Importance of Natural Resources

Population Ecology Questions 2

In the previous video clip, we addressed
the first two of these questions. This short video clip will address the last
three. First, what is the pattern of reproduction? A species reproductive
strategy can affect its potential for growth. The term “reproductive strategy”
refers to the pattern a population shows with regards to reproduction. For example,
its lifespan, its maturity age, the number of offspring per litter, etc…. These
characteristics are all biologically driven. The population doesn’t sit around
and strategize about how they should approach reproduction. We commonly refer
to two general reproductive strategies, “r-adapted” species and” K-adapted”
species. The “r” in r-adapted species refers to the populations growth rate.
Generally, “r” selected species have high growth rates and can thus easily
colonize areas, and they can adapt well to unpredictable environments. Notably, if
a large percentage of the population is wiped out,
for instance by a fire, the population can grow quickly back up to its original
size. Thus, they are adapted to unstable environments. Notably they often
experience boom and bust cycles, as they overshoot carrying capacities and crash.
Many weeds fall into this category, as do mice and many insects. The “K” in the K-adapted species refers to the population’s carrying capacity. These
populations tend to reproduce slowly and have a lower rate of increase, often
leveling out close to the carrying capacity. These organisms are likely less
able to recover quickly after a disaster and tend to not do as well in an unstable
environment. Thus, they tend to do better in stable environments. Here’s a list that
compares typical traits for r-selected species with typical traits for K-selected species. One way to look at this concept is to
consider that a population’s ultimate goal is to reproduce and continue on for
future generations. r- adapted species accomplish this by reproducing in great
numbers, but allocating very little care or resources to any individual. They have
small bodies, reach reproductive age very quickly, and reproduce many many
offspring, which they don’t spend any energy caring for. The strategy is to
simply put hundreds or thousands or millions of individuals out there, in hopes that a few survive. On the other hand, the K-adapted species puts lots of
energy into a few individual. The organism grows slowly, reaching
reproductive age late in life, has just a few young, and cares for their young for
a long period of time. The strategy is to put all the energy into a few
individuals, such that they are strong and able to survive and reproduce. Lastly,
it’s worth noting that not all species fit nicely into the r-adapted profile
or the K-adapted profile. Some species may have some of the “r” traits and some
of the K-adapted traits. See the infographic on the previous
slide, and note that they displayed the range from r to K as a continuum. Just as the species may have a specific
pattern related reproduction, they may also have a specific pattern related to
mortality. The graph here demonstrates three typical mortality patterns. Type 1
shows a species, in this case humans, that tends to have high survival rates
until later in life at which point the likelihood of mortality is high. Type 2
shows a species that has about the same likelihood of mortality all throughout
its life, and type 3 shows a species that has a high mortality rate
early in life, but if it survives that period it could end to live a long life.
Understanding mortality patterns can help population ecologists understand
how to better manage the population and at what stages in the
life cycle the population may need extra protection. Notably, as with reproductive
patterns, some species don’t fall neatly into one category. In example here, the
herring gull has characteristics of both type 2 and type 3. It initially starts as
type 3 as there’s high mortality when the chick is young, defenseless and
the nest on the ground. However, if the individual survives this initial age, the
mortality reduces, and it remains somewhat constant throughout its
lifetime. The last of our five questions asks what the age structure of the population
is. Just as with human populations, the age structure of a population is
important. Population ecologists analyze age structure along with the
reproduction and mortality patterns to determine if a population is likely to
be increasing or decreasing in the near future.
The age structure diagrams shown here offer 3 hypothetical human populations.
Similar diagrams though can be developed for any type of population’s age structure.
Diagrams typically show the number or proportion of a population of
each gender at each age. Often ecologist are not able to get detailed age and
gender data for a population, but estimates can still be valuable. As you
study the diagram, consider the impact of each type of age structure on the
population. In closing, we have worked our way through the various questions
population ecologist ask when studying a population. Notably demographers people who study human population, often ask the same sort of question as population
ecologist who are studying populations of species other than humans. Now let’s
go back and consider the reintroduction of wolves into Yellowstone National Park,
and think of how these five questions may have helped population ecologists monitor, protect, and restore the population. Often though, it’s also
important to look beyond the population to to the relationship the target
population has with OTHER members of the ecological community, as well as
impact the population has on the ecosystem of which it is a part. In our
next topic we will be addressing COMMUNITY ecology, ,but for now let’s just stress the importance that a predator has on the rest of the ecological
community and the health of the ecosystem. The reintroduction of wolves
in Yellowstone resulted in many other populations being affected, sometimes in
ways that were even surprising to the ecologists managing the program. The
Infographic shown here was from a previous edition of the textbook where
they highlighted the importance of wolves on other populations. For example,
the first edition of the textbook noted that without wolves the willows were
over grazed, which led to the demise of beavers and songbirds. With fewer beaver,
there were less beaver dams which meant less diverse riparian areas and less
pools, which meant less fish. Conversely, when wolves were reintroduced, they noted decreased elk populations, which increased the willows and thus increased the
beavers, which led to more songbirds and also more fish and greater groundwater
levels. The current edition of the textbook has stepped back a bit on the
effects of wolf introduction on beavers, but more recent data now suggests that
indeed the beavers are increasing. I encourage you to research the current
status of different populations in the Yellowstone Ecosystem. How many different populations in the Yellowstone ecological community can you
list that were affected by the wolf reintroduction? Specifically, how is each
population you listed affected? What other ecosystem effects were seen? …
meaning what effects were there on the non-living landscape? Do you think it’s
right for predators to be reintroduced to their home habitat, even if they may
pose a threat to some human property nearby? All great questions!

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