Studying the effect of habitat fragmentation on diversity of wildlife
In the hearts of forests and jungles around the world there exist members of a feral strain of scientist who grew up on a steady diet of the Captain Planet cartoons. They’ve swapped lab coats for khaki shirts, plastic shoe covers for walking boots, and safety goggles for binoculars in order to study and help protect the creatures surviving in the most biodiverse, but threatened, habitats in the world. Rumours of spandex-wearing are entirely unfounded though.
In between the Amazon basin, the hot eastern Brazilian savannahs and the temperate grasslands of Argentina lies the Atlantic Forest. It is home to a diversity of plants and animals to rival that of the famous Amazon Rainforest itself, including a number of species found nowhere else such as the golden lion tamarin, the maned three-toed sloth, and the woolly spider monkey. The forest once stretched along one third of the coast of South America, inland as far as Paraguay. However, today the Atlantic Forest has the dubious status of being one of the most devastated habitats in the world. Shockingly, just 7% remains. What makes this statistic even worse is that the forest consists not of one continuous stretch of woodland but instead thousands of small fragments separated by huge crop and cattle farms.
Deforestation of the Atlantic Forest, like in many places around the world, has had a directly detrimental effect on many species of animals by destroying their habitats. However, in all of these ecosystems across the world the wildlife that remains – even in pristine, untouched habitat – are under threat due to the effects of fragmentation.
Habitat fragmentation spells very bad news for wildlife for many reasons. Firstly, small fragments don’t have the volume of fruit and other vegetation to support complex ecosystems. Low volumes of fruit and vegetation means few plant matter-eating animals, and even fewer large animals that prey on them. Secondly, large animals require not just large territories but also access to the territories of other populations of their species. This helps to avoid inbreeding and instead maintain a healthy level of genetic diversity. Unfortunately for many species of land animal, a fragment half a mile away on the other side of a cattle farm may as well be on the other side of the world.
Finally, fragmentation means that more of the habitat is bordering against agricultural land. This degrades the quality of the habitat along its edges because farming has an effect on the soil, amount of shade and the temperature. This ‘Edge Effect’ is revealed in poor tree growth and the absence of many animal species up to 100 metres in from the forest edge. Additionally, more bordering against human land means more encroachment by hunters and by agricultural development, so fragments get replaced by crop fields and cattle ranches much faster than large areas do – it’s like the way a teaspoon of sugar into your tea dissolves faster than a sugar cube.
There are lots of reasons that biologists may find themselves working in a jungle, savannah or swamp, other than wishing they were Steve Irwin. But one of the key concerns of ecology and conservation biologists is to document what level of biodiversity – that is, the variety of different animal and plant species – that an area can support. Biodiversity is an important indicator of how healthy an ecosystem is – decreases can reflect the detrimental effects of deforestation, disease or hunting, and increases can reflect progress of efforts to protect habitats. For example, biodiversity may be lower, i.e. there are fewer different species, in a small fragment of the Atlantic Forest compared to a much larger fragment. Often it is the biggest species that are absent in small habitat fragments – hefty grazing mammals that need a lot of land and vegetation like elephants, giraffes or tapir, or large animals at the top of their food chain like jaguars, caiman or anaconda. However, small animals are also vulnerable, especially if they have very specific dietary or shelter needs. Indeed, many of the most endangered species in the world are small animals that live a fragile existence.
Assessing biodiversity starts with trying to work out how many species there are in an area, and estimating how many individuals there are of each species based on how many you find. Unluckily for scientists and ecotourists alike, habitats are rarely like safari parks, where animals are pretty much always visible and easy to observe. Biologists and conservationists don’t lament this fact though –wildlife’s fear of humans is ideal for avoiding hunters. But between animals being rare, well-hidden, fast, living in impenetrable habitat such as thick forest, or spread over a huge area, it can be difficult or even impossible to find a species. That’s why assessing a location’s biodiversity uses a wide range of different scientific techniques. These techniques range from a very simple bucket dug into the ground, to complex DNA analysis techniques in the lab. The technique you use depends on the kind of habitat you are assessing and the kind of animals you’re looking for.
The most basic method is to simply walk through a habitat and keep your eyes and ears open for the animals themselves or signs of them. This works for conspicuous, noisy animals such as apes, or birds with identifiable nests, or creatures with predictable hiding habits such as some snakes. Despite being simple, this technique does need careful consideration – timing and location are everything.
Trapping methods are useful for a wide range of different animals that are harder to spot. Small, ground-dwelling animals such as rodents, scorpions, tarantulas, flightless insects, frogs, lizards and small snakes can all be assessed with drift fences and pitfall traps: they bump into a temporary fence (usually a long sheet of plastic) and in trying to get around it fall into buckets dug into the ground. With a layer of soil and shelter at the bottom, these buckets house the animals until they can be checked, recorded and released. Bigger, trigger-mechanism traps can be used to trap mammals and lizards of various sizes, often using bait to entice them in. Bats and low-flying birds can be trapped using fine nets called mist nets.
For spotting cautious and elusive animals, such as jaguar or snow leopards, a camera trap is a wonderful piece of equipment. A weather-proof camera with a motion sensor can be strapped to a tree trunk or rock and left for days or sometimes weeks to digitally capture any animal that happens to pass by. They can be expensive, but they’re worth it: camera traps often provide the only picture or video of a species in an area.
For those animals that can be caught, small microchips can be swiftly injected under the skin so it can be identified if caught again – calculating how many captures are actually ‘recaptures’ provides an estimate of how many of that species there are in total. For those animals that are elusive and more difficult to trap, camera traps are great for determining the presence of that species but are unlikely to be able to provide a good estimate of the number of individuals in a population.
This is where laboratory science comes back into the picture. Genetic analysis is now one of the most useful and widespread tools in conservation science – and usually involves poo! Faeces give us a whole host of information – not only can it help to identify the diet and diseases of a species, the DNA of the ‘donor’ can be extracted and amplified in order to determine the individual’s identity, gender and family heritage. Obtaining the genetic code of just a few individuals can provide enough information to estimate the population number and determine the level of genetic diversity.
In short, both field and lab techniques combine to help us determine if an ecosystem fragment is large enough, and has sufficient access to other fragments, to maintain healthy populations of its resident species. With this information, conservationists can go about providing these animals with what they need in order to survive well into the future. In fact, Captain Planet probably did some of his best work in a lab coat...
Is it ME???
No, it's you! chris, Thu, 23rd Jan 2014