Transcript Powerpoint

Resistance to invasion
• Invaders:
– occupy an empty
niche
– split existing niches
– oust a niche-occupier
– construct a new niche
by reaction
More on invasion...
• Selection effect — here, success of invasion
depends on the resistance of the dominant
species
• Niche complementarity – ↑# species, ↓invader
biomass
• Complementarity effect is favored over the
selection effect--the more rich the community, the
more resistant to invasion
– Fargione and Tilman (2005):
Biodiversity effects on productivity (Fargione and
Tilman 2005)
• Complementarity- Diverse plots resisted
invaders more successfully than the best
monoculture (S. scoparium).
• Sampling effect (selection e.)- C4 grasses:
– Increased root biomass
– More competitive in soil nitrate utilization
• Species diversity ↑, success of invasion ↓
Rebuttal?
• Productivity and stability positively correlated w/
invasion
• BUT, invasion has shown to increase extinctions
in species-rich mixtures (Pfisterer et al. 2003)
If invasion causes extinction in species-rich communities,
can invaders more successfully invade areas of higher
diversity?
Some q's
• How is the invasive ability of any species different
than other competitive abilities of species?
“Resistance to competition”
• What is the benefit of a community resisting a
native invader?
– Wouldn't a native invader help to restabilize the
community in question? (presuming that native
invaders helped establish the communities in
the first place)
• Are diverse communities more brittle? (less
stable)
Reminder: Chapter 3
Watt (1947): self-generating cycles
are a widespread cause of
vegetation mosaics
W&A: “Has never been reliably
confirmed by observation over
time”
“One of the best examples”: O.
leptocaulis under L. tridentata
Community X
- Does cyclic succession have to be
completely autogenic and/or
facilitation-driven?
- What was Watt’s contribution to
ecology?
Time
Community Z
Community Y
Watt’s cyclic
succession
Background
Cowles, Clements
- Communities progress to a steady-state
climax
- Disturbance is outside the system
Gleason
- Random, homogenous species associations
within a community
- Minimal importance of spatial, temporal
relationships between species
Watt: a departure from prevailing ideas
Pattern and Process in the Plant Community
-A.S. Watt 1947
Community as a “working mechanism” defined
by relations between its components in space
and time
Processes of succession in time manifest
themselves in spatial pattern – the unit pattern
Several examples of cyclic succession driven by
wind, drought, treefall gaps
Cyclic Succession
How do this example and the
others compare with the
description of cyclic
succession in Chapter 3?
Ricklefs 2001, after
Watt 1947
Succession Leads to Pattern
Unit Pattern: the full
representation of pattern in
all of its phases
Relative abundance of
each phase corresponds to
its duration
Small-scale disturbances
are part of the system
A progression toward
steady-state equilibrium
Patchiness is
widespread because:
1. Species create patchy
environments
2. Establishment is
restricted in space and
time
3. Once established,
plants are unable to
invade surrounding
areas
*Current climate and soil
may not explain
mosaic
The Unit Pattern and Communities
A neutral model for community analysis: a
constant environment should lead to a phasic
equilibrium, but this may rarely be achieved
Description of communities should be based on
all phase types
Can these ideas help in defining communities?
Are they random assemblages as Gleason
argued?
Expanding on Watt’s Unit Pattern
1950s-1970s: Pattern, equilibrium, succession
Whittaker 1953: “climax pattern”
Bormann and Likens 1979: “shifting mosaic steady state”
1970s-1980s: Patch dynamics, non-equilibrium, emphasis
on disturbance as an internal community process
(Levin and Paine 1974, White 1979, Pickett 1980, Pickett
and White 1985, others)
Landscape Ecology: landscape metrics, hierarchical patch
analysis, modeling, historic range of variability
Questions
• Do the flaws in many of Watt’s interpretations detract
from his overall message?
• Do you think Bastow’s case that cyclic succession has
never been reliably confirmed is justified?
• Should Chapter 3 include information about pattern and
process? Would this help in defining plant communities?
• Why does Bastow not discuss more of Watt’s ideas?
Other textbooks show his examples… (Ricklefs, Begon
et al., Gurevitch et al., Fundamentals of Ecology) Is
Bastow’s book just for a different audience than the rest?
Overall Questions… from last week
• How useful are descriptive theories versus
mechanistic explanations? When is one
more useful than the other?
• Is the framework for vegetation change in
this chapter logical, helpful, clear? How
could it be improved?
• Was an appropriate amount of time spent
on topics within the chapter?
• Was anything left out?