How often, how far, how risky and how biased?

Movement behaviour mediates the impacts of habitat fragmentation at multiple scales


KEY MESSAGES
  • Different scales of fragmentation have lethal consequences for animals with certain movement traits
  • For at-risk species, the impact of fine-scale fragmentation was accentuated when fragmentation also occurred at the coarse scale
  • The land use to target with conservation actions to reduce fragmentation depends on the scale at which fragmentation occurs and the movement behaviour traits of the species of conservation concern

 

Imagine you are an adult koala sitting on the top of a tall blue gum in a patch of forest located in an urban area of the Australian east coast. You mainly move among nearby trees and rarely venture away from your patch of trees. Your young offspring, however, is growing fast and will soon need to find a new tree away from yours.

There are two are major obstacles you and your offspring are facing if you want to move away. First, a few residential lots have recently sprung up in the area, with artificial ponds and child playgrounds, which have made it more difficult to reach the other trees within your forest patch. Second, the wheat crops at the foot of the hill have recently been expanded and now the forest patch on the hill looks much further away. To make matters worse, a few unfriendly dogs roam those fields, making it a quite inhospitable place for a koala. You are quite concerned: Will I be able to breed again the next reproductive season? Will my offspring be able to find a new home and reproduce? Will we still be around at all in a few years’ time?

Fragmentation at different scales

Fragmentation of a species habitat – ie, the subdivision of intact habitat into small and isolated habitat fragments – is a major threat to the long-term persistence of species. Fragmentation can also occur at different spatial scales (eg, within a forest patch and between different forest patches), driven by different land uses (eg, urban development and agricultural expansion; See Decision Point #82).

Example of fine-scale vegetation fragmentation (individual trees fragmented by urban development, NSW). (Image http://www.wagga. nsw.gov.au/city-of-wagga-wagga/recreation/lake-albert)

Example of fine-scale vegetation fragmentation (individual trees
fragmented by urban development, NSW). (Image http://www.wagga.nsw.gov.au/city-of-wagga-wagga/recreation/lake-albert)

We know that when habitat is fragmented simultaneously at multiple spatial scales, species need to move a lot more than when habitat is fragmented at one scale at the time (Cattarino at al. 2003). However, moving more requires effort (=energy that could be used for other purposes such as reproduction) and increases exposure to predators.

What we still do not know is how fragmentation at multiple scales affects the chances of an individual to reproduce and survive. Is fine-scale fragmentation more important? Or, is it coarse-scale fragmentation? Do different scales of fragmentation interact? Also, will the impacts of fragmentation within the forest patch, and between forest patches, be the same for our adult koala (that does not move much) and its travelling offspring? These questions are of fundamental importance to develop conservation strategies to mitigate habitat fragmentation and aid persistence of species in human-modified landscapes.

Example of coarse-scale vegetation fragmentation (forest patch surrounded by crops, South Brigalow Belt). (Image by Clive McAlpine)

Example of coarse-scale vegetation fragmentation (forest patch surrounded by crops, South Brigalow Belt). (Image by Clive McAlpine)

Building model landscapes

To shed some light on these questions, we first constructed a computer model of the life cycle of an individual animal that adopts different types of movements:

  • short and tortuous movements within foraging areas; and
  • long and straight movements between foraging areas.

We made sure that the chances that an individual survived and reproduced during the cycle depended on how far the individual moved to find habitat.

Next, we constructed artificial landscapes where habitat could be simultaneously fragmented within foraging areas (fine scale) and between foraging areas (coarse scale).

We then simulated the fragmentation of the habitat occurring at fine scale and coarse scales at the same time. To detect how this manipulation affected individual reproduction and survival, we recorded the total number of offspring produced by an individual during its lifetime. We repeated the analysis for individuals that adopted different types of movement with different frequencies, covered different inherent distances when moving between foraging areas, and incurred different risks of mortality when moving between foraging areas. We also considered individuals who were unable to distinguish suitable from unsuitable habitat when moving (‘low habitat selection’) and individuals who could move in the direction of suitable habitat (‘high habitat selection’).

Traits of movement behaviour are the key

According to our analysis, different scales of fragmentation could turn out to be potentially lethal for animals with a suite of movement traits. These include: (1) high frequency of movements between foraging areas, (2) large inherent movement distances between foraging areas, (3) high risk of mortality when moving between foraging areas, and (4) high habitat section. For such animals, the impact of fine-scale fragmentation was accentuated when fragmentation occurred at the coarse scale as well (Figure 1).

Figure 1. Bar chart showing the decrease in the number of offspring (average ± 1 Standard Error) produced when fragmentation occurred at different scales, relative to the case when no fragmentation occurred. The bar shows the case of animals that frequently move between foraging areas, covered large inherent distances between foraging areas, and have high risk of mortality when moving between foraging areas. Results for animals with different degrees of habitat selection are also shown.

Figure 1. Bar chart showing the decrease in the number of offspring (average ± 1 Standard Error) produced when fragmentation occurred at different scales, relative to the case when no fragmentation occurred. The bar shows the case of animals that frequently move between foraging areas, covered large inherent distances between foraging areas, and have high risk of mortality when moving between foraging areas. Results for animals with different degrees of habitat selection are also shown.

How might these findings help protecting species in fragmented landscapes? By guiding our efforts to manage habitat fragmentation. For example, if we want to improve the long-term survival of sedentary individuals, then we should aim to minimize fragmentation at finer scales.

In addition, thanks to our recent work on the causes of fragmentation at different scales, we can also improve land-use management. For instance, the future of sedentary individuals might look brighter if we direct conservation actions, such financial schemes that promote revegetation or retention of native vegetation, towards the land use that is responsible for fine-scale fragmentation. On the other hand, if our priority is to conserve highly mobile individuals, then we should target with management actions the land uses causing fragmentation at both fine and coarse scales.

More info: Lorenzo Cattarino l.cattarino@gmail.com

Reference

Cattarino L, CA McAlpine & JR Rhodes (2016). Spatial scale and movement behaviour traits control the impacts of habitat fragmentation on individual fitness. J Anim Ecol. 85: 168–177. doi:10.1111/1365-2656.12427 http://onlinelibrary.wiley.com/doi/10.1111/1365-2656.12427/abstract?campaign= wolacceptedarticle

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