Agriculture Reference
In-Depth Information
lower than in soil-stored seedbank species, this predation presents a substantial
threat to successful reproduction. In this context, the absence of this strategy in
Chile and the limited role in northern hemisphere MTC regions may not be
surprising since they do not face such nutrient-limited conditions, and much larger
seedbanks are feasible for soil-stored seeds, as indicated by generally greater
seedling recruitment density. Another potential factor is the much higher fuel
loads and higher fire intensities experienced in MTC shrublands on more fertile
soils (see
Chapter 2
), which would make aerial seed storage more vulnerable than
in shrublands of less fertile soils. For example, it was estimated in the serotinous
fynbos shrub
Widdringtonia nodiflora
that on the majority of sites
80% of the
>
cones were destroyed by high fire intensity (Keeley
et al.
1999b
).
Seed dispersal
Postfire seeders disperse seeds in time more than in space (Keeley
1992d
) and thus
most have relatively passive dispersal. One exception is myrmecochory (a dispersal
syndrome characterized by specialized seeds that attract ant dispersers); the con-
trast between MTC regions in the importance of this form of dispersal is striking.
On infertile soils in Cape region fynbos and Western Australian heathlands there
are literally hundreds of species that disperse seeds by myrmecochory (Bond &
Slingsby
1983
). Although myrmecochory is found in the other MTC regions, the
total number of species in all of the other three is about an order of magnitude
lower than in either fynbos or heathland. The connection between myrmecochory
and poor soils is inescapable, even within regions (Milewski & Bond
1982
), and
stands as one of the more dramatic cases of evolutionary convergence (Lengyel
et al.
2010
). One of the longstanding hypotheses has been that in order for
seedlings to establish successfully they need to find pockets of high-nutrient soil
and myrmecochory is a mechanism for targeting such sites (Westoby
et al.
1991
).
However, tests of this hypothesis in both Australia and South Africa have failed to
support it (Rice & Westoby
1986
; Bond & Stock
1989
; Schatral
et al.
1994
).
An alternative strategy to targeting dispersal to high-nutrient sites is to produce
large seeds with high nutrient concentrations (Stock
et al.
1991
), but one consequence
is that such seeds are ready targets of predation on the soil surface (Andersen
1982
).
Two means of reducing this impact would be serotiny and myrmecochory (
Fig. 9.7
).
The advantages of serotiny for reducing predation are discussed above and themeans
by which myrmecochory reduces predation is as follows. Myrmecochorous ants
remove seeds from exposed sites on the soil surface and thus reduce predation by
other ground-dwelling granivores (Bond & Breytenbach
1985
;Westoby
et al.
1991
).
The lipid-rich elaisome is the reward these ants receive for not consuming the
seed (Keeley
1992b
). This elaisome reward directly reduces predation as well as
indirectly affecting predation since the presence of these myrmecochorous species
would be expected to displace other ground-dwelling invertebrates and vertebrates
that do prey on seeds (e.g. Brown & Davidson
1977
; Andersen & Yen
1985
).
Myrmecochorous ants further reduce predation by producing chemicals deceptive
to other seed-predator ants (Pfeiffer
et al.
2010
).
