how to find a dispersal agent

As seed plants evolved, they invented many different ways to disperse their seeds away from the maternal plant to new sites. We observe various adaptations for dispersal by wind (maple seed wings, fireweed and dandelion fluff) or on the outsides of animals (stick-tights and burrs) or by bribing an animal with a food reward. The many varieties of bribery have evolved many times in separate lineages.

The food rewards for vertebrate animals that disperse seeds are typically what we call fruits, which vary in size and composition; they also develop in a variety of ways on the maternal plants. Quite different plant tissues can be turned into rewards for seed dispersers—seed plants have been quite inventive!

If you venture farther into this essay, be warned that our common terminology often differs from that of botanists. The common parlance has little relationship to botanical technicalities and often reflects culinary uses rather than biology (and we are not about to change, of course). Furthermore, botanical terminology itself is confusing; it’s meant to distinguish different anatomical relationships and developmental pathways, but these definitions are not always precise and vary even among botanists. Rather than try to untangle the semantic snarls in a way intelligible to readers, I’ve curtailed the elaborate descriptions.

In the angiosperms or flowering plants, the food reward is commonly a fruit of some type, formed from the seed-containing ovary and sometimes additional tissue from the parent. Gooseberries, currants, peppers, and tomatoes are fruits that are simply mature ovaries containing multiple seeds; cherries, plums, peaches, and olives are similar, but with a single hard seed inside. All of those could be called botanical berries, broadly defined. But sometimes considerable material from the base of the flower and the upper stem gets incorporated into the fruit. Thus we get apples, pears, rowan or mountain ash fruits, and serviceberries in which the ovary is now just the core and the co-opted floral base and upper stem are the edible part. And in pomegranates and rose hips, those upper stem tissues make the covering of the fruit.

What we call ‘berries’ of raspberry and blackberry are really aggregates of numerous small fruits borne on an enlarged base of a flower. Mulberries are multiple fruits from several flowers, all lumped together, and so is an ear of corn. Strawberries are not fruit at all (botanically); the tasty part is an expanded flower base, and the things we call seeds are actually tiny fruits.

All of this got started as I peeled a mandarin orange for lunch one day. I’ve peeled many of them over the years, but I suddenly got curious about its botanical description. The citrus fruits are actually specialized berries! They are, however, unique in having the juicy little sacs inside the segments; even other members of their taxonomic family do not have this feature. Most of the commercial varieties of citrus are said to be hybrids of various sorts, but mandarins are one of the original types, coming initially from the Himalayan region, spreading first to southeast Asia and eventually, via trade routes, to Europe, North Africa, and beyond. This made me wonder just how did that initial range expansion occur–Was it all due to human transport or did fruit-eating animals play a role too? In citrus orchards today, lots of different animals feast on the fruit, including rats, possums, raccoons, squirrels, parrots. Could similar animals have served as dispersal agents in Asia?

In some cases, the food reward for animal dispersers comes, not from the ovary and associated tissues, but from the maturing ovule. Again, definitions vary, but broadly defined, these outgrowths of the ovule are called arils. The seeds of pomegranates are covered by arils, as are those of the invasive Asian bittersweet vine and passion flower. 

Even the gymnosperms get into the aril story: juniper ‘berries’ and yew ‘berries’ are really made from modified cone scales that cover the seed like an aril, although the edible part arises from maternal tissue (the cone) rather than the ovule, as in angiosperms.

Many flowering plants make seeds with appendages called elaiosomes; these too have varied developmental pathways. This type of animal dispersal has evolved quite independently from those involving vertebrates. The appendages look like little lumps on the outside of the seed. They are usually full of oils and are attractive to ants and sometimes other insects that carry away the seeds and later eat the elaiosomes. Examples include at least some species of Trillium, Claytonia, Corydalis, and Dicentra.

Splash power

using raindrops for dispersal

Some weeks ago, I wrote about spore dispersal in bird’s nest fungi, in which the mature spores are held in a small cup and when a raindrop falls into the cup, the spores are splashed out. I decided to learn more about what other species use raindrops for dispersal. It turns out that raindrops have been put to work, so to speak, to disperse spores, seeds, little asexual propagules, and even sperm.

Splashcups are apparently the most common means of using raindrops. Seed dispersal from splashcups has been reported for many genera of plants, including some that grow in our area: Veronica (brooklime or speedwell), Sedum (stonecrop), Sagina (pearlwort), and Mitella (bishop’s cap). However, I’ve not been able to confirm that our particular species of these genera have this adaptation. Something to look for!

Splashcups for seed dispersal are typically small, just a few millimeters across, and more or less funnel-shaped, with the sides of the funnel not too steep and not too spread out. Small seeds are splashed out at various speeds, up to a meter or so away from the parent plant. Splashcup plants are generally small, herbaceous species; they grow in a variety of habitats.

Some mosses use splashcups too, for dispersal of sperm from male individuals. The raindrops give the sperm a head start on their way to receptive females, but once started, they have to swim to their final destination (thus needing a film of water to complete the journey). The juniper haircap moss (Polytrichum juniperinum) here in Southeast is one of these, and other local haircaps may also do things this way. Other moss genera in Southeast with this adaptation for sperm dispersal include Atricum, Plagiomnium, and Mnium, but again I’ve not succeeded in confirming that our local species do.

Another moss uses splash cups made of modified leaves to disperse ‘gemmae’, which are asexual clusters of cells that can germinate and form new individuals. This moss is called pellucid four-tooth moss (Tetraphis pellucida); it disperses its sexual spores by wind. Although it is found in south-central Alaska, Yukon, and B.C.—that is, all around our area, but there’s apparently only one record, so far, for Southeast (in Sitka).

The lung liverwort (Marchantia polymorpha) occurs all over Southeast and belongs to a genus known for gemmae dispersal by splashcups and raindrops. In addition, males bear their sex organs on little, stalked ‘saucers’ (not the technical term!) and sperm are released onto these saucers. When a raindrop hits the loaded saucer, the sperm can be dispersed as much as sixty centimeters away. Then they have to swim, in a film of water, to a female.

A familiar lichen genus is Cladonia, some of which are known as ‘pixie cups’. These make stalked cups that contain little asexual granules (technically called soredia) composed of bits of fungus and algae that are enough to start a new lichen individual. These tiny granules can be splashed up to a meter away by a raindrop, but they may also travel by wind.

Another type of raindrop-assisted dispersal is thought to occur in Tiarella (foam flower), which we often see here. The seed capsule is shaped like an old-fashioned sugar scoop, with a long lower lip. A rain drop hitting the lower lip could flip out the seeds. This spring-board mechanism is also seen in some club-mosses (Lycopodium, including our local L. selago), which disperse little vegetative, non-sexual propagules (called bulbils) this way.

Fungi have a couple of other unusual ways of using raindrops or at least water drops for dispersal. Some of the puffballs release spores when the tender top of the mushroom is struck by raindrops. Certain fungi (not specified in the available literature) release spores by a water-drop-driven catapult: There is one drop (apparently produced by the fungal spore) at the base of a spore and one (source not given) along the side of the spore. These two drops merge, creating a big enough drop with reduced surface tension so it breaks open, pushing the spore from its attachment and popping it loose.

See this video by Bob Armstrong for an example of fungal dispersal by raindrop.

Raindrops even get involved in pollination of flowering plants. In some buttercups (Ranunculus) and marsh marigolds (Caltha), rain splashes pollen from the anthers (where pollen is produced) to the receptive stigma of the same flower.

All those splashcups and springboards and catapults are evolved adaptations of the respective species. They function to the benefit of the plant or fungus by increasing reproductive success via successful dispersal.

However, raindrops may also have non-adaptive effects, such as transmission of foliar diseases. If a raindrop hits a leaf infected by bacteria, viruses, or fungus, it bounces, potentially carrying the infective agents with it in a water drop. The distance carried depends on many factors, including water-drop size, leaf shape and orientation and its motion when hit by the rain drop, and location of the original infection on the leaf. A complicated matter, indeed, but of some importance especially in agricultural monocultures. 

Late summer flowers

pollinators, seed dispersal, and other end-of-season floral stories

One sunny day, I sat in a small meadow in the Kowee Creek drainage, just taking in the peacefulness and a snack. Some small, red-eyed flies found me and crawled over my arms, dapping at the skin with their nonbiting mouthparts—possibly tasting a bit of salt. Several of them had substantial amounts of yellow pollen on the thorax. All over the meadow, little white flowers showed their heads among the mosses and sedges. I knew these to be flowers of swamp gentian (Gentiana douglasiana). They were about a centimeter across, with odd little pleats in between the five white petals. Because they were the only flowers to be seen, I suspected that the flies are among the pollinators of those flowers—although I did not actually see them visiting the flowers.

What a contrast with the goldenrods that bloom along the road above Eaglecrest. There, on another nice summery day, the golden inflorescences were often thronged with visiting insects: flies of various sizes, a wasp or two, hoverflies, and bumblebees.

As I traipsed through another meadow, this time on the Spaulding trail, I saw tiny white flowers that were borne on branching stems that reminded me of wee candelabras. Looking more closely, I saw that each flower, only a few millimeters across, looked like miniature flowers of the swamp gentian flowers I’d seen earlier. Could this be the same species? The answer turned out to be Yes. By diligent searching, I found a few plants that bore single large flowers on one stem and acandelabra of tiny flowers on another. So I learned that this species produces flowers that vary two- or three-fold in flower size, apparently depending on how many flowers are borne on a stem. This suggests that there may be a trade-off between size and number of flowers per stem: more flowers, then small size; one flower, then large size—as if the plant had made some hormonal decision about how to allocate its floral displays.

In the muskeg ponds, buckbean flowers were finished and the seedpods were maturing. Some had opened, shedding their seeds onto the water surface. The seeds floated nicely, but the opportunity for dispersal by water was limited, because there was no surface-water connection among ponds. That would mean that all the buckbeans in one pond are closely related to each other—the seeds being genetically at least half like the mother plant. However, insects probably transfer pollen among plants in different ponds, increasing the genetic diversity within each pond while reducing the potential differentiation among ponds.

The big tall fireweed that is so common here is producing lots of fluffy seeds to blow on the breezes. That, however, doesn’t work when rain flattens those fluffy parachutes (into nets that are beautiful with captured raindrops). I wonder how many of those seeds get airborne. Landing in a suitable place is always a lottery, but getting into the lottery in the first place is a necessary step. Rain gets in the way of seed dispersal for these plants.

Some of the tall fireweeds still had some flowers and even buds at the top of the stem. Although most of the flowers are about the same size, on one plant I noticed flowers that were only about half of the normal size. Weird! I have read that drought conditions can lead to production of significantly smaller fireweed flowers, but that would not explain this single small-flowered plant. Then I happened to peer more closely within a normal flower (shame on me for not having done this sooner!) . First, I was reminded that fireweeds produce blue pollen; although blue pollen is known from a few other species elsewhere, it seems to be unusual here. That simple observation raises the immediate question: Why blue?

The flowers mature in sequence from the lower part of the inflorescence to the tip, where buds can still be found when the older, lower flowers have already produced mature pods. Within each flower, the pollen-producing anthers mature before the female-receptive surface (the stigma). The lengths of the male phase and female phase of each flower varies; the duration of the male phase is longer early in the season, when there are fewer female-phase flowers available to receive pollen and mating opportunities are few.

Fireweed is self-compatible, meaning that pollen from the same flower, or another flower on the same plant, can be effective in fertilizing seeds, although outcrossing, with pollen from different plants, also occurs. Perhaps because seeds can be produced either by selfing or outcrossing, fireweed flowers seem to produce pods very successfully. Furthermore, as a flower ages, the lobes of the (unpollinated) stigma bend back and down, bringing the receptive surface closer to the nectaries, where a visiting bee would be likely to forage, perhaps bringing in pollen and thus increasing the probability of fertilizing seeds.

The flowers have several means of increasing the probability of outcrossing. The difference in timing of male and female parts reduces the likelihood of pollen landing on the stigma of the same flower (but not between flowers on the same plant). The flowers start to close about four hours after pollination, well before any seeds are fertilized (it takes many hours for sperm from the pollen to reach the eggs in the ovary). The flowers close faster if outcross pollen is deposited than if self-pollen lands on the stigma (slower closing of selfed flowers leaves more time for outcross pollen to arrive), and faster if there is lots of pollen deposited than if few grains are deposited.

Photo by Bob Armstrong

Footnote: in a recent essay I commented that I had found marsh felwort only in two places. No sooner than I had written that, I found lots of the little blue, starry flowers in Cowee Meadows, mostly over toward the creek. Although this plant is an annual, it’s probably been there all along; I simply was not there are the right season.

Summer ending…

…and musings on seed dispersal

Summer seems to be closing down all too quickly. Bird song is past, replaced by the chips and squeaks of juvenile birds scuttling in the underbrush. Even the late-hatching mallard ducklings have their full-on adult plumage and no longer hang out with Mama all the time. Fireweed is already done blooming in most places (in very early August, yet!), a sure sign that fall is upon us.

Here and there we can see some late-season wildflower stragglers, putting out their last flowers of the year. However, a few wildflowers are just getting started: the little blue-flowered felwort (a.k.a. star gentian) along the Boy Scout trail in the grassy meadow near the slough; the purple-flowered northern gentian and the sky-blue gentian on Gold Ridge.

For many plants, this is a time for dispersal of seeds (although cottonwoods and most willows accomplished this earlier in the summer). Mature seed capsules have opened on fireweed, releasing the seeds with their fluffy, white parachutes to float on the breezes. Lupine seed pods have begun to pop open on warm days, scattering the ripe seeds that pitter-patter as they fall through the surrounding vegetation. All over town, the non-native mountain-ash offers its orange, fleshy fruits to willing avian foragers that gobble up the free lunch and excrete the seeds elsewhere.

The whole point of seed dispersal is to send a plant’s offspring away from the mother plant, landing in new sites where they may be able to germinate and grow. If all the seeds just dropped at mama’s feet, so to speak, the competition among those densely packed seedlings would be ferocious, and seed-eaters would be likely to come and demolish the lot in one go. So there are advantages to traveling, but it is always a sort of lottery: most seeds and juvenile plants die, landing in a bad site or picked off by a predator. On the whole, however, it seems to be better to disperse than to congregate.

Plants have evolved many different ways of dispersing their seeds. Here is just a sample, from species here in Southeast.

–By wind: Some seeds bear devices that can lift them on a puff of wind: the fluffy parachutes of fireweed, willow, cottonwood, and dandelion; the propeller-like blade of a twirling maple seed; the small flat wing around the seeds of alder, spruce, hemlock, and rattlebox. Orchid seeds are as small as fine dust, because they (unlike other seed plants) contain no stored material to nourish seedlings; so they easily waft away on a breeze.

Fireweed seeds. Photo by Bob Armstrong

–By water: The seeds of the yellow pond lily are surrounded by a buoyant matrix that keeps the seeds afloat for a few days.

–By animal consumers: The succulent, fleshy fruits of blueberry, salmonberry, twisted stalk, mayflower, lingonberry, devil’s club, and crabapple are eaten by birds and bears, which digest the fruit and excrete or regurgitate the seeds.

–By animals that pick up seeds on fur or feathers (or socks): the prickly seed-heads of avens, the spiky seeds of some grasses, the micro-hooks on capsules (and stems and leaves) of bedstraw and the seeds of sweet-cicely.

–By explosive opening, ballistically shooting out the seeds forcefully: Lupine pods snap open; touch-me-not capsules fly open at a touch, expelling the seeds; mistletoe seeds are expelled with force (and may also sometimes get stuck on passing animals). Wild geranium holds its seeds in five small cups at the base of a long ‘crane’s bill’ with a hinge at the tip; when the mature seed container is dry, the hinge pops opens suddenly and flings seeds vigorously (with a backhand toss). Long ago, I measured the distance achieved in this way, for the eastern species of geranium, and found that seeds could travel at least thirty feet from a plant no more than twenty inches tall. Not bad.

–By shaking: Fern-leaf goldthread bears a whirligig of capsules, each with an opening near the end. If the stem or the capsules are jostled at just the right time and in the right way, a seed flies out. Foamflower puts its seeds in capsules that look like old-fashioned sugar-scoops, with the bottom part longer than the top part. Again, the right kind of jostling releases a seed, which gets an extra impetus from the lever-like action of the lower part of the scoop. Seeds of chocolate lily are stacked tightly in their capsules; the capsules split open and the seeds can be shaken out (they also have small wings that might give them a little glide).

–By unknown means: Some plants produce seeds with no evident means of dispersal, either on the seed itself or on the mother plant—no edible fruit, no hooks, no wings, no ballistics…These species generally have shorter dispersal distances than those with specific dispersal adaptations, so their seeds are likely to be more clustered than those of the other plants; but what are the ecological consequences of their more clustered distribution? Seed shakers probably do a little better. Ballistically dispersed seeds achieve a somewhat wider distribution, and both wind- and animal-dispersed seeds can travel far.

There are other ways for seeds to get around. As Darwin pointed out, they may ride in the mud on the feet of ducks. They may travel in the guts of herbivores that eat the greenery but don’t digest the seeds. Floods may wash them way downstream. These, however, are largely serendipitous events, not specific adaptations resulting from evolution; they may nevertheless be quite effective in moving seeds around.

The pattern of seed distribution around a parent plant is called a ‘seed shadow’. Most seed shadows exhibit a concentration of seeds relatively close to the parent with a long ‘tail’ of fewer seeds, extending to greater distances. Most studies of seed shadows have focused on distances that include the majority of the dispersed seeds. But the tail of the distribution cannot be ignored. It is much more difficult to measure, because there are fewer seeds at greater distances from the parent, and some of the distances can be very long indeed (miles!). But it is those far-travelling seeds that make it possible for plants to colonize new areas.

Here is a little natural history game to play, if you are inclined, as you walk the trails. Try to find as many seed dispersal mechanisms as you can; there are differences among habitats. Can you find plants that have other dispersal mechanisms, and how do they work? Think about other factors that influence the distance that seeds travel (such as the height from which the seed is released). And special kudos to anyone who can send me a good local photo of the seed container of northern geranium after it has flung out the seeds.

Hilda Meadows

musings on winged seeds

Typical Juneau weather! We get a nice snowfall, for good skiing and snowshoeing, then it thaws and rains and makes the snow soggy. Then we get a hard freeze, so the footprints and postholes created by walkers during the thaw are frozen solid. After that happens, walking there is a misery of lurches and ankle-turnings. The popular Dredge Lake trails are a good example. Then the cycle starts over again—lovely snow, then rain…

Before the last rains, however, Parks and Rec hikers did manage to fit in one excellent junket to Hilda Meadows above Eaglecrest, on the one perfect day with good snow underfoot and more coming down all day. Some of us were on skis, others on snowshoes. There was enough firm snow that it was easy to find places to cross Hilda Creek on our way to the chain of meadows.

Critters had been active overnight, so here and there we saw tracks of hare, squirrel, and weasel, rapidly being covered with new snow. Trundling porcupines left numerous furrows as they wallowed in the fresh snow from tree to log to tree. One hiker inadvertently flushed a feeding ptarmigan, which flew complainingly off into the conifers. We all inspected its tracks; it had circled several small blueberry bushes, nibbling on buds.

The surface of the snow was dotted with wind-blown spruce or hemlock seeds. I wasn’t sure which kind they were, so I checked the forestry literature to see how to distinguish them. Western hemlock seeds are smaller, on average, than Sitka spruce seeds, but mountain hemlock seeds are larger. Foresters don’t usually measure individual seed weights; instead, they count the number of seeds per unit weight. So they estimate that there are 570 western hemlock seeds per gram, 400 Sitka spruce seeds per gram, and 250 mountain hemlock seeds per gram. (There are 28.35 grams per ounce, if you want to convert those numbers to the English system of weights and measures.)

Spruce (left) and hemlock (right). Photo by Katherine Hocker

I also collected some spruce and western hemlock cones and extracted the seeds. In general, spruce seeds look fatter and the wing seems a bit wider, but there is a lot of overlap in apparent size of the wing.

When seeds are released from cones, most of them land fairly close to the maternal tree. Many of the seeds that land near their mother or each other usually die, because seed predators focus on high densities of seeds. Or, if those seeds germinate, the crowded seedlings compete with each other, stunting growth and eventually causing mortality. Therefore, it is important to the tree’s reproductive success that some seeds disperse to greater distances. This can happen in a big wind or, as we saw on our way to Hilda, when fallen seeds are blown over the surface of the snow in a breeze.

The convenient name for the distribution of seeds around a parent tree is ‘seed shadow.’ The seed shadows of trees are difficult to measure, especially for far-traveling seeds, and most studies have ignored the ‘tail’ of tree seed shadows. With that limitation, a review of published studies of tree seed shadows suggests that Sitka spruce seeds may tend to fall closer to their parents than do western hemlock seeds.

It might be interesting to do a little experiment with our wind-dispersed trees (spruce, western or mountain hemlock, Sitka or red alder), all of which have winged seeds. Take seeds from ripe cones, an equal number of each species. One by one, drop the seeds from a standard height, letting them land on a clean surface. Mark the landing site of each seed. Then compare the average and maximum distances achieved by each species. Try it all again, in a breeze.

Are some wings better than others for carrying seeds away from a source? Does the ‘wing-loading’ of each kind of seed differ, and is that related to dispersal distance? (Wing-loading is the ratio of seed weight to area of wing.) Is the total weight of the seed-plus-wing important? How does the height of release or breeziness affect the outcome?

!Prospective Science Fair students take note!

Jays and seed dispersal

when a predator doubles as a disperser

On a recent hike, I heard a volley of high-pitched screams coming from a thick stand of small spruces just beside the trail. They sounded very much like the cries of a red-tailed hawk, but that bird would be highly improbable in such a place and at this time of year (February). Surely it was a Steller’s jay, which is well-known to mimic redtails and some other birds as well.

That small incident set off some musings about our Steller’s jay and jays in general. Steller’s jays are omnivorous, eating all sorts of things, including bird eggs and nestlings, carrion, insects, seeds, and fruits. I was thinking in particular about their role in seed dispersal; when they eat fruits, the seeds pass through the digestive tract and get deposited, sometimes in a place where they can germinate and grow. The jays share this important ecological task of seed dispersal with thrushes, waxwings, crows and ravens, bears, coyotes, marten, and other animals.

They are also seed predators, along with sparrows and finches, chickadees, squirrels, mice, and others. In this capacity, they raid bird feeders and train humans to offer peanuts. Sometimes they cache their seedy prizes, under a bit of moss or a stick. Peanuts and most seeds offered in seed feeders don’t grow well here, but cached sunflower seeds sometimes produce seedlings in improbable places.

This seed probably won’t grow. Photo by Bob Armstrong

I don’t know how often our jays consume the seeds of our trees (spruce, hemlock, pine, cottonwood, alder, willow); all of these seeds are small and typically dispersed by wind; they probably don’t offer much nutrition to a relatively large bird such as a jay. However, in more southerly portions of their geographic range, Steller’s jays are known to harvest and cache the seeds of several conifers. Not all of these caches are retrieved, and the seeds germinate, so there the Steller’s jays are contributing (along with other kinds of jays and Clark’s nutcracker) to seed dispersal, a critical portion of a plant’s life cycle.

Steller’s jays are closely related to the blue jays that live in wooded areas of eastern North America. The blue jay is also an omnivore; among its varied dietary items are acorns and beechnuts. Blue jays harvest and cache these items, sometimes several kilometers from the mother trees. They are much better dispersal agents than squirrels, which cache nuts but don’t travel as widely. Researchers think that blue jays were important in the development of northern forests as the Pleistocene glaciers retreated, by carrying nuts northward to ice-free zones and stashing them.

Neither of these jays is as specialized to a diet of seeds as the pinyon jay of the southwestern U. S. or the more distantly related Clark’s nutcracker of the mountain west. Both of these species depend on conifer seeds year-round, even feeding cached seeds to their chicks early in the summer. Both species travel long distances, sometimes many kilometers, to cache their harvested seeds, and both have excellent spatial memories for retrieving those seeds. But, as usual, not all seeds are retrieved and, because they are often cached in good sites for germination, they become important for forest regeneration. As our climate changes, they and other seed-caching birds could facilitate altitudinal shifts in tree distribution.

Without the seed-caching jays and their relatives, some forests cannot regenerate. For example, pinyon pines are dispersed by western scrub jays (along with nutcrackers and pinyon jays). A study in New Mexico, where scrub jays were the main pine-seed dispersers, showed that near a very noisy industrial area, the jays became extremely rare and, correspondingly, there were many fewer pinyon pine seedlings in the forest, while at the same time, in a relatively quiet area, both jays and seedlings were common. Noise pollution drove out the jays and severely reduced pine forest regeneration.

There isn’t room here to sketch out the whole story of jays and nutcrackers, their adaptations for seed harvesting and caching, the adaptations of trees that facilitate seed dispersal by these birds, and the sometimes complex interactions with other seed-eaters, such as mice and squirrels. For the time being, suffice it to note that the interdependence of these birds and certain trees means that if one side of the mutualism fades away, the other side declines too.