LZ lecture 4 Flashcards
Minimum viable population (MVP)
- For a given species in a certain habitat
- The smallest isolated population having a good chance (e.g. 99%) of surviving in the long term (e.g., 1000 years)
- Despite the foreseeable effects of demographic, environmental and genetic stochasticity, and natural catastrophes (Shaffer 1981) (populations do fluctuate!)
- Years and percentages can vary
- Difficult to estimate (large data requirements of models –
abundance, ecology, genetics, …). - Also, extinctions take place in the future, maybe in the long term, but you cannot wait to measure them: estimates (and management decisions) must be made now
Ecologically Functional Populations (EFP)
Populations large enough to maintain their ecological roles in nature, thus maintaining the integrity of ecosystems, communities
Oyster Reefs at Risk
Why do we need oysters?
- Biodiversity (ecosystem engineering)
- Water filtering
- Storm protection
Causes of vulnerability in small populations
- Genetic issues: loss of genetic variability
- Demographic issues: random demographic fluctuations
- Environmental issues: random variations in environmental conditions (included changes in climate, predation, competition, illness, resources or random natural catastrophes)
Loss of genetic variability
(as a cause of vulnerability in small populations)
- Populations experience changes in allele frequencies over time and loss of genetic variability due to chance (genetic drift, bottleneck effect, founder effect)
- Small populations are especially vulnerable to the loss of genetic variability
- Alleles with low frequency have a significant probability to be lost at each generation due to chance
- Compared to large populations, small populations more easily undergo other negative effects such as inbreeding depression and the loss of evolutionary flexibility (i.e. capability to adapt to environmental changes)
Genetic bottleneck
A genetic bottleneck occurs when a significant portion of a population is drastically reduced in size, leading to a loss of genetic diversity within the population.
Consequences of genetic bottlenecks?
- Genetic bottlenecks can increase the vulnerability of populations to diseases and environmental changes, limit their ability to adapt, and raise the risk of extinction for certain species.
- Populations can sometimes recover over time through mutation, gene flow, and natural selection.
Founder effect
The genetic founder effect occurs when a small group of individuals establishes a new population in an isolated location, leading to reduced genetic diversity compared to the original population.
Characteristics of the genetic founder effect?
The genetic founder effect involves a small founding population, isolation from other populations, influence of the founder’s genetic makeup, and significant role of genetic drift in shaping the new population’s genetic composition.
Inbreeding depression
- ‘Inbreeding’ describes different related phenomena referring to cases in which mating takes place between individuals with similar genotypes. This tends to reduce genetic variability or fitness (inbreeding depression)).
- Offspring is less abundant, vital, with low reproductive success or sterile
Why is inbreeding depression a concern for conservation?
Inbreeding depression poses a significant threat to endangered species and small populations by reducing survival rates, reproductive success, and overall population health.
Conservation efforts aim to mitigate its effects through strategies like introducing genetic diversity and managing breeding programs.
Effective population size
- Number of individuals which reproduce effectively contributing to preserve the genetic variability of the population
- Always a fraction of total population (some individuals do not reproduce)
- The real effective size can be lower than the expected one, e.g. if sex ratio is unequal, there are bottleneck or founder effects, etc.
Random demographic fluctuations
(as a cause of vulnerability in small populations)
- In a real population the number of reproducing individuals,
offspring and dying individuals can vary a lot - In populations of large size, individual variations tend to be absorbed by the mean
- If population size goes below a certain threshold (for vertebrates generally 50 individuals), single variations in the number of newborns and dead can make population size vary randomly
- Important risk factor: it increases the probability that a population get extinct by chance
- Leads to genetic variability loss, e.g. due to bottleneck effects
How do random demographic fluctuations impact population dynamics?
Random demographic fluctuations can lead to short-term variability in population size and demographic parameters, influencing population dynamics and ecological processes.
While they may not have long-term consequences, they are crucial to consider in population management and conservation efforts.
Geometric growth model
«discrete version» of the exponential growth
ChatGPT:
- The geometric growth model you provided is a discrete-time model commonly used to describe population growth in ecological and biological contexts. Let’s break down the components of the model:
- Nt+1 = the population size at time t + 1 which is the population size in the next time interval
- Nt represents the population size at time t, which is the population size in the current time interval.
- Bt represents the number of births in the time interval t
- Dt represents the number of deaths in the time interval t
- R represents the net reproductive rate, calculated as the difference between the birth rate (b) and the death rate (d). In other words, R=b−d.
- λ (λ=1+R) represents the geometric growth factor or the multiplier for population growth from one time interval to the next.
What is demographic stochasticity?
Demographic stochasticity refers to non-deterministic population growth due to random fluctuations in birth and death rates, particularly noticeable in small populations. This effect is similar to genetic drift’s role in small populations, influencing variations in allele frequencies.
What are per capita rates in population dynamics?
- Per capita rates, such as birth (b) and death (d) rates, represent the average number of births or deaths per individual per unit of time.
- For example, if b=0.55 births/(individual per year), on average, each individual is expected to produce 0.55 children per year.
- b and d are averages that make sense in large populations
- In small populations it may be that due to chance there are strong deviations between the expected average value of deaths and births and what actually occurs.
Every individual in a population has 4 possibilities in a unit of time
1) survive and not have children;
2) survive and have children;
3) die and have children;
4) die and not have children
Geometric growth model in small populations (geometric vs stochastic)
- Geometric processes are deterministic and predictable, following smooth patterns of change
- Stochastic processes are random and unpredictable, involving variability and uncertainty in outcomes
Logistic growth model for environmental stochasticity
Incorporating environmental stochasticity into the logistic growth model allows for a more realistic representation of population dynamics by accounting for the random fluctuations in environmental conditions that influence population growth and carrying capacity
Logistic growth model, discrete version with env stocasticity
(2 graphs)
Logistic growth model, discrete version with env stocasticity
(fluctuations graph)
