Species interactions

Question 1

Types of interactions

Answer 1

1) Competition
2) Predations
3) Herbivory
4) Symbiosis
- Mutualism
- Commensalism
- Parasitism

Question 2

Stress gradient hypothesis

Answer 2

The frequency of facililitative and commensal interactions increases as an environment becomes more stressed and the frequency of competitive interactions decreases.

Examples: Nursing plants under hydraulic stress
- Under hydraulic stress nursing plants bring water to the surface for the seeds
- Other surrounding plants benefit from this

Tends to be for abiotic factors but evidence it also stands for biotic factors

Example: large mammals in African Savana
- under predation risk, dyadic associations with giraffes were more common
- but dyanic interactions were weakest correlated with primary productivity

Question 3

Competition among species and using the Lotka- Volterra model to predict the outcome

Answer 3

The LK model can be used to predict the outcome of competition between species, predicting which species will win and whether co-existance would be possible.

This is very important to predict how biodiversity will change (e.g. with the introduction of invasive species).

The model predicts whether competitive exclusion occurs: no two species can co-exist in exactly the same niche.

Question 4

LK equation

Answer 4

Growth of species 1= r1N1x (K1-N1-aN2/K1)
Growth of species 2= r2N2x (K2-N1-bN1/K2)

The equation predict the population growth of species 1 or species 2 and includes:

• Carrying capacity of that species
• The intrinsic population growth
• the population size
• Alpha (effect of 2 on 1) of beta (effect of 1 on 2)

When a/ b>1 interspecific competition is greater than intra specific competition

When a/b<1 intraspecific competition is greater than interspecific competition

Overall, this means that the growth when competiting with another species is predominantly determined by it’s carrying capacity and the effect that the other species has on it - whether inter/intra specific competition is greater

N1=K1- alphaN2

N2=K2-betaN1

Question 5

The possible outcomes of competition

Answer 5

The outcome of competition can be predicted in most scenarios by using the LK equation and plotting isocline graphs.

**One species drives the other species to extinction: **

This occurs when one is a good competitor and the other is not.

Good competitor: K is high and the impact from the other species is low

Bad competitor: K is low and the impact from the other species is high.

The lines on the plot do not cross and the top line is the winning species.

The population of the bad competitor would have to be very high before it had an effect on the good competitor.

(This occurs when the carrying capacity of only one competitor is greater than the population size required to make the other go extinct)

N2> K1/alpha
Or
N1> K2/beta

**One species drives the other to extinction but it cannot be predicted **

Both species have a similair competitive ability and carrying capacity therefore it cannot be predicted which will win.

Species 1: Large K and low alpha

• This means they both have the potential to reach the carrying capcity at which the other will go extinct.

(This occurs when both species are able to reach a population size that drives the other extinct)

N1 > K2/alpha
And
N2> K1/beta

**Species are able to co-exist **

Species are able to co-exist if intra specific competition is greater than interspecific competition and if the carryig capacity is low.
Species 1: Small K and small alpha
Species 2: Small K and small beta

Species are also able to co-exist even if there is high interspecific competition but the carrying capacities are low.

Species 1: Small K and large alpha
Species 2: Small K and large beta

(species are unable to reach population size to make the other go extinct)

N1 not able to reach K2/beta
N2 not able to reach K1/alpha

Question 6

What does the LK model not include?

Answer 6

1) Habitat stochasticity (vary K)
2) Individual heterogeniety (Use stage structured model)
3) Multiple species interacting (expand equation to n number of species)

Question 7

Overview

Answer 7

There are many different interaction that occur within and between species and it is important to understand them when studying populations.

The LK model can be used to predict the outcome of competition which is very important to understand how biodiversity changes.
- Which species will win?
- Can they co-exist?
- You may not be able to predict the outcome- unstable.

Question 8

When can do-existence occur

Answer 8

Coexistence can occur when

K1<K2/beta
K2<K1/alpha

When the carrying capacity of the competitor is less than the population size needed to drive the other to extinction.