Ecological applications of niche theory - Ecological Applications: Toward a Sustainable World - page 53

60

50
40

30
20

10
0
-500 0 500 1000 1500 2000 2500 3000 3500

Profit margin per ha (euro)

Fig. 2.10 An index of species richness of vascular plants and bryophytes in Austrian meadows
in relation to farming profi t margin in euros per hectare. Relative species richness expresses for
each meadow the number of plants that occur there as a percentage of the total known to
occupy meadows in the landscape as a whole. Evidently, moving to a less intensive farming
regime would both increase species richness and reduce profi t margins. Thus, the average cost
of maintaining each species can be estimated. (From Zechmeister et al., 2003.)

If species richness is to be restored, Zechmeister and his colleagues believe that
farmers will need increased fi nancial incentives, through agroenvironmental subsi-
dies, to counter the decline in profi t margin that managing meadows for species
conservation would involve.

2.4.4 River
restoration - going
with the fl ow

One of the most pervasive of human infl uences on river ecosystems has been regula-
tion of discharge. Water abstraction for agricultural, industrial and domestic use
changes the hydrographs (discharge patterns) of rivers both by reducing discharge
(volume per unit time) and altering daily and seasonal patterns of fl ow. River resto-
ration often involves re-establishing features of the natural fl ow regime.
The rare Colorado pikeminnow ( Ptychocheilus lucius ), which eats other fi sh, is
now restricted to the upper reaches of the Colorado River. Its present distribution
is positively correlated with prey fi sh biomass, which in turn depends on the
biomass of invertebrates upon which the prey fi sh rely, and this, in its turn, is
positively correlated with algal biomass, the energy base of the food web (Figure
2.11a-c). Osmundson et al. (2002) argue that the rarity of pikeminnows can be
traced to the accumulation of fi ne sediment on the riverbed, where it reduces algal
productivity in downstream regions of the river. Fine sediment is not part of the
fundamental niche of pikeminnows. Historically, spring snowmelt often produced
fl ushing discharges with the power to remove much of the silt and sand that would
otherwise accumulate. As a result of river regulation, however, the mean recurrence
interval of such discharges has increased from once every 1.3-2.7 years to only once
every 2.7-13.5 years (Figure 2.11d), extending the period of silt accumulation.
High discharges can infl uence fi sh in other ways too, for example, by maintaining
side channels and other elements of habitat heterogeneity, and by improving sub-
strate conditions for spawning. These are all elements of the fundamental niche of
particular species. Managers must aim to incorporate ecologically infl uential aspects
of the natural hydrograph of a river into river restoration efforts, but this is easier
said than done. Jowett (1997) describes three approaches commonly used to defi ne
minimum discharges: historic fl ow, hydraulic geometry and habitat assessment. The
fi rst of these assumes that some percentage of the mean discharge is needed to