Download Indirect commensalism promotes persistence of secondary

Synopsis by Mary F. Paul
Indirect commensalism promotes persistence of
secondary consumer species
Dirk Sanders and F. J. Frank van Veen
Biology Letters (2012) 8, 960-963
Introduction
Primary extinctions within an ecosystem can often
lead to cascade effects, causing secondary extinctions
in other species. By looking at food webs, it is easy to
see direct interactions such as competition and
predation, but they often fail to highlight the
importance of top-down and horizontal indirect
effects among species .1 Higher trophic species, such
as predators, are especially vulnerable to secondary
extinction. 1,2
Competition between prey species can lead to an
indirect mutualism between their consumers1,3,4
because a predator that reduces the density of its
prey also reduces competition at the prey's trophic
level, positively affecting other prey species and
their respective consumers..
The 2012 study by D. Sanders and F. J.F. van Veen
aimed to test the effects of removing a secondary
consumer from a population, simulating the
extinction of a predator species. By removing one of
two parasitoid wasp species, they tested if
interspecific competition for plant resources among
prey aphid species would have cascade effects.
Key terms:
•
Commensalism: relationship where one
species benefits without affecting the other
•
Mutualism: relationship where both
species benefit
•
Parasitoid: an organism that parasitizes a
single host organism for a part of it’s life
and ultimately kills the host
•
Food web: networks of trophically
interacting species
•
Cascade effect: a series of secondary
extinctions triggered by the extinction of a
key species in an ecosystem
•
Aphis fabae: herbivorous aphid species
•
Lysiphlebus fabarum: parasitoid wasp
species that only uses A. fabae as a host
(pictured below)
•
Hypothesis:
“Exclusion of either parasitoid
species will have a negative
effect on the population of the
other as a result of increased
resource competition between
respective host species.”
•
Acyrthosiphon pisum: herbivorous aphid
species
Aphidius ervi: parasitoid wasp species that
only uses A. pisum as a host (pictured
below)
Treatment I
Both predator species present
L. fabarum
Experimental communities consisted of:
a shared plant resource (Vicia faba), 2 herbivorous
aphid species (A. fabae and A. pisum), and either one
or both of the parasitoid wasp species (L. fabarum and
A. ervi).
There were 7 replicates of three different treatment
groups :
Treatment I: Both parasitoid species present
Treatment II: Only L. fabarum present (A. ervi extinct)
Treatment III: Only A. ervi present (L. fabarum extinct)
A. ervi
A. fabae
Methods
A. pisum
Results
Parasitoid L. fabarum ultimately went extinct within
six weeks in all Treatment II communities where the
other wasp species , A. ervi, was absent. However, the
same reciprocal effect was not seen in Treatment III
communities where L. fabarum was absent.
This effect was due to asymmetric competition
between aphid species, where A. pisum are better
competitors. When populations of A. pisum boomed,
A. fabae was out-competed from the common
resource, Vicia faba, and went extinct, which then
cascaded to the extinction of its consumer, L.
fabarum.
Vicia faba
Treatment II
Only L. fabarum present
L. fabarum
Conclusions
A. fabae
The results highlighted an indirect commensalism
within a community, where the survival of L. fabarum
is dependent upon the presence of A. ervi to control
interspecific competition of prey species.
This is an example of how a top-down indirect effects
can be transmitted through four trophic links,
resulting in extinction cascades. This simple
microcosm experiment served as an interesting
example of how complex population dynamic effects
must be considered when modelling robustness of
communities to primary extinctions.
A. pisum
Vicia faba
Cited Works & Further Reading
1.
2.
3.
4.
Sanders, D., & van Veen, F. J. F. (2012). Indirect commensalism
promotes persistence of secondary consumer species. Biology Letters,
8(6), 960-963. doi: 10.1098/rsbl.2012.0572
Borrvall C., Ebenman B. (2006 ). Early onset of secondary extinctions in
ecological communities following the loss of top predators. Ecol. Lett. 9,
435–442.
Levine S. H. (1976) Competitive interactions in ecosystems. Am. Nat.
110, 903–910.
Vandermeer J. (1980) Indirect mutualism: variations on a theme by
Levine. Am. Nat. 116, 441–448.