Dung beetles’ responses to changing environments; implications on ecosystem service delivery

Dung beetles are coprophagous insects belonging to the families Scarabaeidae and

Geotrupidae. They are a very important insects’ group, acting as significant ecosystem

engineers through dung burial, decomposition, nutrient cycling, bioturbation and seed dispersal

in natural and human managed systems. Despite these benefits, most essential ecosystem

services are not economically quantified and as such, their roles as natural capital in natural

and human managed systems have largely been neglected. To date, the global climate change

and anthropogenic activities such as the widespread use of synthetic pesticides for livestock

endo- and ectoparasites, has largely led to loss of or shifts in some the biodiversity and richness

of this natural capital. It has been hypothesised that in order to determine a suitable time for

activity, insects may monitor environmental variables such as light, temperature and diel events

such as sunrise and sunset. However, information on how environmental stress resistance may

influence diel activity times and ecosystem services provision remains scant, especially on

dung beetles in arid, tropical environments such as Botswana that are more vulnerable to the

impacts of climate change and associated anthropogenic activities. This work is therefore

aimed at investigating i) the diversity and activity patterns of dung beetle species in Botswana

(ii) the effects of temperature (and adaptation mechanisms thereof) as an environmental

variable in influencing activity times and (iii) implications on ecological services provision.

This study found that Botswana is richly abundant with Scarabaeinae dung beetles belonging

to 8 tribes namely, Canthonini, Coprini, Dichotomini, Gymnopleurini, Oniticellini, Onitini,

Onthophagini and Carabaeini. Most of the species belong to the Onthophagini tribe, and this

tribe comprised the most abundant species. Coprini and Scarabaeini were however the least

abundant tribes. Furthermore, the study also found that small-bodied beetles from the

Gymopleurini tribe such as Allogymnopleurus indicageous, Gymnopleurus aenescens and

Gymnopleurus ignitus preferred diurnal activity, compared to large-bodied beetles from the

Coprini tribe (Copris elephenor and Catharsius calaharis), and the Scarabaeini tribe

(Scarabaeus zambezianus, Scarabaeus goryi) which preferred crepuscular/nocturnal activity

(Chapter 2). Second, an investigation between activity time and temperature tolerancefound

that day active dung beetle species generally had higher maximum temperatures for activity

measured as critical thermal maxima (CTmax) than crepuscular and nocturnal species. This

implies that heat may significantly play a role in choice of diel activity time in dung beetle

species by constraining some species to be active at specific period of the day corresponding

to their thermal limitations. However, there was no clear correlation between body mass and

CTmax suggesting complex associations across species, body mass and heat tolerance that

warrants further investigation (Chapter 3). Third, this study revealed that tropical dung beetles’

physiological and ecological activities were generally negatively affected by low temperatures.

Results showed that low temperature stress may offset dung beetles’ ecological services

through reduction in dung removal. Dung ball creation between diurnal and nocturnal species

interacted with temperature, with diurnal species producing significantly fewer balls at low

temperatures, whilst nocturnal beetles were not significantly affected (Chapter 4). Fourth,

based on the trade-off theory, plastic responses to variable high (VT-H) and variable low

temeperatures (VT-L) were investigated in a day active dung beetle, Allogymnopleurus

thalassinus. Results showed that effects of acclimation were significant for heat tolerance,

significantly increasing and reducing CTmax values for variable temperature high (VT-H) and

variable temperature low (VT-L) respectively. Similarly, effects of acclimation on HKDT were

significant, with variable temperature high significantly increasing HKDT, while variable

temperature low reduced HKDT. In addition, effects of acclimation on ecological traits showed

that beetles acclimated to variable high temperatures were ecologically more efficient in their

ecosystem function (dung removal) compared to those acclimated at variable low temperatures.

Allogymnopleurus thalassinus nevertheless, had low acclimation response ratios, signifying

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limited scope for complete plasticity for ULTs tested here. This result supports the 'trade-off'

theory, and that observed limited plasticity may unlikely buffer A. thalassinus against effects

of climate change, and by extension, albeit with caveats to other tropical ecological service

providing insect species (Chapter 5). Fifth, functional responses experiments were conducted

to help quantify the effects of increasing mean temperatures consequence of climate change

and dung beetle species density on dung removal, an essential ecosystem service on three

telecoprid species: Allogymnopleurus indigaceous, Scarabaeus zambezianus and Khepher

prodigiosus (Chapter 6). Results showed that K. prodigiosus, exhibited greatest dung utilisation

efficiency overall across dung masses, compared to the other two species. Nevertheless, both

S. zambezianus and A. indigaceous utilization increased with both warming and beetle density,

whereas K. prodigiosus performance was less temperature and density-dependent. Results on

dung ball number differed across species and correlated positively with temperature and

densities, with S. zambezianus producing significantly most balls overall. These results show

that temperature and competition largely have a significant effect on dung beetle species’

fitness and thus functional efficiency and ecosystem service delivery. This work provides

insights on the survival mechanisms of tropical species against environmental stress and

provides a framework for the conservation of these natural capital species that inhabit arid

environments under rapidly changing environmental climate. Such studies are critical in

serving as early warning systems for predicting the potential effects of climate change on

biodiversity and fore warning against threats to the integrity of ecosystems and their essential

services. Incorporation of such scientific information in conservation policies may help

safeguard this neglected but essential natural capital and consequently ecological services provision in the future.