if you ask your neighbor to describe their Community they might talk about the other folks on the street or at their church or maybe the breakfast club-esque group of lovable Misfits that they call friends but you can be pretty sure that their answer will focus on a single species humans ask an ecologist to describe their Community though and they might tell you about the birds that build a nest in their gutters or the Gophers swiping carrots from their Garden or even the fleas that their dog is always scratching that's because in ecology a group of
different species living in the same area is called a community studying Community ecology helps us to better understand The Good the Bad and the Ugly when it comes to interactions between species in an ecosystem hi I'm Dr Sammy your friendly neighborhood entomologist and this is Crash Course biology foreign is sort of like an ecosystem minus all the non-living abiotic stuff like rocks water or weather science is all about asking questions so an ecologist might ask a big question like how do these animals coexist in the African Savannah or for that matter why did Mufasa think
it was a good idea to put an African hornbill in charge of babysitting a lion and a community ecologist might look for Answers by studying the interactions between the groups of a species within a particular environment like how a predator hunts for its prey or how a parasite can sap nutrients from other species now that might seem a little Bleak but the interactions between species in a community are not always violent or one-sided just like your community is not only the guy honking behind you at a red light or the neighbor that steals your Wi-Fi
a community can be huge with lots of interactions among populations or members of the same species and tons of interactions among different species including animals plants and even microbes by the way if you're wondering how we draw the line between species the simple answer is that if they can't make babies together they're usually different species having different species in a community is really beneficial because diversity contributes to community stability making the community more resistant to disturbance for example if a harmful parasite is introduced diversity could prevent the whole Community from becoming infected because some species
might be resistant other examples of disturbance include fires landslides flooding and hurricanes and some disturbances can wipe an area clean destroying much of the life present but the awesome thing about communities is that they rebuild to find out how let's head to the thought bubble following a couple months of Tremors Mount St Helens in the Pacific Northwest region of the United States erupted in May of 1980 blasting the top clean off the volcano killing trees in a 42 square mile Scorch Zone and spreading Ash for hundreds of miles it was a spectacular display that made
news worldwide but not everything died bigger animals like elk perished but mice survived large trees were blasted but seedlings of the same species managed to make it the frozen lakes and snow still covering the ground shielded some Aquatic Life and bacteria from the blast too in a phenomenon called ecological succession the organisms that survived and grew back fastest repopulated the area these pioneer species paved the way for slower growing species to eventually move in and out populate them the community continued to shift until it became stable in what is known as a climax community in
other words you're going to make me say it aren't you fine life finds a way thanks Bob bubble you always find a way too but let's move away from cataclysms and talk about life on an ordinary day in a community where members of different species are frequently connecting through what's called inter-specific interactions these aren't just one-off Random Encounters like when you're riding your bike really fast and you accidentally swallow a fly we wouldn't call you a fly Predator flies just need to be more careful instead with inter-specific interactions we're talking about ongoing long-term relationships that
are categorized by how the relationships affect each species involved positive if the relationship benefits an organism negative if it harms it and neutral if it doesn't really have an effect in either direction when one species eats another in an act of predation a positive negative scenario it doesn't exactly conjure up the warm fuzzies but that predator-prey relationship can play an important part in maintaining the balance in a community without enough Predators communities can get all out of whack like in 1859 this wealthy Australian guy had 13 live European rabbits shipped to him sounds adorable right
yeah only this guy had them shift in for sport well if I learned anything from 80s movies it's that the scrappy Underdog will always always beat the rich preps the rabbit outsmarted the hunters and well they did what rabbits do and today because they don't have a lot of natural predators to keep their population size in check the European rabbit is an invasive species in Australia contributing to the decline of native plants and animals predation is not just animals eating other animals animals eat plants too we call that herbivory of course when there are plenty
of predators the prey need a way to help maintain the balance on their side enter our good friend Evolution some animals respond to predation by evolving defense mechanisms like the ginormous super sharp quills of a porcupine but what do plants do when danger is near let's set the stage and find out over in the theater of life so Dr anurag agrawal an ecologist showed that animals are not the only ones with defense mechanisms some plants develop techniques for responding to predation as well Agarwal and his colleagues were studying a population of radishes commonly eaten by
caterpillars to figure out if the plants could fight back and how when he tested some plant samples he found the caterpillar Munch plants were producing 10 times more of a defensive chemical compared to unmunched plants but agrawal isn't just an ecologist he's also an evolutionary biologist who wondered if this defensive mechanism might get passed on to new generations so in a later experiment he compared seedlings from the damaged plants to the undamaged ones and he compared caterpillars that munched on each he found that the caterpillars who snacked on seedlings whose parents had also been nibbled
gained 20 percent less weight this tracked with his hypothesis that the plants changed from generation to generation to better protect their offspring from Hungry caterpillars whoa they really upped the production value today so Dr agrawal's Discovery helps us understand how plants can fight back but what about those species that don't use protective chemicals sometimes they evolve to look like other more threatening species like the owl butterfly which gets its name from The prominent yellow spots on its wings which resemble the eyes of an owl this mimicry can provide them an evolutionary Advantage by scaring off
predators and then you have your lose-lose scenarios when two species are in competition for the same resources the situation kind of stinks for everyone involved it it negatively impacts both species neither has enough of what it wants and it limits the survival and reproductive ability of both this happens because the organisms have an overlapping ecological niche which includes where and how they live take Virginia's warbler for example which contrary to their name lives in the American southwest not in Virginia in central Arizona the orange crowned warbler occupies the same niche as Virginia's warbler ecologists removed
one species to see if the other would Thrive and sure enough it did indicating that the two were in direct competition with each other kind of like a roommate that just eats anything in the fridge without asking who it belongs to oh wait that was me so some organisms that share the same Niche use a technique called resource partitioning which is like drawing a line down the middle of their habitat it alleviates the mutually negative competitive relationship take the golden spiny Mouse and the common spiny Mouse whose niches overlap in Rocky habitats in the Middle
East they both like to stay up all night they're they're nocturnal but the golden spiny Mouse adjusts his biological clock when it has to share space with the common spiny Mouse becoming active in the daytime instead but sometimes a species is just really bad at getting along with the other species around it and it kicks out its competitors entirely this competitive exclusion happens when two species actually share the same Niche and one species has an advantage over the other like the single-celled paramecium Aurelia and paramecium chordatum independently they grow quickly until they exhaust their resources
but put them together and you have got the paramecium equivalent of an old-time Western Showdown and then there are parasites unlike competition which is negative negative for both parties parasitism is a positive negative ship where one species benefits a parasite lives in or on another species called a host and benefits by taking nutrients or resources from the host of course not all interactions between species are antagonistic take the bacteria in our guts they help us break down complex carbohydrates and get a tasty snack and a cozy place to live in return that's a mutualistic relationship
where we both benefit and even some bacteria that aren't actively helping with digestion usually aren't hurting us so it's a form of commensalism meaning a positive neutral relationship so ecological communities much like the human ones that we're a part of consist of many different types of relationships and some of those relationships exert more influence than others for example elephants and beavers change the availability of resources for other species by altering the biotic and abiotic environment elephants knock down trees and stomp giant mud holes beavers build dams changing the environment and making them what's called ecosystem
Engineers the influence of ecosystem Engineers on other species in the community is just one example of how keystone species disproportionately impact their Community relative to their population size without keystone species the community in its current state wouldn't be able to exist in quite the same way anymore and even something as small as a Bee which pollinates flowers helping plants reproduce in an act of mutualism can serve as a keystone species that's because all of the species that feed off of the plants and the animals that in turn feed off of them rely on those bees
and of course big strong Predators like wolves and lions and bears can serve as keystone species as well by reducing the number of plant-eating herbivores in turn increasing the number of primary producers like plants or algae when these Predators on higher trophic levels affect the population size of organisms below them we call this top down regulation regulation can also start from the bottom since the energy available from primary producers limits the number of predators at the top of the food chain this type of control is called bottom up regulation this type of ecological regulation isn't
an intentional thing a wolf doesn't wake up one day and decides there should be more plants in their community and then Gorge itself on a buffet of local deer that's because top-down regulation is an emergent property meaning it arises organically and collectively from organisms all surviving in their own unique ways and through that whole complicated systems of relationship and energy transfer emerged if think about a food chain which follows a single path of energy through a community showing how energy passes from the Sun to plants to animals and then to bigger animals and so forth
it's actually way more interesting than that because it's rare for the energy sources near the bottom of the food chain to only have one Predator like the same plant may be eaten by a rabbit and a tortoise and a sheep so trophic structures emerge with many interconnected food chains that end up looking more like webs really and these food webs are also an emergent property of a community meaning they arise in response to many little actions from members of the community acting all at once Community ecology helps us understand how species interact in direct and
indirect ways it helps us to come to terms with the violent acts like predation but also see the beauty and mutual relationship between a bee and a flower and like in Dr agarwal's radish plan it can reveal ways that interactions between species can lead to evolutionary changes through generations and of course we humans are a part of these communities as well we might not always realize it but every day we interact with different species in our communities at times our behavior Can drastically alter living conditions for all of us humans might be one of the
most consequential keystone species around and that's a big responsibility but the more we understand our relationships too and our Reliance on other species the better roommates will become this series was produced in collaboration with hhmi biointeractive if you're an educator visit biointeractive.org crash course for classroom resources and professional development related to the topics covered in this course thanks for watching this episode of Crash Course biology which was made with the help of all of these nice people and if you want to help keep crash course free for everyone forever you can join our community on
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