Small-scale vs large-scale tropical agriculture impacts on frugivorous avian species: A review

As primary forest abundance continues to decline as land use practices continue to expand with global economic demand (Renwick et.al 2014), understanding land use impacts on native forest inhabitants has never been so important. With the aid of scientific literature, this study aims to review the various levels of tropical agriculture and its impacts on primary forest biodiversity and native species, with particular interest in frugivorous birds.

Tropical agriculture

A number of studies classify small-scale tropical agriculture as small managed areas of altered forest land (approximately 1-5 hectares) aimed at optimising production of a single or variety of crops by integrating with primary forest or adapting and utilising secondary forest (Chandler et.al 2013: Tremblay et.al 2014: Kuppler et.al 2015). Agroforestry utilises primary forest vegetation i.e canopy trees, to provide shade whilst retaining water (Hernandez et.al 2013) to help drought-proof crops, as found in Gutierrez-Velez et.al 2014. Another example of small-scale agriculture are home gardens and small swidden (also known as slash and burn) practices. Scales and Marsden 2008 study illustrates that swidden practices farm over a number of years (approximately 1-5 years) followed by the burning of all vegetation. The area is then allowed to be fallow to promote soil fertility for approximately 4-15 years in preparation for replanting. Though swidden crop production is usually intended for local markets (Tremblay et.al 2014), home gardens are often located in or around the grounds of a residential home for the purpose of food production for that household via integration with primary forest flora, particularly canopy cover (Helbig-Bonitz et.al 2015). When compared to large-scale monoculture, research into small-scale agroforestry indicates that these practices accommodates a wide range of native flora such as primary forest trees, which sustain and support a greater fauna biodiversity than intensive large-scale agriculture (Perfecto et al. 2003; Philpott et al. 2008). However, as most small-scale agricultural practices are land sharing, more land must be cultivated to produce yields similar to large-sparing methods which leads to the production of more secondary forest and degradation of primary forest (Chandler et al. 2013).

Large-scale monoculture practices (50+ hectares) involves the complete deforestation of primary forest for the purpose of establishing a plantation of a single crop to produce maximum yield, over a prolonged period of years; as documented throughout many studies (Koh & Wilcove 2008: Avhar et.al 2011: Senior et.al 2012: Azhar et.al 2014). The most commonly researched example of large-scale monoculture is the plantation of regimented palm oil trees for the production of palm oil kernels (over 16 million hectares globally (Lees et.al 2015)). Which is a commercially utilised product throughout the globe (Koh & Wilcove 2008). Other examples of large-scale monoculture are the production of rice, Cocoa and bananas which often utilise practices similar to swiddens only on a much larger scale (Azhar et.al 2014).

The structure and function of the primary forest, impacted upon by the differing types of agriculture, changes depending on the practice being utilised. As small-scale agriculture often manipulates primary forest by preserving key species for the benefit of crop plants, mixed crop species are often planted to optimise profit and yield throughout the year and not

in a single seasonal harvest. This is due to the production scale not being as vast as large-scale agriculture (Renwick et.al 2014). The landscape created by agroforestry is a mix of sporadic canopy trees in combination with mid-story shrubs and low ground crops such as rice and coffee. Shrub growth is often controlled to produce optimal conditions for crop growth over a period of years (Hernandez et.al 2013). The altered primary forest function serves not only to create produce, but often biodiversity composition of native fauna changes; creating a beta diversity of native fauna as more hardy species establishes themselves amongst the newly available niches (Gilroy et.al 2014). Hernandez et.al 2013 study demonstrated that agroforestry often increases insect diversity thus encouraging insectivores. Which consequently reduces the requirement for pesticides, promoting microbial soil diversity and prolonging soil quality and crop yield (Scopel et.al 2012). Agroforestry cropland subsequently become multifunctional, with production of crops as well as conserving some migrating native fauna by creating succession habitat from remnants of primary forest.

As large-scale monoculture removes all primary forest habitat, it creates hectors of uniform forest with no understory, grass cover or mid-story. This often leads to reduced crop yield in dry conditions as the lack of groundwater retention from complex root systems, cause plantations to become less resilient to drought as detailed in Gutierrez-Velez et.al 2014 investigation. Many studies have shown that plantations focus on optimising yield by regimented growth management with the aid of pesticides and chemical fertilisers (Senior et.al 2012: Azhar et.al 2014). Consequently the function of monoculture production is solely to produce maximum profit over a period of years via maximising yield and minimising produce loss. Though a highly productive and profitable practice, Scopel et.al 2012 study demonstrates that overtime yield will often reduce due to soil degradation. This practice serves minimal function in the conservation of any primary forests inhabitant species, flora or fauna. As decline in species can be seen throughout monoculture plantations with retention of only the hardiest and most niche diverse species (Azhar et.al 2014: Najera & Simonetti 2010). However this is a land sparing practice which allows for the protection of the primary forest in the form of reserves and parks, by limiting the area of crop production to a limited, intensely farmed area (Malhi et.al 2013).

Comparatively, small-scale and large-scale tropical agriculture causes less damage to primary forest biodiversity than some other forms of land use. Investigations found that although large-scale monoculture may destroy primary forest, the plantation forest it creates can retain and protect some of the primary forest biodiversity. Whereas practices such as commercial logging and mining can remove primary forest for the sole purpose of extracting resources without replacement of any secondary forest, causing significant and detrimental degradation to soil and groundwater which in turn prevents and delays primary forest recovery; devastating primary forest biodiversity (Edwards et.al 2010: Deikumah et.al 2014).

Impacts on native fauna

Agricultural practices can affect primary forest residents both negatively and positively depending on the type of practice used. However, most scientific investigations have highlighted that frugivorous avian species, and most specialist feeders, are negatively impacted upon wherever primary forest is disrupted (Deikumah et.al 2014: Gilroy et.al 2014a: Gilroy et.al 2014b) This is due to a number of reasons and often a combination of factors contribute towards declining numbers of frugivorous avian species. Chandler et.al

(2013) and Hernandez et.al (2013) studies compared shade grown agroforestry coffee production to secondary forest sun grown coffee and found that the integration of open canopy coffee farming amongst primary forest canopies, produced higher species richness of birds in comparison to secondary forest farming. However, Chandler et.al (2013) found that of the species recorded none were frugivorous, yet 82% of the forest dependent species did remain within shade grown coffee farming. This gives the suggestion that specialist feeders such as frugivorous birds, are fewer in number and favour primary forest over cropland; potentially due to poor food availability. The investigation furthermore supports this theory as it also identified that in the absence of frugivorous bird species, insectivorous species increased in number due to rising insect abundance. Hernandez et.al (2013) investigation also indicated that shade grown coffee plantations reflected the composition of secondary forest communities with minimal primary forest dependent species being recorded, which could also explain the lack of frugivorous birds as increased competition could be reducing their abundance. However, the validity of their investigation can be questioned as the method used gave no explanation for the high levels of recaptures during the data collecting process. One explanation could be that the radius of analysis was too small or too far away from the primary forest parameter to give a clear indication of any native bird species migrating in and out of the coffee plantations.

Though frugivorous bird species abundance is often dependent on food availability (Garcia et.al 2014), nesting availability has also been documented as a key factor involved in determining the presence of most avian species. Sheldon et.al (2009) and Styring et.al (2010) found that industrial tree plantations such as those used for the purpose of pulp production, often fluctuated in species richness depending on the age of the plantation. Young plantations were dominated by insectivorous bird species yet as trees matured more specialist birds arrived. As tree cavities developed, nesting availability increased and in seven-year-old plantations small frugivorous bird species began to settle within the plantation, coinciding seed blooming trees. However the study highlighted that throughout the investigation no large frugivorous birds were observed. This is potentially due to increased competition from nesting amongst smaller frugivorous and insectivorous species, alongside the small size of the young plantations cavities and lack of food availability. Though the study did indicate as the plantation boundaries began to transition into secondary forest and epiphyte growth became abundant; larger primary forest species migration did increase. This finding was mirrored by Helbig-Bonitz et.al 2015 study which indicated that a complex ecosystem encourages the migration of native fauna from primary forest to within secondary forest and agricultural farmland. Sheldon et.al (2009), Styring et.al (2010), Gilroy et.al (2014), Hernandez et.al (2013) and Chandler et.al (2013), all found that agroforestry practices which incorporate a combination of crop plants whilst retaining tall canopy trees from primary forest provide the best dwellings for birds and promote both species richness and diversity. Increased mid-story, grassland and ground cover have also been identified as having a positive impact on bird species richness as a positive correlation is often found between insectivorous, granivorous and some frugivorous species (Najera & Simonetti 2010: Sheldon et.al 2009).

Many studies have however demonstrated that one particular agricultural practice is severely detrimental not just for frugivorous bird species, but all native fauna within a primary forest. Palm oil monoculture as indicated in both Azhar et.al (2015) and Najera & Simonetti (2010) studies, lacks the complexity and diversity of plant species found within a natural forest.

Consequently, birds, mammals and insects significantly decline in abundance and in plantations were understory is completely removed, species richness of birds can be reduced by up to 41% (Najera & Simonetti 2010). This finding supports the theory that of the native bird species that migrate into agricultural land, most are insectivorous and not frugivorous. The Najera & Simonetti 2010 study further supports this in its identification that the use of pesticides among palm oil plantations significantly reduces bird species richness. Azhar et.al 2015 study found that regular and intensive harvesting cause structural and food availability changes which impacted further upon any generalist species dwelling within the plantations. In addition bird species richness decreased as distance from major roads and oil palm smallholdings decreased. This was also found to be the case with fruit bats, though increased understory helped promote their numbers. As frugivorous birds share similar food sources as fruit bats, their decline in number with increased proximity to developed areas could also be reflected in frugivorous birds which could indicate the presence of humans is also a deterrent to some species. This is supported by Azhar et.al 2014 study which found the uniform, linear structure and vigorous maintenance of oil palm plantations even negatively impacting upon mammal species, with a decline in number where human activity was abundant.

These investigations draw attention to the importance of vegetation variation not just to diversify plant species, but to create complexity in habitat structure from understory to mid-story and canopy. Furthermore, not just within a plantation or cropland but around the parameter of these cultivated lands too. Without careful consideration of these key habitat characteristics, native species of primary forest fauna all experienced significant and detrimental decline due to dramatic alteration in food and habitat availability. With the greatest detriment being felt by frugivorous birds and mammals.

The future

In order to conserve primary forest biodiversity it is clear that a balance between habitat complexity in and around cultivated land is needed (Sheldon et.al 2009) whilst maintaining moderate yield over a number of years. All the reviewed agroforestry investigations provide strong support for land-sharing, mixed crop agriculture. As it can sustain healthy beta populations of native species whilst producing a resilient crop yield of diverse produce throughout the year. However, agroforestry is severely lacking the amenities required to sustain specialist feeders, such as frugivorous birds. Reconciliation of this downfall could come from the implementation of unmanaged, mixed vegetation, boundaries around cropland. This could provide a buffer zone to aid the transition between primary and altered forest, which could encourage frugivorous species residency. Though, realistically, as initial cost of adapting to agroforestry practices is high and cropland is already limited (Chandler et.al 2013), the application of a “buffer zone” is at an economical loss to landowners. Cost motivates change and unfortunately due to this, land-sparing practices such as intensive monoculture are favoured due to their low implementation cost and high profit (Tremblay et.al 2014). Consequently, in order to encourage conservation in tropical agriculture, it must be either a profitable alternative or a regulated enforcement by a legal body.

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