18 Biological Invasion

Prof. Daizy Batish

1   To understand the concept of Biological invasion

2.  To understand the different terminologies and hypothesis of biological invasion

3.  To understand the mechanism of invasion

4.  To understand pathways which led to biological invasion

5.  To understand the ecological, social and economic impact of biological invasion

2.      Concept Map

3.   Description (Biological Invasion)

3.1 Introduction

Dispersal and movement of species from one place to another is a natural process and important for the distribution of life in this world. On a longer time scale, we can say that dispersal is one of driver for evolution of species and responsible for life. Generally, these dispersal and movement is confined to an area and often limited by multiple barriers such as mountains, rivers, lakes, oceans and deserts. Species limited to such confined area are known as native species. Every species take advantage of dispersal and make profit in distribution in range from these processes to flourishes itself.

Man had overcome all these barriers and distributed itself in every part of the world. The success story of humans in evolutionary time scale is also due to his mental skills and the capability to domesticate wild species of plants and animals in different region of the world and utilize them for his benefit. For example, wheat, maize and rice which may be originated and domesticated in different part of world are now the major cereal crops of the world. There are hundreds of plant species such as vegetables (potatoes, tomatoes, chilies, brinjal etc.), commercial crops (groundnut, sugarcane, coffee, tea etc), medicinal plants (cinchona, ephedra, aloe etc) and animals (sheep, horses, cow, pigs etc) which were always accompanied by man, transported and introduced from one place to another all around the world for his benefit.

Every process has its pros and cons, while the brighter side is the benefits and economic prospects of the new species introduced in non-native lands, but darker side is the that some species unintentionally accompanied with useful species such as weeds, pests, bacteria and viruses. These species created problems such as epidemics, famines, habitat destruction and homogenization of the biota. Weeds and pests impacts our agriculture yields while human pathogens can directly linked with humans and costs millions of valuable lives. Some species escaped from cultivation lands, botanical gardens and other such human repositories and spread in the nearby forest and other habitat. These species always accompanied us and move with the human movement to the new places. Globalization and fast convenient means of transportation with intercontinental connection rapidly increased the distribution of hundreds and thousands of species intentionally or unintentionally worldwide. Species introduced to new habitat in a distant land outside their native range separated through geographical barrier are known as alien species or exotic species or non-native species.

In the medieval period, pathogen Yersinia pestis, which resides in mice and rats causes Plague and killed about one third of the European population. The potato blight caused by Phytopthora infestans, a fungus pathogen spread from North America reduced the Irish population to half its size in 1840. The present times biggest pandemic is AIDS caused by human immunodeficiency virus spread from central Africa to the rest of the world. At present HIV has infected 1% of the world population. Besides pathogens, animals like in mammals such as domestic cat (Felis catus), macaque monkey (Macaca fascicularis), birds such as Indian Myna (Acridotheres tristis), Starlings (Sturnus vulgaris) and in plants Acacia mearnsii, Lantana camara and Rubus ellepticus are all included in “100 most worst invasive alien species list” compiled by Global invasive species programme (GISP). Every year lots countries spend huge amount of money to manage and control these alien species Biological invasion refers to the disruption of natural communities and ecosystems by increase in distribution and abundance of alien species outside the native range. Studies on biological invasion documented the role of alien species in extinction of native species, disruption of essential ecosystem processes, ill effects on human and animal health and losses to economy of the region they invade. That’s why it is important to understand alien invasive species behavior and generate strategies to manage alien invasive species.

3.2 Historical Backgrounds

Earlier in 18th century, many naturalists like Charles Darwin, Alphonse De Candolle, and Joseph Hooker had mentioned invasive species. They studied these species more as a curiosity as an adaptation of species in new environment rather than major threat to global biodiversity. Charles Darwin gave the first theory in ‘On the Origin of Species’ (1859) whereas Alphonse De Candolle learned that floras gain by naturalization. Darwin in the last edition of Asa Gray’s ‘Manual of the Flora of the northern United States’ enumerated 260 naturalized species that belong to 162 genera. It was Charles Sutherland Elton in 1958, who realized the typical pattern of biological invasion and write the book on ‘The ecology of invasions by animals and plants’ first time in the world. He was considered as the founder of biological invasion. He was first to realize that mixing of numerous organism of different part of the world will cause problems to the world.

Though the work on biological invasion remained in slow pace upto 1980, the Scientific Committee on Problems of the Environment of UNESCO (SCOPE) in second half of the 1980s set a programme to revisit the key assumption and generalized proposals in Elton’s pioneering book and to review the current status of invasions in many parts of the world. The programme based on three questions: 1) what makes certain species more invasive than other? 2) What features of ecosystems make them susceptible to invasion? and 3) How can we use such knowledge to manage invasion?

2.3 Terminologies and Frameworks in Invasions

Biological invasion is new field and due to increased number of research after 1980 many new terminologies were used in research publications to explain the process of biological invasion from all over world. These terminologies lead to confusion to the readers and it was realized soon to clarify these terminologies. Different terms used in biological invasion such as ‘invasive’, ‘naturalized’, ‘alien’ and ‘noninvasive’ need are defined in box 1

3.4 Major Hypothesis and Theories

There are multiple hypotheses to explain invasions and researchers classified them in two categories: ecological hypothesis and evolutionary hypothesis. These hypotheses help to understand the species adaptation and establishment of a species in a non-native land. Williamson and Fitter (1996) have proposed the “tens rule” for a species to become invasive species in non-native land. This rule states that the probability of a species to become invasive is 10% at every stage from introduction to invasion.

3.4.1    Ecological hypothesis

It explains invasions and successful pre adaptation in a new range with long association of humans and ecosystem modified by humans. Some of the hypothesis explained in table 1.

Box 1 Key terms for biological invasions

Alien species: Plant species whose occurrence in a given area is due to their introduction, intentionally or accidentally, as a result of human activity [synonyms: exotic species, introduced species, non-native species; non-indigenous species].

Casual species: Alien species that may flourish in an area, but which do not persist for more than one life cycle without further introductions.

Naturalized species: Alien species that reproduce and sustain populations over more than one life cycle without direct intervention by humans (or in spite of human intervention); they often recruit offspring freely, but often just near adult species, and do not necessarily invade natural, semi-natural or human-made ecosystems.

Weeds: Plants species (not necessarily alien) those are undesirable from human point of view in gardens, agricultural fields, plantations and lawns. These are usually taxa with detectable economic or environmental effects [synonyms: pests, harmful plants, problematic plants].

Invasive species: Alien species that recruit reproductive offspring, often in very large numbers at considerable distances from parent species, and thus have the potential to spread at a considerable rate.

Propagule pressure: A composite measure of the number of individuals (or propagules such as seeds) released into region to which they are not native. It incorporates estimates of the absolute number of individuals involved in any one release event (propagule size) and the number of discrete release (propagule number). As the number of releases and/or the number of individuals released increases and/or time increase, propagule pressure also increases [Synonym: introduction effort].

Transformer species: A subset of invasive species which change the character, condition, form or nature of ecosystem over a substantial area relative to the extent of that ecosystem

Table 1. Ecological hypotheses:-

3.4.2     Evolutionary hypotheses

It is developed due to potential role of evolution are realized in the success of invasive species. Evolution contribute important role in the success of introduced species in relative fitness of the individuals in the population as compare to the native populations, leading to strong invasion. Some of the hypothesis explained in table 2

There are various hypotheses given by many researchers from all over the world, most of them based on small subset of invasive species under particular environment conditions, in which few became popular and discussed whereas other were discarded. Many of these hypothesis works mutually on the species at different stages of its adaptation and establishment in the new range. To understand the biological invasion, there is a need of daunting task which will be responsible for further explanation to understand the invasion in future.

Table 2. Evolutionary hypotheses:-

3.5 Invasion pathways

Introduction of any species to non-native range requires the pathway or means to cross the geographical barriers such as vast oceans, deserts, rivers and mountain ranges etc. Species sometime pass these barriers through natural forces like storms, ocean currents or vectors like birds and animals, but their chances of overcoming these hurdles are meager. Man is one of most powerful driving force behind all the introduction of species in the non-native range. Throughout ancient times humans always classified and selected the plant species according to their needs and exploited them. With the dawn of civilization and advancement in agriculture, trades in-between the civilization, humans mobilize many alien species to distant lands and continents. In the last century, with increasing and fast means of traveling, human-aided introduction of species increased manifold by intentionally and unintentionally means. Many alien species established and naturalized in the non-native range, but few species spread in vast area and became threat to the local flora and fauna in that region, these species are known as invasive species. Whereas, the whole set of processes which results in the introduction of an alien species from one geographic region to another with vectors including dispersal mechanism and means of introduction is defined as invasion pathways.

The main pathways used by invasive species for their rapidly spread in the local area are air, rivers, railway tracks, roads, and their dispersal by vectors (man by vehicles and ships, birds and animals from one region to other). The detailed analysis of human mediated introduction or pathways includes “causes” for introduction, “vectors” as physical means or “agents” for transportation, “routes” and “corridors” as geographic paths for transportations. Horticulturists have always given higher preferences to alien species from native species while native species are always valued by the ecologists and environmentalists. In case of invasion pathways, human beings are the prime vector responsible for transportation of invasive species through vehicles, ships and airplanes others than insects, birds and animals which disperse locally or occasionally long distances. Routes can be roads, railways, waterways, corridors and air. In Global invasive species databases (GISD), the invasion pathways were divided into 6 categories:

1)Release (release in nature for use, biological control, erosion control, hunting and fishing in wild, improvement of landscape/fauna and for conservation and other unintentional release)

2)  Escape (Agriculture, botanical gardens/zoos, farmed animals, forestry, ornamental, horticulture, research facilities)

3) Transport/contamination (contaminated nursery material, food contamination, contamination of plants, animals and seeds, trade and transportation of material)

4)  Transport (containers, bulk, ship, plane, vehicle, others means of transportation)

5)  Corridors (Interconnected waterways/basins/seas, tunnels and land bridges)

6)  Natural dispersal across borders.

The phenomena of different ways invasive species arrive in the non-native range are known as propagule pressure. The human activities such as intensifying agriculture, clearance of the forest for building road, railways, urbanizations and over exploitation of native species (timber, fuel woods, food, fodder, medicines and other commercial activities) are the reasons behind the spread of alien invasive species. These activities creates disturbance in the natural established ecosystems hence, creating pathway for the establishment of invasive species into further deep in the ecosystem. Human induced disturbance was associated with the 97% establishment of invasive species. Once the species survived in the non native land and starts producing new recruits, it further spread in the local area by different means of dispersal such as seeds, suckers, roots, propagules and cuttings.

3.6 Mechanism of Invasion

Many theories and framework to explain the mechanism of the biological invasion which created lot of confusion, hence Blackburn (2011) tried to unify these all framework (Figure 1). First widely accepted framework for biological invasion was by Williamson and Fitter (1996) in case of animals and Richardson (2000) in case of plants. Blackburn took base from Williamson and Fitter (1996) and Richardson (2005) and combine all other available framework to explain the mechanism of invasion. The invasion framework was the combination of barriers and stages. There were five vertical bars which signify barriers and four stages written above the bars in the framework proposed by Richardson. In invasion, the species has to overcome these barriers to pass into next stage. Blackburn added the additional barrier of captivity or cultivation after geographical barrier (Figure 1). The arrows signify the species flow toward next stage and letters in the arrow signify the name of the arrow which is the categorization scheme for the population. He included the arrow B3 for the many species such as some plants, fungi, protists, invertebrates, small fishes and other taxa which can skip captivity or cultivation barrier. The environmental barrier in Richardson’s framework was replaced by survival barrier which together with reproduction barrier helps in establishment stage. Once the species was established it remained in C1 and C2 cycle to build enough population (lag period) for dispersal to long distance. It was very important stage need to overcome by long distance dispersal for naturalized species to spread and successfully invade. Failure in any stage due to barrier is a failure for a species to become invader which is shown in the diagram with the horizontal bar and the arrow. After the alien species had spread, it undergoes through ‘boom and bust’ dynamics which suggest the increase of the population to its peak and then decline. Terminologies were labeled in red colour above the diagram whereas management strategies were drawn in yellow arrow at different stages below the diagram. Conceptualization of invasion is a staged process and not a linear one. In every stage number of species diminishes explained as invasion failure and filtered to reach the next stage.

Terminology

Figure 1 Framework developed to understand the process of invasion by Blackburn et al 2011 in Trends in Ecology and Evolution.

3.7 Successful characters of invasive species

It was assumed that invasive species success outside their range is due to some characters which helped them establish and spread. These traits are as follows:

1.High ecological amplitude i.e. species have larger range of tolerance and can withstands various environmental condition including long range of temperature, altitude, pH of soil, different terrains etc e.g. Superior competitor

2.Short juvenile period i.e. starts reproducing from very young age.

3. High fecundity and various mode of reproduction

4. Diverse means of dispersal e.g. disperse seeds through aboitic (wind, air and water) and biotic means (insects, animals and birds) , Long distance dispersal (LDD)

5. Least enemies in the non native range e.g. enemy release hypothesis, and the evolution of increased competitive ability hypothesis (EICA)

6. Allelopathic effect e.g. Novel Weapon Hypotheses

7.      Gaining genetic diversity e.g. through hybridization and founder effects.

3.8 Genetic and evolutionary aspect

Researchers believe that the Evolution of increased competitiveness (EICA) helps the invasive species to shift their resource allocation from the defense against natural enemies in outside their native range. A small amount of researches had conducted on the evolutionary studies of the invasive plants and to sort out the answer that certain genotypes within the invading population are more successful. The ability of exotic plants to establish and rapidly expand in a novel range is an evolved trait with underlying genetic characteristics in a population. To understand the genotypes various studies conducted for genetic diversity of invasive species. Genetic variation occurs due to mutation, selection, gene flow, genetic drift and breeding system in a population vary over a time. In invasion, ecological adaptation is achieved by exotic species driven by colonization and subsequent invasion into completely novel range. Local selection occurred from genetically diverse population which leads to fitness of the species in the new range helps in the additive genetic variation.

Many researchers have found that founder effect associated with the initial colonization of the species which significantly reduce the genetic variation in invasive populations. It might reduce their capacity to adapt in novel condition which leads to reduced genetic diversity due to inbreeding depression limiting propagule production and population growth whereas other suggest there have been a lag period between initial colonization and subsequent population of successful invader to rebuild the genetic diversity on which local population act for selection. Genetic variation is increased in the case of reintroduction of the invasive plant several times from diverse gene pool of its native range and due to hybridization of the invasive species with native plant species.

However on the bases of the studies on genetic diversity it was not decided yet that increased genetic diversity leads to invasion because there are many species which has shown invasive success due to low genetic diversity. It was found that cross-pollinating plant species have high levels of genetic variation within populations but genetic differentiation among populations is low, whereas self-pollinating species have less genetic variation within population and more diversity among populations. Some invasive plants have attained high genetic diversity within the invaded range such as Phalaris arundinacea, Alliaria petiolata, Pueraria lobata, and Euphorbia esula where the historical records tell about their multiple introductions which may be the source of such diversity. Other plant species invaded aggressively despite having little or no genetic diversity such as Fallopia japonica, Polygonum cuspidatum. Invasive populations of alligator weed (Alternanthera philoxeroides) and water hyacinth (Eichhornia crassipes) in China are also reported to have very low levels of genetic diversity. Reason for these invasions suggested the rapid clonal spread and dispersal via root or stem fragments in these species for lack of genetic diversity and population genetic structure.

Comparison of the genetic diversity of introduced and native population of invasive plant can pinpoint which native population is the source of invader and shows the loss of genetic diversity through founder effect. Diversity data is used to reconstruct invasion histories, reveal how a non native plant adapts and expands into new territory, can also lead to more effective management strategies.

3.9 Ecological and socio-economic impacts

The impacts of invasive species such as threat to native flora and fauna, disruption of essential ecosystem processes, ill effects on human and animal health, and losses to economy of the region they invade is an object of socio-economic interest. Impacts can be measured onto two types: 1) direct impacts on ecosystem functions and on human welfare, 2) management and control cost of invasive species plus side effects of introduction of biological control agent. On single species have varieties of effects on ecology, ecosystem services and socio economic effects e.g. In Cape region of South Africa, black wattle (Acacia mearnsii) affects the water table, species composition and also recreational functions due to scarcity of water in rivers and lakes. Ecosystem services categorized into four main categories are as follows:

1. Supporting services which are necessary for the production of other ecosystem services e.g. Soil formation, nutrient cycling, Primary production and habitat stability.

2. Provisional services include the production of products obtained from ecosystem services e.g. Food, fuel, fresh water, genetic resource.

3.Regulating services are benefits supplied by self-maintenance properties of ecosystems e.g. water regulation, water purification, waste regulation, biological control, pollination, disease control, natural hazard protection and erosion regulation.

4.Cultural services generate non-material benefits derived from ecosystems e.g. recreational, aesthetics, education and cultural diversity.

Alien invasive species has been observed to disrupt all these types of ecosystem services which are directly and indirectly linked with human wellbeing. There are almost 500,000 species of plants, animals, and microbes have invaded in different countries of the world in which 50,000 have been reported in US alone. However, it is difficult to calculate the economic cost caused by invasive species. Pimentel (2005) had calculated $120 billion/year USD damage cost from the invasive alien species in US alone. In Australia the cost estimated to $13.6 billion USD/year and cost is increasing with each finance year. Most evidence of economic impact of IAS comes from the developed world. Similarly, there are strong indications that the developing world is experiencing similar problems due to invasive species. Global invasive species programme (GISP) was initiated in 1997, internationally to address the growing problems and adverse effect of invasive species on both natural and managed ecosystems. It had compiled the list of “100 worst alien invasive species” in its website which had cost trillions of USD damage to the different countries along with ecological and social impacts. Some of the other important international initiatives includes Mountain Invasion Research Network (MIREN) and Assessing Large scale Risks for biodiversity with tested Methods (ALARM) and Delivering Alien Invasive Species Inventories for Europe (DAISIE) was initiated to address the growing problems and adverse effect of invasive species on both natural and managed ecosystems in large scale. The ultimate goal of all these initiative is not only to control and eradicate invasive species but the conservation of biological diversity, protection of human health and prevention of economic losses. These goals can be achieved by ecosystem restoration through reintroducing the native species, preservation of undistributed ecosystem for the establishment of sustainable use by local people. Biological invasion studies programmes are launched continentally mostly by developed nations for understanding the behaviors of invasive species and managing them. Since the cost of these programmes are too much high that developing and underdeveloped world are still not able to initiate such programmes. It’s still a long way to go, for preserving and restoring the ecosystem by saving them from biological invasion and restoring the ecosystem services to its normal cycle.

4. Summary

In this lecture we learn about:

• Concept of Biological invasion
• New terminologies used in the topic biological invasion Mechanism of biological invasion
• Pathways of biological invasion
• Successful characters of alien invasive species
• Ecological, social and economic losses due to spread of invasive species.