Seldom do we consider ‘edge effects’ in EIA

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Edge effects can be abiotic or biotic. Murcia (1995) indicated that abiotic edge effects included lower biomass and lower or different structural complexity that alters the microclimate, with the air temperature, air moisture, soil moisture and light intensity varying between edges and the interior in forest fragments, and these were modulated by orientation and physiognomy. Biotic variations include differences in plant biomass and species composition.

Fragmented habitats and landscapes have increased habitat edges and small mammals can respond both positively and negatively to edges depending on their ecological traits and the biotic and abiotic features of the edges (Laurance 1991, 1994, Goosem and Marsh 1997, Goosem 2000).

Edge effects include disruption to ecological processes such as predation and dispersal (Paton 1994), food availability (Mills 1995), animal movements (Oxley et al. 1974, Goosem 2001) and can change assemblage structure (Laurance 1991, Mills 1995, Temple 1998, Luck et al. 1999, Schlaepfer and Gavin 2001, Harding and Gomez 2006, Craig et al. 2015), with responses to edges varying among taxa. For example, Craig et al. (2015) reported that mammal communities showed no edge response along unmined Jarrah forest edges, however, the overall reptile abundance was higher, even though some reptile species had lower abundances along edges (e.g. Egernia napoleonis). Contrary to this, Harding and Gomez (2006) reported an increase in medium sized mammal abundance along edges in tropical rainforests.

Anderson and Burgin (2002) indicated that in small remnant woodlands the abundance of three species of small skinks diminished in close proximity to the clearing edge, and Carvajal-Cogollo and Urbina-Cardona (2015) reported a similar result in the Caribbean Region of Colombia. In the open Tomago forest in New South Wales, Bragg et al. (2005) reported lizard assemblage patterns were altered by edges, with some species negatively and others positively correlated with canopy cover. Lehtinen et al. (2003) reported edge-avoiding reptile and amphibian species are more prone to local extinction than non-edge avoiders, and that wind speed were generally higher and humidity lower around edges at their study site in Madagascar. Andrews (1990) and Yahner (1988) in an earlier review of the fragmentation caused by roads and utility corridors and edge effects reported they contributed to:

  • habitat loss and modification with accompanying effects on populations;
  • increase spatial heterogenetity, which can increase fauna diversity;
  • edge effects intruding into the core of natural areas;
  • barriers to distribution and dispersal; and
  • subdivision and isolation of populations.

Atkinson (2003) reported there were no community edge effects for avian species in karri forest, however, the only guild to avoid edges was nectarivores. Atkinson (2003) reported avian species potentially disadvantaged by the creation of clear-fell edges in karri forests were the Purple-crowned Lorikeet, New Holland Honeyeater, White-naped Honeyeater, Little Wattlebird and Rufous Treecreeper.

Implications and what does it mean for EIA or fauna surveys?

The main consequence, and one that is very rarely reported, is that the impact area will be much larger than the actual cleared area (Strevens et al. 2008). Tracks and power line assessments, and newly created ‘edges’ also provide improved access to areas for feral predators including the cat, fox and wild dogs (Goldingay and Whelan 1997).

Linear corridors and clearing for mineral exploration

The potential impact of clearing linear corridors of vegetation that are associated with developments such as railway lines, haul roads, pipe lines and power lines are likely to have a different level of impact than if the same size area was to be cleared in a concentrated configuration, such as a mine site or housing development. Linear corridors often result in the clearing of quite a narrow track of vegetation, but they can have an equally significant or greater impact in the adjacent areas because of ‘edge effects’ (Strevens et al. 2008). Linear corridors and highly fragmented ecosystems (i.e. exploration tracks) have a higher propensity to develop weed infestations which can impact on natural fauna habitats. Cleared corridors can also provide improved predator access to areas, enhance the invasion of pest species into areas and may act as inhibitors or disrupt fauna migration and movement patterns.

See two examples below, where a mining company has undertaken exploration drilling and vegetation clearing prior to obtaining mining approval. It looks like the damage has already been done.

habitat frag 1habitat frag 2

Example

The Department of Mines and Petroleum has just released revised ‘Guidelines for Mining Proposals in Western Australia‘. This document requires a mining company to define and map the disturbance area (i.e. maximum area within which the activities will be located). We would argue that this is not the actual disturbance area as it often fails to take into account the edge effects on the ecosystem. This document goes on to state (p12):

The Mining Proposal assessment will consider the likely environmental impact/s of the proposed activities within the extent of the disturbance envelope, and on the surrounding environment. DMP will require the proponent to undertake a review of the environmental impacts of the proposed activities within the envelope and mitigate against the impacts identified. This may result in the requirement to survey the envelope area (and possibly surrounding areas) to identify environmental values that are likely to be impacted and/or require protection. A Mining Proposal may also employ the use of a disturbance envelope to illustrate areas that will be excluded from the proposal and within which no activities will occur (e.g. sensitive areas such as Aboriginal Heritage Sites, reserves, wetlands, waterways, prominent ridges, etc.). All areas included in the disturbance envelope will need to be adequately addressed in the risk assessment and the environmental outcomes. An example of a disturbance envelope showing sensitive areas intentionally excluded (eg. river and nature reserve) is provided in Figure 3.

Figure 3 shows a disturbance envelop adjacent to a creek line and Jelly Nature Reserve and it clearly implies that because the disturbance area has excluded the creek line and the nature reserve, it is not having an impact on these areas. Disturbance areas are often much larger than the area cleared of vegetation.

Mining disturbance area

Within the current EIA framework is it actually possible to measure and assess the ‘real’ impacts of edge effects (i.e. impacts on mobile fauna, plants impacted by windblown dust, downstream impacts on ground and surface waters, increased visitation of feral and pest animal species, etc)?

In light of this discussion on edge effects and the unknown or poorly measured impacts that are happening outside of the actual disturbance area, do you think appropriate buffers should be applied to proposed disturbance areas during an EIA process?

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References (can be supplied on request)

Anderson, L., and M. Burgin. 2002. Influence of woodland remnant edges on small skinks (Richmond, New South Wales). Austral Ecology 27:630-637.

Andrews, A. 1990. Fragmentation of habitat by roads and utility corridors: a review. Australian Zoologist 26:130-141.

Atkinson, P. I. 2003. Edge effects and birds across karri forest (Eucalyptus diversicolor) clear-fell edges: A study of theory and conservation management. Murdoch University, Perth.

Bragg, J. G., J. E. Taylor, and B. J. Fox. 2005. Distributions of lizard species across edges delimiting open-forest and sand-mined areas. Austral Ecology 30:188-200.

Carvajal-Cogollo, J. E., and N. Urbina-Cardona. 2015. Ecological grouping and edge effects in tropical dry forest: reptile-microenvironment relationships. Biodiversity and Conservation 24:1109-1130.

Craig, M. D., V. L. Stokes, G. E. S. Hardy, and R. J. Hobbs. 2015. Edge effects across boundaries between natural and restored jarrah (Eucalyptus marginata) forests in south-western Australia. Austral Ecology 40:186-197.

Goldingay, R. L., and R. J. Whelan. 1997. Powerline easements: do they promote edge effects in eucalypt forest for small mammals? Wildlife Research 24:737-744.

Goosem, M. 2000. Effects of tropical rainforest roads on small mammals: edge changes in community composition. Wildlife Research 27:151-163.

Goosem, M. 2001. Effects of tropical rainforest roads on small mammals; inhibition of crossing movements. Wildlife Research 28:351-364.

Goosem, M. W., and H. Marsh. 1997. Fragmentation of small mammal community by a powerline corridor through tropical rainforest. Wildlife Research 24:613-629.

Harding, E. K., and S. Gomez. 2006. Positive edge effects for arboreal marsupials: an assessment of potential mechanisms. Wildlife Research 33:121-129.

Laurance, W. F. 1991. Edge effects in tropical forest fragments: application of a model for design of nature reserves. Biological Conservation 57:205-219.

Laurance, W. F. 1994. Rainforest fragmentation and the structure of small mammal communities in tropical Queensland. Biological Conservation 69:23-32.

Lehtinen, R. M., J.-B. Ramanamanjato, and J. G. Raveloarison. 2003. Edge effects and extinction proneness in a herpetofauna from Madagascar. Biodiversity and Conservation 12:1357-1370.

Luck, G. W., H. P. Possingham, and D. C. Paton. 1999. Bird responses at inherent and induced edges in the Murray Mallee, South Australia. 1. Differences in abundance and diversity. Emu 99:157-169.

Mills, L. S. 1995. Edge effects and isolation: Red-backed voles on forest remnants. Conservation Biology 9:395-403.

Murcia, C. 1995. Edge effects in fragmented forests: implications for conservation. Trends in Ecology and Evolution 10:58-62.

Oxley, D. J., M. B. Fenton, and G. R. Carmody. 1974. The effects of roads on populations of small mammals. Journal of Applied Ecology 11:51-59.

Paton, P. W. C. 1994. The effect of edge on avian nest success: How strong is the evidence? Conservation Biology 8:17-26.

Schlaepfer, M. A., and T. A. Gavin. 2001. Ecological Effects of Roads and Traffic: A Literature Review. Conservation Biology 15:1079-1090.

Strevens, T. C., M. L. Puotinen, and R. J. Whelan. 2008. Powerline easements: ecological impacts and contribution to habitat fragmentation from linear features. Pacific Conservation Biology 14:159-168.

Temple, S. A. 1998. The edge of the cut: implications for wildlife populations. Journal of Forestry 96:22-26. Yahner, R. H. 1988. Changes in Wildlife Communities Near Edges. Conservation Biology 2:333-339.

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Comments

2 Responses to “Seldom do we consider ‘edge effects’ in EIA”

  1. Melanie Dixon on June 30th, 2016 1:30 pm

    Great article on edge effects.
    I am currently working on a maintenance project for transmission towers and have observed the encroachment of weeds and change in vegetation along the edge of the corridor.
    I think edge effects are an important consideration of future developments and should be incorporated into EIA, and other environmental approval, processes.

  2. Scott Thompson on June 30th, 2016 1:40 pm

    Thanks Mel. Too often everyone is focused on the immediate vegetation clearing impacts and don’t consider the secondary impacts (i.e. increased rate of invasion by weeds and feral fauna etc). Lets hope that these concepts are addressed more frequently during the EIA process in the future.

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