In our recently published paper we studied the effect of historical landscape changes on plant and arthropod communities harboured by grassland habitat islands.
Deák, B., Bede, Á., Rádai, Z., Tóthmérész, B., Török, P., Nagy, D.D., Torma, A., Lőrinczi, G., Nagy, A., Mizser, S., Kelemen, A., Valkó, O. (2021): Different extinction debts among plants and arthropods after loss of grassland amount and connectivity. Biological Conservation 264: 109372. doi: 10.1016/j.biocon.2021.109372.
The paper is open access and can be freely downloaded from here.
Loss and fragmentation of natural and semi-natural habitats pose a major threat for biodiversity worldwide. Their effect are often coupled with additional effects (such as altered abiotic conditions, abandonment, invasion of plant and animal species) that lead to the degradation of the habitat, which on the one hand threatens the survival and reproduction of habitat specialist species and on the other hand supports the establishment of generalists. Habitat loss, fragmentation and degradation often lead to a decrease in population size and fitness of habitat specialist species that can finally result in the local extinction of area- and disturbance-sensitive species. However, such extinction processes are not necessarily immediate. Many populations show a delayed extinction causing an 'extinction debt' that is typical of recently fragmented landscapes in which the area and/or the spatial configuration of the habitat fragments are just below the extinction threshold for certain species. Existence of extinction debt can be assumed when the characteristics of the past landscape explain current species richness better than the characteristics of the present landscape.
In our study we aimed to explore the temporal dimensions of extinction debt across plant and arthropod taxa (ants, orthopterans, true bugs and rove beetles) in grassland habitat islands embedded in agricultural landscapes. We related the species richness of habitat specialist and generalist species to the current and past landscape metrics referring to the total grassland area and patch connectivity in the surroundings of the focal patch. We collected data from 60 dry focal grassland fragments (situated on ancient burial mounds so called 'kurgans'). Our study covered a period of 158 years including four time periods.
Grassland habitat island on an ancient burial mound.
We hypothesised that (i) historical landscape factors explain the current species richness of habitat specialists more than that of generalists, (ii) habitat connectivity has a larger effect on specialist species than habitat amount and (iii) area and environmental heterogeneity of the focal grassland patch increase the relaxation time of specialists whereas encroachment of woody species decreases it.
Map of the surveyed grassland fragments (n = 60). Changes in the grassland cover around one of the studied focal habitat patches over time: B – (1858–1864), C – (1881–1884), D – (1956–1975), E – (2014–2016).
We found that in the study area the amount of dry grasslands has been declining constantly from the 1850s until present days. Interestingly, the constant loss of habitats during the four studied periods did not result in a significant loss of habitat connectivity for an extended time interval during the first three periods (from 1858 to 1975). Even habitat loss accelerated after the WW II, the still existing network of farms and the related heterogeneous land use system with the many small grassland fragments could hinder the loss of connectivity. Habitat loss was severe in the later phase of the communist era and during the change of regime in the 1990s when crop monocultures appeared.
Effect of grassland amount and connectivity on the species richness of specialist (green dots) and generalist (grey dots) plant species in Periods III–IV. Solid lines represent significant, dashed lines non-significant relationships.
In our study sites current species richness of specialist plants correlated stronger with grassland amount and connectivity typical to the landscape five to seven decades ago than with present landscape characteristics. And the effect of past grassland connectivity was larger on specialist plants than the effect of past grassland amount, likely because loss of connectivity in recent times had more severe effect on the functional connections of remaining populations. The fact that the populations of specialist plants in the focal habitat patches still have not come to equilibrium with the attributes of the present landscape suggests that further extinctions can be expected especially in the case of specialist species sensitive to the loss of metapopulation connections. The reason for the long relaxation time is that in general, plants as sessile organisms react slowly to changes in the neighbouring landscapes and in local conditions. Traits typical to many plant species such as long life-span, ability for clonal reproduction, possession of seed banks can support the long time survival of plant species even under altered environmental conditions leading to a considerable time lag between the changes in landscape changes and changes in the species composition of specialist plant assemblies.
Interestingly high slope inclination that is a good proxy for high environmental heterogeneity (see also in Deák et al. 2021 Science of the Total Environment), had a significant positive effect on the species richness of specialist plants and assumed to mitigate extinction process (and increased time lag). The reason for that might be that high environmental heterogeneity expressed in topographic, micro-climatic heterogeneity and heterogeneity in soil properties can also increase the chance for the long-term persistence of plants within the focal grassland patch since even short distance disperser plant species can achieve short shifts within the focal habitat to establish in the microsite that is edaphically and climatically most favourable for them.
In contrast to specialist plants, the species richness of the studied specialist animal taxa (ants, orthopterans, true bugs and rove beetles) was not related to grassland amount or connectivity; thus, they were either unaffected by landscape changes or have already paid their extinction debt induced by landscape transformations in the past. The reason for that is despite plants, animals can actively disperse and leave habitats that have become unsuitable due to unfavourable changes. However, when animals are not able to relocate to another suitable habitat patch, local extinction processes might be much faster for them because of their short life cycles and high sensitivity to the lack of metapopulation connections.
However, the fact that dry grassland specialist plants showed a partially paid extinction debt suggests that their observed current species richness is not in equilibrium with the composition and configuration of the present landscape yet. This can be considered an early warning signal suggesting that even if grassland loss will be reduced in the landscape, further extinctions may be expected, which will reduce the biodiversity of the focal habitat fragment and also the biodiversity of specialist plants on a landscape level. Mitigating these negative processes should be considered a conservation priority as in heavily fragmented lowland agricultural landscapes, a considerable amount of grassland-specific species is only maintained by small habitat islands.
Abstract
A decrease in habitat amount and connectivity causes immediate or delayed species extinctions in transformed landscapes due to reduced functional connections among populations and altered environmental conditions. We assessed the effects of present and historical grassland amount and connectivity as well as local habitat factors typical of the present landscape on the current species richness of grassland specialist and generalist plants and arthropods in grassland fragments. We surveyed herbaceous plants, ants, orthopterans, true bugs and rove beetles in 60 dry grassland fragments in Hungary. We recorded the area of the focal grassland, the slope and the cover of woody vegetation. By using habitat maps of the present and historical landscape, we calculated grassland amount and connectivity for four time periods covering 158 years (1858–2016). We found evidence for an unpaid extinction debt in specialist plants, suggesting that they have not come to equilibrium with the grassland amount and connectivity of the present landscape yet. This localised and typically long-lived group responded slowly to the landscape changes. Specialist arthropod taxa with short generation times responded much faster to habitat loss than plants and did not show an extinction debt. Generalist plants and animals adapted to a wide range of habitats were affected by the landscape-scale decline of grassland habitats. Despite decreased habitat connectivity, grassland fragments with dry environmental conditions and high environmental heterogeneity can sustain specialist plants in transformed landscapes. Unpaid extinction debt should be considered an early warning signal: Restoration of grassland connectivity is necessary to halt ongoing extinction processes.
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