Ecology Notes – Oct. 2/12
- An organism’s life history is a record of events relating to its growth, development, reproduction, and
survival. Life history characteristics include – age and size at sexual maturity, amount and timing of
reproduction, survival and mortality rates. The timing and nature of life history traits are products of
adaptation to the environment in which the organism lives.
- Nemo grows up – case study: Clownfish spend their entire adult lives within a single sea anemone.
Anemones are upside-down jellyfish with stinging tentacles. It is a mutually beneficial relationship.
The anemone protects the clownfish by stinging their predators, and the clownfish help the anemone
by eating its parasites or driving away its predators. In real life, two to six clownfish spend their
entire adult lives within one sea anemone, but they are usually not related. The largest fish is a
female. The next largest is a breeding male. The remaining fish are immature nonbreeders. There is a
strict pecking order in the group, based on body size. If the female dies, the breeding male becomes a
female, and the next largest fish becomes the breeding male. Hatchlings move out of the anemone,
and juveniles must find a new anemone to inhabit. Clownfish within an anemone regulate their
growth to maintain a hierarchy in which each fish belongs to z distinct size class. The bigger fish have
a higher rank. How do they maintain the strict hierarchy within the anemone? Experiments with
clownfish show that hierarchy is maintained by regulating growth rates. If two fish become similar in
size, a fight results and one is expelled from the anemone. If they’re not near the anemone, they
become vulnerable to predators. Why do the clownfish maintain the hierarchy? They are completely
dependent on protection by the sea anemone. They are easy prey outside the anemone. Conflicts
result in expulsion and death, probably without them having reproduced. There is strong selection
pressure to avoid conflict. Sea anemones are a scarce resource for clownfish. Growth regulation
mechanisms have evolved because individuals that avoid growing to a size that necessitates conflict
are more likely to survive and reproduce.
- Life history diversity: Individuals within a species show variation in life history traits due to genetic
variation or environmental conditions. The life history strategy of a species is the overall pattern in
average timing and nature of life history events. It is shaped by the way the organisms divides its
time and energy between growth, reproduction, and survival.
- The timing and nature of life history events shapes the overall life cycle of an organism. The life
history strategy is determined by effects of natural selection. For example – A tadpole must know
when to metamorphose. A baby frog must know how fast and large to grow. An adult frog must know
when to begin reproducing, and how many offspring and of what size. They must also know how long
- Some life history traits are determined genetically. Natural selection favours individuals whose life
history traits result in them having a better chance of surviving and reproducing. Ideal or optimal life
histories maximize fitness (genetic contribution to future generations). But no organism has a perfect
life history. All organisms face constraints and ecological trade-offs.
- Phenotypic plasticity: One genotype may produce different phenotypes under different
environmental conditions. For example, growth and development of plants and animals may be
faster in higher temperatures. Changes in life history traits can cause changes in adult morphology.
Pine trees grown in cool environments tend to be taller and have more leaves than trees grown in
desert climates. Phenotypic plasticity may result in a continuous range of sizes or it may result in
discrete types called morphs, with no intermediate types. Polyphenism is when a single genotype
produces several distinct morphs. Spadefoot toad tadpoles have small omnivore morphs (feed on
detritus) and larger carnivore morphs (feed on shrimp). Carnivore tadpoles grow faster and
metamorphose earlier. They are favoured in ephemeral ponds that dry up quickly. Omnivores grow more slowly and are favoured in ponds that last longer. They metamorphose in more favourable
conditions and have more chance of survival. In toads, the relative growth rates of the jaw and the
rest of the body determine whether the tadpole is a carnivore or an omnivore. Different body
morphology results from different growth rates of body parts in both the pines trees and the
spadefoot toads. Allometry – Different body parts grow at different rates, resulting in differences in
shape or proportion. Allometry is a common mechanism of variation within and among species. The
opposite of it is isometric growth. Phenotypic plasticity can be a physiological response or it can be
an adaptive response.
- Modes of reproduction: Asexual reproduction is simple cell division (binary fission) and is done by all
prokaryotes and many protists. Some multicellular organisms reproduce both sexually and asexually
(ex. corals). Coral life cycle – A polyp is formed by sexual reproduction. This polyp then reproduces
asexually by forming buds. Budding continues to form a colony of genetically identical polyps. The
mature colonies in the reef release gamete clusters that contain eggs and sperm. These clusters break
up and the sperm fertilizes the egg to produce a zygote. The zygote then becomes a polyp.
- Benefits of sexual reproduction: Recombination promotes genetic variation and increased ability to
respond to environmental challenges. Disadvantages: An individual transmits only half of its genome
to the next generation. Also, the population growth rate is slower. One cost of sex is referred to as the
cost of males. Assume that each sexual or asexual female can produce four offspring per generation,
but half of the offspring produced by sexual females are male and must pair with a female to produce
offspring (all asexual offspring are females because they are clones of the parent). Under these
conditions, the asexual individuals will increase in number more rapidly and will make up nearly
100% of the population in less than ten generations. The fraction of asexuals in the population
increases by 1/6 every generation. Sexual reproduction originated in single-celled protists.
- Gametes are egg and sperm. Isogamy is the production of equal-sized gametes. An example is the
green alga Chlamydomonas reinhardi. Anisogamy is when the gametes are different sizes. Usually the
egg is much larger than the sperm and contains nutritional material. Most multicellular organisms
- Some species have direct development, which is where the fertilized egg develops into a juvenile
without passing through a larval stage. Most vertebrates have simple life cycles without abrupt
transitions. But complex life cycles are common in insects, marine invertebrates, amphibians, and in
some plants, algae, protists, and fish. Complex life cycles have at least two distinct stages, with
different body forms and different habitats. The transitions between stages in complex life cycles are
abrupt. Metamorphosis is an abrupt transition in form between the larval and juvenile stages.
- Life history continua/scales/ranges: Classification schemes for organizing reproductive patterns
place the patterns on continua with extremes at each end. One way of classifying the reproductive
diversity of organisms is by the number of reproductive events in an individual’s lifetime.
Semelparous species reproduce only once. Iteroparous species can reproduce multiple times.
Semelparous species include annual plants and agave. Annual plants complete their life cycle in one
year. Agave has a prolonged stage of vegetative growth that can last up to 25 years and then it
undergoes sexual reproduction once. When it is ready to reproduce, it produces a single stalk of
flowers that grows really tall. After pollination, the flowers produce seeds that drop to the ground.
The part of the plant that produced the tall stalk dies after this event. So it is semelparous. But this
plant also produces clones asexually that surround the original plant. Iteroparous species include
trees such as pines and spruce and most large animals.
- r-selection and K-selection describe two ends of a reproductive strategy continuum. r-selection is the
intrinsic rate of increase of a population, which is a measure of how rapidly a population can grow. r-
selection refers to selection for high population growth rates. This type of selection can occur in
environments where population density is low and is an advantage in newly disturbed habitats and uncrowded conditions. Genotypes that can grow and reproduce rapidly will be favoured over those
that cannot. r-selected organisms (live fast, die young) have short life spans, rapid development,
early maturation, low parental investment, and high reproduction rates. Examples are most insects,
small vertebrates such as mice, and weedy plant species. K is the carrying capacity for a population.