CHAPTER 4: General Discussion
4.1 Will genetic diversity be lost with future climate change?
A multidisciplinary approach using genetic data and species distribution models (SDMs) is becoming increasingly useful in assessing the impacts of future climate change on the genetic diversity of populations. Such integrated studies have provided useful information for the conservation and management of species. For example Weaver et al. (2006) used SDMs in conjunction with
phylogenetic data to speculate on the biogeographical forces that influenced the observed patterns of genetic diversity and the observed distribution of the Black Hills mountain snails Oreohelix cooperi of Wyoming and provided useful information for their conservation and management. Similarly, Alsos et al. (2009), using genetic data and future climate change models, predicted a 5% loss of the genetic diversity of the dwarf willow Salix herbacea, due to range contractions. The glacial history of this species (using fossil data and model predictions of the Last Glacial Maximum) also provided information on how S. herbacea had dealt with past climatic changes. This information was useful in determining the possible reactions of the species to future projected climate changes (Alsos et al.
2009).
The modelling results of this study similarly indicate a range contraction in the distribution of E.
andersoni if current changes in sea surface temperature continue into the future. These contractions are predicted to mainly occur at the range limits, notably the southern and northern range limits.
Although there was a lot of mixing throughout the species distribution (approximately 39% of the sampled diversity was mixed among localities), both range limit areas also contained range- restricted genetic diversity, indicating that some degree of isolation exists at both range limits.
Chapter 4: General Discussion
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Occurrence records used for the SDM included localities that were much further south of Port Alfred and north of Xai Xai, which were the southern and northern limits of the genetic data (see Figure 4.1), i.e. it is possible that unique populations, that were not represented in the genetic samples, exist even closer to the range limits. This may be unlikely, however, considering the general declines in abundance of species towards their range limits (Bahn et al. 2006). The predicted range
contractions did not indicate a complete loss of E. andersoni at any of the localities where genetic samples were collected, precluding the making of precise inferences about the degree of genetic diversity that might be lost. However, it is fair to assume that if these range limits continue to contract at the projected rate, there is the potential for unique alleles within the populations near these range limits to be lost.
Although the cytochrome b data showed little restricted genetic diversity toward and at the northern and southern range limits of E. andersoni; the RPS7-1 region indicated that private alleles were present close to the northern range limit at Quissico (1 private allele), Xai Xai (5 private alleles) and Inhaca (6 private alleles) (Table 4.1). The E. andersoni distribution projected for 2030 indicates that the northern range limit will contract from Inhambane to close to Quissico (Figure 4.1). This equates to a stretch of distribution approximately 80 km long being lost. If further contraction occurs at the same rate, it is likely that the northern range limit will contract to Xai Xai by 2040 or 2050. If this occurs, the loss of the private alleles present in samples from Quissico would equate to 2.6% of the genetic diversity of E. andersoni being lost. If further contraction occurs at the projected rate, in the long-term, the northern range limit could shrink to south of Xai Xai and result in a further significant loss of up to 15.4% of the genetic diversity.
The vulnerability of northern range limit populations is exacerbated by the fact that the fishery in Mozambique is essentially unrestricted (van der Elst et al. 2003). Compounding the gravity of these findings, is the fact that studies have observed that rear range limits (the low-latitude species limit), being the northern range limit in this study, are of critical importance as they act as a store of genetic diversity and are foci of speciation (Hampe and Petit 2005). This is particularly true where these populations have originated from historical remnants of the species existing in refugia habitats that existed during the Last Glacial Maximum. These populations could consequently have been the source which gave rise to much of the current distribution of these species (Hampe and Petit 2005).
This may be true for E. andersoni considering the oceanographic conditions within its range that would promote northern spawning populations to supply populations further south, since larvae are presumably carried southward in association with the southward-flowing eastern boundary Agulhas Current and Mozambique Channel eddies (Hutchings et al. 2002; Lutjeharms 2006).
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Table 4.1: The private haplotypes and alleles (Hp), and haplotype diversities (Hd) for the cytochrome b and RPS7-1 regions respectively, for each locality. The total number of haplotypes (H) for each gene region (bottom of Hp columns) and overall Hd is also shown.
RPS7-1 Cytochrome b
Locality Hp Hd Hp Hd
Quissico 1 0.819 0 0.362
Xai Xai 5 0.322 0 0.174
Maputo 0 0.763 0 0.308
Inhaca 6 0.492 0 0.267
Cape Vidal 4 0.875 2 0.271
Richards Bay 1 0.844 1 0.414
Port Edward 0 0.842 0 0.345
Msikaba 2 0.883 0 0.507
Port St Johns 5 0.473 1 0.352
Coffee Bay 0 0.284 0 0.417
Mbashe 3 0.787 0 0.271
Port Alfred 0 0.810 0 0.345
Overall 12 0.801 39 0.309
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Figure 4.1: Binary current (top) and 2030 (bottom) projected distributions for Epinephelus andersoni. Occurrence records are shown (black dots) and localities that were sampled for genetic data are indicated (blue-filled dots, bolded names).
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Marine fish populations were originally thought to be characterised by relatively high connectivity (Hauser and Carvalho 2008), as fishes are generally thought to be able to migrate to escape unfavourable conditions (Ebersole et al. 2001; Rijnsdorp et al. 2009). However, there is increasing evidence that isolation does occur in marine populations (Cowen et al. 2007; Hauser and Carvalho 2008). The fact that private or isolated alleles were found in certain localities within the E. andersoni population, suggests that this species may be isolated in these localities due to various
environmental factors. These factors appear to be similar to the life-history factors that have been found to cause genetic structuring in other groupers (e.g. De Innocentiis et al. 2001; Carlin et al.
2003; Rivera et al. 2004; Antoro et al. 2006; Chen et al. 2008; Silva-oliveira et al. 2008; Buchholz- sørensen and Vella 2010; Wang et al. 2011).
Firstly, E. andersoni has been shown to be a relatively sedentary species and the majority of the population does not appear to migrate (Dunlop and Mann 2012a), which may play a role in the isolation among populations. Secondly, larvae are more fragile and susceptible to unfavourable environmental conditions and they are also less capable of avoiding them (Potts et al. in press;
Pörtner and Farrell 2008; Rijnsdorp et al. 2009; Pankhurst and Munday 2011). Therefore, larval mortality is likely to increase with projected climate change (Berlinsky et al. 2004; Pörtner and Farrell 2008; Rijnsdorp et al. 2009; Pankhurst and Munday 2011), which would result in reduced larval dispersal (Hoegh-Guldberg and Bruno 2010) and, in turn, further decrease connectivity between genetically distinct populations (Cowen et al. 2007) (such as those on the northern and southern range limits). The projected intensification of upwelling cells and the Agulhas Current (Rouault et al. 2010) could potentially augment this fragmentation, which will eventually reduce the resilience of affected populations (Jones et al. 2007) making them less able to deal with further climate-induced changes (Brander 2007). Despite some differentiation being present within the E.
andersoni stock, there is no clear separation of Mozambique from South African localities. The sustainability of the shared E. andersoni stock, therefore relies to some extent, on the co-operation between South African and Mozambican linefish managers (Lamberth et al. 2009)