From the 2015/16 Critical Thinking Alumni Gergana and John and a year in the making, here are the final results of our exciting research project, are rare species really more likely to be declining in the UK?
Around this time last year, we, Isla’s 2015/2016 Critical Thinking group, started a research project – it was a great experience for all of us and it was fun to go through the different stages of a research project together. We have since graduated, but our project, and in particular the results, are still on our minds. We are preparing a manuscript for submission to a peer-review journal, so hopefully you’ll be able to read all about the outcomes of our research soon. Until then, here is a summary of our project’s goals, outcomes, as well as the key things we learned in the process.
Are rare species (those with smaller geographic ranges and lower local abundance) more likely to have declining population trends than common species?
H1: As geographic range increases populations will have a lower rate of population change.
H2: As geographic range increases populations will have a higher rate of population change.
H0: The rate of population change will not vary with geographic range size.
To address out research question, we compiled population trends data for 211 UK vertebrate species from the Living Planet Index (LPI) with occurrence data for the same species from the Global Biodiversity Information Facility (GBIF). We thought about how we will define ‘rare’ and ‘common’, and decided on using a combination of geographic range extent and local abundance. Habitat specificity is another possible measure of rarity and commonness, which we could include in the analysis as we take it further. Nevertheless, we already have exciting results from our analysis using geographic range and local abundance.
We extracted the slopes of experienced population change for our species during the time period for which LPI data were available (at least 5 years), with positive slopes indicating a population increase, and negative slopes – a decline. We then plotted slopes against geographic range and local abundance. We estimated species’ geographic range in two ways: firstly, we looked at occurrence maps and categorised the range of each species as UK, partEurope, Europe, partWorld, and World. Secondly, we quantified range as the maximum differences in latitude and longitude of occurrences in the GBIF dataset.
We found that the species with the smallest ranges, those in the UK and partEurope categories, are actually the ones that are experiencing the smallest amount of population change, and most of them have positive slopes, indicating population growth. There is a big spread in the slopes of species with a partWorld – World distribution, so this is geographic range at which both most declining and most increasing populations are found. Comparing levels of population change across all the range categories did not reveal clear evidence in support of one of our hypotheses. We did not find strong evidence that species with more local ranges are showing declines across the time period of their respective LPI data (H1). There is a weak trend of rates of population change becoming higher as geographic range increases (H2), but overall, we didn’t detect much variation in population change based on our geographic range categories (H0).
Figure 1. Rate of population change versus extent of geographic range for 211 UK species populations. Data obtained from LPI and GBIF.
Looking at how our second measure of geographic extent, maximum differences in latitude and longitude of occurrences in the GBIF dataset, influences rates of population change provided support for our null hypothesis – rate of population change does not appear to vary significantly with range size (Figure 2). We find no evidence that population declines or increases are occurring more frequently for UK species with a smaller geographic range.
Figure 2. The geographic range (maximum differences in latitude and longitude of occurrences in the GBIF dataset) versus the population change overtime for the 211 LPI species monitored in the UK.
Investigating rates of population change versus local abundance also showed that population declines or increases are not occurring more frequently for UK species with a smaller population size (Figure 3). These results are part of an ongoing investigation, but for now, we conclude that for the UK species populations we studied, rate of population change does not vary with geographic range size (H0).
Figure 3. The log population size (number of individuals) versus the population change overtime for the 211 LPI species monitored in the UK.
With the data present unwrapped, the next steps of our research project are to ponder what are the implications of our findings, and write a manuscript to communicate our key findings. Questions we will consider include why are patterns of rarity and commonness poor predictors of rates of population change for our UK species, what does this mean for conservation, and since we know that geographic range doesn’t appear to be influencing population change in those species, we could hypothesise what other factors are affecting species’ population trends.
By Gergana and John