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Biodiversity Transients in Degrading and Recovering Mosaic Landscapes

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Biodiversity Transients in Degrading and Recovering Mosaic Landscapes The Balance between Biodiversity Conservation and Sustainable Use of Tropical Rain Forests BIODIVERSITY TRANSIENTS IN DEGRADING AND RECOVERING MOSAIC LANDSCAPES Cees J. Nagelkerke ABSTRACT Tropical forest landscapes generally consist of a mosaic of habitat patches with different disturbance characteristics. Anthropogenic degradation may drastically change the frequency distribution of the different habitats or alter their disturbance rates. I use metapopulation theory to model the fate of ensembles of species that inhabit such mosaic landscapes. Species differ in their habitat needs and a central assumption is that dispersal behaviour is moulded by natural selection. This results in a dispersal rate that increases with the pristine disturbance rate of the habitat patches on which a species depends. The speed with which population levels react to changes in the amount of suitable habitat therefore also increases with the disturbance rate. Consequently, when frequently disturbed habitat types become relatively more abundant regionally, the increase in number and density of disturbance-dependent species is much faster than the decrease in species in the declining, more stable habitats. Landscape degradation may therefore result in a temporary increase in regional biodiversity before a lower equilibrium is reached. When the landscape is allowed to recover, a temporary decrease in biodiversity may occur. Hence, there can be a marked asymmetry in the reaction to degradation compared with recovery. When degradation leads to an increased disturbance rate for all habitats, however, species from the originally most stable habitats suffer the greatest and fastest decline. Accordingly, there is little asymmetry between degradation and recovery. The biodiversity “debts” and “credits” studied here can arise without there necessarily being any competitive interactions between species. Time lags, as studied here, are expected to be of major significance for the future of biodiversity in tropical forests. INTRODUCTION I investigate how the biodiversity of a whole landscape will change in reaction to various kinds of landscape change, and how fast those changes will occur. The work is theoretical, using mathematical and computer models. I address two important theoretical needs of conservation biology. The first need is to take a multitude of species and habitats into account; this is crucial when studying biodiversity change. Most existing work is on one species or on one habitat. The second need is to study time-delayed processes in the reaction to environmental degradation. An example is the occurrence of extinction debts, i.e. the continuing existence of species that are doomed to extinction. Time delays in the reaction of biodiversity to changed circumstances are of major importance because they may determine how much extinction or other ecosystem change is awaiting us. Habitat destruction, especially of tropical forests, is generally seen as the dominant threat to biodiversity (e.g., Pimm et al., 1995; Pimm, 1998). Recorded species extinction, however, is much less than what would be expected theoretically (Heywood and Stuart, 1992; Whitmore, 161 The Tropenbos Foundation, Wageningen, the Netherlands 1997). Some authors therefore reason that the predictions are too pessimistic (e.g., Budiansky, 1994). Others argue that extinction takes place with a delay and that many extant species are doomed (e.g., Heywood et al., 1994; Whitmore, 1997; Pimm, 1998). As yet, very little is known about time lags. Hardly any theory is available and estimates about the time taken by extinction differ widely. For example, Brooks et al. (1999) claim a time scale of about 50 years for birds in tropical forest fragments, whereas Kellman et al. (1996) argue that 10.000 years is too short for trees, but 20 million too long. It is therefore extremely important to investigate the generality, magnitude, time rates, and exact mechanisms of time-delayed biodiversity changes. For example, how do they differ between organisms, landscapes and kinds of degradation? Degradation results in natural habitats becoming fragmented and alters their disturbance dynamics. Disturbance frequencies often increase. Whereas rather stable patches predominate in “pristine” systems (Figure 1a), anthropogenic degradation often results in a drastic shift in the distribution of disturbance towards the more often disturbed patch types (Figure 1b). One way of studying the effects of such changes is to analyse the landscape as being inhabited by subpopulations distributed over habitat patches, which together form a metapopulation (Levins, 1969). This approach assumes a collection of habitable patches embedded in an uninhabitable, but permeable matrix. Moreover, local populations in patches have a limited lifetime, some migration between patches is possible, and the balance between colonisation and extinction determines how large a proportion of the patches will be inhabited at one time. Consequently, the dispersal rate between patches and the rate of local population extinction are crucial parameters. When patches become too isolated, or local extinction rates too high, metapopulations may become extinct. Because many populations are naturally fragmented, metapopulation theory can also be applied to the analysis of more natural situations (e.g., Hanski and Gilpin, 1997). Recently, a metapopulation approach has also become prominent in analysing the influence of degradation on the biodiversity of tropical forests. Because many tropical species are specialised, rare, patchily distributed, and bad dispersers (Laurance, 1994), metapopulation structures are thought to be common. However, existing metapopulation approaches have two main shortcomings for investigating the effect of landscape degradation, especially of species-rich landscapes: (1) only one species is treated at a time and (2) patches have only two possible characteristics (habitable or not habitable).We obviously need to take a multitude of species into account when studying biodiversity, especially in the species-rich tropics. We have to focus on biodiversity, while retaining the realism of the metapopulation approach. Moreover, two states – hospitable vs. inhospitable, often equated with “undisturbed” vs. “disturbed” – will often not be an adequate habitat description. In reality, there will be a range of habitats differing in disturbance characteristics, while species also differ in their tolerance of, or dependence on, disturbance. The problems we are interested in relate not so much to how one particular species will react to the fragmentation of its habitat, as to how the biodiversity of a landscape will react to degradation that causes a change in the distribution or frequency of disturbance. In order to address these shortcomings, I investigate a multi-habitat landscape inhabited by a range of species, but analyse the individual species as metapopulations. This makes it possible to investigate the influence on biodiversity of different types of landscape degradation. 162 The Balance between Biodiversity Conservation and Sustainable Use of Tropical Rain Forests Landscapes are modelled that consist of a mosaic of habitat patches with different disturbance frequencies. (Figure 1a). Disturbance is a fact of life, but some habitats will be seldom disturbed, whereas others will be disturbed frequently. Metapopulation theory is adapted to model the fate of an ensemble of species that inhabit such mosaic landscapes and that differ in their reaction to disturbance. Each species is assumed to depend on a given habitat type (Figures 1c and 1d). Because disturbance frequencies depend on the type of habitat, species differ in the frequency of local extinction they experience. Some species will live in very stable habitats, but others in habitats that have a high disturbance frequency. It is assumed for simplicity that each species can use only one kind of habitat and that species do not interact. I first study the effects of different kinds of impact on single species, and then apply the results to the multitude of species in a mosaic landscape. a) Pristine landscape b) Degraded landscape Relatively stable habitats Often-disturbed habitats dominate dominate Often Rarely disturbed disturbed The habitat patch network of one of the species c) d) Figure 1 The landscape as a mosaic of different habitats that vary in disturbance rate. (a) pristine landscape, (b) degraded landscape, (c) and (d) the habitat network of one species in the pristine (c) and the degraded (d) landscape. 163 The Tropenbos Foundation, Wageningen, the Netherlands TIME LAGS IN INDIVIDUAL METAPOPULATIONS To analyse individual metapopulations I use the classical Levins model (Levins, 1969), which has the form (dn(t))/(dt) = cn(t)(N • n(t)) • en(t) (1) where occupancy n(t) is the density (or number) of occupied patches at time t, N is the total density of patches, c is the colonisation rate, and e is the extinction rate for an occupied patch. The equilibrium occupancies n* are n* = N –e/c (cN > e ) (2a) and n* = 0 (cN ¾ e ) (2b) Hence, when cN > e the metapopulation is viable; when N drops below e/c the metapopulation becomes extinct. Metapopulations react with a time delay to landscape changes, such as habitat destruction (decreasing N) or increased disturbance (increasing e) that change the equilibrium occupancy n*. The reason is that the basic processes responsible for metapopulation dynamics, local colonisation and extinction, take time. For example, at an extinction
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