Metapopulations: how distinct are patch and matrix?

3 Focusing on Patches or an Entire Landscape Approach?

3.1 Metapopulations: how distinct are patch and matrix?

The focus on patchworks, patch and matrix, developed with metapopulation modelling (Gilpin and Hanski, 1991; Hanski and Gilpin, 1997; Ehrlich and Hanski, 2004). This is an extension of island biogeography to terrestrial situations in which dynamic homeostasis (equilibria) is envisaged between coloniza- tion and extinction within a patchwork of potential habitat units (Levins, 1969, 1970; Gotelli and Kelley, 1993). In these models population size is largely equated with patch size and isolation with distance across the matrix between patches.

Increasingly, attention has been given to the quality of patches and matrix; the former has been demonstrated to influence population incidence, in some cases more than either patch size or isolation (Dennis and Eales, 1997, 1999; Thomas et al., 2001; Matter et al., 2003; Valimaki and Itamies, 2003), and the latter equally profoundly to influence transit of individuals (Dover et al., 2000; Roland et al., 2000; Ouin et al., 2004). Quality has been regarded by the proponents of metapopulation models as being subsumed in patch area (Nieminen et al., 2004), but this is clearly not the case, substantiated by firm examples (butterflies Coenonympha tullia Müller, Satyrinae; Dennis and Eales, 1997, 1999; Parnassius spp., Matter and Roland, 2002; Auckland et al., 2004); patch area makes no reference to the com- position and structure of resources comprising habitat patches nor to the connec- tivity amongst resources within patches (Dennis et al., 2003, 2006). The emphasis has also been primarily on a single consumable resource, larval host plants. In exactly the same way, the matrix has been treated as if sea, without content or structure. This is one reason why traditional metapopulation models may rea- sonably refer to patchworks of habitat but not to more sophisticated topologies, such as networks of habitat.

To the extent that a patch (= habitat) is not a discrete, homogeneous entity, i.e. where there is virtual 1:1 correspondence between resources and a vegetation unit typically a host plant with a NVC unit (Rodwell, 1991 et seq.), there will be difficulty in distinguishing patch from matrix. Such situations are atypical of industrial farming systems (e.g. East Anglian cereal farmland, UK), which generate landscape simplification and severe fragmentation.

Normally, where environmental conditions are described as ‘semi-natural’, the sheer difficulty in establishing rules for determining habitat patchworks is all too clear and commented on elsewhere (Dennis et al., 2006); matrix is an extension of non-resource space within habitats (Fig. 5.1c). Metapopulation modellers and empiricists have to face up to two uncomfortable axioms. The greater the fraction of the complement of resources that make up habitats used to define them, inevitably the more resource types and elements will be found in the matrix. But, the fewer the resource types that are used to define habitat bounds, the more resources that should be included within the habitat space will be allocated to the matrix. Metapopulation modellers, whether they limit their definition of a habitat to a single resource or encompass the entire com-

plement in the process, will find that they have resources defining a habitat for the organism dispersed throughout what they categorize as matrix. Suddenly, purely by changing our view of what is or not a habitat, the matrix becomes a zone of resources. Another truth is that the fewer the resources used to define a habitat patch the smaller it will be; simultaneously, the bigger becomes the matrix around it, and the more likely it contains resources. Thus, patch dimensions become truncated. The reason for these artefacts in studies is that arthropods, notably butterflies, which have been extensively studied, use different substrates and vegetation structures for different activities, particularly utilities compared to consumables (Dennis et al., 2003).

There are also problems associated with scale and effort. Human observers have a tendency to filter out small items, simply because of inaccessibility or lack of resources for fine-scale surveys (Dennis et al., 2006). Units or packages below a certain size tend to be ignored. But, this is what is distinctive about consumable resources, if not utilities, within the matrix: the resources are often in small, even tiny, pockets and they are disparate. Among these resources may be found host plants in the right condition for exploitation by a target species, but they are difficult to pick up on survey if only because of access, limitations of search time and numbers of surveyors compared to the area being covered.

In plots of patch area against isolation, account is rarely made of small resource (host plant) patches <0.01 ha and certainly not patches of 1⁻⁰⁴ ha (0.0001 ha or 1 × 1 m²) (Thomas et al., 1992; Hanski and Thomas, 1994; Lewis et al., 1997;

Baguetteet al., 2003). Yet, these can occur in matrix contexts for oligophagous species (e.g. P. argus on Great Ormes Head, North Wales; R. Dennis, personal observations) and are much more likely for species that exploit a wide range of larval host species (e.g. butterflies Maniola jurtina L.; Pyronia tithonus L.; Dennis, 2004a;Pieris spp.; Dennis and Hardy, 2007). In the case of many grass feeders we know very little about host plant preferences (but see Pararge aegeria L.;

Shreeve, 1986; Lasiommata megera L., Nymphalidae; Dennis and Bramley, 1985) and even less about their other highly complex resource requirements and are thus more likely to misinterpret the role of matrix resources.

Small resource elements and items in the matrix are frequently regarded either as below the scale to which insects respond or, if used, as inflicting a cost, slowing movement and acting as sinks in reproduction (Pulliam, 1988).

However, in the business of defining patch and matrix, very little attention is actually paid to what arthropods perceive and respond. Pertinent questions are: Do arthropods actually experience the environment polarized as patch and matrix or as landscape with variable resource distributions? How much does size matter when it comes to resource recognition and use? Empirical studies of just what arthropods do in landscapes can reveal how behaviour is related to biotopes, vegetation units and substrates. It is expected that, with a typical habitat (= vegetation patch) model, movements will be of two basic types: routine searching, sinuous flights and direct linear flights (Van Dyck and Baguette, 2005). Direct linear flights will dominate what is sup- posed to be the matrix. However, two studies on butterflies (e.g. M. jurtina;

P. tithonus; Dennis, 2004a; Pieris spp.; Dennis and Hardy, 2007) reveal that they treat the matrix as comprising resources. In the matrix – defined either

on the basis of traditionally accepted unsuitable biotopes or very low density of the target organism – they engage more in resource searching and resource using than they do in direct linear flights, typical of dispersal and escape (Table 5.1). However, pierid butterflies switch between direct linear flight and search flight in response to resource cues across a variety of substrate or vegetation surfaces (Fig. 5.3). Close attention to resource attributes indicates that movements are affected by more than just inter-patch distances, by at least five aspects of resource geography and timing (Fig. 5.4). This lies at the root of the occurrence of both types of movements observed in Pieris spp.

across landscapes; frequent switches between the two types are expected (Fig. 5.3). The key is that consumer resources, if not utility resources, in the matrix will tend to be in smaller lots, scattered and different in composi- tion (type) and structure (shape). Host plants occur in the matrix but are often so small that they are missed by human observers. But, they are found and used by arthropods even when they are not visible to the observer (e.g.

DLF

Cue

SF n y

y TR

y RU

IR y

RC n y

y n

CRT

Fig. 5.3. Flowchart of suggested switches in fl ight behaviour in response to resource cues in Pieris butterfl ies within both habitats and matrix. Boxes: hexagon/diamond shading, RT = resource targeted; round cross-shaded squares: DLF = direct linear fl ight and SF = search fl ight; white squares: resource variables, TR = targeted resource, CR

= complementary resource, CRT = complementary resource targeted, RU = resource use; diagonal shaded boxes: IF = interaction in transit (with butterﬂ y or predator), IR = interaction on resource; round boxes: connectors, ‘y’ yes or ‘n’ no; diamond box, proximate ‘cues’ including visual and scent stimuli triggering switch in ﬂ ight types.

host plants in Pieris napi; Courtney, 1988). There is a serious lack of data on interactions between behaviour and matrix components, yet such data are critical to understand the potential of the matrix and the functioning of species in industrial farming landscapes. There is an urgent need to know more about scales of resources used and distances over which resource elements can be sensed and the part played by vision and olfaction in resource tracking (Vane-Wright and Boppré, 1993; Cant et al., 2005).

Resource synchronization

with stadia

Resource disturbance

Complementary resources

required

Abundance of resources

(per site)

Frequency of resources (landscape)

Population size

Resource aggregation

Movement threshold

Distance between resource outlets

Success of movements (fitness)

Population density

Competition harassment predation

Allee effect Conditions

– –

+ ↑+

↓−

– –

Resources

Populations +

– +

– –

–

Migration

–

Fig. 5.4. Resource variables inducing movement and migration in butterfl ies. Initial conditions include a variety of agents (e.g. vegetation succession, human management, weather and climate). This has impacts on fi ve basic attributes of resource distributions that, in turn, over different space–time frames infl uence the tendency to move over the landscape. Resource disturbance refers to vegetation changes (Grime, 1974) and host plant dynamics (i.e. generation time). Complementary resources refer to non-substitutable resource outlets (Dunning et al., 1992). Some degree of dependence occurs among the resource variables (illustrated). No attempt is made to expand on individual (e.g. lifespan) and population infl uences on movement and migration distances in this simple process-response model other than to indicate a link to resources, nor on the direct infl uence of conditions (e.g. weather; Dennis and Bardell, 1996;

Dennis and Sparks, 2006). In Baker’s (1978) initiation factor model, the probability of an individual initiating migration depends on its migration threshold being exceeded. According to this model, both population density and environmental conditions interact to effect mobility, including changes to resources generated by individual resource use (e.g. the ideal free distribution; Calow, 1999).

Patch and matrix bounds are further confounded by temporal changes (Wiens, 1996; Thomas and Kunin, 1999) and spatial (regional) variation. Just what appears to be a habitat patch changes on scales of seconds to decades.

Those engaged in conservation practice are constantly faced with succes- sional changes on sites, as well as changes in conditions induced by human activities (Sheppard, 2002; Offer et al., 2003; Underhill-Day, 2005). Change is integral for sites and in the following sections we argue for conservation to be geared to managing dynamics. Change on fine timescales has important implications for recognizing just what resources are important for organisms;

there are also inevitable implications for habitat recognition. The habitat space used by P. argus (Lycaenidae) on the Carboniferous limestone head- land of the Great Orme (North Wales) oscillates upslope and across slope from the vicinity and shelter of scrub with changes in sunshine, temperatures and wind speeds; the warmer the conditions, the larger the area used, and the response is immediate on weather changes (Dennis and Sparks, 2006). Such changes are known for other invertebrate taxa (e.g. Oedipoda caerulescens L., Orthoptera, Acrididae; Maes et al., 2006). Changes with weather and seasonal conditions have also been recorded in mate location surfaces and elements in Inachis io (Nymphalidae) at three different spatial scales: the landscape, the surface substrates and in relation to microfeature topography (Dennis, 2004c;

Dennis and Sparks, 2005). Seasonal shifts are well known in vegetation and biotope occupancy in both sexes of P. aegeria (Shreeve, 1984, 1985, 1987) and for vegetation and host plant use by different generations of the butterfly Polyommatus bellargus Rottemburg (Lycaenidae) (Roy and Thomas, 2003).

Distinction of habitat and matrix has also to contend with regional changes in apparent resource use, involving substantial shifts in biotope occupancy (e.g.P. napi; Dennis, 1977; Anthocharis cardamines L. Pieridae; Courtney and Duggan, 1983). Changes in resource use, including movements and resource tracking are under evolutionary change and not static (Dennis, 1977; Merckx and Van Dyck, 2002; Van Dyck and Baguette, 2005). All this frustrates the distinction of habitat and matrix for management based, as it is, on limited resources. At the very least it means that attention should be given to the matrix (surrounding landscape) and the resources it can usefully provide, as well as the obstacles it presents, for a target species.

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