Sexual Reproduction In Marginal Habitats

4.2.1 Pre-zygotic and post-zygotic limitations to seed production

Inability to reproduce sexually may arise from numerous causes, which can be grouped under two headings, namely pre-zygotic and post-zygotic limitations (i.e. before or after fertilization). Flower development may be prevented or delayed due to unfavourable environmental conditions. Even if a flower and receptive ovules are produced, pre-zygotic limitations may occur due to a lack of suitable vectors for pollination. If pollination succeeds, fertilization of the ovule may still not be achieved, either because the pollen grain does not germinate or else the pollen tube fails to reach the ovary. A brief exposure to low temperatures may be sufficient to prevent fertilization. The small-leaved lime (Tilia cordata) is limited in its distribution in the northern parts of the British Isles as pollen fails to germinate below 15 °C, which is sufficient to prevent fertilization, especially when coupled with a short period of stigmoid receptivity (Pigott, 1991).

This restriction in the northern limit to the natural distribution of T. cordata in the British Isles is in sharp contrast to its higher latitudinal distribution in Finland (Fig. 4.3a). However, despite the warmer midsummer temperatures of the more continental Finnish climate and therefore avoiding the pre-zygotic limitation of fertilization, the subsequent rapid fall in temperature in late summer causes a post-zygotic restriction by preventing embryo and endosperm development (Pigott, 1981). Thus, in both Britain and Finland a failure to produce seed defines the northern boundary of T. cordata, but in the British Isles the cause

Fig. 4.2 Artemisia norvegica; an extreme example of a disjunct distribution and a species that maintains its presence in small isolated populations (data from Hulten & Fries, 1986). (Top) European distribution. (Centre left) Norwegian plant from a population that readily produces seedlings. (Centre right) Scottish plant, from a population that reproduces mainly vegetatively. (Below) Scottish habitat (Norwegian photos Dr A. Moen; Scottish photos Professor R. M. Cormack.)

Fig. 4.2 Artemisia norvegica; an extreme example of a disjunct distribution and a species that maintains its presence in small isolated populations (data from Hulten & Fries, 1986). (Top) European distribution. (Centre left) Norwegian plant from a population that readily produces seedlings. (Centre right) Scottish plant, from a population that reproduces mainly vegetatively. (Below) Scottish habitat (Norwegian photos Dr A. Moen; Scottish photos Professor R. M. Cormack.)

Fig. 4.3 Probability density plots ofthe potential range ofsmall leaved lime (Tilia cordata) modelled on probability distribution under January and July temperatures from European Gridded Climatology for 1961-1990 (Hulme et al., 1995). The four maps show the consequences of an increase in mean annual temperature of 2 ° C obtained by distributing the relative amounts of summer and winter warmth in different ways to obtain three contrasting amounts ofadditional seasonality. (a) The modelled probability distribution under the current European (1961-1990) climate. (b) The summer temperature is increased by 3 °C and the winter temperature by 1 °C. (c) The temperature increase is applied equally, 2 °C each to summer and winter temperatures. (d) The winter temperature is increased by 3 °C and the summer temperature by 1 °C. These maps were made available by courtesy of Dr C. E. Jeffree.

Fig. 4.3 Probability density plots ofthe potential range ofsmall leaved lime (Tilia cordata) modelled on probability distribution under January and July temperatures from European Gridded Climatology for 1961-1990 (Hulme et al., 1995). The four maps show the consequences of an increase in mean annual temperature of 2 ° C obtained by distributing the relative amounts of summer and winter warmth in different ways to obtain three contrasting amounts ofadditional seasonality. (a) The modelled probability distribution under the current European (1961-1990) climate. (b) The summer temperature is increased by 3 °C and the winter temperature by 1 °C. (c) The temperature increase is applied equally, 2 °C each to summer and winter temperatures. (d) The winter temperature is increased by 3 °C and the summer temperature by 1 °C. These maps were made available by courtesy of Dr C. E. Jeffree.

is pre-zygotic and in Finland post-zygotic. Climatic warming can therefore be expected in both cases to permit a significant northward advance of T. cordata.

The above reproductive studies on T. cordata have shown that specific temperature conditions at certain times of the year can be crucial for successful reproduction. It is therefore essential to include seasonal sensitivity in modelling the consequences for species responses to climatic change. When this is done clear seasonal effects can be demonstrated in relation to the geographic distribution of both insects and plants

(Jeffree & Jeffree, 1996). When these methods are applied to T. cordata (Fig. 4.3) it is possible to examine the distribution of T. cordata in Europe based on the limits to distribution under current (1961-90) temperatures with the modelled potential distribution under three seasonally different climate change scenarios in which the mean annual temperature is increased by 2 °C in each case, but with contrasting seasonal differences (see figure legend). The marked contrasts in the direction of potential migration of the species between the scenarios shown in Fig. 4.3b-d emphasize the importance of differences in seasonal temperature. The more continental seasonality with a 1 °C rise in winter and a 3 °C rise in summer (Fig. 4.3b) shows an expansion into Ireland and many areas of Britain, except the far north of Scotland, and a very marked northward advance in Scandinavia to northern Norway, as well as through the Kola Peninsula to the shores of the White Sea (Crawford, 2000). It is also of interest to note that in scenario (d) where the winter temperature is increased by 3 °C and the summer temperature by only 1 °C the maps indicate a marked reduction in the probability of the occurrence of T. cordata throughout its current British range and in the neighbouring oceanic regions of Western Europe.

North American dwarf birch (Betula glandulosa) is an example of a species which can reproduce both sexually and asexually. At the northern limits of the species, less than 0.5% of the seeds (samaras) are viable and populations are maintained by asexual reproduction (Weis & Hermanutz, 1993). The potential role of pollination dynamics in the loss of sexual reproduction in North American dwarf birch has been compared at Tarr Inlet on Baffin Island (64° N), near the northern limit of the species, with populations at Kuujjuaq, Quebec (58° N) at the centre of its distribution where sexual reproduction is the primary mode of reproduction. It was found that plants at the peripheral site at Tarr Inlet attained only 15-30% of the pollen production achieved at the more southerly site due to a combination of a lower density of staminate catkins and less pollen per catkin. Potential seed productivity appeared to be further constrained at the northern limit with pistillate catkins producing 50% fewer flowers in the north than in the south. While stigmatic pollen loads were similar at both sites, the northern site had lower pollen viability (68% versus 93%). There was also at the northern site a reduction in cross-fertilization due to clonal growth, which increased the probability of geitonogamous pollination (pollen from different flowers on the same plant and as a result self-sterility, a pre-zygotic limitation) and became the main cause of reduced seed production.

Post-zygotic causes of reproductive failure encompass all the varying aspects of ecology that control species survival. These may start with low or shortlived seed viability, inadequate seed dispersal, as well as destruction by predators and pathogens, and failure to be able to compete with better adapted species.

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