**2. Habitat loss and conversion**

There is a gradual but steady loss of mangrove forests in the Niger Delta due to uncontrolled deforestation for the purpose of sand dredging and canalization (**Figure 1**). Mangrove forest is also cut to recover stems, which are used in the production of firewood and wood for the construction of houses. Similarly, numerous oil and gas exploratory activities all over the Niger Delta area open up the forests to further exploitation of resources and invasion by foreign species such as nypa palms (*Nypa fruticans*) [7]. Habitat loss is amongst the three important factors that are responsible for recent species extinction. The other two are overexploitation and the introduction of exotic species [8]. The implication of the loss of mangrove habitat is the loss of ecosystem services it renders to the society [9].

#### **2.1 Habitat loss**

*Habitats of the World - Biodiversity and Threats*

events. Biodiversity hot spots are important zones because they contain 25% of all terrestrial species. Hot spots are small areas on the map (i.e., about 2%), but wield so much ecological influence on the surface of the earth. They contain disproportionately large numbers of species on earth. Tropical forest has the largest number of species in the world. It has a lot of taxonomic diversity, which includes plankton, macrophytes, arthropods, fungi, protists, mollusks, crabs, vertebrates including birds and mammals. Mangrove forest is the most dominant amongst the plants found in the coastal region of the Niger Delta area. The reasons for high species diversity in mangrove forests of the Niger Delta are because of high reproduction and productivity [3]. The tropics are areas that have high speciation and less species extinction. Secondly, the stability hypothesis postulates that the tropics have been more stable than present in the last 1000 years and will remain so for a long period of time. It is stable with respect to the abiotic conditions that prevail in the area. Similarly, the equator has been stable with respect to global warming and cooling with lower extinction rate recorded over the years. Furthermore, intense solar radiation had resulted in an increase in overall productivity resulting in high biodiversity turnover. The tilt of the earth also affects how the photon of sun hits the earth. The sun position right overhead the equator serves as a major source of energy. Plants capture solar energy, which they convert to food for other organisms in the ecosystem. The captured energy is converted into biomass, which is transferred to the production of more resources. This is exemplified in the high productivity of mangrove forests recorded in the Niger Delta [3]. Increased litter fall facilitates high decomposition [1] and makes food available for different species of organisms in the food chain. The tropical environment is highly specialized. High specialization and speciation lead to high biodiversity. But the problem of mangroves globally is the gradual loss of their habitat due to anthropogenic activities [4]. This study showed that 35% area of global mangrove forest had been lost as a result of some human activities such as shrimp culture, forest use, fish culture, diversion of freshwater, land reclamation, herbicides, agriculture, salt ponds and coastal developments. In Africa coastal development is a major factor of habitat loss [4]. Although, other studies had revealed that Nigeria has one of the least carbon CO2 emissions from soil as a result of mangrove losses [5]. Similarly, previous studies have shown that habitat conversion far exceeds habitat protection by a ratio of

A forest is an ecosystem, which is dominated by trees. Forest produces timber and non-timber products. Non-timber forest includes anything other than timber. Forest products are all renewable and can be sustainably managed. More often than not mangrove forest is looked at from the economic resource angle. This is because they are the sources of revenues such as wood, firewood, latex, dyes, thatches, bamboo, reptiles, insects, roots, shoots, stems, flowers, honey resins, gum, silk, fabrics, rope, animal oil, cosmetics, water from streams and lakes. The value of these resources is often difficult to quantify. We also have nature reserves and

Nigeria has a wide and diverse range of habitats from arid zones in the north to the swampy wetlands in the south. Many forest types (tropical rainforest, Sahel savanna, Guinea savanna, Sudan savanna, Montane savanna, etc.) are associated with these zones and have an array of plant and animals species. Some of these

**50**

8:1 globally [6].

recreational activities.

biodiversity include:

**1.1 Mangrove forest resource in Nigeria**

Habitat loss leads to extinction because species are adapted to a particular habitat type. For instance, mangroves are habitat specialist that survives in swampy wetland soil, therefore, when they are removed from their native environment and taken to other environment they do not survive. There is also a relationship between the numbers of habitats and the species present. This is because the larger the habitat size the higher the number of species, and the smaller the habitat size the fewer the number of species present. There are many ways humans cause the loss or conversion of mangrove forests, these include tree cutting for production of firewood, agriculture (e.g., fish farming or rice paddies), construction of houses and hydrocarbon pollution from oil and gas exploration activities. Currently, there are numerous

**Figure 1.**

*Consequences of biodiversity loss on species population: (A–C) shows deforestation for fire wood production; (D–F) show canalization of river bottom to create way for sea craft; and (G–I) show the displacement and migration of birds (heron) to urban areas for breeding due to the loss of their natural habitat in the Niger Delta, Nigeria.*

locations in the Niger Delta where sand dredging and mining activities are taking place. Usually before the sand dredging the entire mangrove forests in such vicinity are cut down to pave way for the entry of heavy duty machinery (e.g., **Figure 1E, F**).

#### **2.2 Habitat conversion**

It is a situation whereby mangrove forests are converted into sand fill or other land use systems that are inimical to the existence and proliferation of coastal species. For instance, in sand filled land only weed grow as the main vegetation cover. This leads to a drastic change in the total number of species. In this context, loss of mangrove forests lead to the loss of forest canopy that houses other species such as birds, tree crabs, monkeys, etc. Increased human activities such as industries, living quarters, and marine transportation result in the increase in waste disposal in aquatic environment. The disposal of liquid and solid wastes into the river contaminates adjoining mangrove forests. Accumulation of organic products promotes the proliferation of algae, which accelerate the process of eutrophication. Fertilization of the aquatic system leads to the increase in biological oxygen demand (BOD) and decrease in dissolved oxygen (DO) in the water. Accumulation of organic waste in mangrove forest leads to anaerobic condition which causes slow death of mangroves. The implication of these activities is that once the habitat type is converted protection becomes extremely difficult [6].

### **2.3 Mangrove habitat fragmentation**

Mangrove fragmentation is the process whereby large contiguous areas of mangrove forests are divided into small habitats as a result of urban development [10].

**53**

activities.

**Figure 2.**

*out in few years time.*

*Mangrove Habitat Loss and the Need for the Establishment of Conservation and Protected Areas…*

When this happens, fragmented forests become physically isolated from each other, which has a negative effect on the population dynamics of organisms in the forest (**Figure 2**). Fragmentation of mangrove forests has an effect on the connectivity of the forest, which affects the movement of organisms within the forest. The island biogeography theory vividly throws more light on this situation when [11] in his seminal work showed that big island has more species than small islands. In the same way, big mangrove forest will have more biodiversity and less extinction than small mangrove forests. Furthermore, island biogeography theory shows that island close to main land has more species than island far away from mainland. In contrast, the closer the mangrove forest is to urban area the lower the number of species due to human disturbances. Mangroves are only adapted to coastal areas, which prevent them from migrating upland [12]. However, their seeds can migrate to foreign lands through tidal pressure far away from their place of origin. Some individual mangrove seedlings can migrate to oceanic habitat patches. The distance between the patches will affect the rate at which the species get there. This is because human activities that lead to land fragmentation such as urban development and sand filling can lead to the compartmentalization of the mangrove forests. The mangrove in the Niger Delta area has become a sink population, and thus there is a negative population growth due to anthropogenic

*Isolated and fragmented mangrove stands surrounded by invasive nypa palm species (*Nypa fruticans*) and an overlooking urban area at Eagle Island, Niger Delta Nigeria. This mangrove stands are subject to impending stochastic events such as flooding, tidal surge, sediment erosion at the root of the plants which may easily wipe it* 

*2.3.1 Impact of edge effect and fragmentation on mangrove forest*

Edge is a location of an abrupt transition between two habitat types. When a large area is fragmented, it increases the amount of edges on a habitat and decreases core habitat, thus exposing the edges to more impacts by humans who gain access to the forest to cause more plundering of its resources. An example is the entry of

*DOI: http://dx.doi.org/10.5772/intechopen.89623*

*Mangrove Habitat Loss and the Need for the Establishment of Conservation and Protected Areas… DOI: http://dx.doi.org/10.5772/intechopen.89623*

#### **Figure 2.**

*Habitats of the World - Biodiversity and Threats*

locations in the Niger Delta where sand dredging and mining activities are taking place. Usually before the sand dredging the entire mangrove forests in such vicinity are cut down to pave way for the entry of heavy duty machinery (e.g., **Figure 1E, F**).

*Consequences of biodiversity loss on species population: (A–C) shows deforestation for fire wood production; (D–F) show canalization of river bottom to create way for sea craft; and (G–I) show the displacement and migration of birds (heron) to urban areas for breeding due to the loss of their natural habitat in the Niger* 

It is a situation whereby mangrove forests are converted into sand fill or other land use systems that are inimical to the existence and proliferation of coastal species. For instance, in sand filled land only weed grow as the main vegetation cover. This leads to a drastic change in the total number of species. In this context, loss of mangrove forests lead to the loss of forest canopy that houses other species such as birds, tree crabs, monkeys, etc. Increased human activities such as industries, living quarters, and marine transportation result in the increase in waste disposal in aquatic environment. The disposal of liquid and solid wastes into the river contaminates adjoining mangrove forests. Accumulation of organic products promotes the proliferation of algae, which accelerate the process of eutrophication. Fertilization of the aquatic system leads to the increase in biological oxygen demand (BOD) and decrease in dissolved oxygen (DO) in the water. Accumulation of organic waste in mangrove forest leads to anaerobic condition which causes slow death of mangroves. The implication of these activities is that once the habitat type is converted

Mangrove fragmentation is the process whereby large contiguous areas of mangrove forests are divided into small habitats as a result of urban development [10].

**52**

**2.2 Habitat conversion**

**Figure 1.**

*Delta, Nigeria.*

protection becomes extremely difficult [6].

**2.3 Mangrove habitat fragmentation**

*Isolated and fragmented mangrove stands surrounded by invasive nypa palm species (*Nypa fruticans*) and an overlooking urban area at Eagle Island, Niger Delta Nigeria. This mangrove stands are subject to impending stochastic events such as flooding, tidal surge, sediment erosion at the root of the plants which may easily wipe it out in few years time.*

When this happens, fragmented forests become physically isolated from each other, which has a negative effect on the population dynamics of organisms in the forest (**Figure 2**). Fragmentation of mangrove forests has an effect on the connectivity of the forest, which affects the movement of organisms within the forest. The island biogeography theory vividly throws more light on this situation when [11] in his seminal work showed that big island has more species than small islands. In the same way, big mangrove forest will have more biodiversity and less extinction than small mangrove forests. Furthermore, island biogeography theory shows that island close to main land has more species than island far away from mainland. In contrast, the closer the mangrove forest is to urban area the lower the number of species due to human disturbances. Mangroves are only adapted to coastal areas, which prevent them from migrating upland [12]. However, their seeds can migrate to foreign lands through tidal pressure far away from their place of origin. Some individual mangrove seedlings can migrate to oceanic habitat patches. The distance between the patches will affect the rate at which the species get there. This is because human activities that lead to land fragmentation such as urban development and sand filling can lead to the compartmentalization of the mangrove forests. The mangrove in the Niger Delta area has become a sink population, and thus there is a negative population growth due to anthropogenic activities.

#### *2.3.1 Impact of edge effect and fragmentation on mangrove forest*

Edge is a location of an abrupt transition between two habitat types. When a large area is fragmented, it increases the amount of edges on a habitat and decreases core habitat, thus exposing the edges to more impacts by humans who gain access to the forest to cause more plundering of its resources. An example is the entry of


**Table 1.**

*The level of environmental impact on fragmented mangrove forests in the Niger Delta, Nigeria.*

invasive nypa palm species into mangrove forest due to the direct introduction by humans [7]. Fragmentation of large mangrove forest into small fragments lead to the formation of little mangrove islands that are prone to forces of denudation such as tidal pressure, erosion and flash floods which erodes it banks leading to a gradual loss of the entire mangrove stands (**Figure 2**).

The difference between core and edge mangrove forests in relation to habitat loss is because of the impact of some climatic parameters (**Table 1**).

A major cause of mangrove deforestation in the Niger Delta, Nigeria is its use in the production of firewood, which reduces mangrove forest sizes. The fragmentation of mangrove forest can lead to the formation of numerous edges that subject the forest to further climatic and anthropogenic damages.

#### **2.4 Rarity of species**

Rarity of species can be described in three ways [13, 14]:


#### **2.5 Danger of small populations of mangrove forests**

There are basically six problems associated with small populations of any kind of species [15], they are:

1.**Environmental stochasticity**: there is no constant environment, because of the action of fluctuation. Environmental stochasticity is an unpredictable event such as changes in weather (i.e., climate variability), invasive species, parasites and diseases, soil nutrients, etc. that has to do with environmental variation, and causes variation in survivorship of individuals. If population is large, it is not threatened with the possibility of extinction. But if the population is small it has high probability of extinction. The stochasticity, which

**55**

is exemplified in Eq. (1):

smaller and finally goes extinct.

*H* = 1 −

generation to the next and *Ne* is the effective population size.

*Mangrove Habitat Loss and the Need for the Establishment of Conservation and Protected Areas…*

affects survival affects small population size more, which eventually leads to zero. Moderate population size will also go to zero with time. This can occur during major disturbances such as flood, mudslide, deforestation, canalization, sand filling and sand dredging activities. Deforested mangrove will also suffer from reduced population, which will further lead to habitat loss.

2.**Demographic stochasticity**: this is the chance of fluctuation or randomness that is common in small populations. Small populations of mangroves are at a risk of low seedling turnover, which leads to low growth rate. It is easier for small mangrove stands to be wiped out due to environmental and anthropo-

3.**Inbreeding depression**: this can occur in mangroves especially when population size drops resulting in reduced fitness as a result of cross pollination between similar species. This is because individuals in small inbred populations may have small germination rate when compared to large inbred populations. This situation is observed in two mangrove forest communities situated in Buguma and Eagle Island in the Niger Delta. There is a rejuvenated and increased germination rate in red and black mangroves in Buguma as compared to Eagle Island. This is because the mangrove forest in Buguma is isolated from human disturbances resulting in high canopy cover whereas mangroves in Eagle Island are scanty with few populations because of poor growth rate as a result of human intrusion into the mangrove forest. Low population size can also lead to loss of genetic variation.

4.**Genetic variation:** The theory of natural selection states that the rate of evolutionary change in a population is proportional to the amount of genetic variation in the population. Based on this hypothesis all populations can respond quicker to any change in the environment where they have higher genetic variation than lower genetic variation. Populations with high genetic variation can easily adapt to any change. Although, this situation operates at a long term scale. The mangroves are resilient species and take between 20 to 50 years to attain maturity, therefore, any genetic change in mangroves based on environmental perturbations such as pollution from oil spillage will take along time to manifest in the population.

According to [16], there is a relationship between population size and a probability of genetic variation that would be lost from one generation to the next. This

where *H* is the proportion of heterozygosity remaining in population from one

Effective population size is the number of adults breeding that contributes to equal genetic material in the population. This implies that small populations tend to lose heterozygosity over time, which is an argument for maintaining large populations and thus larger reserves wherever possible to conserve these species. Years of population abundance can be interrupted by stochastic events leading to population bottlenecks, a situation, which occurs when a population experiences a severe temporary reduction in size. Thus population bottleneck reduces effective population size, thus, when population size is small genetic variation declines. Small population size creates extinction vortex, where abundant population continuously gets

\_1 2 *Ne*

(1)

*DOI: http://dx.doi.org/10.5772/intechopen.89623*

genic factors than large mangrove stand.

*Mangrove Habitat Loss and the Need for the Establishment of Conservation and Protected Areas… DOI: http://dx.doi.org/10.5772/intechopen.89623*

affects survival affects small population size more, which eventually leads to zero. Moderate population size will also go to zero with time. This can occur during major disturbances such as flood, mudslide, deforestation, canalization, sand filling and sand dredging activities. Deforested mangrove will also suffer from reduced population, which will further lead to habitat loss.


According to [16], there is a relationship between population size and a probability of genetic variation that would be lost from one generation to the next. This is exemplified in Eq. (1):

## 1

\*\*еemplumes un :\*\* ( $\epsilon$ ):

$$H = 1 - \frac{1}{2N\_{\epsilon}}\tag{1}$$

where *H* is the proportion of heterozygosity remaining in population from one generation to the next and *Ne* is the effective population size.

Effective population size is the number of adults breeding that contributes to equal genetic material in the population. This implies that small populations tend to lose heterozygosity over time, which is an argument for maintaining large populations and thus larger reserves wherever possible to conserve these species. Years of population abundance can be interrupted by stochastic events leading to population bottlenecks, a situation, which occurs when a population experiences a severe temporary reduction in size. Thus population bottleneck reduces effective population size, thus, when population size is small genetic variation declines. Small population size creates extinction vortex, where abundant population continuously gets smaller and finally goes extinct.

*Habitats of the World - Biodiversity and Threats*

loss of the entire mangrove stands (**Figure 2**).

**2.4 Rarity of species**

**Table 1.**

upland locations.

of species [15], they are:

is because of the impact of some climatic parameters (**Table 1**).

the forest to further climatic and anthropogenic damages.

exhibit a rare form of the habitat size).

risk increases as population size declines.

**2.5 Danger of small populations of mangrove forests**

Rarity of species can be described in three ways [13, 14]:

invasive nypa palm species into mangrove forest due to the direct introduction by humans [7]. Fragmentation of large mangrove forest into small fragments lead to the formation of little mangrove islands that are prone to forces of denudation such as tidal pressure, erosion and flash floods which erodes it banks leading to a gradual

**Climate/human parameters Core mangrove habitat Edge mangrove habitat**

Temperature Lower Higher Humidity Higher Lower Light level Lower Higher Wind Lower Higher Human impact Lower Higher

*The level of environmental impact on fragmented mangrove forests in the Niger Delta, Nigeria.*

The difference between core and edge mangrove forests in relation to habitat loss

A major cause of mangrove deforestation in the Niger Delta, Nigeria is its use in the production of firewood, which reduces mangrove forest sizes. The fragmentation of mangrove forest can lead to the formation of numerous edges that subject

1.Geographic range: it means how much of landscape is covered by the mangrove forest. It can be wide (i.e., found over a large area), or narrow (i.e., it will

2.Habitat specificity: this involves whether the forest is narrow or restricted to coastal area or broad and found in different habitat type such as riverine and

3.Population size: this is important because large population of a given species is better than medium or small populations of species. This is because extinction

There are basically six problems associated with small populations of any kind

1.**Environmental stochasticity**: there is no constant environment, because of the action of fluctuation. Environmental stochasticity is an unpredictable event such as changes in weather (i.e., climate variability), invasive species, parasites and diseases, soil nutrients, etc. that has to do with environmental variation, and causes variation in survivorship of individuals. If population is large, it is not threatened with the possibility of extinction. But if the population is small it has high probability of extinction. The stochasticity, which

**54**
