Populations continued

I.

A.

B.

Metapopulation Theory

What is a metapopulation?

Assumptions of the metapopulation theory

II.

A.

Stochastic Perturbations & MVPs

Sources of “uncertainty”

B.

C.

MVP PVA

I. Metapopulation theory

A. What is a metapopulation?

– Populations are seldom independent • Metapopulations = Series of small, separate populations (in discrete habitat patches) united together (Levin 1970). – In this scenario, even if the individual populations go extinct, other populations survive and they supply dispersing individuals who recolonize “extinct” patches

Huffaker’s mite experiment: examining predator-prey relationships Prey = six spotted mite, prey = Western predatory mite Habitats = oranges, Nonhabitats = rubber balls When prey sp. was forced to feed in oranges concentrated in large areas, they were exterminated, and predators died out… when oranges were widely dispersed, prey survived longer and the predator/prey populations followed predictable cycles

Also demonstrates the value of a heterogeneous environment in which predator and prey are widely dispersed

Classic metapopulation Source-Sink Metapopulation -population survival may be dependent upon a few key habitat patches

• Source populations = “ • • Sink populations = habitat poorer in quality and mortality exceeds reproduction

dp/dt = cp(1-p) - ep

• Must take into consideration

both

temporal and density-dependent changes in population demography, sources/sinks change

Rescue effect

• When a subpopulation is very small, local extinction can be prevented by occasional immigrants that arrive from neighboring patches   – Proportion of occupied habitat patches constant, even though populations in individual patches may go extinct Extinction threshold –

cp > ep

Remnant populations – long-lived species may persist because of life history traits rather than immigration  i.e. plant species that can survive a long drought

B. Assumptions of the metapopulation theory

 reproductive success of individuals within populations is dependent upon habitat quality  not all habitats have equal quality within the range of the species  Local population dynamics are density dependent  Population dynamics in different patches are independent of one another  Limited dispersal links population subunits

Significance of Metapopulation theory

1) Provides incentives 2) 3) Fragmented group of population subunits could enhance population structure and persistence Small habitats may

II. Stochastic Perturbations & MVPs

A. 4 Sources of “uncertainty” that can affect the size of a population: 1) Genetic stochasticity 2) Environmental stochasticity  If r av > V e then the expected persistence time of a population increases directly with increasing population size  If V e > r av then the population reaches a situation where further increases in population size don’t significantly increase expected time of population persistence

Extinction vortex

The Heath Hen, and example of the stochastic effects-

B. MVP – minimum viable population

• Focus of population ecologists  one that meets

"the minimum conditions for the long-term persistence and adaptation of a species or population in a given place"

(Soulé 1987).

 N theoretically sufficiently large to protect against extinctions caused by harmful and unpredictable genetic,  MDA – minimum dynamic area –

C. PVA: population viability analysis

1). Construct a mathematical model

using the following data:  Average mortality rates  Average recruitment rates  Current age distribution  Current size

2). Add stochasticity to the model

(Environmental, genetic, demographic stochasticity) – elements of variation, for a realistic approach

Benefits of PVA

 Simulations of individual populations can be run using this random variation to determine the probability of population extinction within a certain period of time or the mean time to extinction.

 Can determine which parameter or combination of parameters most influences extinction probabilities  Management regimes that affect population parameters can then be developed and analyzed  Can compare alternatives  Can evaluate the effectiveness of management efforts

Problems with PVA

     Different definitions – not restricted to a mathematical model, but should be Estimating parameters – totally dependent upon field data which is not always available

(the more data, the better the analysis)

Can only be used on a population by population basis Can’t diagnose the cause of decline, or prescribe a remedy for it • Level of uncertainty may be large Management actions like betting on a game of chance – spatially complex stochastic population hard to predict

PVA examples

Eastern barred bandicoot, endangered Australian ground-dwelling marsupial In 1989, only 150-300 individuals remained.

Leadbeater’s possum, Until recently, it was regarded as extinct, after disastrous fires throughout its range in 1939. However, in 1961 a small colony was rediscovered near Marysville, Victoria. As of 1996 it was confined to an area of about 3500 sq km (1350 sq mi) near the western limit of Victoria's Central Highlands.