Spatial Ecology: Metapopulations

1. Spatial Ecology: Metapopulations Peter B. McEvoy Oregon State University

2. Outline • History of metapopulation theory in the past 50 years • Basic model – persistence via a balance in colonization and extinction rates • Case study of the Glanville fritillary butterfly • Interaction web of the Glanville fritillary • Metapopulation structure and dynamics • Influence of area and isolation • Influence of parasitoids • Abiotic forcing by rainfall • Principal messages

3. The past fifty years(Hanski 1999 Metapopulation Ecology) • Local populations connected by migration; colonization and extinction (Andrewartha and Birch 1954) local demes (Sewall Wright 1940) • ‘Bottle Experiments’ with blowflies (Nicholson 1933) and mites (Huffaker (1958) • Island biogeography – the balance between immigration and extinction (MacArthur and Wilson 1967) • Evolution of dispersal in a metapopulation (Gadgil 1971) and spreading the risk (den Boer 1968) • Metapopulation persistence reflecting a balance between extinction and recolonization (Levins 19969, 1970) • Theoretical development takes off beginning in 1990 (Hanski 1999)

4. Patch occupancy and population dynamics covary in a mite predator-prey system:a ‘bottle experiment’ Prey Predator

5. Application of the Theory of Island Biogeography Distance Effect Area Effect

6. Experimental Island Biogeography: distance effect Equilibrium number of species higher on islands closer to mainland source

7. Model of Levins (1969, 1970) • The rate of change in the fraction of occupied habitat (patches, p) dp/dt = mp(1-p) – μp where μ is the rate of local extinction of patches and m is the rate of recolonization of empty patches Structurally identical to the logistic (Hanski 1994) dp/dt = (m - μ)p {1- p/[1- (u/m)]} The total metapopulation will reach a stable equilibrium with a fraction p* =1- u/m patches occupied

8. Alternative Stable Equilibriabiomodal distribution of patch occupancy p • Heterogeneity among habitat patches may give rise to a bimodal equilibrium distribution of the fraction of patches occupied in an assemblage of species (the core-satellite distribution).

9. Central Result • It is possible to have instability on local scales: Even if none of the local populations is stable in its own right • Yet have stability on more global scales: …a metapopulation can stably persist as a result of a balance between random extinctions and recolonizations • Local extinction and colonization play organizing roles. Persistence in a metapopulation depends less on local rates of birth and death, immigration or emigration and more on the rates of extinction and colonization • Adding Relevant Detail. Levins’ model does not take account of variation in potentially influential variables/processes such as size of patches, their spatial locations, nor the dynamics of populations within individual patches

10. Persistence in Glanville fritillary butterfly is lower for populations than metapopulations Metapopulations Populations

11. A problem for empiricists:When is a ‘patchy population’ a ‘metapopulation’? • Subpopulations • Frequent extinction and recolonization • Sufficiently high colonization rate • Sufficient asynchrony in local dynamics • Concept applies easily to insects, but plantspose problems– does recolonization following extinction apply to plants with a seed bank? To succession following disturbance? (Husband & Barrett 1996; Bullock et al. 2002) • In general, the processes influencing the dynamics of ecological systems are disturbance, colonization, and local interactions (e.g. competition, predation, mutualism) that set in motion a successional process. Spatial ecology is not tied to any particular model (e.g. a metapopulation is a special case of more general theory of spatio-temporal dynamics)

12. Glanville Fritillary and Host Plants Ilkka Hanski Melitaea cinxia Glanville fritillary (UK) Veronica spicata Plantago lanceolata http://www.helsinki.fi/science/metapop/english/cinxias.htm

13. Interaction Web for Glanville Fritillary

14. Primary parasitoids of the Glanville fritillary A gregarious endoparasitoid, laying one to about 40 eggs inside a host larva, depending on the size of the host. Multivoltine. Limited dispersal ability, less vagile than host. A solitary endoparasitoid, laying eggs in first instar host larvae just before the larvae hatch from the egg. Univoltine. High dispersal ability. • http://www.helsinki.fi/science/metapop/english/Species/Hypsoter.htm Hyposoter horticola  (Gravenhorst) (Ichneumonidae: Campoplaginae) Cotesia melitaearum  (Wilkinson) (Ichneumonoidae:Braconidae)

15. Influences on local population size • Competition for resources – quantity and quality can be limiting on local scales; drought reduces host plant quality • Natural enemies • Predators • Parasitoids • Hyposoter horticola (Ichneumonidae) – solitary, univoltine, large dispersal range • Cotesia melitaearum (Braconidae) – gregarious, 2-3 generations per host generation, shorter dispersal range than butterfly host • Interactions and movements of adults – males and females differ in emigration rates; Allee effects; migration distances generally<500 m; 4000 habitat patches divide into ~tens of networks • Abiotic conditions – temperature influences development;larvae exploit thermal heterogeneity in the environment; wind reduces fight; precipitation affects mortality • Habitat Loss and Alteration – natural and anthropogenic (e.g. grazing) disturbances

16. Summary of DD process known to influence local dynamics • Food shortages at local scales • Parasitism by primary parasitoidCotesia melitaearum only significant in regions and years with large-well connected host populations • Inversely DD emigration and immigration and difficulty of finding mates in low-density populations contribute to an Allee effect on local dynamics • Inbreeding depression in small populations increases extinction risk

17. Glanville FritillaryMap of a Metapopulation Occupied (filled) and empty (open) habitat patches suitable for the Glanville fritillary in the Åland Islands off west coast of Finland in the autumn 2005.

18. A related butterfly with a different metapopulation structureBay checkerspot butterflies • Euphydryas editha bayensis butterflies occur in discrete patches ~ metapopulation south of San Francisco on serpentine grassland • Patches occupied in 1987 indicated by arrows • Occupancy of suitable environments changes from year to year • Some populations are ‘sources’ and others are ‘sinks’ Wilson 1992

19. Ups (▲) and Downs (▼) in metapopulation size over time in the Glanville fritillary

20. Effect of Connectivity on Colonization Proportion of colonized patches Related to Distance to nearest population) No. Empty No. Colonized Related to Connectivity S

21. Effect of Patch Area: Larger patches increase population size and occupancy of M. cinxia

22. Influences on colonization • Connectivity increases with Increasing number, increasing sizes, and decreasing distance to local populations that occur within the migration distance from the focal habitat patch • Apart from connectivity – increasing propagule (= found population) size, increasing size of habitat patch, increasing host plant quantity and quality, reduced grazing, increasing abundance of nectar plants…all increase the rate of successful colonization

23. Influence of Abiotic ConditionsIncreasing July precipitation yields large changes in Population Size NandOccupancy O Temperature Precipitation Change N and O

24. Abiotic Forcing of Metapopulation DynamicsPrecipitation in Space and TimeWeather events can be spatially correlated

25. Classic Metapopulation Structure and Dynamics well represented by Glanville Fritillary • Suitable habitat occurs in small discrete patches • Local populations have a high risk of extinction • Patches are not too isolated to prevent recolonization • Local and regional dynamics are asynchronous enough to make simultaneous extinction of all local populations unlikely

26. Is there a balance between Colonization and Extinction?Plotting Data in Table 4.2Nieminen, Siljander, & Hanski 2004

27. Principal Messages of Metapopulation Ecology • Population size is affected by migration • Population density is affected by patch area and isolation • Asynchronous local dynamics • Population turnover, local extinctions and establishment of new populations • Presence of empty habitat • Metapopulations persist despite population turnover • Extinction risk depends on patch area • Spatially realistic models can be used to make predictions about metapopulation dynamics in particular fragmented landscapes (e.g. Glanville fritillary) • Metapopulation coexistence of competitors (e.g. inferior competitor is superior colonizer) • Metapopulation coexistence of predator and prey (e.g. two-spotted spider mite Tetranychus urticae and predatory mite Phytoseiulus persimilis in greenhouses Nachman 1988, 1991)