Another reason why 10ha is too small

Subject: Another reason why 10ha is too small
Date: Wed, 9 Jul 2003 08:35:12 +1000

Thought that this paper may be of interest (although I am not sure that you can do much about aggressive native species other than use this as evidence that further fragmentation of eucalypt forest habitat threatens populations of some bird species):

Piper SD.  Catterall CP.
A particular case and a general pattern: hyperaggressive behaviour by one
species may mediate avifaunal decreases in fragmented Australian forests
Oikos. 101(3):602-614, 2003 Jun.
We quantitatively assessed edge effects associated with elevated abundance
of a hyper aggressive bird species, the noisy miner Manorina
melanocephala, in fragmented eucalypt forest adjoining developed land.
Long-term data from Toohey Forest, subtropical Australia, show that noisy
miner colonies intensively occupy a zone of 20 m from the forest edge,
with frequent use occurring up to 100 m from the edge, but little beyond
200 m. Within noisy miner colonies, the abundance and species richness of
other birds were both about half those recorded at nearby transects which
were outside the colonies' main activity area. Bird species smaller than
noisy miners, which are also those with similar diets, were collectively
20-25 times more abundant, and their species richness tenfold greater,
outside miner colonies than within them, whereas larger species, which
have less dietary overlap, did not differ. Exclusion of small
insectivorous birds has been hypothesised to cause elevated insect
herbivore density, but we found no difference between tree crown
defoliation or dieback rates within versus outside miner colonies.
Aggression by noisy miners can be viewed as a mechanism of interspecific
competition, since miners have a relatively large body size for their diet
and are hence able to exclude virtually all potential competitors at
relatively little cost. We examine evidence indicating that reduced bird
diversity in eucalypt forest fragments of eastern Australia is often
simply the effect of noisy miner occupancy of edges, acting directly on
the densities of other species through their aggressive behaviour. With an
edge effect 200 m deep, a remnant 10 ha in size is likely to become
entirely occupied by noisy miners, and this is a size threshold that has
been commonly reported in association with area-standardised avian
diversity reductions. Convergent patterns of species loss from small
forest fragments in different continents are the result of different
underlying ecological processes. [References: 49]


David Geering
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