Structural, functional and temporal fragmentation: the impossibility of pleasing everyone
Authors: Hinsley, S.A., Hill, R., Bellamy, P.E., Ferns, P.N. and Harrison, N.M.
Conference: 25 Years of Landscape Ecology: Scientific Principles in Practice
Dates: 8-12 July 2007
Journal: Proceedings of the 7th IALE World Congress
Abstract:As landscape ecologists we are familiar with the concepts of habitat fragmentation, mainland/island relationships and metapopulations. These envisage discrete habitat patches dispersed in some type of non-habitat matrix and the standard “habitat patch” is often a uniform entity with extent, but no internal structure. In reality, habitat structure is often highly heterogeneous, and different organisms’ resource requirements e.g. food, shelter, nest sites etc. are usually unevenly distributed in time as well as space. Therefore, fragmentation need not be only structural. Habitat composition, and phenology of both the habitat and the inhabitants, may also contribute detrimental fragmentation effects which are not immediately apparent from patch structure and extent alone. For arboreal insectivorous birds such as great tits, Parus major, and blue tits, Cyanistes caeruleus, the availability of trees around the nest hole is vital for breeding success. For 22 nest boxes in woodland, the mean percentage gap fraction (gaps defined as heights < 1 m obtained from a Digital Canopy Height Model derived from airborne remote sensed data acquired using Light Detection and Ranging, LiDAR, Hinsley et al., 2002) within a 25 m radius circle of each box was 0.64 ± 1.18%, with a range of 0-4.3%. In contrast, in an urban park, the corresponding mean value of the gap fraction for 26 boxes was 32.7 ± 22.6%, with a range of 0.1-88%. Such gaps are likely to contribute to the poor breeding success typical of park-breeding tits (Table 1). Furthermore, the costs to the adults of rearing young (measured as female daily energy expenditure, kJ day-1, using doubly labelled water) appeared to increase rapidly above a threshold of about 35% gap (Hinsley et al., in prep.).
For woodland tits in the UK, oak trees are favourable as foraging sites (e.g. Naef-Daenzer et al., 2004). Again, using airborne remote-sensed data (multi-spectral Airborne Thematic Mapper, ATM), for the same sample areas around the 22 woodland nest boxes, the canopy cover of English oak, Quercus robur, was 19.6 ± 16.9%, range 1.7-66%. In the park, the incidence of English oak was zero for most boxes. Unlike the wood, many of the tree species in the park were exotics. Compared with native species, exotics may be poor foraging substrate for invertebrates and thus also poor foraging substrate for insectivorous birds. Therefore, in addition to the spatial gaps around their nest sites, the park birds may also encounter functional gaps. The significance of functional gaps was demonstrated by the woodland birds where the daily energy expenditure of female great tits rearing young increased as the proportion of oak around their nest boxes decreased (Hinsley et al., in prep.).
Recent work on the effects of climate change and increasing spring temperatures in Europe has identified the possibility of a temperature-induced mismatch between birds’ timing of breeding and that of their food supply (Both and Visser 2005). Exotic tree species may also present birds with phenological problems; should they prove suitable as invertebrate hosts, their timing of leafing may be too early or too late for the birds. Urban great tits tend to breed earlier than their woodland counterparts, whereas timing in blue tits remains similar, e.g. in 2006, mean first egg date for great tits (n = 11) and blue tits (n = 7) in Bute Park, Cardiff was 20.6 ± 5.3 and 26.1 ± 2.3 respectively, compared to 27.6 ± 5.7 for great tits (n = 16) and 26.1 ± 4.8 (n = 16) for blue tits in woodland (where April 1st = 1). Overall, tree leafing in the park was about 12 days earlier, but the range was wider due to the much greater number of tree species. Thus there is considerable scope for mismatching between birds, trees and invertebrates.
An organism’s susceptibility to deleterious effects of fragmentation may also vary with other factors. For example, when egg laying, a female tit is relatively free to move around its habitat, fragmented or not. However, when feeding young, all food must be taken to the nest increasing the costs of exploiting patchy habitat (Hinsley 2000). Exposure to inclement weather may also be greater in fragmented habitat and probably contributes to the greater variability between years in breeding performance of tits in parks compared to large woods. Therefore, habitat may be more fragmented than is simply apparent from its structure at a particular time. While management often seeks to increase habitat diversity and maintaining or increasing biodiversity in general is desirable, or even essential for long-term ecosystem function, there may be circumstances where diversity favours generalists at the expense of specialists. In the UK, most semi-natural habitat occurs in small patches and tends to be highly fragmented. Networks of patches and linear features have high edge to interior ratios. In densely populated countries where most of the landscape is highly modified and managed, is it inevitable that habitat specialists will lose out to generalists?
Source: Manual
Preferred by: Ross Hill