Habitat Fragmentation

2 components

1. reduction (habitat loss) in the area covered by a habitat or habitat type, in a landscape

2. A change in habitat configuration, with the remaining habitat apportioned into smaller and more isolated patches

Habitat Fragmentation

usually caused by human settlement, agriculture, or resource extraction

Patchwork of varying habitat suitability (from the standpoint of species’ habitat requirements)

The landscape matrix that surrounds The landscape mosaic can be may/may not be

hospitable to species, and may/may not be used for movement among fragments

The landscape mosaic can be

be described as small, isolated natural areas (patches) in a sea of altered land (matrix)

Thousands of studies in fragmented landscapes have documented:

1. Local extinctions (extirpations)

2. Species declines

3. Shifts in composition and abundance patterns favoring weedy species or invasive species

4. Isolation into small populations with reduced capacity for survival

Fragmentation & Heterogeneity/patchiness

A naïve view is that the fragments within a fragmented landscape are homogeneous/even patches

Heterogeneity/patchiness is created by the physical environment and by natural disturbances

Patchiness is also temporal

Together they make up the ‘space-time mosaic’

Fire mortality patches in Oregon>>>>>>>

Metapopulations

systems of local populations linked by occasional dispersal

populations are spatially separated and linked by dispersal of individuals or propagules (e.g. seeds)

if dispersal becomes impossible due to distance or lack of corridors, stepping stones, or a suitable matrix through which species will travel, the meta population will go extinct

Source population

population growth rate is increasing, feeds individuals into other populations

Sink population

population growth rate is decreasing, needs individuals from the source population to persist

Source and sink populations are NOT necessarily determined by size. They are determined by

whether or not the population is increasing in size (source) or decreasing in size (sink)

Fragmented vs. naturally patchy habitats

1. Fragmentation results in a reduction of the extent & connectivity of habitats, and species may or may not adjust to these changes

2. Naturally patchy landscapes have rich internal patch structure (e.g, treefall gaps, logs, layers of vegetation), whereas a fragmented landscape typically has simplified patches and matrix (e.g, parking lots, cornfields, clearcuts, tree farms)

3. Due to the above, natural landscapes have less contrast between adjacent habitats than do fragmented landscapes

4. Certain features of fragmented landscapes, such as roads and various human activities, pose specific threats to population viability

Fragmentation

results in a reduction of the extent & connectivity of habitats, and species may or may not adjust to these changes

naturally patchy

have rich internal patch structure (e.g, treefall gaps, logs, layers of vegetation), whereas a fragmented landscape typically has simplified patches and matrix (e.g, parking lots, cornfields, clearcuts, tree farms)

the fragmentation process

• fragmentation and edge effect studies in landscapes that are still largely forested typically fail to find negative effects; sometimes effects are positive

• fragmentation effects or thresholds of fragmentation are species specefics

Time lags are known: for example

site tenacity in birds may cause birds to return to a site where they bred successfully in the past, sometimes years after the habitat has been altered or fragmented

extinction debt

the time lag between habitat loss and eventual extinction

fragmentation studies

• Researchers may see outcomes of fragmentation without observing the process; conversely, they may view the process but not the long-term consequences

• Fragmentation occurs over a number of spatial and temporal scales

  • The Bering land bridge or Isthmus of Panama were critical to the prehistoric movement of species. Imagine if those passages had not existed!

• Fragmentation currently is preventing long range movements such as might be needed for species in response to climate change

The biological dynamics of forest fragments project

• born out of the SLOSS debate (single or several small reserves of equal area) during the 1970s

• large-scale, long-term project investigating the effects of habitat fragmentation on tropical rainforest

• began in 1979 in the amazon near Manaus, Brazil

• collaboration between Brazil and the US

• Fragments created with sizes of 1 hectare, 10 hectares, and 100 hectares

• >600 publications have arisen, along with >150 PhD disserations and masters theses

Example study: Bierregaard and Stouffer studied changes in bird presence and abundance with remnant size, time since isolation, and nature of the surrounding vegetation

• Obligate army ant following bird species disappeared from the 1 Ha and 10 Ha remnants within 2 years

• Frugivores and insectivores showed declines after isolation

• Nectivores (e.g., hummingbirds) were less vulnerable to fragmentation

initial exclusion

loss of species that occurred only in the portion of the landscape that was destroyed

• example: 90 spp. of (endemic) plants were lost when a small patch of forest was logged in ecuador

Crowding effect

• Increased population densities due to ‘packing’ of individuals that lost their home ranges when former surrounded habitat is destroyed

• It is generally followed by population collapse

• Example: observed in the Amazonian forest fragmentation experiment: capture rates of understory birds doubled in the first few days after clearing, but fell rapidly in subsequent days

insularization and area effects

• When the area of a habitat declines, so does the number of species (Island Biogeography Theory)

• A small habitat island may be smaller than the home range of a single individual (e.g., cougar home ranges can exceed 400km2)

• Large (wide-ranging) animals are often more vulnerable to local extinction due to human-caused mortality via vehicles, roads, and guns (human-animal conflict)

• Some species are sensitive to fragment size much larger than their home ranges (example: songbirds in the Eastern U.S., Japan); reasons are unknown

isolation

• Isolation of habitats and populations affects species that require a constellation of patches for food, roosting and breeding

• Barriers to those patches can cause extirpation or extinction

• Barriers tend to be species-specific (e.g., a hedgerow can be a barrier to livestock but a corridor to native species such as a skunk)

• Includes a reduction in genetic diversity (e.g., inbreeding)

Edge effects

• Edge zones of forests are usually drier, less shady and warmer than forest interiors; these conditions affect plant composition

• The width of an edge effect can vary, depending on how far an edge species penetrates; some fragments will be considered “all edge”

• Ecological trap: edges can attract more predators than forest interiors; for example, predation and parasitism of bird nests can increase near edges

ecological trap

edges can attract more predators than forest interiors; for example, predation and parasitism of bird nests can increase near edges

matrix effects

• The amount of structural contrast between habitat fragments and the matrix in which they exist

• As this contrast increases, animals can be less willing or able to travel from one habitat patch to the next

• For example, bird nest predation rates are higher in woodlots surrounded by suburbs than those surrounded by agriculture, because garbage and other food subsidies in suburban landscapes encourages the proliferation of mesopredators (e.g., raccoons, skunks)

What can we do to reverse fragmentation?

“regrow” the matrix

Create wildlife corridors