from each other, unless they have a life stage that disperses over land (some amphibians), through the air (many insects with freshwater juvenile stages), or by the sea (amphidromous fishes, snails, shrimps). Many freshwater habitats are quite young: e.g., pretty much all of the freshwater habitats north of Washington, DC in North America were likely severely impacted by the Pleistocene glaciations, and their communities may have assembled from scratch in most cases. Many terrestrial environments are intermediate in age and habitable volume between the two other realms.

A thought experiment to attempt to tie this cavalcade of strangeness in organisms and habitats together in the context of our course: What would have more gastropod species: A tropical freshwater lake, a marine lake, or a terrestrial island? Assume the photic zone in all three is the same area (and there’s no aphotic zone, so the lakes are really “ponds” in the strict sense). Assume they are all 1 million years old. Now, same question but for vertebrates. Same question but for arthropods. Same question but for insects. Same question but for fungi. Same question but for nematodes. All islands are equal area, but do any of the answers depend on which “realm” that area is? We’ll circle back to this gedankenexperiment when we’re talking about Phanerozoic trends (the answer seems to have been different at different times).

Thought experiments like this are fun, but people who primarily focus their research on one of these systems might have differing views on what the most urgent conservation priorities are. Fertilizing fields is much more problematic for nearby freshwater and marine systems than it is for adjacent terrestrial areas, whereas direct habitat destruction is much more of a problem for terrestrial and freshwater systems than it is for marine systems (the three-way-intermediate mangrove ecosystem is arguably an example of a marine system that’s under the most pressure from humans right now… although negative impacts form ocean dredging and the mining of seamounts are not to be underestimated. Ocean acidification (driven by an increase in the proportion of CO2 in the atmosphere) is a direct problem for the oceans, just as acid rain is a direct problem on a more local scale for freshwater and terrestrial systems. Extinction seems to be happening much more rapidly in terrestrial and freshwater habitats, but changes in the biomass of marine fisheries rival or exceed any swings in biomass in the freshwater or terrestrial realms.

I know I’ve quoted this before, but, in the words of Robert MacArthur (1972):

“I predict there will be erected a two- or three- way classification of organisms and their geometrical and temporal environments, this classification consuming most of the creative energy of ecologists. The future principles of the ecology of coexistence will then be of the form "for organisms of type A, in environments of structure B, such and such relations will hold." This is only a change in emphasis from present ecology. All successful theories, for instance in physics, have initial conditions; with different initial conditions, different things will happen. But I think initial conditions and their classifications in ecology will prove to have vastly more effect on outcomes than they do in physics.”

Taking a cue from MacArthur, Schoener (1986), Pianka and others (2017) have contributed to systematically categorizing organisms and habitats (see also Bambasch’s (e.g., 1983) characterization of ecological guilds in the marine fossil record). But for the moment, let’s just have fun talking about how different things are weird. The enormous variety of organisms and environments might necessitate more of an atlas than a periodic table.