Life-history theory and management - Ecological Applications: Toward a Sustainable World - page 77

3.4.3 When
competitiveness
matters - CSR
theory, grazing and
habitat fragmentation

The herbaceous species of grasslands differ in the extent to which they can with-
stand the pressure of grazing by vertebrates. In grazing trials in Argentina and Israel,
Diaz et al. (2001) a sked whet her grazing vulnerability could be predicted on the
basis of species traits. If so, managers in a new area with different species could
predict (and manage) their grasslands without the need to conduct their own time-
consuming grazing trials. Diaz and his colleagues found that plant species that
decreased in abundance under increased grazing pressure ('decreasers') were char-
acterized by tallness, perennial life history, large leaves and low specifi c leaf area
(cm 2 leaf g leaf −1 ). The ' increasers' possessed the opposite traits. If grazers attack plants
from above, as they do where the vegetation is relatively dense, tall plants are most
vulnerable because they are the fi rst ones encountered and, by their presence,
smaller species are protected to some degree. Other things being equal, large-leaved
species are doubtless preferable to grazers because they offer bigger mouthfuls;
small-leaved species require more bites per unit leaf material consumed and the
bites probably include more unwanted stem material. Perennial species, in compari-
son to annuals, remain exposed to grazing for longer and also allocate less to repro-
duction, reducing their ability to compensate for grazing mortality. Similarly, species
with low specifi c leaf areas may be relatively more affected by intensive grazing
because they do not have such a capacity for rapid regrowth.
So we see a number of predictable responses to grazing, some that can be related
to the r / K continuum as in Section 3.4.2 (small annuals with large reproductive
allocations do better under grazing pressure), and others to relative competitive
status in the face of grazing pressure (small species gain a greater share of resources
if larger species protect them by their presence). These species seem to combine
Grime's competitive and ruderal strategies (CR) - competing for resources best when
disturbances are relatively frequent, whereas the vulnerable species lack these traits.
Such generalizations will be helpful to managers, but Vesk et al. (2003) warn against
extrapolating too far. They note that in more arid rangelands, with their more open,
patchy vegetation, simple predictions based on species traits do not hold so well.
The importance of sensitivity to disturbance and competitive status receives
further support from a variety of studies of tropical trees in forest fragments (e.g.
Metzger, 2000). For example, shade-tolerant species typical of dense forest are vul-
nerable because, in comparison to 'edge' species, they have higher mortality, lower
growth rate and lower dispersal capability. This results in a competitive disadvan-
tage under the modifi ed light and increased disturbance regimes that dominate a
larger percentage of the area of smaller forest fragments. Poor dispersal ability
(fl ightlessness) is also linked to extinction risk of beetles of woodland in Australia,
which has become increasingly fragmented through conversion to agriculture
(Driscoll & Weir, 2005).

Summary Life cycles and species traits

To under st a nd why one species is successful in a particular location while another
is not, we need to focus on differences in their life cycles - birth, juvenile period,
reproductive period, and possibly a post-reproductive period. Some species have one
generation each year (annuals), but others (perennials) have life cycles that last for
several or many years. Many can spend a substantial time lying dormant (e.g. as
seeds or eggs). Individuals of different species may be small or large, invest little or
heavily in their offspring, grow fast or slowly, and be more or less vulnerable to