Reversal of whitefish species diversity took place due to closer breeding

Plant and animal species diversify to occupy specific environmental niches created by several ecological factors. But can a reduction or elimination of environmental niches force the otherwise diversified species to come together and hence resulted in a reduction in species diversity?

A study published today (February 16) in Nature proves that reversal in species diversity can indeed result when different environmental niches in a given system, say lakes, are reduced or removed. The study highlights the case of whitefish species seen in 17 Swiss lakes.

Reproductive isolation

Diversification in the case of whitefish species is manifested in the form of body size and gill rakers (cartilaginous structures that protrude from the gills).

The two species — large-size species with a few widely spaced gill rakers and small-size whitefish with plenty of densely packed rakers — show very clear reproductive isolation.

The large-size ones spawn in winter in shallow littoral habitats; the small-sized ones spawn in deeper water in winter.

But eutrophication changed the ecosystem in the 17 Swiss lakes. Eutrophication results when there is excessive plant growth and microbial life due to excessive nutrient supply. The enhanced growth and death of plants and animals result in increased decomposition. This results in depletion of oxygen in the lakes.

Oxygen reduction

In this case, the reduction in oxygen affected the deep-water spawning whitefish species. It also killed benthic life that served as food for this fish species.

These changes “compressed the depth range in which whitefish could spawn,” thus bringing them closer together to breed.

The convergence occurred in the case of lakes that were moderately and strongly polluted, but not in the mildly polluted ones.

The coming together of the species to breed resulted in “increased [the] gene flow between previously ecologically differentiated species.”

In theory, natural selection between species can continue, even when gene flow between the species is present, thus keeping the species apart.

But in this case, the human intervention was so pronounced that selection could not surpass gene flow between the two distant species.

The loss of genetic variability is the proof that diversity loss had indeed taken place. The authors found that reversal of speciation had set in the whitefish species in 13 lakes.

“Most whitefish assemblages have lower species and functional diversity today than historically,” they assert. But it is not just the whitefish that suffered due to eutrophication.

“The lakes have lost 38 per cent of species diversity, 14 per cent of functional diversity and 28 per cent of functional disparity among species. “Eight endemic species and seven distinct populations of species [seen till recently] have become extinct,” they write.

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