http://www.earthday.net/footprint/index_reset.a sp?pid=8696863066519484 From a sustainability perspective, when humanity's Footprint exceeds the amount of renewable biocapacity a draw down in natural capital is required and this is considered unsustainable.
1. Island biogeography theory paved the way for understanding fragmentation effects Biogeography = the study of distributions of organisms, both past and present, and of related patterns of variation over the earth in the numbers and kinds of living things –Where do species occur, why do they occur there, and where are the greatest concentrations? Island theory:
I.B.T. applied to fragments – size matters - Probability of common forest interior neotropical migrant birds nesting in mid-Atlantic forests of various sizes
Hyperabundance phenomenon Animal populations “trapped” on small & medium sized islands have high population densities – WHY? –Reduced competition? –Reduced predation?
Predictions based on I.B.T. 1)Compression hypothesis = on islands, as more species invade & are packed into the landscape, the niche occupied by each species becomes smaller 2)Filtering hypothesis = reduction in the # of species during dispersal 3)Stepping stone hypothesis = smaller islands that lie between the main recipient island & its mainland colonizing source increase the rate of exchange
The I.B.T. & Conservation 1.larger areas are better than smaller ones 2.one large habitat area is better than several smaller ones of equal total area 3.closely spaced habitat patches are better than widely spaced ones 4.habitat patches arranged at equal distances from one another are better than those arranged in a linear sequence of increasing distance from the first patch to the last 5.connected patches are better than unconnected patches 6.a circular patch is better than a noncircular patch
2. Fragmentation = Formerly continuous natural habitats broken up due to the encroachment of civilization 2 components of habitat fragmentation:
Natural edge areas are important Inherent edges are usually long- lasting features of the landscape related to: –Topographic differences –Soil type shifts –Presence of open water –Or geomorphic factors (peaks, ridge-crests)
The problem with unnatural edges: Abrupt edges alter the microclimate conditions - resulting in changes in plant community composition, mortality rates, regeneration processes
Anthrophogenic fragmentation/edges Anthropogenic edges –Alter patterns of local diversity –Population dynamics
Edge effects can cause some species to thrive, whereas others perish - Deforestation brought the cowbird into contact with naive populations of potential hosts, many of whom have not yet had time to evolve strategies for dealing with brood parasitism. 2 Brown-headed Cowbird eggs in a Blue-winged Warbler's nest
% of quail nests preyed upon as a function of distance from forest edge Edge habitat Core habitat -
Species vulnerable to fragmentation Wide-ranging species Nonvagile species Species w/specialized requirements Large-patch or interior species Species w/low fecundity or recruitment Species vulnerable to exploitation
Checkerspot butterfly – poor disperser, needs serpentine native grasslands Iberian lynx, hunted to near extinction Green Sea turtle – low reproductive capacity, takes 20-50 years Grizzly Bear – wide ranging, 70 to 400 square miles