“contagious catastrophes” “ecological fire escape” species-area relationship
But first, some background:
geometric module (Cheng and Spetch 1998)
“sense of place”
Most organisms can recognize "landmarks" (which may be visual, olfactory, auditory, chemosensory, magnetic, or tactile), indicating that they recognize that the world is heterogeneous and can identify landscape structure. In this way they become oriented and consequently know where they are (Healy 1998). But in addition to landmarks, most other organisms also have an instinctive “positional awareness.” Experiments on rats, for example, have shown that they utilize geometric space more than other cues like odors in orientation. This trait has also been found in domestic chicks and in human children and is called a geometric module (Cheng and Spetch 1998). This "sense of place" is a strong instinctive response in many animals, including humans. These facts indicate that animals recognize that they exist within a spatial context–in other words, they recognize that they exist within a landscape.
The features that are recognized/used as landmarks and to reinforce the geometric module are patches, corridors, and boundaries. These to be embedded in a matrix, the background land‑use type, characterized by extensive cover, high connectivity, and/or control over dynamics that occur within patches and corridors and along boundaries.
The matrix can have a PROFOUND effect on pattern-process relationships happening within patches (much more in future lectures); see recent review by Prevedello and Vieira 2010 for more info.
What is a landscape?
Consider these varied definitions:
"the total spatial entity of human living space" (Troll 1968)
"[the] physical, ecological and geographical entities [that integrate] all natural and human (‘caused’) patterns and processes" (Naveh 1987)
"heterogeneous land area composed of clusters of interacting ecosystems [that] is repeated in similar form throughout" (Forman and Godron 1986)
"a particular configuration of topography, vegetation cover, land use and settlement patterns which delimits some coherence of natural and cultural processes and activities" (Green et al. 1996)
A landscape, by the way, need not be terrestrial. It can also be aquatic.
Landscapes are (usually) heterogeneous in composition (vs. homogeneous, which would be rather boring). Landscapes are comprised of three main elements: patches, corridors, and boundaries (all embedded in a matrix). Different landscapes differ in the configuration of these elements. Recall that landscape ecology is the study of the effect of pattern on process. Therefore, it is important to describe landscape structure in terms of its pattern of patches, corridors, and boundaries:
for comparative purposes, to quantify the differences between or among study areas; &
to infer underlying agents (causal mechanisms) of pattern formation.
Of course, this latter goal is fundamental to much of landscape ecology as it is to ecology in general. As we shall see, the task of attributing causal mechanism (process) to observed pattern is a daunting task.
Patches are defined as regions that are more or less internally homogeneous with respect to a measured variable (a set of spatially proximate homogeneous units). There are several approaches to defining specifically what a patch is:
Simple aggregation of like-valued regions: A straightforward method of defining patches is simply to aggregate all adjacent areas that have the same (or similar) value of the variable of interest.
Contrast methods: An alternative approach is to define patches by finding the edges around patches, which contrasts the patch from its surroundings. In some cases, this is easy to do; in other cases, edges can be subtle (more later in the course). An edge in this case is an area where a variable (e.g. canopy height) changes abruptly (will have high variance).
Patch Characteristics A patch (once found/defined) is usually quantified (described) by its:
Area - the size of the patch, in units of map scale (e.g., m2) or as a proportion of the total map area. Actually very difficult to measure. Often crucially important to organisms (species-area relationship).
Perimeter - patch circumference
Shape - often summarized in terms of edge/area ratio. In many instances, this ratio is normalized to take on a value of 1.0 for the most compact shape possible (e.g. a square for raster data)
Type - usually a categorization of some internal patch attribute, such as "forest," "urban," etc.
Components of Landscape Structure
There are three main components of landscape structure (O'Neill et al. 1988):
connectivity - Corridors – see book Corridor Ecology by Hilty et al. (2006) & recent review by Gilbert-Norton et al. (2010)
Various definitions of corridor (some structural, others functional):
“narrow strips of land which differ from the matrix on either side” (Forman and Godron 1986, p. 123)
“a linear landscape element that provides for movement between habitat patches” (Rosenberg et al. 1997, p. 678)
“routes that facilitate movement of organisms between habitat fragments” (Hilty et al. 2006, p. 5)
“a landscape element that plays a key role in connectivity” (Anderson and Jenkins 2006, p. 3)
Corridors have been of especial concern in conservation with respect to reserve design to combat fragmentation-induced habitat isolation (“bandages for wounded landscapes” according to Laurance and Laurance 2003). They are presumed to decrease extinction risk by boosting connectivity (click here for a modeling example). Why would connectivity be important for this?
Click here for some properties of corridors.
benefits of corridors (Noss 1987) -
deleterious aspects of corridors (Simberloff and Cox 1987, Simberloff et al. 1992) -
But see a recent review by Haddad et al. 2014 - No evidence that corridors increase unwanted disturbances or invasives, and there were as many positive as negative edge effects…
Although the value of corridors has been debated (Hobbs 1992), there is a growing body of empirical work of corridor use, although many studies have been on small mammals that aren’t in need of reserves (see, for example, Coffman et al. 2001) and short-term. More good data/studies on how corridors affect large, endangered organisms are needed (although some work has been done on Florida panthers, Australian marsupials, plants, insects, birds, Iberian lynx, and gray wolves). As a great example of some empirical work, check out the following news link (from Scientific American) that discusses a study of how corridors affect animals, plants, and processes such as pollination: http://www.sciam.com/article.cfm?articleID=00039786-33B9-1D86-B3B9809EC588EEDF .
(The paper cited in the article won the 2002 Best Paper in Landscape Ecology award.)
Other empirical examples:
-bat species richness increased in patches linked by forested corridors in French Guiana (Brosset et al. 1996)
-small mammal species richness increased in patches of Atlantic forest in Brazil that were linked by corridors compared to isolated patches (Pardini et al. 2005)
Gilbert-Norton et al. (2010) conducted a meta-analysis of corridor use and effectiveness in 78 experiments from 35 studies; you should know what their key findings were (from assigned reading).
Keep in mind that something that is a corridor to one species may not be perceived or used as such by another species (i.e., corridors [and, thus, connectivity] are taxon-dependent). And corridors may have both positive and negative effects on the same species at different life stages (Orrock and Damschen 2005). And the landscape matrix surrounding a corridor may influence its use (Pearson 1993; see Turner et al. text pp. 236-238 for more details).
How wide does a corridor need to be? Too narrow and it may expose organisms to adverse weather conditions, predation risk, etc. Too wide and it may cause animals to wander around in it, spending energy without finding a suitable territory. A “10% rule” has been proposed (see Laurance and Laurance 2003): the corridor’s width should be 10% of the area of the patches it is connecting.
The transition zone between two distinct landscape elements (e.g. patch and matrix) is variously called:
• an edge
• an ecotone
• a boundary
This transition area possesses some characteristics of both landscape elements but is neither completely one nor the other. Therefore, some people refer to an ecotone as a landscape element (habitat type) unto itself. Edges are often drier and hotter, with more weedy species, than the patches of which it is an edge. Others recognize that although it may have some emergent properties, an edge/ ecotone/boundary’s primary function is as a barrier (permeable or impermeable) to ecological flows (movement of matter [including organisms] and energy). The transition may be subtle (a "soft" boundary, e.g. edge between mixed oak-beech stand to pure oak stand) or quite sharp ("hard," e.g. edge between forest and clear-cut), with implications for movement of matter and energy across the landscape. Soft boundaries may be traversed with relative ease, whereas a hard boundary may hinder, slow, or even deflect ecological flows. Edge effects create a "patch within a patch" situation (i.e., interior patch within patch as a whole). For example, Wilcove (1985) showed that nesting birds suffer greater predation the closer to forest edges they nested.
Similar deleterious edge effects have been found for plant growth and seed dispersal. Although certain taxa may be favored by ecotonal conditions (such as nest predators), boundaries can act as ecological traps: because they often have more sunlight, edges favor growth of weeds, which have lots of seeds and insects that songbirds feed upon; songbirds may be fooled by apparent high habitat quality and nest there, unwittingly exposing themselves (literally) to increased nest predation. See Battin (2004) for a nice review of the topic of ecological traps.
Determining boundary "hardness" (or "softness") is extremely difficult because it is more a posteriori than a priori: difficult to predict whether, when, and how a particle (especially an animal) will actually flow from one element to the other. Closed boundaries create a patch.
See Johnston et al. (1992) for quantitative methods for studying boundaries. One often-used technique is "sliding windows" (e.g. along a transect or within a quadrat), which is used to detect areas where variance changes suddenly (i.e., a boundary). Compare variances between one window position and the next using squared Euclidian distance (SED). Click here for an example of how changing the window size changes the results.
An alternative to detecting boundaries involves variance. Click here for an example.
We will discuss movement in greater depth later in the course, for movement is a key ecological process with implications for conservation biology, wildlife management, pest management, and many other areas.
How do these landscape elements affect ecological processes?
The composition, configuration, and connectivity of these landscape elements, singly and interactively, determine the abundance and distribution of organisms, where dispersal and flows occur, and the rates of biogeochemical reactions (Dunning et al. 1992, Taylor et al. 1993). These 3 descriptors should always be at the back (if not the front!) of your mind throughout this course.
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Battin, J. 2004. When good animals love bad habitat: Ecological traps and the conservation of animal populations. Conservation Biology 18:1482-1491.
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Coffman, C.J., J.D. Nichols, and K.H. Pollock. 2001. Population dynamics of Microtus pennsylvanicus in corridor-linked patches. Oikos 93:3-21.
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