LZ lecture 5 Flashcards
Biodiversity: human impacts.
A brief recap from the Millennium Assessment
- Changes in biodiversity due to human activities were more rapid in the past 50 years than at any time in human history
- The drivers of change that cause biodiversity loss and lead to changes in ecosystem services are either steady, show no evidence of declining over time, or are increasing in intensity.
What are the most important direct drivers of biodiversity loss and ecosystem service changes?
- Habitat change (such as land use changes, physical modification of rivers or water withdrawal from rivers, loss of coral reefs, and damage to sea floors due to trawling)
- climate change
- invasive alien species
- overexploitation, and pollution
The effect of habitat loss on biodiversity
- The disappearance of a habitat in a given place will result in the loss of the species usually found in that habitat.
- In addition, considering the well-known relationship between area and species richness (see Rosenzweig, 1995 or Drakare et al. 2006 for recent discussion)
-We expect that when the area available for species is reduced, the number of species declines.
Habitat loss for a single species
- we expect a similar relationship between patch size (area of the patches of available habitat) and population size (big patches have more resources and more microhabitats and can maintain bigger populations).
- This indeed provides a mechanism to explain the species richness area relationship: when smaller habitat contains smaller populations their higher extinction risk result in species disappearance.
Impact of habitat fragmentation on biodiversity
- The impact of habitat fragmentation is less clear, as fragmentation frequently accompanies habitat loss
- Even if the total area remains relatively constant, fragmentation leads to an expansion of edge habitat at the expense of interior habitat.
- This shift can result in a loss or reduction of ideal habitat for species that thrive in the interior.
- Nonetheless, some species may favor edge habitats, and distinct fragments can support subtly different communities.
- In principles then, the consequence of fragmentation in isolation may range from negative to neutral or even positive, contingent on the specific species involved.
Habitat loss vs fragmentation
“Empirical studies to date suggest that habitat loss has large, consistently negative effects on biodiversity. Habitat fragmentation per se has much weaker effects on biodiversity that are at least as likely to be positive as negative. “
Metapopulation
- Set of local populations occupying various habitat patches and connected to one another by the movement of individuals among them (Ricklefs and Miller 2000).
- Subdivided populations with demographically significant exchange among them (Gutierrez and Harrison 1996). A ‘population of populations’
Local population
- Individuals of a species that live in a habitat patch.
- Sometimes used synonymously with subpopulation and population (Ricklefs and Miller 2000).
- In some species, local extinction is common, and regional persistence is related to the existence of a metapopulation
Habitat patch
Discrete areas of habitat characterized by the resources and conditions for a population to persist
Turnover event
When a habitat patch becomes unoccupied through extinction and is then recolonized by individuals from other local populations (Ricklefs and Miller 2000).
Metapopulation persistence time
Length of time all populations persist within a metapopulation until all go extinct (Ricklefs and Miller 2000).
Metapopulation structure
- Describes the specific characteristics of natural metapopulations that are explicitly included in a metapopulation model.
- The intended result is to make the model more realistic.
- For example, the assumption that patches are all equally isolated could be made more realistic by specifying different migration rates among local populations.
- The types of modifications to the metapopulation model make the concept more useful (Ricklefs and Miller 2000).
Rescue effect
Migration between local populations prevents local extinctions (Gutierrez and Harrison 1996).
Possible metapopulation structure
- The destruction of a core population can cause the extinction of its
satellite populations (some areas are sources, others are sinks) - Description picture: The size of the population is indicated by the size of the circle representing it. Arrows indicate the direction and intensity of dispersion from one population to another
- A: 3 independent populations
- B: Simple metapopulation with three interacting populations
- C: Metapopulation with 1 big central population and 3 satellite populations
- D: Metapopulation with complex interactions
What are examples of connectivity within the metapopulation?
- Ecological corridors
- Stepping stones
- Buffer zones
Continuum of mobility relative to patch structure (patchy population, metapopulation, separate population)
Figure 9 of F Boero et al 2016
Hypothetical dispersal kernels for three
populations (A, B and C) that differ in size.
The horizontal line differentiates between ecologically-important
sustaining dispersal and evolutionarily-important seeding dispersal.
As population size decreases (A→C),
the distance over which it provides sustaining dispersal decreases (z→x), and the distance over which seeding
dispersal occurs may decrease as well.
Population C is too small to provide any sustaining dispersal.
Figure 10 of F Boero et al 2016
Simplified conceptual model illustrating the
effect of MPA size and spacing on connectivity in a MPA
network.
- numbers (1-3) designate the centre of the MPA,
- white space the distance over which ecologically-important “sustaining” dispersal occurs
- grey space the distance over which evolutionarily- important “seeding” dispersal occurs.
- A. Spacing between two relatively large MPAs appropriate to insure sustaining dispersal between them. Note that
the area of overlap of the seeding dispersal kernels for the two MPAs may or may not be sustaining. - B. 50% smaller MPAs than in A but the same distance apart. MPA populations are smaller, producing fewer
propagules, thereby decreasing the spatial scale of dispersal resulting in no connectivity between them. - Re-establishing connectivity between small MPAs can be accomplished by decreasing the distance between them or C the addition of a third small MPA to the network.
