Habitat Loss and Fragmentation
83% of Earth s Land Surface Transformed by Human Activity Temperate Grasslands, Savannas, Shrublands: > 80% Loss Mediterranean Habitats: 72% Loss Coral Reefs: 20% Destroyed + 20% Degraded Mangroves: 35% Destroyed Ecosystems Adjacent to Oceans: 20% Highly Modified
The Human Footprint, based on population density, land transformation, accessibility, and electrical power infrastructure From: Sanderson et al. (2002)
6.10 Distribution of cultivated systems worldwide
Problems of Terminology Habitat Degradation (Alteration) vs. Habitat Loss (Destruction/Conversion) vs. Habitat Fragmentation At some point, changes in ecosystem quality (habitat degradation) lead to a quantitative decline (habitat loss)
Habitat Fragmentation Loss of habitat area Apportionment of remaining area into smaller and more isolated pieces Curtis (1956)
Can species adapt to fragmentation?
The theoretical background for much fragmentation research was provided by island biogeographic theory (MacArthur and Wilson 1967). However, many of the severe problems related to fragmentation (e.g., edge and matrix effects, ecological processes) are not considered by the theory (Laurance 2010).
Relaxation and the Extinction Debt A time lag exists between loss and fragmentation of habitat and extinction of species Many habitat patches contain the living dead Species with relatively low rates of population turnover ( slow species) will persist longer on small fragments, leading to under-estimation of extinction rates and a prolonged extinction debt.
7.7 Predicted species richness over time for land-bridge islands (A) and oceanic islands (B) Relaxation through extinctions Incremental colonization
Fragmentation Studies Researchers may see the final outcome of fragmentation without observing the process. Alternately, they may observe parts of the process, but not the long-term consequences.
Biological Consequences of Fragmentation Initial Exclusion Crowding Effects Insularization and Area Effects Isolation Edge Effects Matrix Effects Disruption of Ecological Processes The Special Problem of Roads
Despite decades of research, the effects of patch size and edge on animal populations are still debatable because patch size and edge are confounded in most studies. - Parker et al. (2005)
Large blocks of habitat are better than small blocks
From Fahrig (2001)
Threats to Small Populations Demographic stochasticity, i.e., random changes in birth and death rates Genetic deterioration (e.g., inbreeding) Environmental stochasticity, i.e., vulnerability to disturbances, weather, and catastrophes Social dysfunction (e.g., failure to breed) Deterministic threats (e.g., collecting, hunting, edge effects)
Woodroffe and Ginsberg (1998, Science) Our results therefore indicate that humaninduced mortality contributes more to the extinction of populations of large carnivores isolated in small reserves than do stochastic processes. Conservation measures that aim only to combat stochastic processes are therefore unlikely to avert extinction. Instead, priority should be given to measures that seek to maximize reserve size or to mitigate carnivore persecution on reserve borders and in buffer zones.
Blocks of habitat close together are better than blocks far apart
7.8 A constellation of separate habitat patches may be critical to the survival of individuals or populations
7.9 Many animals require a suite of different habitats or resources to meet life history needs
7.13 Roads can be significant barriers to the movement of small vertebrates and invertebrates
Habitat in contiguous blocks is better than fragmented habitat
Fragmentation disrupts the spread of natural processes, such as fire. In Florida, as little as 10% anthropogenic landcover (especially roads) caused a 50% decline in fire extent (Duncan and Schmalzer 2004)
Fragmentation-Sensitive Species Area-limited species (e.g., large home ranges, low density) Dispersal-limited species (e.g., non-vagile) Ecological specialists (e.g., resourcelimited or process-limited) Naturally rare species Interior (edge-avoiding) species Species vulnerable to human exploitation or persecution
One Thing we Know: Artificial Edge Effects are Bad Microclimate, regeneration environment, and mortality for plants changes near edges Vegetation structure changes near edges Some animal species avoid edges Other animal species thrive near edges (e.g. nest predators) and in the landscape matrix, to the detriment of sensitive interior species
Changes in soil quality and various reproductive parameters of an unpalatable plant, Arisaema triphyllum, with increased browsing by deer of a co-occurring palatable plant, Trillium grandiflorum, in Pennsylvania From: Heckel et al. 2010 Ecology
Nest Predators
7.11 Percent of experimental nests preyed upon as a function of distance from forest edge
Space
Findings from a synthesis of patch occupancy data from 89 studies of terrestrial fauna (785 spp.) (Prugh et al. 2008; PNAS) Patch area and isolation are important factors affecting occupancy of many species, but poor predictors of occupancy for most species (full area x isolation model: pr 2 = 25%). The patch/nonpatch dichotomy appears to be a gross over-simplification for many species in fragmented landscapes. Many studies reported a strong influence of habitat quality on occupancy patterns; if habitat quality varies among patches, patch area may be a poor proxy of population size and extinction risk (e.g., amphibians). Landscape context has a strong effect on occupancy patterns; for many species, improving matrix quality may lead to higher conservation returns than manipulating the size and configuration of remnant patches.
Interconnected blocks of habitat are better than isolated blocks
Fragmentation Research: Next Steps More large-scale experiments like Biological Dynamics of Forest Fragments project (Brazil) and Lago Guri (Venezuela) More work on effects of matrix quality on effective isolation for various species Research to disentangle habitat loss effects from fragmentation per se and identify thresholds Research to disentangle area effects from edge effects Research on effects of alternative land-use scenarios along real wildland-urban gradients