Have you ever been out on a nice walk and noticed a cute squirrel eating a pecan and thought, where did he get that pecan? Where does he live? Is he afraid of heights? How big is his family? Does he prefer grasshoppers or berries for dinner?
How many squirrels live around here anyway? And what if there aren't enough pecans for all the squirrels? Surprisingly, ecologists have the job of asking those questions, most of those questions, and the answers reveal important information about the ecosystem and the organisms living in it. Today, we will be looking at a few different species and the carrying capacity of their environment. The carrying capacity of an environment is the maximum population that environment can sustain long term with the resources available.
A population is a group of individuals of the same species who interact regularly. So our squirrels here on the nature trail are a population. The resources we're talking about are the things that an organism needs to live and reproduce. These resources vary for different species, but are mostly things like food, water, shelter, and space. For our squirrel friends, this nature trail ecosystem that we're in has a higher carrying capacity than, say, a city block.
The trail follows this creek that provides plenty of water. There are even tons of grasshoppers and seeds, like sunflower seeds, for them to eat. The population is strong here, so why aren't there hundreds of squirrels running all over the place? Because even though there are tons of resources here, there are limiting factors that are keeping the squirrel population in check.
We have already covered a few limiting factors like food, water, and shelter. Not enough of those and your population will decline. But what about things like space and predation? Squirrels aren't territorial like bears or lions would be, but they do need space for their nests and they do not like other squirrels getting close to their babies.
So if nesting space is limited, it could force some squirrels out of this area. Squirrels'main predators are birds of prey, animals like red-tailed hawks, kites, and owls. But they can also fall prey to cats, dogs, snakes, and raccoons, especially when they're young.
Predation limits the population. the more squirrels an ecosystem has, the more predators it can sustain. In this area, there is a family of kites that I see all the time, and I can hear calling to each other when I'm out on the trail. I have also seen several snake species that would absolutely eat a young squirrel, like this copperhead.
Hi little chickadees! Don't poop on my head. The number of squirrels in this specific area is the population density.
It's the number of individuals divided by the size of the area. The population density changes when individuals are born or die, move into the area or leave the area. The limiting factors that we talked about earlier can be broken down into two types, density dependent factors and density independent factors.
Things like food, water, shelter, space, predation, and even things like disease are density-dependent factors. If this area was impacted by something like a wildfire and the squirrels fled to escape the danger, that would be a density-independent factor. The fire has nothing to do with how many squirrels were here to begin with, but absolutely limits the amount of squirrels the area can support after the fire comes through.
If you're still here liking this video, Hit that subscribe button so you never miss an episode. What about another example? This is passionvine. It's the host plant to a species of butterfly called the Gulf Fritillary. There is a population of butterflies that lives in this neighborhood because this plant is here.
In most seasons, the chief limiting factor to this butterfly population is the passionvine itself. Butterflies lay so many eggs that the population grows exponentially. Tons of caterpillars means tons of butterflies.
Then those butterflies mate and lay their eggs back on the same passion vine. Once the population grows to a certain level, the caterpillars can eat almost every leaf, leaving it looking like a skeleton vine. When the leaves run out, the caterpillars no longer have anything to eat and they will die before they reach their pupa phase.
The vine is a fast-growing plant and will bud new leaves to support the next generation. But this population will have reached its carrying capacity. The amount of individual caterpillars it can support with the given resources.
This year that same initial growth occurred and there were lots of caterpillars. But now the leaves are just covered in holes. The carrying capacity is being greatly impacted by a predator, the yellow jacket. Once the host plant became covered in butterfly eggs, it attracted the predators.
Yellowjackets search for butterfly eggs and caterpillars to consume and even destroy the protective chrysalis. I love these butterflies, so I decided to become a factor in their environment. I've started collecting the butterfly eggs and tiny caterpillars and moved them right next door to this screened enclosure.
The caterpillars grow happily, eating the leaves of the passion vine I place inside for them. Then go into their pupa stage and emerge as a beautiful butterfly. All inside here, protected from predators.
I then release the butterflies back into the environment, so the cycle can start all over again. Take that, yellow jackets! The carrying capacity for different populations of species varies from ecosystem to ecosystem and even season to season.
There's a lot going on, even in your own backyard, so go check it out! And... If you want to learn more science, you can check out this video next. You're positioning yourself right over my head and I feel like you're gonna poop in my mouth.