Spider Ecology

Spider ecology is a very diverse topic which includes reproductive and feeding ecology, both of which have their own pages. Other aspects of spider ecology include growth, dispersal, survival and their effects on the environment they live in. I will write a little on each of these here, but please realise that I am only offering you a very small glimpse into the fascinating world that is spider ecology.

Conservation

Spiders are undoubtedly fascinating, despite their bad press in many people's minds and they are also an extremely valuable part of the balance of the world we live in.

 

They are not greatest of pest control agents, unless you consider all insects to be pests which is a very erroneous point of view, although they are cheap, and very useful. Spiders are, for the most part, such generalist feeders that they kill many useful insects as well as many pest species, however because pest species are by their nature more numerous than beneficial species they are still effective.

They are of great value to the environment. While they tend to sample insect populations randomly, any population that becomes explosive in its growth will find more of its kind taken by spiders than otherwise. Thus, rather than specifically controlling certain insects, spiders have a general blanket or buffer effect, promoting evolution, helping keeping insect numbers under control in a broad sense and supplying food for many other animals and therefore keeping the flow of life moving.

In particular the ability of spiders to exist in huge numbers on relatively little food gives them a useful role in damping out population fluctuations in a diverse environment.

Some spiders are however being promoted in the battle against agricultural pests. Linyphiids against aphids in wheat fields in the UK and Pardosa ramulosa in California where it has been shown that 20 spiders per square metre can reduce leaf hoppers in rice fields by up to 90%.

As with most conservation, spider conservation is habitat conservation. While this can involve lots of money and special reserves it can also be simple as leaving some of you lawn unmown or setting a side strips of weedy ground around fields. This also supports other friends such as Ground Beetles (Carabidae) and birds. As a simple rule the larger you crop field is the less ecologically sound it is. Huge monocultures of crops my be easy to work and run but they are a death knell to the biodiversity of the land.

The 1990 Red Data book lists 16 species of spiders as threatened including the Great aft Spider in Europe (Dolmedes plantarius) and the No-eyed Big-eyed Wolf spider in the USA. This small number reflects how little we actually know about spider populations than the real extent of endangered species. The Mexican Red-kneed Tarantula has been so endangered by collecting for the pet trade that it is currently a Cites Appendix II species.

Moulting

Like all arthropods spiders grow inside their skeleton, the skeleton itself does not grow. Therefore they need to shed their old skeleton in favour of a new one as their bodies get larger. The abdomen of a spider is fairly flexible and will stretch as the spider grows, but the cephalothorax and the legs do not stretch. As its body acquires sufficient resources the spider develops a new cuticle inside its existing skeleton. When the time is right it climbs out of this old skeleton, allows the soft new cuticle to expand and harden and then gets back to the business of hunting and eating. Spiders stop eating for a few days before moulting, the larger the spider the longer this period can be.

During this time the spider's body is secreting a lubricating fluid between the old hard skeleton and the new cuticle and dissolving the inner layers of the old cuticle, this recycles nutrients and makes the whole exoskeleton weaker so it will be easier to escape from. First off the carapace of the cephalothorax opens up like a trapdoor and swings away from the body, then the sides of the abdomen split and the spider is able to to start pulling its body out of the old skin. The legs and palps are always the last parts of the spiders body to be extracted and occasionally one or more of them may fail to come free properly. Most spiders perform this amazing feat hanging upside down from a few strands of web. Either in their normal web home, tunnel etc., or from a some small web they have spun on the local vegetation specifically for this purpose. Tarantulas do not hang from a thread and they moult lying upside-down (on their backs) in their burrows.

Looking a little more deeply at this process we see that as the spider begins to moult its heart rate increases and its haemolymph pressure doubles. The spider's body redistributes its internal mass so that the weight of the cephalothorax increases by about 80% and the abdomen decreases by about 30%. The cuticle of the carapace then splits along the front and the sides allowing it to lift away from the body. As the spider flexes its muscles the abdomen then splits on both sides. As the spider works its abdomen free the spinnerets attach a thread of silk to the inside of the old cuticle giving he new spider a new life-line to hang from. The legs are extracted in a series of movements with two legs on each side being worked free a bit, then the other two. Once the legs are free they are regularly exercised until the cuticle hardens into a new skeleton. A failure to flex the legs results in the joints becoming stiff and immovable.

Moulting can be over in less than 15 minutes for small spiders but the larger the spider gets the longer the process takes and adult tarantulas take a number of hours to completely shed their skins. Adult spiders that live for more than one year may shed their skins once a year even though they are adult already. Moulting is controlled physiologically by a hormone called ecdysone. Like other arthropods spiders can regenerate lost limbs, but not always. It depends on when the limb was lost, if the limb is lost early on after a moult the spider will be able to regenerate it a bit at a time through successive moults, however if the leg is lost closer to the time of the next moult the spider will be unable to regenerate it. The time between moults is called the intermoult period and legs can generally be regenerated if they are lost in the first 25% of this intermoult period. Spiders have to moult a number of times during their lives, smaller species have to go through less moults than larger ones. Small spiders will normally have less than ten moults to reach maturity while tarantulas have many more. In species where the female is larger than the male the female will also normally have one or more additional moults.

Growing Up

Having developed within the egg a spiderling eventually hatches, not alone however, all the eggs in one eggsac will hatch at roughly the same time. This makes for a crowded web, but at first this is not a problem because the young spiders are still living off yolk-material stored in their abdomens. Eventually however, normally after their first moult, the young spiders need to start finding something to eat. In most species this means it is time to start hunting and that means it is time for the young to disperse, otherwise they will end up eating each other. Some species disperse simply by walking away, others however like to disperse more quickly and for these species ballooning is the answer. The young spiders climb to the top of a blade of grass or out to the edge of a branch or leaf and raising their abdomens into the air they release a string of silk that catches the wind and eventually develops enough lift to take them with it. So they fly off. Many will be eaten by Swallows and Martins as they hoover up everything floating in the air, but some will be lucky enough to reach land somewhere distant from all their brothers and sisters. Here they will seek out a suitable home and set about catching food in what ever manner is used by that species.

However while many young spiders never see their mother, as she will have died after securing the egg cocoon, others are more lucky. For these species not only does mother guard the eggs until they hatch but she may supply a safe place to learn about the world and even supply food. Young Wolf spiders in the genus Lycosa spend time clustered on their mother's abdomen after hatching. Segestria sp. keeps her young in her burrow with her and Pisaura sp. builds a special nursery web for hers. The genus Theridion contains a number of small but attractive spiders who surprise us with their maternal care, a number of species allow their young to share in the flies caught in the mother's web, and at least one, Theridion sisyphium, seems to rebuke us for rough image of spiders as she gently feeds her young from her own mouth. Another unusual spider is Amaurobius terrestris who allows her young to share her burrow until she dies, they then take their last meal from the dead mother's body. Yet even in cases like these the young spiders, usually by their second moult are forced to leave home and find a place to live on their own. Except for those few species that are social or subsocial this is how most spiders spend their lives, alone. From now until they reach sexual maturity they will only have two real drives in their lives, finding food for themselves and avoiding becoming someone else's dinner.

 

Staying Alive

Like everything that lives spiders die. We can divide the causes of death into two types, biotic and abiotic. The abiotic causes are weather, storms, floods and droughts and they can all take their toll on spiders directly and indirectly. Indirectly because the weather effects not just spiders but also all the animals they prey on and those that prey on them. Weather conditions that reduce the numbers of flies and or moths flying around can easily result in spiders not getting enough food to mature properly. They can also result in birds and other predators taking a greater portion of spiders in their diet than they would normally.

Spiders are very numerous in many habitats, it is therefore logical that there are many spider predators, either full-time or part-time, to take advantage of this abundance. Amongst the predators are many mammals, particularly that group known as insectivores, but also hundreds of species of bats and many small primates. In England it is likely that Shrews (Sorex sp.) are the most frequent of the mammalian spider gourmets. Many birds enjoy a spider snack and it is not unusual to see small birds that live near houses hunting the eaves and window ledges for spiders, and their webs. Also fish, particularly trout (Salmo trutta), frogs and toads as well as numerous lizards will enjoy a meal of spider whenever the opportunity offers.

However the most serious predators on spiders are invertebrates, and particularly other spiders, Bristowe believed that spiderkind, like mankind, are their own worst enemy, and scientific research has since been found this to be true. One family of spiders, the Mimetidae, otherwise known as Pirate spiders, feed exclusively on other spiders. Another family with a particular liking for other spiders as prey is the Pholcidae, there are also individual genera that seem to specialise on other spiders rather than insects as their prey, for instance Portia in the family Salticidae. Other than their fellow arachnids spiders that are to reach maturity need to avoid Ichneumons (Pimpline and Cryptine), Wasps (Sphecids, Pompilids, Trypoxylonids, Chalcids and common Yellow Jackets), various parasitic flies such as the family Acroceridae, beetles, ants, centipedes and even some large crickets. Moving away from arthropods we find several species of Nematodes (round worms) parasitising spiders. It is not easy surviving this barrage of predators and this perhaps explains that while spider webs are often easy to find, the spiders themselves often prefer to remain hidden in rolled up leaves, under dead bark and rocks inside their own home made silken hideaways.

Even then their lives are not guaranteed, in some parts of the world many spiders are killed by predatory fungi, particularly members of the genus Cordiceps some of whose species only develop in the bodies of living spiders.

Other Considerations

When thinking about the ecology of spiders we have to look at the world not only from the point of view of the spider but also to consider the effects their existence has on the rest of the world, none of us, not even the most solitary spider lives in isolation, our lives are all interconnected. One simple example is to realise that if all the web-spinning spiders were to removed from the Americas, amidst the many other effects that would occur, the Hummingbirds would also most likely disappear as well. This is simply because the Hummingbirds are dependant on spider web to build their nests. Many plants are dependant on Hummingbirds for the pollination of their flowers, soon these flowers would also disappear and with them would go all the insects dependant on them as well. The effects would spread through the environment like a series of dominoes falling and knocking the next domino over as they fall, and so on and so on.

While spiders are poikilothermic (cold blooded) animals it is not true to say that they are always limited to operating at a temperature equal to that of their environment. Small size and behavioural adaptations can allow a spider such as Geolycosa godeffroyi to maintain its body at a temperature about 4-5 degrees higher than the ambient. In North America species of Nephila experience cold temperatures in winter, they respond by building their webs so that the maximum amount of sunlight falls on the spider's body. Tropical orb-web spiders may have a different problem and can help keep themselves cool by changing their orientation in their webs as the sun moves across the sky.

In many temperate climes the winters are severe, not only is the cold too great for most spiders to be active but their is a great shortage of insect food. Different spiders us different strategies to survive this difficult time. Many simply dig down into the leaf littler and go to sleep, this subterranean hibernation is the most common method of surviving winter. Other species simply die off and leave only their eggs to survive the cold, their lack of cellular development prevents them from damage when they freeze. Once the weather becomes more clement the eggs develop and the young spiders can hatch in the spring when food is again available. There are some exceptions to every rule however and this applies to spiders as well. In the milder temperate climates some species will remain active throughout the winter and can survive temperatures down to about minus 5 or 6. Below this they die, but at zero they are still spinning webs. They live mostly on soil surface invertebrates such as Collembola.

Known Spider Over wintering Strategies
As Nymphs only 45%
As Nymphs and Adults 23%
As Adults only 11%
Inside Egg Sac 7%
Winter Active 9%

Habitat Selection

Most spiders live in very specific habitats, in this way they divide up the world between them and avoid undue competition. Their environmental choices are made depending on their ability to withstand such abiotic variables as temperature, both the average as well as the seasonal maximums and minimums, wind presence, moisture, humidity and light intensity. It is also effected by abiotic considerations such as vegetation type, competitors and predators.

The amount of space a spider needs to live in is small, especially for web building spiders, they can go a long time without food and expend little energy waiting fro their prey. Spiders that hunt out their prey obviously use more energy but even these can live for weeks without eating. This, and the small size of most spiders means their can be a great diversity of spiders in every habitat with different species occupying different niches only a few centimetres apart.

Scientists studying the distribution of spiders within a given habitat have found that vertical distribution up a tree trunk have found that microclimate, bark structure as well as height are important features controlling which spiders live where. In another study scientists found that two spiders, Pardosa pullata and Pirata piraticus both live in sphagnum bogs, however P. pullata lives near the surface and P. piraticus down inside the moss. The temperature, which varies near the surface is much more stable lower down.

In the above case, and in many others, we have some idea of what separates out the species. But there is still a lot for us to learn about spiders, especially about their ecology. For instance in America Lycosa carolinesis and Lycosa timuqua live in the same habitat, hunt the same prey items and are both nocturnal. Also in Europe Linyphia triangularis and Linyphia tenuipalpis also live in exactly the same habitat and hunt the same prey. What the parameters are that separate the two species is yet to be learned.

Finally on habitat it should be mentioned that some spiders change their habitat throughout the year as they grow, thus the orb web spiders Argiope auranitia and Argiope trifasciata make their webs at different heights in the vegetation during their juvenile stages, but once adult they both use the same vegetation level to build in.

Camouflage and Mimicry

Many spiders are mimics of one sort or another, mimicry helps them to catch their prey, avoid other predators and to live in other peoples homes. Mimicry can be either or both morphological and physiological. Many spiders that are active during the day are cryptically coloured, their body's patterned in ways that make them difficult for their predators to find. Take a close look at Pisaurids, Salticids or Tetragnathans. Night active, and hole dwelling spiders however tend to be drably coloured.

Some spiders look quite spectacular when taken out of their natural habitat. Species like Araniella cucurbitina and Micromata virescens are bright vivid green whereas Misumena vatia can be seen to change colour from yellow to white, from white to yellow as it is moved from one flower to the next. The colour change does take a day or two to be complete.

Other spiders such as Cyrtophora cicatrosa can change their colour rapidly, while dropping from their perch on a thread so that they are different colour on the ground than they were in the vegetation. This undoubtedly confuses some predators.

Catalepsy, or playing dead, is a defensive technique that the spiders share with many insects and vertebrates. A spider will fall from a leaf or branch and curl its legs around it and lie still for several minutes. It does look convincingly dead when it does this, but if you are patient it will suddenly come back to life and run off.

Mimicry of ants is very popular among spiders that are active during the day. This mimicry often involves the possession of an exaggerated waist and short front limbs which are held but like antennae while the spider walks, ant-like, on only six legs as well as colouration. Several hundred species of spider are know to be spider mimics, though some are obviously better than others. Ant mimicry appears to offer the spider some protection from both wasps, for instance Myrmarachne, and birds, for instance Synageles.

To end this page, perhaps the most astounding example is the Central American spider Castianeira rica. The juveniles, 2nd and third nymphal stages are black and shiny mimicking Myrmicine ants, the 4th and 5th instars are mimics of the bright orange Attines (leaf-cutting ants), as are the adult males. The mature females however are mimics of Ponerine ants.

Spiders are really fascinating and often attractive animals and if you can learn to share your world with them in peace you will have added a little to your enjoyment of life. Good luck.

 

 

 


 
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Introduction Spider Anatomy The Silken Web
Feeding Ecology Reproductive Ecology Spiders and Man
The Fear of Spiders Tarantula Myths Spiders and Evolution
Caring for your Tarantula A House-spider Safari Bibliography and Reviews

 

 

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