Edge effects

One of the problems with fragmentation is that the remaining fragments may be effectively much smaller than they seem.

• Vegetational changes are detectable for at least 10-30m from the forest edge in temperate zones and sometimes more.

• Edge-related increases in nest predation may extend as far as 300-600m into temperate forests. To take an extreme example, suppose that a single breeding pair of forest birds requires a patch of interior forest 10m in radius, where we define a patch of interior forest as the area not affected by increased nest predation. Then
  • With a 300m edge buffer, we need a circle 310m in radius, an area of 301,907m$^2$ or 30ha, about 75 acres.

  • With a 600m edge buffer, we need a circle 610m in radius, an area of almost 120ha.

  Predation effects extend a similar distance into forests (Figure 4). No wonder persistence of many forest interior songbirds requires areas of unfragmented forest at least several hundred hectares in extent.

  Figure 4: Fraction of quail eggs removed from experimental nests as a function of distance from forest edge (Figure 7.11 [3]).

  

• Laurance et al. [4] show that edge effects can have an unexpected and large effect on biomass accumulation in tropical forest trees.
  • Experiments begun between 1980 and 1986 with clearing to create replicate forest patches of 1, 10, and 100 ha in an area 80 km north of Manaus, Brazil.

  • Estimates of above-ground biomass assessed in 2-5 censuses through early 1997.

  • The regression of biomass change on distance to forest edge was significantly positive. Those plots that were the deepest in the interior of the remaining fragments showed no change in biomass. Those that were closest to the edge lost as much as 5 tons of biomass per hectare per year, a rate of about 1.3% per year.

Circular areas minimize the edge to interior ratio, so if you have a choice in designing a reserve you may be better off to have one that's fairly round than one of equal area that's square or rectangular, unless your objective in designing the reserve is to provide habitat for ``edge'' species.