FW 404: McComb Ch15 and Ch16
Chapter 15: Landscape Structure and Composition Notes
Defining and Conceptualizing Landscapes
Ecological landscapes are viewed as a mosaic.
A landscape has structure (the pattern of patches), composition (the types of patches), and function (defined by resources, such as habitat for a species).
The pattern of habitat patches differs greatly depending on the species.
For example, Swainson’s thrush seeks patches of shrubs for foraging and nest sites, often disregarding tree species or stand structure.
Northern flying squirrels are concerned with snags, tree size, and canopy cover.
Cottontail rabbits require a linear strip of habitat at the edge between grassy areas and dense shrubs.
Habitat quality is a spectrum, more appropriately viewed as a collage of values or shades of gray rather than simple source/sink patches.
The structure, composition, and function of a landscape are dependent entirely on the species or other values that managers wish to produce.
Some researchers suggest that the future of landscape ecology lies in gradient-based analysis (viewing landscapes as gradients of resource availability) rather than patch-based approaches.
Landscape Attributes (Scaling Properties)
When considering habitat function, three attributes of a landscape must be defined: grain, extent, and context.
Grain is the smallest unit of space identified and used (by us or the species). It might be an individual tree, or a minimum size for a patch that is meaningful and practical for management (e.g., 0.5 ha for deer browse patches).
Extent is the overall area over which resources are being managed (e.g., a 20,000 ha forest).
Context is the larger, surrounding area that is not managed but significantly affects the function of the managed landscape. For instance, managing pileated woodpeckers in Central Park is constrained by the urban context.
The size of a landscape is functionally dependent on the species; for example, a decaying log can be a landscape for fungal colonies, while the migratory route of Swainson's hawks is a landscape extending from North America to Argentina.
From a practical standpoint, landscapes are usually considered hundreds to thousands of hectares. When managing mobile animals, the extent must include all owners/properties because animals do not respect property or political lines.
Habitat Quality and Edge Effects
Habitat quality relates to individual and population fitness. The location and distribution of high-quality patches are crucial; if a species needs 40% of its home range in high-quality food, those patches must be distributed appropriately across the landscape.
Edges (boundaries between patches or linear features like roads/rivers) have a disproportionate effect on habitat quality.
Induced edges occur between patches of different successional conditions, usually caused by disturbance, representing structural differences (e.g., forest next to agricultural land).
Inherent edges are formed by differences in forest composition, representing a floristic interface.
Edges can be detrimental for some species:
Induced edges near agricultural lands are ideal for brown-headed cowbirds (brood parasites). Cowbird parasitism and increased predation by animals like raccoons and snakes along edges can make these areas "sinks" for neotropical migrant birds.
Species classification based on edges:
Edge associates find the best habitat quality when they have access to resources in two or more patch types.
Edge specialists (like cottontail rabbits) are likely to only occur where edges exist.
Forest interior species avoid edges and use the core of a patch.
Species richness is often high along edges due to the presence of specialists, associates, and species from adjoining patches. However, increasing edges can be detrimental to forest-interior species.
Edges affect habitat through microclimate changes (drier, more wind, more sunlight), influence on disturbances (e.g., windthrow, fire), and human effects (hunting, grazing, feral animals).
Edge Geometry and Fragmentation
Edge Density is influenced by patch size and shape.
To minimize edge (and maximize core area), management should aim for fewer, large patches that approximate a circle or hexagon.
To maximize edge, management should create many smaller, irregularly shaped patches.
Core conditions are defined as the area of a patch away from an edge (which could be 5 m or 500 m depending on the species). Small or irregularly shaped patches may have zero core area.
Habitat Fragmentation is the process where a specific habitat is subdivided into smaller, geometrically more complex, and more isolated pieces.
Fragmentation is distinct from habitat loss (reduction in area). Habitat loss may occur without increasing isolation or geometric complexity.
The process of fragmentation is species-specific (e.g., fragmentation for ovenbirds means reducing mature hardwood forests; for bobolinks, it means forest encroachment upon grasslands).
Species-Area Relationships: Large patches support more species (species richness shows an asymptotic relationship with area).
Human intervention and forest management often lead to a narrower domain of spatial scales (fewer very small patches and fewer very large patches) than natural disturbance regimes, potentially putting species adapted to those scales at risk.
Chapter 16: Landscape Connections Notes
Dispersal and Movement
Movement between patches can occur via corridors/connections or by dispersing across the matrix (intervening conditions).
The matrix conditions represent a differentially permeable membrane; some intervening conditions allow animals to move easily, while others pose significant survival risks.
Dispersal is the movement of an animal from its home range, typically caused by natal displacement, dominant individuals, or habitat disturbance.
Dispersal is critical for population growth, metapopulation stability, recolonization of vacated habitat, and gene flow.
Dispersal Capabilities: Dispersal specialists (e.g., red tree voles) have limited capabilities, while dispersal generalists (e.g., Pacific jumping mice) have extensive capabilities.
In mammals, natal dispersal distances are often predicted by body mass, trophic level (carnivores disperse farther than herbivores), geographic range size, and home range size.
Dispersal Pattern: Dispersal distances typically follow a negative exponential curve; most individuals disperse short distances, and few disperse long distances. Animals may choose to settle in sub-optimal habitat rather than risking death by expending too much energy dispersing through low-quality patches.
Probability of Successful Dispersal
The probability of successful dispersal across a landscape is a function of three interacting probabilities: encounter, survival, and continuing.
Probability of Encounter ($P_e$):
Many animals disperse in random directions.
$P_e$ is defined by the angle ($\alpha$) formed from the center of the source patch to the outermost edges of the target patch (if dispersal is random, $P_e = \alpha/360$).
To increase the likelihood of maintaining a metapopulation through dispersal, managers should ensure there are more, larger target patches located closer to the source patch, as this increases $P_e$.
Probability of Survival: This depends on the animal’s fitness as it moves across different patch types. Survival is the key requirement during dispersal (reproduction is typically not a concern). Survival can be measured using a time-specific probability, such as daily probability of survival.