Unraveling the Threads of Predator-Prey Interactions

Introduction

The Evolutionary history of interactions between predator and prey has been detected in the fossil records as early as around 550 million years ago in the Cambrian period. The largest predators that have ever lived were dinosaurs, such as Tyrannosaurus which used to prey on herbivorous dinosaurs: Hadrosaurs, Ceratopsians and Ankylosaurs (Switeck, 2012). Nowadays, the majority of top predators are large carnivores (such as whales, lynx, bears, wolves, sharks and eagles), which are represented at the top of the food web. If you are delving into the intricate world of biology dissertation help, understanding all these ancient interactions is going to provide the most valuable insights into modern ecosystems. The apex predators occur in low populations and they require a vast special environment to live in (Edward, 2014). Predation is a vital species interaction that has implication for biological populations, communities and ecosystems. According to Ripple (2014) 64% of apex predators are in danger of extinction and 80% are experiencing a declining population. The main threats as to why species are declining is linked to habitat loss and fragmentation, invasive species, diseases, hunting, overfishing, using animal’s body parts as traditional medicine or trophies and loss of prey base. All these factors have impacted on diminishing a species population range and even caused the extinction of many apex consumers which can have detrimental effects on biodiversity and on ecosystem as whole with unknown and complex implications.

Top-predators have been affected by substantial threats by a human-wild conflict that has lasted for over 500 years. As a result, species have decreased in numbers and are now threatened with extinction. Extirpated large carnivores have had a devasting negative impact on the ecosystem, as apex predators play an important and fundamental role in driving the structure, balance and ecological function of the ecosystems such as a disease regulation, population behavioural variability, biodiversity and carbon storage. Nevertheless, the process of reintroduction of extirpated species to their former ranges is an important carnivore conservation tool and the restoration using a top-down model may encourage ecological regulation.

The purpose of this review is an analysis and interpretation of theoretical knowledge, hypothesis and modelling of interactions between predator and their prey; a comprehensive critique and analysis of the implications for re-introducing top predators to an area, through an understanding of the important place of top predators in ecosystems, the historical developments of these sites, and will involve comparing different theories and practices applications.

Background

Leading to a trophic cascade of ecological change, grey wolves work on helping the beaver population to increase and bringing back the vegetation (Barton et al. 2019). The absence of wolf resulted in increase of elk population. The increase of elk population did two things: the elks pushed the limit associated with carrying capacity e of Yellowstone in the field move around during the winter and browsed heavenly on cottonwood below and Aspen plants making it take for the beavers to survive in winters that need willows (Schell, 2020).

This worked on creating a counter intuitive situation when the l population was about three times as today vegetation cover willow was limited. However keeping the populations same the plantation survives due to the Predator re pressure created by the wolves keeping the else on the move so that the field to intensely browse on the same vegetation for a longer time. Research projects by the geological survey in the US combination of elk browsing on willow stimulated the beaver cuttings producing stunted willow stands (Newsome et al. 2016). Similarly Beaver cutting in absence of browsing produced healthy stands of willow plantation. A decade of research related to an invention started 10 times better biomass on and browse plans as opposed to the browse plant. With elk movement during the course of winter the elks receive plenty of time to recover from the continuous browsing and the Beaver discovered the food source that had been absent earlier. As the beaver spread building new dams and ponds the cascade effect continued. The dams built by beavers create significant impact on stream hydrology (Found et al. 2018).

Dams built by beavers, even out the seasonal purposes of water runoff storing water for recharging the water table and providing shaded water for the fish while the robust willow stands provide habitat for birds. The ecosystem is increasingly complex and the addition of wolves is changing the feeding habits of the large herbivores in the park (Wortley, 2017). Biologists are often subjected to the challenge of documenting the cascade effect that takes place when a species is removed from the ecosystem. However in the Yellowstone park biologist can have a new experience of understanding the possible effects of an animal reintroduced into the ecosystem. The wolves in the Yellowstone are quoted as food distributors. Resources determined at the wolves are the prime reason for mortality of elks (Pilot et azl. 2018). Before the introduction of wolves the snow was the determinant of elk's death. Combination of less snow and more wolves has benefited the scavengers both big and small from grizzly bears to ravens. Instead of a sudden increase and bus cycle of elk carrion, availability existed before the world and the vintage harder and equitable distribution of elk bodies were witnessed (Tallian et al. 2017).

Coevolution predator-prey

The term predation is based on an interaction where the predator of one species kills and consumes a fraction of the biomass of another species. Charles Darwin in his book On the Origin of Species (1859) wrote about evolutionary interactions between flowering plant and insects, he observed how plants and insects could evolve through reciprocal evolutionary change. Nowadays, the theory of coevolution is well-developed and proves that coevolution can play an important role in the structure and function of ecological communities, for example: the evolution of groups of mutualists such as plants and their pollinators and/or the dynamics of infectious diseases (Nuismer, 217). Predators and prey interact and coevolve with the predator developing a more effective behaviour and physical skills for catching their prey and the prey developing strategies for eluding being eaten. The coevolution of the mutability applies a selective pressure which leads to an evolutionary arms race between prey and predator, finishing in anti-predator adaptations.

Analysis of the Lotka-Volterra equations

The main aim of the Lotka-Volterra equations or the predator-prey equation is to describe the dynamics of biological systems, which involve one species of each of predator and prey. The populations changes through time based on the pair equations (Volterra, 1926):

x is the number of prey (for example rabbits),

y is the number of predator (for example foxes),

t represents time,

and represent the instantaneous growth rates of the two populations

α, β, γ, δ are positive real parameters describing the interaction of the two species.

This system of non-linear differential equations represents a more general version of a Kolmogorov model because it includes only the predator-prey interactions and eliminates other competition and the impact of disease, and mutualism which the Kolmogorov model has. The Lotka-Volterra equations (Lotka,1925; Volterra 1926, 1927) are based on the hypothesis where:

plentiful source of food for the prey population,

the amount of food to the prey is related to the size of the prey population,

the rate of change of population is directly proportional to its size,

the environment is constant,

predators will never stop eating.

The Lotka-Volterra equations is successful in theoretical understanding, but in reality there are a range of other issues within the natural environment such as: predators can only kill as many prey as encountered, the possibility of migration in the habitat, the presence of other mesopredators, the possibility of hiding shelters, different prey individuals( old, sick and young) (Genovart, 2010). In the laboratory this model has often failed, because of the model has an unstable structure, so any changes in equation can stabilize or destabilize the dynamics. Another aspect, that predators have a linear functional response to prey, where the rate of kills increase in proportion to the rate of encounters. If this rate is limited by time spent handling each catch, then the prey population can expand to become an out control size (Levin, 2009).

However, the model Lotka-Volterra gave a start for others describing models of interactions between living organisms and processes and they have been able to predict widely differing and often chaotic predator-prey population dynamics (Berryman, 2006). From the basic model of predator-prey more complex additional models in different fields have been developed which help to analyse variable processes from biological to economical. Since the chosen equation and model example of the other model giving rise to issues. The most important problem in the chosen model is the ability of the prey population to bounce back when subjected to a highly low population number (Nelson et al. 2016). This is really a real life occurrence as likelihood would be that the pre population could possibly go extinct which then causes extinction of the predator population. The inevitable population to go extinct in the chosen model can be rightly explained from the stability point. However it is generally difficult for the population to reach zero (Gómez‐Sánchez et al. 2018)

Can top-predators create high Biodiversity?

Resource facilitation

Top-pre predators or in other words alfa predators, are those animals at the top of a food chain with no natural predators. For years they have been defined as majestic, powerful, beautiful, dangerous animals (Kruuk, 2002). It has been proven that top-predators have possibilities to enhance biodiversity, so they have been used in conservation as keystone, umbrella, flagship, sentinel and indicator species. These species descriptions were used to engage public support for biodiversity conservation, raise funds, protect and restore ecosystems, prioritize reserve size, and plan the size and configuration of protected areas (Gittleman, 2001). According to Craighead (1968) top-predators can be ‘ecological engineers’ in the ecosystem. For example, American alligators (Alligator mississippiensis) through digging and maintaining can provide important habitat for fishes, frogs and snakes during droughts in Florida.

The status of the wolf population with the United States was jeopardize during the early 20th century. As the wolves disappeared the population of elk kept on increasing. The top predators are important for a smooth functioning of the ecosystem. This statement may be perceived as a paradox as a predators eat prey and other animals their by causing death and not life (Schweizer et al. 2016). However predators work on keeping check on other population making sure that there are varieties of species occupying the environmental niche. As for the wolf in Yellowstone park, predators a crucial important for healthy ecosystem making sure that there are greater variety of species surviving by keeping the population of prey in check. Several small species are subjected to threat due to human activities the wolf case shows immense hope. The public were involved in the Yellowstone wolf project and where us to send pictures and images for tracking the wolf (Lesniak et al. 2017). The involvement of public works on spreading awareness. Simultaneously if the general people a proactive then the conservation programmes is successful ensuring positive future for the other species. Hence it can be conclusively stated increasing awareness of effects associated with species decline is important step in conserving biodiversity

Trophic cascade

According to Errington (1946) for some time it was believed that predators did not influence prey populations and only removed individuals that were surplus to the population while prey abundance was determined by density-dependent processes such as intraspecific competition and density-independent processes such as weather. However, Leopold (1947) established that loss of top predators prompted increase of wild ungulates and subsequent impact to plant communities. Hairston et al. (1960) came up with an idea based on Elton, Leopold and other studies with their Green World Hypothesis (GWH) and proposed that predators maintained global plant biomass at high levels by limiting herbivore density. The trophic cascade term was first used by Paine (1980) to characterize a progression of direct and indirect effects of native predators across successively lower trophic levels.

Today it is well known that predators play a vital role in the structure and function of ecosystems, where they remove sick, old, injured species and consequently leave more food for the survival and success of healthy prey animals. Trophic cascade is a powerful indirect set of interactions that can effectively control entire ecosystems, occurring when a trophic level in a food web is suppressed. By regulating the populations of other organisms, predators keep a balance in a food web and increase species richness. Trophic cascade can also have indirect effects, where the presence of top predators can influence behavioural changes in prey and as a result prevent overgrazing area (Pace et.al, 1999).

A good example can be seen in the way sea otters manage the kelp forests in the Pacific. The main diet of sea otters are sea urchins. In some areas sea otters have become extinct and at the same time the number of sea urchins have significantly increased and the kelp forests were reduced, which had a negative impact on other species dependent on the kelp (Estes, 1974). Through processes of competition and intraguild predation, predators can influence the abundance, and distribution of key resources for other species (Schmitz et al, 2000).

Top-down influences upon community structure can be found in both ecosystems: terrestrial and aquatic. It can be seen in literature review, that there was not enough research done on the trophic cascade in the terrestrial ecosystem, where top predators are involved. That might explain the difficulty of assessing their effects on long temporal and large spatial scales (Shmitz et al., 2000).

Numerous cutting edge developmental scholars accept that predation has assumed a significant job in deciding examples throughout the entire existence of life on this planet (Gould, 1977). It has been seen that the advancement of qualities identified with predation in predator-prey is a troublesome point to comprehend in principle and in the field. Charles Darwin in his book On the Origin of Species (1859) expounded on transformative associations between blooming plants and bugs, he saw how plants and creepy crawlies could develop through equal developmental change. These days, the hypothesis of coevolution is very much evolved and demonstrates that coevolution can assume a significant job in the structure and capacity of environmental networks, for instance: the advancement of gatherings of mutualists, for example, plants and their pollinators or potentially the elements of irresistible sicknesses (Nuismer, 217). Predators and prey connect and coevolve with the predator building up a progressively compelling conduct and physical aptitudes for getting their prey and the prey creating procedures for avoiding being eaten. The co-evolution of the changeability applies specific weights which prompts a developmental weapons contest among prey and predator, completing in hostile to predator adjustments.

Trophic cascade research in the western US as potential role in structuring ecosystem

The study was conducted in the western US in five National Parks (NP): Olympic, Yellowstone, Yosemite, Zion and Wind Cave with the aim to understand what the effect the absence of apex predators (top-down cascade) had on the plant scarcity in the areas over the century. Previously each of the parks supported top predators, however, gradually the carnivores had been extirpated or displaced from the habitat. As a result, the elk population started to increase and as a consequence massive vegetation loss occurred. In this case tri-trophic cascades including apex predators, ungulates and plants represented an essential model for comprehensive understanding of trophic level changes subsequent predator disappearance.

It was clearly observed there were crucial changes in the five parks, where following the extirpation of top predators from the area had an impact on the reduction Aspen trees (Populus tremuloides) due to increasing elk (Cervus elaphus) grazing (Fig. 1a). Nowadays, only tall and large in diameter trees stand but with remarkable damage on the bark as highlighted in Fig. 1b (Ripple and Larsen, 2000). Fig. 1c represents the normal growth of aspens when it was properly fenced from ungulates browsing. A significant loss of aspen from Yellowstone NP was directly connected with elk overgrazing and revealed a major possible role of the large predators (NRC, 2002a).

The method of study in five separate parks included many aspects such as: the time of extirpated top-predators, the increase in elk population, gathering information about flora before and after loss of predators, the age structure, frequency and diameter (breast height) of deciduous trees( Beschta, 2005). The method of study measured forest density and determined the general strength of the trophic cascade for a specific park. However, the most significant aspect of the study was an assessment of the potential climate impacts (droughts and precipitation) on ecosystem changes with its further calculations of Palmer Drought Severity Index (Cook et al., 2004). Furthermore, the study included other factors that could have influenced temporal patterns of tree growth such as moisture availability, fire history, climatic patterns.

Fig. 1. (a) Aspen age structure from 1840 to 2000, (b) Aspen bark damage from elk grazing with missing tall saplings, (c) normal aspen growth in fenced area and an absence of saplings outside enclosure. With the loss of wolves from Yellowstone NP 95% aspen have been lost due to overgrazing (Larsen and Ripple, 2003).

Observations of ecosystem changes in the Western NP

At the end of the nineteenth and the beginning twentieth centuries many observations and reports were conducted. It was clearly noticed a huge degradation of ecosystems occurred with acknowledging a loss of large carnivores. Many biologists reported a significant deterioration of plant communities and suggested the top-predators had played an important role in the elk control population (Bailey, 1918). In Yellowstone, Grimm (1939) reported that due to elk numbers there was a threat to the aspen woodlands on the winter range. Ungulate overgrazing had an impact on soil erosion and the loss of the rich surface layer by washing away. Cahalane (1941) stated that the ecosystem needed a management approach to restoration and conservation of the fauna to its previous state. However, there was a lack of understanding how it could be reversed back to its previous state. The strength of the trophic cascade has become stronger over time, as the culling method was not successful. Furthermore, the fencing approach could protect only some areas from elk browsing.

The impact of Climate change on study area

Nevertheless, the most important approach of the study was the inclusion of the hypothesis of climate change on the plant scares in the area. Climate change appeared not to have had an impact on ecosystem degradation as the same climatic patterns inside or outside parks revealed. The research also calculated the potential role of climate in a consistent manner across park by assessing long-term records of the summer season using Palmer’s Drought Severity Index (PDSI) (Cook et al, 2004). The calculated PDSI has shown a significant difference when top-predators were present or absent during a study time. Thus, suggesting that climate change did not have an impact on tree loss during the study period.

Furthermore, the current study would argue with Keiter (1994) who stated that there is no definite evidence that the wolves have helped the ecosystem recovery. One of the potential reasons for Yellowstone Park’s degradation could be climate change. For years the park has been affected by droughts which could explain the elk reduction due to the scarcity of grazing. However, some parts of the park are in recovery state with dry wetlands being reclaimed to prevent fast-flowing creeks and poor biodiversity, which suggests that land management has had an impact. Based on the results, it can be concluded that the research in the western US has been successful and the findings of that study are quite convincing that climate change did not affect ecosystems degradations.

The results and acknowledgement of trophic cascade theory

The study in the Western NP US has proved the theory of trophic cascades, where the loss of top predators created a massive scale of trees and vegetations reduction by ungulates overgrazing. It can be clearly seen, how large predators are important in structuring and functioning ecosystem. However, other predators had influenced on a trophic cascade too including human actions which had a devastated impact on ecosystem in disturbance of top-down forces (Attwood, 2006). A recent study in the Western NP US concluded, that ungulates had significant effects on lower trophic levels and ecosystem function in the absence of apex predators. An important approach of reconstruction and recovery of ecosystems needed not only in the US, but globally. There is a great demand in further studies of top-down and bottom-up relations over a range of temporal and spatial scales.

The importance re-introduced top predators into an area

The re-introduction of large predators into areas from which they have been eradicated in the past is a globally used conservation action. Some of the benefits of rewilding are linked to the reduction in species and extinction risks; providing opportunities for natural range expansion beyond the introduction area; boosting the possibility of ecotourism; and the possibility of re-establishing predators’ ecological effects (Berger, 2007). The process of re-introduction should be seriously taken into consideration before establishing it, as it involves a range of risks such as a death of animals during translocation into new area, raising the conflict between humans and animals, through the possibilities of putting people and their livestock at risk of attacks and spreading diseases (Bruscotter, 2017).

The main goal of re-introduction projects was re-establishing lost spices back into their historical ranges (IUCN, 1998). In the beginning of such programmes the research and monitoring was very limited and it was applied more as management exercise. It can be seen, with increasing levels of projects more studies and research has been undertaken (Wolf, 1996). However, re-introduction research of top predators has been fragmented and less organized around attempts to gain the knowledge needed to improve the success of re-introduction programs. It can be seen that there is a big gap in the research of understanding predator-prey interactions and the implications of re-introduction into an area. This means that resources will be taken from different areas of ecology, zoology, and conservation biology for comprehensive research.

Biodiversity works on boosting the ecosystem productivity where the species irrespective of their size have a critical role to play. The vegetation of willow Aspen allows shelter to various types of songbirds and as a source of food for the beavers. Decrease in late winter snow pack, helps reduce the possible stress associated with the winter (Lu et al. 2016). Small snowpack allowed gaining easy access to nutritional resources and decreasing the energy associated with locomotion. Additionally there is the harvest plant growth initiated within a few days of the last snow cover increasing the quantity and quality of food intake in the year shortening the physiological stress of the winter period (Hoffmann et al. 2017). This is likely to reduce the frequency and timing of carrion, as the mortality rate of the elk tends to decline. The change in climatic condition provides a short production in the amount of late winter dead body availability to the scavengers of Yellowstone. This effect can be important for the scavenger species that are highly dependent on the spring and winter carrions food reproduction and survival. In the winter scenario the species would be subjected to food bottlenecks. The magnitude of this effect is dependent on how quickly the species can adjust to the changing environment and other food resources in response to shortening of the winter period (Droghini and Boutin, 2018).

Based on the above statement, it can be easily stated that the Yellowstone park is an interesting subject for the biodiversity enthusiasts. It becomes important to understand that the advent of new species is influenced by a series of external factors. It becomes important to note that the wolf acts as a keystone species as it influences the population of the other species in the ecosystem. In other words it can be clearly stated the importance of the grey wolves in the park worked on improving the existing niche (Hoffmann et al. 2017). This dealt with making a strange circumstance when the populace was around multiple times as today vegetation spread willow was restricted. Anyway keeping the populaces the same the ranch get by because of the predator re pressure made by the wolves keeping the else progressing so the field to strongly peruse on a similar vegetation for a more extended time. Investigating the venture by the land study in the US blend of elk perusing willow invigorated the beaver cuttings creating hindered willow stands (Barton et al. 2019). A time of research identified with a development of multiple times better biomass on and peruse designs rather than the peruse plant. With elk moving over the span of winter gets a lot of time to recuperate from the non stop pursuing and the Beaver found the nourishment source that had been missing before. As the Beaver spread structure, new dams and lakes the course impact proceeded. The dams worked by beavers make a huge effect on stream hydrology.

Management problem during re-introduction large predators

The re-introduction programs consist of two stages: the first stage conducts the selection, breeding and keeping of suitable animals for future release. Norton (1995) admitted that there are ethical and welfare problems in keeping animals in artificial environments. Teixeira (2007) argues that animals can develop different stress levels which can have a negative effect on an animal’s memory, natural behaviour when mating, hunting and feeding. The second stage involves the re-introduction program with the release of animals into the wild environment. The main problem starts with the attitude of stakeholders due to human-carnivore conflict. In the reviewed study Mishra (1997) argued that compensation schemes for livestock loses have mitigated the negative approach of human-carnivores conflict. Snyder et al.’s (1996) study described many cases were conservation programs failed due to disease. One of the challenges of a re-introduction program is in finding a suitable genetic stock. Meretsky et al. (2000) conducted a study where California condors were released in the wild despite being fed by artificial mother-bird (glove puppet) the surviving age was of about only four years. Another crucial aspect is disease control which is necessary for successful re-introduction. Screening for disease or parasites before release will not guarantee, that a disease may have developed over a long period of time without noticeable symptoms (Snyder et al., 1996). Brandon (1997) claimed, that successful re-introduction can be achieved by using compensation schemes for stakeholders that aims to relieve the human-carnivorous conflict.

Contemporary evolution in captivity

To achieve successful re-introduction, individuals must be both genetically and physiologically true to type, captive animals quickly adapt to artificial life. Belyeav (1979) in his studies recognised changes in a few generations of silver fox (Vulpes fulva). Boice (1981) proved the development of differences in physcial struture in wolves and jackals (Canis spp.) highlighting a change in the bone structure and tail carriage in one generation being in enclosure. These changes called ‘contemporary evolution’ can lead to significant disadvantages in wild. Less active individuals with lower aggression are preferred in captivity and anthropogenic selection for traits better suited to captivity may mean that heterogeneity is reduced as the action of natural selection are reduce (McDougall, 2006). The consequences of it can lead to inbreeding depression and their fitness.

Conclusion & Recommendations

To assess the importance of predation in driving the structure and function of ecosystem, this review investigated the theoretical base of predator-prey interactions. Where can be seen is that some theories cannot be applied in real natural world such as the Lotka-Volterra equations, however, others such as trophic cascade theory has proved itself and were effectively used in the study of the Western NP US. The implications for re-introducing grey wolves into Yellowstone Park were closely studied and the results have been discussed above reinforcing the thesis that large carnivores are believed to play a key role in determining ecosystem properties via trophic cascade.

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