Hello students, Plants and Animals of Southern California.  We're on the seashore today at the edge of the sea.  Later on we're going to go out onto the low tide area, intertidal area.  And so, I wanted to talk a little bit about the nature of communities, the nature of ecological communities.  And these communities are made up of populations of species.  But I'll just call them species usually.  Of course, it's not the whole species, it's just the representatives of the species in the local community that are acting upon one another and interacting with their environment.  They are sometimes also called Avatars.  They are actors in a play.  These species or avatars then each have their own niche.  And what I mean by that is that they have a range of tolerances that they can handle and then they have a certain requirements or needs.  And those limit their abundance and their distribution--the abundance and distribution of each of these species.  And the species, I'm imagining are not particularly closely relative to one another, whereas in other times when I'm talking about species, you know, I'm talking about closest relatives that have diverged a little bit and have undergone speciation or something like that.  Here, I'm talking about barnacles and mussels.  So these species then, their distributions are shaped by a number of things, and that's because they all have their kind of peculiarities to them.  We might first consider the abiotic factors that would be responsible for the differential distributions of the various species in the community.  Along the seashore, I think I might divide it into three classes of things.  First of all, there's the tidal height.  So, there's organisms that are very sensitive to exposure to the air or changes in temperature or changes in salinity, and so they can't handle high tide.  They can only live at low tide or even subtidally.  Whereas there's other organisms that are able to withstand the daily cycle of exposure and so they can live at higher tide.  So, the first of my categories would be tidal height.  The next category would be the nature of the substrate.  So, there are certain organisms that can only live on rocks, you know, bed--they're just solid bedrock, that's what they need.  And then it could even be the nature of the rock.  Some rock has to be soft enough for them to drill into or hard rock, you know, that could differentiate where two different types of organisms could live.  Some live on gravel and can tolerate sands.  Some live in the sand and require the sand.  And then some, of course, live on very fine mud.  They have to dig into the mud or something like that.  So, there's always different types of organisms that live on the seashore and they're kind of sorted out based on what kind of substrate they can tolerate and what kind of substrate they require.  And then the third category of abiotic factors that I think is really important along the seashore is the amount of wave exposure.  So like mussels really thrive or the waves are crashing right into them.  And a lot of organisms can't handle that crashing wave, whereas, mussels are not going to grow very well, you know, on the backwater here where there's hardly any flow of water.  Then that's partially because the waves are bringing in food and so animals that are living off of that food need the wave action.  Also, the wave action it reduces the boundary layer and that means that carbon dioxide and oxygen can be exchanged with the organism if there's greater flow of water.  Now, there's other things I'm sure you could think of is abiotic factors that affect the distributions and abundances of various species in the community.  All of those abiotic factors we might call the fundamental niche, right?  It doesn't maybe mean it's more fundamental than other things.  But anyway, the abiotic places where an organism could live in theory are called the fundamental niche.  And then, the realized niche is some modification of that, it's also based on the other organisms that are there.  On other words, the biotic interactions that these focal organisms that are having with other organisms.  And then, just for the sake of symmetry, I'll also classify those down to three kind of categories, although you could go for two or five or, you know, those all sorts of different ways to do it.  But I'll go for three.  The first one I'll mention is competition.  So, if we look along the shore here, we can see a place where there's kind of a zone of small barnacles, and then below that there's a zone of larger barnacles.  And I think those small barnacles would love to grow down lower where they get more water.  And they do--you do find a few of them there but I think that they're actually physically displaced by the larger barnacles.  They just kind of pushed them off the rock.  Now, the larger barnacles are more sensitive to exposure to the air and so they don't grow as high up.  And so there is this partitioning of the habitat by these two barnacles into different zones and part of that is based on competition between them.  The second category, I'll just sort of generally call facilitation.  Those are positive interactions between organisms.  So for instance, there's places where we'll see rockweed on the rocks.  And those rockweed, they allow a series of other species to do really well there, you know, like there are snails and limpets that are kind of hiding in amongst the rockweed.  If the rockweed weren't there, those species would not be doing as well.  So, there's this positive interaction that the rockweed has on other members of the community and allows for their distribution to be greater than it would've been just based on the abiotic factors.  So the third category that I want to talk about is predation.  I could have lumped predation in with competition and call them both antagonism, but I think it's helpful and at least in this kind of community, to distinguish it out is kind of a third type of thing.  And that's because the enemy of my enemy is my friend.  And some of these predators here eat different things that are competitors.  And so, for instance, if we'll see a mussel bed, if it were just for competition in abiotic factors, that mussel bed would be just kind of solid mussels.  But then what happens is sea stars come along and they eat the mussels and they make little holes in the mussel bed.  And those holes on the mussel bed make it so that goose-neck barnacles can thrive or just get a little patch of goose-neck barnacles that can thrive because the mussel beds were disturbed by the predator, the sea star.  So, predators are another thing that shape the structure of the community.  We might even go as farther and say that in addition to niche differences that are based on abiotic factors and biotic factors, there's also niche differences that are based on the ability to take advantage of disturbance.  So there are species that are really good at going in and when there's a disturbance, thriving for a while, and then there are other species that are not so good at thriving right away but then their population eventually kind of catches up and pushes out these early colonist.  So that again is another kind of access whereby we might differentiate the different avatars in this community and how they are spreading themselves out on--along the shore.  The niche is an interesting term and it something that I kind of come back to over and over again.  On the one hand, the niche is those different aspects of the habitat that are important to one organism or another.  And on the other hand, the niche is also the characteristics of the species that allow them to take advantage of those habitats, or that keep them from spreading into other habitats.  So it's this word that has this kind of dual meaning to it.  People have tried to separate the meaning, and if you want to, you can.  The habitat one is called the Biotope.  In other words, species partition to biotope.  And they partition it based on their characteristics which would be aspects of their niche.  But if you wanted just use one term for the whole thing, it would be niche.  I don't think the biotope term has taken on that much, nor has the avatar term taken on that much.  So, as you are walking along the shore and looking at the different wave of exposures and the different substrates and different tidal heights, try to pay attention to each of the avatars in this community, they are like--they are the actors in a play, right?  So, that's all that I have to say about that.