Transcript Predator-prey interactions: lecture content

Introduction to disturbance:
lecture topics
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What is stability, how is it defined, and what
kinds of stability are there in biological
communities?
How is stability related to diversity of ecological
communities?
How do ecologists reconcile divergent views of
the relationship between diversity and stability?
What is the intermediate-disturbance hypothesis,
and how is it related to community diversity?
David Tilman’s Minnesota grassland
study is one of most celebrated tests
of relationship between diversity &
stability…what does it tell us?
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Experimental study of grassland (prairie) plots
well designed, 2m2 plots
 Controlled numbers and types of species planted
 Followed plots for long time periods (years)
 Replicated,
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Results? (See Figure 17-10, Stiling text, next slide;
& more recent work)
 Total
community biomass fluctuation decreased with
diversity (thus diversity promotes biomass stability)
 Fluctuations in population (biomass): no connection
stability with diversity (fluctuations increase with div.)
What is community stability?
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Intuitively, stability means that the population sizes
and number of species remain constant over time
(“equilibrium”)
Components of stability
 Resistance
defined as the force needed to change the
community
 Resilience defined as the ability of the community to
return to prior state (equilibrium) after perturbation
 Elasticity
= how quickly community returns to equilibrium
 Amplitude = how much disturbance community can
tolerate, and still return to some kind of equilibrium
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Stability may be global (applies to entire system or
community) or local(applies to response to lowamplitude disturbances); confer prior discussions
Are community diversity and stability
related?
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Charles Elton (1958) proposed that the diversity of
biological communities promotes their stability
 Accordingly,
impact of disturbance would be
cushioned by large number of species
 This is a “balance of nature” view, held widely among
lay people
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Elton’s arguments in favor?
 Island
communities (not very diverse) often
vulnerable to invasions, a kind of instability
 Invasive exotic species (pests) often become
abundant in disturbed (e.g., cultivated) habitats
 Tropical rainforests (extremely diverse) tend to have
few insect outbreaks, few exotic species
However, Elton’s argument of diversitystability link has been challenged:
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Many examples of exotic species known today that
invade continents, national preserves, even forests
Alternative explanations for invasion of cultivated
habitats by exotics: Biological control agents not
present (& other simple communities--salt-marshes,
mangroves--not susceptible to invaders)
Even tropical communities experience biological
disturbances
 Herbivore
pest outbreaks have been observed
 Disease epidemics can wipe-out trees, other spp.
 Locust (& other insect) outbreaks well known
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Mathematical studies of food webs (Robert May): no
relationship of stability to diversity
Some support for idea that
diversity promotes stability exists:
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Addition of species via addition of trophic levels tends
to destabilize populations in experimental (and
natural) communities
Effects of buffalo grazing in E. Africa savanna
(biomass reduction) greater the less the diversity,
supporting diversity->stability link
Study of Yellowstone grassland communities (Frank
and McNaughton) showed that resistance increased
with diversity (next slide; Fig. 17-9, Stiling)
 Resistance
defined as inverse of summed component
population changes, before versus after a drought
 Diversity measured by Shannon index (Hs)
Resistance to change as a function of
species diversity in Yellowstone National
Park grassland ecosystem
Result of
Tilman’s
(Minesota
grassland)
study:
Ecosystem
stability
increases with
species
richness, but
not population
processes
More recent results of Tilman’s (et al.)
work (next slide):
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Aboveground grassland plant biomass (per plot) and
total biomass both increase with the number of
species in the plot, and the trend becomes stronger
with time (2001 Science paper--see reference next
slide)
Multiple species plots have more biomass than the
most successful monoculture plots
Thus ecosystem (community-wide) characteristics of
plots do appear to depend on the number of species
present, thus diversity does seem to affect ecosystemlevel measures of stability
No evidence that diversity influences stability of
individual species (which compete, & are independent)
(Tilman et al., 2001, Science 294: 843-846)
Intermediate-disturbance hypothesis
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Hypothesis proposed by Joseph Connell (1978):
 At
high disturbance levels only a few species can
persist, which have special adaptations to survive
 At low levels of disturbance, again a few species
predominate in community, namely those that can
out-compete all the others in a stable environment
 At intermediate levels of disturbance, both kinds of
species exist, leading to higher levels of diversity
Species
diversity
Hi
Lo
Disturbance frequent-------------------------> infrequent
Soon after disturbance-------------------------> long after
Disturbance large-------------------------------> small
Tests of intermediatedisturbance hypothesis
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Sousa (1979), intertidal invertebrates on boulders,
CA coast
 Small
boulders (frequently disturbed) had mean of
1.7 spp. (disturbance tolerant)
 Intermediate-sized boulders mean of 3.7 spp.
 Large, sessile boulders dominated by 2.5 K-selected
species
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Hiura (1995) beech forests, Japan (next slide):
species diversity (Hs) highest at intermediate levels
of disturbane (interval between windstorms)
Work of Reice (see Kareiva text reading, discussed
in class)
Test of Intermediate-disturbance
hypothesis (Hiura 1995)
Does stability promote diversity??
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Maybe there is a relationship, but it is not cause-andeffect, as Elton suggested, but rather relatively stable
environments (like tropical rainforests, coral reefs,
deep ocean trenches) allow the evolution of
ecological specialists, which allows greater number
of species coexisting
Jury is out on this (see lecture on latitudinal diversity
gradients)
Work of Sherry (1984) on tropical rainforest
flycatchers: Insectivores are far more specialized
ecologically than any temperate insectivores yet
studied; this would promote small niches, many spp.
Long-tailed flycatcher = specialist on stingless bees (67%),
ruddy-tailed flycatcher specialist on fulgoroid Homoptera
 Homogeneous diets among individuals suggests environment is
predictable from perspective of these rainforest interior animals
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A topic related to community
stability is concept of
keystone species
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Define keystone species as any species
whose impact on, or role in a community is
proportionally greater than its actual relative
abundance, biomass, or energy flow
Distinguish keystone species from dominant
species, which (latter) is simply a particularly
important species in community as measured by
relative abundance, biomass, or energy flow
Examples of different types
of keystone species
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Keystone predator (e.g.,
sea otter “O”, starfish
Pisaster “P”, bass “B”);
keystone parasite such as
distember virus (“VD”),
possibly nuclear
polyhedrosis virus
Keystone prey (e.g.,
tropical fruit such as figs &
palm nuts that sustains
animals in dry season)
Keystone habitat modifier
(e.g., beaver, prairie dog,
gopher tortoise)
The importance of keystone species
from perspective of community
stability is that they can stabilize or
destabilize community out of
proportion to their abundance.
Conclusions:
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Intermediate disturbance hypothesis is reasonably
well supported in a number of species, and
emphasizes role of disturbance in promoting and
maintaining species diversity in ecological
communities
Relationship between diversity and stability not well
established
 Species
diversity does not generally promote greater
species and population stability
 Species diversity does promote ecosystem stability (at
least in grassland communities)
 Stability may promote diversity more than vice versa,
but this idea is not well tested yet, not yet widely
accepted (evolutionary ecological idea)