Invasions and resident communities Richard Law University of York, UK

Invasions in a community context Hieracium pilosella Mimulus guttatus Cytisus scoparius Pinus

Invasions in a community context invasions are stochastic invasion depends on propagule pressure invasion implies persistence (usually) invasion resistance and regional processes invasion depends on resident community Invasion: arrival of some propagules at some location, leading to local population, large enough for the probability of extinction by demographic stochasticity to be small increasing speculation!

Invasions are stochastic theory microcosms field

theory: from birth-death processes Probability P(n,N) that a population starting with n independent individuals reaches a size N before it goes to extinction (Bailey 1964; Goel and Richter-Dyn 1974) where b and d are constant probabilities per unit time of giving birth and of death (b > d ) Can think of an inoculum of n individuals, rare enough not to interact, added to a resident community

theory: probability of establishment Warren, Law and Weatherby (2006) In M. W. Cadotte, et al. n r/b P(n)P(n) depends on r/b how important is r? To get established, require N to be large enough for stochastic effects to be small. For d < b, large N

microcosms: protists Weatherby, Warren & Law Journal of Animal Ecology 67, controlled experiments short time scales high replication but not a substitute for field work

Warren, Law & Weatherby Ecology 84: microcosms: background A. persistent sets of species B. which absent species can invade persistent sets C. the new community which emerges from invasion Gives a graph showing invasions, and changes in communities. Can build a map of community assembly Experimental work looks for:

microcosms: initial growth of Blepharisma Law, Weatherby & Warren Oikos 88, stochasticity is important in establishment Blepharisma introduced to Paramecium microcosms Example of stochasticity

field: releases of a hemipteran to control broom in New Zealand 5 (100) (80) (60)1030 4(40)10 3 (30)104 2 (20)102 No. (%) colonies extant after 1 year No. of sites Intended release size hemipteran: Arytainilla spartiophila (European specialist on broom) Memmott et al 2005 JAE 74:50-62 many other examples demonstrate stochasticity

Invasion depends on propagule pressure theory microcosms field

theory: propagule pressure n r/b P(n)P(n) Warren, Law and Weatherby (2006) In M. W. Cadotte, et al. assuming independence but there may not be independence e.g. Allee effect, founder effects, local interactions in space

field: biocontrol of broom in NZ Memmott et al 2005 JAE 74: (100) (80) (60)1030 4(40)10 3 (30)104 2 (20)102 No. (%) colonies extant after 1 year No. of sites Intended release size release of hemipteran: Arytainilla spartiophila (European specialist on broom) increasing propagule pressure increasing probability of colony survival

Invasion implies persistence (usually) theory microcosms field

theory: permanence all orbits starting in the skin move further into the interior boundary is a repellor skin of finite thickness  x1x1 x2x2 permanence: from dynamical-systems theory (not well known)

theory: permanence: Lyapunov functions fixed point test by Lyapunov-like methods extends to multispecies systems with k species, but have to check 2 k subsystems boundary of phase space 2D phase plane

theory: invasion implies persistence resident community invading species augmented community invading species persists augmented community permanent Case A: collapse to a subset S of aug. comm. augmented community not permanent Case B:

theory: invasion implies persistence can S be the resident community? no: new species can invade it can S be a subset of the resident community? no: resident community is permanent and contains no attracting subsystem leaves only subsets which contain new species invasion implies persistence caveats resident community not permanent heteroclinic cycles

microcosms: exceptions: invaders as catalysts of change: {B,P} {P} E prop. microcosms new species established prop. microcosms new species persisted invasion usually implies persistence

field: Exceptions? rabbits on islands biological control agents others?

invasion depends on resident community theory microcosms field

theory = Can think of the resident community as providing an environment E, into which a new species is introduced

omnivore: feeds on bacteria and small protists eats T as well as competing with it holds its own in competition with C microcosms: introducing Blepharisma Law, Weatherby & Warren Oikos 88, struggles to compete with P

microcosms: initial rate of increase GLM  i : effect of resident communities  i : effect of introduced species  ij : introduction x resident interaction a lot depends on idiosyncrasies of particular ecological interactions

field: invasion resistance less invasion into species-rich communities: more niches filled (Elton 1958 etc) treating every invasion as a special case not very helpful attempts to generalise not obvious why introduced species should be a weaker competitor than resident species

field: confusion? At large spatial scales more exotic species get into species-rich communities (e.g. Lonsdale 1999) At small spatial scales, species-rich communities less readily invaded (e.g. Naeem et al 2000)

Invasion resistance and regional processes speculations about species pools speculation: evolution of biotic interactions theory: community assembly and invasion-resistance

theory: community assembly algorithm Law & Morton Ecology 77: {1} {1,3} 3 {2} 2 4 {1} species pool new species resident comm {1,2,3,4…}

theory: community assembly and invasion resistance Law & Morton Ecology 77: invasion resistance increases invasion resistance: prop. species from pool unable to invade community statistical result invasion resistance depends on history of community, not on its species richness

speculations about invasion resistance and species pools larger species pools get communities closer to invasion-resistant states richness of pool matters more than richness of community size of species pool: composition of species pool species pool lacking major parts of a flora/fauna unlikely to be near invasion resistant states communities from such species pools are especially vulnerable to invasions by introduced species

evolution of biotic interactions differences in specificity of mutualistic symbionts and enemies where do the rhizobia come from? mycorrhizal fungi? etc

Conclusions invasions and extinctions are part of the natural turnover process in communities community context is crucial for understanding invasions NZ species pool is special