The Relationship Between Diversity and Invasibility

1
The Relationship Between Diversity and
Invasibility

• Metha Klock
• Louisiana State University

2
Eltons legacy
Richardson Pysek 2008
3
Eltons Hypothesis

• High diversity communities are more resistant to
  invasion (Elton 1958)

http//academic.sun.ac.za/cib/events/images/elton.
jpg
4
Eltons evidence

• Mathematical models
• Laboratory experiments
• Vulnerability of islands
• Invasions of cultivated lands
• Resistance of the tropics
• Orchard pest control

http//img2.allposters.com/images/NPLPOD/1075370.j
pg http//tinyfarmblog.com
5
Mathematical models

• Invasion resistance arises in strongly
  interacting species-rich model competition
  communities (Case 1990)
• Constructed stable communities that differed in
  species number and strength of interactions
• Lotka-Volterra equation
• Stable communities invaded by a new species
• Determined invasion success and fate of residents

6
Models promote Eltons hypothesis

• Increasing diversity decreases invasibility
• Equivalent invasive competitors unsucessful in
  more diverse communities
• Priority effects Initially abundant species
  dominate
• Colonization decline in the face of many,
  strongly interacting species

Case 1990
7
Laboratory experiments

• Testing the invulnerability of laboratory island
  communities to invasion (Robinson Dickerson
  1984)
• Developed 100 communities of microscopic
  invertebrates and algae
• 2 size categories (small and large islands)
• After three months introduced invaders

Platydorina
Staurastrum
Dictyosphaerium
8
Results

• Communities invulnerable to Platydorina
• Partially vulnerable to Staurastrum and
  Dictyosphaerium
• Invasion success independent of community
  complexity

Robinson Dickerson 1984
9
Contradictory results

• Elton revisited (Levine DAntonio 1999)
• Reanalyzed Robinson Dickerson (1984) data
• Positive diversity-invasibility relationship
• Invaders persisted longer in more diverse
  communities

Levine DAntonio 1999
10
Field experiments

• Seed addition experiment in mature oak savannah
  (Tilman 1997)
• Small scale plots (1-m2)
• 10 seed addition treatments from 0 54 seeds
• Invasibility correlated negatively with richness

http//miriameaglemon.com/San20Diego20Bird20Pag
es/Sites/Site20Photos/P619043320Foggy20Pamo.jpg
11
Sampling effect

• Invader biomass reduced primarily by presence of
  one or a few highly competitive species
• Greater probability of including competitive
  native species in experimental manipulations
• Negative relationship found in experimental
  studies may be caused by sampling effect or
  incorrect statistical analysis (Wardle 2001)

12
Sampling effect

• Diversity decreases invasion via both sampling
  and complementarity effects (Fargione Tilman
  2005)
• Negative effect of residents on invaders
• C4 grasses strongly inhibit invaders
• Less invaded plots had high root biomass and low
  soil nitrate
• Negative effect of C4 grasses on invaders
  associated with specific functional traits

13
Arguments against Elton

• Evidence mainly theoretical
• Observation of lower invasibility in tropics
  based on casual conversations with three
  foresters
• Human impact on agricultural systems varies
• High diversity site may promote invasions

Pimm 1984
14
Diversity may promote invasion

• Does diversity beget diversity? (Palmer Maurer
  2007)
• Experimental crop plantings
• 5 species, monocultures and polycultures
• Positive relationship between weed and crop
  richness
• Structurally complex community may provide more
  microenvironments for invaders

15
Relationship in riparian zones

• Invasibility of species-rich communities in
  riparian zones (Planty-Tabacchi et al. 1996)
• Surveyed exotic and native richness
• One river in France, three in western US
• Watershed and patch scales

http//commons.wikimedia.org/wiki/FileFR-65-Adour
.JPG
16
Diversity promotes invasibility

• Positive relationship independent of scale
• Connected networks facilitate species spread
• Anthropogenic modification increases invasion
• Changes in hydrology increase specialization
• Climate warming favors exotic species

17
Scale and species invasions

• Experimental studies negative relationship
• Observational studies positive or neutral
• Plots gt 30-m2 have positive relationship

(Herben et al. 2004)
18
Scale and species invasions

• The invasion paradox (Fridley 2007)
• Conflict Experimental and observational studies
• Conflict Fine and broad scale studies

Fridley et al. 2007
19
Scale and species invasions

• Fine-scale processes focus on species
  interactions
• Fine-scale constrained to have negative
  relationship because cant support as many
  species
• Broad-scale processes focus on abiotic conditions
  and environmental heterogeneity
• Large scales any process creating variance in
  total community richness lead to positive
  relationship

Fridley et al. 2004
20
Scale and species invasions

• Exotic plant species invade hot spots of native
  plant diversity (Stohlgren et al. 1999)

Stohlgren et al. 1999
21
Invasions in high diversity sites

• Measured native and exotic richness
• Three scales Plot, landscape, biome
• Recorded soil characteristics
• N, C, percentage sand, silt, and clay
• Foliar cover and frequency of exotics

http//www.kanagat.com/kanagat/images/2005-Colorad
o/BeaverMeadow_IMG_0911_lg.jpg
22
High diversity spots more invasible

• At subplot (1-m2) and plot (1,000-m2) scales in
  grasslands saw negative relationship
• Positive relationship for all scales in Rockies
• At biome scale more species rich sites are more
  invasible

NATIVE SPECIES RICHNESS
Stohlgren et al. 1999
23
Positive diversity-invasibility

• Processes controlling local richness may be
  different from those controlling regional
  diversity
• Heterogenous sites more invasible because natives
  cannot use all available resources
• Pattern related to resource availability,
  independent of species richness
• Sites high in foliar cover, soil fertility, hot
  spots of native plant diversity are invasible

24
Is scale trumped by productivity?

• Productivity alters the scale dependence of the
  diversity invasibility relationship (Davies et
  al. 2007)
• Native-exotic diversity at three scales
• Serpentine environments, usually fewer invasives
  and sites of endemic species
• Measured richness and site productivity

http//www.statesymbolsusa.org/IMAGES/California/s
erpentine_path.jpg
25
Productivity vs. scale
High-productivity
Across scales
Low-productivity
Davies et al. 2007
26
Productivity and invasibility

• Environmental heterogenity
• Environmental favorability
• Facilitation

High productivity
Low productivity
Exotic richness (no. species)
Exotic richness (no. species)
Native richness (no. species)
27
Does diversity deter invasions?

• Small-scale negative relationship
• Broad-scale positive relationship
• Both natives and invasive may respond positively
  to available resources
• High species richness does not ensure invasion
  resistance
• Species-specific response more important than
  broad generalizations

28
Literature reviewed

• Case, T. J. 1990. Invasion resistance arises in
  strongly interacting species-rich model
  competition communities. Proc. Natl. Acad. Sci.
  879610-9614.
• Davies, K. F., S. Harrison, H. D. Safford, and J.
  H. Viers. 2007. Productivity alters the scale
  dependence of the diversity-invasibility
  relationship. Ecology 881940-1947.
• Elton, C.S. 1958. The ecology of invasions by
  animals and plants. Wiley, New York, New York,
  USA.
• Fargione, J. E., and D. Tilman. 2005. Diversity
  decreases invasion via both sampling and
  comlementarity effects. Ecology Letters
  8604-611.
• Fridley, J. D., J. J. Stachowicz, S. Naeem, D. F.
  Sax, E. W. Seabloom, M. D. Smith, T. J.
  Stohlgren, D. Tilman, and B. Von Holle. 2007. The
  invasion paradox Reconciling pattern and process
  in species invasions. Ecology 883-17.
• Herben, T., B. Bohumil, K. Bimova, and Z.
  Munzbergova. 2004. Invasibility and species
  richness of a community A neutral model and a
  survey of published data. Ecology 853223-3233.
• Kennedy, T. A., S. Naeem, K. M. Howe, J. M. H.
  Knops, D. Tilman, and P. Reich. Biodiversity as a
  barrier to ecological invasion. Nature
  417636-638.
• Levine, J. M. 1999. Indirect facilitation
  Evidence and predictions from a riparian
  community. Ecology 801762-1769.
• Levine, J. M. 2000. Species diversity and
  biological invasions Relating local processes to
  community pattern. Science 288852-854.

29
Literature reviewed, continued

• Levine, J. M., and C. M. DAntonio. 1999. Elton
  revisited A review of evidence linking diversity
  and invisibility. Oikos 8715-26.
• Palmer, M. W, and T. A. Maurer. 1997. Does
  diversity beget diversity? A case study of crops
  and weeds. Journal of Vegetation Science
  8235-240.
• Pimm, S. L. 1984. The complexity and stability of
  ecosystems. Nature 307321-326.
• Planty-Tabacchi, A. M., E. Tabacchi, R. J.
  Naiman, C. Deferrari, and H. Decamps. 1996.
  Invasibility of species-rich communities in
  riparian zones. Conservation Biology 10598-607.
• Robinson, J. V., and J. E. Dickerson Jr. 1984.
  Testing the invulnerability of laboratory island
  communities to invasion. Oecologia 61169-174.
• Shea, K., and P. Chesson. 2002. Community ecology
  theory as a framework for biological invasions.
  Trends in Ecology Evolution 17170-176.
• Stohlgren, T. J., D. Binkley, G. W. Chong, M. A.
  Kalkhan, L. D. Schell, K. A. Bull, Y. Otsuki, G.
  Newman, M. Bashkin, and Y. Son. 1999. Exotic
  plant species invade hot spots of native plant
  diversity. Ecological Society of America
  6925-46.
• Tilman, D. 1997. Community invisibility,
  recruitment limitation, and grassland
  biodiversity. Ecology 7881-92.
• Wardle, D. A. 2001. Experimental demonstration
  that plant diversity reduces invisibility
  evidence of a biological mechanism or a
  consequence of sampling effect? Oikos 95161-170.