Summary: The Impact Of Climate Change On Leopard Seals

The Impact of Climate Change on Leopard Seals (Hydrurga leptonyx)

Leopard seals (Hydrurga leptonyx) are a species of ice-obligate seals that are widely distributed in Antarctic and sub-Antarctic waters (Hückstädt, 2015). The most recent circumpolar estimate of leopard seal abundance suggests more than 35,000 individuals, and there does not appear to be a declining trend on these numbers. The IUCN, for this reason, list the leopard seal as Least Concern (Hückstädt, 2015).

However, climate change is happening. The global average land and sea temperature has increased over the twentieth century, with the North and South Poles being particularly affected (Learmonth, et al., 2006). In turn, this has caused ice cover to decrease and sea levels

to rise. Changes to salinity, CO2 concentrations and pH effect the behaviour and abundance for some fish species and marine mammals which, in turn, can create changes in the food web and therefore could change the behaviour and abundance of higher trophic feeders (Learmonth, et al., 2006).
This change is climate could affect the leopard seal, either directly (through changes to the ice-floes leopard seals are dependent on) or indirectly (through the effects of changes in ice-floe on other species such as the leopard seal’s prey). These effects are discussed below, and are compared to the effects on another ice-obligate seal species, namely the Crabeater seal (Lobodon carcinophagus), to demonstrate the severity of these effects in context.

Leopard seals show a preference for water-ice; they are pagophilic (Meade, et al., 2015). This means that they have a circumpolar distribution (they inhabit the area around the south pole), with the main population occurring within the circumpolar sea-ice. They use this sea-ice as a haul-out platform (they temporarily leave the water for a range of purposes, including for pupping, moulting and as resting platforms throughout the year) (Meade, et al., 2015). For these reasons, it could be argued that leopard seals are dependent on sea-ice, and so any changes to the size or structure of the sea-ice could have a negative impact on the species.
Over the last 50 years, the mean temperature of the Western Antarctica Peninsula has increased by 3ºC, and the mean winter temperature by 6ºC (Meade, et al., 2015). This is likely to have caused the 47% decrease in sea-ice coverage and sea-ice duration decrease of 100 days over the last 35 years. This means that the sea-ice is expanding and contracting at different points at the year than usually. There being less sea-ice than in previous years (Meade, et al., 2015).
If sea-ice extent was reducing, then you would expect more leopard seals to be found closer to the coastline (Meade, et al., 2015). This is because most available ice would be found there and so, any seal wishing to haul-out would have no other option than to inhabit the areas close to the coastline. When sea-ice extent was low, leopard seals spent relatively longer periods of time closer to the coast line compared to seals in higher sea-ice extent areas. This suggests that leopard seals are limited by their habitat selection, and so when sea-ice is low they will spend more time close to the coastline (Meade, et al., 2015). The habitat of leopard seals has been predicted to decrease by 1.1%yr-1, so leopard seals are going to be further restricted to the coastline in the future (Meade, et al., 2015).
The change in the timing of sea-ice expansion is likely to impact the behaviour of leopard seals. Pups are born usually between early November and December on sea-ice and they are weaned for the following month, again on sea-ice (Hückstädt, 2015). However, if the sea-ice is contracting earlier than usual at that time, there will be a reduced amount of sea-ice for the expectant mothers, and so competition for these areas could increase. As leopard seals are usually solitary animals, this could cause increased disputes between these individuals, leading to injuries or casualties of mothers or pups. However, as leopard seals are usually solitary animals, very little research has been done looking at interactions between individual leopard seals, and so there is little evidence to suggest fighting between adult individuals (Hiruki, et al., 1999). Therefore, it is possible that decreased sea-ice won’t cause an increase between the competition for areas for birthing.
Other Antarctic seals, such as the Crabeater, predominantly use pack-ice for hauling out (sea-ice that isn’t attached to the land and ‘drifts’ because of the wind or current), whereas leopard seals usually use fast-ice (ice that is ‘fastened’ to land) (Hückstädt, 2015) (Hückstädt, 2015). However, leopard seals are known to use different types of ice floes for resting, moulting, and pupping (Siniff, et al., 2008). This means that if the fast-ice in their habitat begins to decrease, it is possible for the seals to move to another area, independent for that area containing pack-ice or fast-ice. This would be more difficult for the Crabeater seal, as they do not have this flexibility of habitat (Siniff, et al., 2008).
Compared to the Crabeater seal, leopard seals have the advantage of very little predation. Out of all the Antarctic seals, the Crabeater is the most vulnerable to predation, with leopard seals focusing mainly on pups and killer whales a constant threat to all age classes (Siniff, et al., 2008). In comparison, killer whales are the only known predator of leopard seals. This means that Crabeater seals must carefully choose a size of ice-floe as larger ice-floes provide better protection. Leopard seals are less restricted, and so, despite a preference for large ice-floes, can use smaller ones if necessary (Siniff, et al., 2008).

One indirect effect of climate change on leopard seals is changes to the food web (Siniff, et al., 2008).

If a prey species decreases in abundance, this leads to increased intraspecific and interspecific competition, which could have negative impacts on the species. One area of concern is the effects of climate change on the abundance and behaviour of krill, one of the leopard seal’s source of food during the winter (Siniff, et al., 2008).

The reduction if sea-ice coverage is likely to effect krill population (Meade, et al., 2015). The reduction in the duration of sea-ice coverage will allow greater opportunity for humans to exploit the Antarctic ecosystem. Extended periods of ice-free water allow the temporal and spatial expansion of krill fisheries. This leads to increased krill catch, which means a reduction in the number of krill for leopard seals (Meade, et al.,

2015).
Additionally, krill is indirectly effected by changes in salinity (Learmonth, et al., 2006). The increased temperature will lead to an increase in the amount of salt within the water. This could be due to increased evaporation of sea water (therefore increasing the proportion of salt to fresh water within the ocean) as well as the pack-ice melting, releasing previously frozen salt into the water. Many cephalopods (including some species of phytoplankton) are very sensitive to these changes in salinity, causing the community structures in near-shore coastal waters to shift into open waters. This directly effects the zooplankton community, and in turn, will cause a shift in the spatial distribution of krill. This will affect the higher trophic levels in the food web, including the leopard seal (Learmonth, et al., 2006).
It is not just the leopard seal that will be effected by a decrease in the abundance of krill.

Crabeater seals are a smaller species of pack-ice seals that feed primarily on Antarctic krill (Siniff, et al., 2008). This means any environmental changes that will affect the availability of Antarctic krill will probably have an impact on crabeater seals. This (along with changes to ice floes which significantly impact the crabeater seal’s pupping and protection behaviours) mean that it is likely that this species of seal will experiences a decline in abundance as the Antarctic continues to be effected by climate change (Siniff, et al.,

2008).
This should cause little impact to the leopard seal, as a decline in the number of crabeater seals would decrease interspecific competition for krill. However, a relatively large proportion of a leopard seal’s diet during the summer months is juvenile crabeater seals (Siniff, et al., 2008). In fact, in nearly every region of the Antarctic, between 60 and 70 percent of crabeater seals show scars that indicate an escape from a leopard seal, demonstrating the importance of the species to the leopard seal. If the abundance of crabeater seals decreased, it is very likely that this would have a negative impact on the abundance of leopard seals (Siniff, et al., 2008).
However, leopard seals are unusual apex predators. They have a diverse diet which varies seasonally and spatially, ranging from relatively tiny sources (such as krill), as well as much larger species, such as penguins, squid, and other juvenile seal species, such as Crabeater, Southern Elephant and Fur Seals (Siniff, et al., 2008). Stomach analysis and direct observations suggest that during the winter, krill dominates as the main source of food, switching to crabeater seal pups in during the spring and fledging penguins during the summer. Additionally, all year-round fish are a common prey for this species (Siniff, et al., 2008).
This diverse diet should mean that any changes to the abundance or distribution of a specific prey species shouldn’t significantly affect the leopard seal’s ability to feed, as they can easily move onto a different prey source (Siniff, et al., 2008).
It is because of this diverse diet and ability to use multiple haul-out sites that many people believe that Leopard Seals are unlikely to be effected by climate change. Siniff et al. (2008) suggested that like Ross seals, leopard seals would be less affected by changes in sea ice than Crabeater or Weddell seals (Siniff, et al., 2008). After being given arbitrary scores between 4 (positively affected) to -4 (negatively affected) in 7 areas relating to effects of climate change, the net effect for Leopard Seals was -10, just over half the that for the Crabeater seal (-19) (Siniff, et al., 2008). It also suggests that changes to sea ice extent and concentration are the most likely of the environmental effects to cause negative impacts for the leopard seals (Siniff, et al., 2008).
However, as most of the research done in this area involves adult leopard seals, very little is known about the effects of climate change on juvenile seals. Leopard seals sometimes visit islands in in lower latitudes and a couple remain there for the entire year. Most of these are juveniles and are often in poor condition (Siniff, et al., 2008). It is thought these individuals are moving further north in search of better suited land for hauling-out, because of the aggressive behaviour of older individuals in response to reduced suitable habitat (Aguayo-Lobo, et al., 2011). In other words, interspecific competition has increased, causing the dispersal of the population (Aguayo-Lobo, et al., 2011). More adult seals may follow this patterns as climate change proceeds. This lack of information may mean that leopard seals are effected by climate change than previously thought, as juvenile survival is crucial for the survival of the species (Siniff, et al., 2008).

In conclusion, it is possible that the leopard seal is negatively affected by changes in climate, both directly (through loss of sea-ice used for hauling-out behaviours) and indirectly (through changes to prey abundance and behaviour). However, compared to species of other ice-obligate seals, such as the Crabeater and Wendell, these effects are likely to be less significant. This is due to the leopard seal’s ability to use many different ice floe types (so can move away from a location is that ice becomes limited). It is also due to their diverse diet, which allows them have a food source, even if prey reducing in abundance.