week 21 Flashcards
Most ecological communities have some balance between species, why is this important?
allowing them to co-exist. There are occasions, however, when a population becomes so numerous that it can create problems.
Why may it be necessary to attempt to control a population?
Species which become introduced to a new environment can cause problems for native species. Attempts to control populations may be in the form of mechanical control, e.g. cutting or removal, chemical control, e.g. spraying insecticides, or biological control, e.g. introducing a natural enemy to reduce the population of a pest species.
3 main variables of a life history includes:
1) When reproduction begins
2) How often the organism reproduces
3) How many offspring are produced per reproductive episode.
The cost of reproduction
Finite resources
Resources that are used for one thing cannot be used for another
This means that an organism’s life history will reflect these trade-offs
One trade-off that is much explored is between growth and reproduction
This is the cost of reproduction
Well reproducing takes a lot of energy and means consuming lots of resources from the environment. Resources that are used for one thing cannot be used for another, meaning that organisms face trade-offs in how they uses these resources
How do organisms use their resources? Well, this is reflected in their life histories.
At the centre of idea of life histories is trade off between growth and reproduction – termed ‘cost of reproduction’
The cost of reproduction in trees-
In trees, growth is visible as rings
The thickness of the ring reflects the investment in growth.
Very easy to quantify investment in growth each year with trees as the growth is visible as rings with the thickness of the rings reflecting the investment in growth.
The growth of the trees was assessed by taking core samples, and measuring the ring width.
Seed traps were placed under individual trees over 33 years.
On good years, you see masting — heavy crops of seeds — whilst bad years will see small crops.
Easier for trees which produce big seeds such as oaks and chestnuts.
Trees are perennial – produce seeds each year.
Where it produces absolutely loads we call this masting – may have heard of beech mast.
Both seedfall and growth were variable between years.
When they compared the growth data and seed data they saw that in mast years the ring size is much reduced. So there appears to be a trade off whereby the trees invest in growth or reproduction at any particular moment, but not both at the same time. Only an observation so correlation rather than causation…
So let’s think about unstable or ephemeral habitats, what sort of organism are we going to get living here?
Can’t predict when you’re going to die, so evolutionary pressure is to put as much of your energy into reproductive output as quickly as possible, not put into growth, but into reaching sexual maturity asap. So short lived and small. Survival of your offspring is also highly unpredictable so evolutionary pressure is to produce as many offspring as possible with the energy you have. As a result they’re all going to be small. But then they won’t be so great at buffering changes in the environment.
An r-strategist tends to be short-lived, meaning that it can exploit new habitats and resources quickly, with rapid population growth
•produce lots of offspring (many and small)
•reproduce early
•invest in reproduction rather than growth or survival
This strategy tends to be seen where the habitat is highly disturbed or transient.
What are the differences in where K and R strategists live?
K- Deterministic environmental change- More suited to life in a stable environment in which they can rely in a long lifespan and a low mortality rate that will allow them to reproduce multiple times with a high offspring survival rate.
R- Stochastic environmental change- suited to unstable environments because: reproduce early and abundantly; allow for a low survival rate of offspring.
There are three main ways in which organisms tend to be distributed.
They can be uniform like this albatross sitting on their chimney-like nest structures, random like these daisies in a field, or aggregated like these starfish all clumped together on an underwater column of rock. or random.
Aggregated distributions
§Most common type
§Found in environments with patchy resources; animals “clump” around resources
§Also clustered due to social factors e.g. family groups or inability of offspring to disperse
§Prey clumped distributions in areas where they can hide and detect predators
The sample distribution affects:
- How many samples we need to take to measure the abundance accurately
- Our ability to compare populations
- How we handle, analyse and interpret the data
Change in pop. size =
Births + Immigrants – Deaths – Emigrants
logistic or sigmoidal growth-
Growth rate slows, and eventually stops, as resources are depleted
“carrying capacity”
Population size at which growth stops
What is K and r in relation to population growth?
(K) = the number of individuals that the environment can support
At K, birth rate equals death rate, and population growth is zero
So when the population is relatively small it tends to grow exponentially and the rate of increase is the population’s intrinsic rate of increase, r
