Lecture 7 Take Home Points
Disturbance
Any discrete event that disrupts ecosystem, community, or population structure and
changes resource availability or physical environment
► Effects are often complex and system dependent
► Passengers versus Drivers
Passengers – invasive species passively taking advantage of existing or
ongoing disturbance.
Drivers – the invasive species themselves are the disturbance, altering the
ecosystem through their activities.
Disturbance is linked to other processes and it is hard to separate the effects of
disturbance alone apart from other factors
e.g., Interaction with propagule pressure
Invasional Meltdown Hypothesis
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With  number of introductions, even if not successful, the ecosystem
is incrementally weakened, facilitating the invasion of other species.
• As established invaders , future invasions succeed more easily, AND may
have larger impact.
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The Biology of Small Populations
• Several processes act independently and synergistically on small populations
that can drive population dynamics toward extinction
• Environmental Stochasticity
Unpredictable (stochastic) changes in external forces acting on a population
(climate, natural enemies, resources, etc).
Density independent – but small populations may be more vulnerable due to
spatial constraint.
Catastrophe – special class of environmental stochasticity – occur very
infrequently but have dramatic, large scale effects.
• Demographic Stochasticity
Internal (intrinsic) properties of the population such as births, deaths, sex ratio,
etc. that fluctuate from generation to generation.
Density dependent
• Genetic Stochasticity
Random changes in allele frequencies that are independent of selection.
Density dependent
• In turn these make populations susceptible to Allee Effects, inbreeding and
genetic bottlenecks.
• May contribute toward long lag times in some invasions and the extinction of
others.
Environmental Stochasticity
Temporal changes in vital rates (births and / or deaths) that affect all
individuals of a given age or stage similarly; the sampling variances of the
vital rates are nearly independent of population size
• = largely a density independent process
• Random environmental variation that affects a population
• Can affect even large populations
• May result in population extinction
Demographic Stochasticity
• Density dependent – the smaller a populations gets, the more important it
becomes
• Random fluctuation in vital rates (births deaths, sex ratio, etc) that is an
intrinsic function of the population (not driven by external factors)
• Variance increases with smaller populations – the larger fluctuations in the
vital rates over time lead to a significant increase in extinction risk.
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Assisted Colonization
-Deliberate extra-range movement of threatened or endangered species.
-Move species into areas outside of their historical range in order to provide
more suitable habitat for them.
-Element of unpredictability:
(1)
(2)
(3)
(4)
(5)
How will the species respond in the new environment
How will the recipient environment respond to the new species
Inadvertent introduction of associated organisms
Plastic or adaptive change in new environment
No matter the species, the element of risk is never zero.
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