NEWS
New global study positioned
to dominate thinking about
herbivores and plant bio-
diversity and savannas
Dr Dave Thompson
Current Address: Biodiversity Scientist, SAEON Ndlovu Node
E-mail Address: [email protected]
Reprinted From: http://bit.ly/2GTLGqH
H
erbivores impact on plant bio-
diversity in many of the world’s
ecosystems, but the magnitude
and direction of these herbivore effects
(positive, negative or no effect) vary
widely within and among ecosystems.
Understanding such impacts is vi-
tal for conserving plant diversity and
ecosystem functioning in a human-al-
tered world, especially in disturbance-
dependent grasslands and savannas
where herbivore communities are typi-
cally species-poor, and often numeri-
cally reduced versions, of their former
selves.
Prevailing theory predicts that the activ-
ity of herbivores – primarily being feed-
ing by grazers, should increase plant bi-
odiversity in environments where rainfall
is high (high productivity), and have the
opposite effect in dry environments (low
productivity). However, isolated studies
reveal that not all grassy systems con-
form to this theory, with deviations from
the pattern being seen.
This calls into question the generality of
the role of system productivity in gov-
erning herbivore effects on diversity and
suggests that alternative mechanisms
may be driving how animals impact on
their environments.
New global study
Now, a study recently published in Na-
ture Ecology & Evolution is providing
just that alternative mechanism, and
is offering rather compelling evidence
which explains how and why herbivores
impact plant biodiversity.
In grappling to explain different pat-
terns in how herbivore-exclusion plots
at the Konza Prairie LTER (Long Term
Ecological Research) station (USA) and
in the Kruger National Park (South Af-
rica) affected plant biodiversity imme-
diately after and in the years following
herbivore removal, a group of research-
ers hit on an intriguing idea.
Could the ability of herbivores to
change the abundance of the dominant
plant species – which is linked to those
species being palatable or not to the
herbivores, affect resource availability
and so either encourage or prohibit ad-
ditional species?
To test this hypothesis, the researchers
– led by Dr Sally Koerner of the Univer-
sity of North Carolina Greensborough
and including Dr Dave Thompson from
SAEON’s Ndlovu Node, established the
‘Grazing Exclosure Consortium’ to con-
duct a meta-level analysis comparing
findings from large herbivore exclosure
experiments from around the world. Ul-
timately plant species composition data
from 252 sites spanning six continents
and a large rainfall gradient (mean an-
nual precipitation 45-1511 mm) were
‘donated’ for inclusion in the analyses.
To be included in the Grazing Exclosure
Database, sites had to meet five crite-
ria: (1) exclosures had to be located in
herbaceous dominated communities –
sites ranged from tallgrass prairie to al-
pine meadows to desert; (2) herbivores
with adult body mass > 45 kg were ex-
cluded from plots using fencing, with
adjacent plots exposed to herbivores;
(3) data had to be collected after at least
three years of herbivore exclusion; 60%
of sites provided data reflecting 10 or
more years of grazing manipulation and
18% of sites provided herbaceous data
following 50 years of exclusion; (4) plots
inside and outside the exclosure had to
be sampled at the same time and sam-
pling intensity; and (5) data had to be
available at the species level.
And the result?
Figure 1 (a + b): Herbivore exclusion plots erected at Konza Prairie LTER station
(left) and in the Kruger National Park in 2006 showed differing effects of grazing
(or rather, the lack of grazing) on plant diversity. In attempting to explain this,
researchers were forced to challenge conventional thinking.
21
A positive, but weak relationship be-
tween grazing-induced change in spe-
cies richness and annual rainfall (as a
proxy for productivity) was found. But
the pattern emerging from the data
showed a much stronger relationship
Grassroots
Vol 19
No 1
March 2019