Landscape Ecology
Fragmentation
Terms/people:
Fragmentation
Loss, isolation, alteration
Ecological trap
Land use
Microcosm
edge effects (Aldo Leopold)
species-area relationship
ecotone
land cover
mesocosm
Like the term “disturbance,” “fragmentation” is a single term for a complex subject (see
Fischer and Lindenmayer 2007). Fragmentation is a leading cause (arguably the leading
cause) of the loss of biodiversity and ecosystem services and as such is a crucial topic in
landscape ecology, conservation biology, and wildlife management (Collinge 2009).
Fragmentation is both a pattern and a process.
There are two agents of pattern formation in landscapes: natural and anthropogenic ones.
But first, a distinction:
land use land cover Natural landscape heterogeneity:
often created by disturbance, such as:
-fire
-flood
also:
-topography
-slope
-aspect
-elevation
Anthropogenic landscape heterogeneity:
Delcourt (1987) discussed the five main ways in which humans have historically affected
the Earth’s surface:
-humans have changed the relative abundances of organisms (especially trees)
from forest clearing for firewood and making agricultural fields
-humans have extended or truncated the distribution of several species (including
apples and olives, deliberately spread, and bald cypress, overused by Native Americans
and becoming locally extinct as a result)
-opportunities were created for ruderal species to invade at the edges of humandisturbed areas
-nutrient status of soils was altered both thru ag depletion and fertilization
-landscape mosaic was altered, especially the distribution of forest vs. nonforest
(In the next lecture, we will discuss the three main activities that humans do that create
these results.)
Both natural and anthropogenic factors result in habitat fragmentation.
Fragmentation - entails 3 changes:
1.
2.
3.
Why care about fragmentation? See reviews by Saunders et al. 1991 and Fahrig 2003.
I’ll summarize in a chain:
fragmentation  brood parasitism, nest predation, inbreeding, disruption of
symbioses or coadapted gene complexes, inducement of dispersal, Allee effect from
isolation, etc.  increased extinction risk
Species most vulnerable to fragmentation:
-specialists
-species with large home ranges (e.g. large-bodied species, species at high trophic
levels [e.g. carnivores])
Species-area relationship – one of the most venerable patterns in all of ecology:
Results may not be as dramatic as anticipated if remaining habitat is high in quality
(can offset loss of habitat): Summerville and Crist 2001, Fleishman et al. 2002, Franzén
and Nilsson 2012, Robles and Ciudad 2012; see review by Prugh et al. 2008
One of the consequences of fragmentation is creation of edges.
A recent review (Haddad et al. 2015) showed that 70% of the world’s forests are within 1
km of the forest’s edge!!! This fragmentation has reduced biodiversity by 13-75% in the
past 20 years, depending on region. The only relatively intact forests left are the Amazon
and the Congo. That’s it—TWO forests. The rest are beset with edges and edge effects.
Edge effects The term "edge effect" was popularized by Aldo Leopold in his classic text Game
Management (1933). The term refers to the fact that many organisms exhibit responses
to the presence of habitat edges. Because edges represent the boundary between two
habitat types, one may encounter representative species from both habitat types there.
(This blended area is sometimes considered to be a unique habitat type called an
ecotone.) Leopold noted that the "simultaneous access to more than one [habitat type]"
led to higher abundances and species richness at edges, meaning that edge effects were
initially seen as being positive. Many wildlife management and conservation efforts
were initially focused on creating and maximizing edges.
However, while it is true that some species do tend to congregate at habitat edges for a
variety of reasons:
- change in microclimate present at an edge that represents an intermediate
between 2 habitat types means that a species can experience a change in conditions
without necessarily being exposed to completely different conditions
- difference in visibility (useful for both predators and prey)
- a simple unwillingness to venture out into the other habitat type while being
crowded or wanting to disperse
many other species tend to avoid edges, meaning that the effective patch size is smaller
than the actual patch size (an insidious trait because actual patch size can be measured
but mislead us into thinking that we have more effective habitat than we in fact do have):
"edge" and "interior" species (esp. in the ornithological literature) Although many "edge" species are economically important game species (e.g. quail,
pheasant, partridge, rabbits), many more edge species are quite common, "weedy"
species. In addition, there are many deleterious aspects of the presence of edges,
including:
- edges serve as access points to interior habitats for invasive/weedy species,
brood parasites, and nest predators
- organisms may experience crowding there
- species near edges that have passively dispersed propagules (e.g. plants with
wind-borne seeds, ballooning spiderlings) may lose more of their propagules into
unsuitable habitat
- edges may act as "drift fences" that funnel organisms along set pathways, where
they may be vulnerable to predation
- may act as "ecological traps" (see Battin 2004) –
And many interior species were species with declining population numbers and were thus
of conservation concern. Thus, the term "edge effect" came to be seen as something
negative, and these negative effects were noted to extend into the adjacent habitats by as
much as 300 m! Wildlife management and conservation efforts now often perform
actions that are 180o different than just a few decades ago regarding edges.
However, the terms "positive" and "negative" are value-laden terms. It is far more useful
to not categorize but rather to focus on what is now known about edge effects.
1) Edges do influence temperature, light availability, and moisture, which in
turn influence the abundance and distribution of organisms (Harris 1988, Kapos 1989,
Chen et al. 1992, Newmark 2001).
2) Edge effects are species-specific: some species benefit from the creation of
edges whereas others fare worse.
3) Edge effects do not extend some fixed distance into a habitat type: instead,
they vary with topography, local climate, and the species being considered, and the
distances may be considerable (Harris 1984, Laurance et al. 1997).
4) Species do not necessarily respond to the same type of edge in all
situations: a species may not cross an edge in one location but may be perfectly happy to
do so under other circumstances. And edge effects are context-dependent: nest
predation patterns are stronger in woodlots in agricultural landscapes than in forested
ones (because contrast between patch and matrix greater?) (Hartley and Hunter 1998,
Chalfoun et al. 2002).
5) Edges do influence movement patterns of organisms, propagules, wind
currents, and other things.
6) Not all edges are created equal: there are "soft" and "hard" edges.
Question: do small patches have fewer species because of the species-area
effect, or because of edge effects? Most studies have confounded these two
factors, so how would we answer this question? (Parker et al. 2005)
How has fragmentation been studied?
There are logistical difficulties in studying fragmentation, particularly from an
experimental standpoint. A review by Debinski and Holt (2000) over a 14-yr period, for
example, only found 20 experiments on fragmentation.
Fragmentation experiments may be divided into three size categories:
-small: microcosm e.g. Huffaker 1958 - classic study on the role of spatial heterogeneity on
permitting predator-prey coexistence (mites on oranges)
Forney and Gilpin (1989) - role of connectivity on extinction risk (lab
Drosophila)
McIntyre and Wiens (1999a, b) - experimental model system (EMS)
-medium: mesocosm e.g. ECOTRON
(http://www3.imperial.ac.uk/cpb/research/biodiversityandecosystemfunction/theecotron)
Collinge (2000) - insects in grassland patches in Colorado
-large -
e.g. Biological Dynamics of Forest Fragments Project (née Minimum
Critical Size of Ecosystems) in Brazil, the largest and longest-running experiment on
fragmentation in the world
(http://en.wikipedia.org/wiki/Biological_Dynamics_of_Forest_Fragments_Project)
(Laurance et al. 1997)
Wog Wog system in Australia (Margules 1992, Davies et al. 2001)
Savannah River Site corridor experiment (Tewksbury et al. 2002)
What have these experiments taught us?
1. Fragmentation is difficult to study.
2. There is a tradeoff between size (realism) and replication (statistical power).
3. Size isn’t everything: patch context and connectivity are important, too.
4. Different response variables may exhibit different responses to fragmentation.
(In other words, make sure you’re comparing apples to apples.)
5. There may be time-lagged effects (“extinction debt”).
Epilogue:
There are both natural and anthropogenic factors that generate landscape patterns.
Anthropogenic factors are not "modern," for humans have been modifying their
environment for as long as humans have existed. The question of what matters most–
habitat area, contiguity, or quality–is context (species, system)-dependent.
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