Content uploaded by Tatenda Dalu
Author content
All content in this area was uploaded by Tatenda Dalu on Oct 09, 2020
Content may be subject to copyright.
Does invasive river red gum (Eucalyptus camaldulensis) alter leaf litter
decomposition dynamics in arid zone temporary rivers?
Ryan J. Wasserman ,
a,b
Sia Sanga,
a
Mmabaledi Buxton,
a
Tatenda Dalu ,
c
and Ross N. Cuthbert
d
a
Department of Biological Sciences and Biotechnology, Botswana International University of Science and Technology, Palapye, Botswana;
b
Department of Zoology and Entomology, Rhodes University, Makhanda, South Africa;
c
Aquatic Systems Research Group, Department of
Ecology and Resource Management, University of Venda, Thohoyandou, South Africa;
d
GEOMAR, Helmholtz-Zentrum für Ozeanforschung Kiel,
Kiel, Germany
ABSTRACT
Riparian zones are important for the maintenance of aquatic ecosystem functional integrity, yet are
considered to be particularly vulnerable to plant invasions. The role of terrestrial riparian plant
invasions in compromising aquatic ecosystem processes is, however, still poorly understood. This
issue is particularly relevant for temporary rivers, which are understudied compared to
permanent river systems, despite their ubiquity and largescale contributions to biogeochemical
processes. Here we experimentally assessed leaf litter breakdown dynamics in situ in a
temporary river in arid southeastern Botswana, Southern Africa. We contrasted aquatic leaching
and microbial and invertebrate litter breakdown contributions to the native leadwood
Combretum imberbe and invasive river red gum Eucalyptus camaldulensis in the Lotsane River.
Fine-mesh (detritivore exclusion) and coarse-mesh (detritivore inclusion) bags were separately
filled with leaf litter from each species and deployed in the river during a hydroperiod (wet
phase), with decomposition measured over a 6-week period. E. camaldulensis shed significantly
more leachate than the native C. imberbe. Significantly more microbial and detritivore
breakdown was, however, observed in native than in invasive leaf litter. Overall, invertebrates
contributed little to biological leaf litter breakdown processes compared to microbial breakdown
contributions. Although significantly higher in native leaves, low invertebrate numbers were
found in leaf litter in the study. This study highlights the role of microbial contributions to
detrital decay in temporary arid zone rivers, whereas invertebrate contributions were relatively
minor. The study further contributes to our understanding of how invasive riparian plant species
alter aquatic detrital pool dynamics in invaded temporary wetland ecosystems.
ARTICLE HISTORY
Received 25 May 2020
Accepted 22 July 2020
KEYWORDS
biological invasions; cross-
ecosystem connectivity;
detritivore decomposition;
leaf litter decomposition;
microbial decomposition;
riparian zones
Introduction
Freshwater ecosystems are typically fringed by riparian
or waterside plant communities, composed of both obli-
gate and facultative species (Rood et al. 2010). Rivers and
their riparian zones are among the best examples of
meta-ecosystems given their cross-ecosystem connectiv-
ity of materials and energy (Loreau et al. 2003, Abelho
and Descals 2018). Up to 90% of all terrestrial plant
organic carbon production is estimated to end up in
the detrital pool (Cebrian 1999). Of considerable impor-
tance to productivity dynamics in aquatic ecosystems is
the contribution of allochthonous dead and decaying
plant parts from terrestrial riparian zones (Gulis and
Suberkropp 2003, Abelho and Descals 2018). Compo-
nents of biological communities are reliant on decay of
organic matter for nutrient provision, and, as such,
nutrient availability from allochthonous plant matter
can be important for primary and secondary
productivity in receiving aquatic environments (Dalu
et al. 2017). Carbon and nutrient recycling in this regard
is largely determined by plant species characteristics,
such as lignin and tannin levels (Webster and Benfield
1986, Hobbie 2015). Changes to the composition of
riparian plant communities may therefore result in
changes in terrestrial plant-derived energy flow from
detrital pools in aquatic ecosystems (Lecerf et al. 2007).
Biological invasions are among the most prevalent
features of global change, with invasive species negatively
impacting native communities and natural ecosystem
processes (Vitousek et al. 1996, Sala et al. 2000, Ricciardi
et al. 2017). While much progress has been made in the
field of biological invasions, our understanding of their
implications for ecosystem processes is often lacking
(Kennedy and Hobbie 2004, McGeoch et al. 2010, Crys-
tal-Ornelas and Lockwood 2020). Of terrestrial ecosys-
tems, riparian zones are considered particularly
© 2020 International Society of Limnology (SIL)
CONTACT Ryan J. Wasserman r.wasserman@ru.ac.za Department of Zoology and Entomology, Rhodes University, Makhanda 6140, South Africa
Supplemental data for this article can be accessed https://doi.org/10.1080/20442041.2020.1802196.
INLAND WATERS
https://doi.org/10.1080/20442041.2020.1802196
vulnerable to plant invasions, with rivers serving as con-
duits for dispersal and river banks often suitable for
propagule establishment (Hood and Naiman 2000).
Shifting riparian communities as a result of invasions
by riparian plants will have implications not only for
the riparian ecosystem, but also for aquatic environ-
ments receiving terrestrial plant-derived energy from
dead and decaying plant parts (Kennedy and Hobbie
2004).
The rate of recycling of biologically essential nutrients
from plant litter is determined by the rate at which it is
changed to alternate forms of organic and inorganic car-
bon (Heimann and Reichstein 2008, Boyero et al. 2016,
Mutshekwa et al. 2020). Decomposition of leaf litter is
considered to be a result of leaching of soluble com-
pounds, microbial degradation dynamics, and fragmen-
tation through invertebrate shredding and mechanical
abrasion (Boling et al. 1975, Abelho 2001). Within the
context of riparian plant invasions, litter dynamics may
be compromised through shifts in leaf litter input rates,
leaf litter quality, and consumer compatibility, the latter
dependent on coevolutionary history with native biota.
Here we assessed aspects of leaf litter decay within the
biological invasions context in an austral arid region
temporary river system. Among the most hydrologically
dynamic freshwater ecosystems, temporary rivers are
common in arid landscapes (Corti et al. 2011). However,
organic matter decay in temporary rivers has been exam-
ined relatively little (Larned et al. 2010, Datry et al. 2018,
Shumilova et al. 2019). This knowledge gap in these envi-
ronments is further pronounced within the context of
biological invasions because of the limited scope for con-
trasting native and invasive plant matter dynamics, given
the lack of baseline studies and natural processes on
native leaf litter in the region.
Eucalyptus is among the most widely introduced gen-
era globally, with numerous species categorised as inva-
sive in various parts of the world (Rejmánek and
Richarson 2011). The river red gum Eucalyptus camaldu-
lensis Dehnh. is particularly widespread, and in Southern
Africa it was introduced for commercial and non-com-
mercial reasons (Wate et al. 1999, Tyynelä 2001, Aweto
and Moleele 2005, Hirsch et al. 2019). In a review article
on E. camaldulensis in South Africa, Hirsch et al. (2019)
highlighted that 76% of all occurrences of
E. camaldulensis are along watercourses in the list of
the Southern African Plant Invaders Atlas (SAPIA).
Few studies have, however, provided quantifiable data
on detrimental environmental effects of the species in
the region, despite its wide distribution and known
impact (Hirsch et al. 2019). Here, we contrast leaf litter
decay dynamics between E. camaldulensis and a native
tree species in situ in an arid subtropical temporary
river system. Unlike most boreal investigations on leaf
litter decay, there is no native adopted model species
for studies in arid systems of the region. Leadwood Com-
bretum imberbe (Wawra) leaf litter was, however,
observed to be among the most prevalent native litter
in our study system, with evidence of invertebrate
decomposition (R. J. Wasserman pers. observ.), suggest-
ing that the leaves of the species were palatable for detri-
tivores (Graça et al. 2001,Boyero et al. 2011). As such,
C. imberbe was identified as the native comparator tree
species.
Both E. camaldulensis and C. imberbe are large peren-
nial tree species reaching heights of >18 m (Carr 1988,
Colloff2014) and are frequently found in riparian
zones where they occur (Coates-Palgrave 2002, Hirsch
et al. 2019). Given their size, prevalence, and proximity
to streams and rivers, both species likely contribute con-
siderably to cross-ecosystem organic matter transfer
between terrestrial and aquatic systems where they co-
occur; however, no such investigation has been con-
ducted. The prevalence of both species along dry water-
courses (Coates-Palgrave 2002, Capon et al. 2016)
suggests that in regions such as the study area, where
temporary rivers dominate, incorporation of
E. camaldulensis litter may have implications for aquatic
nutrient cycling dynamics.
Using an established leaf litter bag approach (Boyero
et al. 2011), we conducted a combination of in situ and ex
situ experiments to assess leaf litter breakdown dynamics
in the Lotsane River, Central District, Botswana. We
were particularly interested in contrasting leaching and
microbial and invertebrate (biological) litter contribu-
tions to the breakdown of native C. imberbe and invasive
E. camaldulensis. We hypothesised that (1) leaching rates
would differ between the species, and (2) microbial decay
contributions would be greater than macroinvertebrate
contributions, given that rudimentary aquatic inverte-
brate communities are expected in temporary rivers dur-
ing their hydroperiods. We finally hypothesised that (3)
leaf litter decay through biological processes would be
slower in the invasive E. camaldulensis than in the native
C. imberbe, given that native biota are likely better
adapted to breaking down the native tree leaf litter.
Materials and methods
Study site
The study was conducted on the Lotsane River (22°33′S,
27°08′E) near the town of Palapye, situated in Central
District, Botswana, Southern Africa. The study area is
considered arid with a mean annual temperature of
22.7 °C and total rainfall of 351 mm (Batisani and Yarnal
2R. J. WASSERMAN ET AL.
