Week 9.1

Lecture 4.3: Interspecific Competition and Predation

Overview of Species Interactions

Outcomes of Interactions Among Species:
- Mutualism: Both species benefit from the interaction.
- Antagonism: One species benefits at the expense of another.
- Competition: Both species are negatively impacted, competing for resources.
- Commensalism: One species benefits while the other is not significantly affected.
- Amensalism: One species is harmed while the other remains unaffected.
- Neutralism: Neither species is significantly affected by the interaction.

Intra- vs. Interspecific Competition

Intraspecific Competition (previous lectures):
- Involves competition for mates, resources, or space among individuals of the same species.
- Can significantly slow population growth and regulate density.
Interspecific Competition:
- Does not involve competition for mates but applies to key resources such as space and food. → mostly doesn’t apply to this
- Influences morphological evolution, leading to divergence and specialization.
- Affects species distributions.
- Can lead to scenarios where resource availability is insufficient to sustain all individuals at high population sizes.
- Higher interspecific competition can lower the carrying capacity (K) of a given environment.
  - plays a role in setting carrying capacity → how many animals can exist in a space at a given time

Impact of Interspecific Competition on Evolution

Niche Overlap and Divergence:
- Niche overlap can drive divergence in key traits, influencing specialization and trait differentiation due to competition.
- get specialization in different taxa… ex. bill shape in different birds based on what they forage

Influence of Interspecific Competition on Species Distribution

Realized vs. Fundamental Niches:
- Fundamental Niche: The full range of conditions under which a species can survive and reproduce, based on abiotic tolerances.
  - based on abiotic factors
- Realized Niche: The actual conditions under which a species exists, considering the presence of other species, including superior competitors and available prey.
  - incorporates positive and negative biotic factors
- Understanding the shape of realized niches can explain species distributions influenced by intra- and interspecific interactions.
- competitor 1 doesnt just overlap blue part by a l
- higher number has a higher impact on the number

Lotka-Volterra Models for Interacting Populations

Basic Concepts:
- The Lotka-Volterra equations model interspecific competition and describe how the populations of two species interact based on their densities.
- helps us describe what happens to pop of dif species when they interact

Lotka-Volterra Competition Model Equations

Change in population size for species 1:
$\frac{dN_{sp1}}{dt} = f(S_{sp1}, S_{sp2})$ where:
- $\frac{dN_{sp1}}{dt}$ = change in population size of species 1
- $f$ = a function describing interaction effects
- change in size of pop over change in time
- often plot outcomes in a graph where its abundance of one species vs another…. no time on the graph
  - there’s vectors on the figure that describes hoe pop changes based on the interactions
  - isocline: they arent changing… net affect balances eachother
Change in population size for competitor 1:
$\frac{dN_1}{dt} = r_1 N_1 \left( K_1 - N_1 - \alpha N_2 \right) \cdot \frac{1}{K_1}$
Where:
- $K = \text{carrying capacity}$
- $\alpha = \text{competition coefficient (effect of species 2 on species 1)}$
- affect of alpha is dependent on N2
- when species one below line they want to increase when above line species 2 decreases
Change in population size for competitor 2:
$\frac{dN_2}{dt} = r_2 N_2 \left( K_2 - N_2 - \beta N_1 \right) \cdot \frac{1}{K_1}$
Where:
- $\beta = \text{competition coefficient (effect of species 1 on species 2)}$
- get beta instead of alpha
- have a different isocline now

Outcomes of Competition Models

Case 1: Species 1 Outcompetes Species 2
- Conditions lead to an increase in species 1 while species 2 declines and possibly gets excluded.
- take isocline of both … in bottom they both increase inmiddle species 1 gets rid of species 2 or if isocline a is higher than b then species 2 will take over
- above both species decrease
Case 2: Species 2 Outcompetes Species 1
- Similar dynamics occur, leading to the decline of species 1.
Case 3: Stable Coexistence
- Occurs when intraspecific competition is stronger than interspecific competition, allowing both species to coexist through niche differentiation.
- interspecific comp is stronger than inter specific competition and can lead to competitive exclusion, where one species outcompetes the other for resources, ultimately resulting in the decline or extinction of the less competitive species.
- shrew species that are similar that eat the same stuff live in same environment and get eaten by the same things and they coexist in certain places so when pigmi shrews are very abundant they diminish the population of common shrews
  - common shrew is well below carrying capacity and are more likely to grow but pigmi shrew is more likely to decrease because they are approaching or past their carrying capacity
- competition coefficient is how much the zones overlap

Implications for Species Coexistence

Limiting Similarity:
- Refers to the maximum level of niche overlap that allows two species to coexist peacefully.
- Species with similar resource requirements face stronger competitive influences, leading to competitive exclusion.
- max level of niche overlap that they can have that allows them to exist in the same area
- 2 species that compete for the same resource can’t coexist in the long term if their niche overlap exceeds this maximum, as one species will outcompete the other for these essential resources.
  - in long term if they do stay in the dame area their differences will be strongly selected for

Ecological Dynamics in Nature

Effects of Competition in Sympatric vs. Allopatric Populations:
- Populations exhibit lower densities in sympatry (coexisting in the same area) compared to allopatry (existing in separate areas).
- Example: Pygmy shrews and common shrews—density suppression occurs due to competition.
- sympatric: live in the same place and time
- Allopatric: live in separate places and times, often leading to reduced competition and resource exploitation in distinct environments.
- pygmy shrew density is compressed by common shrews
- difference in abundance has to be attributed to competition in species not food availability
- clif chipmunks and colorado chipmunk.. when there’s only one chipmunk it will occupy whole mountain range
- if there is multiple species there will be elevation boundaries which are shaped by interactions w competitors
Ecological Release:
- Increase in available niche space following the removal of a competitor, leading to potential population explosions.
- the niche figure at the beginning w circles… competitor 1 snd 2 are keeping it away from blue circle
- European mink vs american mink
  - both species voracious predators
  - eat mostly the same things and are very efficient predators
  - american mink was accidently introduced to european
  - europen mink was the loser in these interactions → in some areas they have been extrapated
  - on studying diet of pboth species , when we compare the diets we see the relative proportion
  - despite being competitively superior its behaviour was influenced by competition in areas where tehy over lapped
  - geneeralyl where comp exists theres a couple dif ways that competing species minimize the cost of competition

Niche Partitioning Strategies in Animals

Resource Partitioning:
- Differentiation in resource use, leading to specialization. E.g., honey bees versus orchid bees.
- narrow niche,few eseouce types really efficient at ovtaining them (specialist)
- broad niche use many resources, ecploit each with so-so efficiency
- orchid bees specialist
  - good example of mutualist
- honey bees are good example of a generalist that can thrive in various environments, pollinating a wide range of plants, thus showcasing their adaptability and contribution to biodiversity.
Spatial Partitioning:
- Species adapt to utilize different areas to reduce competition. Example: Brown anole species in Florida.
- eating same things but eating the in dif places
- brown and crested anole were both introduced to florida upon contact the places they forage has shofted
- when allo patric hthey forage just above the grouns
- in symparty the brown anole forages on the ground and the crested anole will forage higher up
Temporal Partitioning:
- Timing of activity shifts to minimize overlap. Example: Six shark species in the Gulf of Mexico.
- 6 species of sympatric sharks
  - monitor them based on accelerometers can quantfy the percent of time that each species spends active and not actie and plot them against time
  - coloured rings show when 80% of the activity happesn… don’t see more than 2 species at a time
so theirs less competition for resource competing

Traits of Competitors in Various Contexts

Character Displacement:
- Natural selection promotes evolutionary differences in sympatric species to reduce competition for resources.
- evolve differences through resource use
- can be morphology, behaviour, physiology etc
- affects the acquisition of the key resource that animals are competing for
- in generalized figure if species 1 and 2 are very similar…over time species thend to fibide in what they do
Morphological Divergence:
- Differences in morphology, such as leg length in anole species, can enhance locomotion efficiency and resource acquisition.
- morphologically we see this in Anoles in limb characteristic so those that move down to ground will have longer legs… that trait allowed them to be more efficient at foraging on the groundf
Vocal Divergence:
- Changes in song patterns in closely related bird species influence mating behaviors and resource competition.
- tinekerbirds that occur in central africa
- when they overlap their songs get more different (function of song is mate attraction)
- cost of attracting species of different species is that there might be genetic incapbaility
- when in sympatry we see differences in white and see allopatric in black and
- see that frequency and song rate in spectogram

Types of Antagonistic Interactions

Amensalism:
- Involves unilateral interactions benefiting one species while ignoring the other.

Types of Antagonisms in Ecology

Various forms including parasitism, kleptoparasitism, and predation.
- Predation:
- Examined through the simple Lotka-Volterra model, where predator-prey dynamics define population changes.

Simple Lotka-Volterra Predation Model Equations

Change in population size of prey: $\frac{dV}{dt} = r V - \alpha PV$
1. Change in population size of predators:
  $\frac{dP}{dt} = \beta PV - qP$

Population Cycles:
- These dynamics produce cyclic patterns in abundance, showing how prey and predator populations respond to each other.

Theoretical Examples

Isle Royale Case Study:
- Moose and wolves showcase predator-prey dynamics resembling the simple Lotka-Volterra model, with population densitites influenced by mutual interactions.