Invasions and resident communities Richard Law University of York, UK

Invasions and resident communities

Richard Law

University of York, UK

Invasions in a community context

Hieracium pilosella

Mimulus guttatusCytisus 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 .

nr/b

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 1998. Journal of Animal Ecology 67, 554-566

controlled experiments

short time scales

high replication

but not a substitute for field work

Warren, Law & Weatherby 2003. Ecology 84:1001-1011.

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 2000. Oikos 88, 319-326.

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)5270

8 (80)1090

6 (60)1030

4(40)1010

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

nr/b

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 74:50-62

5 (100)5270

8 (80)1090

6 (60)1030

4(40)1010

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

x1

x2

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 2k subsystems

boundary of phase space

2D phase plane

theory: invasion implies persistence

resident community

invading speciesaugmented 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

caveatsresident 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

),( nEP nEbEd )()(1 =

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 2000. Oikos 88, 319-326.

struggles to compete with P

microcosms: initial rate of increase

Model Number of parameters

Degrees of freedom

Deviance

i + j + ()ij 44 187 3.08

i + j 16 215 8.04

i 11 220 26.6

+ j 6 225 10.7

1 230 31.4 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 1996. Ecology 77:762-775.

{1} {1,3}

3

{2}

24

{1}

species pool

new species

resident comm

{1,2,3,4…}

theory: community assembly and invasion resistance

Law & Morton 1996. Ecology 77:762-775.

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