When I ask my introductory biology or ecology students what they think the biggest threat to Earth’s biodiversity is, climate change or pollution typically get the most votes. Perhaps the (much warranted) public attention and debate on these issues leads students to focus on these particular problems, but in fact, habitat loss and degradation have the largest impact on biodiversity. Further, many of the other major threats to biodiversity (e.g., climate change, pollution, etc.) can be directly or indirectly linked to habitat loss.

It is easy to connect complete habitat loss to loss of biodiversity – if a forest is cleared of trees, the diversity and abundance of associated flora and fauna will likely be reduced. But what about fragmented habitat? How does the size and shape of patches of forest or grassland influence remnant communities and ecosystems? The theory of island biogeography gives us hypotheses of how fragment size and isolation might influence populations and diversity. However, habitat fragments are embedded in an anthropogenically-influenced landscape and just how much that landscape influences the structure and function of the remaining habitat is an important question for conservation.

Nick Haddad and colleagues recently reported on the state of the world’s fragmented habitats; including a meta-analysis of long-term experiments specifically designed to test how area, isolation, and edge (distance to perimeter) of fragments effect the remaining communities and ecosystems. High-resolution satellite data revealed that 20% of the world’s forests were within 100 meters of a forest edge and 70% were within 1 kilometer, meaning most forests today are in close proximity of human activity.

A series of long-term (20+ years) habitat fragmentation experiments (see below), spanning multiple continents and biomes, have provided a data set of 76 studies testing how this proximity to human activity influences ecosystems. Specifically, this synthesis enabled Haddad et al. to test the effects of reduced habitat area, increased isolation, and increased habitat edge on a variety of community and ecosystem variables. Not surprisingly, all three treatment variables had negative effects on processes such as organismal abundance, species richness, pollination, nutrient retention, etc. and reduced habitat area and increased isolation appear to have the strongest effects.

Most striking however, was the accumulated long-term consequences of habitat fragmentation. By comparing changes in species richness, immigration, and ecosystem functions (e.g., biomass, total organic carbon, etc.) over time, a delayed effect of fragmentation appeared. That is, the proportional (negative) change in community structure and function increased over time. The negative effects of habitat fragmentation are not necessarily seen immediately after deforestation, and those effects may get worse over time – extinction and ecosystem function debts yet to be realized.

Large, expanses of forest still exist in South America, Africa, and boreal regions. Given that biodiversity loss itself can have strong detrimental effects on ecosystems, that climate change will likely exacerbate effects of fragmentation, and our economic incentives for protecting habitat, the analysis by Haddad et al. present a strong argument for maintaining these large stretches of uninterrupted forest.

Long-term experiments included the meta-analysis:

Biological Dynamics of Forest Fragments (Brazil, in Portuguese)

Kansas Fragmentation Experiment (USA)

Wog Wog Fragmentation Experiment (Australia)

SRS Corridor Experiment (USA)

Moss Fragmentation (UK, Canada)

Newly established experiments:

Metatron (France)

S.A.F.E. Project (Borneo)

April 1, 2015