The SGDC pattern in recovering forest herb communities:

Explained by drift?

Critique:Vellend, Mark (2004)

Parallel effects of land-use history on species diversity and genetic diversity of forest herbs.

Ecology 85(11): 3043-3055

Jim Stasz @ USDA-NRCS PLANTS Database

Vellend 2003: Results from 14 island studies

THE SGDC

1. Correlation not Causation

2. Is there a common process (X)?

3. Vellend 2004: Recovering forest herb communities…

X = drift…

Vellend 2004: Methods

•http://en.wikipedia.org/wiki/Agriculture

10 1° (untouched)

17 2° (abandoned pastures)

Sites

http://www.pbs.org/wnet/nature/holycow/images/top_pic2d.jpg

Vellend 2005

http://terraserver-usa.com/usgsentry.aspx?T=2&S=16&Z=18&X=29&Y=366&W=1&qs=%7cTompkins+County%7cNew+York%7c

Characteristics of each site:

1. Trillium grandiflorum

2. Isolated

3. Similar Soil Associations

Tompkins County NY

Community

Species of Forest Herbs

Within site (Diversity) Between Sites (Divergence)

Genetic

(T. grandiflorum)

For each 1º & 2º site measured…

SR E AR He Brc Fst-C Fst-G

Drift?

Selection?

How SD & GD might change in parallel…

1° > 2° 1° < 2°

SD & GD

SD & GDDiversity Divergence

Within site…

Between site…

In general, Vellend found the SGDC…

….but was this pattern primarily caused by drift?

Factors best at explaining variance,

were indicators of drift:

Land-use history (1 vs. 2)**

Population size**

Area**

Other factors:

pH

Habitat

Isolation

Environmental heterogeneity

** They contributed the most to the ‘full model’

BUT not all the variance is explained by these factors…

Missing factors?

Other factors: (i) Biotic environment!

Herbivore behaviour?

Vellend 2005 results…

1° 2°

Controlled all abiotic factors…

Still a difference!

Selection? Contradiction to 2004: drift…

x lots… x few…

Tompkins county overrun with deer…

Competition with other plants?

http://efl.htmlplanet.com/xmas_songs.htm

vs.

local bookstores

..

•Only measured native forest herbs!

•Invasive species?

•Remnant open-field species?

Other Factors: (i) Biotic:

Selection explains lower abundances in 2º sites as well as drift?

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

Lower abundances in 2º sites

E

Other factors: (ii) Soil Heterogeneity

Selection explains greater divergences in abundances between sites as well as drift…

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

20 Soil Samples from each site, mixed together to get an average.

No measure of within site variance!

More patchy distribution of species in 2º sites?

Fst-C

Greater divergence in abundances between 2° sites

Other factors:

(iii) Hedge Rows as source populations?

Included only 3 as 1° sites

Corbit et al (1998) finds them to be important source populations for surrounding forest

http://www.jaynecotten.com/images4/hedge1.jpg

Other factors:

(iii) Hedge Rows as source populations?

Most common forest herbs in hedge, same in 2 ° and 1°.

1° Sites 2° Sites

Other factors: (iii) hedge rows:

Migration explains lower divergence in species types in 2° stands as well as selection?

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

QuickTime™ and aTIFF (LZW) decompressorare needed to see this picture.

Brc

Is Drift the main process?

Vellend interpretations:

Drift?

Selection?

Further interpretations:

Drift?

Selection?

Migration?

SR E AR He Brc Fst-C Fst-G

SR E AR He Brc Fst-C Fst-G

So, still see SGDC pattern… but as for a common process…

SD & GD

SD & GD DivergenceDiversity

1° > 2° 1° < 2°

X=?

A few more comments…

• Applications…– Only to isolated secondary stands – Only to forest herb communities – Only with history of pasturing

• Non-equilibrial community?

• Supports SGDC pattern– Unification of community and genetic disciplines…– E.g. Hubbell’s ecological drift parallels genetic drift…

• Common process to SGDC pattern?– More studies…– Oversimplification of model?

http://www.news.cornell.edu/Chronicle/03/9.11.03/deer_dispersal.html