Download marine mammals and their environment in the

Journal of Mammalogy, 82(3):630–640, 2001
MARINE MAMMALS AND THEIR ENVIRONMENT IN THE
TWENTY-FIRST CENTURY
JOHN HARWOOD*
Natural Environment Research Council Sea Mammal Research Unit, Gatty Marine Laboratory,
University of St. Andrews, St. Andrews, Fife KY16 8LB, United Kingdom
Critical habitat for marine mammals is defined in terms of the functioning ecological units
required for successful breeding and foraging. This definition is used to consider the potential effects of environmental change in the 21st century on abundance and distribution
of marine mammals. Critical habitat for breeding can be identified relatively easily for
pinnipeds and some coastal and freshwater cetaceans. Critical habitat for foraging is more
difficult to define, particularly for pelagic species. However, telemetry-based studies have
indicated that relatively localized areas may be particularly important for some species.
Habitat degradation, as a result of reduction in prey density and increased risks of mortality
due to human activity, is likely to be a problem for most species. Ice-breeding seals,
particularly those that are endemic to inland seas and large lakes, are most likely to be
affected by climate change. Climate change will also affect distribution and availability of
prey in the short and long term. Although highly mobile species, such as marine mammals,
can respond more rapidly to effects of climate change than their terrestrial counterparts,
central-place foragers, such as many otariid seals, may still be seriously affected.
Key words: climate change, critical habitat, habitat loss, habitat fragmentation, marine mammals,
resource limitation
Ecologists agree on few things; nonetheless, there seems to be general agreement
that the most important agents responsible
for recent extinctions are habitat destruction
and fragmentation, the impact of introduced
species, chains of extinction, and overkill.
Diamond (1984, 1989) referred to these as
‘‘the Evil Quartet,’’ an analogy to the 4
horsemen of the Apocalypse, and concluded that habitat destruction and fragmentation were the most important agents of species extinction in the 20th century.
Historically, overkill was the most important human activity affecting abundance
of marine mammals, and many species
were reduced to low levels of abundance by
overexploitation in the 19th and early 20th
centuries. The 2 known, or presumed, extinctions of marine-mammal species in re-
cent history (Steller sea cow, Hydrodamalis
gigas, and Caribbean monk seal, Monachus
tropicalis) were almost certainly the result
of overkill, although Boyd and Stanfield
(1998) suggested that the latter species may
still be extant and the former species may
have been involved in a chain of extinctions
(Anderson 1995).
Unlike the situation for terrestrial and
freshwater species, competition and predation from introduced species do not seem
to be serious problems for marine mammals. However, introduced pathogens and
toxin-producing organisms (e.g., diatoms
and dinoflagellates) may be important causes of mortality (Geraci et al. 1999).
Mammals may function as keystone species in some marine communities. As a result, a serious depletion in their numbers
can cause major changes in species com-
* Correspondent: [email protected]
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SPECIAL FEATURE—MARINE MAMMALS
position and initiate a chain of extinctions
that cascades through the food web. The effect of the abundance of sea otters (Enhydra
lutris) on the structure of kelp communities
(Estes et al. 1998) is the best known example. Anderson (1995) suggested that the
extinction of Steller’s sea cow in the Komandorskiye Islands at the end of the 18th
century may have been part of such a chain,
initiated by overkill of the local populations
of sea otters. Sea otter predation on sea urchins is essential for the maintenance of the
dense kelp beds that provided the main food
supply of sea cows.
However, habitat destruction and fragmentation seem likely to become as important for marine mammals in the 21st century as they were for most other taxonomic
groups in the 20th century. Although marine mammals are observed widely across
the world’s oceans and freshwater bodies,
their distribution is patchy, and some areas
are used more frequently than others. These
preferred areas are probably particularly
important for survival and reproduction,
and changes to these areas are most likely
to affect distribution and abundance of marine mammals. This observation implies
that these areas may constitute critical habitats for marine mammals, and I first consider whether the concept of critical habitat
is relevant to this group and how such habitat may be identified. I then examine how
availability of critical habitat for marine
mammals is likely to change in this century,
and how these changes may vary geographically. Finally, I discuss whether habitat
fragmentation is a useful concept in the marine environment and its implications for
marine-mammal populations.
CRITICAL HABITAT
Critical marine habitat was defined by
Ray (1976) as ‘‘those identifiable areas
which are vital to the survival of a marine
species at some phase in its life cycle’’ (Ray
and McCormick-Ray 1995:24), and he advocated the use of this concept in choosing
marine protected areas. Subsequently, Ray
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suggested that a focus on the requirements
of individual species was not an optimal approach and recommended that marine protected areas should be selected to protect a
mosaic of ecosystems (Ray and McCormick-Ray 1995). This move toward protecting ecosystems rather than species is
supported by many other conservationists
(e.g., Christensen et al. 1996; Noss 1996;
Schwartz 1999; Sherman and Duda 1999).
However, identification of individual ecosystems is difficult because they rarely have
distinct boundaries, and their definition is
difficult because an ecosystem has no obvious objective or purpose (Grimm 1998).
Determination of whether or not an ecosystem has changed, and therefore whether
protection has been successful, is only possible if the ecosystem can be identified and
defined.
Jax et al. (1998) suggested that this problem can be overcome if ecological units are
defined in terms of their function or topography, their internal relationships, and the
spatial scale on which they are being studied. In principle, an ecological unit can be
defined operationally and unambiguously
by its location in this 3-dimensional space.
When ecological units are defined in this
way, the terms ecosystem and critical habitat clearly are simply different ways of labeling parts of this 3-dimensional space.
Unless critical habitat is defined only in
terms of its physical features, it must be a
functioning ecological unit. Otherwise, critical habitat will not persist over time in an
identifiable form, and cannot provide a useful basis for a protected area or for the species that depend on the area. Although the
situation in aquatic environments may be
more complicated than on land, because the
location of ecological units that involve pelagic organisms may vary over time, in
principle, no difficulty exists in applying
this concept to these environments.
The concept of critical habitat is central
to national conservation legislation in many
countries. For example, European Commission Directive 92/43/EEC on the conser-
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vation of natural habitats and of wild fauna
and flora requires member states of the European Union to ensure the favorable conservation status of 632 species of animals
and plants (including 4 marine-mammal
species). The main mechanism for achieving this involves establishment of a network
of Special Areas for Conservation that protect critical habitat for each species. Using
the framework proposed by Jax et al.
(1998), the critical habitat for a particular
species is the ecological unit whose existence is essential for that species’ persistence. For example, in the Northeast Atlantic, the grey seal (Halichoerus grypus) requires islands or cliff-bound beaches that
are relatively inaccessible to humans and
where seals with pups have access to salt
or freshwater for successful breeding. The
activity of the seals, and activities of other
species (such as seabirds and rabbits) that
breed there, affect characteristics of this
habitat, which must therefore be considered
as an ecological unit rather than a purely
physical feature.
The extent and spatial arrangement of
critical habitats required by a species, or by
a population of that species, will determine
the abundance of the species or population.
Thus, an ecological unit can be identified as
providing critical habitat for a population if
changes in the unit’s characteristics affect
survival, fecundity, or movement rates resulting in a change in the size of the population. Critical habitats can be ranked in
importance by carrying out a sensitivity
analysis of the effect of small changes in
each habitat on the intrinsic rate of increase
of the population in question. In contrast,
the only way to identify a critical habitat
using Ray’s (1976) original definition is to
remove it completely and see if the species
of concern can still persist.
For an ecological unit to provide critical
habitat, it must provide the resources that
determine the abundance of a species
through density-dependent or Allee effects
(Courchamp et al. 1999; Stephens and
Sutherland 1999). Harwood and Rohani
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(1996) reviewed factors that are known to
limit the abundance of marine mammals
and concluded that the most important were
availability of safe areas to breed and forage. Thus, critical habitat for marine mammals can be defined in terms of the ecological units that provide those 2 resources.
Some cetacean species (e.g., grey whale,
Eschrichtius robustus) have clearly identifiable and well-known breeding areas, but
many species give birth in the open sea. For
those species, the social environment in
which females breed provides the critical
ecological unit. Pinnipeds, which must give
birth on land, require predator-free islands,
sandbanks, or ice floes that remain stable
for at least the duration of lactation. High
densities may occur at localities that have
those characteristics, resulting in densitydependent mortality of young from physical
injury or disease.
Remarkably little evidence exists that
marine-mammal populations can be limited
by food availability. Major reductions in
food availability as a result of oceanographic events, such as the El Niño Southern Oscillation, have had devastating effects on
pup and adult survival in many pinniped
species (Trillmich and Ono 1991). But the
magnitude of those effects was not related
directly to abundance of the marine-mammal species in question. In fact, the greatest
effects probably occur when population levels are relatively low.
Changes in fertility rate (Gambell 1973)
and age at sexual maturity (Lockyer 1972)
of baleen whales in the Southern Ocean
during the course of commercial whaling
between 1910 and 1970 have been interpreted as a response to changes in prey
availability (Laws 1977). However, this
conclusion must be treated with caution because no direct measurements were made of
abundance of the whales’ main prey species
(Euphasia supurba) over that period, and
the evidence of changes in reproductive parameters came entirely from whales killed
by the whaling industry, which was highly
selective.
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SPECIAL FEATURE—MARINE MAMMALS
Nevertheless, a decrease in prey availability eventually must have an effect on
the size of a marine-mammal population.
Provided per-capita prey availability remains above a threshold level, a reduction
in availability can be compensated for by
increasing the amount of time spent foraging. If availability falls below this threshold, species must respond by emigrating to
areas where prey availability is higher, by
slowing their growth rate, or by reducing
reproductive effort. All these responses will
affect local abundance, although such effects may be difficult to detect.
Evidence is increasing that marine mammals may require relatively high and predictable local levels of prey availability to
forage effectively. Areas used by right
whales (Eubalaena glacialis) and fin
whales (Balaenoptera physalus) in the Bay
of Fundy, Canada, were characterized by
high densities of their principal prey species
and physical conditions that facilitated capture of those prey (Woodley and Gaskin
1996). Female southern elephant seals
(Mirounga leonina) travel long distances
from South Georgia to small local areas
near the Antarctic Peninsula after the pupping season, presumably to feed (McConnell and Fedak 1996). Male beluga
whales (Delphinapterus leucas) in the Canadian Arctic exhibit similar behavior
(Martin and Smith 1999). Grey seals in the
North Sea also travel long distances to
small well-defined areas of the seabed, and
these preferences persist over a number of
years (McConnell et al. 1999).
Should these preferred areas be considered as critical habitat? The answer depends
on the nature of the ecological unit that is
associated with each area. On a relatively
fine spatial scale, loss of 1 of these areas
could be critical because the marine mammal in question might vacate the region that
contains the area. However, the effect of
this movement on the rate of increase of the
population might be trivial on a larger
scale. If the area is preferred because prey
are concentrated there by a combination of
633
topographic and oceanic features, then the
area can only be considered as critical habitat if a discrete prey population is associated with the area. Otherwise the area does
not constitute an ecological unit, and no
guarantee can be made that the resource
will persist over time, even if the area is
given complete protection.
HABITAT LOSS
AND
DEGRADATION
Diamond’s (1984) conclusion that habitat
loss and fragmentation were the principal
causes of extinction in the 20th century was
based primarily on studies of terrestrial
communities, where effects of human activities on the extent and distribution of natural habitats have been most evident. Similar effects can be observed in freshwater
habitats, where demands for water for human consumption, irrigation, power generation, waste disposal, and transport have resulted in dramatic changes in the structure
and quality of freshwater habitats.
Because pinnipeds breed on land, they
also might seem to be vulnerable to the loss
of suitable terrestrial habitat. However, with
a few exceptions (e.g., Mediterranean monk
seal, Monachus monachus) changes in the
pattern of land use have resulted in a net
increase in the number of potential breeding
sites for many species. For example, the
number of grey seal colonies in the United
Kingdom increased substantially during the
20th century (Summers 1978) as many offshore islands were abandoned by humans.
The trend toward the depopulation of remote islands may be reversed in the 21st
century, but this seems unlikely because
these islands offer few, if any, of the facilities that are most highly valued by modern
human societies.
Effects of human activities on the marine
environment have been less obvious than
those on land. Although this may be because change has been less extensive, it
may also be because change has been more
difficult to detect. The other major factor
that may cause habitat loss and degradation
in the 21st century is climate change. Mean
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ambient temperatures certainly increased
during the 20th century, and this trend
seems likely to continue in the current century. However, because the precise cause of
this trend is disputed, I have treated the
likely effects of climate change separately
from other direct or indirect results of human activities.
Habitat loss.—The marine environment
offers few barriers to movement, particularly for marine mammals that are large and
mobile. Marine species can, in theory, compensate for loss of a particular area of critical habitat by migrating to other suitable
areas. Effects of this migration may only be
detectable on a relatively fine spatial scale,
and such data are rarely available for most
species of marine mammal. However, the 4
freshwater species of river dolphins in the
superfamily Platanistoidea (Platanista gangetica, P. minor, Inia geoffrensis, and Lipotes vexillifer) and the tucuxi (Sotalia fluviatilis) live in a habitat where major barriers to migration exist. For these species,
effects of habitat loss may be profound and
readily detectable.
A growing demand for freshwater is predicted to be one of the most important features of the early years of the 21st century,
which will probably lead to increasing regulation of water flow through the use of
dams and dredging (Gleick 1993). Such activities have already created small isolated
populations of freshwater dolphins and rendered some areas of otherwise suitable habitat completely inaccessible. Flood control
will result in loss of shallow-water habitats
that are often used extensively during rainy
seasons.
Habitat loss in the marine environment is
more difficult to quantify. Mineral, oil, and
gas extraction from the seabed could result
in the complete destruction of some patches
of critical habitat, although such effects are
likely to be on a relatively fine scale. Effects of fisheries on marine mammals are
dealt with elsewhere in this special feature
(DeMaster et al. 2001). However, repeated
use of mobile fishing gear can transform
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large areas of benthic habitat and thus destroy patches of critical habitat for marine
mammals (e.g., Kaiser 1998; Watling and
Norse 1998). All of these effects are likely
to increase both in magnitude and geographical extent during the 21st century, as
technological advances and market demand
encourage extension of resource extraction
and fishing activities into deepwater areas.
The effect of an extension of resource extraction on marine mammals will depend on
what precautions are taken to prevent loss
of critical habitat. Although many countries
oblige companies that are registered there
to complete environmental impact assessments, even when these companies are operating in international waters, the fishing
industry is rarely obliged to carry out an
environmental impact assessment before
opening a new fishery. International agreements can sometimes be used to close or
limit a fishery if it can be shown to have a
negative effect on a marine-mammal population (Slooten et al. 2000). But such evidence is often contested, and the resulting
disputes can allow potentially damaging
fisheries to continue for long periods.
Habitat degradation.—In addition to the
outright loss of critical habitat through oil
and gas extraction, dam construction, and
fishing, the quality of remaining patches of
habitat and areas between those patches can
also be reduced by those activities. Although effects of habitat degradation within
patches of critical habitat may seem to be
more important than those that occur outside them, this is not always the case. For
example, if a species of marine mammal
spends more time traveling between patches
of critical habitat than it does within patches, then an increase in risk of mortality outside these patches is likely to have a greater
effect on its abundance than an equivalent
increase in risk within patches.
Survival and fecundity rates may fall because of an increased risk of entanglement
in fishing gear or marine debris (DeMaster
et al. 2001), exposure to chemicals that reduce immune system function or diminish
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SPECIAL FEATURE—MARINE MAMMALS
reproductive performance (O’Shea and
Tanabe 1999), and exposure to new pathogens. Moreover, animals may be excluded
from particular areas by various forms of
disturbance, such as noise pollution.
Substantial evidence exists (O’Shea and
Tanabe 1999) that persistent man-made organic contaminants, such as polychlorinated
biphenyls, can affect reproductive performance and immune function of marine
mammals. Those effects seem to be the result of structural similarity between such
chemicals and compounds produced by the
endocrine glands. Reijnders (1999) reviewed the way in which such chemicals
may affect survival and reproduction. Although production and discharge of many
of these contaminants are now restricted,
less than one-third of the total production
of polychlorinated biphenyls in the 20th
century has been disposed of adequately.
The remaining production is present in machinery and equipment, particularly in developing countries, where it is usually unlabeled. Thus, large quantities of polychlorinated biphenyls are likely to continue to
enter the marine environment during the
21st century, as this equipment becomes
obsolete. Global transport mechanisms will
distribute these chemicals over large areas
(Bard 1999). A number of new compounds
(e.g., polybrominated diphenyl ethers) with
similar chemical properties have also begun
to enter the marine environment (de Boer
et al. 1998). Controls on the way in which
these compounds enter the environment
may lead to a net reduction in input rates,
but their biological impact will depend on
toxicity and persistence of the compounds
involved, and of their breakdown products.
Growing evidence exists (Harvell et al.
1999) that the frequency of mass mortalities
of marine organisms caused by disease increased in the latter one-half of the 20th
century. One likely cause of this increase is
the greater frequency of movement of humans and their domestic animals. That has
resulted in the introduction of novel pathogens into naive populations, a process
635
analogous to the widespread species extinctions on oceanic islands associated with human settlement (Diamond 1997). The discharge of ballast water by cargo ships has
also resulted in the widespread distribution
of toxin-producing marine organisms, such
as dinoflagellates and diatoms, that previously had restricted distributions. For example, toxins produced by the dinoflagellate Gymnodinium catenatum were implicated in the mass mortality of 70% of the
Mediterranean monk seal population on the
coast of the former Spanish Sahara in 1997
(Forcada et al. 1999; Harwood 1998). Until
recently, that dinoflagellate was confined to
the east coast of the United States. Since
1970, G. catenatum has become more widely distributed (Anderson 1994). Therefore,
mass mortalities of marine mammals seems
likely to become more frequent during the
21st century. Many of the populations that
suffered mass mortalities in the 20th century recovered remarkably rapidly (Geraci
et al. 1999), but they may not be so resilient
to more frequent exposure to these organisms.
Potential effects of disturbance on seal
populations have been reviewed by Reijnders et al. (1993). The most dramatic evidence has come from changes in numbers
and distribution of Hawaiian monk seals
(Monachus schauinslandii—Gerodette and
Gilmartin 1990). The number of seals using
Kuril Atoll declined from .100 to ,20 after the establishment of a United States
Coast Guard station in 1960. Conversely,
numbers of seals using Tern Island at
French Frigate Shoals increased sharply after the Coast Guard station there was vacated in 1979. Although the depopulation
of remote islands will probably continue
throughout the 21st century, increased human mobility and demand for wildlife-related tourism may mean that seal breeding
sites on such islands will be visited more
frequently. Unless access and behavior are
carefully controlled, this may result in increased disturbance. Consequences of this
disturbance are difficult to evaluate, be-
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cause the most obvious effects of disturbance (emigration of individuals from a disturbed site to a less disturbed site) may have
a smaller impact on a local population than
less easily detectable changes, such as a reduction in survival or fecundity, among the
individuals that remain at the site (Gill et
al. 2001).
Oceanic sound levels in the frequency
range 10–1,000 Hz are now dominated by
noise produced by ships (Richardson et al.
1995). Use of high-intensity, low-frequency
sound for geological surveys, military purposes, and monitoring of sea-temperature
change has further increased ambient noise
levels in some regions. Marine mammals
respond to such increased noise levels by
moving away from them, or by changing
their patterns of vocalization and surfacing
(e.g., Richardson et al. 1995). However, the
implications of those responses for survival
and reproduction are unclear. Disturbance
may have long-term effects other than redistribution. Disturbed animals may forage
less efficiently and may become more vulnerable to pollution and disease because of
increased stress (O’Shea and Tanabe 1999).
Climate change.—Although at least a
preliminary concensus now exists on the
way in which climate change may affect
terrestrial weather patterns in the early
years of the 21st century, the implications
of such changes for biodiversity are likely
to be much less predictable (Davis et al.
1998). Effects on the marine environment
are even more difficult to predict because
of the complex interaction between ocean
processes and climate (Stocker and
Schmittner 1997).
Similarly, productivity of marine ecosystems is strongly influenced by patterns of
circulation and water stratification. Nevertheless, some broad-scale predictions are
possible, and they were conveniently summarized by Fields et al. (1993), from which
the rest of this account is taken. Many of
these predictions are based on analogy with
changes that occurred in the transition between previous glacial and interglacial pe-
Vol. 82, No. 3
riods. A reduction in primary productivity
seems likely to occur along western continental margins because of diminished winddriven upwelling and a poleward shift of
subequatorial water masses. However, higher temperatures and higher levels of carbon
dioxide may allow local increases in primary productivity, especially where sufficient nutrients are available. The effect of
climate change on primary productivity
through the stability of the mixed layer is
also complicated. In some areas, increased
stability will increase productivity by reducing advection of nutrients and biomass.
But too much mixing will favor communities dominated by motile dinoflagellates,
rather than diatoms, and they will tend to
have reduced productivity. Changes in circulation will also affect distribution and
survival of pelagic larvae of a wide range
of marine species. All these processes will
affect the integrity of the ecological units
that currently constitute critical habitat for
feeding by marine mammals.
Although the long-term effects of climate
change on ocean productivity are clearly
difficult to predict, short-term changes may
be relatively local. Marine mammals are
highly mobile and should be able to locate
new areas of high productivity as they develop. However, development of ecological
units that are associated with those areas
will almost certainly take much longer. As
a result, overall productivity levels may be
reduced for decades and the density of prey
items in these areas may be below the critical threshold for efficient foraging by marine mammals. Central-place foragers, such
as many colonially breeding otariid seals
that rely on local areas of high productivity
during lactation, may be particularly disadvantaged if new areas of high productivity are far away from suitable pupping sites.
Convincing evidence now exists that the
extent of Northern Hemisphere sea ice has
decreased over the last 50 years, and that
this decrease is much larger than would be
expected from natural climate change (Johannessen et al. 1999; Vinnikov et al.
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SPECIAL FEATURE—MARINE MAMMALS
1999). Such changes are likely to have an
effect on pinniped species that rely on sea
ice for resting, pupping and moulting, and
for cetaceans such as the bowhead whale
(Balaena mysticetus) that seem to rely on
high levels of productivity associated with
the ice edge (Dyke et al. 1996). Tynan and
DeMaster (1997) discussed the likely effects of regional and local changes in the
extent of sea ice in the Arctic Ocean on its
marine-mammal populations. Similar effects can be expected in the Southern Ocean
where the distribution of a number of marine-mammal species is strongly associated
with the particular ice conditions (Ribic et
al. 1991). The same effects are also likely
to be observed, at least initially, in the Norwegian and Barents seas. However, increased melting of the ice sheet in those
areas could lead to the breakdown of the
North Atlantic thermohaline circulation, resulting in a rapid increase in the extent of
winter sea ice. Changes in ice extent during
the year induced by climate change can also
be important. For example, Stirling et al.
(1999) concluded that an observed decline
in condition of polar bears (Ursus maritimus) in western Hudson Bay was a result
of the trend toward earlier breakup of winter sea ice.
Although a reduction in the extent of sea
ice and changes in its nature will probably
reduce the extent of critical habitat for
many polar marine mammals, some suitable
habitat will almost certainly remain. This
may not be the case for seals that are endemic to inland seas and large lakes. The
Caspian seal (Phoca caspica), the Baikal
seal (Phoca sibirica), and 2 subspecies of
the ringed seal (Phoca hispida lagodensis
and P. h. saimensis) are confined to such
areas and rely on the presence of relatively
stable winter ice for breeding. If suitable ice
does not develop in winter, as a result of
increasing winter temperatures, they will be
unable to breed. Some evidence already exists that this may occur. The 1998–1999 and
1999–2000 winters in the Caspian Sea were
very mild and the extent of ice suitable for
637
seal breeding was much reduced (C. Duck,
pers. comm.). The increased crowding that
resulted may have contributed to a mass
mortality during the breeding season in
1999–2000 caused by canine-distemper virus (Kennedy et al. 2000).
REGIONAL CHANGES
The effects described in the previous section will not be uniformly distributed across
the planet. Major changes in abundance of
marine mammals are most likely to occur
in the major river systems of Asia and
South America and within the former Soviet Union where large enclosed bodies of
water are occupied by endemic species or
subspecies of seal. Changes also may occur
in the Arctic Ocean, where a dramatic reduction in the extent of sea ice could reduce
availability of breeding habitat for icedwelling seals below a threshold level. Colonies of otariid seals, particularly in the
Southern Ocean and along the western
coast of the North and South American continents, that depend on local concentrations
of prey species may be severely affected by
changes in the distribution of primary productivity. The number of potential breeding
sites for these species is limited, and suitable sites may not be close to new concentrations of productivity.
HABITAT FRAGMENTATION
The average distance between the remaining areas of suitable habitat is likely to
increase as a consequence of habitat loss.
Such habitat fragmentation can have important consequences for biodiversity because small populations have a greater risk
of extinction through chance effects and because they may lose genetic diversity
through drift, inbreeding depression, and
increased mutation (Lande 1995; Lynch et
al. 1995). Consequences of habitat fragmentation are least serious for highly mobile species and where areas between patches of suitable habitat are not particularly
hostile (Debinski and Holt 2000). Because
both of these conditions apply to marine
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mammals, habitat fragmentation might be
assumed not to be a problem for these species. However, this is not necessarily the
case. Increasing fragmentation of patches of
critical habitat for feeding will increase
costs of traveling between patches and may
reduce rewards that an individual receives
once it locates a patch. As a result, average
net intake will decline as habitat fragmentation increases, with consequences for
growth or reproduction.
Although the number of potential breeding sites has increased for many terrestrial
breeding pinnipeds, this is not universally
true. For example, Mediterranean monk
seals require secluded beaches in late summer for pupping. Most suitable beaches in
the Mediterranean Sea are heavily used by
tourists, and the species is now confined to
breeding on small beaches at the back of
isolated caves. Pup mortality from storms
can be high on such beaches. Such beaches
are relatively rare in the western Mediterranean. As a result, only 2 viable Mediterranean monk seal populations now survive,
1 breeding on remote islands along the
Mediterranean coasts of Greece and Turkey
and the other in the North Atlantic with
small colonies on 1 island in Madeira and
along a short stretch of coast in the former
Spanish Sahara (Reijnders et al. 1988). The
2 populations are separated by more than
4,000 km of unsuitable habitat (Harwood et
al. 1996), and the chance of any exchange
of individuals between the populations is
small. As a result, Mediterranean monk
seals are exposed to the same risks from
habitat fragmentation as much less mobile
species.
CONCLUSIONS
The environment in which marine mammals live during the 21st century is likely
to deteriorate as a result of commercial fishing, pollution, disturbance, freshwater
dams, and increased risks of mortality from
novel pathogens and biotoxins. With the exception of 4 species of freshwater dolphin,
whose populations are already known to be
Vol. 82, No. 3
at risk, it is not possible to assess the extent
to which this deterioration will affect particular marine-mammal populations. A better understanding of the ecological units
that are associated with areas used preferentially by foraging marine mammals could
substantially improve our predictive abilities.
The extent of breeding and resting habitat for ice-dwelling pinnipeds in the Northern Hemisphere declined substantially in
the last decade of the 20th century. That
trend is likely to continue during the 21th
century and could have severe consequences for species that are confined to enclosed
bodies of water (e.g., Caspian Sea and Lake
Baikal).
Climate change will also affect the magnitude and distribution of primary productivity. Changes in the immediate vicinity of
breeding colonies of otariid seals could
have a substantial effect on abundance of
these species, because potential breeding
sites are limited.
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Special Feature Editor was Michael R. Willig.