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)

Community YCommunity Z

Community X

Watt’s cyclicsuccession

Time

Reminder: Chapter 3Watt (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

- Does cyclic succession have to be completely autogenic and/or facilitation-driven?

- What was Watt’s contribution to ecology?

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

Ricklefs 2001, after

Watt 1947

Cyclic Succession

How do this example and the others compare with the description of cyclic succession in Chapter 3?

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

Succession Leads to Pattern

Patchiness iswidespread 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 Pattern1950s-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?