Habitat connectivity is a crucial factor for the survival and thriving of various species and, thus, biodiversity. It enables species to access critical resources, find mates and adapt to environmental changes. In other words, it drastically reduces the risk of extinction.
In this blog, we’ll break down the current state of habitat connectivity, deep diving into National Statistics Indicator 3: Habitat connectivity in the wider countryside, and look at how it can serve as a measure to gauge biodiversity health.
A Brief History of Biodiversity Indicator 3
This state indicator was originally developed by Forest Research in collaboration with the UK Centre for Ecology & Hydrology. It used Countryside Survey data from 591 one-square-kilometre plots of land throughout Great Britain in 1990, 1998 and 2007. Expert opinion was used to assess the relative likelihood of movement by species characteristic of each habitat, between habitat patches, and across different intervening land cover types found in the survey - primarily woodland and grassland (these habitats were chosen due to their widespread occurrence and differing ecological dynamics).
As depicted in the graph below, the collected data indicated that for woodlands, the connectivity remained mostly the same from 1990 to 2007. Even though the area of these woodlands increased, other factors like changes in the landscape might have offset this, keeping the connectivity relatively stable. For grasslands, the data showed an improvement in connectivity. This is likely because the overall area of neutral grassland increased during this period.
While, at the time, the data promised to provide valuable insights into habitat fragmentation and connectivity, experts determined that more research was needed to understand why connectivity had changed. They concluded that too much was left to ambiguity when evaluating why the size of the habitats was growing or shrinking, or otherwise changing in the areas around them. As such, the specific data set has not been updated since 2007. In other words, despite the apparent improvements seen in grasslands, measuring and assessing the effects of habitat connectivity on these changes can’t be confirmed until there is more holistic and consistent data available.
As such, the UK Biodiversity Indicators Steering Group decided to reclassify the indicator as “experimental” or “under development” while they review the accuracy and application of alternative data sets. This allows for the exploration of alternative headline measures, acknowledging that the original data is now archived and no longer serves as a current benchmark.
The latest measurements of habitat connectivity (which were last updated in 2012) come from a variety of new data sources and apply different methodologies and measurements than those of the original indicator. They are still under peer evaluation and are being assessed to determine whether or not this indicator adds any real value to the understanding gains or losses in biodiversity and how accurate it is in measuring biodiversity health.
Before we dive into the details of the more recent habitat connectivity data, it’s important to have a thorough understanding of the terminology used in its evaluation:
Structural Connectivity. This is focused on the physical layout of habitats. In other words, how patches of forests, grasslands, or other environments are physically linked or separated by other land features or human developments (think roads and roads or farmland and housing developments). It evaluates the potential for species to move between different habitat patches.
This concept is often used in land-use planning and conservation to identify potential wildlife corridors or to assess the impact of human activities on natural habitats.
On the other hand, functional connectivity goes beyond just the layout; it considers how species actually move and interact between these patches. Put simply, while structural connectivity might show that two habitats are close, functional connectivity tells you whether species are actually able to or likely to move between them. It does this by taking into account the behaviour of the species in question, including their ability to navigate the landscape, their dispersal distance, and their specific habitat requirements.
Finally, we have population synchrony, a phenomenon where populations of the same species, located in different geographical areas, exhibit similar trends in their growth rates or numbers over time. This synchrony can be driven by various factors such as shared environmental influences, migration between populations, or even similar responses to human activities. The level of synchrony is often measured using statistical correlations between time-series data of population sizes or growth rates from different locations.
Understanding population synchrony is crucial for conservation efforts. High synchrony might indicate that populations are susceptible to the same threats. Low synchrony could suggest that different populations are influenced by local factors, which might require targeted conservation strategies.
The Latest Results on Habitat Connectivity
In its latest iteration, with data updated up to 2012, the indicator focuses on 33 butterfly and 29 woodland bird species in the UK. It employs population synchrony to assess functional connectivity, examining the correlation between population growth rates of each species at different monitoring sites over time. By quantifying this, the indicator aims to guide more effective landscape conservation strategies, ultimately reducing the risk of species extinction.
The following graphs give a quick visual idea of the experimental indicator’s results:
In Figure 3.1, depicting two graphs that monitored the 33 butterfly species, it’s shown that UK butterflies' functional connectivity levels remained fairly consistent from 1985 to 1995. However, it took a significant hit in 2004, dropping to just 48% of its original level. The causes of this, or theory as to why this may be, is unlisted in the indicator’s report. Fortunately, by 2012, not only did it recover, but it also saw a 10% improvement compared to 1985.
In the late short-term, between 2000 to 2012, 72% of butterfly species experienced an increase in their ability to move between habitats. However, in the long term, covering the entire breadth of data between 1985 - 2012, the data showed that only 33% of species improved, while 19% worsened, and 48% remained the same.
Figure 3.2, focusing on woodland birds, shows a different situation. Their functional connectivity was stable from 1985 to 1996 but started to decline from 1999 to 2012. It reached its lowest point in 2005, being only 44% of its 1999 level. Any explicit explanation as to why this may have occurred is not stated in the reports.
While there have been some signs of recovery, the majority of these bird species (57%) have seen a decline in their ability to move between habitats in the more recent years from 1999 to 2012. It's important to note that long-term trends for birds were not fully assessed due to a gap in the data between 1996 and 1999.