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Habitat fragmentation

发布时间:2017-04-22
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Three different approaches to understand declining population

: the case of habitat fragmentation 

Introduction

The IUCN Red List indicates that over 6000 species of vertebrates are threatened (IUCN 2009a). The most significant factor of threatening species is habitat fragmentation (Caughley and Gunn 1996; Hanski 1998; IUCN 2004), which is defined as “an agent of decline” in the declining-population paradigm (Caughley 1994). Although species-specific approaches are employed to address the agent of decline, the more theoretical approach should be developed to provide more efficient management strategy of endangered species conservation (Caughley 1994). However, several methods associated with habitat fragmentation have been developed on classical approaches (Fischer and Lindenmayer 2007). Therefore, the aim of this review is to examine recent approaches to addressing population decline in fragmented landscape and if these approaches have been developed in the way to understand population decline.

Approach to addressing population decline

INDIVIDUAL BEHAVIOUR

Central to this approach is that loss of core areas, which are frequently used for individual behaviour such as foraging or breeding, is the key to an understanding of the impact of habitat fragmentation on population.

Case study: Habitat selection of the Iberian Lynx (Lynx pardinus) at dispersal stage

The Iberian lynx (Lynx pardinus) occurring in the Iberian Peninsula (Delibes et al. 2000) is a medium-size felid and the most threatened felid in the world (Nowell and Jackson 1996). One of the major factors of decline in the population is habitat destruction and fragmentation (Rodriguez and Delibes 1992).

Palomares et al. (2000) conduct radiotracking research with 42 individuals for 14 years in the Donana region. Their findings indicate that radio-collared Iberian lynxes at dispersal stage most frequently use the Mediterranean scrubland, where vegetation type is more suitable for hunting and breeding. Palomares et al. (2000) found postdispersal individuals tend to settle in lower quality habitat compared to habitat at predispersal stage. In conclusion, it is suggested that the core area for the Iberian lynx might have already reached carrying capacity, thus, to maintain population, the Mediterranean scrubland should be protected.

The major strength and weakness of the individual behaviour approach

This paper focuses on individuals in a fragmented habitat with the emphasis on behaviour at a critical stage for survival, where individual dispersal behavior is vital for population dynamics (Bowler and Benton 2005).

The advantage of approach might be detailed description of the relationship between habitat and species by radiotracking. Based on individual behaviour, Palomares et al. (2000) highlights the importance of a specific vegetation type. Although this approach is similar to the behaviour-based model (Norris 2004), which is based on individual behaviour strategy with evolutionary theory, the approach on here is not complete enough to include evolutionary However, the strength of the behaviour-based model might be able to partially apply since both approaches are developed from behavioral ecology. From this point of view, the strength of this individual behaviour approach is that a plausible assumption of population response to habitat fragmentation can be established based on individual behaviour, as Norris (2004) supposes.

On the other hand, this approach could be criticized as an insufficient method to understand population decline theoretically owing to complexity. This objection is difficult to counter, since this approach tends to be species-specific and case-by-case on the ground that individual behaviour is influenced by various factors such as fitness, intra- or interspecific competition, resource availability, or environment (Begon et al. 1996). Therefore, the weakness of the approach must be difficulty in applying results to other species.

METAPOPULATION APPROACH

Metapopulation dynamics is often considered as a sufficient tool to understand theoretically the impact of habitat fragmentation (Hanski 1998). This approach is derived from island biology, hence, it is possible to understand the impact of habitat fragmentation by integrating a patch structure with well-established small population theory (Hanski and Gilpin 1997).

Case study: Metapopulation dynamics of the Iberian lynx (Lynx pardinus)

Gaona et al. (1998) characterizes the spatial configuration of Iberian lynx population as the metapopulation structure in the Donana region, and conducts modeling and simulation research of metapopulation dynamics with demographic parameter based on field data gathered from 1983 to 1992.

According Gaona et al. (1998), Iberian lynx metapopulation has a source and sink structure, and the survival rate of territory-holding adults in sources play a prime role in the metapopulation dynamics. They conclude that increasing carrying capacity in the sources and decreasing mortality rate in the sinks are effective in maintaining the population of lynx.

The major strength and weakness of the metapopulation approach

In the case study, it is assumed that population decline of Iberian lynx cased by habitat fragmentation in the Donana region depends on the balance between sources and sinks. In addition, the metapopulation model shows several possible determinants of population dynamics. Therefore, it could be argued the metapopulation model has three strengths to understand population decline by habitat fragmentation. Firstly, the model could enhance theoretical understanding of the impact on population dynamics at regional level (Hanski 1998). Secondly, hypothetical scenarios of population dynamics can be proposed for future management (Gaona et al. 1998). Thirdly, metapopulation model can integrate the main two frameworks for research in habitat fragmentation; one is species-oriented (e.g. demographic or genetic), another is pattern-oriented (e.g. connectivity or edge effect) (Fischer and Lindenmayer 2007).

However, the two fundamental problems of metapopulation theory for the impact of habitat fragmentation can be identified. One is an ambiguous definition of fragmentation in metapopulation theory. The term of fragmentation originally means the process of habitat change, thus, it is difficult to consider that a spatial structure of fragmented landscape is the criteria to understand population decline (Fahrig 2003). Another is insufficient parameters for demographic and environment in the model due to either oversimplifying of or lack of ecological data and environmental factors (Harrison and Bruna 1999; Bowler and Benton 2005)

PHYLOGEOGRAPHICAL APPROACH

Phylogeography can contribute to identifying geographic barrier and the impact of dispersion on genetic variation (Freeland 2005). Therefore, it assists to understand the impact of habitat fragmentation on species dispersal and genetic diversity.

Case study: Phylogeography and conservation genetic of Jaguars (Panthera onca)

Although Jaguar once ranged from south part of US to Argentina, current range is restricted from Mexico to south part of Argentina (Nowell and Jackson 1996). The population trend is declining (IUCN 2009b) caused by mainly habitat fragmentation (Nowell and Jackson 1996).

Eizirik (2001) et al. examines mtDNA control region and microsatellite loci from 44 Jaguars. They point out that jaguar may have high gene flow across their range, because 22 different haplotypes and the 4 haplotype subgroups do not indicate a historical-geographical barrier for subspecific differentiation. However, their mtDNA and microsatellite analysis shows considerable differentiation between subgroups. Eizirik et al. (2001) assumes that some dispersal of Jaguar is limited by Amazon River and the Darien Strait, and possibly by the recent geographical barrier. Added to this, they also find low level of genetic diversity from mtDNA analysis while high level of individual variation is found from microsatellite loci analysis. Although Eizirik et al. (2001) consider sampling bias, they conclude that it is necessary to keep high gene flow and genetic diversity for Jaguar conservation.

The major strength and weakness of the phylogeographical approach

The major strength of phylogeography may be providing insight of the distribution process from individuals to population and species. Regarding individuals, the case study reveals that some jaguars have a high ability to disperse within the range, although major geographical barriers exist. This serves as evidence of long-distance dispersal because the study of long-dispersal has been superficial, even though much research has done for short-distance dispersal (Waser et al. 2001). In addition, phylogeography shows genetic diversity, on which dispersal/gene flow has the crucial impact, at population and species level (Frankham et al. 2002). Therefore, the phylogeographical approach may be a sufficient tool for understanding the impact of habitat fragmentation by identifying individual dispersal and genetic diversity in population and species. However, this approach might not be suitable to recognize possible indications of decline such as high mortality rate in a habitat which are invisible in genetic analysis.

Discussion

It was observed in previous part of this review that the three approaches show the different dimensions and the advantages and disadvantages. To compare all of these differentiations is beyond the scope of a brief paper. Therefore, the fundamental difference, spatial and temporal scale, in the three approaches is focused on here. Regarding spatial scale, the first approach focuses on individual behaviour, while metapopulation approach attempts to understand population decline at regional level. However, phylogeographical approach allows consideration from individual to species level. Phylogeographical approach is also able to consider the impact of habitat fragmentation within a broad temporal scale. Compared to phylogeographical approach, the temporal scale in other two approaches is relatively small.

Having noticed this underlying differentiation, it could go on to consider how they complement each other with their strengths and weaknesses. We shall concentrate on the individual behaviour approach to examine how to integrate approaches. When integrating it with metapopulation model, it might not be effective. The reason is that the model simplifies individual behaviour (Heinz, et al. 2006), even though the metapopulation model allows expanding time scale by simulating scenarios. Namely, the metapopulation model may reduce the advantage of individual behaviour approach.

On the other hand, combining with phylogeographical approach has high potential for understanding the impact of habitat fragmentation on population dynamics. Since phylogeographical approach provides evidence of historical and current dispersion, integrated approach can maximize the ecological data of species by comparing historical dispersion and present dispersal behaviour. In addition, genetic diversity revealed by phylogeographical approach can track the consequences of behaviour strategy on population dynamics. Therefore, it seems reasonable to suppose that integrating individual behaviour and genetic analysis is sufficient for future understanding of the impact of habitat fragmentation. This integrated approach has already started to develop (Gebremedhin et al. 2009).

In conclusion, even though this integrated approach has started to develop and several approaches including a theoretical model have emerged, our understanding for population decline might not be beyond Caughley's declining-population paradigm in the sense of species-specific and case-by-case approach. The main reason is that detailed ecological data of the target species is essential to use models or an integrated approach effectively for understanding the impact of habitat fragmentation, which is a major threat for most species and the agent of decline.

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