Today, I'm going to be talking to you about biodiversity and its maintenance. And I'm going to talk a little bit about glittering generalities, but then I'm going to focus down and talk about a specific plant community in the Sonoran desert, the desert winter annuals, where we can focus down and really understand what's going on. So, people have long been impressed by the great diversity of life on Earth. And ecologists in particular ask, why there are so many species and why there aren't more, why aren't there less and how do they coexist without displacing each other? Also, everybody is worried about what's the future of biodiversity. Well, the traditional explanation of the biodiversity and species coexistence is niche differences. And the ecological niche is loosely thought of as an organisms address and its occupation. So, what it eats, where it lives, when it's active. And to coexist then, species need to have different niches. A kind of cool example from the deserts of southwestern United States is the desert iguana and the chuckwalla, which are two big lizards, they're second and third largest lizard we have in the deserts. And they're big daytime lizards that eat plants. And their species ranges largely overlap. They're kind of hard to tell the difference if you squint your eyes. And so, they have similar range, similar diet, and similar activity times. So, how do they coexist? Well, it turns out that the desert iguana likes to live in the sandy creosote flats, whereas the chuckwalla prefers the rocky terrain, where it likes to hide in crevices where it can blow itself up and escape predators. And so, they each have their own habitat and they can coexist despite all these commonalities by having that niche difference. Well, I'm going to focus on biodiversity in plants as I said. And plant communities can have lots and lots of species, hundreds of species. So when people first started thinking about it, they kind of scratched their head and they said, "Oh, no. How do you get hundreds of niches? Let's see what the plants use. They use water, CO2 and a set of minerals. How do you divide that up in 150 different ways?" So they scratch their heads initially, but being clever people, ecologists eventually came up with too many explanations. So they had a chaotic diversity of coexistence mechanisms that were proposed by the end of the last century. And today, it sort of settled down into this categorization here of coexistence mechanisms. And I'm not going to talk about every single one, but I'm going to talk about the ones that I think are important for desert winter animals. So, some of these dependent upon environmental fluctuations, if you don't have a fluctuating environment these don't work. Others are independent of fluctuations, and so they're fine even in a constant environment. So, the first one that's important for plants in general and also for the desert winter annuals, is resource partitioning. And resource partitioning is pretty well understood. That's what I just described with the chuckwalla and the desert iguana. So with plants, some plants like the north side of the hill, some plants like rocky environments, some like sandy environments. There's a whole bunch of just different things, wetter, drier, hotter, colder the plants partition so that some plants do better under some circumstances, some in others. In that way, they all coexist without excluding each other. I'm not going to talk much more about that. But the next thing that could be very important for plants and for desert annual plants is frequency dependent predation. And so frequency dependent predation is when predators eat more of organisms that are common. And so this is sort of a Robin Hood effect taking from the successful and giving to the unsuccessful. So in deserts, there are lots of seed eating animals: rodents, ants, and birds. And for annual plants, they're not even above ground for most of the year, but the seeds are there all the time. And so a very important source of predation for desert annual plants is seed predation. And so, if they switch their preference to common seeds, that could do a lot to keep a lot of species coexisting in nature. So in the places we've worked, there's a diverse community of seed eating rodents, there's cactus mice, pack rats, pocket mice, kangaroo rats, and these things are all roaming around at night and eating the seeds. So the question we had was, do these rodents preferentially eat the seeds that are common? So, to study this, we just took some standard cafeteria trays, fill them up with sand and we put seeds of these three species in it. These species here, these are the seeds, these species here and these species here. And a third of the trace, these species was common and the other two were rare. And then in a third, each of the other species was common. So then, we put these out at night and we left the rodents go and see what they did. And what we found is that, yes indeed, the species are preferred when they're common. So this is erodium cicatarium, and when it was the common species, it was preferred, pectocarya carrier was preferred when it was the common species and plantago was preferred when it was the common species. So, yes indeed, each species is eaten more when it's common, less when it's rare and this should have an important stabilizing effect on coexistence in desert plants and desert annual plants. So the next coexistence mechanism I want to talk about, is something called the storage effect. And the storage effect is one of these fluctuation dependent mechanisms. Probably it could work pretty well in deserts which are famous for being highly variable and unpredictable; especially in the rainfall, which is a big driver in deserts. So, the storage effect. And so the way the storage effect works, is that species need to have different niches in time, so that they can coexist. So they have to respond differently to the environmental variation that's out there and each species have to respond differently enough that they can coexist. Well, so this environmental variation involves things like temperature and rainfall. So, in deserts, they're so dry that rainfall is really sort of the thing that limits everybody. And so, that's how much rain there is in a given year? The timing, is it early in the season, late in the season? Lots of little events or many big events. All these things have a big impact on the species that make their populations vary not necessarily in synchrony. Well fortunately, we are in a good position to say this, because we have 35 years of weather and population data for lots of species of Sonoran Desert annual plants. And we've done this work at the Desert laboratory, in Tucson Arizona. And this was founded in 1903 by the Carnegie Institute of Washington, to study desert ecology and it's been fenced in and gray since 1906, and we set up permanent plots to look at these desert winter animals in 1983 and have been doing it ever since. So individual plants may germinate, survive, they may die. They might germinate, survive, grow and reproduce make more seeds. And alternatively, the seeds don't have to germinate. Seeds might not germinate and then they could survive to the next year as a seed or maybe not, they might die as well. So we've documented this for a lot of species and for a lot of years. And so, we can use this to address this temporal niche, see if plants are responding differently enough for them to coexist. The storage effect can work if the species are responding differently in terms of their germination, what fractions of the seeds germinate each year or in terms of their reproduction, their survival growth and reproduction. If they're a little bit desynchronized and are different enough, they may be able to coexist. So specifically, the coexistence mechanism involve in decoupled germination requires two things. First; there has to be storage, which is what the storage effect comes from. So, there's got to be a buffer due to the surviving and germinating seeds in this situation. So, it might be a really good year or a really bad year, but if it's a really bad year, there's at least some seeds that didn't germinate and can come back next year. So that's the first part. And then the second part is that the deep germination fractions have to be decoupled between species. So year one, species A might germinate more, year two species B might germinate more. Same thing can work with decoupled survival growth and reproduction. So annual plants should be able to bounce back from rarity, low numbers when there's buffering again due to seed bank storage and then there's decoupling in the vegetative phase. So some years species A might have higher survival and reproduction, and in some years species B might have higher survival and reproduction. So these are two pathways, the germination, variation and the reproductive variation. And so, what we do, is we took all this long term data that we have and we partitioned, we measured the storage effect and we partitioned it into these two things, decoupled germination and decoupled reproductive variation. And the way people measure the coexistence promotion, a really good way specially for things that are varying through time, is can things bounce back from low numbers? Because it doesn't matter so much if the species is more common and abundant in different years, but the important thing is when it's rare, it doesn't keep going down to zero. So when things are rare, can they bounce back? And so we measured the storage effect as the low density advantage. So on average, these species, do they increase when they're rare or do they not? And it turns out that they have about a 10 percent boost when they're rare in their population growth rate. And about half of that is due to this decoupled germination variation, about a quarter of it is due to the decoupled reproductive variation, and then another quarter due to the co-variation between germination and reproduction. So we were able to sort of partition out the storage effect coexistence mechanism. And then the key point is that when species are rare, they have this growth rate advantage, population growth rate advantage, which is sort of equivalent to a doubling time of about seven years, which is pretty good. So that should be a lot of stabilization due to the storage effect. The strength of the storage effect was estimated and we could estimate it from long term data and it depends on to work, it depends on species responding differently to environmental variation in time. So the conclusion so far is that, differences in population responses to environmental variation promote coexistence. We have environment varying in the amount and timing of rain and the populations are responding to that going up and down but they're doing it differently. They're not totally synchronized and germination and in survival and growth. Then that leads to an important community property which is species coexistence and biodiversity. But the question arises as to, why are these species out there in the same environment, they kind of look the same as little annual plants, why are they responding differently to the environment? And the hypothesis is that it must have something to do with their functional traits, their physiology, anatomy, and how those plants work. So they're experiencing the same variation but they're responding to it differently. And so, we want to know then, what causes this population dynamic decoupling in germination and in reproduction. And I'll talk about that in another module. So, thank you for today.
