Available at Metapopulations Dane McDonald [email protected].

Available at http://planet.uwc.ac.za/nisl/Eco_people/Presentations/

Metapopulations

Dane McDonald

[email protected]

Introduction (1) Richard Levins introduced the term “metapopulation”

in his work on biological pest control1,2

He used models of migration, extinction, and local fluctuation to study the population processes of pests in a heterogeneous environment2

Levin’s work marked the beginning of contemporary metapopulation biology.

Metapopulation literally means a ‘population of populations’

DEFINITION: a set of local populations within a larger area, where migration from one local population to other habitat patches are possible1

These groups of local populations usually occur in suitable, discrete (i.e. separate and discontinuous) habitat patches that are scattered in a landscape.

Richard Levins

Introduction (2) This spatial arrangement allows populations to interact via

dispersal of individuals across a matrix of unsuitable habitat3

The result is dynamic interactions between local populations through migration4

These interactions are explained and interpreted by metapopulation modelling and theory.

The aim of this presentation is to introduce metapopulation theory within the context of butterfly metapopulations

Why butterflies?

..Simply because their populations are often structured in a way that is largely consistent with the metapopulation concept.

Thus the concept will be more clearly illustrated (within the context of relevant environmental issues). Melitaea cinxia

Butterflies existing as metapopulations (1) European butterfly populations have suffered major declines since

the 1950’s and throughout the 20th century 2,4

Main causes were agriculture and other land use changes4

These human activities caused various levels of habitat loss and fragmentation.

Thus butterfly populations and their habitats are not homogenously distributed in space, hence spatial heterogeneity5

In the figure lanscapes are seen as idealized habitat patches or fragments containing species that occur as discrete local populations connected by migration9

Figure 1

This spatial heterogeneity is shown in Hanski (1998)9’s illustration of metapopulation ecology as one of the approaches to spatial ecology (Figure1).

Butterflies existing as metapopulations (2) This spatial distribution can be observed in the habitat of the butterfly,

Pyrgus amoricanus, which occurs in Southern Sweden (Figure 2)6

Figure 2

The occupied (black dots) and unoccupied (clear dots) are connected by migration.

Migration is synonymous with dispersal and forms part the broader transfer process of individuals within a metapopulation1

This process can be divided into three-stages (Figure 3)1:

Emigration (individual leaves its home site).

Migration stage (displacement process where the individual moves away from its home site).

Immigration stage (settlement occurs/immigration into an empty patch followed by successful establishment of a new population is termed colonization).

Migration in metapopulations (1)

Migration or dispersal patterns have very important effects on the dynamics and survival of animal populations5

Most metapopulation models (to be dicussed later) assume that dispersal in Butterflies occur randomly5

Figure 3


In a study of two British butterfly species (Maniola jurtina and Pyronia tithonus), Conradt et al (2001)5 gave reasons to believe otherwise.

In P.tithonus, 22% of the butterflies released in unfamiliar habitat flew in loops about the release point before moving in a particular direction (Figure 4).

Case study

Figure 4


Migration habits (as in case study) are particularly important when habitat fragmentation has caused small ‘local populations’ to become spatially isolated.

Furthermore the survival of the metapopulation depends on the efficient and regular recolonisation of extinct local populations through dispersal from other local populations5,9

Indicative of searching strategy and perceptual range whereby butterflies actively direct movement towards a familiar habitat patch from distances >85m and towards unfamiliar patches from >65m

These distances are comparable to the usual scale of P.tithhonus habitat within the UK- as a result they usually find themselves well within the perceptual range of their starting habitat patch or a suitable new habitat patch.

Extinction operates at two levels9, namely:

Metapopulation level

Local population level

The main unit of the metapopulation is the local population, thus it would be apt to define local extinction.

DEFINITION: Local extinction may be defined as the extirpation (i.e. the loss of a local population as distinct from an entire species ‘extinction’) of any population segment sufficiently isolated from immigration that, once extinct, typically remains so for several or more generations1

Hanski (1998)9 provides a table illustrating the extinction processes (metapopulation and local level) that operate in the glanville fritillary butterfly (Melitaea cinxia) (Table 1).

Extinction-colonization dynamics (1)

Habitat loss and fragmentation, extinction typically delayed

Extinction-colonization

Specialist enemies

Migration in small populations

Metapopulation extinction

Genetic

Persecution by humans

Environmental

Habitat lossDemographicLocal extinction

Extinction due to extrinsic causes

Extinction due to stochasticity

Scale of extinction

Extinction-colonization dynamics (2) Survival of the metapopulation is a function of the counteracting

processes of the extinction of populations and the recolonization of empty patches7

Meaning that extinction of local populations and subsequent recolonization of empty patches are what drive metapopulation dynamics1

Ockinger (2006)6 identified the threatened Swedish butterfly, Pyrgus armoricanus population as a probable metapopulation based on its dependence on extinction-colonization dynamics.

Case study

In this study, Ockinger (2006)6 found that many of the occupied patches were small and contained low numbers of individuals

Environmental or demographic stochasticity could easily cause the extinction of these small populations, but they seemed to persist…

Extinction-colonization dynamics (3)

As evidence for the role of extinction-colonization in this persistence, Ockinger (2006)6 refers to a previous survey of a P.armoricanus population in 1985.

This survey of two supposedly ‘occupied’ habitat patches yielded no observations. At the time of the present study, however, the habitat patches were occupied.

Therefore the patches must have been colonized at some time during the period 1986-2004.

Metapopulations that consist of small extinction-prone local populations as in the Ockinger (2006)6 study can only persist regionally, in a balance between local extinctions and colonizations, hence extinction-colonization equilibrium9

However, ecologists still debate the frequency with which species persist as metapopulations in extinction-colonization balance

The other prevailing scenario is where metapopulations persist as result of stable large populations, also called “source populations”

Source-sink dynamics (1) DEFINITION: A source-sink metapopulation is one in which there are

patches where the population growth rate at low density and in the absence of immigration is negative (sinks) and patches in which the growth rate at low density is positive (sources)1.

Source populations…

large in size

occupy habitats of the best quality

net reproductive rate, R0, greater than 1

Sink populations…

deaths exceed births

emigration exceeds immigration

R0 is consistently less than 1

Sinks are prevented from extinction by a constant supply of immigrants from more productive sources.

Source-sink dynamics (2)

Case study

Hanski (1997)1 uses a detailed example to illustrate source-sink dynamics of a Euphydryas editha (checkerspot butterfly) metapopulation.

Before 1967, butterlies were restricted to natural outcrops that supported its host, oviposition plant Pedicularis semibarbata

In 1967, logging caused P.semibarbata to dissappear from the clear-cut areas.

The butterflies reinvaded the clear-cut habitat and adopted Collinsia torreyi as a host (with the exception of unlogged areas where P.semibarbata was still preferred).

By 1985 there was a patchwork of host use over an area >100km.

There are two likely reasons why individuals would immigrate into low quality habitat patches inhabited by sink populations8:

Interference competition Passive non-directed dispersal

Source-sink dynamics (3)To illustrate

Rocky outcrops

(P.semibarbata) Clear-cut

areas

(C.torreyi)

Butterfly

movement (1967)

1980’s

Newly adopted clear-cut areas with C.torreyi as the preferred host acted as population sources during the 1980’s.

Clear-cut habitats received less eggs- note: P.semibarbatus was the preferred host for oviposition- but yielded higher adults due to higher survival than at outcrops.

Butterfly movement from clear-cut to outcrop occurred about twice as frequent as movement in the opposite direction.

Source-Sink dynamics (4) In the case study, clear-cut areas acted as sources and rocky

outcrops as sinks. It is thus clear that source populations are indeed very important for

metapopulation persistance and should be considered a conservation priority.

But what about Sink populations? Sinks may be important on both ecological and evolutionary grounds8

In terms of ecology Sink populations can… stabilize species interactions foster coexistence in ecological systems Increase metapopulation persistence under variable conditions

From an evolutionary perspective… Geographically peripheral sink populations tend to be

genetically divergent from central populations. …importance for future speciation, considering the probability

that mutations with large effects on individual fitness occur in these sink populations.

Source-Sink dynamics (5)

From an evolutionary perspective…(continued) Significant mutations can overcome selective bias towards

increasing adaptation to source habitats, as opposed to sinks. Hence under some circumstances Sink populations can be of

conservation value through maintaining genetic variability8

This may prove to be useful for future adaptation, especially in the light of global environmental change (E.g. climate change).

Global environmental change, Modelling, and Conservation implications (1) Destruction and fragmentation of natural habitats have proved to be the

main causes of modern-day extinction 4,8,9

Example: Endangered bird species of the world where habitat loss has been identified as the main threat in 82% of the species9

A majority of species display a metapopulation structure within their geographic ranges8

Their global extinction is mediated by changes in metapopulation dynamics as a direct result of habitat loss associated with human encroachment of natural habitats8

As mentioned earlier, the butterfly species of Western Europe have suffered greatly due to human activities such as agricultural practises and land use changes since the 1950’s3

Many species are now listed as threatened (E.g. the marsh fritillary, Euphydryas aurinia in Belgium and Pyrgus armoricanus in Sweden) 3,4

Global environmental change, Modelling, and Conservation implications (2)

Population Viability Analysis (PVA) has emerged as a tool to provide practical conservation guidelines for threatened and endangered species3,4

PVA projects metapopulation abundance to predict the risk of extinction or viability of metapopulations in a particular landscape under specific conditions. It also allows for the most important factors that contribute towards the persistence of metapopulations and the ranking of different habitat management options4

Case study

Schtickzelle et al (2005)4 used PVA to explore quantitatively the viability of the threatened E.aurinia metapopulation in a successional landscape from south-eastern Belgium, where invasion by shrubs and trees such as Crataegus monogyna and Prunus spinosa Formed unsuitable habitat for larval development.

The study also assessed the effeciency of various management measures to ensure its persistence.


The density-dependent population regulation was investigated using the time series of nest counts.

The chosen density-dependence function produced a population growth rate (Rt ) as a negative exponential function of the number of nests in the previous winter.

The first step was to estimate metapopulation dynamics parameters…

Intrageneration, Intergeneration, and between-patch dispersal parameters were used.

For intergeneration demography (as an example), the data of yearly autumnal nest censuses from one of the populations were used (Table 2)

Table 2. Winter nest numbers (Wt ) and associated population growth rates (Rt ) for E.aurinia

After the function was fitted to the data and parameters were estimated, it provided estimates of (Figure 5):

Maximum growth rate, Rmax =3.11

Carrying capacity, K=224 winter nests for the 5.75 ha of suitable habitat

Global environmental change, Modelling, and

Conservation implications (3)

Figure 5.

This carrying capacity corresponded to 39 winter nests per hectare….which was further extrapolated to K=253 adults per hectare

The second step was the PVA model parameterization.

The model incorporated the internal dynamics of every local population and was based on a density dependence function, the dispersal between sites, and the correlation of local dynamics.

Other factors such as environmental and demographic stochasticity were also incorporated into the model.

Global environmental change, Modelling, and Conservation implications (3) The analysis of the PVA model..

Four parameters were used as viability indices.

Only the first (metapopulation trajectory) will be here discussed.

This parameter represents the number of butterflies present in the metapopulation at each time step (i.e. generation) during 100 simulated years

The viability results for the natural succession (thick line) and status quo (thin line) situations are shown in Figure 6 .

Figure 6.

The metapopulation will be driven deterministically to extinction if the natural succession of vegetation was not controlled by some form of management.


The study further assessed metapopulation viability scenarios representing realistic management options that would increase the overall carrying capacity of the system by:

1. Improving habitat quality via restoration and maintenance of existing patches (scenario A)

2. Increasing the quantity of habitat by enlarging existing patches or creating new habitat patches in the matrix (scenarios B1-B4)

The results of these scenarios are shown in Figure 7. In comparison with the status quo situation, it is clear that the restoration

and maintenance of habitat quality (A) as well as new habitat creation (B1-B4) would increase both overall abundance and viability of the metapopulation.

In conclusion the study suggested urgent and substantial habitat restoration for E.aurinia in Belgium to counteract the predicted extinction.

Figure 7

Conclusion

The metapopulation concept as introduced by Levins (1960’s and 70’s) has become increasingly useful in contemporary conservation biology.

This evident from the large body of literature (E.g. butterflies) that use the concept to explain the dynamics of fragmented animal and plant populations.

Habitat loss and fragmentation will become widespread in the future, due to global environmental change resulting from human activities (E.g. land development, climate change).

In the light of these environmental challenges, I think that the metapopulation concept will continue to develop useful conservation tools such as population viability analysis (PVA) which aid in the management of biodiversity.

References (1)

1. Hanski IA, Gilpin ME (1997) Metapopulation Biology- Ecology,Genetics, and Evolution. Academic press, San Diego. ISBN 0 12 323446 8

2. Levins,R (1969) Some demographic and genetic consequences of environmental heterogeneity. Entomological Society of America 15:237-240

3. Baguette M,Schtickzelle N (2003) Local population dynamics are important to the conservation of metapopulations in highly fragmented landscapes. Journal of applied ecology 40: 404-412.

4. Schtickzelle N, Choutt J, Goffart P, Fichefet V, Baguette M (2005) Metapopulation dynamics and conservation of the marsh fritillary butterfly: Population viability analysis and management options for a critically endangered species in Western Europe. Biological Conservation 126: 569-581.

5. Conradt L, Roper TJ, Thomas CD (2001) Dispersal behaviour of individuals in metapopulations of two British butterflies. Oikos 95: 416-424.

6. Ockinger, E (2006) Possible metapopulation structure of the threatened butterfly Pyrgus armoricanus in Sweden. Journal of Insect Conservation 10: 43-51.

References (2)

7. Levin, DA (1995) Metapopulations: an arena for local speciation. Journal of Evolutionary Biology 8: 635-644.

8. Marquet, PA (2002) Metapopulations in Encyclopedia of Global Environmental Change, Edited by Mooney, Prof. HA and Canadell, Dr. JG. John Wiley & Sons, Ltd, Chichester. ISBN 0 471 97796 9

9. Hanski, I (1998) Metapopulation dynamics- review article. Nature 396: 41- 49.

Available at Metapopulations Dane McDonald [email protected].

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