Indirect effects: Apparent competition:

1
Indirect effects Apparent competition Two
non-competing species sharing a predator.
P
Prey do not compete (no arrows between Vs)
-
-


V
V
- In the absence of the predator each prey
species population is self-regulating. -
Coexistence of prey elevates predator abundance
above the level maintained by single prey
species.
2
- If predation rate increases with predator
density then both prey species could occur at
lower densities when they occur together than
when they occur separately - Pattern would
appear to be interspecific competition between
the prey species even if prey do not directly
interact. Does it happen?? Not many relevant
studies
Schmitt (1987) yet more rocky shores(sorry!)
Prey species (1) Chama and Mytilus
Crevice dwelling bivalves Filter feeders
3
Prey species (2) Tegula and Astraea Opportuni
ty for competition between gastropods and
bivalves appears to be small Common
predators Lobsters, octopi, whelks. If the
local abundance of bivalves is increased by
transplanting them to the gastropod environment
does that increase predator density?
Graze on algae on exposed rock surfaces. Mostly
Lower reef areas
4
- Yes and gastropod (snail) density over time
relative to control sites
Control areas
With added Chama and Mytilus
- Similar results for bivalves - more eaten in
sites with high gastropod density
5
Apparent predation Occurs when a predator
species indirectly harms a non-prey species by
feeding on a species that facilitates the
non-prey in some way
P (whelks)
-

V (barnacle)
Snail (non V)
- Whelks are predators on barnacles. Littorine
snails decline when whelks are abundant because
barnacles provide shelter for small snails and
become more vulnerable to other predators when
barnacles are absent
6

• Indirect mutualisms and commensalism
• Activity of one organism maintains resources
  needed by another
• Dodson (1970) Two groups of small alpine ponds
• Ponds with Ambystoma
• Ponds with Ambystoma and Chaoborus

Ambystoma
Chaoborus
Feeds on large zooplankton
Feeds on small zooplankton
7
Hypothesis Size selective predation of plankton
by Ambystoma maintains the feeding niche of
Chaoborus Some evidence for this removal of
Ambystoma resulted in a shift in body size of
plankton and a decline in Chaoborus abundance in
one pond (Giguere 1979)
8
How important are indirect effects?
Menge (1997) review of 23 experimental studies of
rocky shores - Considered only ecologically
significant effects (that caused at least a 10
change in the abundance of one or more
species) Indirect effects accounted for 40 of
the changes seen in community structure caused by
manipulations (eg predator or prey
removal). Most of indirect effects attributable
to keystone predation (35 ) and apparent
competition (25 ) Exploitative competition
constituted only 3 of indirect effects.
9
Habitat selection
For mobile animals, habitat selection may be as
important as strong direct interactions among
organisms in structuring communities - Community
composition may as much result from choices made
by animals before joining a community as
interactions that occur afterwards Choices Avoid
ance of physiological stresses Availability of
food and other resources Avoidance of
competitors Avoidance of predators Habitat
selection often involves a trade-off between
these factors
10

• Evidence for habitat selection
• MacArthurs (1958) warblers
• Pacala and Roughgardens (1982) Anolis lizards.
• Many other bird and bat examples
• - Description of association patterns aided by
  use of ordination techniques to evaluate habitat
  characteristics (e.g. structure and composition
  of vegetation) most strongly associated with
  species distributions
• - Need to perform experimental manipulations to
  determine the causal mechanisms

11
Habitat selection based on prey availability
(Sebens 1981)
Anthopleura anemones sessile predators common in
patches of their prey - Mytilus beds. Juvenile
anemones preferentially settle in Mytilus patches
Discriminatory settling behaviour may be common
in marine invertebrates? Grosberg (1981) Species
avoid settling on plates with high densities of
tunicate Botryllus - dominant competitor
(tunicates are blobby creatures called sea
squirts - our closest invertebrate relatives!)
12
Resetarits and Wilbur (1989, 1991) Selection of
breeding sites by Hyla chrysoscelis. Does it
assess predation risk to its offspring?

• 7 treatments in artificial ponds
• Control (no predators/competitors)
• Intraspecific competition (tadpoles)
• Interspecific competition (Bufo)
• Adult salamander
• Larval salamander
• Fish
• Larval dragonfly

Assess selection indirectly by measuring Hyla
activity
13
fish
Rana
Hyla
Larval salamander
Larval dragonfly
Adult Salamander
14
Temporal resource partitioning - Avoid
competition by segregating shared resources
Stiles (1975) Costa Rican Heliconia phenology
Flowering times of different hummingbird
pollinated flowers
Looks convincing?
Jan
Dec
15
Need an appropriate null model Is the pattern
significantly different from random? Randomly
drop flowering periods of the observed length
onto an interval scale of same length as observed
flowering season. Count days of overlap. Repeat
over and over again. Compare actual number of
days with the probability of that number of days
occurring by chance.
Randomly dropped on time scale
Measure overlap period
Last flower day
First flowering day
16
Reanalysis of these data suggest that actual
overlap in timing of flower production is no less
than expected by chance. Ashton (1982) mast
flowering events in Asian tropical forest trees -
Dipterocarpaceae. Very recently shown that mass
flowering is cued by low minimum temperatures.
Masting occurs every 9-11 years on average.
Members of genus Shorea (section Mutica) of
the family are pollinated by the same thrip
species. Overlap in flowering periods among
these Shorea species is only one day more than
the theoretical minimum!
17
(notice also that flowering periods get
progressively longer)
18
Resource partitioning need not reflect
competition May reflect tracking of temporal
variation in resource availability Bee species
that track flowering phenologies of species for
which they depend for pollen and nectar
production Orchid insect mimics flower in
concert with insect breeding Bird examples??
19
Effects of food availability on timing of breeding
Ahumada (2001) Timing of breeding in two wren
species in NE Colombia Highly variable
rainfall - dry and wet years T. leucotis
understory wren, feeds on insects on leaf
surfaces Insect abundance is strongly dependent
on rainfall T. rufalbus feeds on litter on
ground. Little effect of rain on food
availability
20
Comparison of length of breeding season and mean
date of clutch initiation for 2 wren spp.
Cumulative rainfall on y-axis (thin line 1993,
thick line 1994)
Bars breeding season, circles first clutch
21
Phenological patterns reflecting physiological
constraints Differences in the seasonal timing
of reproduction, dispersal, leaf flush etc may
reflect different physiological constraints
imposed on an organisms biology according to
its evolutionary history John-Alder (1988)
Looked at temperature dependence on locomotion
in frogs (jump or freeze)
Pseudacris crucifer, the Spring Peeper can jump
at near zero temps (trait is unique to Pseudacris
spp)
22
- Later breeding spp need temp gt15oC to leap
happily Early breeding is advantageous because
later breeders suffer from resource depletion and
increased populations of predatory invertebrates
in ponds Low temperature tolerance appears to
have evolved only once among temperate frogs. All
early-breeding species are in the family Hylidae
and genus Pseudacris Phylogeny may, in general,
be an important determinant of phenological
patterns