# Premise

On Earth the seas have numerous filter feeding species, large and small, sustaining themselves on the microscopic nutrients and organisms that drift in the current. The largest animal evolution has produced here feeds on some of the smallest. Tropical coral reefs are the go-to example, but as a strategy there are examples from pole to pole and from the sun-warmed shallows to the abyssal depths.

Imagine a world with terrestrial environments in which numerous organisms – both sessile and motile – have evolved to do the same, living off pollen, spores, seeds, and other biological matter drifting through the air. Without necessarily using webs or slime, just opening its mouth or purpose-evolved appendages and scooping the air an organism can sustain itself for a season and/or supplement its diet. As on Earth there are biomes (perhaps the equivalent of tropical rain forests) where this is omnipresent and others where it may be less prevalent (perhaps the poles), but it is essentially as prevalent as filter-feeding in the ocean.

I expect a world that can support this sort of widespread feeding strategy would need prolonged periods of pollen-, seed-, and spore-production, which suggests minimal seasonal variation through the year and a high degree of competition. It also suggests there aren't enough pollinators to do the job manually, or the dispersal of pollen/spore to the air is done to combat pollinators and predators that consume more than they spread about.

Note: I am making a distinction between filter feeding and predation of flying organisms with webs and such, as that is already a successful strategy.

# Question

What conditions – atmospheric composition and density, tilt (as above, I suspect a low-to-no degree of tilt), evolutionary history, etc – would directly contribute to the success of such a global strategy and how?

Related: Land coral and filter feeding asked by metal_maelstorm

• Terrestrial environment? How terrestrial? Can it perhaps have a denser atmosphere? – AngelPray Nov 19 '17 at 22:36
• That's the question. – rek Nov 20 '17 at 1:39
• Possible duplicate: worldbuilding.stackexchange.com/questions/3303/… I would answer the same way. – IchabodE Nov 20 '17 at 23:39

First requirement is low gravity to make it even easier for materials to get and stay air borne.

Second is a relatively dense atmosphere or as dense as possible in the gravity. This would aid strong air currents and storms that would also help keep things air borne.

If there was a lot of rough terrain such as high mountains with heavy storms in the lowlands it might disrupt a lot of plant material and sweep them high up into the air. This might in turn encourage species to move to higher altitudes. The low gravity, relatively high density and strong winds encourage plants to spread pollen and seeds by air rather than relying on pollinators.

There are in fact animals on earth that remain air borne for very long periods. Swifts stay aloft for months on end and do not generally land between breeding seasons. Swifts feed mainly on insects such as bees, wasps, dragonflies, flying ants etc.

So Swifts are not exactly what you are looking for but they are getting close. Such a species might be of interest and would certainly be encouraged in a low gravity environment.

• In a low-G/high-atmospheric-density environment, you might even wind up with natural aerostats - animals that produce lifting gas to stay afloat (methane, maybe), much like an airship. Given that the general appearance would be rather akin to a floating whale, that hits the "filter feeder" nail on the head. – Sebastian Lenartowicz Nov 20 '17 at 7:55

The viability of filter feeding depends entirely on the viability of aeroplankton and the other organic material in the air. There's really no way around this, as filter feeding is only useful if there are a high abundance of things floating around.

Simply put, aerial filter feeding will happen wherever there are high concentrations of organic material floating through the air. There are a few ways to optimize this.

## Planet-scale effects

### 1) Decreased gravity

This is the most obvious one. If it's hard to stay in the air on Earth, then the obvious thing to do make that easier on our planet. It's easy to get to 0.25$g$ and you can probably push it to 0.10$g$ without too many large-scale planetary problems.

### 2) Increased atmosphere density

Again, we're going to be using the buoyancy equation so the heavier we can make the material we're displacing then the easier it'll be to stay afloat.

## Local effects

### 3) Focusing mechanisms

Filter feeders aren't found everywhere in the ocean. They choose their locations very specifically, often selecting for areas where there's a constant flow of plankton and nutrients. In the ocean, this means areas of upwelling and areas of high flow.

If our aerial filter feeders are still similar to Earth aquatic ones, we'd find them at areas of consistent atmospheric upwelling- the rising branch of the Hadley and Ferrel cells. Here, any materials in the atmosphere would be borne towards each other and upward until they can no longer be carried by the air, then they'd drop out and onto our filter feeders. Just like the rainforest temperate forest biomes, these biomes would be found near the equator and at 60 degrees N/S.

They'd also be found in more localized areas of flow- in valleys, for example. When wind passes through valleys, it's often focused, making it an ideal place to filter it out. Organisms living here would probably inhabit caves during the day and creep out with their feeding appendages to hunt at nighttime, very Avatar-style.

### 4) High-nutrient areas

Just as in the ocean, areas of high nutrient concentration would be vital. In the ocean, these are areas like hydrothermal vents. For the air, these would be areas near deserts. Many minerals can be found in dust, and deserts are large sources of dust and thus minerals, which would be important for anything trying to grow nearby. This would be another biome for the aeroplankton and thus the aerial filter feeders.

Water has a much higher capability of keeping things in suspension. Airborne particles tend to be much smaller (otherwise they would fall on the ground quite quickly). This pose a major limitation in the dimensions of the filter-feeders.

I strongly doubt this is a viable solution, but you may try:

• have no flying insect.
• have pollination via dispersion of pollen in the wind (like conifers).
• have dry air (rain washes-down any particles in air) with constant winds.
• have seeds to be airborne.
• devise some reason why flying is a very bad idea (e.g.: strong winds) so that plants need to rely on airborne seeds and pollen to spread.
• I would have assumed having flying insects would be a source of food that your filter feeders would catch and eat. See a large swarm of flying insects, time to dig in. – Anketam Nov 20 '17 at 2:55
• @Anketam: OP explicitly states: "Note: I am making a distinction between filter feeding and predation of flying organisms with webs and such, as that is already a successful strategy". I take this to mean fliers are "another issue" so we should concentrate on drifters. – ZioByte Nov 20 '17 at 8:30
• I took that to mean OP wanted the creature to use a filter to catch them and not a typical mouth or web to catch flying items. So if insects can be caught with a filter method I assumed it was valid. – Anketam Nov 20 '17 at 11:06
• @Anketam: I see. Let's have the OP decide. – ZioByte Nov 20 '17 at 12:02
• The focus of the question is how to maximize airborne organic particulates/matter of the non-insect variety, because we already know what it takes to get bugs in the air. I'm not sure how eliminating flying insects would increase spore/pollen/etc volume though. – rek Nov 22 '17 at 16:23

First: what is a filter feeder?

Filter feeders are a sub-group of suspension feeding animals that feed by straining suspended matter and food particles from water, typically by passing the water over a specialized filtering structure.

Swapping in "air" for "water" are there any existing animals that fit the bill? I think nighthawks do.

Nighthawks swoop through the air, scooping up small insects as they go. Their beaks can gape very widely, similar to the gape of filter feeding fish. screenshot from video https://www.youtube.com/watch?v=XqDL-HzrfA4

Filter feeding marine animals have baleen (whales), gill rakers (fish) or similar structures to effectively increase the surface area for catching food. Nighthawks beaks have bristles all around them that serve the same function.

Nighthawks are not eating the insects they catch one at a time. They are accumulating them in a big wad as they scoop them up, like a whale accumulates krill and plankton on the baleen.

https://atowhee.wordpress.com/2011/06/13/chordeiles-minor-a-wonder-to-be-held/ This site also shows the "windshield" of feathers nighthawks have around their eyes to protect them as the bird plows through clouds of insects.

Here an adult delivers a wad of insects to its young. http://what-when-how.com/birds/common-nighthawk-birds/

Other birds lead similar lifestyles. Limiting factors may not be specific to airborne food - the chimney swift similarly filters the sky of insects, and their populations exploded after people started putting up brick chimneys everywhere (and then fell off as chimneys fell into disuse and were demolished). Higher density of airborne food facilitates this lifestyle - if you pay attention during outside sports games at night, you might see nighthawks swooping through the dense clouds of insects attracted by the lights. I wish I could have found a photo of this!

Some of the largest water creatures are filter feeders - I suspect because their indolent lifestyle allows it, larger size allows larger gape, and there is not much downside in the water. But lots of smaller water creatures are successful filter feeders too - like cod. I think in existing air animals the difficulty staying airborne works against very large sizes, although I could certainly imagine a pelican-sized bird being a successful air filter feeder in an environment with consistent loads of insects (or swarms to follow).

• I'm not sure sure how this answers the question. I noted that catching insects is outside the focus (we already know the conditions necessary for an insect-based diet). – rek Nov 22 '17 at 16:31

This hasn't received the attention I'd hoped for and the initial answers haven't garnered follow-ups to my comments, so I decided to do a bit more research and speculation and post it. Is answering your own question taboo?

Note: I've said "plants" below but that should be understood as the alien equivalent, and include analogues to fungi, lichens, etc.

# Planet Attributes

• Low/no axial tilt and orbital eccentricity: Seasonal variation would be at a minimum, meaning pollination and growth are possible year-round. It also means perpetual twilight at both poles and extends the tree line beyond the current ~60-70° and should help reduce permafrost at lower elevations.

• Thicker atmosphere, more greenhouse gases: This will raise the global temperature and stabilize any variation that may exist, further reducing permafrost and extending the growing range of plants. A denser atmosphere also makes it easier for small particulates to stay aloft, increasing the range pollen and seedlings and such can drift and the value of those strategies.

• Increased greenhouse gases are necessary because removing carbon dioxide from the atmosphere, as plants do, has to be offset to keep the temperature stable over geologic time. I'm unsure how this could be achieved naturally and reliably – volcanism? more frequent forest fires?
• Lower gravity, stronger magnetosphere: This would aid the suspension of pollen, but more importantly it would increase the potential height of tree-analogues. A stronger magnetosphere would help retain lighter atmospheric gases (namely hydrogen) in the weaker gravity. Taller trees are critical because of the next point.

# Ecosystem Attributes

• Greater botanical parasitism: With taller trees and a warmer, stable climate, there will be more parasitic plants growing without any direct connection to the ground. This obviously multiplies the number of plants producing pollen and seedlings, but it also means more plants that can't take advantage of the mycorrhizal networks that facilitate inter-plant and inter-species communication and cooperation.

• Airborne signal networks: With parasitism more common the lack of access to the mycorrhizal networks will have to be adapted to. Parasitic plants (and regular plants) will rely more heavily on pollen and biological volatile organic compounds to communicate with parasites on neighbouring trees and beyond, meaning more particulates.

• Increased competition: On the flip side, this also means more plants will employ and rely on chemical warfare to compete for resources, discourage rivals, and combat (or selectively attract) predators. More particulates produced.

• Late start for land animals: On Earth the evidence suggests it was a relatively short time in the Ordovician after the first simple plants took root on land that the first animals followed.

• A stable surface world thoroughly dominated by plants and fungi with advanced defenses would be a harder place for aquatic animals to transition to, but it would also mean more plant species have the aforementioned signalling and warfare mechanisms in place when predation and zoophily start to factor.

• The delay of animal-aided pollination would also mean plants are more reliant on airborne reproductive methods for longer.

Planetary Conditions Aren't necessarily the critical factors

Sure things like lower gravity and increased air pressure would aid in this kind of evolution but the KEY FACTOR IS ECOSYSTEM.

Technically, Earth could support such an evolution as a flying filter feeder, but what is needed is the available aerial energy.

• Lots of insects or small birds flying around like constant swarms of locusts or larks.
• and/or lots of aerial pollen or seeds drifting in the wind.

After that it is a matter of evolutional roulette but its not hard to fathom adaptations to allow large filter feeding animals.

Buoyant algae via hydrogen generation.

Several types of photosynthetic algae can metabolically produce hydrogen.

https://www.sciencedirect.com/science/article/pii/S0960852417312014

A colonial organism like a volvox could sequester bubbles of metabolically produced hydrogen inside its body to the point where it was buoyant in air. Buoyancy would have benefits for a photosynthetic organism as it could rise above earthbound competitors for sunlight. Such organisms are very small and so the bubble needed to lift them would not be big.

Such organisms would need a method to keep oxygen away from the hydrogen, a trick because these organisms also usually produce oxygen. In an analogous situation, legumes use leghemoglobin to scavenge oxygen and keep it away from the nitrogen fixing bacteria that reside inside them - oxygen poisons the nitrogen fixing biochemistry. Floating algae could do the same to protect their hydrogen. It would make them red.

Floating algae would need nitrogen from the air which probably means N2. They would need the ability to fix nitrogen or have onboard commensal bacteria which could do it. Good thing they have that oxygen scavenging system!

Floating algae would need moisture from the air as well as they could easily dessicate. High ambient humidity would be the best way to do this.

Walking through a wet humid planet dense with these things would very quickly look like they were covered with blood. Aerial filter feeders could eat this aerial plankton up just fine.