Shrinking forests - what lizards can tell usabout fragmented rainforest habitats

November 1998

Tropical forests are the Earth’s mostbiologically diverse ecosystems. Toproperly manage these unique areas, itis important to understand the effectsthat reduced size and fragmentation ofthe forests have on their inhabitants. Itseems inevitable that small ‘islands’ ofrainforest will become the last refugesof many rainforest species. These‘islands’ or fragments may become the‘seeds’ from which to re-establishextensive forests in the future.

Why study fragments?

Studying the effect of fragmentation isimportant because small isolatedpopulations of animals and plants tendto decline in genetic diversity. This is aproblem as it can reduce their ability torespond to changing environmentalfactors or to new diseases or parasites.Another effect of fragmentation is thechanges caused to the physicalenvironment. Plants and animals aremore likely to be exposed to what are

known as ‘edge effects’. As a block offorest becomes fragmented, there is anincease in the amount which is edge.

Edge effects are numerous and includefactors such as increased sunlightreaching the forest floor with associateddrying effects. Temperatures areconsequently hotter and more variable,and humidity is lowered. Increasedwind shear results in greater numbersof fallen trees which further opens upthe canopy. The changed environmentalconditions cause changes to sensitiveinsect populations, and predatorybehaviour by birds increases due to themore open canopy. These are just someof the direct effects of increasing theamount of edge.

Why use lizards to study forestfragmentation?

Most previous studies have focused onbirds and mammals, and theirresponses at the community rather thanindividual level. To make statistically

valid conclusions regarding individualspecies, you need large sample sizes.

The scale that is investigated is verydifferent when looking at lizardscompared to mammals or birds. This isbecause lizards are usually muchsmaller than birds and mammals, theydo not move such large distances, andhave smaller home ranges. In terms ofhabitat fragmentation, this means thatthe size of a forest fragment which canhold a healthy population of reptiles islikely to be different to that which canhold a healthy population of eitherbirds or mammals.

Studies of the effects of fragmentationon reptiles are rare, but since they areknown to adapt quickly toenvironmental change, especially in‘island’ systems, they are an idealcandidate for studying changes to shapeor size. The two research storiesdescribed here are about forest skinkswhich are abundant in the wet tropicsand easy to capture. Therefore, it hasbeen possible to have good sample sizeswhich are suitable for individualstudies.

Skinks are a family of lizards widelydistributed across Australia. They haveevolved into many species in theAustralian desert and Australiaoutranks other countries in terms of thenumber of skink species. Despite thisfact the biology of skinks is poorlyunderstood, and very few Australiansare even aware of their existence. This isprobably due to the fact that they are,for the most part, small, cryptic andoften overlooked in favour of larger,more obvious animals.

Researchers,

Joanna Sumner

and Craig

Moritz, find

skinks by rolling

logs and catching

them by hand

Researching skinks in rainforestfragments

Simon Cook is a PhD student who isstudying at James Cook University inTownsville. The purpose of his researchis to determine the diversity of skinkspecies living in tropical rainforest andthe difficulties they might have faced inthe long evolutionary path needed toevolve from desert skinks to rainforestskinks. Simon’s studies areconcentrating on the importance oftemperature. Lizards rely on gainingheat from the environment to performnormal bodily functions such asdigestion of food and reproduction, andmay also rely on heat from the sun towarm them up in the morning so thatthey can commence their dailyactivities.

Joanna Sumner, a PhD student at TheUniversity of Queensland in Brisbane,has been studying the effects of forestfragmentation on one particular skinkspecies—the prickly forest skink,Gnypetoscincus queenslandiae (Figure 2).In particular she has studied populationdensities, and differences in size andshape among populations of the lizardskink in both rainforest fragments andareas of continuous rainforest. Shechose to study the skink populations ofthe Atherton Tableland where there area large number of rainforest fragments.

Simon and Joanna are both participantsin the Rainforest CRC. They haveundertaken their research separately,but there are some interestingsimilarities in their findings. Bothstudies are summarised in thisinformation sheet.

Hot and cold running skinks

There are two major strategiesdeveloped by skinks to deal withliving in a cool rainforest environment.Skinks in the rainforest are either‘heliotherms’ or ‘thigmatherms’.Heliotherms actively seek sunny spotsto warm their bodies. In the rainforestthe opportunity for basking in sunlightis limited by the closed canopy of theforest, therefore most of the heliothermspecies are limited to areas such asrecent tree falls, where sunlight canreach the ground. Thigmatherms don’tneed to seek sunny positions to warmtheir bodies, they are able to be active atthe temperature of the surroundingenvironment.

Simon conducted laboratoryexperiments by putting skinks in a longtank which is hot at one end and cold atthe other (Figure 1). He allowed thelizards to choose the place they liked inthe tank, and then measured theenvironmental temperature in thatpreferred location. This preferredenvironmental temperature in the tankwas then compared with thetemperature in their rainforest micro-habitats. The results show that theirdistribution in rainforest is largelydictated by their temperaturerequirements, although Joanna’s results(below) show that the availability ofsuitable homes is also a factor.

While heliotherms may be vulnerable topredators such as birds as they sit in thesun warming up, they are extremelyquick and rely on their speed to avoidbeing caught. Thigmotherms move

more slowly but rarely exposethemselves to predators, as they forageunder the leaf litter and in the darkerareas of the forest.

Skink lifestyle may determine wherethey prefer to live

These differences in lifestyle mayexplain the different distributions of thespecies in these wet tropical forests. Theheliotherm species are capable of livingin eucalypt forests, as they have notgiven up the heat-loving lifestyle oftheir more open country relatives. Thethigmatherms, however, are incapableof moving into hot, dry eucalypt forests- they have become rainforestspecialists. The result of this is thatalthough thigmatherms can use agreater area of the rainforest, they areless capable of moving between isolatedareas of rainforest. Heliotherms such asthe red throated rainbow skink (Carliarubrigularis, Figure 3) are fairly common

Figure 2Prickly forest skinks are dark brown in colour, with a roughrather than a shiny skin like common garden skinks. Aftermoulting, the striped orange of the skin on their legs is muchmore visible - most of the time they are too muddy for thestripes to be obvious!

The lengths of these skinks is usually between 30 and 80 mmfrom their snout to the base of their tails, and they reachmaturity at about 50mm. They give birth to between one andfive live offspring once a year in the late wet season (Jan - Feb),and groups of baby lizards can often be seen under the same logas their mother during this time.

40cm

180cm30cm

lightbulb

leaf litter

Figure 1 The thermal gradient used to determine

preferred temperatures of different skink species in the

lab. The tank sits in a cold room at a constant

temperature of 16 oC

in all the rainforest areas of the wettropics and can be easily found baskingin sunspots for the majority of the year.

Are changes in light gaps makingprickly forest skinks vulnerable toextinction?

Simon’s research has shown that thereis a minimum canopy opening sizewhich the red throated rainbow skinkcan occupy to ensure that it gets enoughsunlight to have time to forage anddigest food. These skinks cansometimes be seen chasing each otherout of a particularly favourable sunspot. Artificial canopy openings such asroadsides, have produced far morehabitat for this species than wouldnormally be available, as tree fall gapsare usually quickly filled by youngtrees. This has allowed the species toexpand its range within the rainforest.

The thigmatherm skinks such as theprickly forest skink (Gnypetoscincusqueenslandiae), are more restricted inrange, being found only on theAtherton Tableland and around theDaintree area. This skink is a truerainforest resident. It lives under andwithin rotting logs on the rainforestfloor. Studies have found that pricklyskinks tend to remain in the vicinity ofthe same log for their lifetime and arenever found outside the rainforest.Prickly skinks are generally active onlyon wet nights. They have become suchspecialised rainforest animals that it isextremely difficult for them to colonisenew areas, and so they are basicallyrestricted to the rainforest patch theyare presently inhabiting. This may mean

that increased forest fragmentation andclearing may make thigmathermicspecies more vulnerable to extinction.

Simon is continuing his research on thedistribution patterns of these speciesand their preferred temperatures, andthis will enable prediction of the effectsof changes to the environment on thesefascinating creatures.

For more information about Simon’swork, please contact him directly:Simon CookZoology Department, James CookUniversity, TOWNSVILLE, Qld, 4811Ph: (07) 4781 5742 Fx: (07) 4725 1570Email: simon.cook@jcu.edu.au

Are shrinking forests shrinkingskinks?

Joanna’s research has concentrated onthe prickly forest skink (Gnypetoscincusqueenslandiae). She made several fieldtrips to the Atherton Tableland duringseveral wet and dry seasons andsurveyed 11 rainforest fragments andseven continuous forest sites. Joannafinds the skinks by rolling logs andcatching them by hand. Capturedskinks are weighed, measured,allocated to a sex if mature enough todetermine and then released at thepoint of capture.

The availability of suitable habitat hasalso been determined. The diet of thisspecies consists of the insects that sharethe rotting log homes, such ascockroaches and pill millipedes. Themain predators of skinks are snakes

such as crowned snakes (Cacophischurchilli) and swamp snakes (Hemiaspissignata), as well as birds. On oneoccasion Joanna put her hand over askink to catch it and noticed that rightnext to her hand was a small crownedsnake curled up waiting for the rightmoment to grab that same skink!

Results

Joanna’s results indicate that two factorsaffect population density—the size ofthe forest and the number and qualityof the logs available in the forest.Smaller forest fragments were lessdensely populated with the skinks, aswere fragments containing fewer andlower quality habitat sites.

As for the skinks themselves, Joannafound that those in the forest fragmentswere smaller, weighed less and showedsome changes in shape compared tothose captured in continuous forest.

Figure 3The red throated rainforest skink is usually grey or brown onthe back with a pale belly and a copper coloured head. Duringthe breeding season adults of both sexes develop a bright scarletthroat and chin. This colour extends down to the front legs infemales and further down the body in males. Some males havea red underside right down to their back legs at the height ofthe breeding season. The colour on the throat is believed to beused to signal other skinks while not attracting predators.

Females lay two eggs at a time and may have two or threeclutches of eggs during a wet season if conditions are good.The eggs are layed at the edge of sun spots in the rainforest andgenerally take 4-6 weeks to hatch.

Mature rainbow skinks are generally 70-80mm from the tip ofthe snout to the end of the tail.

Researcher, Simon Cook

What do the results mean?

The differences in size and weight mayreflect either differences in growth rateor survivorship. There are two reasonsfor differences in growth rate - geneticdifferences between populations andenvironmental factors.

One environmental factor that couldaffect growth rate in some species oflizards is food availability. Edge effectshave been studied in Amazonianrainforest fragments and these havebeen found to change both theabundance and community structure ofinsects. If this is the case in the WetTropics also, the preferred prey ofprickly forest skinks may be in lowerabundance in small fragments.

Edge effects may also play a role in thechanges to body shape that Joannaobserved in the skinks. Edge effectsresulting from forest fragmentation tendto increase ambient temperature andsoil temperature on the forest floor, andmay also increase temperaturevariation.

It is known that if skinks experienceeither increased average temperaturesor variation in the temperatures duringpregnancy, newborn lizards havesignificantly different shapes and aresmaller. The prickly skinks Joannacaught in forest fragments showed thekinds of changes that would beexpected from increased or variedtemperatures during pregnancy.

Differences in survivorship offer analternative interpretation for the sizedifferences seen between lizardpopulations from fragmented versuscontinuous forest. For example,

increased predation could occur due toa relative scarcity of decaying logs inthe forest fragments. This wouldaccount for the average decrease inbody size of skinks found in thefragments.

However, the changes in bodyproportions as well as overall sizesuggest that the decreases in populationdensity are due to more than justpredation. Further study will be neededto determine the causes for the changesthat are being observed.

What are the implications of thisresearch?

These studies examine the quality of therainforest in fragments. A measure ofhabitat quality is provided by studiesexamining the changes in the number ofdifferent species and in individualanimals.

Joanna’s study and that of otherresearchers shows that there are largenumbers of different animal species inforest fragments, so maintenance andmanagement of these areas isworthwhile. The simplest way to do thisis to link the rainforest fragmentsthrough rainforest corridors of nativevegetation.

Joanna believes it is important toencourage landowners to reconnectisolated forests by planting trees alongrivers and to keep cattle out of the forestfragments.

Joanna will compare her results on theeffects of fragmentation on reptiles toother researchers results for mammalsand birds, in order to work out how

best to manage rainforest areas tomaintain the greatest amount ofbiodiversity possible.

The animals and plants we find inrainforest today evolved together toform the intricate web of interactionsthat scientists are just starting tountangle. We don’t know theconsequences of losing even one speciesfrom this web that we rely on for cleanwater and air. For this reason it is vitalthat we try to maintain rainforest,reduce the drastic changes resultingfrom fragmentation and conserve thesevital interactions.

For more information about Joanna’swork, please contact her directly:Joanna SumnerZoology Department, The University ofQueensland, ST LUCIA, Qld 4072Ph: (07) 3365 1032 Fx: (07) 3365 1655Email: jsumner@zoology.uq.edu.au

Joanna Sumner with a prickly forest skink