The City in the Tropics: Lizard Community Structure and Spatial Resource Use Along an Habitat Gradient of the Dahomey Gap (West Africa)

doi:10.21203/rs.3.rs-310688/v1

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The City in the Tropics: Lizard Community Structure and Spatial Resource Use Along an Habitat Gradient of the Dahomey Gap (West Africa)

https://doi.org/10.21203/rs.3.rs-310688/v1

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Studies on habitat gradients are particularly interesting for the community ecology theory, but almost no investigation has been carried out on community structure of any vertebrate groups along gradients of habitat that were modified by humans in historical or prehistorical times, such as for instance the Dahomey Gap in West Africa. Here, we analyze the community structure of lizards in suburban Lomé (Togo) and in comparison with nearby savannah and forest sites, with a suite of statistical methods. Overall, we recorded 25 lizard species, with a heavy reduction in species richness from forest (18 taxa) to savannah (13) and suburbs (9). 24% of the species occurred in all habitat types, 40% exclusively in forest, and only two were exclusive of suburban habitats. Suburban habitat types were relatively homogeneous in terms of number of observed species (maximum number of taxa per habitat = 6). There were significant interspecific differences in both substratum type preferences and vertical spatial niche by species, but with no evidence of a nonrandom niche partitioning pattern, and hence with a competitively assembled community structure. There was a nonrandom “clustered” distribution of the various species along the available resource categories, thus indicating that species-specific preferences instead of community-driven mechanisms are more likely to explain the observed patterns. We concluded that lizard communities in tropical cities are (i) less species-rich than in the surrounding more natural habitats, (ii) usually clustered into specific habitat/substratum types (often artificial ones), and (iii) not assembled through competitive interactions.

Ecological Modeling

Sauria

Togo

niche partitioning

habitat use

community ecology

ecological gradients

Studies on habitat gradients are particularly interesting for the community ecology theory because the environmental continuum may be a useful axis for understanding how attributes of individuals do generate structure at the level of the community (e.g. see Wellborn et al. 1996; Pillsbury and Miller 2008; Khatiwada et al. 2019). It has been shown that community structure across a given gradient may be determined by both physical factors limiting the potential breadth of species distributions (for instance, proximate eco-physiological performances; see Barbault 1991; or habitat characteristics, e.g. Avila et al. 2017) and biotic effects mediated by ecological interactions, such as predation (Nurminen et al. 2018; Freestone et al. 2020) and interspecific competition (Pianka 1986; Salinas-Ramos et al. 2020; Powell et al. 2021).

Most studies on community structure along tropical habitat gradients have focused so far on habitats that have been modified by humans during recent times, for instance burnt versus non-burnt areas (Akani et al. 2018; Costa et al. 2020) or forest-plantation mosaics (Palmeirim et al. 2017; Supsup et al. 2020). However, almost no investigation has been carried out on community structure of any vertebrate groups along habitat gradients that were modified by humans in historical or prehistorical times, despite these studies may be important for a better under standing of the community dynamics along habitat gradients. In this regard, the Dahomey Gap in West Africa may offer a excellent scenario where to perform such studies. The Dahomey Gap is a savannah-like vegetation zone that extends all the way to the coast in Benin, Togo, and Ghana, thus separating the forest zone that covers much of the south of the region into two separate forested regions: the Upper Guinean forests and the Lower Guinean forests (Demenou et al. 2018). The Dahomey Gap has been created, according to some studies, by deforestation during the late Holocene, whereas for other researchers it should be a natural transitional vegetation zone between forest and savannah (Demenou et al. 2018). Anyway, now the Dahomey Gap is also characterized by the presence of a myriad of urban centres, with Accra (capital of Ghana), Lomé (capital of Togo) and Cotonou/Porto Novo (Benin) being the largest urban agglomerates.

In this study, we analyze the community composition of sympatric lizards along a habitat gradient, forest-savannah-suburbs (hereby FSS), in south-western Togo, inside the Dahomey Gap of West Africa. We will focus (i) on the taxonomical composition and species richness of lizards within each habitat type, and (ii) on their resource partitioning patterns along the spatial niche axis with regard to the suburban habitat. We selected the suburban habitat for the detailed resource partitioning study because this is the simplest habitat type of the Dahomey Gap in terms of number of sympatric species, thus allowing a easier quantitative assessment and a better understanding of the community dynamics.

More specifically, in the present paper, we examine the following key questions:

Are there any species richness differences among lizard assemblages inhabiting the various habitats in the FSS gradient? This question is relevant because there are empirical evidences that tropical forest communities are more species-rich than savannahs or deforested areas (Estrada et al. 1993; Addai and Baidoo 2013; Yeo et al. 2017; Schyra et al. 2019), but quantitative data are not available for the Dahomey Gap region in West Africa.

If so, are the inter-habitat differences in species richness due to proximate ecological factors (i.e. recent events of local deforestation) or to broad historical/biogeographical events at the West African scale? This is important because the Dahomey Gap has been considered an anthropogenic vegetation zone that originated thousands year ago (Iloh et al. 2017; Demenou et al. 2018; Leaché et al. 2020), so it is possible that not only proximate factors may influence actual community structure.

Focusing on the ecologically simplest habitat type (in terms of species richness) in the gradient, what are the main assembly rules of the lizard assemblage? That is: is interspecific competition for the spatial niche an important determinant of the community structure in the simplest habitat type of the FSS gradient? This aspect is relevant because it is easier to detect interspecific competition mechanisms in simple than in complex arenas, and thus a relatively simple natural scenario may help considerably our understanding on this issue.

Study areas and field protocol

In order to compare community composition of lizards across a gradient of habitat alteration (FSS), we used data from Segniagbeto et al. (2015) for forest and forest-derived savannah, supplemented by original opportunistic field observations (years 2013–2020), and compared these data with those originally collected by us in the suburbs of Lomé (see below). The map of southern Togo, showing the various study areas, is given in Fig. 1. For forest, we considered the presence data of lizards for Missahohé, Agome Yo, Kpalimé and Agou, i.e. four hilly, adjacent sites situated nearby the border with Ghana in south-western Togo. For forest-derived savannah,we considered the presence data of lizards at Tsévié, Tgblekope and Agoé, also situated in the south-western Togo.

The field study at the urban area was carried out in July-August 2013 and in July-August 2014 in a suburban area of Lomé (Baguida), the capital city of Togo, West Africa. Baguida is a residential area situated along the coast, at the western periphery of Lomé, nearby the city port and along the highway connecting Togo with Benin. The climate of this area is tropical, with dry season from October to March, wet season from April to September, but with a short phase of relative rain shortfall during August.

Suburban lizards were searched for along random transects, walked slowly and by two-three researchers, passing through the Baguida landscapes, from about 0900-1200hrs and 2000-2200hrs, for a total of 21field days in 2013 and 23 days in 2014. Visual Encounter Survey (VES) methodology was applied, and each lizard was observed at a distance ranging 10–150 cm. During night hours, we used a torch (model Weltool T10) to examine the various substratum types and recording the lizards. We considered in the analyses only the above-ground active individuals, so, we did not consider those lizards (mainly Agama agama) that were observed while sleeping perched on walls, trees or other substrates.

Each observed lizard was identified to species and sex; however, H. angulatus and H. mabouia were pooled due to their difficult discrimination in the field. We recorded the habitat characteristics of each lizard individual along a horizontal and a vertical spatial axes. In order to evaluate the horizontal spatial resource use, we recorded the substratum where each individual was recorded. Lizard sightings were assigned to one of the following six different substratum typesΛ1) cement/asphalt, (2) grass, (3) trees/wood, (4) sand, (5) bushes, and (6) stone-piles. The relative availability of each substratum type was determined by eye by inspecting the relative area occupied by each habitat type in 1000 random spots of 5 x 5 m that were selected along the surveyed area. N order to evaluate the horizontal spatial resource use, we determined the height from the ground of each lizard individual at the sighting time. We defined five height categories: (a) on the ground (up to 10 cm height); (b) 11–50 cm height; (c) 51–100 cm height; (d) 101–150 cm height; (e) > 150 cm height.

Statistical analyses

Comparisons of lizard community compositions among the three FSS gradient habitats were made just qualitatively as there were no sufficient quantitative data in Segniagbeto et al. (2015) to allow any statistical analysis. Concerning the XXXystemat lizard community, the differences in the frequencies of observed lizards by species and by substratum type, as well as within species (i.e. males versus females versus juveniles) and by substratum type, were assessed by contingency table χ² test. Pearson’s correlation coefficient was used to analyze the correlation between substratum type availability (independent variable) and number of observed lizards by species (dependent variable). When running the above-mentioned analyses, we considered that, in order to process the data statistically, the sufficient number of individuals per species was 30. Thus, we did not analyze the data by contingency tables and correlations for those species with less than 30 sightings.

Table 2
Raw data of the lizard sightings at the suburban area, divided by species and by substratum type, at Baguida (Lomé), Togo.
		A. agama	A. agama	A. agama	T. quinquetaeniata	T. quinquetaeniata	T. quinquetaeniata	T. maculilabris	T. perrotetii	Chamaeleo gracilis	Hemidactylus sp.
substratum	% availability	males	females	juveniles	males	females	juveniles
cement	46	26	48	40	0	4	0	2	0	0	54
grass	45	48	62	86	5	9	0	0	0	0	10
bushes	1,4	7	5	4	8	10	0	0	0	0	42
sand	7	4	6	5	13	17	0	0	0	0	0
trees/wood	0,5	2	1	0	17	23	7	0	2	1	12
stone piles	0,1	0	5	3	1	1	7	2	6	0	8
total	100	87	127	138	44	64	14	0	0	0	126

In order to differentiate the various types of substrates in terms of their quantitative lizard community composition, we used a One-Way Analysis of Similarities (ANOSIM). ANOSIM is roughly analogous to an ANOVA in which the univariate response variable is replaced by a dissimilarity matrix, i.e. with distances that were converted to ranks (Clarke 1993). Significance was computed by permutation of group membership, with 9,999 replicates, and Bray-Curtis was used as distance measure. Sequential Bonferroni correction was used for post-hoc comparisons of pairwise ANOSIMs between all pairs of groups.

We calculated Chao-1 index in order to predict the theoretical number of species that can be expected to use each substratum type on the basis of the sampling regime. In addition, the following univariate community diversity metrics were calculated for each substratum type: (i) Species richness (total number of species recorded at each site); (ii) Dominance (D); (iii) Simpson index (S) S = 1 – D; (iv) Shannon entropy index (H’; Shannon and Weaver 1963) and (v) Evenness (e), calculated by Buzan and Gibson’s formula (Magurran 1988). For each diversity metric, we also generated upper and lower 95% confidence intervals by performing a bootstrap analysis with 9999 random samples, each with the same total number of individuals as in each original sample generated (Harper 1999).

Niche breadth (B) in resource uses was assessed by Simpson’s (1949) index. In order to evaluate the “structure” (sensu Gotelli and Entsminger 2003) of the lizard community in terms of substratum use and height of perches, we firstly calculated the similarity XXXystema species/population groups by Pianka’s (1986) symmetric equation (with 0 indicating no similarity and 1 total similarity), and then performed a statistical simulation study. Indeed, to assess whether the various observed overlap values between species/population group pairs occurred by chance, we randomized the original species utilization matrices by shuffling the original values among the resource states by using the randomization algorithms RA2 and RA3 (Lawlor 1980), with 30,000 random Monte Carlo permutations generated by Ecosim software. RA2 tests for structure in the generalist–specialist nature of the resource utilization matrix by conserving guild structure, but destroying observed niche breadth (Gotelli and Graves 1996). RA3 tests for guild structure by conserving niche breadth for each species, but destroying guild structure manifested by the zero structure of the resource utilization matrix (Gotelli and Graves 1996). If the observed overlaps were significantly lower than the randomized overlaps, there was evidence of community-wide niche partitioning of the analyzed niche dimensions (substratum type and perch height). If the observed overlaps were significantly higher than the randomized overlaps, there was evidence of community-wide convergent use of the niche dimensions (i.e. a “contagion” effect), whereas if observed and randomized overlaps did not differ significantly, the lizard community used the niche dimensions by chance, i.e. randomly. The substratum type availability (%) was added to the XXXystematicXXX in the Ecosim software.

The pairwise similarity of each lizard species with the availability of each substratum type was assessed by non-metric multidimensional scaling (nMDS), using cosine as similarity measure between items in the item-item matrix. A Principal Component Analysis (PCA) was used to displaying the various lizard species by substratum type in the multivariate space, retaining only factors with eigenvalues > 1 in the analysis.

General community characteristics

Pooling all the study areas, there were 25 lizard species in total, belonging to five different families (Table 1). Comparing the species composition along the FSS gradient, there was a heavy reduction in the species richness, from 18 taxa in the forest to 12 in the savannah up to 9 in the suburbs (Table 1). Six species (24% of the total) were found in all habitat types along the gradient, ten (40%) were found exclusively in forest, four (16%) only in savannah, two were found in both forest and savannah (8%), in both forest and suburbs, and in only suburban habitats. This exclusively suburban species were the geckos Ptyodactylus ragazzi, that was recently introduced by merchants of charcoal and firewood transported from the northern regions of Togo (our unpublished observations), and Tarentola ephippiata. These latter species are currently common in some areas of Lomé, for example in the Be-Kpotà sector of the main city centre.

Table 1
Comparison of species richness along the forest, forest-derived savannah, suburbs gradient in South-Western Togo. 1 = indicates presence of a given species in a given habitat type along the FSS gradient
Genus	Species	forest	savannah	urban
Scincidae
Trachylepis	affinis	1	1
Trachylepis	albilabris	1
Trachylepis	maculilabris	1	1	1
Trachylepis	perrotetii	1	1	1
Trachylepis	polytropis	1
Trachylepis	quinquetaeniata	1	1	1
Cophoscincopus	simulans	1
Mochlus	guineensis	1
Panaspis	togoensis	1
Gekkonidae
Hemidactylus	albituberculatus		1
Hemidactylus	angulatus	1	1	1
Hemidactylus	fasciatus	1
Hemidactylus	kyaboboensis	1
Hemidactylus	mabouia	1	1	1
Hemidactylus	muriceus	1
Ptyodactylus	ragazzi			1
Tarentola	ephippiata			1
Agamidae
Agama	agama	1	1	1
Agama	sankaranica		1
Chamaeleonidae
Chamaeleo	gracilis	1	1
Chamaeleo	necasi	1
Chamaeleo	senegalensis		1	1
Varanidae
Varanus	exanthematicus		1
Varanus	niloticus		1
Varanus	ornatus	1
TOTAL		18	13	9

There were significant differences in the frequency of species by ecological niche (arboreal/climbers; arboreal-terrestrial; terrestrial; subterranean) across the FSS gradient (contingency table: χ² = 23.4, df = 6, P < 0.001), although arboreal-terrestrial and arboreal/climbers were dominant in all habitat types (Fig. 2). The forest lizard community had however a higher breadth of utilized niches compared to the other two habitats (Fig. 2).

Community structure in the suburban area

In the suburban site, during our surveys we observed a total of 611 individuals belonging to 6 species of lizards. Three of the observed species were Scincidae (2 individuals of Trachylepis maculilabris, 8 Trachylepis perrotetii, and 122 were Trachylepis quinquetaeniata), one Agamidae (352 individuals of Agama agama), one Gekkonidae (126 individuals of Hemidactylus sp.), and one Chamaeleonidae (one individual of Chamaeleo senegalensis). Only three species were observed in sufficient number for statistical analysis to determine their niche characteristics patterns: A. agama, T. quinquetaeniata and Hemidactylus sp. The former two species were observed only during the period 0900–1200 hrs, whereas Hemidactylus sp. mainly during the period 2000–2200 hrs, with four individuals sighted in the morning sampling.

Species richness was adequately sampled within each habitat type, as revealed by the plateau phase reached by the respective saturation curves (Online Supplementary Figure S1). Habitat types were relatively homogeneous in terms of number of observed species, with the highest number (n = 6) being observed in bushes (Table 2). Chao-1 estimated that the maximum number expectable per habitat type is 7 (Table 3). In terms of diversity metrics (calculated from the number of individuals by lizard species sighted on each substratum), the various substrates showed relatively similar values, with bushes and trees/wood being used by a slightly higher diversity of species and with a slightly higher evenness than the other substrates (Table 3). However, there were no statistically significant differences among substrates for any of the diversity metrics using ANOSIM (mean rank within substratum types = 29.46; mean rank between substratum types = 37.6; R = 0.05, at least P > 0.155 in all metrics).

Table 3
Mean values of diversity metrics, with upper and lower 95% confidence intervals (CI), for the various substratum types surveyed in Baguida (Lomé), Togo.Values are calculated on the basis of the number of lizard individuals by species that were observed in each substratum type during the field surveys.
	Species richness	Individuals	Dominance	Simpson	Shannon	Evenness	Chao-1
cement	3	178	0.547	0.453	0.713	0.68	3
Lower CI	3	178	0.3541	0.5472	0.9604	0.4867	3
Upper CI	6	178	0.4523	0.6454	1.162	0.8652	7
Grass	3	179	0.7591	0.2409	0.4851	0.5415	3
Lower CI	3	179	0.3556	0.5412	0.9555	0.4938	3
Upper CI	6	179	0.4588	0.6442	1.166	0.8845	7
trees/wood	3	76	0.4058	0.5942	0.9969	0.9033	3
Lower CI	3	76	0.3393	0.5083	0.8908	0.5555	3
Upper CI	5	76	0.4917	0.6586	1.196	0.9428	6
Sand	3	49	0.4969	0.5031	0.7982	0.7405	3
Lower CI	3	49	0.3294	0.4715	0.8477	0.5827	3
Upper CI	5	49	0.5277	0.6706	1.215	0.9612	6
Bushes	6	71	0.4767	0.5233	1.077	0.4891	6
Lower CI	3	71	0.3402	0.4983	0.8908	0.5569	3
Upper CI	5	71	0.5013	0.6594	1.182	0.9513	6
stone piles	3	25	0.3344	0.6656	1.097	0.9984	3
Lower CI	3	25	0.3184	0.3904	0.7038	0.5911	3
Upper CI	5	25	0.6096	0.6816	1.241	0.9784	6

Suburban lizards: horizontal niche resource use

The synopsis of the observations by species and by substratum type, and in relation to the relative availability of each substratum, is given in Table 2. The three species used the various substrates in a significantly different way (χ² = 225.68, df = 5, P = 0.00001), with A. agama being observed mainly in cement and grass, T. quinquetaeniata mainly in bushes and sand, and H. angulatus in cement and trees/wood. Within species, there were neither intersexual (χ² = 7.2, df = 5, P = 0.209) nor adults versus juveniles (χ² = 8.6, df = 5, P = 0.111) differences in A. agama; similarly, there were no significant intersexual differences in T. quinquetaeniata (χ² = 3.2, df = 5, P = 0.668), with the number of juveniles being too small for any statistical analysis.

Substrates were used in relation to their relative availability by A. agama, but not by T. quinquetaeniata and H. angulatus (Table 4), showing that the former species was more a substratum type generalist than the two latter species.

Table 4
Correlation values between substratum type availability and number of individuals observed in each substratum type at Baguida (Lomé), Togo. Statistically significant P-values would indicate generalism in substratum type usage by lizards. Significance is in boldface.
	Pearson's corr. Coeff. (R)	P
A. agama males	0.915	< 0.01
A. agama females	0.979	< 0.0001
A. agama juveniles	0.896	0.0156
T. quinquetaeniata males	̶ 0.526	0.283
T.quinquetaeniata females	̶ 0363	0.479
T. quinquetaeniata juveniles	sample too small
Hemidactylus sp.	0.370	0.470

A nMDS analysis, with cement and grass being placed along the Component 1 (positive values) and trees, bushes and stone piles along Component 2 (negative values), revealed that T. quinquetaeniata resulted positioned towards high positive values of Component 1 and negative values of Component 2, whereas A. agama at negative values of both Component 1 and 2, and H. angulatus at negative values for Component 2 but highly positive values for Component 2 (Fig. 3). In this analysis, Component 1 explained 77.02% of the whole variance and Component 2 explained 22.35%. The three species were also clearly spaced apart in the multidimensional space, with A. agama being very close to the habitat availability centroid, in a PCA diagram (Fig. 4). In both analyses, Hemidactylus sp. centroid was spaced far away from that of the other two species within the multidimensional space.

Suburban lizards: vertical niche resource use

The raw data on the distribution of lizard sightings by vertical height from the ground are given in Table 5. As for A. agama, males had much higher niche breadth (B = 4.46) than females (B = 2.41) and juveniles (B = 2.03). This difference was due to the fact that males (1) used more frequently perches at > 101cm height (26.4% of observed individuals) than females (7.4%) or juveniles (2.9%), and (2) they were observed more frequently at 10–50 cm height, whereas females and juveniles mostly on ground (Fig. 5), Thus, males usually perched higher than females and juveniles and overall, the differences between the three groups of individuals were statistically significant (χ² test, P < 0.001). On the other hand, in T. quinquetaeniata the three groups of individuals did not differ significantly (χ² test, P < 0.001), with all of them being found essentially on ground (Fig. 6). However, also in this species the males exhibited a wider niche breadth (B = 3.26) than females (B = 1.82) and juveniles (B = 1.56).

Table 5
Raw data of the lizard sightings, divided by species and by height from the ground, at Baguida (Lomé), Togo.
Species	Population category	on ground	10–50 cm	51–100 cm	101-150cm	151 cm or more	Total
Agama agama	males	19	27	18	15	8	87
Agama agama	females	70	41	8	5	3	127
Agama agama	juveniles	86	44	4	4	0	138
Trachylepis quinquetaeniata	males	21	7	8	6	2	44
Trachylepis quinquetaeniata	females	44	15	3	1	1	64
Trachylepis quinquetaeniata	juveniles	11	2	1	0	0	14
Trachylepis maculilabris		2	0	0	0	0	2
Trachylepis perrotetii		7	1	0	0	0	8
Hemidactylus sp.		9	18	32	51	16	126
Chamaeleo gracilis		0	0	1	0	0	1

Suburban lizards: simulations study

Horizontal niche resource use.--- As for the substratum type use by lizards, the mean observed overlap between pairs of species was 0.625. The mean of the simulated overlaps with RA3 was 0.365 (variance of simulated indices = 0.0089; random seed = 1942475120), with P(observed < = expected) = 0.983 and P(observed > = expected) = 0.017. Using RA2, the mean of the simulated overlaps was 0.545 (variance of simulated indices = 0.0125; random seed = -523877646), with P(observed < = expected) = 0.753 and P(observed > = expected) = 0.247. Thus, RA3 differed significantly from RA2 in that it showed a non-random preference for same substratum types by the whole lizard community, whereas there was no preference according to the latter algorithm, with the substratum type selection being random.

Vertical niche resource use.---The mean observed overlap between pairs of species was 0.641. Using RA3, the mean of the simulated overlaps was 0.421 (variance of simulated indices = 0.00156; random seed = 956343435), with P(observed < = expected) = 0.999 and P(observed > = expected) = 0.001. With RA2, the mean of the simulated overlaps was 0.560 (variance of simulated indices = 0.00178; random seed = 1521330926), with P(observed < = expected) = 0.976 and P(observed > = expected) = 0.024. In this case, both RA3 and RA2 gave consistent results, showing that there was a non-random preference for same height category by the whole lizard community.

General community characteristics along the habitat gradient

Although not methodological consistent across habitat types, our study revealed that along the FSS gradient (i) there was a clearly higher species richness in the forest than in the savannah and suburbs, and that (ii) the three communities were quite taxonomically distinct. Thus, there was a “forest-specific” assemblage of species (including e.g. Varanus ornatus) and lowly diverse assemblages in savannahs (including e.g. Varanus exanthematicus) (Fig. 7) and suburbs, mostly characterized by ecological generalists with wide habitat tolerances. Both these diversity patterns were not unexpected (McKinney 2008). Concerning pattern (i), there is wide available literature showing that lizard communities are significantly more species-rich in tropical forests than in more open or degraded environments (Lewin et al. 2016), the same as generally observed in both vertebrates and invertebrates (e.g., Basset et al. 2008; Watling and Donnelly 2008). For instance, in the Philippines Supsup et al. (2020) found that higher species richness was found in secondary forests than in mixed-use agricultural areas. Since habitat heterogeneity is an important predictor in explaining lizard richness in Africa (Lewin et al. 2016), it is likely that the multiple microniches available in the hilly forests of the Togo hills may explain the much higher species richness that we observed therein in comparison with savannah and suburban habitats of the same study region.

Concerning pattern (ii), the fact that we observed habitat-specific assemblages of lizards at our study area mirrors observations from Brazilian Cerrado lizard communities (Nogueira et al. 2009). In the Brazilian Cerrado, in fact, habitat-specialists with patchy distributions in the forest-savannah-plantation mosaics create habitat-structured lizard assemblages with faunal overlap between forest and savannah being limited and forested versus open areas acting as mutual barriers to lizard distribution (Nogueira et al. 2009). In our study case, the taxonomical composition of the savannah assemblage was similar as that of the suburban habitat (although in the savannah we also observed Agama sankaranica and a few other species), thus suggesting that there is no clear barrier acting against lizard distribution between savannah and suburban areas in southern Togo.

The species richness reduction from forest to urban areas may also be interpreted as an effect of habitat loss on forest-specialists (Turner 1996; Lea et al. 2003; Edwards et al. 2013; Decena et al. 2020). Indeed, studies from tropical Africa have revealed that forest loss (by slash-and-burn shifting cultivations or by direct tree cutting) at the local scale produced a considerable reduction in the species richness of lizards (Akani et al. 2018). In a Niger Delta area, the number of sympatric lizards species was 9 in a unburnt forest patch, whereas it was just 2 in a nearby forest patch up to 12–16 months after fire (Akani et al. 2018). However, at the scale of southern Togo, the Dahomey Gap savannahs, albeit of likely anthropogenic origin, are there since few thousands years (XXXystemat; see Salzmann and Hoelzmann 2005). Thus, it is likely that the lower species richness of savannah lizard assemblages was due more to broad historical events and geographical/climatological reasons (Salzmann and Hoelzmann 2005) than to local-scale recent habitat loss.

Suburban lizards: analysis of community structure and niche characteristics

Our data revealed interspecific differences in both substratum type preferences and vertical spatial niche, but with no evidence of a mathematical structure consistent with a nonrandom niche partitioning pattern, and hence with a competitively assembled community structure. Instead, we detected a nonrandom “clustered” distribution of the various species along the available resource categories. Therefore, the main conclusion of our study is that species-specific preferences instead of community-driven mechanisms are more likely to explain the observed patterns at the study area.

Huey and Slatkin 1979). When interspecific competition cannot be advocated, the proximate external ecophysiological constraints are often considered the drivers of species-specific differences in niche characteristics (Barbault 1988; Barbault and Stearns 1991). In our case, thermal constraints pushing the selection of given substratum types for lizard basking (Huey and Slatkin 1979; Hailey 1982; Huey 1991) are unlikely to explain the observed patterns, as the study area has a tropical climate with ambient temperatures constantly ranging 27–32°C all throughout the year (https://it.climate-data.org/XXXystem/togo/maritime-region/lome-764237/). Interspecific differences in substratum type uses due for foraging reasons are also unlikely as the various species have a relatively similar insectivorous diet: at the study area, the diet of A. agama consisted mainly of Coleoptera adults and Formicoidea (Akani et al. 2013) and that of T. quinquetaeniata of Coleoptera adults and larvae, Lepidoptera adults and larvae and Formicidae (Dendi et al. 2019). For Hemidactylus sp. we do not have data for Lomé, but in a urban population from Port Harcourt (Nigeria) the diet was composed mainly by Lepidoptera adults and Formicoidea (Amadi et al. 2020a). In this latter species, however, the use of cement and elevated heights are likely linked to the need to exploit artificially lighted spots for optimal foraging over the insects that are attracted by night around electric bulbs (Amadi et al. 2020a). The same strategy was also employed by A. agama in Nigeria (Amadi et al. 2021), but in Lomé we never observed any shifts from diurnal to nocturnal foraging activity in contrast with the Nigerian conspecifics (Amadi et al. 2020b, 2021), whereas dietary shifts have been observed in other tropical lizards from urban environments (Balakrishna et al. 2016). Thus, we suggest that antipredatory reasons may be behind the species-specific substratum type selection and the height of perches, given that predation risk is well known to be a crucial determinant of community structuring of tropical communities (e.g., Pianka 1973, 1986; Poulin et al. 2001; Olsson et al. 2005, Reyes-Olivares et al. 2020). We also noticed that the artificial substratum (cement) was predominantly used by both A. agama and Hemidactylus sp. In this regard, our data mirror other studies showing that lizards in urban areas use artificial substrates a large proportion of the time, and that these substrates tend to be broader than substrates in natural forest (Winchell et al. 2016; Amadi et al. 2020a, 2020b).

Intraspecific differences in the two species for which we collected such data (A. agama and T. quinquetaeniata) were minor, but A. agama males tended to select higher perches than females and juveniles. This positive selection for elevated perches is likely related to the complex hierarchic structure of the A. agama groups, with dominant males often XXXystematic adult females from higher height (Anibaldi et al. 1998).

General considerations on lizard communities in tropical cities

It is well known that urbanization, involving the conversion of natural habitats into human-modified ecosystems, usually reduce both the diversity and the abundance of indigenous animal communities, up to leading to the extinction of some species or even facilitating the establishment of non-indigenous communities in cities and towns (Hamer and McDonnell 2010). So, the general observations that we made (i.e. a reduced species richness in the suburban habitat of the FSS gradient) is not surprising. However, as mentioned above, our study also suggests that the lizard species used the available resources in a clustered way within the suburban habitat in Lomé. Whereas many studies analyzed reptile assemblages in secondary forests and altered habitats (e.g., Luya et al. 2008), the data available in the literature with regard to communities of lizards (or other reptiles) in tropical cities are relatively few and based on different methodologies from ours, and so comparisons with our study are not straightforward. However, in a snake community inhabiting a Brazilian city, Franca and Franca (2019) observed a clustered distribution for two species and dispersed distributions for eight species, and in a Indian city, Janiawati et al. (2016) demonstrated that the various reptilian species were clustered around water sources and vegetation cover patches. In fact, in urban fragments embedded in an “artificial matrix” hostile to the movement of individuals, most species tend to be concentrated into suitable microhabitats (and using clustered types of substratum) as extinction rates (stochastic or anthropogenic) are not compensated by colonization rates (Laurance et al. 2006a, 2006b; Salomão et al. 2019).Thus, we may conclude that lizard communities in tropical urban areas are (i) less species-rich than in the surrounding more natural habitats, (ii) usually clustered into specific habitat/substratum types (often artificial ones), and (iii) not assembled through competitive interactions. Further studies should verify the validity of these conclusions.

Funding – This study was indirectly supported by funds provided from Mohamed Bin Zayed Species Conservation Fund and Turtle Conservation Fund.

Conflicts of interest – none

Ethics approval – not applicable

Consent for publication – All authors read and approved the final manuscript, and accepted to be co-authors in the published paper.

Availability of data and materials – Data will be available by the first author (LL) on request

Competing interests The authors declare no competing interests

Author contributions – LL generated the idea and designed the field protocol. All authors participated equally in data collection, analysis and writing of the various drafts. All authors agreed on the final submitted version of the manuscript.

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FigS1.png
Figure S1 Saturation curves for lizard species discoveries across habitat types in suburban Lomé, Togo

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The City in the Tropics: Lizard Community Structure and Spatial Resource Use Along an Habitat Gradient of the Dahomey Gap (West Africa)

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Abstract

Figures

Introduction

Materials And Methods

Study areas and field protocol

Statistical analyses

Results

General community characteristics

Community structure in the suburban area

Suburban lizards: horizontal niche resource use

Suburban lizards: vertical niche resource use

Suburban lizards: simulations study

Discussion

General community characteristics along the habitat gradient

Suburban lizards: analysis of community structure and niche characteristics

General considerations on lizard communities in tropical cities

Declarations

References

Supplementary Files

Status:

Version 1