Applications from food web and ecosystem theory - Ecological Applications: Toward a Sustainable World - page 253

Fig. 9.9 Tot al i nver te-
brate biomass and algal
biomass (chlorophyll a )
(±SE) in an experiment
performed in summer
in replicated artifi cial
channels placed in a
small New Zealand
stream. N, no fi sh; G,
native Galaxias sp.
present; T, brown trout
present. Trout cause a
trophic cascade by
reducing grazing by
invertebrates, thus
allowing algal biomass
to accumulate. (After
Tow n send, 20 03.)

(a) (b)

3 4

3

2

2

1

1

0

0

N G T N G T

Fish predation regime

times greater in a trout stream than in a neighboring Galaxias stream; Huryn, 1998).
This, in turn, resulted in more effi cient uptake of nitrogen from the overlying water
and, thus, lower nitrogen losses downstream (Simon et al., 2004). Thus, the two
most important elements of ecosystem functioning - energy and nutrient fl ux - were
altered by the invading brown trout.
The related rainbow trout ( Oncorhyncus mykiss ) has invaded fi shless lakes in
North America and a similar increase has been recorded in plant biomass, this time
the phytoplankton that occur in open water. To some extent the cause is similar,
with a fi sh-induced reduction in grazing (by zooplankton). But the main reason for
increased primary production is that the fi sh feed on invertebrates living on the lake
bed and then, via their excretion, transfer phosphorus (the nutrient that limits plant
production in the lake) into the open water where the phytoplankton live (Schindler
et al., 2001).
Invading zebra mussels ( Dreissena polymorpha ) in North American lakes (Section
4.4.1) also exert a profound ecosystem effect. As super-effi cient fi lter-feeders that
live on the lake bed, these can remove up to 25% of phytoplankton cells from the
open water each day, slowing energy fl ow from lower to higher trophic levels. They
have a further stoichiometric effect (Box 9.1). Zebra mussels excrete soluble waste
products at low nitrogen to phosphorus ratios that favor the growth of cyanobacte-
rial blooms - these are less preferred food of zooplankton, further reducing the
transfer of energy between trophic levels (Conroy & Culver, 2004).
Biosecurity managers need to pay particular attention to animal invaders that
have a novel method of resource acquisition (such as brown trout in New Zealand)
or that link previously unlinked ecosystem compartments (such as rainbow trout
and zebra mussels in North America) (Simon & Townsend, 2003).

9.5.2 Ecosystem
effects of invasive
plants - fi xing the
problem

One class of plant invaders with particularly pervasive ecosystem effects are nitro-
gen-fi xing woody plants, including Myrica faya in Hawaiian forest, Acacia spp. in
South African shrubby fynbos communities and Lupinus arboreus in Californian
coastal prairies (Corbin & D'Antonio, 2004). These plants, with their symbiotic
bacteria that are capable of fi xing atmospheric nitrogen into dissolved inorganic
form, alter patterns of nutrient fl ux and dramatically increase soil nitrogen content.
Not only are they highly successful as invaders, but even after removal they can
leave a long-term legacy of high soil and litter nitrogen - features that may favor
other invaders at the expense of native species. Managers need to take such legacies