Large herbivores are considered to be landscape engineers as they modify habitat structure both spatially and temporally (Gonnet 2001; Landman et al. 2012). As a megaherbivore, African elephants (Loxodonta africana) are known to cause landscape-level transformation and, especially when in high densities, can directly influence the structure and composition of the vegetation as well as impacting on an array of ecological processes (Ogada et al. 2008; Landman et al. 2014). These impacts, unevenly distributed across the landscape, not only directly affect habitat availability but also influence foraging opportunities for other species and in so doing initiate a ripple effect within the system (Cumming et al. 1997; Kerley & Landman 2006; Landman et al. 2014). An example of one of these ripple effects would be the creation of paths by elephants in the Addo Elephant National Park (AENP) through previously inaccessible Sundays Thicket (Kerley & Landman 2014). This process benefits smaller browsing species as it allows access to this newly exposed vegetation; on the other hand the opening up of intact thicket could compromise refuge sites of browse-sensitive vegetation such as small geophytes (Kerley et al. 1999a; Kerley & Landman 2006).
The uneven distribution of elephant-induced impacts across the landscape produces a gradient of transformation ranging from intact vegetation to the severely degraded sites of open habitat dominated by grasses (Landman et al. 2012). It has been shown that this type of habitat transformation elicits community structure and composition responses from birds that are sensitive to changes in their habitat. Shifts in community attributes such as richness and abundance have been observed during a comparison between transformed and untransformed thicket environments (Chabie 1999). In determining the nature of bird community responses to changes in Sundays Thicket structure brought about specifically through elephant-induced impacts across the landscape, the degree to which elephants influence bird assemblages in the AENP can be established. This is further beneficial as bird communities can be used to monitor changes in the environment due to their status as sensitive environmental indicators; with shifts in resource availability becoming readily apparent in the structure and composition of these communities (Pomeroy 1992).
The distribution of resources across a landscape and thus its transformation as a result of herbivore-induced impacts, such as foraging and trampling, is spatially heterogeneous (Coughenour 1991; Washington-Allen et al. 2004). The intensity and diversity of these impacts are dynamic and determined by a spatiotemporal hierarchy of factors resulting in a phenomenon known as the “piosphere effect”, a radial pattern of diminishing impacts from a patch centre (Chamaillé-Jammes et al. 2007; Landman et al. 2012). The most common example of this being the radial vegetation gradient found around water sources (Coughenour 1991; Chamaillé-Jammes et al. 2009). The impacts on vegetation dynamics have cascading effects that are apparent throughout the system; reverberations being apparent across the spectrum of biological elements, ranging from individual species to the overall health and functionality of the ecosystem and culminating in shifts in biodiversity patterns (Owen-Smith 1996; Landman et al. 2012).
As a result of landscape transformation, documented for elephant populations in both the Kruger National Park (KNP) and the AENP of South Africa, conservation goals have been revised by reserve management (Owen-Smith et al. 2006; Landman et al. 2012). Focus has transferred from species-specific conservation to the state of biological diversity, with the minimisation of single species impacts being top priority to ensure system resilience (Rogers 2003; Owen-Smith et al. 2006; Chamaillé-Jammes et al. 2007). The move from species-specific conservation to the preservation of biological diversity acknowledges the substantial influence that elephants have on ecological patterns and processes spanning both abiotic and biotic system components (Kerley et al. 2008; Landman et al. 2014). The location and availability of key resources such as surface water and the quality and quantity of food have been described as being fundamental factors in determining the variation of the intensity and heterogeneity of elephant-induced impacts (Chamaillé-Jammes et al. 2007). Due to the limited research available on the variations of the intensity of these impacts between the biodiversity elements, the magnitude of elephant-generated influences will be difficult to quantify (Landman et al. 2014).
MECHANISMS OF ELEPHANT IMPACTS
African elephants are characterised in many ecosystems as keystone species due to the intensity of their impacts, relative to their abundance, on vegetation community structure and ecological processes (Owen-Smith 1992; Kerley & Landman 2006). There are a number of direct and indirect mechanisms through which elephants can impact on vegetation. Foraging and trampling directly influence both the structure and composition of vegetation communities by way of ingestion of a wide variety of species together with the physical removal and compression of vegetation (Kerley & Landman 2006). Movement across the landscape, in the vicinity of water sources as well as skirting barriers, establishes pathways opening up previously inaccessible forage to other smaller, browsing species (Kerley & Landman 2006; Skead 2007; Landman & Kerley 2014). Elephants are also able to substantially contribute to other ecological processes such as nutrient cycling, subsequently altering soil chemistry, and seed dispersal, due to certain physiological characteristics (Owen-Smith 1992; Cowling & Kerley 2002). These impacts have the capacity to substantially alter the suitability of habitats thus influencing the species able to occupy them (Cowling & Kerley 2002).
Elephant-altered vegetation is generated as a result of an array of feeding behaviours displayed by these animals, long since known as robust and wasteful feeders, which include the use of their trunk, tusks and feet to browse the vegetation, strip bark, fell or uproot trees and forage for geophytes (Selous 1881; Kerley et al. 2008). Elephants are unlike most other herbivores in that the consequences of their feeding is substantially more than simply the selective removal of plant tissues typically inhibiting plant growth and reproductive potential. Their foraging behaviour, specifically the felling and uprooting of trees, has the potential to directly result in the death of mature trees while the stripping of bark is the first step in exposing these trees to other processes resulting in their eventual mortality (Kerley et al. 2008). Throughout periods of foraging elephants not only demolish trees but also break off and discard large quantities of uneaten vegetation, which is believed to represent up to a half of the biomass consumed in the AENP (Lessing 2007; Kerley et al. 2008). Discarded vegetation, typically more coarse than the normal litter, is not uniform in size and nutrient levels as well as not being evenly distributed across the landscape subsequently influencing the litter dynamics of the subtropical thicket ecosystem (Kerley & Landman 2006; Kerley et al. 2008). Although this behaviour is not unique to elephants, it is an aspect that has received little description and as such, the extent of the cascading effects on the ecosystem and its biodiversity patterns and ecological processes is poorly understood (Kerley et al. 2008).
Even though the extent and complexities of the impacts of elephants on the environment has not been fully explained as yet, it is broadly understood that these megaherbivores substantially influence a diverse selection of ecological processes owing not only to their foraging behaviour but simply their movement throughout a system (Kerley et al. 2008). Their influence is evident in the subtropical thicket environment of the AENP where it was demonstrated that the number of broad ecological processes affected by the presence of elephants, 14, was equivalent to the number affected by the remaining species in the vertebrate herbivore community, totalling 21 (Kerley & Landman 2006). It has been established that plant species can be negatively impacted as a result of elephant feeding with subsequent extirpation, localised species extinction, being a key concern but elephant impacts on a system need not only be negative.(O’Connor et al. 2007; Kerley et al. 2008). Elephants can stimulate plant growth and regeneration within a system, as observed in their browsing of spekboom, Portulacaria afra, a prominent component in the subtropical thicket ecosystem (Stuart-Hill 1992; Kerley et al. 2008).
The ecological repercussions tend to be species-specific and vary between the nature of the impacts on the vegetation with a number of factors determining the vulnerability of an individual to destruction although as long as a plant’s roots remain in the soil, resprouting is possible for several species (Eckhardt et al. 2000; O’ Connor et al. 2007; Kerley et al. 2008). Seedlings are particularly vulnerable to elephant impacts, mortality being common and produced as a result of elephants either simply removing the plant from the soil or inhibiting its growth due to their top-down foraging behaviour (Kerley et al. 2008). In the subtropical thicket environment these species-specific variations are evident. As mentioned, P. afra is able to persist, despite the heavy browsing pressure imposed by elephant, due to the fact that the top-down browsing method promotes vegetative reproduction; as long as the plants are not uprooted and their roots damaged (Stuart-Hill 1992; Lessing 2007; Kerley et al. 2008). Aloe species on the other hand are relatively sensitive to the effects of elephant and even at very low levels of herbivory, tend to disappear rapidly from the system (Landman et al. 2008; Kerley et al. 2008). Thus contradicting the fact that elephant herbivory is held as the sole element responsible for the disappearance of the species and suggesting that an alternative impact mechanism such as direct trampling or cascading effects may be responsible (Landman et al. 2008).
CASCADING EFFECTS – AFFECTED ECOSYSTEM PATTERNS AND PROCESSES
As mentioned, elephant-impacted vegetation has numerous effects that cascade throughout the system influencing both biotic and abiotic ecosystem components. The cycling of nutrients is a key element in the healthy functioning of any ecosystem. Elephant act to accelerate this recycling process through their wasteful foraging behaviour and tree felling together with their consumption of the more coarse, fibrous plant parts, bark and roots, that are generally avoided by other herbivorous species (Owen-Smith 1988; Lessing 2007). As keystone species, elephants provide a number of vital roles in this ecological process, especially evident in nutrient deficient ecosystems, ranging from nutrient release from tree trunks and their roots to influencing the efficiency of mineral extraction and soil nutrient composition through tree removal (Botkin et al. 1981; Treydte et al. 2007). A change in the soil nutrients could result in a change in the type of vegetation being able to establish and propagate in the area leading to shifts in forage quality and thus the species utilising the habitat. The nutrients found within the soil matrix are not the only elements to be influenced by megaherbivores such as elephant. Due to large sizes, elephant trampling not only effects the vegetation on the soil surface but also substantially compacts the soil in their path having consequences, albeit currently unclear, for vegetation in the vicinity and thus shaping community composition (Plumptre 1994). It has been demonstrated in the AENP, through the use of elephant exclosures, that the proportion of the landscape utilised by elephants had a higher incidence of run-off zones thus resulting in a decline in soil nutrients in regions traversed by elephants (Kerley et al. 1999b).
The dispersal of seeds, through endozoochory, is another ecological process greatly impacted by elephants with these animals vitally contributing to the promotion of regeneration in a number of plant species through enhanced germination and survival rates (Cochrane 2003). While some plant species, Balanites wilsoniana a forest canopy species, are solely dependent on elephants to ensure long-term persistence, it has been found that this trend does not hold true in the subtropical thicket of the AENP where it was found that in spite of their broad diet, elephants are relatively poor in their seed dispersal influences (Cochrane 2003; Kerley et al. 2008). While the diversity of plant species dispersed in this manner was relatively low, when compared to other herbivores such as black rhinoceros (Diceros bicornis), it is possible for large numbers of seeds to be dispersed across the landscape owing to the large volume of vegetation that is processed by these animals (Owen-Smith 1988).
Elephants as well as their influences on vegetation have been relatively extensively documented in the subtropical thicket of the AENP through the comparison of established exclusion zones, botanical reserves, to areas supporting free-roaming elephants. Studies using this framework however are therefore based on the assumption that any observed difference in the vegetation would be due to the presence/absence of elephants (Kerley et al. 2008). While in general it has been established that elephants negatively influence various plant community attributes, such as reducing both richness and biomass, it has also been countered that a selection of species in the thicket biome have adapted to the aforementioned beneficial top-down browsing behaviour of elephants (Stuart-Hill 1992; Lombard et al. 2001). In addition, elephants do not impact on every component of ecosystem functioning, influencing only fourteen of the nineteen broad ecological processes assessed in the AENP, suggesting that alternative mechanisms of impact could be responsible for the differences observed between the exclusion and inclusion zones within the park (Kerley & Landman 2006; Landman et al. 2008).
Although it is broadly accepted that foraging elephants promote regeneration in select thicket habitat species; there is evidence that this vegetation type, due to its slow recovery rate, is vulnerable to transformation with intense browsing pressure resulting in species loss and replacement (Kerley et al. 1995). A shift in the species composition and structure of the plant communities in thicket alters the suitability of a habitat and could thus lead to a shift in the faunal species being able to inhabit the region. It has been documented that after extensive herbivory from domestic livestock, thicket undergoes a gradual change whereby the shrubs present in the canopy are replaced by ephemeral grasses and instead of achieving a balance between browsing and regeneration, transformed thicket will continue to decline (Kerley et al. 1995; Lechmere-Oertel et al. 2005b; Landman et al. 2014). This decline has been attributed to the loss of key ecological processes in the altered environment (Lechmere-Oertel et al. 2005b; Landman et al. 2014).
The probability that elephant-transformed habitats affect other ungulate species has been asserted and that the location and availability of water resources plays a pivotal role in this assessment (Valeix et al. 2007). Essentially the intensity of elephant impacts is determined largely by water availability, areas in the vicinity of water holes being the most transformed, elephant population density, higher densities causing the most destruction, and soil nutrients, influencing the forage quality (Cumming et al. 1997; Cowling & Kerley 2002; Landman et al. 2012).
Through their destruction of the vegetation, elephants have the potential to indirectly affect resource availability, such as foraging opportunities and shelter, and alter habitats for other species with a decline in herbivorous vertebrates being noted in the Hwange National Park of Zimbabwe (Jones et al. 1994; Valeix et al. 2007). During the same study, it was observed that the smaller herbivore community was the strongest impacted by the changes in vegetation density with a sharp reduction in numbers being a result of these animals being more prone to predation and therefore more vulnerable to the changes in visibility brought about through a reduction in vegetation density (Sinclair et al. 2003; Valeix et al. 2007).
IMPORTANCE OF VEGETATION STRUCTURE TO BIRD COMMUNITY COMPOSITION
Transformed vegetation structure will not only affect resource availability for other ungulate species but will reduce the foraging opportunities and shelter available for the bird community inhabiting the area with reduced canopy cover increasing visibility for raptors and thus making closed canopy species more vulnerable to predation (Estrada et al. 1997; Maya-Elizarrar & Schondube 2015). Birds provide vital ecological services in many ecosystems influencing such processes as seed dispersal and survival as well as the reduction of herbivorous arthropods (Hooks et al. 2003; Ingle 2003). Consequently they have the ability to influence the survival and reproduction of plants while in addition being prey to other species thus suggesting that they represent both bottom-up and top-down forces within the system (van Bael et al. 2003; Ogada et al. 2008). As a result, any factors influencing bird community structure and composition could have substantial effects on the structure and functioning of a system (Ogada et al. 2008). The combinations of vegetation structure along with its floristic composition are known to be key factors in determining the composition of bird communities (MacArthur & MacArthur 1961; Skowno & Bond 2003).
Avifauna are particularly sensitive to changes in the volume and composition of the vegetation with height being key in terms of not only providing an increased number of niches but also in making spatial and temporal coexistence possible (MacArthur 1964; McShea & Rappole 2000). The response of bird communities to herbivore impacts, as facilitated by their impacts on habitats, was observed in a study where the reduction of the density of a deer population changed the structure of the bird community through browsing-induced changes in the density and diversity of understory vegetation (McShea & Rappole 2000). A simplified vegetation structure, as brought about through the transformation of forest habitat to cattle pastures in Mexico, limits the diversity of food resources and subsequently reduces the number of birds that the habitat can support (Saab & Petit 1992; Maya-Elizarrar & Schondube 2015).
BIRD COMMUNITY RESPONSES TO VEGETATION DISTURBANCES
Disturbances in habitat structure and composition as a result of herbivorous pressure alter the bird species present in the community, when transitioning from an intact vegetation state to one of a transformed nature. Due to changes in habitat suitability, studies have demonstrated that shifts in bird feeding guilds would be expected when comparing intact and transformed vegetation with a loss of frugivores as well as an invasion of granivores being evident (Chabie 1999; Skowno & Bond 2003). A decrease in the habitat heterogeneity of as well as the loss of fruiting trees from transformed areas leads to a decrease in the diversity of the foraging guilds due to the reduction and possible loss of groups such as gleaners and nectarivores (Chabie 1999).
Not only is there a variation in bird assemblages, there is also an overall loss in species diversity and richness due to the simplification of vegetation structure limiting the diversity of food resources (Chabie 1999; Esquivel et al. 2008). It was found that the elephant-impacted Miombo woodlands in Zimbabwe display a reduced vertical and horizontal heterogeneity and as such possess reduced species richness with shifts in domination from woodland to non-woodland species (Cummings et al. 1997; Kerley et al. 2008). Through the uprooting of large trees, elephants reduce perching and refuge sites as well as possibly decreasing the availability of raptor nesting sites in the savanna habitat, although limited studies have been carried out to determine the specific requirements of these birds and therefore the influence of elephants on their nesting behaviour (Eckhardt et al. 2000; Monajem & Garcelon 2005; Kerley et al. 2008).
Biodiversity is greatly impacted through habitat change with additional implications for ecosystem health and functionality (Owen-Smith 1996; Landman et al. 2012). The abundances of birds within the community are dependent on species-specific habitat requirements with a general shift being observed from thicket specialists, in intact thicket, to open habitat birds, in the transformed areas of the AENP (Chabie 1999). The spread of species within an open habitat bird community would be anticipated as being restricted to only a few dominant grassland species of higher relative abundance and as such, reducing community evenness along the elephant-induced transformation gradient. As the landscape is transformed from intact vegetation it would become more suitable to other species and therefore be utilised differently, thus modifying the types and abundances of the bird species present in the area.
BIRDS AND ELEPHANTS IN SUNDAYS THICKET
The loss of resources between intact and transformed vegetation as a result of elephant movements has implications for the species utilising these habitats. The best example of this intensity variation of elephant impacts on biodiversity is apparent in the Sundays Thicket of the AENP. With a shift from intact thicket to transformed habitat, there is potentially a loss in thicket-dependent species however this transformation generates opportunities for other species to utilise the landscape generating a possible overlap of bird species. Little is understood however about the cascading effects of elephant presence in a system on other faunal groups (Kerley & Landman 2006; Kerley et al. 2008). Chabie (1999) provided a general idea of the effect of thicket vegetation transformation on bird communities but this was not carried out in relation to elephant effects and only used a thicket-grassland approach with no intermediary habitat phases. Therefore the proposed theory that the changes observed in the structure and composition of the AENP bird community as a whole can be directly attributed to elephant-induced impacts on the Sundays Thicket habitats within the park requires further investigation (Kerley et al. 2008).
The key driver in thicket structure is herbivory but due to slow regeneration, a large-scale disturbance, such as the intense utilisation of the canopy shrubs, will promote a transformation fundamentally irreversible resulting in reduced ecosystem health and functionality (Kerley et al. 1995; Landman et al. 2012; Landman et al. 2014). One of the key objectives in reserve establishment and management is the preservation of the species within its boundaries. However, with a shift from species-specific conservation, such as promoting elephant population growth, to the more holistic approach of ecosystem-based conservation, a more integrated understanding of the patterns and processes involved in a healthy functioning system is required (Owen-Smith et al. 2006; Landman et al. 2014).
Elephant-induced impacts on vegetation have been well described, but besides the existence of a few examples in the literature, including a select few focusing on bird responses to structural vegetation changes, the impacts on faunal groups are not as yet extensively defined (Chabie 1999; Kerley & Landman 2006; Ogada et al. 2008; Landman et al. 2012). Often the approach taken on is simply the assessment of the presence-absence of elephants; however this is probably less useful in the understanding of how bird communities are subsequently influenced. We know elephants transform landscapes and we know vegetation structure and composition influences habitat suitability, it is however important to know to what extent elephant-induced impacts on vegetation structure and composition influence the diversity of species utilising the affected habitat (Skowno & Bond 2003; Landman et al. 2012). Therefore there is a need for a comprehensive and integrated understanding of the mechanisms driving shifts in biodiversity across space and time (Landman et al. 2014).
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