Biotic homogenization
Oyomoare Osazuwa-Peters
Graduate Seminar; Lost in Space
October 12, 2011
Overview
• History of Biotic homogenization (BH)
• What exactly does BH mean?
• What is the evidence for BH?
History
• Episodic mixing of
biotas when physical
barriers are removed
– Formation of
Panamanian land bridge
between N and S
America
• Modern recognition of
concept by Charles
Elton
What is BH?
• Laliberte & Tylianakis (2010) refer to it as a
phenomenon that reduces variability and
uniquess of flora and fauna across regions.
• “A gradual increase in compositional similarity
among formerly distinct biological
communities” (Naaf and Wulf 2010)
• “A temporal increase in community similarity”
(McKinney & Lockwood 1999).
• “Biotic homogenization is the process by which
species invasions and extinctions increase the
genetic, taxonomic or functional similarity of two
or more locations over a specified time interval”
(Olden 2008).
• “Biotic homogenization is defined as an increase
in spatial similarity of a particular biological
variable over time” (Olden et al. 2004).
BH definition
•
•
•
•
Change in similarity
Space
Time
Main drivers
– Species invasions
– Species extinctions
Identity of
species dictates
the outcome
• Multiple levels of biodiversity of organization
– Genetic
– Taxonomic
– Functional
Olden 2008
Olden and Rooney 2006
EVIDENCE FOR BIOTIC
HOMOGENIZATION?
• Goal: compare patterns of species invasion,
dispersal and impacts on three Eurasian seas
• Ponto-Caspian= The Black Sea + Sea of Azov +
The Caspian Sea
• Low diversity low salinity temperate waters
• Black Sea has become an international
shipping destination
• Goal: to determine how floristic similarity is
affected by exotics on a continental scale
• Data: native and exotic flora of America North
of Mexico
• Measure: Jaccard index of similarity
J= a/(a + b + c)
J ranges from 0 to 1
a is the number of species shared between two localities
b and c are the numbers of species unique to either locality
• Result: Exotic floras differ more among
neighboring communities, but have a broader
and more uniform distribution.
• Goal: to quantify extent of functional and taxonomic
homogenization across Great Britain between 1978
and 1998
• Data: National ecological surveillance data for Great
Britain
• Scale: random sampling plots of 10 – 200 m2 within 1
km2 regions
• Functional traits: canopy height, specific leaf area,
dispersal vectors, seed bank longevity
Space
&
Time
Space
• Positive correlations between change in α
diversity and change in trait variation between
1978 and 1998
• Conclusion: Plant communities became
taxonomically less similar but functionally
similar
• Goal: to explore regional and elevational
patterns in site similarity throughout the
Holocene.
• Data: eight fossil pollen datasets from
Romania
• Method:
– They divided time into 250 years intervals from
11500 years BP till recent.
– Used PCA and Bray-Curtis similarity analysis.
• Conclusion:
– Biotic differentiation = anthropogenic activities + climate
change
– BH= biotic interactions as immigration and competition
– Most studies that do not account for time represent single
snapshots in time
• Goal: to understand the importance of
patterns of extinction at a regional scale
• Data: species list of amphibian species before
and after extirpations associated with a
pathogenic fungus
• Approach: null model
• Results
Predecline
Postdecline
• Conclusion: Non random extinctions resulted
in the decline of regional diversity of Central
American amphibians.
• Goal: to determine whether parasitoid host
networks can be homogenized across a
gradient of habitat simplification
• Data: 48 quantitative food webs
– parasitism events
– parasitoid and host composition
– unique parasitoid-host interactions
– Strength of interactions
Composition
and
frequencies
of
interactions
Host relative
abundances
Parasitoid
relative
abundances
• Goal: Explicitly test the effect of landscape
fragmentation and disturbance on functional
homogenization of birds in France
• Data: French Breeding Bird Survey
• Method: Community Specialization index (CSI)
• Results: Functional homogenization
Clavel et al. 2010: Worldwide decline of specialist
species: toward a global functional homogenization?
• Goal: to validate a theoretical model predicting
the outcome of distinct invasion and extinction
scenarios
• Data: freshwater fish faunas in the USA at three
spatial scales
– Country
– Provinces in California
– Watersheds within provinces
• Method:
– used regression analysis
– Seeded the model with empirical data
• Conclusion
– Fish communities homogenization was at different
scales was due to
• Introduction of ubiquitous species
• No extinctions
• Differential patterns of native species extinctions
Take home message
• There is evidence for BH at different scales
– Most BH studies focus on taxonomic
homogenization
– Neglect of temporal comparison
– Most studies are performed at continental scales
• What are the implications of BH?
– Disruption of potential for local adaptation
– Reduced resilience of ecosystems to disturbance
Papers for discussion
1. Olden et al. (2004) Ecological and evolutionary
consequences of biotic homogenization
2. Smith et al. (2009): Selecting for extinction:
nonrandom disease associated extinction
homogenizes amphibian biotas
3. Abadie et al. (2011): Landscape disturbance
causes small scale functional homogenization,
but limited taxonomic homogenization in plant
communities.