224
Furthermore, stable isotope compositions in tissues of marine mammals are also
used as an alternative way to investigate the spatial, trophic, and temporal dimensions of the ecological niche (i.e. n-dimensional hypervolume [Hutchinson 1957])
(e.g., Bearhop et al. 2004; Franco-Trecu et al. 2012). In this context, some dimensions of the ecological niche of a predator can be determined through δ
15
N (trophic
component) and δ
13
C and δ
15
N (space component) values (Bearhop et al. 2004;
Newsome et al. 2007). This isotopic niche can be represented in a multivariate space
forming a δ-space with isotopic values as axes and is related to the mixture of food
items consumed by a predator. With the increase in the use of this methodology,
tools have been developed to visualize and measure the isotopic niche as proposed
by Layman et al. (2007). In this approach, the δ
13
C and δ
15
N values of organisms are
the base for the construction of convex hulls, considered the extent of their trophic
diversity. Then, statistical mixing models, also called stable isotope mixing models
(SIMM), were criated, allowing researchers to deal with various food sources to the
predators they were studying under the estimation of the proportional contribution
of each source (Phillips 2012). They are mathematical equations that are used to
explain the diet of a consumer taking into account the mixture of the isotope values
of its assimilated diet (Phillips 2012). Examples of them are the Mix-SIR (Moore
and Semmens 2008) and the Stable Isotope Analysis in R (SIAR) (Parnell et al.
2010). Later, Bayesian metrics were incorporeted to represent the isotope niche of
an individual, using standard ellipses that represent a species isotope niche (e.g.
Jackson et al. 2011; Phillips 2012). This tool can considerably improve determination of diet, especially when the source geometry of the mixing model is poor
(e.g. Ward et al. 2010). One of the most used is the Stable Isotope Bayesian Ellipses
in R (SIBER) (Jackson et al. 2011), a package developed for the free statistical software R (R Development Core Team). It has the great advantadge of allowing the use
of relative small data set and the comparison of the niches among diferente sample
size data sets. One of the limitations to which mixing models are subject is the overlap or great similarity that can occur among the food resources of a consumer; the
more isotopic distinct they are, better the diet estimation is (Phillips 2012). With the
advantages of providing information on the isotope variation within a population,
and then on how its ecological niche varies, a novel approach was developed
(Semmens et al. 2013). It is called Mix-SIAR, and it can indicate variations in the
diet a population in relation to diferente aspects, such as sex, age and geographical
location (Semmens et al. 2013).
Carcasses of marine mammals washed ashore or incidentally caught in fishing
gear are valuable sources for sampling different tissues, at low cost, for biological
and ecological studies through SIA. In different regions of southwestern Atlantic
Ocean (SAO), research groups survey the coast and collect carcasses or samples from them. In addition, scientific biopsy sampling of animals from different
environments and at different time frames also provide useful material for these
studies. As a result, several single- and multi-species studies have been published in
recent years, which represent an important contribution to the ecology of these
marine mammals.
E. Seyboth et al.
Furthermore, stable isotope compositions in tissues of marine mammals are also
used as an alternative way to investigate the spatial, trophic, and temporal dimensions of the ecological niche (i.e. n-dimensional hypervolume [Hutchinson 1957])
(e.g., Bearhop et al. 2004; Franco-Trecu et al. 2012). In this context, some dimensions of the ecological niche of a predator can be determined through δ
15
N (trophic
component) and δ
13
C and δ
15
N (space component) values (Bearhop et al. 2004;
Newsome et al. 2007). This isotopic niche can be represented in a multivariate space
forming a δ-space with isotopic values as axes and is related to the mixture of food
items consumed by a predator. With the increase in the use of this methodology,
tools have been developed to visualize and measure the isotopic niche as proposed
by Layman et al. (2007). In this approach, the δ
13
C and δ
15
N values of organisms are
the base for the construction of convex hulls, considered the extent of their trophic
diversity. Then, statistical mixing models, also called stable isotope mixing models
(SIMM), were criated, allowing researchers to deal with various food sources to the
predators they were studying under the estimation of the proportional contribution
of each source (Phillips 2012). They are mathematical equations that are used to
explain the diet of a consumer taking into account the mixture of the isotope values
of its assimilated diet (Phillips 2012). Examples of them are the Mix-SIR (Moore
and Semmens 2008) and the Stable Isotope Analysis in R (SIAR) (Parnell et al.
2010). Later, Bayesian metrics were incorporeted to represent the isotope niche of
an individual, using standard ellipses that represent a species isotope niche (e.g.
Jackson et al. 2011; Phillips 2012). This tool can considerably improve determination of diet, especially when the source geometry of the mixing model is poor
(e.g. Ward et al. 2010). One of the most used is the Stable Isotope Bayesian Ellipses
in R (SIBER) (Jackson et al. 2011), a package developed for the free statistical software R (R Development Core Team). It has the great advantadge of allowing the use
of relative small data set and the comparison of the niches among diferente sample
size data sets. One of the limitations to which mixing models are subject is the overlap or great similarity that can occur among the food resources of a consumer; the
more isotopic distinct they are, better the diet estimation is (Phillips 2012). With the
advantages of providing information on the isotope variation within a population,
and then on how its ecological niche varies, a novel approach was developed
(Semmens et al. 2013). It is called Mix-SIAR, and it can indicate variations in the
diet a population in relation to diferente aspects, such as sex, age and geographical
location (Semmens et al. 2013).
Carcasses of marine mammals washed ashore or incidentally caught in fishing
gear are valuable sources for sampling different tissues, at low cost, for biological
and ecological studies through SIA. In different regions of southwestern Atlantic
Ocean (SAO), research groups survey the coast and collect carcasses or samples from them. In addition, scientific biopsy sampling of animals from different
environments and at different time frames also provide useful material for these
studies. As a result, several single- and multi-species studies have been published in
recent years, which represent an important contribution to the ecology of these
marine mammals.
E. Seyboth et al.