Biological response of birds and mammals

This page is part of the topic Marine biology over the next 100 years

Biological responses to increasing climate-driven habitat and ecosystem fluctuation, and hence to climate change, can be articulated along four major axes: individual physiology and behaviour; species distribution; community structure; and ecosystem dynamics (Walther et al., 2002). Focussing on individual physiology and behaviour is important, because that is the level at which natural selection works, and other responses ultimately depend on physiology and behaviour.

The most observable changes in physiology and behaviour are the changes in phenology (Berteaux et al., 2004), which is the annual timing of life history events in populations (migration, arrival to and departure from breeding or feeding grounds, and reproductive events). These changes are thought to evolve by natural selection to match the environmental conditions and maximize the fitness of individuals (Futuyma, 1998). Given the extreme seasonality of high latitudes, phenology is a key aspect of the adaptation of Antarctic organisms and populations to change, and can be used to evaluate the match or mismatch in variability and trend between the rates of environmental change and of phenological response. A match between rates will be likely to occur with a stable optimum mean population fitness, and a mismatch to occur with decreasing fitness (Futuyma, 1998; Berteaux et al., 2004). Thus, measuring mean population fitness in long-term studies, which is more difficult than monitoring phenological changes, will be essential to identify and characterise responses of organisms to change.

Organisms will depend on their degree of phenotypic flexibility to cope with environmental change, or will adapt through changes in gene frequencies between generations, which at the population level is known as microevolution. Changes in gene frequencies are irreversible and will permanently modify their phenotypes. Though evolution is generally thought to be a slow process, microevolutionary changes can occur fast in response to climate change (e.g. Berteaux et al., 2004). Microevolution has been singled out as the main driver of Adélie penguin responses to extreme habitat changes caused by giant icebergs (Shepherd et al., 2005). This suggests that studies of fitness-related phenotypic traits collected over time and among related individuals of the same population may help detect evolutionary responses to climate change. Because microevolution occurs across generations, the typically long generation times of most predators are likely to affect their evolutionary responses (Rosenheim and Tabashnik, 1991). In predators, changes in optimum phenotypes of a trait from past adaptations will likely come with a fitness cost in survival or fecundity or in both. These short-term demographic costs may not be compensated fast enough by long-term adaptation, in which case populations are likely to decline (Stockwell et al., 2003).

Insights from long-term studies

The modification of the physical and biological environments around the Southern Ocean is accepted to be a direct or indirect consequence not only of changes in the mean climate, but also of the variance in climatic conditions at different spatial and temporal scales (Trathan et al., 2007). This variance has increased since the 1970s and may continue to increase with the frequency of extreme climatic events expected under many simulated scenarios (IPCC, 2007). The consequences for Antarctic birds and mammals are likely to be an increased variation in life history traits, which may have repercussions for fitness, population density and distribution.

Recent studies of phenological change in Antarctic seabirds with climate change indicate a multi-species delay in mean arrival and egg-laying dates in the Indian Ocean sector of the Southern Ocean (Barbraud and Weimerskirch, 2006). Of the seabird guilds studied, the fitness of three populations - emperor penguin, snow petrel and southern fulmar - has also been evaluated (Jenouvrier et al., 2005a). These species show a stable population growth to date, and only southern fulmars show a significant phenological change, which is a delay in date of arrival to the nesting grounds. In southern fulmars, however, about 4% of the population growth rate may be caused by immigration, and without it the asymptotic growth rate (or mean fitness) becomes negative (Jenouvrier et al., 2003). This could indicate a mismatch between the rate of phenological change and of environmental change, leading also to an increased sensitivity of fitness to the environment. Indeed, this population, like those of the other two species fluctuates with the Southern Oscillation Index (and therefore with ENSO) and with increased sensitivity to sea ice concentration and high SST (Jenouvrier et al., 2005b). Because the propagation of ENSO effects to the southern Indian Ocean is most likely to have ecosystem-wide effects, as opposed to direct effects on the weather, increased ENSO variability is likely to bring more frequent food shortages. Extremely cold weather can sometimes affect the breeding success of emperor penguins or the nesting ability of snow petrels and have other adverse effects. However, these effects are less known to date. Snow petrels and emperor penguins are species that depend on the sea ice to complete their life cycles, and they show increased sensitivity to its loss. An important part of this sensitivity is likely to be the food shortage associated with increased sea ice extent and variability in sea ice duration (Jenouvrier et al., 2005a).

Most Antarctic marine predators are long-lived or relatively long-lived, and their average generation times are longer than the average interval between extreme climatic events. In most of them, rather than a direct impact of temperature or weather (precipitation, snowfall, or sea ice coverage) on individuals, the main impacts of climate are likely to be the changes in abundance, quality or stability of the food resources, which result from food web modifications related to changes in the physical environment. An increase in climate-related food shortages has caused population decline in the otherwise highly successful Antarctic fur seal at South Georgia (Forcada et al., submitted), which recovered from near extinction in the early Twentieth Century. The recent rapid increase in ENSO-related variability, with more frequent extreme events causing frequent food shortages, has led to a local decline in female fur seal fitness, and a loss of buffering of survival and propensity to breed in adult breeders due to the high rates of ecosystem fluctuation. These vital rates, most important to fitness, have increased in variability over the last 25 years (Forcada et al., submitted). When the temporal variation in these types of vital rates increases, the long-term fitness tends to decrease, causing loss of buffering against the environment (Morris and Doak, 2004).

In the fur seal case, changes in phenology were not apparent, which suggests that the loss of stability of the food supply and shortage in food, rather than other habitat constraints, are more likely to affect long-term fitness. An important difference between Antarctic fur seals and other species around Antarctica is their exposure to increasing ecosystem fluctuation derived from extreme climatic events, which are manifest near one of the Antarctic regions with most rapid warming, the Antarctic Peninsula. That makes it likely that the loss of buffering against environmental change occurs in species most sensitive to loss of their critical habitats, but also in those most affected by changes in stability and availability of food supply. In emperor penguins and other ice-dependent species, like Adélie penguins, the sea ice habitat is essential to complete the life cycle, and without it populations cannot persist. Sea ice-retreat is limited by the ice caps and other factors, and therefore there is likely to be little flexibility in phenological changes in these species. In that case, phenotypic plasticity may not be enough to ensure persistence, and it is likely that more successful species will replace those most sensitive to loss in critical habitats (e.g. Forcada et al., 2006). Microevolution may help long-term adaptation, but there is little hard evidence that it is occurring already.

Responses of baleen whales to climate change in the Southern Ocean

Predominately krill predators, baleen whales are a key component of the Antarctic ecosystem, but are currently at only fractions of their historical abundance due to exploitation to near extinction by whaling operations in the last century (Clapham and Baker, 2002). Contemporary data have shown that different species populations are recovering at different rates, both temporally and regionally, but long-term data sets on whale population dynamics are lacking (Leaper et al., 2008). Interpretation of the responses of baleen whale populations to climate change will therefore be especially difficult to disentangle from the effects of exploitation, and may not be detected for some time, as whales are such long-lived species. In the long term, given that some species are only in the early stages of recovery, such as the blue whale (Branch et. al., 2004), means that climate change impacts will almost certainly negatively affect the recovery potential of such species. In the short term, direct effects of temperature increases on baleen whales are unlikely because of their mobility and thermoregulatory ability. Instead, it is likely that climate change impacts will be mediated primarily through, (1) changes in sea ice dynamics that alter habitat characteristics and (2) changes in prey abundance and distribution (Nicol et al., 2008). In those areas of the Antarctic where sea ice is predicted to shrink (IPCC, 2007), pagophylic species such as blue and minke whales will be both directly and indirectly affected. As ocean productivity shifts with changes in sea ice extent, more northerly (oceanic) species, for example fin whales, may track changing prey availability and expand their range south, overlapping spatially with blue and minke whales. Prey availability may also have direct affects on whale demography, and this has already been demonstrated for southern right whales. Leaper et al. (2006) have shown that the breeding success of southern right whales feeding in South Georgia is driven by underlying relationships with the availability of krill, whose population fluctuations are correlated with changes in ocean climate, especially sea surface temperature (Trathan et al., 2006). Paradoxically the very fact that baleen whales depend upon the availability of suitable habitat in multiple locations (i.e. not just Antarctica) may also make them especially vulnerable to climate change, given that their low latitudes breeding areas may be subject to different impacts.