Applications from food web and ecosystem theory - Ecological Applications: Toward a Sustainable World - page 266

Omnivorous
Coleoptera

Collembola

Fungivorous
Oribatida
Fungivorous
Oribatida
Fungivorous
Prostigmata
Fungivorous
Nematoda

Araneae

Predatory
mites

Fungi

Tardigrada

Detritus Enchytraeidae

Omnivorous
Nematoda

Amoebae

Bacteria

Flagellata

Ciliata

Bacteriovorous
Nematoda

Phytophagous
Nematoda

Roots

Fig. 9.18 The food web of a forest fl oor, in which energy and nutrients fl ow towards the left from living plant biomass
(roots) and dead organic matter (detritus). Mites feature as fungivores/detritivores in the fungus-based chain (Oribatida and
Prostigmata) as well as predatory mites. Other important soil organisms include nematodes, springtails (Collembola), pot
worms (Enchytraeidae), fl agellates, ciliates, amoebae, tardigrades, and the predatory spiders (Araneae) and omnivorous
beetles (Coleoptera). Note how predatory mites are high on all three food chains - whether based on living plants (grazer
system) or on detritus/fungus or detritus/bacteria (decomposer systems) (Box 9.1). (After Ruf & Beck, 2005, and Berg et al.,
2001.)

9.8.4 Ecosystem
health of rivers - it's
what we make it

River health has been measured in a number of ways, from assessment of abiotic
evidence of changed state (e.g. nut r ient concentrations and sediment loads), through
community composition (of fi sh, invertebrates or algae) to ecosystem functioning
(such as rate of decomposition in rivers of leaves falling from overhanging vegeta-
tion). Some health indexes include more than one indicator, while in other cases
managers rely on a single measure. In New Zealand, for example, river managers
use the macroinvertebrate community index (MCI, Stark, 1993). Based on the pres-
ence or absence of certain types of river invertebrates that differ in their ability to
tolerate pollution, healthy streams with abundant species that are intolerant of pol-
lution attain high scores of 120 or more, whereas unhealthy streams have values as
low as 80 or less. Land development in the river's catchment area is often the major
pressure . Figure 9.19a shows the relationship for tributaries between MCI and the
percentages of their catchment areas that have been developed for pasture or urban
growth. The positive relationship between pressure and the health index gives confi -
dence that the measure has value when identifying unhealthy rivers for remedial
action ( response ) and for monitoring recovery.
It is worth remembering that the concept of ecosystem health is often, or partly,
a social construct. A healthy ecosystem is one that humans believe to be healthy,
and different groups may legitimately hold different ideas. Anglers may consider a