Lepidoptera and climate change: consequences for UK woodland

 

Summary

With the current climatic trend towards higher temperatures and lower summer precipitation, a positive effect on moth numbers had been expected. Whilst climatic variables are driving changes in numbers and species range, they do not explain the decline in both numbers of species and their frequency.

Studies have suggested that changes in climate attributed to global warming and anthropogenic factors such as agricultural intensification and habitat fragmentation, may have a significant effect on the status and range of macro-moths (Lepidoptera) in the UK. A number of these moths are known defoliators of UK trees and changes in their abundance and the timing of life cycle events could have a significant effect on the health of our trees. A preliminary investigation was undertaken by Forest Research to explore the relationship between climate and a number of key moth species.

Moth trap data collected from within the 850 ha ECN site at Alice Holt was examined in conjunction with meteorological data for changes in moth abundance, diversity and range. This will clarify how the future health and productivity of trees may be impacted by changes in defoliating species if the current climate trends continue.

Analysis

Preliminary results show that, in general, species of macro-moths local to the Alice Holt area are declining in both numbers and diversity. This decline is also clearly seen in a sample of moths associated with herbaceous plants. Similar findings at Rothamstead Research in Hertfordshire suggest the losses may not be local, but be indicative of a wider pattern in the UK (Butterfly Conservation press release, April 2003).

The relationships observed between moth abundance and climate support previous studies which indicated that moth numbers are positively related to high summer temperature and negatively associated with high rainfall in the current year (Pollard, 1988).  Analysis of life-event tables reveal no apparent life-cycle event (that is egg, pupae, larvae or adult) where the effects of climatic variation was clearly evident; however this is most likely to reflect the wide inter-specific variation in phenology which is characteristic of Lepidoptera.
 
While the results reveal that rain and temperature are important factors influencing moth numbers, the results do not clarify why, in general, the number of species and the abundance of individual species are in decline.  Climate change may be expected to have indirect as well as direct effects on moths. For example, changes in tree phenology, ie advancement of tree bud-burst, may mean that larvae hatching is out of synchrony with their food source. Existing evidence indicates that moths are failing to responding to such phenological changes and are  appearing later than in previous decades (Burton and Sparks, 2002).

This loss of moth defoliators may initially appear beneficial. However, local decline and extinction may be indicative of a shift in range, which simply results in the adverse affects associated with moths as defoliators being transferred elsewhere. Migrant and alien species may then colonise vacated habitats not previously available because of the inhibiting effects of resource competition.

Method

Moths have been collected in a standardised light trap on a daily basis at Alice Holt since 1966. This long-term data-set has been analysed alongside the climatic data for the same period.

A sample of 40 key species representing those moths occurring in sufficient numbers which are known to be associated with broadleaved trees were analysed for long-term trends in the abundance, and for individual species, for changes related to variation in climate. Modelling this relationship produced a variable ‘count index’, which was regressed onto each climate variable including a lag of up to three years. Life-event tables were constructed to examine whether relationships between climate, moth numbers and life-events existed. A further sample of 36 moths, primarily associated with herbaceous plants, was compared with the broadleaf associated moth sample to examine whether the trends were specific to the associated broadleaf tree species. 

Trends in climate variables

Precipitation for consecutive autumn (Sep, Oct, Nov) and winter (Dec, Jan, Feb) seasons of each year of the
study show a general increase. In contrast spring (Mar, Apr, May) and summer precipitation (Jun, Jul, Aug) shows a general decline:

Average dry air temperature (^oC) for consecutive autumn/winter seasons, and spring/summer show a general increase:

In general the number of moths collected at Alice Holt has declined over the study period 1966-2001:

Variation and trend in moth species diversity

There has also been a general decline in the number of species occurring:

• 82.5 % show a negative trend across the study period
• 15 % show a positive trend across the study period
• 2.5 % show no trend i.e. remained stationary for the study period.

Data modelling

Distribution of significant relationships (direction indicated) between the count index for each key species and precipitation and temperature across seasons by year, t = current year, t-1 = 1 year lag, t-2 = 2 year lag.

Comparing the key species sample with a sample of herbaceous associated moths

The negative trend and pattern of variation in the numbers of moths primarily associated with broad-leaved trees is not specific to this sample and is mirrored in a sample of species associated with herbaceous plants.

Conclusions

The current and future status of moth populations in the U.K. may be of more concern to entomologists and conservationists than to the forestry sector. However results presented here are only preliminary and future analysis, in particular that which combines longitudinal data sets from other sites with that held at Alice Holt should reveal the nature and extent of species loss and gain, and evaluate changes in abundance. In addition further analysis at the species-specific level is required. Such analysis may help anticipate where in the future variation in moth numbers and species are likely to be a cause for concern from the perspectives of both conservation and forestry.