The effect of exotic invasives on diversity Loren Hintz Bio255 Oct. 21, 2004

outline Handout and extra 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 Effect of invasive plant species on temperate wetland plant diversity. Conservation Biology. 18(4) : Rosenzweig, Michael L, 2001a, The four questions: What does the introduction of exotic species do to diversity? Evolutionary Ecology Research.3 : 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 (S r ). Used a model based upon size, landscape, water quality and sediment quality to predict (S) and (S r ).

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 S r 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% S r 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% S r 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 A i z Intra-provincial z between Island z between Inter-provincial z between (thus 1.0!)

Rosenzweig 2001a  S i = C  A z i

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