Population Ecology

What is biodiversity?

Biological variation at all levels:

• Genetic

• Species

• Ecosystem

Why does biodiversity matter?

• Critical for ecosystem resilience

• Supports ecosystem productivity

• Enhances stability under environmental change

• Essential for human values and welfare

Why is biodiversity multidimensional?

Understanding biodiversity goes beyond species counts

Includes traits and evolutionary history in three broad metrics:

• Taxonomic diversity

• Functional diversity

• Phylogenetic diversity

What is species richness?

A count of species that are taxonomically distinct

- useful as we want to understand it

- accumulation curves are tricky as can go on forever so can define and environment

What are the limitations to measuring species richness when trying to understand biodiversity?

• Species are all treated equally - their abundance is not captured

In simple species richness measurements, we lose information about:

• species dominance

• how rare a species is

• Population stability

• Risk of extinction from disturbance events

Ecological role or functional diversity is missing

• Two communities with the same species richness can vary greatly in:

• Functional traits

• Trophic structure

• Ecosystem processes

Does not acknowledge genetic diversity:

• How will a community adapt to biotic/abiotic change?

• Is a community predisposed for disease resistance or is it at risk?

• What about populations? Multiple populations of one species count as one, even though they could be genetically distinct

Why do we use species richness?

• Quick and simple

• One standardised metric

• Application to broad range of environments

• Allows large-scale biodiversity monitoring

• Great for spotting large-scale patterns

• Global biodiversity hotspots

• Useful to reveal evolutionary and ecological trends

• Helps focus conservations efforts

• High species richness likely has more rare species

• Higher ecosystem services

What is taxonomic diversity?

• This defines the number and relative abundance of species in each ecosystem

• Species richness

• Species evenness

Why is taxonomic diversity important?

• Underpins other metrics of diversity - we need to know what is there before we can assign it to a functional group etc.

• Can be grouped by order, family, genus, species etc. Dependent of the scientific question

• Commonly used baseline for comparisons across ecosystems

• Common metric in conservation planning

What is the simplest metric you can use?

Species richness - but limited by sampling

What can taxonomically ID'ing help you to define?

Taxonomically ID'ing can help to define the diversity of a community but it depends on the parameters that you set

What is functional diversity?

• Variation in species' ecological traits

• How do organisms contribute to community structure?

• How much do their roles vary?

• How do these traits influence ecosystem functioning?

• Forget "how many are there?" - (richness)

• Think "What do they do?"

What is included in functional diversity?

• Trophic level

• Predator or prey

• Pollinators

• Insectivore

• Root depth

• Nitrogen fixation ability

Why does functional diversity matter?

Functional diversity directly links biodiversity to ecosystem function

• Productivity

• Nutrient cycling

• Resilience

• Tolerance and response to disturbance

• Higher functional diversity makes more efficient use of niches

• Communities with high functional diversity generally have stronger resilience to change

What is phylogenetic diversity?

• Evolutionary history (close or distant) among and between species

• Linked to functional diversity, but not always!

• Useful across biomes/taxa

- can study evolutionary closeness

• Captures evolutionary history

• May reflect unmeasured functional differences

• Does not attribute species abundance... Presence or absence

• Highly dependent on reliable phylogeny and branch length

• High phylogenetic diversity does NOT always equal high functional diversity

What are the diversity matrices?

- Simpsons diversity index

- Shannons diversity index

What is simpsons diversity index?

Highlights whether certain species dominate a community

• Useful in highlighting when one species dominates

- calculates the proportion of each species contribution

What is shannons diversity index?

Considers species richness and evenness.

• Rare or underrepresented species have a strong influence

What is simpsons diversity index useful for?

Simpson's index is useful for understanding if a community is dominated by specific species

• Therefore "common species" have a strong effect on the index

• How likely are two species picked at random to be the same

• Simpson's index weakly influenced by rare species

• Evenness has the strongest weighting (not species richness)

• Subtle changes to vulnerable species missed

What does shannons diversity index look for?

• Unlike Simpson's index, Shannon's index looks for uncertainty

• If you picked two species at random, what is the likelihood that they're different?

What do almost all organisms depend on?

Carbon fixers (plants)

How much energy in terrestrial ecosystems is fixed by green plants?

90% of energy in terrestrial ecosystems

What percentage of the diversity of organisms eat plants?

Approximately 30% of the diversity of organisms in the world are planteaters: herbivores and omnivores

How many species of plants and animals are there? What proportion of them are athropods?

• 1.8 million described species of plants and animals

• Approx. two thirds of all species are arthropods!

Total described insect species around 925,000

20% of insects have been described

What would happen if all insects were to disappear?

humanity would not last more then a few months

What do plants rely on animals for?

- Many plants (Angiosperms) depend on animal pollinators for sexual reproduction

- Animals defend plants from other animals

- Animals also help disperse seeds

What percentage of plant species are angiosperms? what does this mean?

87% of plant species are Angiosperms, and, therefore, produce flowers

What are examples of where plants and animals have close relationships?

- Corals are part animal, part plant and represent an intimate symbiosis

- The animal cells are polyps that forage on plankton

- The plant cells are from the genus, Symbiodinium - they are dinoflagellate protozoa that live in coral cells (a.k.a.zooxanthellae)

- Both share nutrients with the other cell type and have evolved as a 'super-organism'

What is evolution?

change between generations within a population lineage of a species

What is an adaptation?

The properties of living things that enable them to survive and reproduce in nature

What is fitness?

The probability of passing on specific genes (associated with successful traits) to the next generation

What is natural selection?

The fact that certain individuals within a population possessing specific traits have a greater probability of contributing more offspring to the next generation than others. Natural selection acts on genes via individuals

What are the 2 types of ecological interactions?

- friendly

- unfriendly

What are friendly interactions?

'Friendly' interactions:

-Mutualism

-Symbiosis

-Commensalism

What are unfriendly interactions?

'Unfriendly' interactions:

-Antagonism

-Amensalism

-Competition

What is an example where organisms dont fall into the friendly or unfriendly categories?

- The red-billed oxpecker removes insect parasite sfrom many animal species -acting as a mutualist

- It becomes a parasite, however, when it feeds directly from wounds inflicted on its mutualist partner

What is mutualism?

Both species benefit from an association

What is the mutualism in frugivory and seed dispersal?

- Plants produce fruits which attract animals

- Animals eat the fruits and then 'deposit' the seeds away from parent plant

What is the mutualism in pollinators and flowers?

- Plants produce flowers with pollen and nectar rewards- Animals visit flowers to obtain nectar and pollen, and transmit pollen to conspecific flowers for plant sexual reproduction

What are examples of mutualisms?

-Frugivory and seed dispersal

- Pollinators and flowers

What are gymnosperms?

a group of seed-producing plants, meaning they have "naked seeds" that are not enclosed within a protective fruit or ovary

What are the features of gymnosperms?

• Distinct and distant male and female reproductive organs

• Exposed ovules and seeds

• Mainly depend on wind for pollination

• Produce vast amounts of pollen

• Non-targeted pollen delivery

• Inefficient without wind

What were found to be some of the first pollinators?

Ancient scorpion flies were some of the first pollinators

Mid-Mesozoic ~250-150 MYA

When did insects start to visit cones to collect pollen?

200 MYA insects started to visit cones, likely to collect pollen

e.g. Palaeodictyopteroidea

When did we start to see a change from gymnosperms?

Angiosperms started to produce inflorescences

- Visual and olfactory

- Stamen and anthers on same flower

-Large anthers with copious, sticky pollen

- Sugary exudate secreted from the petals

When did bees emerge?

125 MYA and coevolve with the expansion of the eudicots (flowering plants)

now have:

- 20,000 species worldwide

- UK: 270 native species

What proportion of plants are pollinated by bees?

20%

How do bees pollinate?

- Bees have specialized structures for collecting pollen and sucking nectar

- Mouthparts arefused into a straw-like 'proboscis'

Why did plants benefit from insect pollination?

Transferral of pollen by an animal has many potential fitness benefits:

1. Pollen deposition is much more targeted and efficient than wind pollination

2. Because pollen is transferred efficiently, plant populations may be able to exist in much smaller population densities, making species less susceptible to extinction and more able to colonize new habitats

3. Because pollen transfer is not dependent on the wind, it can take place in habitats without wind (e.g. dense tropical rainforest)

4. Resources ordinarily put into pollen production can be used for primary growth, defences, and seed development

Why are bees beneficial?

1. Bees are highly efficient vectors of pollen and evolved structures to transport pollen (corbicula)

2. Bees distribute pollen widely - they fly for miles to find conspecific plants

3. Bees learn and remember which floral morphologies provide rewards

Who pollinates flowers?

- Approximately 2/3 of all pollinators are insects

- Most of the remaining 1/3 of pollinators are birds

- A few are mammals

How do flowers reward pollinators?

Flowers produce nectar to reward pollinators

- Total sugar ranges between 8 and 80% and is mostly composed of glucose, fructose sucrose

- Nectar also contains lower amounts of amino acids and other secondary metabolites

Encourages animal to come back

What are multimodal cues?

- Vary in Colour, shape, size, symmetry, scent (more traits you can provide for the pollinator, the more likely it is to remember the plant)

- Highly attractive Bees learn better

- Traits have coevolved

How do bees learn quicker?

- With multimodal ques

- make quicker decisions

What is an example of a specialised reward?

scent

How is the scent reward used?

- Some neotropical plants, including orchids, produce scent which is collected by male

- Euglossine bees for use as pheromones.

- The bees secrete saliva full of lipids onto the floral surface. The saliva absorbs the scent compounds and is then collected by the bees and placed in a modified structure on their hindleg called a 'corbicula.'

- The bees then use the fragrances they collect as a pheromone signal for attracting females

- The orchids have specialised anthers that deposit a 'pollonium' onto the back of the bee while it is taking the scent from the floral surface

When are not all mutualisms mutualistic?

Pollination by deceit is a form of antagonism

e.g.

- The dead horse arum (Helicodiscerus muscivorous) produces oligosulfide compounds that are also produced by carcasses

- This attracts carrion flies and traps them in the . When they leave, they carry the pollen with them.

- Ophrys sp. orchids emit compounds which mimic female wasp pheromones (Andrena sp.) and have visual displays that look similar to female wasps

- Male wasps visit, believing that they are finding a female, and are unwitting pollinators

How do pollinators cheat plants?

- Honest pollinators may have to climb through complex floral structures to get access to nectar

- A dishonest visitor robs the nectar by biting a hole through the flower at the nectary, thus, and failing to come into contact with the anthers/stigma and pollinate the flower!

What does sexual deception by an orchid result in?

results in reduced sensitivity to female pheromones

Does caffeine in nectar effect bees?

- Caffeine is bitter-tasting to bees

- Caffeine amplifies Kenyon cell responsiveness to ACh

- Caffeine in nectar is constrained by pollinators

- caffeine improves their learning

Why has natural selection driven the production of nectar with caffeine?

- makes bee a better learning

- more guaranteed transfer of pollen

Why was the evolution of pollination by animal vectors so important?

- Pollinators can travel directly from flower to flower, and plant to plant, depositing pollen directly

- Pollinators may travel further to reach conspecific plants than pollen transferred by wind

- Most pollinators have wings, and fly from flower to flower

Why do flowers have scent?

- improve pollination success

Floral odours are often complex mixtures of many volatile compounds

- improves speed of pollinator to discover your ques

- Flowers with scent are easier to learn

What is the mutualism: dual benefits within pollinators?

Plants want efficient pollinators

An efficient pollinator

1. Visits only a few of their own flowers to collect pollen

2. Visits many, other conspecific flowers to deposit pollen

3. Visits many of their flowers to deposit pollen from conspecifics

4. Avoids the flowers of other species

Pollinators want to visit flowers that have:

1. Abundant, rich food resources

2. Easy access to resources

3. Little competition from other pollinators for access to rewards

What is the differnce between population ecology and community ecology?

What are the type of ecological interactions?

- Competition

- Exploitative - herbivory, predation of parasitism

- Facilitation - mutualism or commensalism

- phoresis

How do individuals, populations and communities link together?

Population ecology requires consideration of interactions between two species, but interactions within the wider community important for community ecology

What are important considerations in community ecology?

- Energy is expensive

- Organisms are selfish

- Energy is limiting and lost with every action or reaction

- Tracking energy "input" and "output" is a useful tool in community ecology

What are trophic interactions?

• Species cannot live in isolation

• Trophic interactions are feeding interactions that occur between trophic levels

• E.g. predator eating prey herbivore feeding on plan

What are food chains?

• Linear network of trophic interactions showing the sequence of transfer of energy from organism to organism

• Each successive group feeds on the group immediately before it in the chain and is in turn eaten by the succeeding group

What is the concept of trophic levels?

A trophic level is the functional classification of an organism in a community according to its feeding relationship

What is a food web?

• A food web describes the network of feeding interactions with a community

• Ignores non-trophic interactions e.g. mutualism

• Different types of food web - source, sink, linkage etc

What is a linkage web?

showing which species "eat" which other species. Other uses such as pollination networks etc

What is a source web?

based on a primary producer

What is a sink web based on?

based on a predator

Why do we study food webs?

• Summarise complexity of community interactions

• Help understand community structure

• Help understand community dynamics

How can you calculate food web metrics?

• Linkage density (LD)

• Number of species S = 5

• Number of links L = 5

• LD = L/S = 5/5 = 1

How can you calculate food web connectance?

• The fraction of possible links in the web that actually occur

C = Actual Links / Possible Links

C = L/[S(S-1)/2]= solid lines/(solid plus brokenlines)

C = 5/10 = 0.5

What limits food chain length/no. trophic levels?

Energy is expensive and lost...

Productivity

• Energy is lost at each tropic level (heat, respiration, etc.)

• 2nd law of thermodynamics

• Eventually there is not enough energy for a further trophic level

What is commensalism?

a type of symbiotic relationship where one organism benefits while the other is neither harmed nor helped

What are examples of commensal interactions?

1.Habitat/shelter

2.Structure/support

3.Microbiome

4.Nutrition

5.Transport/dispersal

What is the commensalism in habitat/shelter?

- Also referred to as delayed commensalism

- Using old nests

- Existing holes in trees

- Birds and bees

Not always commensalism

e.g.

Brittle stars in sponges

Insects living under bark

What is commensalism in structure/support?

- Epiphytic plants on trees

- Orchids, mosses, ferns, bromeliads

- Observed in tropical and temperate forests

- Gives the epiphyte access to light and moisture

- Allows veritcal expansion to diversify communities

e.g. Lichen growing on inert surfaces, Barnacles

What is commensalism in microbes/microbiome?

"Harmless" bacteria on human skin

e.g. Staphylococcus epidermidis

- Gut microbes with no clear benefit

- Yeasts and fungi on plant surfaces

What is commensalism in nutrition?

- Nutrients are costly to waste/lose, so we need to think carefully about defining commensalisms.

- Cattle egret

- Dolphin bait balls

What is commensalism in transport/dispersal?

- Seed disperal

- Remora's on sharks

- Phoretic mites on insects

- Barnacles

What are benefits of commensal interactions?

Organisms have been able to utilise otherwise "wasted" resources without harming another organism's fitness.

• This has vastly improved biodiversity

• Species have been able to fill new niches (growing veritcally etc.)

• Higher diversity = stronger, more stable community structure

Can commensal interactions be difficult to measure?

- Commensal interactions are difficult to measure. Often assumed.

- Can be context dependent.

- Can shift to mutualism/parasitism depending on context.

- Using the given exampels, think about which factors might push them towards more parastitic of mutualistic interactions

What are limitations to our understanding of commensal relationships?

• Relatively "new" concept and understudied compared to other important ecological interactions.

• Difficult to demonstrate a neutral response.

• Experimental evidence is limited and commensalisms often defined from observations

• Context changes. "The only constant in life ischange..."

What are parasitoids?

Insects which lay their eggs on or inside other insects or other arthropods

Are larvae and adults parasitic or free-living?

Larvae are parasitic, adults are free-living

What happens to the host when the parasitoid matures?

Host is killed

What are parasitoids inbetween?

In-between predators and 'classical' parasites

What do parasitoids play an important role in?

Played an important role in developing theory

What is the Nicholson-Bailey model?

Model developed by Nicholson and Bailey in 1930s to describe parasitoid-host interaction

- Forms basis of many more recent, more sophisticated models

How does the Nicholson-Bailey model work?

- At time t, there are N hosts (Nt) and P parasitoids (Pt)

- Parasitoids search for hosts, leading to E encounters (Et)

- Unlike prey, hosts can be encountered more than once!

- Hosts are encountered at random

Work out What proportion of hosts not encountered:

- Zero-term of Poisson distribution P(x) = e ^-x x^x/x!

- P(0) = e-Et/Nt

Work out What proportion of hosts encountered:

- 1 - P(0) = 1 - e^-Et/Nt

Work out How many hosts parasitised:

- Np = Nt (1 - e^-Et/Nt)

- Et = aNtPt

- Np = Nt (1 - e-aPt)

- Et/Nt = aPt

- a = searching efficiency of parasitoid

Assumption: every encounter leads to parasitism (i.e.no host defense or resistance)

How can you use the Nicholson-Bailey to work it out across assumptions?

What does the Nicholson-Bailey model look like when run on a computer?

Why do we see so many species coexisting outside?

We see heterogeneity and complexity which can stabilise interactions

What do you do when there is an aggregation of hosts?

Hosts are not encountered at random (i.e. not Poisson, but negative binomial)

Proportion of hosts not encountered (zero-term ofnegative binomial distribution)

What is a type 3 functional response?

Low attack rates at low densities

Parasitoids aggregate where host densities are high