Habitat Fragmentation: Part II. Biological Consequences of Fragmentation Edge Effects One of the best documented effects of fragmentation are ‘edge effects’

Habitat Fragmentation: Part II

Biological Consequences of Fragmentation

• Edge Effects• One of the best documented effects

of fragmentation are ‘edge effects’• Edge effects have mostly been

examined in forests• Sunlight and wind alter the micro-

climate at the forest edge, changing which plant species are favored


• In WI forests, edge zones of shade-intolerant plants may extend 10-15m into a forest (N,E,S) and 30m (W)

• In Douglas Fir forests of Pacific NW, increased rates of blowdowns and other physical edge effects may extend over 200m into the forest

• In Queensland, elevated rates of canopy and sub-canopy damage extend 500m in


• In some landscapes, especially in warmer climates, sealing of edges occurs through accelerated growth and increased regeneration of understory trees and shrubs

• In some cases animals are then attracted to these edges, which may then function as an ‘ecological trap’

• In MI, songbirds nest at higher rates


• Consider the impact on birds• Roads and powerline corridors as

narrow as 8m may produce edge effects• Cowbird parasitism may be significant

for 100’s of m into a forest • Predation can also be significantly

higher near the edges as densities and movements of raccoons, opossums, crows, foxes, jays, skunks, are all higher


• Artificial nest study of songbird nesting success


• Similar effects have been observed in a multitude of habitats and locations

• Not all studies find ‘ecological traps’ in edge habitat, but most do find poor reproductive success

• Highly fragmented systems, little edge effects are documented…why?

• How do you think ‘edge effects’ in naturally heterogeneous landscapes compare? Why?


• Size and Shape• If edge effects are relatively

pronounced or fragments are relatively small, the entire fragment may be edge

• Consider a circular reserve of 100ha (250 acres) with a 600m edge


• 16 breeding sp of birds in WI

• Similar total area, but different amount of ‘core’ areas with a 100m edge width

• No ‘area sensitive’ birds nested in A, but 6 sp bred in B


• In tropical landscapes, human influences (i.e. hunting) generally extend several km into a forest, making smaller patches virtually void of game species


• Deleterious edge effects are quite the contrary to what many traditional views expressed in ‘wildlife management’ textbooks

• It is true many of the traditional game species are edge species

• However, in some areas species like deer are becoming a nuisance for people and plants


• Matrix Effects• The matrix surround habitat

fragments in terrestrial landscapes distinguishes these patches from real islands

• The amount of structural contrast between habitat fragments and the matrix in which they exist is one measure of fragmentation (severity of the filter)


• A structurally rich matrix may serve as a corridor for some and even suitable habitat for other species

• For birds in ID, the landscape matrix determined sp richness within patches (relative to patch attributes themselvs)

• When the matrix becomes more impermeable, the more isolated a patch becomes


• Undoubtedly on the main reasons results from fragmentation studies vary widely is the importance of the matrix itself

• In general, the more different the matrix is from the fragments, the more intense the edge effects


• In MD, nest predation rates are higher in woodlot fragments surrounded by suburbs relative to agricultural fields

• Similar results in Ontario• Why?• In CA, in patches that retained

coyotes, songbirds had higher nesting success


• Roads• Habitat fragmentation is frequently

accompanied by road building• It appears relative benign, but is NOT• Trombulak and Frissell (2000)

– Mortality from construction, collision mortality, modification of behavior, alteration of physical environment, alteration of chemical environment, spread of invasives, & increased use by humans


• Most Amazonian understory birds respond negatively to roads and if wide enough (30-50m) will not cross

• The endangered FL panther, auto collisions are the highest source of mortality

• Small mammals in Ontario would not cross roads >20m wide

• In the Mojave, of 387 rodents recaptured, only 1 was ever on other side

• Turtles highly susceptible to roads (♀)


• Populations of the forest-dwelling carabid beetle were almost completely divided by a road and parking loops (unpaved)


• There can also be long-term effects at the genetic and community levels of organization

• A German study from that separation of populations of common frogs reduced the average heterozygosity of both local populations; similar findings for a German study of bank voles


• However, while roads often serve as barriers to many native species, they frequently serve as a conduit for invasive species…why?

• Plants are particularly well transported


• Species Invasions• The addition of undesirable species

can have as large of an impact as habitat degradation or loss

• In the US, this may be the 2nd leading cause of species endangerment behind direct habitat loss


• An additional problem with species invasion is the subsequent biotic homogenization, which can occur at several levels of biological organization (i.e. from genes to ecosystems)

• One of the results of such a process is the loss of distinctiveness of communities (from genes to ecosystems)


• Effects on Ecological Processes• Ecological processes may change

substantially as a result of edge effects

• In addition to impacting avian population dynamics, fragmentation has also been shown to impact on area-sensitive species (particularly of top-predators), which can have cascading effects (Case Study 7.3)


• In forest fragments in OR, those surrounded by clearcuts have densities of deermice 3-4x higher, leading to increased predation on trillium seeds, which impacts recruitment and understory structure


• Invertebrates may be the strongest impacted due to their importance in nutrient cycling, decomposition, agents of disturbance

• E.g. defoliation rates at edges > than core areas, densities of dung beetles being lower in smaller fragments, and lower abundances of soil invertebrates in smaller fragments


• There also appears to vulnerability of animal-plant mutualisms to habitat fragmentation (particularly plant-pollinator interactions)

• As fragmented populations, many plant population become smaller where they may suffer from the Allee effect, where below some threshold size, pollinators no longer visit them


• Extinction rates of an annual plant related to pop(n) size


• One approach to mitigate the potential animal-plant is to establish corridors facilitating movements of seed dispersers and pollinators


• The landscape may also act to stop large-scale processes such as fire


• ‘Nested Species’ Patterns• The loss of species from fragmented

landscapes may follow a predictable sequence

• In studying habitat patches of various sizes, a pattern f nested subsets in the distribution of species is often observed


• A nested subset is a geographic pattern in which larger patches contain the same species found in smaller patches, along with additional species found only in larger patches


• For example, on mountain ranges in the Great Basin, which are natural habitat islands, boreal mammals and birds show a nested distribution pattern that may result from extinction of area-dependent species on smaller islands

• Birds in woodlots show similar patterns• Birds in aspen stands (natural fragments)


• There can be other reasons for nested subsets besides a predictable sequence of extinction events (pattern vs. process)

• Four processes which may generate nested patterns include: differential extinction, differential colonization, habitat nestedness, or simply an artifact of ‘habitat size’


• Species Vulnerable to Fragmentation• How can a sp survive in a fragmented

landscape?• 1) a species might to well in the new matrix• 2) maintain viable pop(s) in single

fragments• 3) be highly mobile and integrate multiple

patches into a single ‘territory’ (corridors)• 4) none of the above…go extinct


• Populations in isolated woodlots in a fragmented landscape are higher in lots connected by fencerows than isolated lots


• Who is vulnerable to extinction?• Examine Box 7.2• Wide-ranging species• Non-vagile organisms• Specialized requirements• Large-area or interior species• Low fecundity• Vulnerable to human exploitation


• Fragmentation vs. habitat loss• Sometimes it is extremely difficult to

distinguish between fragmentation and habitat loss

• We know the matrix is important• We know ‘regional’ composition is

important


• Driving factors maybe different• Arden (1994) hypothesized that

habitat loss would be the most important process explaining species declines in landscapes with a high proportion of suitable habitat whereas patch size and isolation would become more important in landscapes with a low proportion of suitable habitat


• In a computer simulation, Fahrig (1997) found that beyond 20% habitat cover, species persistence was virtually assured, regardless of habitat configuration

• However, other studies have shown configuration can be very important, especially if considered over a relatively long time-scale


• Habitat fragmentation has not been nearly as well studied in the western US

• Is there a need? Why?• Hansen and Rotella (2000) found only a

single species that shared the LHC of eastern deciduous forests birds did that made them so vulnerable to fragmentation

• They also found no edge effect in artificial nest predation studies in the Rockies


• Fragmentation in non-forest habitats• About 37% of grassland ecoregions in

the western hemisphere are reduced to small and few fragments while another 15% are moderately fragmented

• In the BBS, grassland birds have shown the greatest decline since 1966 (very high rates of predation and parasitism)


• Marine Fragmentation• Benthic habitats are subject to

fragmentation• Mangroves, coastal wetlands,

seagrass beds and reef systems are all highly fragmented

• Estuaries and the fish residing within them are very susceptible to fragmentation


• Climate Change• Fragmentation is a threat in a stable

world• If we add climate change, it may be

ominous for many species• Species that are forced to move due to

natural climate fluctuations have to deal with barriers (e.g. dams, pipelines)

• New barriers may make movement tough


• For many species, it will be difficult to track shifting climatic conditions

• High-elevation and high-latitude habitats may be lost entirely

• Simulation models suggest wind dispersal is extremely poor in landscapes with <25% cover

Documents

Habitat Fragmentation: Part II. Biological Consequences of Fragmentation Edge Effects One of the best documented effects of fragmentation are ‘edge effects’