Anthropogenic drivers of plant species coexistence in selected riparian systems, Kumawu district, Ashanti region (Ghana)

Figures & data

Figure 1. Map of Ghana, showing the three study areas (Bomfobiri Wildlife Sanctuary Source and Boufrom Forest Reserve – Protected areas and Soboyo community – unprotected area), in the Kumawu District. The four riparian zones are represented by the following colours in the legend: yellow – Onwame riprian zone (ORZ), violet – Pame riparian zone (PRZ), bright green – Ninian riparian zone (NRZ) and red – Amobia riparian zone.

Table 1. Riparian quality index method (RQI) (Gonzalez del Tanago and Garcia de Jalon 2011), used to classify the four riparian zones according to their different habitat conditions, in Bomfobiri Wildlife Sanctuary and Boufum forest reserve and Soboyo community.

	RQI	Disturbance level	Criteria 1	Criteria 2	Criteria 3
Amobia riparian zone	13–15	Intact condition zone	Vegetation connecting with upland species and covers all land between channel and adjacent slopes)	Usually, different vegetation strata cover the entire length of the segment, showing a heterogeneous distribution pattern, linked to natural fluvial forms and flood dynamics, without alterations related to human actions).	Riparian corridor including a mix of species corresponding to the indigenous vegetation associations of the river segment, with different strata (canopy, understory, ground cover) often including shade and climbing plants. No exotic species.
Onwame riparian zone	10–12	Slightly disturbed zone	Average width of riparian corridor slightly restricted by human action	Riparian vegetation covers the entire stretch of the segment but, with slightly reduced coverage, being higher than 60 % of natural coverage, and includes several strata.	Riparian vegetation slightly altered by human actions, with the presence of some alien invasive.
Ninian riparian zone	7–9	moderately disturbed zone	areas where vegetation covers nearly half of the riparian zone being disturbed	1–3 m active channel width and about 10–30% exotic	ruderal species present
Pame riparian zone	1–6	Degraded zone	areas where 60% of the riparian corridor is reduced by human-led activities	vegetation appears in small patches of < 30% (mainly grasses/herbs and isolated woody species), with channel banks connected to agricultural fields	more than 60% of coverage of exotic species, ruderal or alien invasive, due to land use.

Application of the RQI classification was riparian habitat-specific.

Figure 2. a & b: Principal component analysis (PCA) diagram, showing variability in species response to seven disturbance-related drivers in the 120 plots, across the four riparian zones. Plot codes represent the following: ARZ = Amobia riparian zones; ORZ = Onwame riparian zones; NRZ = Ninian riparian zones and PRZ = Pame riparian zone. The black dots represent plant species (e.g. Kyllinga bulbosa = Kbulb, Mariscus longibracteatus = Mlong, Setaria sphacelate = Sspha andPaspalum distichum = Pdist), while the arrows represent the disturbance-related drivers plotted pointing in the direction of maximum change of explanatory variables of the different sample plots. Axis I = 31.74% and Axes II = 21.74% jointly explained 53.48% influence of disturbance-related drivers on the habitat condition and species coexistence.

Figure 3. Gaussian curve or lognormal distribution curve showing how competing species at plot level coexist in their niche space (n-dimensional environmental space) and niche overlaps, along environmental gradient. Notice that severely disturbed Ninian and Pame riparian zones, recorded high species abundance at plot level, whereas less disturbed zone like Amobia, had less abundant species. The x-axis represents disturbance-related drivers in sample plot/riparian zone, calculated as the ‘driver magnitude’ (i.e. a sum of the value score for scope and severity assigned to a specific driver per plot and ranked from highest to lowest – Salafsky et al. 2003; Battisti et al. 2009 ).The alphabets denote the following: u = optimum, t = tolerance and m = maximum.

Table 2. Mean number of plants sampled in each of the 120 plots, across the four riparian zones.

Site	No. of Individuals	Mean ± SE	Variance (s^2)
	374
Amobia riparian zone (Intact zone)		4.6 ± 0.4	12.81
Onwame riparian zone (slightly disturbed zone)	365	5.35 ± 0.5	18.31
Ninian riparian zone (moderately disturbed zone)	440	9.58 ± 1.4	101.98
Pame riparian zone (degraded zone)	410	10.25 ± 1.1	48.24

Amobia and Onwame are located in Bomfobiri Wildlife Sanctuary (a strict protected area). Ninian and Pame are located in the Soboyo community (unprotected area or off-reserve) and Boufum Forest Reserve (a production reserve), respectively. (n = 1,589).

Table 3 Summary of the average scores for seven disturbance-related drivers assessed in each of the 120 plots among the four riparian zones.

Anthropogenic drivers	Onwame riparian zone scores	No. of plots	Amobia riparian zone scores	No. of plots	Pame riparian zone scores	No. of plots	Ninian riparian zone scores	No. of Plots
Bushfire	4	1	2	3	4	5	3	6
Grazing	1	10	1	7	4	8	2	5
Erosion	1	3	1	4	4	3	3	2
Farming	1	4	1	1	4	6	3	2
Animal trampling	2	6	1	12	3	3	3	5
Bare ground	1	1	1	2	4	1	2	4
Logging	3	5	2	1	4	4	3	6

The average scores (magnitude) is a combination of the scope and four severity scores of each disturbance-related driver.

Table 4. Summary of CCA axis lengths, showing the levels of correlation between four riparian zonesand seven disturbance-related driver gradients and percentage variance in species assemblages in 120 plots.

CCA Axes	I	II	III
Eigenvalue	2.22	1.52	1.06
Total % variance	31.74	21.74	15.27
Cum. Total % variance explained	31.74	53.48	68.76
Bushfire	−0.42	0.82***	0.33
Grazing	0.48*	0.22	−0.18
Erosion	0.64**	0.15	0.61**
Farming	0.84***	0.05	−0.17
Animal trampling	0.84***	0.08	−0.37
Bare ground	0.29	−0.59**	0.59**
Logging	−0.27	0.65**	0.09

Asterisks (*) = disturbance-related drivers thatwere significant. A correlation above/below ±0.70 is significant at ***p = 0.001; ±0.60 at **p = 0.01and > 0.44 at *p = 0.05.

Eigenvalues were significant for axes I (P < 0.011) & II (P < 0.013, Monte-Carlo P)

Table 5. Spearman Rank Intercorrelations between the environmental variables.

	Bushfire	Grazing	Erosion	Farming	Animal trampling	Bare ground	Logging
Bushfire		0.24	0.028*	0.34	0.3441	0.001*	0.0003*
Grazing	0.11		0.0061*	0.036*	0.0002*	0.8	0.44
Erosion	0.22	0.26		0.0017*	0.0709	0.06	0.44
Farming	0.095	0.22	0.31		0.0001*	0.72	0.06
Animal trampling	−0.095	0.42	0.18	0.68		0.87	0.37
Bare ground	−0.33	−0.03	0.19	0.037	−0.017		0.0073*
Logging	0.37	−0.08	−0.076	−0.19	−0.087	−0.26

NB: Values above the black diagonal line represent the level of significant, whereas those below represent their respective correlation statistics (R2).

Figure 4. Comparison of estimated species richness among the four riparian zones.

Figure 5. Hill numbers showing diversity profiles among the four riparian systems, in the protected and unprotected areas. Variability in the shape of the curves is indicative of species evenness distribution patterns in each of the four riparian systems. Thus, shallow curve reflects high diversity of a site, while steep curve indicates less diversity.

Table 6. Summary of Hill numbers (at α = 0, 1 and 2), Hill species diversity and Shannon_Evenness, across the four riparian zones in protected and unprotected areas, at Kumawu District of Ghana.

Alpha values	Onwame	Amobia	Pame	Ninian
0(Number of sp.)	70	81	39	44
1(exp[Shannon])	53.32	63.58	30.53	30.04
2(Inv.[Gini_Simpson)	42.28	50.83	26.79	21.53
Shannon_Evenness	0.76	0.78	0.79	0.67
Hill’s numbers	2.77	2.81	2.86	2.59

The order q, which is mathematically unified family of diversity indices differing among themselves only by an exponent q, and which indicates their tendency to include or ignore the relatively rarer species (Hill 1973). Thus, q = 0, represent number of species in the sample (richness index), q = 1, exponential of the Shannon-Weiner index and q = 2, reciprocal of the Simpson’s index (i.e. equivalent number of species).

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Data availability statement

The code and data used in the analysis of this study, will be publicly accessible at Dryad Digital Repository (https://datadryad.org/).