Estimating the encounter rate of Osteolaemus sp. aff. tetraspis in KMA
This study provides the first-hand urban population status of Osteolaemus sp. aff. tetraspis in Ghana and probably the first of its kind in West Africa. Although dwarf crocodiles are reported to be the most generalist of the African crocodiles, occupying various kinds of habitats (Eaton 2008), the encounter rates generally differed significantly across all sites. This is due to the difference in the vegetation and physical characteristics of the various sites as well as the variation in the levels of anthropogenic disturbances (such as agricultural expansion, and infrastructural development (Eaton 2008; Gonwouo and LeBreton 2010). Encounter rate in KNUST - campus is higher than all the other sites thus envisaging it as a potential urban crocodile refuge but will require an initial detailed and proper establishment and management mechanisms put in place first. Other sites of conservation importance are the CV, and Park as these sites did not significantly vary from KNUST - campus in terms of the encounter rates. It is however tricky conserving such areas as the habitats are opened access and managed by the traditional authorities who commonly value land than conservation as the demand for land in urban areas is very high with high opportunity costs.
The encounter rates of Osteolaemus sp. aff. tetraspis in this urban range are within the encounter rates of similar studies of exploited non-urban range populations of the species (Shirley et al. 2009; Wild 2000; Smolensky 2015) and are therefore considered to be low (Smolensky 2015). However, no significant difference existed between the findings of this study and those of Shirley et al. (2009); Wild (2000); Smolensky (2015) (Fig. 3B). Therefore, though this study is based on a one season’s (wet season) survey with a small sample size, the non-significance in encounter rates in relation to non-urban environments presents an opportunity for the urban conservation and management of Osteolaemus sp. aff. tetraspis in Ghana. The wet season’s survey data of this study are expected not to vary significantly from the dry season's estimates though since the survey period of this study coincided with low water levels and fewer rains with high clarity resulting in easy detection of individuals including the submerged individuals (Shirley et al. 2009; Smolensky 2015). It is, however, also relevant to mention here that the average encounter rate of this study ~ 4.5 percent lower than the unexploited population in Gabon (Eaton 2009). But it is possible that such a difference may not be obvious if the population studied in this study were also unexploited populations because personal observations indicate that the unexploited population of Osteolaemus sp. aff. tetraspis, especially under traditional protection in Ghana, are significantly abundant with relatively high encounter rates. The low general encounter rates are a result of urban-related disturbances such as habitat conversion for farming and settlement, and other developmental purposes. According to Cobbinah and Amoako (2012), and the Ghana Statistical Service (2012), the population of KMA is rapidly increasing thereby serving as a potential driver of change that is negatively (increased exploitation, and demand for land) impacting the encounter rates of crocodiles (Smolensky 2015) and this may have accounted for the low encounter rates observed in this study.
Estimating population structures Osteolaemus sp. aff. tetraspis in KMA
Just as the encounter rates, the population structures may also be affected by the type and quality of the aquatic habitats being studied. At site-level, the population structures of this study were good for KNUST - CAMPUS, CV, and Park as the encounters were evenly distributed across all the three-size classes (Fig. 5, 6, and 7) and the availability of breeding habitats. these three sites were reported by some key informants to have a history (in the past 10–20 years) of high species abundance and this together with the fewer cases of both old and recent hunting may have resulted in their high population structures. Such a balanced population structure together with the site-level high encounter rates give the impression that these three sites might have viable populations that require urgent management and conservation programs and action plans. In the cases of the other sites: FCRI, Ey, and Com, the complete adult bias population structure recorded was possibly due to high level of hunting and habitat encroachment. Additionally, FCRI possibly recorded a very poor population structure because it is a research institution with an established dam for the purpose of research field irrigation. It is suspected that the population in this habitat is an impotent population as they may have been brought from elsewhere as single-sex and released into the dam for experimental purposes. This is because the site based on my site assessment is a potential breeding site capable of providing adequate nesting materials, food resources, and a lack of flooding. But unfortunately, all my daytime surveys involving scouting for nests both old and fresh: as this was the perfect hatching period of dwarf crocodiles, neither a nest nor nesting signs were recorded. This was the opposite in KNUST - CAMPUS, CV, and Park where 2, 3, and 1 successfully hatched nests (Fig. 9C) were recorded respectively. A different situation was observed in Com as one nest was recorded but was unfortunately unsuccessful (Fig. 9D). Also, the Com’s channel is directly connected to the Owabi Wildlife Sanctuary and I suspect that due to urban pressure, the animals have migrated to the sanctuary where breading and population structures are expected to be high. Generally, the population structures of this study when compared with the findings of Smolensky (2015), indicate a better representation of all size classes than in the case of Smolensky. Smolensky found a juvenile, and adult population bias structure, whereas in my study I recorded an even population structure for all the three major size classes of the species. This gives a good justification that the urban population of this species has the potential of even outcompeting some of its non-urban range counterpart and is, therefore, worth conserving.
Environmental Determinants of Osteolaemus sp. aff. tetraspis population patterns in KMA
This section investigated the environmental determinants of the variations observed in the population structures and encounter rates. The species responded differently to all the six environmental gradients (urban agricultural expansion, grassland, settlement encroachment, dredged area, and tree cover (closed and opened)). The KNUST site which has the highest closed tree canopy cover recorded the highest crocodile encounter rates as the abundance of crocodiles was equally highest in this site. Contrary, Osteolaemus sp. aff. tetraspis correlated negatively with the other habitat variables such as settlement, grassland, and dredging. The positive correlation with forest cover explains the higher population structures, encounter rates, and abundance of the species in KNUST - campus as high tree cover supports healthy breeding environment, food availability, and refuge (Ross 1998; Waitkuwait 1986, 1989). Other scientists also found that adequate canopy cover provides more litter to support the nesting of forest-dwelling crocodiles (Ross 1998; Thorbjarnarson et al. 1992; Waitkuwait 1989). Similarly, crocodiles are very elusive and prefer dense vegetation which can maintain their camouflage behavior and make hunting difficult (Bayliss 1987; Hutton and Woolhouse 1989; Shirley et al. 2012).
The existence of a botanical garden could be the reason why the tree cover of KNUST - CAMPUS is high, hence higher crocodile abundance and encounter rate. This is again expected because, the species is a forest animal and a mound nesting creature that uses only dead plant material for nesting (Waitkuwait 1986, 1989; Amoah et al. 2021). Likewise, closed canopies support the elusive mechanism of the species to avoid threats such as hunting, thus corroborating with the findings of Eaton (2008) who reported highest densities of dwarf crocodiles in areas of less/absent hunting. Furthermore, it is widely argued that low human disturbance and high forest cover correlate positively with crocodile densities (González-Trujillo et al. 2014). González-Trujillo et al. (2014) found that vegetation structure and richness correlated with crocodile densities. Interestingly, they found out that the relative density of crocodiles responded positively to increasing vegetation cover which confirms our results.
Agricultural lands and opened canopy areas on the other hand recorded the second highest crocodile abundance because dwarf crocodiles are widely known to be nocturnal mainland foragers (mostly agricultural lands, and open bushes) especially following rains as rains stimulate the activities of many mainland’s preys (snails, small mammals, millipedes, centipedes etc., (Pauwels et al. 2007)) thereby exposing them to predation by dwarf crocodiles (Riley and Huchzermeyer 1999; Waitkuwait 1989). According to Waitkuwait (1989) night terrestrial foraging occurs at varied distances starting from the stream banks and extends several meters into agricultural lands or mosaic bushes. This is not however, to argue that agricultural activities around crocodile habitats should be allowed to escalate. Meaning such activities should be moderated wherever they are present.
Sites (Park, CV, Com, and Ey) have responded similarly to high agricultural encroachment, grassland, and settlement encroachment. This is the case because these sites are all open access to the public in the metropolitan area where infrastructural development and agricultural activities are rapidly increasing. These sites would have benefited from the riparian buffer zone policy of Ghana. Unfortunately, poor implementation and weak compliance have made it currently impossible to moderate anthropogenic disturbances along the freshwater gradients of Ghanaian cities (Gonwouo and LeBreton 2010). Even though all the sites responded similarly, CV and Park are still having the chance of being next to develop for the species conservation after developing KNUST - campus since they have not significantly differed from KNUST - campus in terms of population structures, and encounter rates of the specie.