Lecture 12 (6/15/09): A Biology Bedtime Story...Chapter III

When, in the Lecture 11 Blog ("Mom...Do Birds Grow on Trees?"), I said that, "I think that there has been more 'information' tossed your way in Lectures 11, 12 and 13 than in any other 3 lectures all semester," here's what I was talking about:

In Lecture 12 alone, between 10:20 am-12:10 pm you were presented with the following scientific/technical terms:

• Abiotic factors (including temperature, salinity, precipitation, climate), biotic factors (including predation, herbivory, interspecific competition, intraspecific competition, mutualism, habitat selction, symbiosis), biogeography, biomes, dispersal, distribution, population ecology, demographics, range, habitat, mortality rates, birth rates, population density, survivorship, survivorship curves (Types I, II & III), life history, semelparous, iteroparous, population growth, exponential growth, carrying capacity, logistic population growth, limiting resource, r-selected, K-selected, and a little bit of alphabet soup: K, d, N, t, r, b.

How can I best go about helping you learn to use many of these terms--as biologists would use them--by Friday? But of course!!! Why didn't I think of this sooner? This calls for another installment of...A Biology Bedtime Story! If I can craft some kind of interesting story with which you now have some familiarity, perhaps I can practice using some of the vocabulary so that you can also get some practice seeing it...thinking about it...using it...yourself.

I have just the story that I think will highlight some (but not nearly all) of the main ideas from Lecture 12...

Chapter III

When we last left our friends--the finches of Daphne Major--in Chapter II of our story, the Forts (the medium-beaked ground finches at right) had suffered their way through the 1977-78 drought. Their population on Daphne Major had plummeted due to a natural selection event, which selected AGAINST alleles for below average and average beak size/strength, and selected FOR alleles for above average beak size/strength.

That much you should already know...

But can we take a step or two back in history from the hard times of the Forts in '77-78 and ask a question? Can we ask a question similar to the one that Prof S asked the class at the very beginning of Lecture 12? He asked you:

• Why are there so many species on Earth?

I want to ask you a more modest question:

• Why might there be so many species of finches--between 13-15 depending on which experts you talk to--in the Galapagos Islands?

Professor S answered his question by using two main terms in lecture. Did you catch them? He basically said that there were things called biotic and abiotic "factors" that affect living organisms, and then he gave you a bunch of different terms that could be considered either biotic or abiotic factors. Why don't you and I practice using some of these terms and re-live the drought of '77-78 with the Forts.

Abiotic Factors on Daphne Major

What was the main abiotic factor that changed during the drought of '77-78? Hopefully, this is obvious to you...the drought means that the water changed, more specifically, the average yearly amount of precipitation changed. How did it change? It decreased. Remember, almost no rain fell on Daphne Major during '77-78. Did the temperature on Daphne major change during the drought? Well, no precipitation probably means fewer rain clouds; fewer rain clouds probably means more direct sunlight; more direct sunlight probably means more sunlight hitting both the island's surface and the Forts bodies. Consider these two excerpts from The Beak of the Finch, which describes what two of the finch scientists, Peter Boag & Laurene Ratcliffe, saw during the drought (remember the real name for the Fort species is Geospiza fortis or just fortis:

Some of the very smallest fortis on the island, the ones whose beaks were too small [to open seeds of] caltrop, were poking around the Chamaesyce instead. The herb Chamaesyce has small, soft seeds, but it also has aa milky, sticky latex when its leaves are wounded and its stems are broken. These little fortis...began hunting for seeds in the Chamaesyce in spite of its latex. The feathers on the crowns of their heads got so matted, gummy, and sticky that they rubbed off afterward as the birds raked the cinders and gravel looking for more seeds. Their bare scalps were exposed to the sun all day. Boag and Ratcliffe began to find little bald finches lying deal on the lava. (Page 74)

Down on the crater floor [of Daphne Major] the blue-footeed boobies shifted their weight from one leg to the other to cool off their webbed feet. Boag stuck a thermometer into the ground, in the toutured shadow of a cactus, and the soil was hotter than 50 degrees C (122 degrees F). Even when there were seeds lying out in the open, the heat was keeping the finches from foraging there between the hours of 11 a.m. and 3 p.m. [...] Another time they saw a blue-footed booby wounded by a frigatebird. A fortis stood beneath the wounded booby and drank the blood as it dripped on a rock [...] All of the cactus finch fledglings died before they were three months old. Not a single fortis laid an egg or build a nest. (Page 75)

I hope you're getting the sense of just how severe this abiotic disturbance was compared to a 'normal' year on Daphne Major. Since temperature and precipitation are both components of climate, can we also say that the climate changed on Daphne Major during '77-78? Absolutely. What about geography? Geography was another term that Prof S listed underneath "abiotic factors." Did the geography of this island change during the drought? Probably not. The physical features--e.g., rocks, size of the island, etc.--likely stayed pretty much the same.
Biotic Factors on Daphne Major

What were some of the main biotic factors in action during the drought of '77-78? Was there any predation on the Forts? There was. On Daphne Major all of the finch species, including the Forts, were prey for a species of owl that lived on the island. Was there any mutualism? Perhaps we need to consider a finch-plant relationship here. The Forts eat seeds; many of the plants on Daphne Major produce seeds. Can you imagine an arrangement in which both the Forts and the seed-producing plants benefit from this relationship? I thought so. Imagine a Fort eating a bunch of seeds resulting in some of the seeds passing through the birds body undigested. The bird poops them out somewhere else on the island where this plant can now grow. Voila! Mutualism = both the seed plant and the Fort receive some benefit. What about competition? Was there any competition for resources--food? water? mates? nesting site?--among the living things on Daphne Major? Are you kidding me? Of course! Consider a few more excerpts from the book:

During...June [1976], when the island was wet and green, there had been more than 10 grams of seeds in an average square meter of lava. The finches had already eaten their way through many of those seeds during the dry season of 1976 [...] Day after day they went on pecking over the same square meters for the same diminished supply of seeds. By June [1977] there were only 6 grams of seeds per square meter. By December there would be only 3 grams. (Page 73)

As they always do in dry times, the birds went on looking for the easiest seeds. But now they were sharing the last of the pistachio nuts. They were down to the bottom of the bowl. In June [1976], four out of five seeds that a finch picked up were easy, scoring less than 1 on the Stuggle Index. But as the small, soft, easy seeds of Heliotropium and other plants disappeared, the rating climbed and climbed, peaking above 6. The birds were forced to struggle with the big, tough seeds of the Palo Santo, and the cactus, and Tribulus, [the] symbol of the struggle for existence, a seed sheathed in swords. (Page 73-74)

Now and then a frigatebird harried a blue-footed booby out of its kill of fish. If the fish dropped on the island, as many as ten or twenty finches would flock around it. They also scavenged broken eggs and fresh booby guano. They hung close when the boobies fed their young and fought for the fish scraps, and when owls left something of their kill, finches fought over that too. (Page 75)

In other years the finches had ignored the lava lizards that scuttle about the rocks. But once that year, Peter and Laurene saw a female cactus finch eating a black lizard tail, and nearby they spotted a femail lizard with a freshly broken stump. Some days later, they saw the same bird chase after another female lava lizard, pecking at its tail. (Page 75)

Suffice it to say, there was both interspecific and intraspecific competition for food amongst the Daphne Major ground finches. In the remainder of this Blog, however, I want to focus only on intraspecific competition and its results. In other words, I want to focus only on the competition for food within the Daphne Major Fort population. In the Lecture 13 Blog, I will focus instead on interspecific competition. In other words, I will focus then on the competition for food within the entire ground finch community on Daphne Major (the Fort population + the Fuli population + and the Mag population = ground finch community).

The results of intraspecific competition

At least within the Fort population, what was the result of the fierce competition for food on Daphne Major during the drought of '77-78? Actually, I already answered this question in Chapter II of our story, the overall numbers of the entire Fort population plummeted. Natural selection, in the form of a drought, selected AGAINST alleles for below average and average beak size/strength, and selected FOR alleles for above average beak size/strength. Here's another passage from the book that will make this point clear:

...during the drought, when big tough seeds were all a bird could find, [the] big-bodied, big-beaked [Forts] had come through the best. The surviving fortis were an average of 5 to 6 percent larger than the dead. The average fortis beak before the drought was 10.68 millimeters long and 9.42 millimeters deep. The average beak of the fortis that survived the drought was 11.07 millimeters long and 9.96 millimeters deep. Variations too small to see with the naked eye had helped make the difference between life and death. (Page 78)

And now, back to our original question...

Prof S began Lecture 12 and his discussion of biotic and abiotic factors by asking a question: Why are there so many species on Earth? I asked you a more context-specific question: Why might there be so many species of finches in the Galapagos Islands?

Perhaps we can now bring a few of these ideas together...

Since we know that beak size is a heritable trait in the Forts (i.e., it is passed on through the genes), did we see an example of natural selection during the drought of '77-78? Did we see a shift in the relative frequency of alleles present in the Fort gene pool? YES, WE DID! We saw that the genes that code for ever-so-slightly bigger Fort beaks (and body size) made it through the drought in higher percentages than the genes that code for the ever-so-slightly smaller Fort beaks (and body size).

And how was it that people like Peter Boag & Laurene Ratcliffe were able to come to these conclusions? Because they were population ecologists!!! Just look at the kinds of data (at right) that they were collecting about each bird that they could find on Daphne Major during the drought (click to make it bigger).

If you examine the image a right closely, you can see what Prof S was talking about in class when he talked at length about population ecology. In fact, in one of his slides he asked specifically, "How would you describe a population?"

Can you see basic demographic statistics here (e.g., age, sex, birth, death, etc.)? When Boag and Ratcliffe collected data about the Daphne Major Forts like this, you can see how they could then begin putting graphs and charts together that calculated things like birth and mortality rates. Their field notebooks were full of other information about the Forts social interactions, their food sources, their predators, their habitat, the number of eggs they laid, etc.

In this way, I hope you can see how this data might allow for the scientists to determine other things that Prof S talked about in Lecture 12, like survivorship. You tell me: What data would the scientists need in order to make a survivorship graph about the Forts on Daphne Major?

The scientists could probably also determine whether the Forts were semelparous or iteroparous. You tell me: What data would the scientists need in order to make this determination?

If the scientists had data from the years before 1977 (which they did), they could probably also begin to construct a population growth model for the Forts. From what you learned in Chapter II, you tell me: Would a population growth model for the Forts look more exponential or more logistic? What do you suppose the carrying capacity of Daphne Major was over these years? Would the carrying capacity be the same in a drought year as it was, say, in 1978 when the rainfall amounts returned back to normal?

So, did this abiotic disturbance--which led to intense competition for food resources among the Fort population--actually lead to a new species of finch on Daphne Major? No, it did not. But, what did Prof S tell you about the things needed for speciation, i.e., the creation of a new species? Well, back in Lecture 9, he talked about the Biological Species Concept and also about two kinds of speciation, "allopatric" and "sympatric."

Now, here's where your work comes in: At the end of 1977-78, I have left you with a population of Forts, the majority of whom have above average beak and body sizes. Can you take this Fort population, the Biological Species Concept, and the fact that there are other islands in the Galapagos besides Daphne Major, and can you now tell me a story about how this above average beak/body size Fort population might become two species at some point in the future?

If you can do this, then I think you'll be ready for a lot of the questions that Prof S might throw your way on Exam 2.

Which brings me to the line I used at the end of the Lecture 11 Blog ("Mom...Do Birds Grow on Trees"). When I asked you to think about connections between the ideas in Lectures 11 and 12 I tongue-and-cheek said that the connection between these two lectures is "In the trees..." What I meant by that line was that the bridge between the two lectures is actually the image of the phylogenetic tree. If you understand these representations--and by that I mean understand what each line represents, what the nodes or branching points represent, what causes the branching points, etc.--then you're on your way to being able to see the world more like biologists do.