Lyme disease is caused by infections of the bacterium Borrelia burgdorferi (stained blue in inset, among red blood cells) which is transferred to humans via blood-sucking bites from the tick, Ixodes scapularis. Lyme disease can cause flu-like symptoms in the short term and more serious illnesses in the long term, if not treated. Why is Lyme disease on the rise in eastern North America? White-footed mice occupy a broad ecological niche—they occur in most communities regardless of habitat quality. Many other species, like the opossum, are absent from low-quality forest fragments. Based on this information and the data in Question 12, propose a hypothesis to explain the observed relationship between increased forest fragmentation and increased incidence of Lyme disease.

Video transcript

Hi everyone. Let's look at our next question, It says blank is an organism that helps define an entire ecosystem. When we see a question like that, something like, helps define an entire ecosystem can sometimes feel a little vague or something where we can't pull up off the top of our head um exactly what this might mean, or what words might use to define it. In that case, it's always useful to go straight to our answer choices and see if one of the answers looking through the answer choices makes that more clear for us. So when we look through our answer choices, we see we've got choice. A an endangered species were an endangered species would be a species that's in danger of becoming extinct. So it's numbers are very low. Um and if nothing changes, it will become extinct most likely. Um But if we're talking about defining an entire ecosystem, there's no reason that endangered species would define an entire ecosystem, You know, might be something that's just part of one of many birds or something like that. Um So that doesn't match what we're looking to define. Tours B says an extinct species, extinct species is one of which there is no living members anymore. Um but again, you wouldn't describe an extinct species doesn't necessarily have to define an entire ecosystem, an ecosystem being again, that interaction between living non living things. Um in a hole in a geographical area with distinct characteristics. I see says a threatened species. Well, a threatened species would be one that um population numbers are getting lower and it's at risk. It's weird at it's at risk of becoming endangered. But again, this description of its numbers, how many of the species are. Doesn't tell you whether or not it's definitive or key to an entire ecosystem. And finally we have choice D. A keystone species. And yes a keystone species would be something you could describe as defining an entire ecosystem. Um A keystone species says we can recall from our content. Video is one where without this species its ecosystem would be drastically different or even cease to exist. So something on which the whole network is so dependent that we're missing. It would cause drastic changes or even destruction of its ecosystem. And an example of this is actually the sea otter. Sea otter eats sea urchins which keeps the urchin population and check sea urchins consume help and if their population grew too large that would destroy the kelp forests, well without the kelp forests. Um kelp is of course a prime source of food. It's the largest source of photosynthesis in that ecosystem. So it's removing carbon dioxide from the atmosphere affecting climate. Um It preserves coastlines and of course most importantly is this huge source of food for animals. So without the sea otter in place you would see this destruction of the kelp force which would have this drastic destructive effect on its ecosystem. So the sea otter is an example of a keystone species. So again an organism that helps define an entire ecosystem is choice D. A. Keystone species. See you in the next video.