7 : Climate, climate change, and species distributions: Continued
(iv) Rainfall
Global temperature, winds, and current patterns interaction to influence the global patterns of precipitation. The warm, moisture-laden air rising over the tropics cools as it rises and loses its capacity to hold moisture. Condensation and cloud formation result from the cooling of moist air which eventually results in rainfall or snow when the condensed moisture becomes to heavy to remain airborn. Tropical rains are heaviest when the sun is directly overhead and the rate of heating is most intense (the spring and fall equinoxes). Typically, therefore, tropical rainforest areas of the globe are concentrated between 0 and 30 degrees N and S latitude where the first Hadley Cells drive moist, warm air up into the atmosphere.
The opposite effect tends to occur in the so-called "Horse Latitudes", i.e. around 30-40 degrees N and S latitude (see the global circulation figure, above). These areas are zones where the former moist, warm tropical air descends as it cools (and becomes denser). As this air descends it warms up and increases in capacity to hold moisture. These warming and dry winds (the tradewinds) , therefore, tend to "dry-out" the land masses over which they blow and most of the globe’s great deserts are found in the latitudinal belt between 30-40 N and S (e.g. the Mojave, Sonoran deserts in North America, the Sahara, Gobi deserts of North Africa and Asia, respectively, the large desert area of central Australia – refer to the "climates of the world" figure if a few lectures ago).
Nothwithstanding these global patterns of climate. There can be significant variation in general patterns owing to local topography and interannual variation in climate. A good example of the former is the effect of local topography on temperature and rainfall known as the "rainshadow effect’’. In parts of northwestern North America, westerly winds bring warm, moisture-laden air to the coastal mountain regions. As the air rises over these mountains, it cools and tends to release loads of rain and snow on the western edge of these mountains (anyone living in Vancouver knows all too much about this!). As the now, dry air crosses mountain crests and descends, it warms and picks up moisture, again drying out the landmass. Consequently, areas like the Okanagan Valley and adjacent portions of Washington State tend to be quite "desert-like" as a result of the rainshadow effect.
Elevation changes also result in local temperature changes. For instance, below is a table showing the average January and July tempertures and average annual rainfall for two areas in Arizona (separated by only 25 km) in comparison to a region in coastal Oregon (some 1500 km distant). Note the greater similarity between the Tuscon and Salem sites despite the greater proximity of Tuscon to Mt. Lemmon. The elevational differences between the two Arizona sites result in very different local climates (and associated vergetation!).
Table: The effects of elevation on temperature and ppt levels
| Site | Elevation (m) | Mean Jan. temp (deg. C) | Mean July temp (deg. C) | Mean annual ppt (cm) |
| Tuscon, AZ | 745 | 10.8 | 30.7 | 27.3 |
| Mt. Lemmon, AZ | 2791 | 2.3 | 17.8 | 70 |
| Salem, OR | 60 | 3.2 | 19.2 | 104.3 |
A good example of variation on a scale of a few years is the so-called "El Nino" phenomenon. El Nino refers to the periods of sudden weather changes that occur owing to the strengthening of the equatorial countercurrents. These are the relatively short, west to east flowing currents right along the equator in the tropical Pacific (they run counter to the major North and South Equatorial Currents, north and south the of equator in the figure above). The exact cause of El Nino events, which have a periodicity of 5-7 years, is unclear, but may have to do with variation in solar output.
Regardless, the strengthening of these countercurrents (which also happen in the Atlantic on a smaller scale) result in massive increases in the flow of warm water up the coast of western North and South America. As westerly winds pass over these currents, they pick up moisture resulting in heavy ppt on the adjacent islands and continents particularly in the winter months when the land is cooler than the ocean. As an example of the "power" of such fluctuations in currents, consider that the Galapagos Islands experienced rainfall levels of less than 100 mm during drought years in the late 1970’s and early 80’s to over 1400 mm in the El Nino year of 1983. Further years of drought, including one year of no rainfall, occurred from 1984-86, followed by another El Nino induced drenching of over 600 mm in 1987. These changes in rainfall owing to El Nino had dramatic influences on the composition of the local vegetation and bird communities on the islands (see Grant and Grant, 1993, Proc. Royal Soc. Lond. B 251: 111-117 for further details is interested).
The above represents some of the basics of climate patterns and how they are generated on a global basis. We have already gained some appreciation of how biome distribution and coverage on the globe has changed over time naturally (e.g. owing to glaciation-related climate events). Temperature changes are likely the major determinant of such changes and associated changes in species composition. Even in the aquatic world, temperature appears to be a predominant force structuring communities. A recent study, published in Nature (Rutherford et al. 1999, Volume 400: 749-753) found that over 90% of the variation in Atlantic basin zooplanker community diversity could be explained by variation in sea surface temperature!
Shown below is a graph of the average surface temperature of the earth over the last 800,000 years. Note the frequent and relatively rapid changes that have occurred. The "valleys" are typically associated with the 20 or more glaciation events during the Pleistocene and the "peaks" with so-called interglacial periods (we are in one now!). From the end of the "Wisconsinan" glaciation to the present (some 18,000 yr) the temperature has risen about 4.5 deg. C with most of the increase occurring over the last 5000 yr. This works out to no more than a maximum of 1 deg. C every 1,000 yr.
Owing to the increase in so-called "greenhouse gases" in the atmosphere (e.g. CO2, CFC’s CH4, N02, etc – see the example fig below) associated with an increase in the human population (and associated industrial activity), most climatologists predict an increase in the temperature of the earth’s surface of an average of about 2.5 deg. C over the next 100 yr. This is a huge increase relative to that experienced during the recent interglacial period.
A key question for biogeographers is: given that temperature is such an important aspect of species distributions and community structure and that the world’s climate is probably changing more rapidly than natural "background" levels, can changes in species/community distributions be predicted? How would you go about trying? A hint is given in the figure immediately below.
Experimental data and bioenergetic models suggest that the phoebe is limited to areas south of the –4 deg. C isotherm because it cannot obtain and store enough energy during the day to thermally sustain itself overnight (when it is not feeding) when the nightime temp drops below –4 deg. C. This is a good example of what is known as the "climate space" of a species. The climate space refers to the critical climatic parameters that contribute to the fundamental niche of a species. It is most often a measure of the temperature and moisture (precipitation) requirements for the persistence of a species. The –4 deg. C isotherm above is almost perfectly associated (note the discrepancies) with the northern distribution limit of the phoebe.
The computer-based analyses that can try and estimate the climate space for a give species based on the climate conditions of its current range (e.g. "CLIMEX"). An example of this approach was taken by Samways et al. (1999) where they calculate so-called an "ecoclimatic index" for various species of coleopterans (beetles). The key parameters were temperature, ppt levels, and daylength (related to the developmental progress and growth). They tested the ability of the EI to capture the climate space by calculating an EI for areas not originally having records of each species, but which were subject to introductions of the various species. If the EI were able to predict the climate space well than those areas subject to introductions that had EI’s similar to source areas should have resulted in more successful establishment of beetles than areas with low EI’s (they are introduced as plant pest control measures). The results were mixed: only 4 species showed 100% concordance between predicted and actual establishment, 5 had less than 50% successful introductions despite high EI’s, and 6 species had between 50 and 90% of the introductions being successful despite high EI matching between source areas and those subject to introductions. Clearly, the climate space of a species may be a necessary, but not sufficient requirement for establishment.
The broader implication of such a study is that climate parameters may not necessarily be the dominant factor regulating the distribution of a species and hence, projections of distribution change based on climate change must be viewed cautiously!
This caveat was demonstrated experimentally by Davis et al. (1998) who showed that the abilities of three species of Drosophila to maintain viable populations in experimental systems depended on: (1) temperature, (2) dispersal between systems, and (3) the presence or absence of competitor species. One species (D. subobscura), for instance, could persist at 20 and 25 C only when by itself and only if dispersal was allowed from populations at cooler temperatures (i.e., it was a "sink" population). The species, however, was unable to maintain populations at 25 C (and those at 20 C were reduced) when a 30 C treatment group was added to the experiment (to simulate global warming). This was because the other two species dispersed to the 25 C and 20 C treatment groups and displaced D. subobscura. In nature, therefore, this could drive D. subobscura extinct in a warm, low latitude habitat, part of its geographic range where it would be predicted to occur when alone (Davis et al. 1998). See a nice review by Pearson and Dawson (2003).
Another survey-based approach is that of Sagarin et al. (1999) who surveyed an intertidal site in Monterey Bay, CA in the mid 1990’s and compared the distributions of over 40 species of invertebrates to those recorded at the same site some 60 years early by W.G. Hewitt. They found that most of the species that were lost or decreased in abundance ("deletions" in the figure below) from the more recent survey were "northern species" (i.e. those with extensive distributions north of Monterey Bay) while most new species or those that increased in abundance ("additions") were "southern species" (with distributions tending to be south of Monterey). So-called cosmopolitan species showed no clear trends (12 increased, 16 decreased).
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Over the same time period, air and shoreline water temperatures showed a slow, but detectable increase at the site.
These plots show the trends in maximum, mean, and minimum shoreline ocean temperature near the survey site (upper panel) from 1920-1995. El Nino years have shorter term rises in temperature indicated by the dark shading. Also shown are the seasonal fluctuations in temperature averaged for each month for the 13 years prior to the initial Hewitt survey (lower panel, heavy dashed line) and the 13 years prior to the 1990’s survey (heavy solid lines). The smaller dashed lines above and below the main lines represent standard error trend lines.
The authors suggested that global warming was responsible for the air/sea temperature rise and that the bias in increased abundance of southern species (adapted to warmer conditions) was a direct consequence of this warming.
Does this study convince you that global climate change can influence community diversity and structure as a general process? What are the limitations of the Monterey study (e.g. lack of replication)? What further information or surveys would you include if you were to conduct such a "test" of the effects of global climate change?
A final approach to the general issue is exemplified by the work of Parmesan and colleagues who have published some recent papers on the potential influence of global warming on butterfly distributions. In a 1996 paper, she described a survey of Edith’s checkerspot butterfly in western North America in which she compared historical records of populations with repeat surveys done from 1992-1996 (151 populations). A key prediction of global warming is that species affected should shift their ranges poleward and/or by an increase in altitude when considering the entire range of the species in question. This can be examined by looking at net extinction rates at the northern and southern margins of a species’ range. Parmesan found a significant increase in extinctions (extinct populations are encircled in the figure below) at low latitudes relative to higher latitudes and a significant increase in extinction at sites below 2,400 m relative to sites between 2,400 m and 3,450 m.
Table: Change in net extinction with change in latitude. Sites with populations in the 1992-96 survey were, on average 2 deg. Further N in latitude than sites with extinct populations.
| Latitudinal band (South to North) | No. of sites | Percent revisited that went extinct |
| 1 | 15 | 72 |
| 2 | 74 | 35 |
| 3 | 36 | 37 |
| 4 | 10 | 38 |
| 5 | 16 | 19 |
The final figures below show similar analysis for four distinct butterfly species in Europe (Parmesan 1996; Parmesan et al. 1999). The same pattern of northward shifting geographic ranges in independent lineages (species, in fact all were in different genera) strongly suggests that the pattern in not a random one and that global warming is a plausible cause.
Shift of Pararge aegeria from southern areas (in black and red, 1915-1969) to more northern areas (blue dots, 1979-97).
(a) left, loss of Carterocephalus palaemon in southern England (orange) shows range retraction (populations now extinct), current populations are shown in blue. Similarily in (a, right) the species appear to be a "non-shifting" one with range retraction in southern Finland (orange) and stable populations in the north or high altitude areas (blue).
Finally, (a, left) shows areas of range extension northwards (green) from pre-existing populations in Sweden and Finland (blue). In (b, left) Populations of Argynnis in this figure have been lost in some regions at the southern portion of the range in North Africa (orange dots, but still persist slightly farther north, blue dots).
(a, right) Heodes spp. Shows a retraction of its range near the Spain-France boarder (orange) and a slight increase in range northward into Estonia (green).
For a total of 35 butterfly species, 63% showed range shifts to the north (from 35-240 km) while only 3% shifted their ranges south. The average air temp. in Europe has increased by about 0.8 C this century (climatic isotherms have shifted by about 120 km north on average) which is similar to the range shifts of many species. The earlier example of the checkerspot butterfly in North America reported the mean position of populations had shifted by about 92 km north while isotherms have shifted north by a similar measure of 105 km.
The apparent consistency in response among species within a continent (Europe) and between continents (Europe and North America) is a powerful argument that butterflies are responding in a similar way to a common environmental change (even though each case, on an individual basis, may not be compelling).
Finally, a very interesting application of the climate envelope approach was illustrated by Cheung et al. (2008) who modelled fish species distributions as a consequence of global warming (average rise of 2.5C) and its consequences for the distribution of commerical fish catches and the resulting shifts in economic benefits among different areas of the East China Sea.
Climate and climate change: clearly important factors in animal distributions!
References:
Allen, J.R.M. 1999. Rapid environmental changes in southern Europe during the last glacial period. Nature 400: 740-743.
Brown, J.H. and M.V. Lomolino. 1998. Biogeography. 2nd Ed. Sinauer Assoc. Ltd. Chapter 3, pp. 39-46.
Cheung, W., C. Close, V. Lam, R. Watson, and D. Pauly. 2008. Application of macroecological theory to predict effects of climate change on global fisheries potential. Mar. Ecol. Prog. Series 365: 187-193.
Davis, A.J. et al. 1998. Making mistakes when predicting shifts in species range in response to global warming. Nature 391: 783-786.
Pearson, R. and T. Dawson. 2003.Predicting the impacts of climate change on the distribution of species: are bioclimate envelope models useful? Global Ecology & Biogeography 12, 361–371.
Parmesan, C. et al. 1999. Poleward shifts in geographical ranges of butterfly species associated with regional warming. Nature 399: 579-583.
Parmesan, C. 1996. Climate and species ranges. Nature 382: 765-766.
Sagarin, R.D. et al. 1999. Climate-related change in an intertidal community over short and long time scales. Ecol. Mono. 69: 465-490.
Samways, M.J. et al. 1999. Global climate change and accuracy of prediction of species’ geographical ranges: establishment success of introduced ladybirds (Coccinellidae: Chilocorus spp.) worldwide. J. Biogeog. 26: 795-812.
Thomas, C.D. et al. 2001. Ecological and evolutionary processes at expanding ranges. Nature 411: 577-581.
Rutherford, S. et al. 1999. Environmental controls on the geographic distribution of zooplankton diversity. Nature 400: 749-753.
Welch, D.W., et al. 1998. Thermal limits and vertical migrations of sockeye salmon (Oncorynchus nerka): long-term consequences of global warming Can. J. Fish. Aquat. Sci. 55: 937-948.