Terrestrial biology over the next 100 years

This page is part of the topic Biological responses to 21st climate climate change

Antarctica is extremely isolated and unusually cold as a result of its polar location and ice sheet. As a consequence, climate change will impose a complex web of threats and interactions on the plants, animals and microbes living in the ice-free areas of Antarctica. Increased temperatures may promote growth and reproduction, but may also contribute to drought and associated effects. Furthermore, high amongst future scenarios is the likelihood of invasion by more competitive alien species, easily carried there by humans seeking a place of unspoilt wilderness or chasing scientific knowledge. Such invasions are already a reality on many of the sub-Antarctic islands, with consequential and sometimes drastic consequences for the structure and functioning of native biota and ecosystems (Table 4.2) (Frenot et al., 2005, 2008; Convey et al., 2006b). These invasions carry a clear warning for the future of terrestrial ecosystems on the Antarctic continent where, although a small number of alien species has already become established, none have yet become invasive (Convey, 2008). While dispersal and range changes are also natural processes, sub-Antarctic data indicate that human assistance outweighs the natural frequency of such events by two or more orders of magnitude. Furthermore, regional environmental change in the sub- and maritime Antarctic is likely to act in synergy with anthropogenic transfer, lowering the current barriers to both transfer and establishment that have previously protected Antarctica. Antarctica contains some of the only places on Earth where natural biological phenomena can be studied in their pristine state, but human visitation risks breaking Antarctica’s isolation, and threatens Antarctica’s unique legacy.

The consequences even of direct environmental changes might not always be easy to ascertain. For example, many sub-Antarctic islands show increases in mean annual temperature (Bergstrom and Chown, 1999). To date, there has been no suggestion that, even at the microclimate level, the increases are likely to exceed the upper lethal limits of most arthropods. However, in some areas, such as Marion Island, it is not only mean temperature that is predicted to change in line with current trends. Rather, the frequency of freeze–thaw events and occurrence of minimum temperatures are also predicted to increase, because of a greater frequency of cloud-free skies and a lower frequency of snow (which is a thermal insulator) (Smith and Steenkamp, 1990; Smith, 2002). An increase in the frequency and intensity of freeze–thaw events could readily exceed the tolerance limits of many arthropods, as recent work both on Marion Island and other south temperate locations has shown (Sinclair, 2001; Sinclair and Chown, 2005; Slabber, 2005). Other biota, such as continental bryophytes and lichens, may also be pushed beyond their tolerance limits if freeze–thaw frequency increases, especially given the physiological effects of this stress, such as the loss of soluble carbohydrates (Tearle, 1987; Melick and Seppelt, 1992). Thus, one of the major consequences of climate change might paradoxically not be an increase in upper lethal temperature stress, but rather an increase in stress at the other end of the temperature spectrum. How organisms are likely to respond to this kind of challenge has not been well investigated, though it is clear that lower lethal temperatures show substantial capacity for both phenotypic plasticity and evolutionary change.

In ice–dominated continental and maritime Antarctica, changes to temperature are intimately linked to fluctuations in water availability. Changes to this latter variable will arguably have a greater effect on vegetation and faunal dynamics than that of temperature alone (Convey, 2006). Future regional patterns of water availability are unclear, but increasing aridity is likely on the continent in the long-term (Robinson et al., 2003). Plant species which show high tolerance of desiccation, such as the moss Ceratodon purpureus, or others such as Bryum pseudotriquetrum, which have a high degree of physiological flexibility with respect to tolerance of desiccation, are more likely to persist under increased aridity than the relatively desiccation-sensitive and physiologically inflexible Grimmia antarctici (Wasley et al., 2006). Changes to water availability that cause an increased frequency of desiccation events are likely to negatively impact more strongly those species requiring hydrated habitats (hydric species) than those adapted to surviving shorter or longer periods of water stress (mesic or xeric species) (Davey, 1997).

In many respects, Antarctic terrestrial organisms are often well-adapted to the stresses of a highly variable environment, possessing features that should permit them to handle predicted levels of change that are often small compared with the natural variability already experienced. Indeed, with reference to temperature increase, resident biota will often be able to take advantage of reduced environmental stress, which will allow longer active periods/seasons, faster growth, shorter life cycles and population increase. Impacts of increased water availability are expected to be similar, although in both instances exactly the reverse consequences can be experienced locally, either directly as a result of decreased water input, or of interactions between increased temperature and water leading to greater evaporation and desiccation stress. Impacts of increased UV-B exposure associated with the spring ozone hole, while subtle, are expected to be negative.

With increases in the temperature component of current climate change in many locations of the Antarctic, many terrestrial species may respond positively by faster metabolic rates, shorter life cycles and local expansion of populations. But subtle negative impacts can also be predicted (and are perhaps being observed) with regard to increased exposure to UV-B, as this requires greater allocation of resources within the organism to defense and mitigation strategies, reducing resources available for other life history components (Convey, 2006; Hennion et al., 2006; Robinson et al., 2005; Snell et al, 2009). Changes in water availability will also impact on both terrestrial and more stable limnetic environments. Local reduction in water availability in terrestrial habitats can lead to desiccation stress (Convey, 2006) and subsequent changes in ecosystem structure, as has been reported from Marion Island where there have been dramatic changes in mire communities associated with a substantial decrease in rainfall (Smith, 2002).

The selective pressures experienced by Antarctic terrestrial biota over evolutionary time have resulted in adaptations with emphases on stress tolerance, plasticity and variation in life histories (Convey, 1996a). These adaptations have been at the expense of reduced competitive ability, leaving Antarctic ecosystems vulnerable to the impact of colonization by competitors that may be at more advantage under changed climatic conditions (Bergstrom and Chown, 1999; Convey et al., 2006b). These competitors may be either naturally dispersed or have ‘hitch-hiked’ with humans. As evidenced by the rapid increase in numbers and impacts of non-native species on the sub-Antarctic islands, the frequency of transfer by human agency (anthropogenic introduction) appears to far outweigh that by natural dispersal, not least as it overcomes the ‘dispersal barrier’ presented by the geographical isolation and survival of environmental extremes required in transit (Frenot et al., 2005, 2008; Whinam et al., 2005; Convey et al., 2006a; Convey, 2008). Furthermore, the combination of increased human visitation across the entire Antarctic region, and the lowering of dispersal and establishment barriers implicit through climate warming, are expected to act synergistically and result in a greater frequency of both transfers and successful establishments.

Changes in temperature, precipitation and wind speed, even those judged as subtle by climate scientists, will probably have profound effects on limnetic ecosystems through the alteration of their surrounding catchment, and of the time, depth and extent of surface ice cover, water body volume and lake chemistry (with increased solute transport from the land in areas of increased melt) (Quesada et al., 2006; Lyons et al., 2006; Quayle et al., 2002, 2003). Indeed, Quayle et al. (2002, 2003) show that some Antarctic lake systems magnify the already strong signal of regional climatic warming centered on the maritime Antarctic. Predicted impacts of these changes will be varied. A common factor is the changing influence of reduced lake ice and snow cover, which exert strong controls on the abundance and diversity of the plankton and periphyton (Hodgson and Smol, 2008). With increased warming, more of the lake is made available and production increases. Once the central raft of ice melts completely, the plankton and benthos can flourish, and diversity at all levels of the ecosystem increases. In shallow lakes, lack of surface ice cover will also lead to increased wind–induced mixing. In some areas of East Antarctica, longer periods of open water have led to increased evaporation and, together with sublimation of winter ice cover, have resulted in rapid increases in lake salinity in the last few decades (Hodgson et al., 2006c).

Increased inputs of melt water into the upper stratified layer of deeper lakes may also increase stability, and this, associated with increased primary production, will lead to higher organic carbon flux. Such a change will have flow–on effects including potential anoxia, shifts in overall biogeochemical cycles and alterations in the biological structure and diversity of ecosystems (Lyons et al., 2006). The predictions of Lyons et al. (2006) also serve to illustrate a profound paradigm shift in Antarctic biology that has occurred in the last 20 years. Although they and Convey (2006) state that we are not yet in a situation where we can develop a quantitative predictive model (or even models) that completely qualifies the response of Antarctic ecosystems to climate change, many of the predictions currently made are based on a foundation of long-term studies and monitoring, such as those at the McMurdo Dry Valleys LTER, or British Antarctic Survey sites on Signy Island. The importance of such long-term programmes cannot be overstated, particularly in national and global research funding environments increasingly predicated on ‘short-termism’.