Transcript The effect of exotic invasives on diversity

The effect of exotic invasives
on diversity
Loren Hintz
Bio255
Oct. 21, 2004
outline
• Handout and extra email articles
• Diversity ( number of species vs. higher
taxa or distribution)
• Houlahan and Findlay 2004
• Rosenzweig 2001a
• Discussion
Assigned Readings
• Houlahan, Jeff E. and C. Scott Findlay. 2004.
Effect of invasive plant species on temperate
wetland plant diversity. Conservation Biology.
18(4) : 1132-1138.
• Rosenzweig, Michael L, 2001a, The four
questions: What does the introduction of exotic
species do to diversity? Evolutionary Ecology
Research.3 :361-367
• Extra: Sax & Gaines, Hager & McCoy, Gurevitch
& Padilla
Houlahan and Findlay hypotheses
• Exotic species decrease native species
diversity especially rare species
• Exotic species are more likely to be
dominant species
• Dominant (invasive) exotic species are the
most likely to reduce species diversity
Houlahan and Findlay
experimental procedure
• 58 variable wetlands of Ontario (66.7ha ave.)
• Used dominance scale (0-5) where 4 (many
individuals covering 5-20% area) or 5 (many
individuals covering > 20% area) are considered
dominant.
• Compared number of exotic species to number
of species (S) or rare species (Sr).
• Used a model based upon size, landscape,
water quality and sediment quality to predict (S)
and (Sr).
Houlahan and Findlay results
• Exotic species diversity was positively correlated
with native species diversity (r =.405)even when
controlled for other factors that predict diversity
(area, nearby forest , nitrogen and magnesium;
p=.041) but it showed no significant relationship
(r =.005)with rare species diversity even when
controlled for other factors (area, road density, %
wetland as bog/fen; p=0.168)
• There was no significant difference between %
of native (8%)and exotic (5.5%)dominant
species (chi square =0.70)
Results of comparison of invasive
(dominant) exotic and native species
• Examined the effect of 4 exotic and 4 native dominant
species on S and Sr
• 3 of the 4 dominant exotic species studied showed a
negative effect on rare species (purple loosestrife ,LyS,
canary grass, PhY, and glossy buckthorn, RhF); but only
canary grass showed the effect after controlling for other
factors (14% Sr decrease per dominance score step.)
Only canary grass, PhY, showed an effect on general
native species.
• Only willow, SaP, of the 4 dominant native species
studied showed negative effect on both rare and native
species (13% Sr decrease per dominance score step
after controlling for other factors)
Conclusions Houlahan and Findlay
• Many studies contradict the textbook claim that
purple loosestrife competively excludes native
species
• In Ontario wetlands exotic species do not
decrease plant diversity
• Native and exotic species have similar
dominance probabilities and effects
• Managers need to focus on specific community
dominants irrespective of origin
• Pathogens, predators have more impact; exotic
plants may impact nutrient cycling; fire regimes
Rosenzweig
• Used species area curves (spars) and
immigration (speciation)/ extinction rates to
project species diversity after species invasions
form New Pangea.
• Species number = constant x area to the z
power (z is slope of species area curve in log log space) S i = C x Ai z
• Intra-provincial z between .09-.20
• Island z between .25-.55
• Inter-provincial z between .65-1.5 (thus 1.0!)
Rosenzweig 2001a
Si = C Azi
Conclusions Rosenzweig
• After steady state the sub areas of the New Pangea will
have greater diversity than their current diversity due to
the echo pattern (R & Ziv, 1999)
• The massive steady state diversity reduction of the
Homogocene will not occur due to improper
extrapolation of intra-provincial spars curve
• Due to human disturbance the effective area of New
Pangea is less than its physical area and thus will have a
reduced steady state diversity due to reduced speciation
rate (R, 2001b)
• Over the short term global diversity will decrease
(extinction greater than speciation) while provincial
diversity will increase (immigration greater than
extinction rate)
Sax and Gaines
White: islands
Green:
continent
Wilcove et al.
Discussion questions
• What exotic species characteristics (if any) are most
likely to decrease community diversity?
• Why should (or should not) inter-provincial z be 1.0?
• What role should theoretical models ( e.g. SPARS) play
in responding to the potential threat of exotic species
• How can the historical record be used to test the
models?
• What is the best way to communicate to the general
public about the issue of exotic invasive species?
Points to ponder
• Focus on number of species rather than
distribution of species
• Good and bad; ecosystem services
• Paleontological record used to test
• Diversity of rare species, all species
• We all see areas disrupted by invasive
species, why upset? Change landscape