Comparison against 2009 baseline

The previous assessment of western Africa’s freshwater biodiversity some 11 years ago (Smith et al., 2009) provides a baseline against which to compare the more recent assessments. Smith et al. (2009) assessed some 1,395 species of freshwater fishes, molluscs, odonates, decapods and selected aquatic plants using the Regional Red List categories and definitions. Of these species, 460 (33%) were considered endemic to western Africa, so that their regional Red List assessments were equivalent to global assessments representing the risk of global extinction for these species. They found that 14% of all native species were regionally threatened, but 34% of the regionally endemic species were regionally threatened, and hence globally threatened. Because we have here conducted global Red List assessments for all species native to the western Africa region, this does not provide a complete like-for-like comparison, but we are able to compare the past and present global assessments and to use these to calculate a Red List Index based on the available data for each taxonomic group (see Section 8.4). Previous global assessments were provided by Darwall et al. (2011).

1.0000 0.9000 0.8000 0.7000 0.6000 0.5000 0.4000 0.3000 0.2000 0.1000 0.0000 Red List index of species survivalworsebetter

2006

Plants Molluscs Fishes Odonates Decapods All groups

2006 2006 2006 2006 2006 2006

Figure 8.2 Red List Index for freshwater taxonomic groups in western Africa. Source: Compiled by the report authors using data from the IUCN Red List (2021).

Cumulative no. category changes as a % of all species Increased threatDecreased threat

0.00%

-1.00%

Fishes Molluscs Odonates Decapods Plants All groups

-2.00%

-3.00%

-4.00%

-5.00%

-6.00%

-7.00%

8.4 Red List Index

The background and methods for calculating the Red List Index (RLI) are presented in Chapter 2.

The assessments presented in Darwall et al. (2011) were completed over a number of years (2003–2009) but all were reviewed in 2009. Therefore, 2009 was chosen as the previous time point for assessment. The new assessments presented here were completed in 2018–2019 and so 2019 was used as the second time point for the assessment.

A number of species have been assessed at other times outside of these two periods. However, these assessments were not comprehensive for all species in the taxonomic group and so have not been included in the RLI calculations.

As described in Chapter 2, The RLI is calculated from the number of species in each Red List category and the number of species changing categories between assessments as a result of genuine improvement or deterioration in status (i.e. genuine changes). Changes in category resulting from improved knowledge or revised taxonomy (i.e. non-genuine changes) are excluded (Bubb et al., 2009).

8.4.1 Fishes

The Red List Index was calculated for 555 freshwater fish species between 2009 and 2019. During this time, five species (1%) experienced a genuine change (an increase) in their threat status (Figure 8.3). No species experienced a decrease in extinction threat. This caused a small decrease in the Red List Index of freshwater fishes from 0.8558 in 2009 to 0.8528 in 2019. The five species experiencing a genuine change in conservation status are listed below.

Arnoldichthys spilopterus (EN) has changed from VU to EN as result of increasing loss of habitat and quality due to oil exploration, urban development and deforestation within its range.

Figure 8.3 Cumulative proportion of species undergoing Red List category changes 2009–2019. Source: Compiled by the report authors using data from the IUCN Red List (2021).

Brycinus carolinae (EN) has been uplisted from VU to EN.

Urbanisation is advancing quickly in this region and this has resulted in a reduction in the distribution area of this species.

Bryconaethiops quinquesquamae (EN) has been uplisted from LC to EN. It is mainly threatened in Nigeria by oil exploitation and pollution and potentially by dredging on the Cross River, which are leading to a continuing decline in habitat.

Micralestes eburneensis (EN) has an increasingly restricted range, and there are recent reports of threats from habitat loss and decline in quality across its range caused by ongoing gold mining activities in and around the Cavally River, and increasing fishing pressure from artisanal fisheries (Doffou et al., 2018). This species is therefore reassessed as EN from Near Threatened (NT).

Tetraodon pustulatus (EN) has been uplisted from VU to EN.

Increasing human populations, oil exploration and urban and industrial development in the lower Cross River are threatening the habitat extent and quality of this species, and threats from deforestation and subsequent loss of habitat quality are predicted to occur in the upper Cross River in the near future.

Some 146 species had nongenuine changes in their conservation status since their previous 2009 assessments, including nine species being moved from non-threatened categories (LC or NT) into threatened categories, and an additional 15 species moved from DD into threatened categories. Conversely, 32 species were moved from threatened categories into non-threatened categories, and an additional 15 species moved from DD into non-threatened categories, reflecting new information on the ranges of these species. These nongenuine changes reflect new knowledge on existing threats rather than new or emerging threats since the previous assessment.

8.4.2 Molluscs

The Red List Index was calculated for 100 species of freshwater molluscs for which data were available between 2006 and 2019. Twelve species only had a single Red List assessment in 2019 and were therefore not included in the RLI. Of the remaining species with two assessments, six species (6%) underwent a genuine change in their conservation status. All six species moved into a higher threat category. This manifested as a decrease in the RLI from 0.8440 in 2006 to 0.8154 in 2019. This six species with a genuine change in conservation status are listed below.

Afropomus balanoidea (EN) was uplisted from NT to EN.

Explosive human population growth and subsequent conversion of lowland swamps to rice fields and use of molluscicides for Bilharziosis control is suspected to have caused a 50% decline in the population of this species over the last 10 years, while a further 50% decline in population is suspected as these threats continue to intensify.

Mutela franci (EN) was moved from VU to EN. The area of occupancy and extent of occurrence of this species has continued to decline since the last assessment as VU in 2009. This is due to the increasing drought caused by climate change and water abstraction/damming upstream. The species’ habitat quality and area have been decreasing, and can be inferred to decrease further over the next decade. For these reasons, the species is presently assessed as EN (EN) based on a predicted population decline of at least 50% over the next 10 years (2019–2029).

Potadoma bicarinata (CR) was uplisted from LC to CR. The habitat quality in the aquatic environments in the Oti River and Volta Basin for this restricted habitat specialist has changed drastically since the original assessment in 2006, and it is considered likely to meet the population decline thresholds for CR in the next 10 years, if not already extinct.

Potadoma freethi (NT) was moved from LC to NT. The rate of population decline for this species is suspected to be increasing as the threats of siltation, water abstraction and droughts intensify throughout its wide range, so the species is assessed as NT, reflecting a change from Least Concern in 2006.

Pseudocleopatra togoensis (CR) was moved from LC to CR. The locality Moheyenga, ‘possibly on the Obi River’ fide (Brown, 1994) could not be traced and has been omitted.

The other localities either have been inundated by Lake Volta or are on river stretches that presently are polluted. It is assumed that the species has become locally extirpated in these areas and it has only been recently recorded from a single locality downstream the Akosombo Dam. Considering the highly limited EOO, AOO, number of localities and

the trend of increasing nutrient enrichment in the Lower Volta, which greatly reduced the EOO, the species is now considered CR, a change from Least Concern, which was based on an assumed previously larger range.

Sierraia expansilabrum (EN) was uplisted from VU to EN. The former assessment was based on data mainly of the pre-civil war situation in Sierra Leone. Since the conflict ended, the demographic, industrial (mining, logging,) and agricultural developments have significantly increased and this trend can be expected to continue in the next decade. The lack of efforts to counter the negative effects to the ecosystems, in particular the surface waters, is leading to a situation in which the drainage systems where many of the sensitive, rare aquatic species occur are becoming rapidly degraded.

8.4.3 Odonates

Of the 307 species of odonate considered here to be native to western Africa, some 204 species had two assessments in 2006 and 2015 on which to calculate the RLI. A further 21 species were assessed in 2016 (2015–2018) (18 LC and 3 DD) and these assessments were back-cast and included in the RLI, bringing the total species included in the RLI to 225.

Some 82 species have not been reassessed since their first global assessment in 2009 (34 LC and 2 DD) and these species were therefore not included in the RLI. There have been no genuine status changes in the odonates between 2009 and 2016, and the overall threat level for species in this taxonomic group is relatively low, resulting in a static RLI of 0.9699.

8.4.4 Decapods

The Red List Index was calculated for the 22 species of crabs between 2009 and 2019. The 28 species of freshwater shrimps native to western Africa have only undergone one global Red List assessment (of which 25 were assessed in 2013), so it was not possible to calculate a Red List Index for this group, which includes four threatened species and nine DD species.

There have been no genuine status changes to the freshwater crab species since 2009 and hence the Red List Index has remained stable at 0.67. However, several species have not been observed at all during this time and in reality, the stability of the Red List Index reflects a lack on new information on the status and distribution of these species, rather than a genuinely stable conservation status.

The Lobster Claw Crab Liberonautes rubigimanus (VU) has been downlisted from EN to VU due to new records showing the species to be more widespread than previously

thought. Conversely, Sachs’ stream crab Potamonemus sachsi has been uplisted from VU to EN due to a reduction in the estimated Area of Occupancy and the number of threat based locations. These are non-genuine changes and hence do not affect the Red List Index for crabs. One additional species of freshwater crab, Potamonautes lipkei, was described in 2010 (Dˇuriš & Koch, 2010), and assessed as DD.

8.4.5 Plants

The Red List Index was calculated for 178 species of aquatic plants for which data were available between 2009 and 2019. Of these, no species underwent a genuine change in conservation status. This is reflected as a stable RLI of 0.9747 in 2009 and in 2019. Some 29 species underwent a nongenuine change in conservation status, primarily owing to new information becoming available on these species.

8.4.6 Discussion

The overall RLI for all freshwater biodiversity decreased from 0.9152 in 2009 to 0.9122 in 2019 (Figure 8.2). The RLI shows an increase in extinction risk for 1% for freshwater fishes (Section 8.4.1) and 6% of freshwater molluscs (Section 8.4.2), but no change was detected for the other taxonomic groups. This is despite clear evidence pointing to increased environmental degradation and human pressure in the region during the previous 10 years (see Chapter 1). For the decapods, only the 22 species of freshwater crabs had two assessments upon which to base an RLI, and for many of these species there were no new surveys since the previous assessment in 2009 (with some notable exceptions, see Chapter 6).

It is vital that conservation actions are implemented to halt and reverse the declines to freshwater biodiversity where possible and conservation actions are recommended for each taxonomic group in Chapters 3–7 and for freshwater biodiversity more generally in this chapter (Section 8.7).

However, without monitoring systems in place to track changes to species’ conservation status in response to emerging threats, it is difficult to prioritise conservation efforts and to track the impact and effectiveness of conservation interventions.

RLIs and the trends they depict are only as good as their data inputs. Red List assessments are considered scientifically robust because they follow a standardised method, are based on quantitative criteria, and use the best scientific data available. Red List assessments also undergo a thorough review process before publication. However, Red List assessments may be revised, for example as knowledge of species and their habitats increases, resulting in changes to the Red List categories assigned.

Additionally, the Red List categories are broad in nature with wide thresholds for moving between categories and, as a result, RLIs should be considered only a coarse measure of changes in the status of biodiversity over time. It should also be recognised that time lags often occur between changes in the real-life situation of a species, detection of these change, and incorporation of these changes into Red List assessments (Bubb et al., 2009). Finally, in the absence of regular monitoring, changes in threats to species are often hard to detect and their impacts hard to quantify over the periods used here to calculate RLIs.

We currently lack basic information on the distribution and population for most of the taxonomic groups considered here.

Standardised regional surveys have not been conducted for many years, if at all, and there are no significant long-term programmes for monitoring the state of freshwater biodiversity throughout the region. There is much evidence for declines in water quality and loss of natural habitats through conversion to other land uses, but there are few data available to determine the impact of these environmental changes on the freshwater species themselves. Consequently, many of the Red List assessments are based on inferred declines in species populations or distributions, rather than robust scientific monitoring data. This lack of monitoring means that real time changes in the status of freshwater biodiversity are not being detected.

There is an urgent need, therefore, to instigate surveys of freshwater biodiversity in the region, combined with the establishment of long-term monitoring stations. Such surveys and monitoring programmes must be able to identify species accurately if we are to have sufficient information to manage and conserve the globally unique freshwater biodiversity in the region. It is also important to make the findings of all surveys which are conducted, often as environmental impact assessments, freely available as input to studies such as this one. The results of these surveys can be used to better inform Red List assessments, which can in turn be used to help track trends in the status of freshwater biodiversity in the western Africa region through use of tools such as the RLI.

8.5 Patterns of species richness

Patterns of species richness discussed in this section consider only the mapped (polygon) extant native ranges of assessed species where they are available. Species with only point localities and no mapped ranges were not included in the species richness maps, neither were parts of species ranges mapped as Possibly Extant, Possibly Extinct or Extinct.

8.5.1 Overall species richness

Species ranges from all taxonomic groups are combined in Figure 8.4 Figure 8.1 to highlight areas containing the highest numbers of freshwater species overall. Species richness maps for each of the five taxonomic groups (freshwater fishes, freshwater molluscs, odonates, freshwater decapods and aquatic plants) are presented in Chapters 3–7.

Freshwater species richness is highest in the coastal areas including the Niger Delta, the Bight Drainages, the coastal basins of the Upper Guinea ecoregions (Sierra Leone and Liberia), the lower sections of the Volta (Ghana), the entire Ashanti ecoregion (Ghana) and lower Eburneo (Côte d’Ivoire) with up to 387 species per subcatchment (Figure 8.4). Species richness generally declines towards the north, approaching the Dry Sahel ecoregion, with the notable exception of the Upper Niger and the Inner Niger Delta (Mali).

Relatively high overall species richness can be found in the Gambia drainage (Gambia and Senegal) in the west and Lake Chad (Chad, Cameroon, Nigeria, Niger) in the east.

8.5.2 Threatened species richness

All known ranges for globally threatened freshwater species are mapped in Figure 8.5. Areas emerging as centres of threatened freshwater species richness include the Niger Delta (Nigeria), the upper Cavally/Cavalla River and Mount Nimba (Liberia, Guinea, Cote d’Ivoire), Fouta Djalon and Northern Upper Guinea (Guinea and Sierra Leone). Other areas with threatened freshwater biodiversity include Lake Chad, the Inner Niger Delta and several coastal drainages between Liberia and Nigeria. Many of these areas are also identified as potential Key Biodiversity Areas for freshwater species in Chapter 9.

8.5.3 Endemic species richness

Species with global ranges restricted to the western Africa region were mapped in Figure 8.6. Upper Guinea and the Niger Delta emerge as centres of endemic species richness, with up to 53 regionally endemic species per sub-catchment.

Other areas of endemic richness include the Lower Niger, Upper Niger and Inner Niger Delta, the Ogun River (Nigeria) and Ouémé River (Benin) in the Bight Drainages, the lower Sassandra, Bandama and Komoé Rivers in Côte d’Ivoire and the Senegal and Gambia Rivers in the west.

8.5.4 Data Deficient species richness

Areas emerging as centres of DD species richness include the Lower Niger River, the Volta and the Inner Niger Delta (Figure 8.7).

Maps of DD species should be viewed with some caution as, by definition, they represent the known ranges of species about which we know relatively little and their mapped distributions are therefore putative. A number of DD species, for which we have no data on their distribution ranges, could not be mapped so are not represented here. This can therefore be viewed as a map of ‘known unknowns’, and may be useful to target future surveys, particularly to gather information on DD species.