this is the video for d4.2 on stability and change we'll be covering standard level topics related to stability theme D is all about continuity and change and so continuity really relates to the stability of ecosystems so stable ecosystems are ones that can persist for very long periods of time without changing into other types of ecosystems so for example the Borneo lowland rainforest has been in existence for about 140 million years disruptions May alter some of those stabilizing ecosystems which can lead to those ecosystems being unstable so we'll take a look at these in more detail in order for an ecosystem to remain stable it needs a constant energy Supply maybe from the Sun it needs nutrient cycling so we think about carbon cycle nitrogen cycle phosphorus cycle and all those are going to depend on things cycling through different organisms and abiotic processes we need genetic variation within a species to survive the various selective pressures that an ecosystem can contain at different points of time and a relatively stable climate so thinking about things like temperature and precipitation disruptions could include removal of materials so deforestation is an example of removal of things like trees removal of species so things like poaching can definitely alter um the species diversity in an area utopic so having too many nutrients especially in bodies of water that cause things like algae blooms and it can change um things like oxygen availability and then of course climate change is a big disruptor affecting ecosystems especially when we think about ecosystems like coral reefs that can be very sens sensitive to some of these disruptions sometimes these disruptions can be overcome and then that ecosystem will stabilize again so disruption and then returning and then disruption then returning however a Tipping Point is when a level of disturbance is reached that causes a very quick change and it is difficult to reverse so that ecosystem won't kind of like revert into its more stable form these involve positive feedback loops and we can look at the example of the Amazon rainforest so deforestation okay causes fewer trees and trees are responsible for transpiration they give off a lot of water vapor so if there are fewer trees that means there's less transpiration which means there's drought so that condensation won't be making its way through the water cycle and if there's drought that means there's fires and if there's fires that means even more trees are going to be destroyed which means that there are even fewer trees and even less transpiration and even more drought and even more fires and so you can see how this is an example of a positive feedback loop now the difficult part for ecologists is that the Tipping Point can be uncertain so at what point deforestation will kick off this positive feedback loop that's impossible to come back from we don't always know so from an experimental point of view one of the really interesting tools that we have to investigate some of those variables that impact ecosystem stability are things called mesocosms mesocosms are like little miniature ecosystems they are small replicates of ecosystems that allow for controlled experiments so here I've shown some mesocosms re uh that are replicating an aquatic uh ecosystem so here's an aquatic ecosystem here's one and here's one and these replicates are really important because I can control variables to isolate something that I think might have an impact and I can also do many different trials now the best mesocosms are going to be closed systems and that's because this um really kind of illustrates the idea of nutrient Cy cycling and um why that that's important because the Earth in of itself is a closed system nutrient Cycles have to take place um so these are open systems these are closed systems but they are both mesocosms some of the advantages here are that I can do a lot of replicates um they're much easier to investigate and I can control certain factors but one of the big disadvantages is that I'll never be able to replicate all of the Natural Factors so all of the different interactions with the different organisms and the different biotic factors that are in there so we can learn a lot from mesocosms but our conclusions are limited by the fact that we cannot exactly replicate the natural ecosystem one of the interesting factors to investigate in terms of ecosystem stability is the effect of keystone species now keystone species are organisms that have a disproportionate effect in the community that when you remove them the entire ecosystem May collapse that it's not just their population and those that they feed on um it's not just those inter or like very immediate relationships that are affected it's the entire ecosystem and one of the fun experiments that I urge you to look further into is pay's sea star experiment this is a sea star and some of you may call this a starfish it's the same organism but just biologists don't use the word starfish anymore because they are echinoderms they are not fish um but this is a great example of a keystone species and pay was looking at a few different areas in which he was experimenting with the removal of different species in the control group okay the presence of these sea stars led to relatively stable populations of a whole host of other organisms the removal of these sea stars eventually caused an area to be almost entirely populated by mollusks and those mollusks crowded out every other type of organism and even including different types of seaweed and other um photosynthetic organisms so just a whole host of changes happening in that ecosystem due to the removal of just one organism and that is how we would identify a key Stone species