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FORUM is intended for new ideas or new ways of interpreting existing information. It provides a chance for suggesting hypotheses and for challenging current thinking on ecological issues. A lighter prose, designed to attract readers, will be permitted. Formal research reports, albeit short, will not be accepted, and all contributions should be concise with a relatively short list of references. A summary is not required.
Population response to landscape changes depends on specialization to dvferent landscape elements Henrik Andrb, Annika Delin and Andreas Seiler, Grimso Wildlife Research Station, Dept of Conservation Biology, Swedish Univ. of Agricultural Sciences, S-730 91 Riddarhyttan, Sweden (
[email protected]). A landscape consists of a mosaic of different landscape (Lande 1987, Andren 1996, Bascompte and Sole 1996, elements. The structure and composition of the land- Pagel and Payne 1996). Thus, we also include the scape, as well as the changes in them, influence the numerical population responses due to pure habitat loss distribution, abundance and dynamics of different spe- and true effects of fragmentation in the model. cies (Morris 1995, Wiens 1995). Surprisingly, many studies assume that the preferred habitat fragments are isolated from one another by a hostile matrix, i.e. a divided landscape (Addicott et al. 1987), even though A conceptual model fragments are parts of the landscape mosaic and the Here we describe five different numerical population surrounding matrix might also be used (Andren 1994). responses to landscape changes related to different deIndividuals may utilize several different landscape ele- grees of specialization (Fig. 1). Imagine a landscape ments during their daily activity and, thus, experience composed of two different landscape elements; X and the landscape as heterogeneously undivided (Addicott Y. In a landscape where element X dominates, element et al. 1987). Therefore, if one assumes that the species Y will occur as fragments in a matrix of type X. perceives the landscape as divided (Addicott et al. 1987) Gustavson and Parker (1992) and Andren (1994) have but the species actually uses the matrix as well, then the made artificial maps showing this kind of landscape effect of landscape changes on population size will be change. misinterpreted. Thus, the degree to which a species is Species A uses only landscape element X, and fragspecialized in utilizing different landscape elements will ment size and degree of isolation do not influence the affect the numerical population response to landscape abundance and distribution. Therefore, there will be a changes. Here we present alternative models to test the linear one-to-one relationship between the proportion effect of landscape changes on different types of spe- of element X in the landscape and population size in cies. the landscape, i.e. a halving of the proportion of eleSometimes the landscape can be described as divided ment X in the landscape would result in a halving of (Addicott et al. 1987). The simplest explanation for the the population size. This is the prediction that follows decline in population sizes as fragments become smaller from the random sample hypothesis and from habitat and more isolated is that small fragments can be con- matching rules (Connor and McCoy 1979, Haila 1983). sidered as random samples from larger ones (Connor Species B shows the same specialization as species A, and McCoy 1979, Haila 1983). However, sometimes the but is sensitive to changes in fragment size and degree decline in population size is greater than predicted from of isolation. Models by Lande (1987) and Andren the random sample hypothesis and degree of isolation (1996) predict that the effect of habitat fragmentation and fragment size influence population size, i.e. there will be moderate in a landscape with a high proportion are true fragmentation effects (Andren 1994, 1996). of preferred habitat, i.e. the effect will be almost the There seem to be thresholds in the proportion of pre- same as from the random sample hypothesis (Andren ferred habitat in the landscape where the change in 1994, 1996). However, below a threshold value of the population size is suddenly greater than predicted from proportion of preferred habitat, the change in populathe random sample hypothesis, i.e threshold for habitat tion size will be very rapid. This is the prediction from fragmentation (Andren 1996). The threshold value will metapopulation models (e.g. Lande 1987, Hanski 1991, depend on, for example, the area requirements and Verboom et al. 1991, Andren 1996, Bascompte and Sole dispersal distances typical for members of each species 1996).
Species C does not discriminate between the two landscape elements X and Y. The two landscape elements have the same population density, because X and Y are completely supplementary. Such a species may be described as a true generalist, i.e. it uses all available landscape elements in the same proportion as they occur in a landscape (Roughgarden 1974, Rosenzweig 1987, Brown 1996). Species D uses both landscape elements X and Y. However, the population density is higher in X than in Y. We have arbitrarily chosen the density in Y to be half of that in X. Thus, in a landscape with only Y the population size will be half of that in a landscape with only X. Species E is similar to species D, with the exception that landscape element X is essential for its survival. Thus, the population size will be zero in a landscape with no landscape element X. The density for this kind of species will be higher than for a true specialist like species A, but lower than or equal to species D. However, in landscapes where X dominates, the difference between species D and E will be small. Often one assumes a coarse-grained response to the landscape changes (Andren 1994). However, the model can also describe the changes for a fine-grained response. A coarse-grained response for species A and B means that an individual is restricted to only one fragment of element X, whereas in a fine-grained response an individual uses more than one fragment of
Propoftion of element X in the landscape (%)
I
I
Proportion of element Y in the landscape 1%)
Fig. 1. Relative population sizes for five kinds of species (A-E) with different degrees of specialization in utilizing certain landscape elements in relation to the proportion of two elements (X and Y) in the landscape. Species A - uses only landscape element X; Species B uses only landscape element X, but area and isolation influence population size; Species C uses both landscape elements X and Y. with the same density in both landscape elements; Species D - uses both landscape elements X and Y. but with half the density in landscape element Y compared to landscape element X; Species E - uses both landscape elements X and Y, with half the density in landscape element Y compared to landscape element X and landscape element X, is essential. -
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element X (Levins 1968). For example, individual black woodpeckers (Dryocopus martius) used the landscape in a fine-grained manner and the difference in density between two landscapes could be explained as pure habitat loss (Tjernberg et al. 1993), i.e. as for species A. A coarse-grained response for species C and D means that certain individuals use only element X and other individuals use only element Y, whereas in a finegrained response all individuals use both element X and Y. The response for species E can be complex, some individual might use only element X and have a coarsegrained response, whereas other individuals use both element X and Y and therefore have a fine-grained response.
How to test the model Our conceptual model suggests that there are several different numerical population responses to landscape changes depending on the degree of specialization to different landscape elements. Strictly speaking only Species A and B experience the landscape changes as habitat fragmentation, i.e, a divided landscape (Addicott et al. 1987). The other types of species are generalists to a different degree and the numerical response to landscape changes will depend on overall landscape composition. For example, Andren (1992) found that the density of hooded crow (Corttus corone) was higher in landscapes with a mixture of farmland and forest than in landscapes dominated by either farmland or forest. There are several different ways to determine the effect of landscape change on different species, from detailed studies on habitat selection, movements, reproduction, mortality, etc. in different habitats, to censuses of organisms in different landscape types. One test of the model described here is to estimate population sizes in different landscape mosaics and to relate population size to the proportion of different landscape elements. The preferred landscape element can be identified by using isodar theory (Morris 1987, 1988, 1992, 1995). The random sample hypothesis predicts the same relative change in the proportion of the preferred element in the landscape and change in population size, i.e. large-scale habitat matching rule. In other words, halving of the proportion of preferred element in the landscape will result in a halving of the population size (Species A in Fig. I). This results in a slope of 1 in a log-log plot (Fig. 2). The log-log plot is very powerful to test the one-to-one relationship, as suggested by the random sample hypothesis. It can be used in any parts of the change in landscape. A change from 40% to 20% or from 10%)to 5% of landscape element X should both result in a halving of the population size for Species A. Thus, it is not necessary to know the original popula-
landscape (With a n d Crist 1995, Brown 1996). However, the model presented here can serve as a starting point in studies of landscape changes. The model is very easy t o test a n d deviations can suggest factors t h a t might be important t o study. Acknowledgements - D. Morris, T. Part, J. Swenson and S. Ulfstrand gave valuable comments on the manuscript. The study was supported by the Swedish Environmental Protection Agency and the private foundations "Olle och Signhild Engkvists stiftelser". Log proportion of element X in the landscape (%)
Fig. 2. Log relative population sizes for five kinds of species (A-E) with different degrees of specialization in utilizing certain landscape elements in relation to the log proportion of elements X in the landscape. Species characteristics as in Fig. 1. tion size, as Figs 1 a n d 2 might indicate. F o r a specialist suffering the effects o f isolation a n d area (Species B in Fig. I), the relationship between log population size and log proportion o f preferred element in the landscape will be > 1, because the relative change in population size will be greater t h a n the relative change in the proportion of the preferred element in the landscape, especially in landscapes with a low proportion of the preferred element. Finally, for a generalist (Species D in Fig. l), the relationship between log population size and log proportion of preferred element in the landscape will produce a slope < 1, because the relative change in population size will be smaller t h a n the relative change in the proportion o f the preferred element in the landscape. F o r a true generalist (Species C in Fig. 1) there should be n o relationship between population size a n d any landscape element. The relationship for Species E (Fig. 1) should be non-linear (e.g. second degree polynomial). Generally, a sample of 4050 is probably necessary t o have a reasonable statistical power (Cohen 1988) a n d t o be able t o use complex models, such as multiple regression (Morrison et al. 1992). T h e model in this paper describes a n extreme landscape with only two elements, but most real landscapes are more complex. With a n d Crist (1995) simulated the effects of landscape change in a mosaic of three landscape elements o n the dispersion of species with different habitat requirements a n d dispersal abilities. This resulted in a more complex response, but generally habitat specialists a n d good dispersers were more aggregated than habitat generalists a n d p o o r dispersers. A higher degree of aggregation means a larger proportion of the population in preferred landscape elements, a n d therefore a higher probability for persistence (With a n d Crist 1995). Thus, a n increased complexity of landscape changes will result in a much more complex response o n the abundance and distribution of a species in the
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