Habitat Loss, the Dynamics of Biodiversity, and a Perspective on Conservation

TLDR: Habitat loss has been, and still is, the greatest threat to biodiversity; the conversion of natural habitats to agricultural land, pastures, plantations, built areas and infrastructure continues, propelled by increasing human population size and by accelerating demand for resources.

Abstract: Habitat loss has been, and still is, the greatest threat to biodiversity (Brooks et al. 2002; Hanski 2005; Groom et al. 2006). According to the Millennium Ecosystem Assessment (2005), more than half of several biomes, including the Mediterranean and temperate forests and tropical and sub-tropical dry broadleaf forests, had been converted by 1990; in Western Europe, only 2–3% of original forests remain in natural or natural-like condition (WWF Report 2001). Zooming into more detailed classifications of habitat does not change the picture. As an example, a recent in-depth assessment of changes in the quality and quantity of 368 habitat types in Finland (Raunio et al. 2008) classified the vast majority either as threatened (189 habitat types) or near threatened (105), while only 74 habitat types were considered to be of least concern. The conversion of natural habitats to agricultural land, pastures, plantations, built areas and infrastructure continues, propelled by increasing human population size and by accelerating demand for resources. It is self-evident that populations and species will suffer when their habitat becomes degraded or is lost completely. Nonetheless, many issues concerning the response of biodiversity to habitat loss and fragmentation are less clear-cut or they are not widely appreciated. These issues include non-linearity in the ecological response of species to habitat loss and fragmentation at the landscape level, about which I have more to say in this article. The response of species to habitat loss and other environmental changes is typically not instantaneous, particularly not when we consider changes at large spatial scales. Habitat loss leaves large numbers of species to gradually decline and go extinct. If we are not aware of this “extinction debt” (Tilman et al. 1994) we are prone to underestimate the level of threat to biodiversity (Hanski and Ovaskainen 2002). Habitat loss often involves deteriorating habitat quality, either due to intentional changes in land use, such as the conversion of natural boreal forests to intensively managed forests in northern Europe, or due to unintentional damage, exemplified by increasing edge effects with decreasing area and increasing fragmentation of habitat. Hundreds of studies have examined the relative roles of habitat quality, habitat fragment area and connectivity (inverse of isolation) in influencing the occurrence of species in fragmented landscapes (reviewed by Fahrig 1997, 2003; Prugh et al. 2008). Unfortunately, much of this work is of limited value as it does not adequately recognize that the relative roles of habitat quality, fragment area and connectivity depend greatly on landscape structure and heterogeneity, and hence there cannot be a universal answer to the question “which is more important” (Hanski 2005). Habitat loss and fragmentation have genetic and evolutionary consequences. I shall touch below the question about reduced viability of small and fragmented populations due to inbreeding and random fixation of deleterious mutations. Concerning the evolutionary dynamics, habitat loss and fragmentation are likely to alter many components of natural selection and hence lead to evolutionary change. A prime example is selection on dispersal: several costs and benefits of dispersal are affected by the spatial structure of populations and hence by the physical structure of the environment, which are modified by habitat loss and fragmentation. Whether the net effect is increased or decreased rate of dispersal has been much debated (Heino and Hanski 2001; Ronce and Olivieri 2004), and once again it is apparent that there is not a single answer (Hanski 2005). Furthermore, whatever the answer in a particular case, there is no basis to assume that the evolutionary change would necessarily increase the viability of populations and metapopulations. It is even possible that evolutionary changes increase the likelihood of population extinction (Gyllenberg et al. 2002), though luckily “evolutionary suicide” is more of academic interest than a cause for real concern, even if some convincing examples were reported. The year 2010, the United Nations’ International Year of Biodiversity, was supposed to be the turning point in the loss of biodiversity, but a comprehensive report (Butchart et al. 2010) shows that the 2010 target was not met, biodiversity continues to decline, and the indicators reflecting the various pressures on biodiversity continue to increase. At hindsight, the 2010 target was overambitious and vague, there was no clear idea of how to reach it and how to measure success. The new target year is 2020, and we are now wiser, we have metrics and more specific measures that facilitate reaching the goal. Major subsidiary targets relate to the questions how much habitat should be protected and where. In this article, I first discuss the consequences of habitat loss and fragmentation for the ecological viability of metapopulations at the landscape level with a focus on extinction thresholds (the critical minimum amount of habitat that is necessary for long-term persistence of metapopulations). I argue that apart from the amount of habitat, the degree of fragmentation at the landscape level makes a significant difference. The next section gives a brief synopsis of the genetic factors that threaten long-term viability of populations and metapopulations (inbreeding depression and fixation of deleterious mutations that lead to a permanent reduction of fitness). Based on these biological considerations, I put forward an option for habitat conservation that represents, in my opinion, a cost-effective and realistic approach. This approach could make an important contribution towards reaching the target for conservation agreed in the UN biodiversity summit in Nagoya in 2010 that aims to put an end to the decline of biodiversity by 2020.