THEORETICAL FRAMEWORK FOR THE THESIS

The communities of higher plants and animals crucially depend on primary producers like algae and bacteria for energy and matter transfers, thus it is of utmost importance to investigate the algal assemblages at the base of the Delta’s food webs. Several hundred species of algae are deemed to exist in the Delta (Ramberg

et al., 2006); Cholnoky (1966) observed a total of 327 species of diatoms in the

Okavango River Basin with just a few sampling sites in the Delta. Cronberg et al. (1996b) found some 198 algal species, of which 50 were common in rivers, floodplains and isolated pools (Cronberg et al., 1996a). Mackay et al. (2012) described 164 species (and 3 varieties) of periphytic diatoms following a sampling campaign which also yielded algal samples from open water habitats, used in this study (see Chapter 3). However, these studies either addressed specific groups of algae such as diatoms (Cholnoky, 1966a; Mackay et al., 2012) and/or sampled areas beyond the Okavango Delta wetland, i.e. upstream sites in Botswana and Namibia (Cholnoky, 1966a; see section 4.1) or only the Boro region (Cronberg et al., 1996a). This present study provides the first comprehensive estimates of the biomass and biodiversity of all algal groups in the Delta and interpretations of their distribution patterns in relation to limnology and other environmental factors (see Chapters 4, 5 and 6); it fills an important gap in the knowledge of biodiversity in this wetland of global importance.

The assemblages of algae in water analysed in this study are likely to be composed of mixed planktonic, benthic and periphytic species, especially in more shallow waters, e.g. floodplains. For example, in shallow lakes and wetlands it is common to

find abundant true planktonic algae in the epiphyton due to mixing processes and resuspension of sediments (Schallenberg and Burns 2004). In order to interpret the results of this study several theories are used, namely the Intermediate Disturbance Hypothesis (IDH - Grime, 1973; Connell, 1978; Reynolds et al., 1993), Resource Competition Theory (RCT; Tilman, 1982; Tilman, 2004), Habitat Heterogeneity Hypothesis (HHH; MacArthur and MacArthur, 1961; Ricklefs, 1977; Tilman and Pacala, 1993), and the Species-Energy Theory (SET; Wright, 1983; Stevens and Carson, 2002). These are discussed in the following sections.

1.6.1. Intermediate Disturbance Hypothesis (IDH)

The Intermediate Disturbance Hypothesis (IDH) was developed by Connell (1978) who proposed that frequent changes / disturbances are responsible for high levels of biodiversity of, e.g. plants and corals, because they prevent the competitive exclusion of species. The highest diversity is recorded where the disturbance level, e.g. floods, storm waves or predators, is intermediate in terms of frequency, time lapse and magnitude; whereas at both ends of the spectrum, communities have low diversity (Connell, 1978) (Figure 1.7).

Figure 1.7. Schematic representation of the Intermediate Disturbance Hypothesis (source:

Connell, 1978).

As Grime (1973) had already observed in grassland communities, rapid colonists (R-strategists) with high immigration rates are able to withstand high degrees of disturbance, as they are well adapted to frequent changes; on the other hand the best

competing species (C-strategists) with lower immigration rates are best adapted to low or no degree of disturbance (Biggs and Smith, 2002; see also section 1.2.2). Reynolds et al. (1993) discussed the strengths and weaknesses of the IDH for research on phytoplankton. They defined disturbances as:

“Primarily non-biotic, stochastic events that result in distinct and abrupt

changes in the composition and which interfere with internally-driven progress towards self-organization and ecological equilibrium; such events are understood to operate through the medium of (e.g.) weather and at the frequency scale of algal generation times”.

Various scholars have studied experimentally how algal diversity is influenced by flooding patterns in both freshwater (e.g. Reynolds et al., 1993) and marine assemblages (e.g. Sommer, 1995). A total of 12-16 algal generations, i.e. periods of 35-60 days should allow undisturbed successions to approach competitive exclusion and ecological equilibrium in pelagic successions (Reynolds et al., 1993). Reynolds

et al. (1993) tested the effects of infrequent (2-8 times per year), more frequent (8-

50 times per year) and high-frequency disturbances (50-365 times per year). They concluded that phytoplankton diversity increases with fast replacement rates (e.g. in warm waters with small sized algae) and declines in advanced successions and strongly selective environments, such as highly flushed or very oligotrophic systems (Reynolds et al., 1993). However, in a natural river-floodplain ecosystem, an annual flooding is a predictable event (Junk et al., 1989; Bayley, 1995; Junk and Wantzen, 2004).

The alternation of wet and dry years can lead to very different algal species richness. For example, in the semi-arid wetland Tablas de Daimiel National Park in central Spain (Àlvarez and Cobelas, 2003) observed highest algal species richness induced by hydrological perturbation. This wetland’s hydrology leads to local spatial heterogeneity which enhances plankton species richness in the floodplain as compared to isolated water bodies, as hydrological connectivity enables dispersal (Rojo and Rodrigo, 2010 in Rojo et al., 2012). In dry years species richness was higher than in wet years, unless droughts were severe, in which case species richness was reduced (Rojo et al., 2012).

Hydrological connectivity and conditions have been shown to significantly influence the structure and productivity of planktonic communities in floodplain

lakes (Tockner et al., 1999; de Melo and Huszar, 2000; Paidere et al., 2007). Regular floods do not represent sudden disturbances and the seasonal development of phytoplankton under natural hydrological conditions, e.g. in the Central Amazon, is a progressive phenomenon (Huszar and Reynolds, 1997) analogous to gradual climate change, or in general, environmental changes happening at a longer time scale than algal generation times (Wilson, 1994). Given the short generation times of algae, periods of several months in plankton succession are analogous to decades in successions in grasslands and centuries in forests (Padisák, 1994). Rivers are highly kinetic and open systems in which fixed habitat structures are difficult to form; however, organisms such as algae are tolerant of these changing conditions hence perceiving them as uniform and undisturbed (Reynolds et al., 1993). Indeed, even regular floods can be considered as a disturbance, e.g., in floodplain lakes of large European rivers that are temporally isolated, e.g. braided floodplains (Tocker et al., 2000) and rivers with human impacts on flooding patterns (Roozen, 2005).

Many ecologists still refer to or use the IDH, however a recent paper by Fox (2012) suggests that this hypothesis should be abandoned because its arguments are logically flawed and empirical studies only seldom yield hump-shaped diversity curves like the one shown in Figure 2.12. In his seminal paper “A general hypothesis on species

diversity” Huston (1979) described Connell’s IDH as ‘simply population reductions

which prevent competitive equilibrium’ and Sheil and Burslem (2013) consider the IDH as “one potential explanation when unimodal patterns are observed”. This interpretation has been embraced by numerous scholars. The hump-shaped relationship is the predominant trend observed in plants, but linear positive relationships are most common amongst animal species (Groner and Novoplansky, 2003). Overall, <60% of empirical studies do not support the IDH, thus careful evaluations and accurate sampling of low and high levels of disturbance are required (Sheil and Burslem, 2013). The discussion over the validity of the IDH and related developments and theories goes beyond the scope of this research, but this important hypothesis can be used to evaluate and interpret patterns in the Okavango Delta’s algal assemblages. What is most important for this study is that the IDH can be qualitatively used as a reference point for the interpretation of taxon richness and observed diversity patterns. The extensive sampling of the complex landscape of the Delta allows for comparisons of riverine, lotic and floodplain environments, covering very different limnological conditions. Despite the fact that flood disturbance was not monitored at comparable

temporal resolutions as other works on wetlands (e.g. Paidere et al., 2007), permanently, seasonally and occasionally flooded sites are very different environments with different flooding frequency for hundreds, if not thousands, of years (Wolski et al., 2006).

In the Okavango Delta the frequency of flooding disturbance is once a year with a large flood pulse when rainfall reaches the region from the Angolan highlands; in addition to that local flooding due to rainfall events may take place (Wolski and Murray-Hudson, 2006). Therefore, in this study disturbance is interpreted mostly as long-term flooding frequency and duration, permanently flooded (PF) sites, seasonally flooded (SF) and occasionally flooded (OF) sites (see sections 3.1 and 5.3.1), assuming that environments such as floodplains in the lower Delta have, througout long timeframes, developed different algal assemblages than the Panhandle regions due to the effects of the flood pulse. Hence in this study the IDH is used in agreement with Reynolds et al. (1993): “in spite of unresolved

weaknesses, the concept of intermediate disturbance remains too useful in its potential to reject”. In particular, even though the Okavango Delta flooding patterns

are in the domain of infrequent inundation events not representing a disturbance at algal generation time scale, the IDH is broadly tested to assess whether the long- term flooding frequency regimes in this wetland influence algal biodiversity and how (see Chapters 4 and 5).

1.6.2. Resource Competition Theory (RCT)

According to Hardin’s competitive exclusion principle “Competitors cannot

coexist”, that is to say, out of two species occupying precisely the same ecological

niche (i.e. the function performed by the species in the community of which it is a member; Elton, 1927) the one reproducing faster will outcompete the other (Hardin, 1960). All organisms compete for resources such as nutrients, light and habitat with other organisms of the same species (intraspecific competition) or of different species (interspecific competition); these resources may become limiting when their availability is not sufficient for organisms living in a specific environment (RCT; Tilman, 1982). The RCT predicts that a higher Number of Limiting Resources (NLR) has been shown to provide more opportunities for competitive trade-offs (Harpole and Tilman, 2007) and hence to increase species diversity (Tilman, 1982;

Interlandi and Kihlman, 2001; Grover and Chrzanowski, 2004). However, most studies supporting this pattern were on two-dimensional assemblages, e.g. grasslands and phytoplankton, where species have independent access to resources (Passy, 2008). In 3D-assemblages such as benthic algal assemblages, coexistence of species is promoted by tolerance to lower resource levels (McCormick and Stevenson, 1991), with understorey and overstorey species being able to coexist only if sufficiently abundant nutrients are available in the environment (Figure 1.8) (Passy, 2008).

Figure 1.8. Conceptual model of the interaction between resource supply and biofilm composition in two hypothetical 3D assemblages growing on inert substrates under high

(community A) and low (community B) resource supply (source: Passy, 2008).

Tilman’s RCT (Tilman, 1982) has been mostly used as a heuristic basis for developing new conceptual models, as only 42 well designed tests were made since its publication (Miller et al., 2005). Here RCT is also used as a conceptual reference point, as the quantification of specific limiting nutrient levels for algal taxa is beyond the scope of this study. One of the general paradigms in ecology is that areas that contain more microsite types or a wider spectrum of resource conditions should contain more species as more niche space is available (Lundholm, 2009). However, neutral coexistence models of biodiversity attribute species richness to the size of the regional species pool; stochastic immigration and mortality, rather than niche differentiation, maintain species diversity (Hubbell, 2001). For example, regional species richness has been shown to influence local species pools and richness (Passy 2009) and Ricklefs (2004) suggested new research directions to account for the important relationships between local and regional species pools stating that “What ecologists have called communities in the past should be thought of

as point estimates of overlapping regional species distributions”. Tilman himself

problems associated with neutral models not fitting observations of high correlations between species abundance and environmental conditions as well as using a more realistic model of resource partitioning (Tilman, 2004) than the widely used classic broken-stick model (MacArthur, 1957).

This study is a heuristic investigation which proposes plausible ecological interpretations of the results of our extensive survey on the biodiversity and biomass of algae living in the Okavango Delta; the theories outlined in this section are employed in Chapters 4, 5 and 6 to achieve this goal.

1.6.3. Species-Energy Theory (SET)

Habitat heterogeneity increases with larger areas and hence MacArthur and Wilson (1967) theorised that species diversity increased with island area and species immigration rates on islands or, more in general, habitat patches. The Species Area Relationship - SAR; S=cAz, with S=number of species, A=area, c=a constant which depends on the unit used for area measurement, and z=the slope of the species area relationship in log-log space - entails a ß diversity component. The slope of the linearised SAR is an effective measure of ß diversity or the degree of compositional changes in microbial assemblages (Azovsky, 2002; Smith et al., 2005), as well as macroorganisms, in a wide range of environments (Drakare et al., 2006). Mechanisms generating SAR at large scales include dispersal, speciation, and extinction whereas at local scales disturbance, competition, and herbivory are important (Passy and Blanchet, 2007); also, dispersal mediated by disturbance determines ß-diversity (Matthiessen and Hillebrand, 2006). The species-area theory (MacArthur and Wilson, 1967) was elaborated further to include differences in the physical environment such as climate whereby the available energy in an ecosystem (directly proportional to its area) measures the total amount of available resource production, and provides better information on the variety of resource types present than area alone (Species-Energy Theory; Wright, 1983). Analysis of data on angisperms on land and freshwater birds on islands at different latitudes showed that 70–80% of variation in species number was explained by species-energy theory (Wright, 1983). Furthermore, experiments on plants which isolated the covariant nutrient distribution heterogeneity and average supply components, showed that mean resource levels or other correlates of productivity or ‘available energy’ were

better predictors of species richness patterns than resource heterogeneity (Stevens and Carson, 2002). Hence the importance of nutrient gradients at different spatial scales in terms of contributing to taxon richness and diversity patterns. In this study, habitat heterogeneity and nutrient gradients are dealt with as interconnected and also related to the annual flood pulse in the study region (see section 2.14). SET focusses on how the total availability of all limiting resources influence richness by reducing rates of stochastic extinction, while the emerging field of Biodiversity and Ecosystem Functioning (BEF) attributes a key role to the number of species in controlling how resources are taken up by competing taxa and transformed into biomass (Cardinale et al., 2009b). This is highly relevant when looking at diversity- productivity relationships (see sections 5.1.4 and 5.2.6).

1.6.4. Habitat Heterogeneity Hypothesis (HHH)

Diversity of algae at a large scale can be linked to the size of the areas of investigation, e.g. larger areas of the same habitat will host more species. Habitat heterogeneity, which increases with area, has been identified as one of the major mechanisms generating both α-diversity, as per the Habitat Heterogeneity Hypothesis (HHH; MacArthur and MacArthur, 1961; Ricklefs, 1977; Tilman and Pacala, 1993), and ß-diversity (Connor and McCoy, 1979). More ecological space is available to species in areas supporting a greater range of microsite types or resource conditions (Lundholm, 2009).

In this study, habitat heterogeneity was not measured directly, but the assumption that floodplains contain more microhabitats is made due to the availability of multiple substrata for benthic algae to attach. In shallow wetlands, the availability of substrata such as plant stems and sand / sediments for the attachment of benthic algae is likely important for the establishment of productive and diverse algal communities. Higher algal and total plant (algal plus macrophyte) biomass were observed in wetland mesocosms with a greater richness of macrophyte species (Engelhardt and Richie, 2001). The HHH is used as a broader theoretical reference point to explain biodiversity patterns in relation to habitat heterogeneity in conjunction with the IDH and SET and the RCT in relation to the diversity-biomass relationships.

Chapter 2 – Study region: the