Lecture 21: The role of habitat fragmentation and island biogeography in defining ecological communities

When compared to marine habitats, dispersal between spatially distinct communites is often more limited in terrestrial habitats

Still, movements by terrestrial organisms are also a potentially important aspect of ecology (remember, only four things ultimately influence population changes: births, deaths, immigration and emigration).

 

In general, we expect more species (higher diversity) to persist with increasing size of a habitat

This concept is supported by species - area curves

 

Also see figure D22.1.1 in text

Why?

 

Look at this in more detail.  The concept of Island Biogeography

 

First... is there more than one kind of "island"?

 

Species compositions on islands should be a function of immigration and island size

 

The MacArthur-Wilson Equilibrium Theory

(Figure 22.2 in text)

 

Distance from source populations will influence the frequency with which species can reach islands

 

What kinds of biological factors might influence the potential for a species to disperse to an island?

 

Habitat complexity, along with island size, will influence how many species will persist on an island

 

An expectation of decreasing immigration of new species and increasing extinction rate is as an island is populated with species

This predicts an equilibrium in species number, although species composition will continue to shift.

Do the data from the recolonization of Krakatau fit this prediction?

 

We can consider fragmented habitats as islands

What causes fragmentation and what are the general implications for biodiversity given species/area relationships?

Edges of habitat often support a unique flora and fauna that are well adapted to a broad range of environemental conditions

"Interior" species may require a more narrow range of environmental conditions to win the food/sex/death game.

How do edge/interior ratios change as habitats shrink?

How does habitat shape influence these ratios?

 

Some additional thoughts linking ideas we've already talked about:

Habitats fragments (or islands... or reefs... or "reserves") of varying quality may differ greatly in the degree of reproduction that different species achieve.

Rich or productive regions may be important "source" populations that can resupply less viable habitats

If habitat quality is low enough to severely limit reproduction, a population may be considered a "sink" (i.e., organisms enter the area from outside but very few organisms disperse away).

As we have seen with other systems (e.g., coral reefs) the collective interaction of these movements among fragmented populations may be considered as a "metapopulation"

Understanding how populations are influenced under such conditions is critical to management (think about
"reserves" that are effectively isolated from other suitable habitat by large stretches of lower quality)

Also think about the genetic consequences of such isolation

 

As habitat loss continues, extinctions should rise, though some factors may slow the process (think about successional models). Some species and types of organisms may be less prone to extinction....

What about rare, endemic, or localized species?

 

Later in the term we will think about these factors as they relate to forestry practices in the Pacific NW as well as the establishment of marine reserves along our coast.