Ecology

The Individual
Sher & Molles: Chapter 1.1, 1.2,
Sher and Molles: Chapter 7.3, 7.4, 15.1 (Until ‘Ants and swollen thorn Acacias’),
Brock 23.5, 23.14

Carbon and energy sources
Organisms have different ways of obtaining carbon and energy and can be
grouped by how they acquire these. This is called their trophic biology.
Autotrophs use inorganic sources to built their own organic molecules;
 Photosynthetic autotrophs use CO2 as a carbon source and
sunlight as energy. (e.g. plants, algae, cyanobacteria).
 Chemosynthetic autotrophs use CO2 as carbon source and
inorganic molecules as a source of energy (e.g. nitrifying bacteria,
sulphur-oxidizing bacteria).
Heterotrophs use organic molecules are both sources of carbon and
energy; they take carbon and energy from other organisms. (e.g. animals,
fungi and many soil bacteria).
This means that heterotrophs always depend on autotrophs to obtain their
molecules.
Autotrophs only need a handful of resources. For example, all green plants
need:
 Solar energy,
 CO2,
 Mineral nutrients (N, P, ...)
 Water
Because there are not that many resources, there is not much opportunity
for specialization to specific resources. So instead of specializing on
different resources, autotrophs specialize along the resource gradient. For
example, shade-tolerant vs. light-demanding plants.
This is different for heterotrophs; heterotrophs are much more diverse in
their carbon and energy sources. There are three
major feeding categories:
 Herbivores; these feed on living plants. Even
within this category there are many
specializations.
 Carnivores; these mainly feed on animals.
These further specialize on their prey (e.g.
size/behaviour. Their hunting behaviour is adapted to their prey).
 Detritivores and decomposers; these feed on non-living organic
matter, usually plant remains. They differ in that detritivores ingest
and digest the organic material internally (e.g. earthworms,
millipedes, insect larvae), whereas decomposers break down
organic matter externally by secretion of enzymes (e.g. bacteria and
fungi).
There are also parasites, who feed on a living host but do not kill their
host. Parasitoids develop in a living host and eventually kill it.

Ecological stoichiometry
Organisms don’t just need resources in sufficient amounts, they need
them in the right proportions. Ecological stoichiometry is the balance

,between multiple chemical elements. This is particularly relevant is
organism interactions.
There are 5 elements that make up 93-97% of biomass of plants, animals,
fungi and bacteria. These are Carbon (C), Oxygen (O), Hydrogen (H),
Nitrogen (N) and Phosphorus (P). (Cool Owls Hunt Nighttime Prey).
Many other elements occur in tissues. Essential plant nutrients include
Potassium (K), Calcium (Ca), Sulfur (S), Chlorine (Cl), Iron (Fe), Manganese
(Mn), Boron (B), Zinc (Zn), Copper (Cu), Molybdenum (Mo). (Kind cats sing
clever folk melodies before zebras cook muffins).
Animals additionally require sodium and iodine.
Plants obtain their essential nutrients through their root system; animals
acquire them via food intake.
In case of imbalances in stoichiometry, a consumer needs to eat more to
obtain the limiting nutrient. For example, the C:N ratio is higher in plants
than in herbivores, meaning that the herbivore needs to eat more C to
obtain sufficient N.




If we look at plant tissue, the C:N ratio is much higher than, for example,
in insects. It’s the same for the C:P ratio. If we look within plant tissue,
there is still a lot of variation within the C:N ratio.
This is because plants are abundant in carbon-rich structural compounds;
things like hemicellulose, cellulose and lignin, which is mostly present in
the woody plant parts.
Because the various plant parts differ
in C:N ratio, they also offer different
resource composition to herbivores.
This is very different for animals; the
bodies of different species of animal
have a remarkably similar
composition.

Because of the difference in C:N ratio
between plants (>40:1) and animals
(8:1), herbivores have a substantial
nutritional chemistry problem;
herbivores need to compensate for
the large difference between food
nutrient content (high C) and growth requirements (high N).

,This comes on top of having to overcome plant physical and chemical
defences;
 Thorns on plants,
 Cellulose and lignin are hard to digest,
 Toxins and digestion-reducing substances in plant material, which
further reduce the nitrogen availability.

Carnivores have far less nutritional chemistry problems, as predators and
their prey have both similar C:N and C:P ratios. They mostly need to
overcome prey defences, such as camouflage, anatomical and chemical
features (spines, poisons, etc) and behaviours (flight, flashing bright
colours, etc).
Many prey species make use of conspicuous warning signals to show the
species is toxic, unpalatable or otherwise dangerous (e.g. bright colours in
toxic frogs). This is called aposematism.
Another method of defence is Batesian mimicry; a harmless species
mimics signals of a harmful species to fool predators (e.g. the hoverfly
mimicking the black-yellow colours of the wasp).

Resource mutualism in plants
Many plants, but also animals, cannot acquire
their essential resources on their own and
therefor form mutualisms with other organisms.
Mutualism refers to an interaction that is
beneficial to both parties (+/+).
Plants, for example, cannot acquire water and
mineral nutrients sufficiently on their own, and
form mutualisms with fungi in the soil
(mycorrhizal fungi) and soil bacteria.
The mycorrhizal fungi provide the plants with greater access to inorganic
nutrients (particularly P, in some cases water, and to some lesser extent
also K, Cu, Zn and N). The plant provides carbon (sugars) in return.
Another demonstrated benefit is that the mycorrhizal fungi can help
protect against pathogens and root herbivores, and in some cases even
result in resistance to toxic metals in the soil.

There are two types of mycorrhizal fungi; endomycorrhizae and
ectomycorrhizae.
Endomycorrhizae are fungi that grow inside the root. The most well-
known example is the Arbuscular mycorrhizal fungi (AMF), which
penetrates the root cortical cells, forms arbuscules (exchange sites),
hyphae (fungal filaments) and often vesicles (storage).
Ectomycorrhizae remain outside the root cells. They form a mantle
around the roots and a net-like structure (Hartig net) between root cells.
The benefit to plant growth with mycorrhizae is substantial; this also
means that mycorrhizae inoculation can also be used in conservation
efforts.

Another mutualistic interaction is between plants and rhizobia. Rhizobia

, are bacteria living in root nodules of legumes. The plant gains N (NH4-)
from atmospheric N2, and the bacteria gain C (sugars) and a protected
environment in the nodules.

Mucoromycotina fine root endophytes (MFRE) is an ancient fungal
lineage that provides nutrients (N, P) to plants, where the plant provides
carbon in return.




Resource mutualism in animals
Animals cannot digest their essential resources on their own. For
herbivores, the inability to digest their own resources results from that
nutritional chemistry problem (difference in C:N ratio between plants and
animals).
Plant cells have cellulose walls, lignin and other structural carbohydrates.
The overwhelming majority of animals lack cellulolytic and other enzymes
that can digest these compounds. To overcome this, many herbivore
species have mutualistic associations with bacteria and protozoa to digest
these compounds. However, these need time and suitable conditions to
perform the fermentation of these complex compounds.
Longer intestines for herbivores allow their mutualists time to break down
the plant structures, and more surface area to absorb resources.
Carnivores do not need to digest these complex compounds, and have a
shorter intestine.
Some herbivores are further specialized and have evolved fermentation
chambers. These support fermentation by having a high mutualistic
density, stable anaerobic conditions and optimized pH. They can be
divided into two groups:
 Foregut fermenters, where the fermentation chamber is present
before the acidic stomach, e.g. ruminants, colobine monkeys,
macropod marsupials.
 Hindgut fermenters, who have a fermentation chamber behind
the small intestine (cecum and colon), e.g. horses, rabbits.
The cow is an extreme case of foregut fermentation; it has 4 ‘stomachs’.
1. The rumen, which is the main fermentation chamber,
2. The reticulum, where mixing and particle sorting occurs,
3. The omasum, where minerals and water are absorbed,
4. The abomasum, the ‘true’ stomach, where acid digestion takes
place.
This results in a very long retention time. There is repeated mutualistic
(microbial) processing; this is called rumination. This is a very efficient
way to extract nutrients and energy.
A large number of these microbials are methanogens; these are anaerobe
archaea that produce methane as a byproduct. This is the reason why