Managing Pets

Posted on 2 CommentsPosted in Where We Are

Image credits: Pixabay

In the archipelago of Bermuda two endemic species have been threatened by domestic animals that human occupiers brought along with them.

The Bermuda Rock skink (Plestiodon longirostris) are one of the rarest lizards in the world. They are considered to be a critically endangered species living in small scattered groups; there is only one subpopulation of 240 adults that is viable and will be able to continue according to the International Union for Conservation of Nature and Natural Resources (IUCN) (1). Loss of habitats and predation by domestic dogs, cats, pigs and rats are responsible for the decline in their numbers

The second species, Bermuda petrel (Pterodroma cahow) was considered to be extinct for three hundred years due to predation by domestic animals (dogs, cats, pigs and rats) and hunting by people. 18 pairs of these petrels were rediscovered in 1951, and were conserved by an intensive program that has ensured there were105 nesting pairs by 2013 according to the Cornell Lab of Orinthology. The recovery program which is considered a global success, happens mainly on Nonsuch an island where there are no domestic animals (2).

Bermudas is not an isolated case. The IUCN considers cats as one of the top 100 invasive species that are responsible for the extinction of 14% of birds, reptiles and mammals that have evolved in islands without predators (3). The Scientific American suggests that there are just too many cats out there (4).

The impact that pets have is unfortunately not confined only to islands.

Loss and his associates have published a research paper in Nature that reports that in the USA alone, ‘domestic cats kill 1.3–4.0 billion birds and 6.3–22.3 billion mammals’ each year. Though un-owned cats are more responsible than owned cats, it is important to note that the scientists include farm cats, and semi-strays/pets as unowned pets. Cats are therefore the main anthropogenic or man-made reason for bird and mammal mortality (5).

The doctoral thesis by E. A. Silva, found that 180 mammals worldwide are threatened by stray dogs or semi-strays, so dogs are just as invasive as cats. According to him, ‘Domestic dogs are the most abundant carnivores worldwide.’ The less care and food they receive from their owners, the more dogs turn to hunting (6, page 13).

There are an estimated 600 million cats in the world, and 480 million are stray (7). The same is true about dogs. There are 600 million dogs in the world and 480 million are strays according to World Animal Protection International figures (8).

This begs the question of why people keep pets. And why so many?

This article will not explore these issue. People who have pets or plan to have pets will hopefully ask themselves these questions. Even if (prospective) owners have the means to keep many pets, they need to consider the environmental impact they can have through their pets. In places where dogs are required to be kept on lease in the developed countries, there is no restriction on the movement of cats, leaving them free to roam around killing birds and other small animals. So the responsibility is entirely the owners.

In case of dogs there is a human cost involved too. Dogs are responsible for 99% of the deaths due to rabies, and at least 15 million people are bitten by dogs every year according to the World Health Organisation (9). According to 2016 figures from the World Organisation for Animal Health 60,000 people die each year due to rabies, and most of whom are children (10).

While nobody is denying the rights of people to have pets, they need to be aware of the problems they create for others through their pets. Frequently, owners have more than one dog or cat.

Many people do not like dogs and cats and may even fear them, but have no influence over a situation that is harmful to them or the environment they care about. Animal management programs and regulations should be guided by ecological and human health issues besides welfare for the pets.







6. Silva EA. 2012. Domestic dogs as invasive species: from local to global impacts. PhD thesis. University of Florida.





Carob as Nurse-plant

Posted on 11 CommentsPosted in Nature dynamics

Trees can accumulate seeds from other trees and plants. This happens as trees act as perches for birds or attract them with the fruits they bear. Birds use stones, post or trees to rest. When this habit of the birds is combined with the ideal conditions produced by carob trees the results are interesting.

In the Mediterranean regions, summers are warm and dry, and winters are wet and cold winters. Rainfall is around 27 to 90 centimetres annually. The vegetation is adapted to survive the stresses of the warm and dry summer. Plants are evergreen and leaves are sclerophyllus, with thick surfaces to prevent loss of water vapour (1).

In the Mediterranean Valcenia, in east Spain, the carob (Ceratonia siliqua) a legume acts as an important perch for birds. The tree grows four to six metres high and is used as fodder and or in food as a chocolate-flavour by people. This tree has been around for a long time. People in the middle East have cultivated it for 4000 years. From there it was taken to Europe, and later to the Americas and Asia (2).

Once the orchards of carob are abandoned, they begin their role as nurse-plants collecting bird dispersed seeds under them. While they are being tended, the seeds they attract as perches are cleared away by farmers. After the trees are abandoned, there is nobody to clear away the seeds, and they begin to accumulate, germinate and grow (3).

Many species of trees get collected this way under it, like the Mastic (Pistacia lentiscus), kermes oak (Quercus coccifera), prickly juniper (Juniperus oxycedrus), and Aleppo pine (Pinus halepensis). Most of these are themselves also sought after for their products by people (4, 5, 6, 7). Most individuals of these species are found under isolated carobs, than in any other site. The association of mastic with carob is particularly strong.

The frugivorous birds that bring in the seeds are the common songsters seen in Europe. They are small to medium sized. Among them are the brown and white European robin (Erithacus rubecula), true thrushes (Turdus spp.) and the Mediterranean warbler (Sylvia spp). The true thrushes are grey to brown with speckled underparts. In summer these birds usually eat more insects. By the end of autumn there are no more insects available, so the birds’ diet consists mainly of fruits. This diet continues through the winter (8).

The Mastic seems to be their favourite fruit. A Mastic shrub can produce 2,500 to 5,000 red to black drupes and most of them are eaten by birds. Drupes are small single seeded fruit with some fleshy content, for example cherries or almonds. The Mastic fruits fit even in the tiny gape (12 millimetre) of the songster’ beaks. The birds eat their fruits, and then carry their seeds in their bellies. The seeds are regurgitated, or defecated in roughly half an hour. When this happens as they are perched on the carob, the seeds accumulate under the tree. So a group of shrubs and trees start growing around the carob (3, 4).

However, the carob also acts as a nurse-plant by improving the soil conditions under it (3).

During heavy rains when rainfall is more than 25 centimetres, the bare soil cannot absorb all the water in a short time. Much of this water is lost as surface run-off. The thousands of leaves in the carob trees break the force and flow of rainwater falling on trees. So rainwater falling on trees, reaches the ground gradually, and more of it is absorbed (3).

Hence, it happens that water levels optimum for germination are found for at least five days under the trees, while soil in the open dries up faster. Seeds need water to germinate. Under the trees where seeds get more water, their chance of germinating improves. More seeds can germinate, and the process proceeds faster in the presence of water, boosting their growth. When water decreases below a certain level, germination is no longer possible (3).

Water seeping into the ground also changes the soil structure. Soil remains loose and friable while it is wet, and becomes hard and compact as it becomes dry. Therefore increased levels of soil water under the canopy keeps the soil loose. In the open, soil compaction shows half a kilo of pressure more for every square centimetre, than the soil under the trees (3).

The soil under the trees also become more compact in time, but remain looser than the soil in the open. Three days after the rains, the difference in hardness gets pronounced. Soil with more than 4.5 kilo per square centimetre is considered highly compacted. This degree of compaction is widespread in open areas after five days. Seven times more of the area in the open is shows this hard than under the tree. So the distribution of water and loose soil is patchy also under the canopy. When soil compaction crosses 1.5 kilo per square centimetre, germination is critically affected (3).

There is more water available the year around under the trees than in the open, so it is not just germination but later growth also that is helped. So the combination of attracting seeds and providing the right conditions for plant growth makes carob a nurse plant that helps other plants (3).


1. (Retrieved on 10.2.17)

2. (Retrieved on 10.2.17)

3. Verdu M. and P. Garcia-Fayos. 1996. Nucleation processes in a Mediterranean bird-dispersed plant. Functional Ecology, 10:275-80

4. (Retrieved on 28.5.16)

5. (Retrieved on 26.5.16)

6. oak (Retrieved on 10.2.17)

7. (Retrieved on 10.2.17)

8. Jordano P. 1986. Frugivory, external morphology and digestive-system in Mediterranean Sylviid warblers Sylvia spp. Ibis 129:175-189. DOI: 10.1111/j.1474-919X.1987.tb03199.x

Red Devil Crayfish and the Heni’s Emerald Dragonflies

Posted on 34 CommentsPosted in Nature dynamics

Predators usually decrease the number of their prey. The opposite happens with the Heni’s emerald dragonflies (Somatochlora hineana), whose populations are larger when their predators the Red devil crayfish (Cambrus diogenes) is around.

Heni’s emerald dragonfly is the only endangered dragonfly in the United States of America. Endangered species are organisms that are likely to go extinct because there are very few of them alive. This nocturnal insect has bright green eyes, and yellow stripes on its thorax. The wing-span is nine to ten centimetres and greater than the body which is only six to seven centimetres long (1).

Heni’s emerald dragonfly lives only near streams or in marshes in the temperate areas. It is found only in a few places in Illinois, Michigan, Missouri and Wisconsin. They are endangered because their habitat is being cleared away to make place for urban development. This urban development is also affecting the ground-water level crucial for the survival of dragonflies. In addition, pollution of streams by pesticides is lethal to dragonflies living in them. Hence, these beautiful dragonflies are very rare these days (2).

Dragonflies have been around for 300 million years, so it is a pity the Heni’s emerald dragonfly has to die out now (3). So recognition of help from an unexpected source like the nocturnal Red devil crayfish is timely for their conservation.

The streams and marshes where these two species live dry up completely for a few months in the summer from June to August. Heni’s emerald dragonflies have three stages in their life-cycle, egg, the immature stage called nymph (also called larva) and adult. They spend up to four years of their lives as nymphs, which is most of their lives as they survive as adults for only two to six weeks. The brown-coloured nymphs are only two and half centimetres long. Since they are aquatic as larvae and inhabit streams, they are at risk during summer droughts many times during their life before they reach adulthood and can fly. Being young is not fun for many animals. As it is, the larval dragonflies are a common and important food for several other aquatic animals.

The Red devil crayfish are common in the United States of America. Though they are called crayfish, they are a kind of insect. The head and thorax is fused into a thick cephalothorax which has eyes on stalks to improve the range of vision. Then there is a relatively smaller abdomen. They are green, blue, brown, to brownish red, and are four to five centimetres long. They are omnivorous and feed on worms and insects living in water-bodies and dead animals. Red devil crayfish live near ponds or streams and make burrows to a depth of a metre to reach underground water. They also have a second channel that goes horizontal into the stream-bed. Their burrows are easily recognisable by a characteristic chimney made of loose mud above the ground (4).

The crayfish is entirely aquatic and can survive only in water, and when it is out of the water it needs to remain wet. The underground burrows provide access to water that is not available in summer droughts on the land surface. In winter the burrows also protect it from freezing temperatures.

Dragonflies live in the open streams, when it is full of water, feeding on other small invertebrates and insects (1). When streams dry in summer they move to the burrows of crayfish, as these are the only places that have water the whole year, due to access to underground water. However, the red devil crayfish is their predator, which end up eating 65 percent of dragonfly larvae. Only the three to four year old dragonflies that are too big for the smaller and younger crayfish, are completely safe from predation (5).

If they were to live in the open streams the larvae would all die after just 13 days due to desiccation when there is no water. Since drought usually lasts for one to two months in these regions, no dragon fly would be able to survive. Inside burrows at least some have a chance to survive. Since the Red devil crayfish prey on dragonfly nymphs even in open streams, the threat from predation is not any greater in burrows compared to streams.

Again during winter when the water in the stream freezes, they could die if they were outside. Within burrows the temperature is always above zero degrees Celsius, so both crayfishes and dragonflies are safe throughout winter.

In Wisconsin streams that Red devil crayfishes inhabit, the Heni’s emerald dragonflies were the most common type of dragonflies. There are 95 percent of them, i.e., of 100 dragonflies of all kinds 95 are Heni’s emerald dragonflies. The other dragonflies which have different life-cycles or behaviour were unable to survive the dry summer months. So that left more resources like food and space at the disposal of Heni’s emerald dragonflies, and they manage to thrive. In a nearby stream that had water flowing in the streams even in summer, other species were abundant, and there were few Heni’s emerald dragonflies; only two to three in every 100 dragonflies was a Heni’s emerald. So Heni’ emerald dragonflies have an advantage over other dragonflies in streams that go dry in summer.

Since the Heni’s Emerald dragonfly is endangered, it is important to know as much as possible about them to conserve them. If habitats without the crayfish had been chosen because people thought Emerald dragonflies would be safe from their predators they wouldn’t be able to survive dry summer streams. Or if only ‘best’ and bountiful habitats, in this case perennial streams were set aside to preserve Heni’s emerald dragonflies, they could not compete with other dragonflies in permanent streams.

The obvious interaction between Heni’s emerald dragonflies and Red devil crayfishes is predation and negative. But the previously unknown effect of getting refuge in summer and winter turns out to be more important for the dragonflies and worth the risk of being eaten up.


1. (Retrieved on 29.11.2016)

2. (Retrieved on 29.11.2016)

3. (Retrieved on 29.11.2016)

4. (Retrieved on 29.11.2016)

5. Pintor LM and DA Soluk. 2006. Evaluating the non-consumptive, positive effects of a predator in the persistence of an endangered species. Biological Conservation 130: 584-591. doi:10.1016/j.biocon.2006.01.021