Applications from food web and ecosystem theory - Ecological Applications: Toward a Sustainable World - page 258

Table 9.1 Consequences of manipulation of predation by birds and spiders for cabbage butterfl ies ( Artogeia rapae ) and
broccoli plants. A trophic cascade is evident. The herbivores are signifi cantly reduced in density and crop damage lessened
when birds are allowed access (with or without spiders). On their own, spiders produce a smaller effect. Plant dry weight is
for above-ground biomass and does not include the fl ower head. (Data from Hooks et al., 2003.)

Cabbage butterfl ies Broccoli plants

Large % of plants with Plant dry weight

larvae per Pupae per > 50% defoliation (g) at 54 days

Treatment plant ± SE plant ± SE at 25 days ± SE

Birds only 0.18 ± 0.18 0 0 153.5 ± 6.7

Birds plus spiders 0.44 ± 0.24 0 0 166.2 ± 9.7

Spiders only 3.62 ± 1.15 4.2 ± 0.85 9.1 139.2 ± 9.5

No birds or spiders 4.70 ± 0.46 5.0 ± 0.22 72.7 91.2 ± 7.7

ways to minimize the unintended loss of nutrients from the land. I will focus on
nitrogen, but the problems are much the same for phosphorus and other limiting
mineral nutrients.
Most of the fi xed nitrogen in natural communities is present in vegetation and in
the organic fraction of the soil. As organisms die they contribute to nitrogen in soil
organic matter, eventually transformed into nitrate ions that are leached by rainfall
down through the soil profi le. Both decomposition and nitrate production are gener-
ally fastest in summer, when natural vegetation is growing most quickly. Nitrate
ions may then be re-absorbed by the growing plants as fast as they are produced,
so that few ions are leached out of the plants' rooting zone and lost from the terres-
trial ecosystem. As noted in Box 9.1, natural vegetation is most often a 'nutrient-tight'
ecosystem.
By contrast, there are several reasons why nitrates leach more readily from agri-
cultural land and plantation forests. For one thing, agricultural land for part of the
year carries little or no living vegetation to absorb nitrates - and for many years in
the forest cycle, biomass is below its maximum. Then again, crops and managed
forests are usually monocultures that may capture nitrates only from specifi c rooting
depths, whereas natural vegetation often has a diversity of rooting systems and
depths. Third, the practice of burning straw, crop stubble or forestry waste returns
the organic nitrogen in a pulse back to the soil as nitrate, at a time when plants are
unavailable to reabsorb it. Fourth, fertilizer is usually applied just once or twice a
year rather than in the continuous stream that occurs in natural vegetation. Finally,
there are the subtle consequences of stoichiometry (Box 9.1): when agricultural land
is used for grazing animals, their metabolism speeds up the rate at which carbon is
respired, reducing the C : N ratio, and thus increasing nitrate formation and
leaching.
There are a number of tools to minimize fertilizer loss from the land (thus saving
money) to the water (where a useful resource becomes an irritating pollutant).
Farmers might aim to maintain ground cover of vegetation year-round, practice
mixed cropping rather than monoculture and take care to return organic matter to
the soil. The overriding objective should be to match nutrient supply to crop
demand. Modern 'controlled-release' fertilizers hold much promise in this regard
(Mosier et al., 2002).