In Lecture 13, Prof S moved beyond the idea of populations and began instead talking about "communities." So, I thought it would be appropriate to try on a set of community glasses, which are sometimes similar to population glasses in that they share some of the same terms/concepts. However, when we talk about organisms in terms of communities instead of populations, there are some new terms/concepts that we need to keep in mind (and in sight). Once again, we turn to the finches so that we can practice using some of the ideas that Prof S used when talking about "community ecology."
Chapter IV
Forts, Forts, Forts...Brett, that's all you ever talk about? Why don't you ever talk about the other types of ground finches on Daphne Major? If that's what you've been thinking, then your wish has come true. We're going to 'zoom out' and talk about more than the Daphne Major population of Forts today. We're going to talk about the Forts and the two other species of ground finches found there (pictured at right): the Fulis and the Mags.
You can see from their pictures. These 3 different species of finches are similar, but yet they're different, right? When we allow ourselves to talk about all 3 of these Daphne Major species of ground finches at once, we are entering the world of community ecology.As Prof S said in class, community ecology "studies interactions between more than one species." In population ecology, we concerned ourselves with a single species: the Forts. But when we make the 'jump' to thinking in terms of community ecology, we are thinking about the interactions between more than one species. In this case, the Forts, the Mags, and the Fulis. This is not a difficult jump to make, right?
So, let us continue to look at the drought of '77-78 and examine not just how the Forts were effected, but also how what happened to the Fulis (who, on average, had the smallest beaks of the three ground finch species--smaller than the average Fort) and the Mags (who had the largest beaks--larger than the average Fort).
Community ecology speak...
Right off the bat, lets practice using some community ecology terms. If we pretend, just for a moment, that the 3 species of ground finches are the only species of living things on Daphne Major, what would the "species richness" on Daphne Major be?
Hopefully you said, '3' because as Prof S mentioned in class, species richness is simply a measure of the total number of different species in a defined area (in this case the island of Daphne Major).
What if I asked you what the "relative (species) abundance" on this hypothetical island would be?
Hopefully, you said, 'I don't have enough information to answer that question' because relative (species) abundance depends on how many individuals of each species can be found in a defined area (again, in this case the island of Daphne Major). If I told you how many Fulis, Forts, and Mags were present on Daphne Major when I asked this question, only then would you be able to tell me about the relative (species) abundance.
What if I asked you what the "species diversity" on this hypothetical island would be?
Hopefully, you said, 'I STILL don't have enough information to answer that question' because species diversity depends on both species richness (which you know) and relative (species) abundance (which you don't know). Species richness, relative (species) abundance, and species diversity are all ways that Prof S mentioned are ways of "quantifying a community." In other words, these three terms are ways that biologists use to 'count' living things in different habitats often so that they can compare them to other habitats.
What happened to the ground finch community during the '77-78 drought?
From both Chapter II and Chapter III of our story, you already know what happened to the Fort population during the drought. But the Forts weren't the only population of ground finches on the Daphne Major in the years leading up to 1977; there were also healthy populations of Fulis and Mags on the island too. In years with normal rainfall, each of these species of finches typically eat seeds from the island's plants. Yesterday, I shared with you stories of how the Forts engaged in fierce intraspecies competition for food during the drought with members of their own species. Today, I want to share a few excepts from The Beak of the Finch that give you a flavor for the interspecies competition between the 3 species of ground finches.
Recall in previous chapters of our story I've mentioned the caltrop plant (Latin name Tribulus). Tribulus plants produce the most interesting of seeds (pictured at left). Their seeds are initially housed inside of fruits, but when these fruits dry and fall to the ground they become large amoured pods. The spiny, star-shaped pod breaks apart into separate "mericarps," each of which holds three to six seeds inside.For the most part, the Tribulus mericarps are very awkward in a finch's beak. The significance of this seed is that in normal years on Daphne Major, each of the 3 species of finches have the same general diet--they all eat the same 7 kinds of soft seeds. They don't even bother with the hard-shelled Tribulus seed. But in a drought, when the Tribulous seeds are about the only seed left on the island, they are forced to confront it as it is one of the only remaining food sources.
Here are some passages to describe how the 3 finch species dealt with the Tribulus seeds during the drought (some paraphrased):
The smallest ground finch [the Fulis], have never been seen trying to open them. The only species that do attack mericarps are the [Mags and Forts], and each species has its own tactics.What I've done by selecting these passages is do two things. First, I've selected passages that connect to some of the terms that Prof S used when talking about "community ecology" in Lecture 13. Above, you should be able to see by now that these passages illustrate competitive exclusion: on Daphne Major 3 different species need exactly the same resources and, in the end, there are some winners (mainly the Mags and the larger Forts) and some losers (mainly the smaller Forts and the Fulis). Do the above passages illustrate resource partitioning? Not really, during a drought the 3 species all fight for the same seeds. However, take a look at the following quote. I think it is a perfect illustration of how, in a non-drought year, there is clearly some resource partitioning on Daphne Major. In one of the dry seasons prior to the drought (remember a typical year in the Galapagos involves a wet and a dry season)...
Mags (whose beak is almost twice as wide and twice as deep as the beak of a Fort) picks up a mericarp, holds it near the midpoint of its beak, and squeezes its mandibles together. After a while the mericarp shatters into fragments. The Mag picks up each fragment, holds it one one side of the beak, and crushes it.
To crack a whole mericarp like this takes an average force of more than 200 newtons. Apparently that is more force than a Fort can muster. Instead it braces the mericarp against the ground and bites and twists the woody sheath that guards the row of seeds, as if peeling off a lid. This operation requires about 54 newtons of force, which seems to be about the best a Fort can do.
The finches prefer mericarps with only two spines, and mericarps with four spinces are likely to be dropped. Once indication that Mags have an easier time than Forts at eating Tribulus...is that a Mag cracks many more mericarps than it rejects, while a Fort rejects many more than it cracks.
A few Forts have found a trick that helps them even the score. One of them sometimes trails a Mag around on the lava. As soon as the Mag cracks a mericarp...the Fort rushes up, steals a piece, flies a little way off, and cracks it. Not every Fort on Daphne Major seems to know this trick; the Grants have spotted only about half a dozen of them doing it.
So the trials and tribulations of Tribulus are not only harder on Forts than Mags; they are harder on some Forts than others. Forts with bigger beaks can crack the mericarp and gouge out the seeds faster than those with smaller beaks. Tiny variations are everything. A Fort with a beak 11 millimeters long can crack a Tribulus seed; a Fort with a beak only 10.5 millimeters long will not even try.
Peter Boag walked around the island. Each time he spotted a Fort...he watched the bird until he saw it pick up a seed, and he wrote down what kind of seed it was. Boag found that in the dry season the birds with the biggest beaks eat the biggest seeds, the birds with the medium-sized beaks eat the medium-sized seeds, and the birds with the smallest beaks eat the smallest seeds.The second thing I've done is hopefully give you a story to keep in your head during tomorrow's exam. Should you find you're asked a question about, in this case, community ecology, I would hope that this finch story will circulate in your head as you write. Similarly, should you find you're asked a question about population ecology, I would hope that yesterday's finch story will circulate in your head as you write.
More community ecology speak...
Something tells me that you really don't need me to try and put some of the other community ecology terms into the finch story. Hopefully, you can now speak (and write) intelligently about competition in communities of organisms. And, I'm pretty confident that you already know how to use other terms that community ecologists like to use, terms like predation, herbivory, parasitism, mutualism, and commensalism. All of these specialized terms simply describe different types of relationships between different species.
I'm also pretty confident that you are probably all pretty good by this point in your school careers in terms of knowing what basic food chains and food webs are. Do you know the different terms for important species found inside of the chains/webs? Do you the difference between terms like autotrophs/heterotrophs, producers/consumers, dominant species, keystone species, and foundation species? These are all terms that community ecologist like to use when talking about groups of different species living in a particular area.
Closing Comments...
It occurs to me to remind you that in the last three Blogs (Lectures 11, 12, & 13), I have asked you to wear at least 4 pairs of highly specialized glasses. In the Lecture 11 Blog, I tried to show you what living things look like through two types of glasses: taxonomic glasses & phylogenetic glasses. In the Lecture 12 Blog, I tried to show you what living things look like through population glasses. Finally, in the Lecture 13 Blog, I tried to show you what living things look like through community glasses.
For Exam 2, you'll need to be prepared to wear all of these different types of glasses--sometimes separately and sometimes simultaneously--but the difficult job is knowing when to put each of them on. I'd like to remind you, however, that these aren't the only glasses you'll need for Exam 2. You'll also need a reliables pairs of species and speciation glasses, which I tried to help you learn how to use back in the Lecture 8 (Chapter I & Chapter II) & Lecture 9 Blogs. Don't forget that even though you had a quiz on the material presented in Lectures 8 & 9, these days are also fair game for material for this Exam.
Oh yeah..."And they lived happily ever after."
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