The Problems associated with Invasive Species
Invasive species come in many forms and are numerous throughout varying habitats. It is now known that the second largest threat to all species is the introduction of exotic species. Possibly the most infamous is the American Grey Squirrel Sciurus carolinesis which has decimated the British population of native red squirrels, now confined to managed areas of wilderness in small parts of England and Scotland. Any species introduced to a new habitat or area must be well considered as to it’s possible effects on the surrounding populations. Too many mistakes have been made in the past which have led to the demise of many indigenous and endemic species. All are due to the existence of man, possibly the most feared and unstoppable invasive species to have ever been! Animals do travel to new areas naturally to find better environmental conditions and it is from this travel that many of the worlds strangest creatures have evolved to fill a niche that was open for exploitation. Invasive species do not always thrive and it is only when conditions are viable, and the lack of environmental factors which may control populations in their natural habitats do not exist that invasive species excel. Usually this means a lack of natural predators which control populations in their natural habitat, an increase in food sources, nutrients or a lack of competition for these nutrients. These factors lead to the invasive species reproducing successfully and eventually populations begin to exploit an otherwise balanced habitat.
The Himalayan Balsam Impatiens glandulifera which has spread to woodland areas throughout the U.K may now be known as an established non-native species. Its seed dispersal technique and rapid growth in nutrient rich areas has allowed it to thrive throughout most parts of the country, creating monoculture environments and leaving bare soil in winter which is sensitive to erosion. The Himalayan balsam is a perennial herb which prefers moist soils and generally lives in riparian habitats, along river banks, although many individuals are found in broadleaf woodland due to the plants high tolerance to shade. Originally introduced as a visually attractive garden plant in 1839 from the Western Himalayas or India it has now spread at a rate of 645km² per year (gardenorganic.org.uk). It is aesthetically pleasing to the eye, growing up to 3m tall with broad lanceolate leaves with individuals producing many flowers from a deep purple to a pinkish white colour from June to October. These flowers attract bees and wasps which are the main pollinator of the plant. There are many objections from bee-keepers to the attempted extermination of H. Balsam in many areas by conservationists as these flowers provide a harvest of pollen for bees late in the year. As a prolific nectar producer, ecologists believe that this may become a preferred source of food for bees resulting in native plants struggling to re-produce. It is not uncommon to find large monoculture stands of balsam in areas of woodland which are in almost complete shade. These areas would usually be an ideal habitat for species such as nettles and bramble however the large and numerous leaves of the Himalayan Balsam out-compete other plants for light and nutrients leading many native species to show signs of suppressed growth. Some conservationists believe that I.glandulifera may be capable of out-competing even hardy, well-established, native perennials in riparian habitats.
The main vector for seed dispersal is water. Seed pods explode when mature propelling between 4 and 16 seeds per pod, up to 2 metres in all directions. Because of this seed propulsion, the plant can not only spread downstream with the natural flow of the river but also upstream. Seeds can lay dormant for up to 2 years and require cold weather to crack the impervious covering to allow germination. Although no seed bank is formed and there is no rhizome root system individuals can produce up to 800 seeds. After germination H. Balsam grows rapidly upwards, the stem is simple, with some branches and has hollow compartments running down to a basic root system.
Controlling Himalayan Balsam can be difficult and eradicating the species from nature reserves is a time-consuming task. Areas of balsam can be grazed before seeds are produced however, the most effective method is by hand-pulling or ‘balsam bashing.’ Groups of volunteers regularly undertake hand-pulling and can clear large areas in a relatively short period of time. Plants are then looped over branches of trees so as to ensure they do not re-establish themselves which is possible if left on the ground. Another method is burning. Chemical control by spraying in early spring when the plants are at their most active growth stage can ensure little or no growth for that season however the next year’s seeds spread previously may germinate. Riparian habitats can also be very sensitive to chemical control especially pesticides which can easily enter the water table and affect food chains from algal species, the fish that feed on them to bird species which feed on the fish. Research is currently being undertaken in the Balsam’s native habitat to find host-specific pathogens and herbivorous insects (Fowler ‘91) which may be able to control the ever increasing populations of Himalayan Balsam.
The European Hedgehog Erinaceus europaeus
How attractive an invasive species is often draws public attention away from the harmful effects it has on the environment in which it is has been artificially introduced. The hedgehog is a welcome visitor to gardens all around Britain whether it is feeding on herbivorous invertebrates during summer nights or hibernating under a pile of leaves in the back garden, it is considered an iconic figure of British wildlife along with foxes and badgers. During the 1970s, a gardener on the island of Uist, part of the Western Isles of Scotland, introduced a handful of hedgehogs Erinaceus europaeus to control garden pests such as snails and slugs. Hedgehogs however do not only survive on invertebrates and have been known to feed on carrion and ground-nesting birds’ eggs. In one study it was found that 10% of a hedgehog’s diet consists of bird eggs. The introduction of the hedgehog to this remote island has caused a severe decline in ground-nesting bird species such as the lapwing. In the Western Isles there are few predators of hedgehogs such as large owls, badgers and foxes and as such the species has thrived. For gardeners whose crops are eaten by caterpillars, snails and slugs, this is good news. However there are detrimental effects to the population of native species of Uist as is the case with all invasive species, so much so that that the Uist Hedgehog Rescue (UHR) coalition was formed in 2002.
Since the introduction of hedgehogs it is estimated that over 5000 hedgehogs now inhabit the island, in 1999 an estimate of 57 adult hedgehogs per km². As with all invasive species the environmental factors play a major role in the success of the species. Studies in Europe suggest that litter sizes vary with different temperature climates. At higher latitudes litter sizes are said to increase with individual females in Scandinavia and Germany producing an average litter size of 6 compared with litter sizes recorded in Amsterdam averaging at 4 per individual. In relation to the human induced invasion of Uist where average temperatures are less than mainland Britain it is possible that litter sizes are on average larger than those found in native habitats. It has also been found that in Uist individuals are capable of having three litters each year possibly due to harsher winters and milder summers, meaning the hedgehogs are able to start breeding earlier and go into torpor dormancy later. With few predators and an abundance of seasonal ground nesting bird species such as Oyster Catcher, Ringed Plover, Dunlin, Redshank and Lapwing, which have all suffered population declines since the introduction, it is clear that UHR are a worthwhile cause. The decline of such bird species is in good correlation to the introduction and subsequent increase in the population of the European hedgehog.
In 2003 a decision was made by the SNH (Scottish Natural Heritage) to eradicate all hedgehogs found on Uist by means of a mass cull. Working with the RSPB (Royal Society for the Protection of Birds) the planned cull began in the summer of 2003 and 66 individuals were captured and killed. This sparked controversy from animal rights activists who believed that false claims had been made by the SNH about the problems of re-locating individuals from Uist to the mainland (EHRG). These claims included the welfare of re-located hedgehogs would be compromised during transportation, translocated individuals would be more susceptible to disease and, indigenous hedgehogs would suffer food shortages due to Uist refugees. After one season of active culling the plans were halted, while re-location of individuals began in 2003 and to date over 1000 animals have been successfully transferred to mainland Scotland. Survival rates of ‘soft-released’ individuals vary according to a study conducted in Nottinghamshire by Hedgehog Welfare with averages of 80% in week 1 of release, 92.5% in week 2, 70.6% in week 4, 58% after 6 months and 25% after 3 years (Hedgehog Welfare, Newark, UK). From this data it can be concluded that the re-location of Uist hedgehogs was a wiser decision than culling. The task that is ahead of the UHR and the SNH in Uist is not a small one however this is an example of what humans may do to rectify past mistakes. Government funding is low and as such the effects of the valuable work UHR are undertaking are slow to take place however a there is obviously a bright future ahead for the migratory ground-nesting birds of Uist.