Metapopulations and habitat loss

In the previous chapter we examined how the movement of organisms affects their spatial distribution and their demographics. However, the landscape in which they were able to disperse was always considered as uniform, though possibly bounded. But the hypothesis of homogeneity is very often unrealistic for two reasons.

The first - and increasingly dramatic - is that human activities are negatively affecting even the wildest and most pristine regions of the planet. One the most important consequences is fragmentation, that is the phenomenon of species habitat being divided in fragments (or patches, as they are frequently termed), which are immersed in a matrix of inhospitable territory. Perhaps, the most impressive testimony to the large spatial scale at which habitat destruction perpetrated by man operates is the fragmentation of the Amazon forest, the biggest green lung of our planet. It suffices to use an Internet browser, connect to one of the many sites providing Earth maps obtained via remote sensing and enter the string Rondônia, Brazil to directly ascertain what has been occurring in the recent past. The 400 km long and 100 km wide area that connects Alto Paraıso to Primavera de Rondônia is organized according to a geometric configuration that is so regular that there is no doubt about being the result of human intervention. In fact, an array of concurring circumstances, combined with a prolonged growing season and the low cost of land and labour, has indeed favoured the rapid conversion of entire regions of the Brazilian rain forest into soy-bean fields (Fearnside, 2001). The collapse of the Peruvian anchoveta fishery (see Chapter ) has encouraged the use of soy in animal diet in both Europe and the United States, while an unexpected frost event in southern Brazil in 1975 has favoured the abandonment of coffee cultivation. The impact in terms of land fragmentation is not limited to the fields where soy-bean is planted. The construction of eight industrial waterways, three railways and many highways, all funded by the government to facilitate the production, transport and export of soy-bean has produced an inevitable avalanche effect, efeito de arraste in Brazilian. There have been many private investments (which have doubled the already substantial public investment) into felling trees or breeding cattle, additional elements of destruction and fragmentation. The Rondônia area unfortunately is not the only region of South America being subjected to such a destruction, as demonstrated, for example, by the satellite photos of the region around San Pedro Limón, Bolivia. Not always the destruction of habitats is detectable by remote satellite sensing, because land-use change can still leave areas green, but devoid of the species that are key to the conservation of biodiversity in a given ecosystem. For example, looking at the region of the Gunung Palung National Park, Kalimantan, Borneo, one still perceives those areas as being characterized by contiguous and uniform vegetation. However, the exploitation of the precious timber provided by the trees of the family Dipterocarpaceae in the period 1985-2001 caused the destruction of 56% of the Indonesian Borneo lowland forests (Curran et al., 2004). The plant species that now cover these areas are thus different from the original ones. If one considers that the Dipterocarpaceae are fundamental for 60-80% of birds and mammals living in that biome, the consequences for the protection of both plant and animal diversity are certainly dramatic. What is more dismaying is not only the progressiveness but also the rapidity at which destruction occurs. Even if Kalimantan is a protected area, the rate of deforestation in the park has increased by 9.5% annually from 1999 to 2004. Unprotected areas have even higher rates. And the destruction does not hit forests only, and not the tropics only.

A second reason to abandon the hypothesis of uniform landscape is that even territories without significant anthropic influence that may appear homogeneous at a certain spatial scale or for a certain species may not be so at other scales or for other species. For example, many butterflies are strict specialists - every mother chooses with meticulous care the plant on which to lay her eggs, the only one that can actually serve as food for the offspring (caterpillars). The females of the species Euphydryas editha bayensis, for example, emerge as adults in spring and lay their eggs almost exclusively on annual plants of the species Plantago erecta. These grow mostly on serpentine soils which have very special chemical characteristics (low Mg/Ca ratio and high concentration of heavy metals). Not only the distribution of these soils is very fragmented in Northern California, as shown in Fig. 1, but both the spatial pattern and the area of ​​the different fragments is quite varied and uneven.

Figure 1: Distribution of serpentine grasslands in Santa Clara County, California. White arrows indicate the locations of nine small populations of the butterfly Euphydryas editha bayensis (inset) measured via mark-recapture techniques in 1987. These local populations, with the Morgan Hill population in addition, constitute the metapopulation studied by Harrison et al. (1988).

In particular, the figure shows that there exists a fragment of quite big size (Morgan Hill), where the butterfly population has always been present and plentiful. In other smaller fragments, instead, several extinctions of the butterflies have been recorded. We might call these extinctions local in both space and time, because at a later date some of the fragments that became empty were actually colonized again, as shown in the figure. This kind of dynamics in space and time regarding species spread over a fragmented landscape and subject to frequent local extinctions and colonizations is termed metapopulation dynamics in the ecological literature. Ehrlich and his colleagues have studied the dynamics of this natural metapopulation for about thirty years, starting from the 1960's (Ehrlich, 1965). In more recent years, it has been noted that the geographic range of the species is moving towards North (about 2$^o$ on average) in response to climate change (Parmesan, 1996).

The concept of metapopulation was proposed by Richard Levins (1969) who introduced a model that we will present later and has become a paradigm for conservation ecology. Curiously enough, this model was intended to identify control policies for the eradication of agricultural pest. As is known, most of the pests can spread on a large scale into a territory covered by crop fields thanks to their autonomous motion capacity, via both active movement (as some insects do through their flight) and passively (such as spores transported by the wind). In this regard, Levins remarked that the agricultural area on which to intervene in order to control an infestation is in general more extended than the single population of parasites (namely the single infested field or even the single infested plant). Thus Levins wrote “the control strategy must be defined for a population of populations in which local extinctions are balanced by emigration from other populations”.

As suggested by the etymology of the word, the term meta-population refers to a population whose constituent parts are not individuals, but other populations which are called local (or sub-populations). We can say that while the term local population refers to organisms that inhabit an island of favourable habitat immersed in inhospitable surroundings, with metapopulation we mean the residents of the entire archipelago of habitats. Therefore there exist two spatial scales at which the dynamics of a metapopulation must be described and studied. One is related to the demographic events - birth and death processes as studied in chapter  - which take place exclusively at the level of local populations. The other spatial scale concerns the events of dispersal of individuals that move (or are moved by external vectors) between the different fragments and is thus termed global scale. The case of insect pests is particularly appropriate for emphasizing the difference between the two spatial scales. We could in fact say that there is extinction (tout court, without further specification) when all occurrences of local pests have been eradicated. Conversely, if even a single individual in a single local population does survive, then we say that there is persistence of the metapopulation. In the case in which the fragments are of small size compared to the needs of the species being analysed, local extinction can be quite frequent. Therefore, the likelihood for the metapopulation to escape extinction is highly dependent on the species ability to disperse and colonize empty fragments.

Although the model was originally introduced by Levins with the aim of understanding how to destroy an unwanted population, its importance to conservationists is clear for two reasons. First of all, as we shall see below, it provides insight into the ecological conditions under which a population living in a fragmented environment is doomed to extinction. Second, this model has a very general validity because it allows the application of the island paradigm inherent to the metapopulation concept not only to the case of terrestrial pests, but to many other situations of clear ecological interest, which may seem to be non fragmented while they actually are. Quoting from another article by the same Levins (1970), “...the distribution of many species even on the mainland is insular. Mountain tops, lakes, individual host plants, a fallen log, a patch of vegetation, a mammalian gut, or, less obviously, a region of optimal temperature or humidity are all islands for the appropriate organisms. Therefore the insular model is much more broadly applicable.”