Fresh Coast is the Best Coast: cichlid fish diversity in freshwater rocky shores
By Eun Sun (Lydia) Kim
When most people think of conservation of aquatic biodiversity, they think of the glimmering oceans full of colorful tropical fish in coral reefs — but our freshwater friends cannot be neglected. The ecosystems they inhabit contribute enormously to the Earth’s biodiversity though they only cover a tiny percentage of the Earth’s surface. Britton and colleagues spotlight this splendid freshwater diversity in their study of the cichlid fish in Lake Tanganyika and the disturbance that is affecting the cichlids (2017).
Lake Tanganyika’s ecosystem
The lake, shared by Tanzania, Congo, Burundi, and Zambia in northwest Africa, is home to about 1470 different animal species, but it is most known for its cichlids. The cichlids illustrate a textbook example of adaptive radiation: 97% of the 200 cichlid species in this lake are endemic as a result of speciation from a few individuals that, over hundreds of years, filled up the various niches that were available when the lake was newly formed.
Despite the lake’s support of biodiversity, only 6% of Lake Tanganyika’s coastlines are protected. Those protected coasts were not specifically designed to be freshwater protected areas (FPA), but just happened to be included in the territories of the four national parks that border the lake. The rest of the lake is open for use, so small fishing villages to large towns have settled adjacent to the lake. The expanding urbanization has led to various pollution problems, most noticeably increased sedimentation caused by deforestation. When the trees that hold down the soil are removed, soil loosens and erodes into the lake, building up sandy floors where there were originally rocky shore habitats.
The study design
Changes in lake habitats that occur due to nearby human disturbance are a concern because up to 25% of the cichlid species live in these shallow rocky shores. The researchers of this study assessed the effects of possible human impacts along Lake Tanganyika by comparing cichlid biodiversity at 7 localities varying in levels of human disturbance (HD). The three localities representing low HD were on protected national park grounds while the other four unprotected areas of high HD were chosen based on their relatively lower forest density and higher nearby human density.
A total of 554 site surveys within the localities backed the study. These surveys were conducted by SCUBA diving into the water to visually count and identify the cichlids onsite, with video data to double-check the census. After data collection, the numbers were plugged into statistical programs that helped scientists look for correlations between localities and number of species found, as well as observe diversity on three different scales.
Diversity is as complicated as Alpha, Beta, Zeta
The first scale of diversity was alpha diversity, which is the number of species found per site. They found that more disturbed localities had lower alpha diversity. This made sense, since disturbance may change habitat, forcing inhabiting species to either adapt to it or leave the site.
Results got more interesting when comparing species diversity between two sites through the lens of beta diversity, which counts the number of unshared species between sites. Contrary to diversity at the alpha level, beta diversity increased in localities with more human disturbance. This relationship is due to the strong turnover component in beta diversity for the surveyed cichlids. Turnover is defined as species loss then replacement. The explanation drawn from the data analysis was that most species, especially specialist herbivores, were lost by HD-related changes in habitat, so many of the species counted in the surveys were species that recently moved in. Though the species composition looks quite heterogeneous in these disturbed areas, the settlements of some species are so recent that the number of individuals per species is low.
Zooming out even further, zeta diversity summarized diversity between all localities. It revealed that low HD localities had high turnover, whereas high HD localities had high losses without species replacement. In other words, many specialist species were lost while many generalist species that adapted to changes remained in high HD areas, making beta diversity look high in high HD areas despite that they were actually losing many species.
Saving cichlids, forests, and people at once
This study displayed strong relationships between human disturbance and diversity, though they varied depending on the scale at which diversity was viewed. But taking into consideration all the various levels of diversity, the researchers concluded that there is overall higher diversity in the localities that were less disturbed.
The most important finding from this study is that the least disturbed localities, which supported high cichlid diversity, were also situated on national park land. More trees line the national park coastlines of Lake Tanganyika than urban coastlines and less sediment covers the rocks underwater there as well. These terrestrial protected areas had a trickle-down effect on their nearby watersheds by preserving the pristineness of the original rocky habitat, and therefore preserving the preexisting cichlid diversity. This is comforting news for conservation planners, as project spending can be minimized but have its worth maximized by protecting both terrestrial and aquatic wildlife with a single protected area as long as it includes a watershed.
Even if cichlid diversity alone may not be one’s interest in conservation, imagine what other benefits that protection of these freshwater coasts can have. Freshwater ecosystems are overlooked in conservation, which explains why not many FPAs are created nor studied. Yet freshwater sources like lakes are significantly utilized by humans for washing, fishing, disposing waste, and recreation. By giving more attention to freshwater conservation, we may be able to slow down the degradation of the very resources that humans use as well.
Citation:
Britton, A.W. et al. (2017). Terrestrial-focused protected areas are effective for conservation of freshwater fish diversity in Lake Tanganyika. Biological Conservation 212, 120–129.
