What is survivorship? What does a survivorship curve show? Explain what the three survivorship c

What is survivorship? What does a survivorship curve show?...

Key Concepts

Survivorship

Survivorship refers to the proportion of individuals in a cohort that survive to each successive age. It is a fundamental concept in demography and population ecology used to understand life expectancy and patterns of mortality across different life stages.

Survivorship Curve

A survivorship curve is a graphical representation that plots the percentage or number of individuals surviving at each age for a given cohort. This curve highlights mortality patterns throughout the life cycle, helping researchers infer reproductive strategies and environmental pressures affecting a population.

Type I Survivorship Curve

A Type I survivorship curve is characterized by high survival rates in early and middle life, with most individuals dying in older age. This pattern is often seen in species that invest considerable parental care and resources in fewer offspring, resulting in low juvenile mortality and a concentration of deaths in later life stages.

Type II Survivorship Curve

A Type II survivorship curve shows a relatively constant mortality rate throughout an individual's life, meaning the probability of death is roughly the same at all ages. This pattern can indicate that individuals face consistent risks regardless of their age, often due to factors such as predation or environmental challenges that affect all life stages equally.

Type III Survivorship Curve

A Type III survivorship curve is defined by very high mortality rates in early life, followed by relatively low mortality if the individuals survive the initial stages. This pattern is typical in species that produce many offspring with little parental care, where only a small fraction of the young survive to adulthood, and those that do have a higher chance of continuing to live.

Transcript

00:01 This question asks us about survivorship curves and what they mean and how those apply to humans, squirrels, and clams.

00:07 Okay, so first we have to define survivorship.

00:10 So survivorship is going to be the fraction of individuals in a given age interval that survived to the next interval.

00:21 Okay, so basically, as you can see here, this is a percentage of maximum lifespan on the x axis.

00:25 That's basically telling us the age interval, right? and we could subdivide that further.

00:29 We could divide that into a 25%, and we could divide that into 75%.

00:35 And basically, this is showing how many individuals survive past a certain age, or how many, if you start out with a certain number of individuals at birth, how many individuals continue to survive as time passes on through their life expectancy, right? so survivorship would basically be like a proportion, proportion of individuals in given age bracket which is percentage of max lifespan that survive to the next age bracket so that survive to the next interval right so we can subdivide the percentage of maximum lifespan as much as much we want like we can make this a 10 % as well and a 20 % there but if we just look at the curve that we see here we can just get the general idea of how many uh individuals survive through each interval of their development right so uh i think we just need to look at the curve right we need to look at the curve and explain what the curve shows us before going on to the human squirrel and clam model so basically the curve itself, the curve will tell us the fraction of individuals in the population that survive at each age interval during life span.

02:32 So basically it tells us exactly what the survivorship tells us as well.

02:37 So this just basically makes it into a graphical illustration.

02:41 So the curve, it tells us also the fraction of individuals at...

02:53 A given point because the curve really you can narrow it down and look at a specific point on the graph at which they are so fraction of individuals at a given age or we can say development development through their lifespan so it's the fraction of individuals at a given age that can survive to next interval so the curve is basically a visual representation of our survivorship right and so if we plot the survivorship for humans squirrels and clams we get the following curve that emerges right so we see that for humans there is a high survivorship there's um basically what we're trying to say is that if you start out with let's say a thousand survivors so a thousand people born they will be able to survive for a decent percentage of their lifespan until the fraction of people dying would begin to increase.

04:09 And we would see that the people past that lifespan would decrease as well.

04:16 So basically, the number of people that survive past 75 will be greater than the number of people that survive past 100, but it will still be less than the number of people that survived past 50.

04:30 And as you can see, you can trace that right along this curve right here.

04:34 Right.

04:34 So we see that for humans, very few die young.

04:38 Most live out almost a full lifespan past 75 and just eventually die of old age.

04:47 Now, if we look at, if we look at squirrels, for example, right? we see that for the squirrels, they have a straight line.

04:58 It's not a curve.

04:59 We see that the number of individuals, if they're squirrels, they have a constant mortality.

05:06 So the number of individuals that survive is going to be independent, really, of the life expectancy.

05:15 So basically from this, we can conclude that we see a constant mortality, constant mortality, and equal chance, equal chance of surviving at all ages.

05:39 And we didn't label this for humans, but for humans we see few deaths at young age.

05:52 We see that a lot of the humans live out a full lifespan.

06:01 So a large proportion, live out full lifespan.

06:16 And we also see that eventually they just die of old age.

06:21 So die of old age...

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