This is part of the IB’s NEW Biology syllabus, which means you have exams in May 2025 or later. The videos were designed and recorded by Cheryl Hickman
This video is for the higher level content from d4.2 on stability and change and we’ll be looking at ecological succession ecological succession is really about a transformation of an ecosystem and that can be due to biotic or abiotic changes so biotic changes might be like a change
In species so looking at grasses um being like the predominant plant species to maybe a change over to trees it could also be abiotic factors like having less ground light or lower temperatures we’ll go into types of ecological successions some of them are linear some of them are cyclical but in general ecological
Succession is really about transformation first we’ll talk about primary succession this is really the beginning of ecosystem development so primary succession is succession starting with an environment with very few living organisms so we’re starting from scratch here so we’re going to start with be Rock and I think of this
As being like after a lava flow will totally wipe out an ecosystem and I’m left with just this blanket of ous rock right so that bare rock is then um going to be worked on by bacteria and lyans so they’re generally the first organisms that kind of come into this ecosystem or
They may already be present and what they really do is they cause soil development through the erosion process so they erode um that bare rock and then I start to get soil once you have soil then you can start to grow plants and the herbs are really going to be the
First type of plants that are growing then you will see the development of shrubs and then finally you will see the development of trees and so this is all what we call primary success session and during that process we’re going to note some changes one being the increase in
Primary production so this is mainly going to be through photosynthesis so once we start having things like herbs and shrubs we’re going to see a lot more photosynthesis uh taking place and that will continue as we progress towards trees we’ll see an increase in species diversity so that’s the number of
Species pres and because of that we’ll also see an increase in food web complexity or these feeding relationships and then finally we’ll see an increase in nutrient cycling so if we think about the carbon cycle or the nitrogen cycle things like that they’re dependent on the organisms um that live
In that ecosystem so the more diversity we have the more nutrient cycling will take place which means the more species we can um uh support so on and so forth now we talked a little bit about ecolog iCal succession being maybe like linear or cyclical let’s talk about some
Cyclical succession these are going to be patterns that’s really important patterns of change and succession that occur either naturally or due to periodic disturbances so for example this oak tree um just dominates this oak forest ecosystem now when it drops its seeds these seeds are very intolerant of
Shade and so they can’t grow in the shade um so no new oak trees are produced or very few So eventually when all of these oak trees die out this ecosystem becomes dominated by grass and so it turns into a grassland but some of those oak seeds that dropped previously thrive in the
Grass and so then new oak trees um are uh produced or new oak trees grow I should say and then it reverts back into an oak forest so we get this very cyclical uh succession Evergreen forests which are dominated by Evergreens like conifers like you can think about a pine tree
Often will regenerate after fires so that’s an example of a periodic disturbance that changes or transforms an ecosystem but then it Cycles back around because of this there is no stable climax community and those climax communities are those very stable relatively unchanging ecosystems okay so again we just talked about ecosystems
Like the oak um undergoing ecological succession and turning into the grassland which then goes back into the oak that’s cyclical okay climax communities are going to progress more like this okay and so we’ll get climax communities like something like a tropical rainforest that’s relatively stable and unchanging now
Plagioclimax communities are a type of climax community but they’re the result of human activity so something like fields for livestock uh grazing or Wetlands that have been drained are stable but they’re not the natural climax community they are the result of human intervention so to dive deeper into that grazing areas are usually
Protected and they’re protected from predators which means that there are no Predators or very few Predators um to eat these grazing animals and so you get an artificially High population of these grazing livestock the grasses persist the grass grows back after their blades are eaten but there’s no succession by
Trees or shrubs those are either prevented from the animals themselves um or from the um the farmers so you don’t ever get that succession into a forest Wetland drainage is also interesting so this is where um swamps or bogs um have water diverted from them so those swamps and bogs remember are
Natural carbon sinks they develop into things like Pete which stores a lot of carbon that water tends to be anoxic and so when we create a water logged anoxic environment um there’s really only some organ some organisms like methanogenic bacteria that have the special adaptations for living there when you
Drain that water away what you’re doing is you’re removing this water logged aspect from the environment and you’re going to see the introduction of species that can produce oxygen so it’s no longer anoxic so other plants um and things can thrive in that area now and so you’ve created a stable ecosystem but
Again both of these are examples of plagioclimax communities because they’re due to human intervention and so again here what we want to be thinking is theme D continuity and change okay humans can cause changes to environment which make a stable continuous ecosystem but that might be different than the
Climax community that would normally exist there