Effects of Land Use Change on Amphibians in the Tropics

Introduction-

One of the biggest threats to terrestrial biodiversity in the tropics is the expansion and intensification of land-use by humans, particularly in the tropics where secondary forests are expanding, whilst primary forests are declining (Phillips, Newbold and Purvis, 2017). The worldwide demand for consumer products is leading to severe deforestation and land degradation. Around the world, the conversion of tropical forests for timber production, agriculture and various monocultures has had irreparable consequences for tropical biodiversity, especially in regions with a high degree of endemism (Gibson et al., 2011)

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Small-scale tropical agriculture:

Slash and Burn-

Small-scale agricultures can have cumulative impacts on tropical ecosystems. The main forms of small-scale tropical agricultures include slash and burn agriculture, agroforestry as well as home gardens. Slash and burn agriculture is one of the main drivers of deforestation and habitat degradation in the tropics (Klanderud et al., 2009). Slash and burn agriculture may sometimes be sustainable, but high population densities mean the land is often cropped for fewer years than they are fallowed (Marcus, 2001). The burning of primary forest kills native, regenerating tree species and allows dense canopies of invasive pioneer shrubs to invade fallows. This inhibits the growth of tree seedlings. Over time, cutting and burning cycles have produced landscapes with little secondary forest regrowth (Styger et al., 2007).

Agroforestry-

Agroforestry has the potential to maintain higher levels of biodiversity than conventional agriculture (Sistla et al., 2016). The combination of trees with harvests are an essential component of productive agriculture as resources are utilized more efficiently (Blinn et al., 2013). Various studies have found that agroforestry encourages cultivators to allow secondary forest succession to occur on their land (Montambault and Alavalapati, 2005). This secondary forest regrowth plays a major role in biodiversity conservation as natural reforestation provides habitat for species that can tolerate moderate disturbances. Agroforestry also helps to reduce the conversion of natural habitat by providing a more sustainable alternative. It also creates corridors between habitat remnants and provides ecosystem services. (Udawatta, Rankoth and Jose, 2019).

Large-scale tropical agriculture:

Selective logging-

Selectively logged forests are becoming an increasingly common part of many tropical rainforests. Primary forests are lost, and much of the remaining forest is becoming degraded by selective logging. Species richness of invertebrates, mammals and amphibians are rapidly decreasing as logging intensity increases (Burivalova, Şekercioğlu and Koh, 2014). There is no doubt that selective logging has enormous impacts on disturbance-sensitive wildlife. However, birds in logged forests increase with logging intensity as gaps open up habitats for species that thrive in disturbed environments.

Logged forests also store significant carbon and provide most hydrological functions of primary forests. Selectively logged forests also have the potential to act as buffer zones around protected areas and maintain forest connectivity for wildlife (Edwards and Laurance, 2013). Researchers have also found that selective logging drastically changes the composition of the forest. Canopy openness following logging favours the growth of small-seeded pioneer species. This change in composition persists decades after selective logging has taken place.

Oil palm plantations-

Oil palm is one of the world's fastest expanding tropical crops. Driven by increasing demand, several million hectares of forest have transpired into oil palm plantations in the last decade (Tripathi et al., 2016). Studies have found that oil palm plantations hold fewer than half as many vertebrate species as primary forests, and lead to significant changes in community composition. Oil palm plantations are structurally less complex than primary forests with a low canopy, sparse undergrowth, uniform tree age structure and more significant human disturbance (Fitzherbert et al., 2008).

Oil palm expansion contributes to deforestation indirectly by generating road access to inaccessible patches of forest. Oil palm is also a primary motive for clearance of intact forest and replaces forests previously damaged by logging or fire. Oil palm decreases area and connectivity within forests and contains lower species richness and diversity than other agricultural land-use changes (Fitzherbert et al., 2008).

Why amphibians are most affected by land-change-

Background:

Amphibians occupy both aquatic and terrestrial habitats, making them particularly vulnerable to habitat loss. Nearly one-third of amphibians are classified as threatened because of forest loss and edge effects. Fragmentation caused by human land-use change reduces connectivity in forests and aquatic habitats. Reduced connectivity reduces specialist amphibian species by reducing dispersal success and individual movements. Research on amphibians is important for conservationists because tropical forests harbour the highest diversity and most vulnerable species worldwide (Konopik, Steffan-Dewenter and Grafe, 2015)

Temperature & humidity-

Tropical amphibians with low thermal tolerances have been found to exhibit the greatest declines. Human-altered habitats cause changes in community composition and thermal tolerances among species (Nowakowski et al., 2016). Life cycles of amphibians are associated with seasonal migrations between terrestrial and aquatic resources. Therefore, the alteration of terrestrial habitat through forest degradation influences species diversity.

Dispersal-

In comparison to other taxa such as mammals and birds, amphibians are more limited in their dispersal capability, limiting their capacity to inhabit secondary forests. Nowakowski et al. (2013) found that Strawberry poison-dart frogs displayed increased resistance to movement in pastures compared with secondary forests. Decreased dispersal may be the result of heat stress as some species cannot persist in low humidity, high-temperature conditions characteristic of land with little canopy cover (Miller 1982).

Forest structure-

The habitat components that regulate amphibian composition and density are vegetation and leaf litter structure. Forest structure changes in secondary forest, altering microhabitats for foraging, breeding and fleeing predators. Whitfield et al. (2014) found reductions in litter mass had drastic effects on amphibian numbers in Costa Rica. The Tilaran Robber Frog declined the most with reduced leaf-litter quantity. Ash (1997) found that logging had a significant impact on salamander populations, and that salamander abundance returned in concurrence with the return of the leaf-litter.

Effect of different land-use changes on amphibian diversity

Agroforest-

Wanger et al. (2010) studied amphibian species richness, abundance and community composition across a land-use change gradient. Amphibian diversity and abundance declined as disturbance increased from primary forest to agroforest. Primary forest had more extensive species composition, whereas only a few species dominated disturbed areas. In older plantations shade trees are cut down because the shade they provide decreases yields. Open land created by the clearance of shade trees impacts upon disturbance-sensitive species, thereby increasing mortality rates.

Logging & oil palm plantations-

Konopik, Steffan-Dewenter and Grafe. (2015) compared species richness, density and community composition of anuran assemblages across primary forests, repeatedly logged forests and oil palm plantations in northern Borneo. Six generalist frog species mostly inhabited oil palm plantations but were absent in primary forests throughout the study. Primary forests and logged forests were alike in species richness and community composition, whereas oil plantations did not provide suitable habitat for two-thirds of the original forest species

Scriven et al. (2018) found that oil palm plantations create abrupt habitat edges that alter forest structure and composition. Large trees often die off leaving numerous canopy gaps filled by weeds, vines and other pioneer species. This change in forest structure leads to reduced moisture and increased temperatures. Amphibians vulnerable to these microclimates risk desiccation in drier environments. Many species also are affected by residual chemical runoff near oil palm plantations. 

Long-term deforestation-

Lea et al. (2005) discovered similar findings regarding amphibian communities in Nigerian rainforests undergoing long-term degradation. Researchers identified a shift from a predominance of forest specialists to a predominance of generalists following heavy deforestation. However, species diversity remained stable or even increased despite the destruction of riparian forest. Land-use change can provide ecotypes for adaptable species that can switch between various habitats.

Table (1) Summary of results of studies examining why some types of land use change have greater impacts than others on Amphibians in the Tropics.

Summary-

Human population growth represents the primary driving force in land-use change. Southeast Asia has the highest deforestation rate of any significant tropical region due to increased demand for natural resources. Studies in Southeast Asia show that canopy cover and leaf litter thickness are the most critical drivers of amphibian response to land-use change. Species richness and abundance generally decline as disturbance increases from pristine forest to open areas. Disturbed areas in Asia are dominated by a few generalist species, whereas primary forests have rich species composition. Logged forests have similar species richness and community structure to primary forests and provide suitable habitats. Oil palm, on the other hand, does not provide a suitable habitat for endemic forest species and offers limited overall benefit to conserving Asia’s amphibian diversity.

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North America displays increased rates of deforestation that pose threats to its biological conservation. Over the last 50 years, land-use conversion in Mexico has been extensive. Tropical forests are now disappearing at vast annual rates. Some land-use changes, such as shade coffee plantations, increase or sustain environmental heterogeneity. Various microhabitat types created by this land change support species similar to that of primary forests. Conversely, grazing areas and monocultures have adverse effects on biodiversity as the leaf litter layer, soil, temperature and moisture levels all differentiate from the original biotic and abiotic elements in which amphibians thrive. Monocultures and grazing areas decrease microhabitat complexity and moisture levels, ultimately reducing the number of species (Lara-Tufiño et al, 2019).

South America’s biggest threat is the expansion of agriculture. Species declines and extinctions occur mainly because of the countries lack of adequate and effective conservation policies. In Brazil, loss of suitable habitat is the main threat to amphibians. Species richness was found to be significantly lower in monocultures than old growth primary forest. However, small-scale agroforestry was found to hold some promise as an intermediate degraded forest land-use rehabilitation strategy, promoting secondary forest succession and providing microhabitats for many species of amphibians.

Discussion:

The future-

The future of tropical biodiversity conservation depends on the effective management of human-modified landscapes. Only 9.8% of the entire tropical forest lies within strictly protected areas, and the long-term sustainability of existing reserves is affected by human activity in adjacent areas. Conservationists are aiming to educate people on management strategies that will be most effective at enhancing the persistence of forest species as well as meeting the demand for natural resources (Gardner et al., 2009).

Sustainable Logging-

Logging is one of the most extensive disturbances in tropical forests, and so the need for sustainable methods to conserve biodiversity is essential. Reduced-impact logging is a type of timber harvesting that aims to reduce damage to forest structure associated with selective logging practices. Faster recoveries of secondary forest lead to increased timber yields, motivating forest managers to help ensure that forestry practices reconcile with biodiversity conservation (Gardner et al., 2009).

Land sharing versus land sparing-

Conservations and researchers have been arguing for many years if land-sharing or land-sparing is the best strategy to increase crop yields while promoting ecosystem services and biodiversity. Conservationists encourage land-sharing because the resulting farmland supports extensive wildlife in comparison to the habitat it previously replaced. Conversely, maintaining less natural habitat over more comprehensive ranges leads to further encroachment into the forest, affecting even more species (Salles et al., 2017)

However, studies have found that land sparing works best for the most threatened species, such as the majority of amphibians in the tropics. Land sparing uses high-yielding methods while conserving natural habitats. Species do better with more forest, with high-yield farming in the remaining areas — especially threatened species with smaller microhabitats. Within a tropical forest, amphibian species would benefit by maintaining as much natural habitat as possible, helping specialist disturbance-sensitive species with small ranges and dispersal rates. Land-sharing is also an essential method of agriculture to increase biodiversity (Hulme et al., 2013)

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