Climate Change Impacts on Mediterranean Ecosystems

“Mediterranean ecosystems are among the most threatened on Earth. More than 41 percent of their land has been converted to farmland and urban uses. Worldwide, only 5 percent of their natural area is protected.  Most people understand the plight of tropical rainforests, where habitat loss exceeds habitat protection by 2 to 1. In other words, for every acre of rainforest saved, two have been lost to conversion or development. In Mediterranean habitats, the disparity is much greater. For every acre of Mediterranean habitat saved, eight acres have been permanently lost” (Shaw, 2010).“By 2100, the Mediterranean biome is projected to experience the largest proportional loss of biodiversity of all terrestrial biomes due to its significant sensitivity to multiple biodiversity threats and interactions among these threats” (Klausmeyer and Shaw , 2009).      

According to the IPCC (2007) these Mediterranean ecosystems have the following vulnerabilities to climate change:    

• Warmer and drier conditions will force species to shift.
• These shifts imply a need for migration rates that will exceed the ability of many of these species.
• Land use, habitat fragmentation and intense human pressures will further limit natural adaptation responses.
• Fires may threaten specific species and plant functional types.
• Vegetation structural change driven by dominant, common or invasive species may also threaten rare species.
• Overall, a loss of biodiversity and carbon sequestration services may result in many of these regions.

Mediterranean climates are characterized by hot, dry summers and mild, wet winters.  These regions cover only 2.2 percent of Earth’s land surface, yet they account for 20 percent of all known plant species. Only the tropical rainforests of the western hemisphere and southeast Asia have a greater density of plant species (Shaw, 2010).  Mediterranean climates are found in only five places on Earth: California and northern Baja California, the basin of the Mediterranean Sea, southwestern Australia, the western cape of South Africa and the central coast of Chile as identified in the figure below.  

Mediterranean Ecosystems

 The IPCC (2007) concluded the following impacts to Mediterranean ecosystems:   

• California ecosystems are expected to be impacted by any warming of 2^oC or greater.  Desert and grassland expansion at the expense of shrublands, and mixed deciduous forest expansion at the expense of evergreen conifer forest are likely.
• Between 60 and 80% of current species are projected not to persist in the southern European Mediterranean region if the global mean temperature increases 1.8°C.
• Less rainfall is projected for some Mediterranean regions which will exacerbate drought conditions.  These conditions have already been observed in the eastern Mediterranean.
• Greater fire frequencies are likely the in Mediterranean Basin regions under a warmer and drier climate.  Double CO₂ climate scenarios increase wildfire events by 40-50% in California causing vegetation structural change in California (needle-leaved to broad-leaved trees, trees to grasses) and reducing productivity and carbon sequestration.
• Any carbon fertilization due to rising atmospheric CO₂ appears unlikely to have a major impact in Mediterranean ecosystems over the next decades, especially because of consistent projections of reduced rainfall.
• Range size reductions increase species’ extinction risks, with up to 30 to 40% facing increased extinction probabilities beyond the year 2050.

According to Klausmeyer and Shaw (2009) modeling of Mediterranean ecosystems by other researchers shows that in California, 66% of the plant species will experience >80% range reductions within this century and there is a projected 51–65% reduction in the mediterranean biome in South Africa by 2050.  However, the authors concluded that the models used in these studies were limited due to their use of just one or a few atmosphere-ocean general circulation models (AOGCMs).  For a better understand of future climate change impacts, Klausmeyer and Shaw undertook a biome-level analysis of global climate change using all 23 AOGCMs analyzed in the IPCC Fourth Assessment Report (2007).  Their analysis focused on the mediterranean climate extent (MCE) across all five regions based solely on climatic factors.  

Relative size of the projected future (2070–2099) to the current (1960–1989) MCE.

Projected status of the MCE in 2070–2099 relative to 1960–1989 under high (A2) emissions scenario

As the figures above show, the MCE is expected to expand globally but not uniformly.  The Mediterranean Basin and the Chile/Argentina regions are expected to have a large increase in the MCE while South Africa and Australia are projected to have fairly large contractions of their MCEs.  

This analysis shows that climate change puts areas with the some of the highest levels of plant diversity on the Earth at risk. The Mediterranean Basin, Morocco and Israel contain large areas of projected contraction with no adjacent areas of expansion. The mediterranean portions of Morocco contain almost 13 plant species per 1000 km², while Israel contains 200.  The cape region in South Africa and southwest Australia are the other two regions with large projected losses, and they contain 95.5 and 71 plant species per 1000 km², respectively. For comparison, there is 1 plant species/1000 km² in Europe, 6.5 in Brazil, and 40 in Columbia.  The current MCE in these four threatened countries contain 22,400 plants species, 12,925 of which are found nowhere else (Ibid).  

Climate change has already influenced the wine-making industry with a corresponding threat to Mediterranean ecosystems.  Climate impacts everything from harvest quality and quantity to whether a vineyard can produce whites or red varieties.  Because of the warmer climate, vineyards are being moved to higher elevations or to cooler coastlines.  These regions are home to the Mediterranean ecosystems.  For example, in Mendocino County, California, people are clearcutting to plant new pinot noir species at the expense of the natural species.  This trend is likely to continue under a warming climate where vineyards can be physically moved but the native species cannot (Lalasz, 2010).  

Giannakopoulos, et al. (2009) modeled climatic changes over the Mediterranean basin for the years 2031–2060, where a 2 °C global warming is most likely to occur  The authors investigated climate change impacts on human activities and natural ecosystems.  One such impact modeled was fire frequency.  The Canadian Fire Weather Index (FWI) was used because it is one of the most widely used indices of fire risk.  

The FWI consists of six components that account for the effects of fuel moisture and wind on fire behavior. These include numeric ratings of the moisture content of litter and other fine fuels, the average moisture content of loosely compacted organic layers of moderate depth, and the average moisture content of deep, compact organic layers. The remaining components are fire behavior indices, which represent the rate of fire spread, the fuel available for combustion, and the frontal fire intensity; their values rise as the fire danger increases.  Fire risk is low for FWI<15, and increases more rapidly with FWI>15. A threshold of FWI>30 was selected as a measure of extreme fire risk.  

the increase in the number of days with fire risk (top) and extreme fire risk (bottom)

The figure above (Ibid) shows the increase in the number of days with fire risk (top) and extreme fire risk (bottom).  According to this figure, the increase in the mean FWI translates to:  

•  2–6 additional weeks of fire risk (i.e. more than a month) over all land areas. 
• A significant proportion of this increase in fire risk is actually extreme fire risk (FWI>30).
• South of France, and coastal areas of the rest of Mediterranean Region: significant increase in the number of days with fire risk (1–4 weeks), but not in the number of extreme fire risk.

California Mediterranean ecosystems will also be impacted by fire.  Westerling and Bryant (2007) modeled wildfire risks for California under four climatic change scenarios.  GFDL and PCM global climate models and the A2 and B1 emission scenarios were compared for 2005–2034, 2035–2064, and 2070–2099 against a modeled 1961–1990 reference period in California and neighboring states.  Their results appear in the figures below and show that under either emission scenario, wildfires increase but there is a much more substantial increase under the A2 scenario.  

Percent change in the expected annual number of voxels (i.e., lat×lon×month) with at least one fire >200ha for (top) region (California + neighboring states) and (bottom) California only

Wildfires are not the only hazard for California’s ecosystems.  According to Trumble and Butler (2009), “the elevated carbon dioxide concentrations and increasing temperatures associated with climate change will have substantial impacts on plant-insect interactions, integrated pest management programs and the movement of nonnative insect species into California.  Natural ecosystems will also be affected by the expected changes in insect diversity. Many insects will alter how much they eat in response to changing plant nutrition. Also, we can expect increased problems with many pest insects as they develop more rapidly in response to rising temperatures.”  The table below (Ibid) shows examples of how increasing atmospheric carbon dioxide affects plant-insect interactions.  

Examples of how increasing atmospheric carbon dioxide affects plant-insect interactions

Many plants have two types of chemical defenses that reduce or stop insect feeding. One group of common plant defenses is nitrogen-based compounds (such as alkaloids and cyanogenic glycosides) that either act as toxins and kill the insects or act as repellents and make the plants unpalatable.  In natural ecosystems that have limited nitrogen availability, plants may have lower levels of nitrogen-based toxins and so be subject to greater insect damage (Ibid).

References for this post.