Seasonal variations in the structure of avian community in Zambezi riparian forest

Abstract

Seasonal variations in the structure of avian community in Zambezi riparian forest

Among the different biomes in southern Africa, tropical riparian forests appear to be especially rich in terms of biodiversity, but little is known about avian communities occupying these forests. In this paper, bird counts were conducted in May, July–August, and October–November in 2013 and 2014. The line transect method was used. A transect 16.6 km long was designed along the Zambezi River between S 17.4975 E 24.3478 and S 17.4874 E 24.447. In total, 146 bird species were recorded. The number of species detected each season varied from 95 to 100. The number of dominant species was consistently low, varying between 2 and 3 depending on the month. However, the cumulative dominance and the dominance index were much higher in the dry season (May–August) than in the wet season (October–November). Two species were dominant each month: Euplectes axillaris and Pycnonotus tricolor. Vidua macroura and Estrilda astrild were dominant only in October, while Streptopelia capicola was dominant only in May. The group of subdominants was much more diverse than dominants and included 13 species. The Shannon diversity index increased between July and November from 3.45 to 4.05 but Simpson's diversity index did not show any seasonal trend. Pielou's evenness index was higher in the wet season (0.82–0.88) than in the dry season (0.76–0.79). The proportions of the main feeding guilds were much the same throughout the year, except for November, when the proportion of insectivores markedly increased, while that of granivores decreased. Riparian forest corridors in urbanized environments may be viewed as main instruments for offsetting the negative effects of habitat loss and fragmentation.

Dataset published through GBIF (DOI: 10.15470/0ssr1v)

Key words: Community ecology, Population density, Riparian forests

Resumen

Cambios en la estructura de la población aviar del bosque de ribera del río Zambeze según la estación

Entre los diferentes biomas del sur de África, los bosques de ribera tropicales son excepcionalmente ricos en cuanto a biodiversidad, pero no se han estudiado suficientemente las poblaciones aviares de estas zonas. Para este artículo, se hicieron recuentos de pájaros durante los meses de mayo, julio–agosto, octubre y noviembre de 2013 y 2014. Se usó el método de transecto lineal. Se diseñó un transecto de 16,6 km de longitud a lo largo del río Zambeze entre las coordenadasS 17.4975 E 24.3478 y S 17.4874 E 24.447. En total, se registraron 146 especies de pájaros. El número de especies registrado en cada estación varía entre 95 y 100. El número de especies dominantes es muy bajo, entre 2 y 3 según el mes. Aun así, la dominancia acumulada y el índice de dominancia son mucho más elevados durante la estación seca (mayo–agosto) que durante la húmeda (octubre–noviembre). Hay dos especies dominantes en todos los meses: Euplectes axillaris y Pycnonotus tricolor. Vidua macroura y Estrilda astrild solo son dominantes en octubre, mientras que Streptopelia capicola solo lo es en mayo. El grupo de especies subdominantes es más diverso que el de las dominantes, e incluye 13 especies. El índice de diversidad de Shannon aumenta entre julio y noviembre de 3,45 a 4,05, mientras que el índice de diversidad de Simpson no muestra ningún patrón estacional. El índice de homogeneidad de Pielou es más elevado durante la estación húmeda (0,82–0,88) que durante la seca (0,76–0,79). La proporción de los grupos tróficos no experimenta cambios significativos a lo largo del año, salvo en noviembre, cuando la proporción de insectívoros aumenta notablemente, mientras que la de granívoros decrece. Los corredores de bosque de ribera en entornos urbanizados se pueden convertir en la principal solución para compensar los efectos negativos de la pérdida y fragmentación de hábitats.

Datos publicados en GBIF (DOI: 10.15470/0ssr1v)

Palabras clave: Sinecología, Densidad de población, Bosques de ribera

Resum

Canvis en l'estructura de la població aviària del bosc de ribera del riu Zambezi segons l'estació

Entre els diferents biomes del sud de l'Àfrica, els boscos de ribera tropicals són excepcionalment rics des del punt de vista de la biodiversitat, però no s'han estudiat gaire les poblacions aviàries que viuen en aquestes zones. Per al treball d'aquest article, es van fer recomptes d'ocells durant els mesos de maig, juliol–agost, octubre i novembre del 2013 i del 2014. Es va fer servir el mètode de transsecte lineal. Es va dissenyar un transsecte de 16,6 km de longitud al llarg del riu Zambezi entre les coordenades S 17.4975 E 24.3478 i S 17.4874 E 24.447. En total, es van registrar 146 espècies d'ocells. El nombre d'espècies registrat a cada estació varia entre 95 i 100. El nombre d'espècies dominants és molt baix, entre 2 i 3 segons el mes. Tot i això, la dominància acumulada i l'índex de dominància són molt més elevats durant l'estació seca (maig–agost) que durant la humida (octubre–novembre). Hi ha dues espècies dominants cada mes: Euplectes axillaris i Pycnonotus tricolor. Vidua macroura i Estrilda astrild només són dominants a l'octubre, mentre que Streptopelia capichola només ho és al maig. El grup d'espècies subdominants és més divers que el de les dominants, i inclou 13 espècies. L'índex de diversitat de Shannon augmenta entre el juliol i el novembre de 3,45 a 4,05, mentre que l'índex de diversitat de Simpson no mostra cap patró estacional. L'índex d'homogeneïtat de Pielou és més elevat durant l'estació humida (0,82–0,88) que durant la seca (0,76–0,79). La proporció dels grups tròfics no experimenta gaires canvis al llarg de l'any, tret del novembre, quan la proporció d'insectívors augmenta de manera notable, mentre que la de granívors decreix. Els corredors de bosc de ribera en entorns urbanitzats es poden convertir en la solució principal per a compensar els efectes negatius de la pèrdua i la fragmentació d'hàbitats.

Dades publicades a GBIF (DOI: 10.15470/0ssr1v)

Paraules clau: Sinecologia, Densitat de població, Boscos de ribera

Introduction

Among the various biomes in southern Africa tropical riparian forests are especially rich in terms of biodiversity (Mendelsohn et al., 2009). Most of these forests occur in Zambezi, Limpopo, and Okavango, and a few smaller river valleys. Little is known about avian communities occupying these forests. Their structure has been studied in acacia savanna in Eswatini (Monadjem, 2003, 2005), in acacia savanna along the Vaal River in South Africa (Seymour and Simmons, 2008), and in Tamarix vegetation in Karoo, South Africa (Brooke, 1992).

In other parts of the world, species diversity and community structure of birds associated with tropical riparian forests have been studied along the Paraiba do Sul River in Atlantic Forests, São Paulo State, Brazil (Laurenco and Toledo, 2019); in Alta Foresta, Mato Grosso, Brazil (Lee and Peres, 2007); gallery forests in Costa Rica (Seaman and Schulze, 2010); rainforest in New Guinea (Korejs et al., 2023); monsoonal forest in Hong Kong (Chan et al., 2008); oil palm–forest mosaic in Malaysia (Azman et al., 2011; Michell et al., 2018); savanna in Australia (Woinarski et al., 2000), and forests in south–western Australia (Palmer and Bennett, 2006). All these studies reported comparatively high bird species diversity and high population densities of some species. 

The purpose of this study was to determine month–to–month changes in the structure of an avian community in a riparian forest in Namibia, regarding: 1) species diversity, 2) dominance structure, and 3) population densities of particular species making up the community.  

Material and methods

Study area

This study was conducted in Zambezi riparian forest located on the right bank of the river between Katima Mulilo and Kalimbeza, Zambezi Region, NE Namibia.

This riparian forest is composed of main tree species such as African teak Pterocarpus angolensis, albizias Albizia spp., apple leaves Lonchocarpus nelsii, burkea Burkea africana, combretum Combretum spp., camel–thorn Acacia erioloba, other acacias Acacia spp., jackal berry Diospyros mespiliformis, mopane Colophospermum, pod mahogany Afzelia quanzensis silver cluster–leaf Terminalia sericea, sausage tree Kigelia africana, Sycomore fig Ficus sycomorus, white bauhinia Bauhinia petersiana, Zambezi teak Baikiaea plurijuga. and silver tree Terminalis spp. There are also some rural places (traditional houses with gardens; small arable plots, and pastures for sheep and cattle). This riparian forest is interlaced with water canals (back waters) with reedbeds, rushes and other water vegetation, and grassy depressions that are flooded almost on an annual basis.

The dry season in Katima Mulilo lasts from May to September, and the wet season lasts from October to April. I analysed two types of seasonal variations, month–to–month (May–July–August–October–November) and dry season (May+July–August) versus wet season (October+November).

Figure 1 shows the monthly distribution of rainfall in 2013 and 2014. The total rainfall was much the same in both years (423 mm in 2013 vs. 428 mm in 2014 (http://www.weatherandclimate.com/namibia/zambezi/katima–mulilo; http://www.meteona.com/index.php/climate/rainprogress/katima).

Methods

Counts on transects were conducted in May, July–August, October and November in 2013 and 2014. The line transect method, American version (Sutherland, 1996; Bibby et al., 2012) was used to quantify avian assemblages (linear population density expressed as the average number of potentially breeding pairs/10 km) along the study transect. This transect was 16.6 km long and was designed along the Zambezi River between S 17.4975 E 24.3478 and S 17.4874 E 24.447 (fig. 2). The transect was divided into six sections. Each section was surveyed in one morning, so that to cover the whole transect, four mornings were required in a given month.

Counts were conducted in the mornings by walking slowly from 6 a.m. till 11 a.m. and recording all seen and heard birds. For resident birds, a breeding pair was a census unit, while for non–resident species, the census unit was an individual.

Dominance is expressed as the percentage of the total number of pairs of a given species in relation to the total number of all pairs of all species recorded. Dominant species: > 5 %, subdominant: 2–4.99 %.

The following guilds were distinguished: diet (G, granivorous; I, insectivorous; F, frugivorous; N, nectarivorous; R, carnivorous); nesting (T, in trees or shrubs; H, in holes; B, in/on buildings; V, herbaceous vegetation); habitat (F, forest interior; E, ecotone, forest/open area; O, open area, grassland/savanna); residency (R, resident throughout the year; A, intra–African migrant; C, nomad).

The following indices were used to characterize the diversity and evenness of the communities:

Shannon's diversity index: H' = – ∑pi ln pi
where pi is the proportion of breeding pairs belonging to the ith species. The minimum value of H' is 0 (it indicates no diversity, i.e. only one species was recorded), and there is no upper limit to H'. The maximum value would occur if all species have the same number of breeding pairs.

Simpson's diversity index: D = ((∑n (n – 1))/N (N – 1)
where n is the total number of breeding pairs belonging to a given species; N, the total number of breeding pairs of all species. D ranges between 0 and 1, 1 represents infinite diversity and 0, no diversity.

Pielou's evenness index: J' = (– ∑ pi ln pi)/ln S,
where pi is the proportion of breeding pairs belonging to the i^th species; S, the total number of species. J' varies between 0 and 1. The lesser the variation between species in a community, the higher the J'.

Community dominance index: DI = (n1 + n2)/N
where n1, n2 are the number of pairs of two most abundant species; N, the total number of pairs of all species.

Sörensen's Coefficient: I = 2C / A + B
where A is the number of bird species in one breeding season; B, the number of bird species in another breeding season; and C, the number of bird species common to both breeding seasons.

The X²–test was used to test differences in population densities between months and between wet and dry season. For statistical testing, only those species with at least 10 breeding pairs in all four seasons were taken (expected value > 5).

Systematics and nomenclature of bird species follow Hockey et al. (2005).

Results

A total of 146 bird species were recorded (table 1, appendix 1, dataset published through GBIF, DOI: 10.15470/0ssr1v). The number of species detected each month varied from 95 to 100 (table 1). The number of dominant species was consistently low, varying between 2 and 3, depending on the month. The cumulative dominance and the dominance index were much higher in July–August and October than in May and November, but no differences were found between the dry season and the wet season (table 1)

Only two species were dominant each month: the fan–tailed widowbird Euplectes axillaris and the dark–capped bulbul Pycnonotus tricolor. The group of subdominants was much more diverse than dominants and included 13 species (appendix 1). 

Shannon diversity index increased between July and November from 3.45 to 4.05, while Simpson's diversity index did not show any seasonal trend. Pielou's evenness index was higher in the wet season (0.82–0.88) than in the dry season (0.76–0.79).

Seasonal differences were statistically tested for 41 of the 146 species. Significant month–to–month differences in population densities were shown for 29 species (table 2). Among these nine were insectivores, eight were granivores, and seven were frugivores. The remaining five represented other guilds.

The most marked changes in population densities were recorded for the pin–tailed whydah and the fan–tailed widow (appendix 1). No statistical differences were seen for 12 species (table 2). Of the 41 species, 16 were more common in the dry season than in the wet season, while for 14 other species the reverse was true (table 2).

The proportions of the main feeding guilds were much the same throughout the year, except for November, when the proportion of insectivores markedly increased, while that of granivores, decreased (fig. 3A). There were two main nesting guilds (together > 80 % in each month), birds nesting on trees or shrubs, and birds nesting in herbaceous vegetation. While the proportion of the latter decreased with the advance of the wet season, the proportion of tree/shrub nesting birds increased (fig. 3B). More than 95 % of all birds were African residents in all months, but the proportion was slightly lower in the season than in the dry season. The reverse was recorded for the intra–African migrant, while the Palearctic migrants were, as expected, recorded only in the wet season (fig. 3C).

Discussion

The number of resident breeding bird species recorded in Zambezi riparian forest (n = 146 species) is higher than that in riparian rain forests in other parts of Africa the world. Monadjem (2003, 2005) recorded 128 species (including non–breeding species) in riparian Acacia savanna in Swaziland. In Malaysian riparian forest, 92 species were recorded (Saad et al., 2012); 90 species in Costa Rica (Seaman and Schulze, 2010); and 88 species in the Atlantic Forest in Brazil (Laurenco and Toledo, 2019). The higher number of species recorded in Zambezi riparian forest can be linked to its more diversified environment. Beside typical forest, there are oxbows (with reedbeds, rushes and other water vegetation), wetlands, forest clearings, etc. 

The proportions of three main feeding guilds in Zambezi riparian forest distinguishes this community from other avian communities studied to date in southern Africa, where either granivores or insectivores compose the bulk of communities (Dean and Milton, 2001; Dean et al., 2002; Kopij, 2006, 2013a, 2013b, 2014a, 2014b, 2015, 2017, 2019, 2020a, 2020b, 2021a, 2021b, 2021c, 2022; Monadjem, 2003, 2005; Seymour and Simmons, 2008; Parker, 2012). Insectivores were much more numerous than other guilds, even in the neighbouring seasonal Kalahari Forest (Kopij, 2017). In riparian forests, the fruit trees are usually abundant and may benefit frugivorous birds (especially bulbuls), while seeds for granivorous birds may not be as abundant as in more open savanna or grassland biomes. Typical granivores, such as doves and sparrows, may therefore breed in riparian forests in lower density than in a neighbouring, more open habitats dominated by grasses. In a riparian rain forest in Malaysia, insectivores and frugivores were the main guilds, but there was no granivore guild (Saad et al., 2012). 

In regard to the number of breeding species and their densities, species composition differs only slightly from month to month. The amount of water is probably the main limiting factor, governing both the distribution and population densities of most bird species in Africa. However, water is always available in riparian forests and, therefore unlikely to be a limiting factor, and it probably has little effect on the primary productivity and the abundance of seeds, fruits or insects, which constitute the main food resources for most bird species breeding in this biome.

The high number of species and high population density of some species suggest that riparian forests play an important role as breeding and feeding habitats for birds, especially for species of frugivores and nectarivores. For some forest species, riparian forests may act as corridors that allow for dispersal, migration, and free movements within a mosaic of natural and human modified environments (Seaman and Schulze, 2010). In this way, corridors may also increase the level of biodiversity. Riparian forest corridors in urbanized environments may be viewed as main instruments for offsetting the negative effects of habitat loss and fragmentation, such as reductions in population sizes, reductions in immigration rates, changes in community structures, and invasions of alien species (Turner, 1996; Lee and Peres, 2007).

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