Mean temperature is one of the major influences that drive species distribution, especially along an altitudinal gradient. Change in species distribution is one of the consequences of global warming because of poleward and upward shifts across a wide variety of taxa. The global distribution of biodiversity and climate change scenarios predict greater than average warming in the tropics, which results in the loss of current climate conditions. This will cause the rate of species extinctions to increase, suggesting that global warming may be one of the most serious threats to tropical biodiversity. Inner tropics species cannot adapt to climate change by latitudinal shifts because of the lack of latitudinal temperature gradient within the tropics. Plants regulate water cycle, secure slope stability, and provide microhabitats for other species. Evidence from the past provides knowledge that tropical species react to climate change by altitudinal range shifts. Kreyling et al. (2010) illustrates the potential altitudinal range shifts using a model that accounts for the expected warming by shifting the altitudinal range of species according to the shift of isotherms. Four ecological challenges were identified when applying this model to a selected study area. This includes lowland attrition, range shift gaps, range contraction, and extinction. The model was applied to a data set of altitudinal vegetation surveys in southern Ethiopia. The three hypotheses tested were (1) global warming will result in lowland attrition, range shift gaps, range contractions, and increased extinction risks of the plant species; (2) some groups of plants face a higher than average risk because of their current altitudinal distribution; (3) endangered species are most vulnerable to climate change. The study area was located in a physiologically diverse area of the southwest Ethiopian highlands.—Lauren Lambert
Kreyling, J., Wana, D. and Beierkuhnlein, C. (2010), Potential consequences of climate warming for tropical plant species in high mountains of southern Ethiopia. Diversity and Distributions, 16: 593–605
It is assumed that species that currently occupy warmer habitats at their lower range limit may be able to shift to cooler places at the same elevation as the climate warms. The model does not account for other parameters such as change in moisture or land usage but is able to identify vulnerability to warming in a given landscape.
During fieldwork, a total of 475 species belonging to 101 plant families were encountered which represented about 46% of the known species belonging to the data set of the study area. These species are predicted to be subjected to range shift gaps that are already under warming scenarios, and extinction risk will potentially increase under warming scenarios of 3.5°C or higher.
A temperature of about 1°C is predicted to increase the risks of lowland attrition and range shift gaps for the study area. If warming does exceed 3.5°C, extinction of surrounding species will become more likely. Tropical lowland species are already living at their thermal optimum. A forecast of an increase in primary productivity may lead to an increase in the frequency of fires. Since the plants existing in the study area are not adapted be fire tolerant, this will increase the extinction risk and rate of these species, and therefore will accelerate lowland attrition.
Range shift gaps are related to the speed of warming and the concern about species’ responses on the rate of warming. Species that must face range shift gaps will need to compete with the inhabitants of the new potential range. Vulnerability is particularly high for species that are restricted to narrow altitudinal ranges, especially those that are at high altitudes. For example, the fern growth form appears to be more susceptible to climate change than others because it is restricted to high altitudes. This evidence is in contrast to what is found in cloud forest transects, suggesting that differences in vulnerability are regionally specific. Herbs and ferns are predicted to be the most affected by climate change, and plant families with high species richness are more resilient to extinction because of their numbers. The potential response of species belonging to Fabaceae and Poacae might have a negative reaction to global warming. This is of concern because of their economic and ecological importance. The loss of these species may result in the loss of nitrogen fixation and range resources in lowland semi arid ecosystems of the tropics.
The adaptation policies in Ethiopia are focused on agricultural production, encouragement of shifting management strategies, and changing target crops (Bryan et al. 2009). It is a definite possibility that the agricultural zone will move upslope faster than the species can adapt to. The habitat may not be suitable for existence of these species in that zone. For example, the soil might not be developed to a point that is required by these species that are moving into the zone. The process is extremely slow and climate change is increasing the speed of upward shift. This is expected to lead to an increase in extinction rick and decline in population size and phenotypic diversity of the inhabitants.
It is apparent that global warming is a threat to tropical biodiversity and that lowland attrition is emerging as an urgent challenge of importance in the tropics. It is unclear to what extent lowland species are able to tolerate a warmer and drier climate than they are currently experiencing, but altitudinal shift as a result of climate change is in fact a risk that all species must take.