Dragonfly (Odonata) distribution patterns in urban and forest landscapes, and recommendations for riparian management

PII:

S0006-3207(96)00032-8

Biological Conservation 78 (1996) 279-288 Copyright © 1996 Published by Elsevier Science Ltd Printed in Great Britain. All rights reserved 0006-3207/96 $15.00 + .00

ELSEVIER

D R A G O N F L Y (ODONATA) DISTRIBUTION PATTERNS IN U R B A N A N D FOREST LANDSCAPES, A N D R E C O M M E N D A T I O N S FOR RIPARIAN M A N A G E M E N T Michael J. Samways & Nicholas S. Steytler Invertebrate Conservation Research Centre, Department of Zoology and Entomology, University of Natal, Private Bag XO1, Scottsville, Pietermaritzburg 3209, South Africa

(Received 7 March 1995; accepted 28 November 1995)

In temperate regions, the greatest threat to many Odonata species is the intensification of modern agriculture (Moore, 1991a). Pollutants, high water requirements for crop production, and conversion of riverine and wetland vegetation into cropland and into built-up areas, are the main causes of species loss. In Richmond Park, London, aesthetic improvements to natural features have caused a 50% local decrease in the number of Odonata species (Fry & Lonsdale, 1991). South Africa is a dry country, and its freshwater resources are heavily utilised for domestic, industrial and agricultural purposes. In KwaZulu-Natal, wetlands have been lost to dam construction, commercial afforestation and urbanisation, with the highest impact coming from agriculture (Begg, 1986). There is evidence that afforestation has transgressed legislation by replacing riverine vegetation with exotic trees (Johns, 1993). Furthermore, since industrial effluent is often discharged into rivers, pollution also poses a major threat to South Africa's freshwater resources. Against this background, this study is aimed to: 1. describe Odonata distributions in relation to disturbed landscapes bordering a small river associated with human activity; 2. assess Odonata species and assemblages as indicators of these types of human disturbance; 3. select species for management recommendations on land use adjacent to small streams, especially to determine the width of riparian strips needed to maintain various Odonata species.

Abstract

Odonata species are particularly sensitive to human disturbances. Their diversity relative to four landscape types (plantation forest, parkland, residential area, industrial area) along a small river (the Dorpspruit) that runs through Pieterrnaritzburg, South Africa, is described. Individual species environment relations were investigated using the multivariate analysis package CANOCO. Four biotope types were identified and characterised. The analysis also illustrated the extent to which the urban, suburban and forestry environments affected the Odonata species. Multispecies assemblages were good environmental indicators. Individual indicator species included Chlorolestes tessellatus and Crocothemis erythraea. Chlorolestes tessellatus is a good indicator of the minimal width ( > 30 m) of the indigenous strip of riparian vegetation between the stream edge and commercial plantations. This study suggests that there should be a riparian strip between the water's edge and plantation trees of at least 20 m (preferably 30 m). This finding is integrated with earlier ones to arrive at a general conservation management recommendation, at least for dragonflies, for rivers in South Africa. Copyright © 1996 Published by Elsevier Science Limited Keywords: dragonfly, urban, forestry, indicator, riparian management.

INTRODUCTION Odonata have increasingly acquired a high conservation status. They are commonly included in red-lists nationally (e.g. Shirt, 1987; van Tol & Verdonk, 1988; von Eisl6ffel et al., 1992) and internationally (e.g. Groombridge, 1993). They are also recognised as being good indicators of the condition of aquatic and terrestrial ecosystems (Watson et al., 1982; Brown, 1991). Furthermore, Odonata are also ecologically important because they are major predators in terrestrial and aquatic ecosystems.

STUDY SITE AND M E T H O D S Study area

A 10-km length of the Dorpspruit river (Pietermaritzburg, KwaZulu Natal, South Africa (29o36'25" S and 30020'43 " E; elevation: ±670m asl) down from its source in the hills was selected (Fig. 1). It is a permanent river, with its source and most of the catchment located in Eucalyptus spp. and Acacia mearnsii De Wild. plantations where there are also remnant patches of indigenous forest. The river then flows through residential areas and a parkland before reaching the industrial

Correspondence to: M. J. Samways. 279

M. J. Samways, N. S. Steytler

280

Only mature adult males were noted, as tenerals and adult females are difficult to identify on the wing and are not confined to the waterside territories held by the males (Corbet, 1962). According to Schmidt (1985) the best time to sample Odonata species is when they are most abundant at waterside territories. Therefore, individuals were recorded on hot, sunny days between 10.00 and 12.00 h. Only the genus Agriocnemis Srlys caused problems with field species identification. Two species occurred in the area: A. falcifera Pinhey and A. pinheyi Balinsky; these could not be distinguished on the wing and were combined as 'Agriocnemis spp.'.

2 km

Fig. 1. Study area. The Dorpspruit river, Pietermaritzburg, KwaZulu-Natal, South Africa, flowing through 'FOREST' (F), 'RESIDENT' (R), 'PARK' (P) and 'CITY' (C). and business centres. There was thus a clear set of different landscapes from relatively undisturbed to highly disturbed.

The four sites Sampling was from 1 December 1992 to 15 February 1993 when most Odonata species were at peak abundances. The river was sampled from near its source, through a variety o f vegetated zones, and into the industrial area of Pietermaritzburg. Four sites of distinct landscapes were selected: (l) plantation forest (FOREST); (2) residential area (RESIDENT); (3) parkland (PARK); and (4) industrial area (CITY). Spatial replication was five sample units (SUs) in the plantation forest, five in the residential area, three in the park, and four in the industrial area, making a total of 17 sample units. Each was a 15 m x 2 m rectangle consisting of a 15 m length of river bank with 1 m out across the water. Sampling of species The number of individuals of each species at each SU during a sampling period of 5 min was recorded. This was done 15 times, making the sample size at F O R E S T 75, at R E S I D E N T 75, at P A R K 45, and at CITY 60.

Environmental variables Adult Odonata respond primarily to visual cues (Corbet, 1962), though ultimately it is larval survival that determines biotope suitability. Considering adult and larval requirements, nine categories of environmental variables were recorded (Table 1). Common exotic tree species along the Dorpspruit river included Melia azedarach L., Eucalyptus grandis W. Hill ex Maiden, Acacia mearnsii De Wild., Cinnamornurn camphora (L.) J. Presl, Quercus robur L. and Caesalpinia decapetala (Roth) Alston. Common exotic bush and herb species included Solanum mauritianum Scop., Canna edulis L., Lantana camara L., Ricinus communis L. and Bidens pilosa L. Emergent and exposed aquatic vegetation included Matricaria nigellifolia DC., Commelina africana L. and Leersia hexandra Sw. Data transformations The species data were not transformed to normality as there were many zero values (ter Braak, 1987a). However, the environmental variables recorded as percentages (Table 1), were arcsin-transformed (Alder & Roessler, 1977). The interval data (water temperature and ambient temperature) and the ordinal data (flow rate) were not transformed. For the purposes of interpreting the canonical coefficients, the data were standardised to zero mean and unit variance. Multivariate analyses The SUs were classified using T W I N S P A N (Hill, 1979), a polythetic, divisive method that uses reciprocal averaging to position SU points in low dimensional space. Those species that characterise the reciprocal averaging axis extremes are emphasised so as to polarise the SUs. Canonical Correspondence Analysis (CCA) (ter Braak, 1986) from C A N O C O version 2.1 (ter Braak, 1987b) was used to relate the species abundance data to the environmental variables. Anisoptera and Zygoptera data were analysed separately for comparison of the solutions. The species and SU points jointly represent the dominant patterns in the community composition insofar as these can be explained by the environmental variable. The arrows of the environmental variables represent axes in the diagram. Dropping a perpendicular from a species point to the arrow (representing

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the environmental gradient) shows the relative position of the species along that environmental gradient. The length of the arrow is equal to the rate of change in the weighted averages as inferred from the diagram, and is therefore a measure of how much the species distributions differ along that environmental variable. The longer arrows are therefore more important in determining species distributions.

RESULTS Overall species diversity The 26 Odonata species (13 Zygoptera and 13 Anisoptera) recorded along the river during the study period, their abundances and abbreviations are given in Table 2. The cumulative curve for species reached its asymptote of 26 species after the ninth sampling replicate (out of 15), suggesting that most resident species were included in the study.

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PARK LANDSCAPE TYPE

Fig. 4. Number of individuals of five selected Anisoptera species and the remaining Anisoptera species together (REST) recorded over the whole sampling period, arranged for each of the four distinctive landscapes 'FOREST', 'RESIDENT', ' P A R K ' and 'CITY'. Species abbreviations as in Table 2.

TWINSPAN classification TWINSPAN classification of SUs according to Odonata species gave five groups after three levels of division (Fig. 2). Groups 1 and 2 include all FOREST SUs, group 3 all RESIDENT SUs, group 4 P A R K SUs, and group 5 all CITY SUs with the exception of one P A R K SU (No. 12) and one RESIDENT SU (No. 13). The FOREST SUs had a species assemblage most dissimilar to the other landscape types, while P A R K and CITY had similar species assemblages. Species diversity in the landscape types Anisoptera and Zygoptera species richness followed similar trends (Fig. 3), with the exception that Zygoptera species richness dropped off more sharply in the CITY SUs. 180~ 120-

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The percentage cover of exposed macrophytes, water temperature and shade variables accounted for most of the variation in the CCA solution (Fig. 7) as they were strongly correlated with axes 1 and 2 (Table 3). Arrow lengths showed the relative importance of the following: cover of exposed macrophytes (EXMAC), water temperature (WTEMP), exposed rocks (EXROCK), shade temperature (STEMP), shade (SHADE), cover of exotic herbaceous vegetation (EXVEG), cover of exotic trees (EXTREE), cover of indigenous trees (INTREE) and stream flow rate (FLOW). Table 4 (column three) shows the main indicator Anisoptera species in relation to environmental variables and biotope.

Although CITY SUs had the second lowest Anisoptera species richness (Fig. 3), they had the highest number of individuals (Fig. 4), indicating that just a few abundant species comprised the species assemblage. The total number of Anisoptera individuals showed a general increase from FOREST, through RESIDENT and PARK, to CITY. P. cognatus and O. julia falsum were the only species that did not follow this trend. Zygoptera had a slightly different trend from the Anisoptera. The P A R K had by far the highest number of Zygoptera individuals (Fig. 5) as well as the highest Zygoptera species richness (Fig. 3). CITY had the second highest number of Zygoptera individuals (Fig. 5) but the lowest number of species (Fig. 3), indicating that the species assemblage in this landscape type comprised a few, very abundant species. Certain species were unique to particular landscape types. Chlorolestes tessellatus, for example, only occurred in FOREST, while Enallagma glaucum was locally abundant only in CITY.

Zygoptera species-environment relationships A total of 46% of the variance in weighted averages of the species was accounted for by the environmental variable arrows in conjunction with the species points. The total of all eigenvalues was 2.657. Relating the axes to environmental variables (Table 5) showed that most of the variation was accounted for by three important environmental variables: cover of exposed macrophytes, water temperature and shade. The environmental variables, in order of decreasing importance as given by the lengths of the arrows (Fig. 8), are: exposed macrophytes (EXMAC), water temperature (WTEMP), shade (SHADE), exotic herbaceous vegetation (EXVEG), exposed rock (EXROCK), indigenous trees (INTREE), ambient temperature (STEMP), flow (FLOW), and exotic trees (EXTREE).

Anisoptera species-environment relationships CCA of Anisoptera species produced a scatter of 3.5 standard deviation units along axis l, and 4.5 SD units along axis 2 (Fig. 6). Trithemis stictica is an outlier, strongly influencing the CCA solution, and omitting it gave a lowering of variation in the species scatter points (Fig. 7). In all, 58% of the variance in weighted averages of species is accounted for by the environmental variable arrows in conjunction with the species points. The sum of all eigenvalues was 1.15. 3.5

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Fig. 6. Canonical Correspondence Analysis (CCA) ordination diagram with Anisoptera species (abbreviations) and sample units (numbers). The axes lengths are given in standard deviation (SD) units. Species abbreviations as in Table 2; l 5 = FOREST, (~9 and 13 = RESIDENT, 10-12= PARK, 14-17= CITY.

Dragonflies in urban and forest landscapes

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DISCUSSION Anisoptera and Zygoptera species-environment relationships

Percentage cover of exposed macrophytes, water temperature and shade were the most important environmental variables for both the Anisoptera and the Zygoptera, but the two suborders responded differently Table 3. Canonical coefficients between environmental variables (with abbreviation in brackets) and the first two axes of Canonical Correspondence Analysis (CCA) of Anisoptera species data with the outlier Trithemis stictica omitted

Axes

Abbreviations

Canonical coefficients 1

Water temperature Ambient temperature Flow rate Sunlight/shade Exposed rocks Exotic trees Indigenous trees Exotic vegetation Exposed macrophytes

(WTEMP) (STEMP) (FLOW) (SHADE) (EXROCK) (EXTREE) (INTREE) (EXVEG) (EXMAC)

-0.44 0.09 -0.01 0.09 0.04 -0.07 -0.26 0.13 -0.50

2

0.66 -0.64 -0.15 - 1.05 0.32 0.12 0.51 0.19 0.43

to these variables. Zygoptera species occurred at both extremes of exposed macrophyte cover, shade and water temperature gradients, reflecting greater ecological diversity in this taxon. Most Anisoptera species occurred in sunny biotopes with a high percentage of exposed macrophytes. Indeed, exposed macrophyte cover made up the most important environmental variable for most Odonata. This is not surprising as macrofloral assemblages are known to provide important nurseries for aquatic insect life (Ormerod et al., 1987; Rutt et al., 1989) and as perches for the adults (Stewart, 1993). Water temperature was also an important environmental variable. Temperature, among other factors, is likely to affect egg development (Corbet, 1962) and it is therefore important that the adult selects ovipositional sites with suitable temperatures. Water temperature and sunlight-versus-shade are interrelated, and a river with a dense riparian strip that shades the water surface will slow warming up of the water. Adult Odonata show preferences for specific sunlight-versus-shade regimes (McGeoch & Samways, 1991; Clark, 1992; Stewart, 1993; Steytler & Samways, 1995) so the association with water temperature is also a reflection of the importance of sunshine. Also, the amount of sunshine probably acts in a proximate way, signalling other important conditions that are not necessarily recognisable to the adults at the time (Wingfield Gibbons & Pain, 1992). In this study, exposed rocks masked the influence of flow rate, as previous studies (Clark, 1992; Stewart,

M. J. Samways, N. S. Steytler

286

Table 4. Biotope types with important environmental variables and indicator species assemblages

Biotope

Forest

Suburban Lotic

Environmental variables

Anisoptera species

Zygoptera species

Low exposed macrophytes

Paragomphus cognatus

Chlorolestes tessellatus~

Low water temperature Low exotic vegetation High flow rate Medium exposed macrophytes

Orthetrum julia Trithemis furva Pantala flavescens Paragomphus cognatus Anax speratus

Allocnemis leucosticta Pseudagrion kersteni

Medium water temperature Medium exotic vegetation High flow rate

Orthetrum julia Trithemis furva Macromia picta Anax imperator Trithemis stictica ~ Pantala flavescens

Allocnemis leucosticta Pseudagrion kersteni Pseudagrion hageni

Trithemis arteriosa Nesciothemis farinosa Anax imperator Anax speratus Orthetrum julia Crocothemis erythraea~ Orthetrum caffrum

Pseudagrion massaicuma Isehnura senegalensis~ Pseudagrion hageni Pseudagrion salisburyense Lestes plagiatus Pseudagrion kersteni

High exposed macrophytes Suburban Lentic

Medium water temperature Medium exotic vegetation Impounded High exposed macrophytes

Platycypha caligata~

Agriocnemis spp. a

Urban

High water temperature

Trithemis arteriosa Pantala flavescens

Pseudagrion salisburyense

Lotic

High exotic vegetation Low flow rate

Trithemis furva Nesciothemis farinosa Paragomphus cognatus Orthetrum julia

Enallagma glaucum Elattoneura glauca

aSpecies occurring exclusively in a biotope type. Table 5. Canonical coefficients between environmental variables (with abbreviations in brackets) and the first two axes of Canonical Correspondence Analysis (CCA) of Zygoptera species data

Axes

Water temperature Ambient temperature Flow rate Sunlight/shade Exposed rocks Exotic trees Indigenous trees Exotic vegetation Exposed macrophytes

Abbreviations

(WTEMP) (STEMP) (FLOW) (SHADE) (EXROCK) (EXTREE) (INTREE) (EXVEG) (EXMAC)

1993; Steytler & Samways, 1995) have shown that flow rate is important for many species. Here, the gross influence of flow rate was seen indirectly through the amount of exposed rock. True lotic species were associated with a high proportion of rocks, while lentic species showed the opposite. Species tolerating a whole environmental gradient, such as T. furva, M. picta and P. flavescens, can be considered as eurytopic. This last species is a global migrant (Corbet, 1962) which inhabits temporary pools (Samways & Caldwell, 1989). Species assemblages

The T W I N S P A N classification clearly showed that the Odonata assemblages were largely influenced by the

Canonical coefficients 1

2

0.36 0.16 4). 14 -0.58 4).02 0.14 0.52 4).31 -1.25

-1.42 0.05 0.17 1.25

0.19 4).78 0.13 1.17 1.17

type of landscape: FOREST, R E S I D E N T , P A R K and CITY, with each type having a characteristic species assemblage (Table 4) (cf. Schmidt, 1985). Biotopes shared many species, with the result that there were few indicator species. Effects of human impacts on species assemblages

Species assemblages reflected the type of human disturbance. The forest biotope was composed mostly of the exotic eucalypts, which can reduce stream flow, acidify the soil and inhibit subcanopy indigenous vegetation (Johns, 1993). Yet these results clearly show, with the exception of C. tessellatus, that it is vegetation physiognomy and not botanical taxa that determines the presence or absence of species in the study area.

Dragonflies in urban and forest landscapes

287

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Fig. 8. Canonical Correspondence Analysis (CCA) ordination diagram with Zygoptera species (light abbreviations) and environmental variables (bold abbreviations) axes lengths in standard deviation (SD) units. See Tables 1 and 2 for environmental and species codes.

In the suburban areas, the. indigenous riparian vegetation was mostly removed and replaced with aesthetic exotic trees and herbs of highly variable physiognomy. The urban biotope was without shading vegetation, the river banks were dominated by mown grass and exposed macrophytes were in the shallows. Mostly Anisoptera species occurred here, and the few Zygoptera species were highly eurytopic. Crocothemis erythraea, which occurred exclusively in this biotope, is known to occur abundantly in disturbed, even eutrophic conditions (Samways, 1993). Cairns (1983, 1986) suggests that species assemblages can be useful for river management. This view is supported here. Each assemblage was indicative of the particular type of human disturbance, and Table 4 provides a reference for river management in the region.

Single species indicators In the forest biotope, the stenotopic Zygoptera species Chlorolestes tessellatus is a good indicator of disturbance to indigenous riparian vegetation, and particularly to disturbance caused by commercial forestry. Where commercial trees replaced indigenous ones, C. tessellatus numbers dropped substantially. Furthermore, repeated tree felling also adversely affected this species because of its affinity for low water temperatures and constant shade. This species then has important implications for determining the width of the native riparian strip (see below).

Crocothemis erythraea occurred exclusively in urban biotope which was characterised by high regular levels of human disturbance. These findings, those of Samways (1993), suggest that this species good indicator of human-degraded areas.

the and and is a

Implications for riparian management Chlorolestes tessallatus needs indigenous bushes as a perch and for oviposition, and unpublished observations suggest that there should be at least 20 m of such vegetation between the edge of the water and the edge of a plantation. When the plantation trees are tall ( > 20 m), the strip may need to be wider; a width of 30 m would probably be a practical working figure. This figure would also coincide with that in local legislation on the recommended width of riparian strips. Interestingly too, Ormerod et al. (1990) found that a 10 m bankside strip without conifers was insufficient to attract the dragonfly Cordulegaster boltoni (Donovan). Valuable biotopes for heliophobic species are destroyed by clear felling of indigenous trees. Management should be for maximum heterogeneity (see Steytler & Samways, 1995), with clear felling staggered over time and place, so minimising the adverse effects. In turn, small-scale damming such as on farms can infill species in a geographical area (Samways, 1989), but additionally, successional effects also need to be taken into account (Moore, 1991b).

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M . J. S a m w a y s , N. S. Steytler

The conservation value o f a small p o n d or that o f a river i m p o u n d m e n t will depend on whether m a n a g e ment is for m a x i m u m n u m b e r s o f individuals and species, or for t a x o n o m i c uniqueness. I f i m p o u n d m e n t is inevitable for e c o n o m i c reasons, biotope m a n a g e m e n t should aim at creating and maintaining a large variety o f biotopes, with a m a x i m u m range o f plant architectures and occasional small areas o f bare g r o u n d (e.g. for species like Trithemis kirbyi ardens) (Osborn & Samways, 1996). M a n a g e m e n t should not see the landscape elements in isolation. A n a p p r o a c h that considers the relationships between separate landscape elements is crucial. It is i m p o r t a n t to recognise that successional change is the norm, and m a n a g e m e n t should aim at change as well as heterogeneity (see also Usher & Jefferson, 1991).

ACKNOWLEDGEMENTS The F o u n d a t i o n for Research and D e v e l o p m e n t and the University o f Natal Research F u n d financed this study. Pamela Sweet helped with processing.

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